Download Chapter 8 Prototype - DTUsat-1

Tether Electronics for DTUsat
Electronics for a Satellite Subsystem,
Detailed Design and Prototyping
Project by
Richard Tøpholm
Ørsted•DTU
AUTOMATION
Richard Tøpholm
Tether Electronics for DTUsat
Tether Electronics for DTUsat,
Detailed Design and Prototyping
Project by
Richard Tøpholm
Technical University of Denmark
Ørsted•DTU AUTOMATION
As supervisors:
Mogens Blanke, Ørsted•DTU
Claus Kjærgaard, Øersted•DTU
Summer and Fall 2002
Richard Tøpholm
Tether Electronics for DTUsat
Abstract
In the summer of 2001, a student project was initiated at DTU, with the
goal of designing and building a satellite.
This student satellite is named DTUSat, and that fall I wrote a report
(“Electrodynamic Tether for DTUsat”, Feb. 2002) on the investigation
and design of an Electrodynamic Tether payload for it. This electrodynamic tether is a long conducting wire, which is deployed from the satellite. As the tether moves through the Earths magnetic field at orbital
speeds, electrodynamic forces act on it. It can thus be considered a means
of propulsion that does not expend fuel, saving weight and money.
The needed electronics to control this tether has been developed from
sketch to prototype in the present project the summer and fall 2002, as
reported here.
Chapters 1 “Introduction” and section 3.3 “Electron Emitter” are
quoted almost verbatim from my earlier report.
Abstract
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Richard Tøpholm
Tether Electronics for DTUsat
Contents
Chapter 1
Introduction
1.1 Space Tether Systems
1.1.1 Free Fall
1.1.2 Vacuum
1.1.3 Magnetic Field
1.1.4 Ionospheric Plasma
1.2 Electrodynamic Tethers
Chapter 2
Motivation
2.1 Requirements
Chapter 3
Theory
6
6
7
7
7
7
8
10
10
12
3.1 Tether System
12
3.2 Voltages
12
3.3 Electron Emitter
13
Chapter 4
Electrical Configuration
4.1 The elements
4.1.1 The Tether
4.1.2 Emitter
4.1.3 Chassis
Contents
17
17
18
19
20
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Richard Tøpholm
Tether Electronics for DTUsat
4.1.4 Plasma
21
4.2 Connection Logic
21
4.3 Considered Connections
23
4.3.1 Simple Passive
4.3.2 Original
4.3.3 Efficient with Grounding
4.3.4 Efficient using EMF
4.3.5 Floating Gate
4.3.6 Power Generating Design
24
24
24
25
25
26
4.4 Selection of Connections
26
4.5 Overall Circuit
30
Chapter 5
4.5.1 Measurements
30
Special Challenges
32
5.1 High Voltage generation
32
5.2 Isolation
33
5.3 Isolated Low Current Measurements
33
Chapter 6
Design Details
37
6.1 Filtering
38
6.2 Measurements
39
6.2.1 Current S-G
6.2.2 Current S-T
6.2.3 Current E-T
6.2.4 Voltage T
6.3 Power Supply
Contents
39
40
41
42
43
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Richard Tøpholm
Tether Electronics for DTUsat
6.4 DC/DC Regulators
45
6.5 High voltage switching
47
Chapter 7
Component Selection
49
7.1 High voltage converters
49
7.2 Isolation
51
7.2.1 Isolation Amplifiers
7.2.2 Optical Switches
7.2.3 Optocouplers
7.2.4 Linear Optocouplers
7.2.5 Magnetic Sensors
7.3 I/O devices
7.3.1 ADC
7.3.2 DAC
7.3.3 References
7.3.4 SPI Switches
7.3.5 SPI Potentiometers
51
52
53
54
55
55
55
56
56
56
57
7.4 DC/DC regulators
57
7.5 Current Sense & Limit
58
7.6 Other Active
58
7.6.1 Linear Regulators
7.6.2 Op-Amps
7.6.3 Transistor
7.7 Passives
7.7.1 Power Inductors
7.7.2 Diodes
7.7.3 Resistor/Capacitor
Contents
58
59
60
60
60
61
61
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Richard Tøpholm
Chapter 8
Tether Electronics for DTUsat
Prototype
63
8.1 Layout
63
8.2 Measurements
64
Chapter 9
8.2.1 Switch leakage
8.2.2 Tether current measurement
8.2.3 ADC
64
66
68
Conclusion
70
Appendix A Thermal Considerations
71
Appendix B Monitoring DC anode current of a groundedcathode photomultiplier tube
73
Appendix C Diagram
75
Appendix D Prototype Layout
77
Appendix E Tether current measurements on Prototype 79
Appendix F Tether Parameters
81
Appendix G Selected Component Data sheets
83
Contents
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Richard Tøpholm
Tether Electronics for DTUsat
Chapter 1
Introduction
The CubeSat Project* is a framework
for picosatellites of fixed dimensions
to be launched in groups as a single
payload, thereby drastically reducing
the price and test requirements for
launch.
Figure 1: CubeSat deployment
mechanism. Stanford University.
This provided a unique opportunity for a group of students at DTU to
begin realistically pursuing a student picosatellites in spring 2001. The
project was formalized in that summer, and the satellite named DTUSat.
In fall 2001, the team behind DTUSat consisted of ~40 students, working
on the definition and design of the sub-systems of the satellite.
In 2002, the project has reached prototype stages for all subsystems,
and a launch opportunity has been identified, for early 2003.
1.1 Space Tether Systems
While a wire or tether is seemingly the very simplest of technologies, its
application to space systems still remains a science project. The reason a
tether in space is considered so different from the earthbound version, is
mainly the very different environment in which they are used, allowing
novel applications.
* CubeSat is a project of Stanford University's Space Systems Development Laboratory
Chapter 1
Introduction
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Richard Tøpholm
Tether Electronics for DTUsat
1.1.1 Free Fall
A tether in space is usually placed in free fall, in either interplanetary
space, or orbiting a massive central body. In true zero gravity there is no
limit to the length of the tether, whereas in the micro-gravity of orbit the
possible length of a tether is limited but still magnitudes above that
which is possible on earth.
1.1.2 Vacuum
Secondly, the tether is moving in vacuum at speed of several kilometres
per second. Vacuum implies drag free movement, which allows very
weak disturbing forces to give rise to significant motions, as they integrate over time in a system where the some of the dynamics can be free
of dampening.
1.1.3 Magnetic Field
Non-geostationary motion will usually imply a movement through a huge
inhomogeneous magnetic field, with a relative motion of several kilometres per second. If the tether is conductive, this movement can induce a
large e.m.f. across the tether.
1.1.4 Ionospheric Plasma
The vacuum of space is by no means perfect. In the ionosphere of earth,
the surroundings are conductive plasma, consisting of free ions and electrons at temperatures of 1000K to 3000K. This will both have corrosive
effects on materials, but also have implications when the induced e.m.f.
puts the ends of a tether at different potentials with regard to the plasma,
which is the subject of the next section.
Chapter 1
Introduction
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Richard Tøpholm
Tether Electronics for DTUsat
1.2 Electrodynamic Tethers
An electrodynamic tether
consists of a conducting
wire, dragged through a
magnetic field. In a prograde* earth orbit, this will
induce a voltage across the
tether, with positive at the
high end, and thus try to
drive a current up the tether.
Ejecting electrons to the surrounding plasma at the low
end, and capturing electrons
from the plasma at the high
end, will close the circuit
externally. With the circuit
closed, a current will run
and power be dissipated in
it. This power comes from
Figure 2: The forces on an electrodynamic teththe mechanical energy of the
er in orbit.
orbit, which is thus decreased (by Lorenz forces on the tether). This is a completely analogue to
the principle of an electrical generator.
The opposite mode of operation is also possible. Ejecting electrons at
the high end, and capturing them at the low end will reverse the current.
Many problems are implicit in this mode of operations. If we assume the
potential of the surrounding plasma to be somewhere in between the potentials of the ends of the tether, the ejection of electrons will be against
the electrical field. This requires power, as does the capture of electrons
*
An orbit which eastwards movement around the Earth (same as Earths rotation) is called pro-grade, while a
westerly orbit (against the Earths rotation) is retro-grade.
Chapter 1
Introduction
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Richard Tøpholm
Tether Electronics for DTUsat
at the other end. This means the power must entered into the electrical
system to force a current. This power is added to the mechanical energy
of the orbit, and the tether system becomes an analogue to an electrical
motor. Practical problems like the ejected electrons trying to short the
circuit by returning at once to the tether in same end as they are ejected
from must be solved by isolating the tether. This is a nontrivial problem,
which caused the failure of the recent Italian/NASA TSS-1R shuttle mission, in which a spark breached the isolation of the tether, which was
subsequently severed by the ensuing discharge arc.
Electrodynamic tethers allow for the direct – propellant-less - conversion between electrical energy and mechanical orbital energy. This can
be used to boost a spacecraft with energy from solar cells, instead of propellant. Alternatively, it can provide a spacecraft with energy, while lowering its orbit. A reversible system can temporarily store excess energy
by raising the orbit, and later retrieve the stored energy when supplies are
low, by lowering the orbit.
The possibilities of combining momentum transfer with electrodynamic tethers are many, as momentum transfer manoeuvres which raises
the orbit of a payload will leave the tether in a lower orbit, which may
later be raised or deorbited by electrodynamics.
Chapter 1
Introduction
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Richard Tøpholm
Tether Electronics for DTUsat
Chapter 2
Motivation
The DTUSat payload group has decided to build an Electrodynamic
Tether Payload for DTUSat.
As systems design of the payload was completed in early 2002, which
is the motivation for this project. Parallel work is ongoing for the other
aspects of the Tether Payload (notably the Electron Emitter, and Deployment Mechanic).
The aim of this project is to demonstrate working electronics for the
subsystem, in a prototype form (i.e. using a prototype layout of a design
as close to final as possible).
2.1 Requirements
The requirements for the electronics are primarily to power the electron
emitters, and measure all operational data, if possible.
The size is obviously to be kept to a minimum, as the total size of the
satellite is 100x100x100mm. This also has the crucial side effect of minimizing mass – which is likewise needed. The total mass of the satellite is
to be kept below 1 kg – of which the electronics should preferably be less
than 40g.

Power electron emitters - expected 1mA at 70-150V

Gather operational measurements

Low power use

Low size and mass
Chapter 2
Motivation
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Richard Tøpholm
Tether Electronics for DTUsat
The satellite orbit is now known to be sun-synchronous near 98° inclination. Orbital height is around 900km.
This is operationally the “worst case” orbit for the tether – however
has the nice side effect, that the tether will be deployed “downwards”
(towards the Earth), which means that the camera will be facing in the
same direction. This could enable oscillations in the tether to be observed
by the camera. Observations or other measurements of the oscillations
are a requirement for using active control for dampening.
Chapter 2
Motivation
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Richard Tøpholm
Tether Electronics for DTUsat
Chapter 3
Theory
3.1 Tether System
The decision to use the CubeSat fixed the weight at 1000g, and the size
as roughly a 100mm side cube. In the first iteration of the following design process, it was assumed that the payload would be allowed to use
one fifth of mass and size. Due to growing subsystems, as specifications
got more solid, and the addition of a camera payload, the allowed mass of
the tether has since dropped to 100g.
As I from the beginning indicated, that the tether could be designed to
any weight, simply by changing dimensions of the tether, it probably
seemed easiest to squeeze this payload. This admission was probably a
tactical mistake, as no other subsystem group indicated anything similar.
As it stands now, the payload has been defined to consist of an electrodynamic tether, and a camera, that will take pictures of the Earth, and
the deployment of the tether. The tether subsystem is allowed to use
100g. including structural supports, and should if possible stay within a
25x65x65mm box. The electronics can be located on a print outside this
envelope
3.2 Voltages
The voltage induced in the tether depends on the horizontal B-field,
normal to the velocity vector.
It will vary from –96V/km to +58V/km, for our orbit. We only conduct a current when it is negative, as we need to expel electrons from the
Chapter 3
Theory
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Richard Tøpholm
Tether Electronics for DTUsat
satellite. The average induced voltage is then –30V/km, when positive
values are set to zero. Since we wont be conducting any current the “other way”, this is the interesting information, i.e. the “effective” average
induced voltage.
The bias of the tether is controlled by several factors. If very small
currents are flowing, the high mobility of electrons compared to ions will
drive the positive (electron collecting) end towards zero (relative to the
background plasma potential). The satellite will thus become negative, if
connected to the tether. If the electron emitter emits large currents, the
satellite will be driven towards positive potential. When the potential of
the satellite becomes positive relative to the plasma, the ejected electrons
will begin to return to the satellite, limiting the net ejected electrons. It
can be assume that the satellite cannot be driven much above zero by
electron emission (unless - perhaps - large accelerating potentials are
used). With the satellite at zero then, the maximum current collected by
the tether will be dictated by the induced bias, when we can’t add to the
bias. This current is determined by OML Langmuir theory, as our tether
is certainly going to be inside the OML regime.
In short: Because we have decided to NOT raise the tether potential,
by generating a positive voltage potential between the tether and the satellite, we are confined to electrodynamic drag operation of the tether, and
the bias voltage of the tether is limited to the induced e.m.f.
3.3 Electron Emitter
Emitting electrons is part of closing the tether circuit through the ionosphere. It can be done passively, in the sense, that a large passive conductor (with sufficient bias) will collect small amounts of ions, which is almost equivalent to ejecting electrons due to recombination on the surface.
In practice however, it is always done by active means.
The basic principle of an active electron emitter is the generation of
free electrons, which are then accelerated away from the emitter by an
acceleration potential. If the emitter is biased negative with regards to the
Chapter 3
Theory
page 13
Richard Tøpholm
Tether Electronics for DTUsat
surrounding plasma, the electrons will drift away, and this bias may do
the job of accelerating potential. Alternatively, an accelerating grid with
a positive potential relative to the electron cloud can be placed in front of
the electrons, for accelerating them. If this grid is the outer grid, and has
a positive bias with regard to the plasma potential, the electrons will be
decelerated again after leaving the grid, possibly even returning to the
satellite. The energy needed to liberate an electron from surface varies
with the material, is known as the workfunction of the material, and usually measured in eV.
The cheapest active emitter is a heated filament, which will boil off electrons. This could be a tungsten wire, which was heated, until a cloud of
electrons exists around the wire (thermionic emission). The energy for
liberating the electrons are supplied thermally, which is not very effective, but has the advantage of not needing a high voltage supply. When
used with an accelerating grid, this assembly is known as an electron
gun. They are best known for their use in cathode ray tubes, i.e. standard
television sets, where the electrons are used to draw the image on the
phosphor.
The emitter used on the prior electrodynamic tether experiments, have
all been a hollow cathode plasma contactor. This ionises a gas, which is
then slowly expulsed, thus augmenting the local plasma density, to a
point where it is sufficiently conductive. Hollow cathodes are popular
because they can reverse the current, and because they can be run “passively”, i.e. not actively driving a current. In this way, they can be used to
ground a satellite with regard to ambient plasma potential. They require a
pressure-vessel with gas, which is used during operation.
The last active emitter is the Field Effect
Emitter. The principle is that electrons may be
liberated from the substrate by intense electric
fields, instead of by thermal energy. The high
fields are accomplished by placing an acceleration grid very close to the source. The source is
Chapter 3
Theory
Figure 3: FEAC from
Agüero and Adamo,
2000. Peak size is ¾μm.
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Richard Tøpholm
Tether Electronics for DTUsat
the shaped with peaks, because the electric field is stronger around a
peak. In order to work with reasonable acceleration potentials, the distance to the grid has to be on the order of single μm. This type of emitter
has many advantages. They are small and light, and the energy consumption is very low compared to the alternatives. The current per peak is
from 0.01μA to 100μA, for controlling voltages of 50-500V. In some
technologies, like the Spindt Cathode, the peaks can be readily integrated
by photolithography, and 10.000 peak arrays have been demonstrated by
Agüero and Adamo, 2000*. As the sharpness of the peak is determining
for the voltage required, nano-wires are sometimes used. The problems
with FEAC’s are their small scale, which makes them susceptible to sputtering and oxidation by atomic oxygen.
This problem can be alleviated
by choosing a tip material, which is
noble, like gold or beryllium. The
problem is that these materials often
also have large workfunktions. Another way of reducing oxidizing and
sputtering, is to use several grids in
front of the peaks. The grids can be
configured to both accelerate the
electrons, and repel ions (Marrese,
2001).
For DTUSat, a field effect based
solution would be optimal, because it
is small and light, and has low power
requirements.
MIC and KU are involved in this
aspect of the project, with a view to
produce FEAC’s with a surface
*
Figure 4: FEAC design with multible grids,
for both performace and ion protection.
Arrays of field effect emitters are called FEAC’s: Field Effect Array Cathode’s, or just FEA’s.
Chapter 3
Theory
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Richard Tøpholm
Tether Electronics for DTUsat
area of 2cm², and able to emit at least one µA with bias potentials around
100V. As the average induced voltage is close to 30V, this voltage has to
be generated by the electronics.
Chapter 3
Theory
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Richard Tøpholm
Tether Electronics for DTUsat
Chapter 4
Electrical Configuration
The primary function of the electronics is to provide the necessary bias
voltage for the emitter. As the tether is being used in the current generating configuration, this voltage could actually be provided by the tether –
had it been long enough. This however is dependent on the configuration.
In prior work, I identified a single configuration, with the emphasis on
safety. It would connect both the tether and the emitter gate to the chassis. The emitter cathode would be supplied with a high-voltage supply.
This is only one of several possible configurations, which will all be explored here.
4.1 The elements
The external elements to be connected are Tether, Emitter Chassis and
Plasma. They are covered as such here, and simple but reasonable equivalents are considered.
G
T
Satellite
Chassis
E
her
Tet
m
0
70
Plasma
Electron
emission to
ionospheric
plasma
FEAC
emitter
Figure 6:
5 The external elements connected by the electronics
Chapter 4
Electrical Configuration
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Richard Tøpholm
Tether Electronics for DTUsat
4.1.1 The Tether
The tether is of course a wire, but as it is deployed, it will induce a voltage. As such, we can model it as a voltage supply. It has an internal resistance, which is the resistance of the tether wire itself. Assuming a
700m aluminium wire with a cross section of Ø 0.2mm, it will have an
internal resistance of 670Ω (see Appendix F : Tether Parameters on page
81).
Unfortunately, this is not quite so simple. While one end of the tether
will be connected to the satellite, the other will connect to plasma by collecting electrons and ions. This connection however will not happen at
the end of the tether, rather along the whole length of it. This means that
the current will not be constant along the length of the tether, and the resistance (which is distributed evenly along the tether) will not act evenly
on the current either.
In order to handle this complexity, I look first at the case where very
little current is running, compared to the collecting capability of the tether. Electrons are more mobile than ions, and so will be collected more
easily. Thus in this case, where almost no current is running to the satellite, electrons will be collected over a short stretch of tether furthest from
the satellite, while ions will be collected over the rest of the tether – resulting in a net current running along the tether (so called ram-current).
This current is not avoidable in our case, meaning it will not be possible
to turn the tether completely off.
The electron collection stops and is replaced by ion collection at the
point of the tether, where the tether potential is equal to the plasma potential (actually there is an offset of around 0.3V, because of the higher
electron mobility, but this is negligible in our case). This point will also
be the point of highest current density in the tether.
Chapter 4
Electrical Configuration
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Richard Tøpholm
Tether Electronics for DTUsat
When the current to the satellite is increased, the zero-point will move
towards the satellite, increasing the effective collecting area of the tether.
The zero-point reaches the satellite when the current is equal to IOML.
A “quick and dirty” model of the tether and plasma connection, is
thus the plasma connected to a voltage generator with an output of VEMF,
in series with a resistor defined as RL = VEMF/IOML. This means that when
the current reaches IOML, all the generated voltage is lost in the resistor,
so the resultant output is the plasma potential. In addition to this comes
the internal resistance from the aluminium.
From the previous chapter, we have a value of IOML = 29mA for 700m
tether inducing 21V. This corresponds to 720 Ω, to which should be added the aluminium resistance of 670 Ω, yielding a total resistance of 1.4
kΩ.
In this case, the tether then can be modelled as a 21V generator, with
an internal resistance of 1.4 kΩ, connected with the positive pole to the
surrounding plasma (P), and the negative pole (T) to the electronics.
4.1.2 Emitter
The emitter will emit a stream of electrons to the surrounding plasma,
with the magnitude dependent on the voltage between the cathode tips
(E) and the gate (G). G should be positive with regards to E, to generate a
field emission.
This corresponds to a positive current from the plasma (P) to the
cathode tips (E).
A small percentage of the electron current will hit the gates, giving a
positive leakage current from the gate (G) to the emitter (E). This is presumably a fixed percentage, which together with the emissions non-linear
dependence on G-E voltage makes the device look like a bipolar NPN
transistor, with G as the basis, E as the emitter, and the collector connected to the plasma P. It is however peculiar in that the G-E voltage will be
in the order of 60-120V to achieve a collector current of a mA.
Chapter 4
Electrical Configuration
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Richard Tøpholm
Tether Electronics for DTUsat
Actually, it might be more correct to refer to it as a Triode rather than
a transistor, with the anode being the plasma. Triodes are devices very
similar to a FEAC, although they are heated in order to allow thermionic
emission, whereas the FEAC work by field emission, i.e. by having a
large electrical field at the cathode.
The expected current amplification is around 100, but this has yet to
be experimentally verified.
4.1.3 Chassis
The chassis of the satellite is connected to power ground at one point
only – in the radio. Due to the radio-transmitter emitting EM waves from
antenna’s located close to the chassis, it must be expected that various
points of the chassis will assume high AC voltages, with a frequency of
435Mhz. It is therefore not possible to connect points on the chassis directly to the electronics, without an appropriate choke in between.
The DC potential of the chassis will however, be connected very effectively to power ground.
The chassis of the satellite (S) will also constitute a considerable electron/ion-collecting surface. For this reason, it can be considered as being
connected to the Plasma (P) with a resistance. The value of said resistance has not been theoretically investigated, however the surface area
of the tether is very close to 10 times the surface area of the satellite. The
tether further has a known optimal shape for collecting electron, which
the satellite does not.
It seems reasonable to assume that the apparent resistance between
satellite (S) and plasma (P) is more than 10 times larger than the one between the tether and Plasma. Further, the resistance was calculated for
the collection of electrons, i.e. with the tether at positive potential re
plasma. The satellite however, can be assumed negative re Plasma (it is
after all connected to the negative end of an electron collecting tether).
This means that the connection to plasma will collect ions, which are less
mobile. The larger dimensions of the satellite may mitigate this to some
Chapter 4
Electrical Configuration
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Richard Tøpholm
Tether Electronics for DTUsat
degree, as a large portion of the collection will be by actually hitting ions,
rather than the ions deflecting to hit the satellite.
I will assume that the satellite (S) is connected to plasma by a resistor
of 70 kΩ.
4.1.4 Plasma
The plasma surrounds the satellite and tether.
The connection to the various parts of the satellite has already been
discussed above. Internally the plasma can be considered a perfect conductor, because of its very large cross section.
The possible connection of terminals T,S,E,G is explored in detail in
the rest of this section.
4.2 Connection Logic
The objective for the connections is to allow a current to run from E to T,
while maintaining a large voltage differential between G and E. This
while using as little power as possible. The emitter- tether current should
reach a few milli-amps, while the expected gate-current is 100 times less.
The electronics have a number of voltage sources available. The in-
Figure 7: Equivalent diagrams for the connected elements. Positive current-arrows shown.
T
Induced EMF
0-200V
Typ 20V
S
r
the
Te
670
70k
Satellite
Chassis
G
Electron
emission to
ionosphere
E
FEAC
emitter
Plasma
720
Chapter 4
Electrical Configuration
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Richard Tøpholm
Tether Electronics for DTUsat
ternal power supplies are V3.6 and the unregulated Vunreg, which are 3.6V
and 2V-5.5V above power ground respectively.
Externally the power ground is DC connected to the chassis S, which
has a rest potential close to zero (re plasma). The tether terminal T on the
other hand will be near -VEMF at rest, which is typically -20V re plasma.
In studying the possible connection, I will adopt the plasma potential
as zero, measuring other potentials relative to this.
There is however the restriction, that the internal power lines should
not be connected directly to external elements, in order to avoid injection
of 430Mhz noise (from our radio transmission) into the electronics.
To keep a positive voltage on Gate relative to Emitter, it would seem
simple and logic to connect Gate to the most positive external point,
which is the chassis (S) and Emitter to the most negative: the tether - T.
This further allows a current to run from E to T, which was needed anyway. This has the great value of being completely passive, i.e. drawing
no current from the power supply. It will thus work, regardless of the
state of the satellite electronics. This “Simple Passive” configuration is
shown below:
FEAC
emitter
E
T
Electron
emission to
ionospheric
plasma
G
S
Induced EMF
0-200V
Typ 20V
r
the
Te
Satellite
Chassis
670
70k
Plasma
720
Figure 8: Simple Passive connection of tether to emitter, and gate to chassis.
Chapter 4
Electrical Configuration
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Richard Tøpholm
Tether Electronics for DTUsat
There are certain problems with this simple passive connection. First
off, the tether might not induce enough voltage for the emitter to work.
Actually, it will typically induce only 20V, which is not enough.
Second, there could be a safety concern with the tether being at a different potential from the rest of the satellite. This implies that there is a
risk of short circuit between the tether and the satellite structure (which
further has an AC potential).
Third, the leak-current into the gate of the emitter must be supplied
by ion-capture on the structure. If the assumption of 70 kΩ from satellite
to plasma holds, this should not be a problem for low emitted currents.
The Simple Passive configuration can be improved in several ways to
improve performance and/or safety. It is however perfect in regard to
simplicity and reliability: It will work even during a complete loss of
onboard power.
In order to augment the voltage supplied, a small boost could be had
from connecting the gate to V3.6 instead of chassis. This is a very small
gain, and hardly worth the bother, as an efficient AC choke will be required. A more worthwhile approach is to insert a “high”-voltage supply
(~100V) to either lower the Emitter potential, or raise the Gate potential
(or both).
4.3 Considered Connections
In the diagrams of the connections, I will try to maintain higher potentials
above lower potentials.
The basis for the numbers, is 30V induced EMF, 1mA tether current,
100V gate-emitter voltage, and 1% gate current (0,01mA).
The diagrams are simplified as much as possible, showing only the
connections between T, S, E and G.
Chapter 4
Electrical Configuration
page 23
Richard Tøpholm
Tether Electronics for DTUsat
4.3.1 Simple Passive
The connection diagram for the Simple Passive
configuration is shown right. It is a short form of
Figure 8.
S
T
G
E
It uses the EMF to power the emitter, and the
tether is not grounded, which might give concerns.
Figure 9: Simple Passive
Performance is very low, however it works even
when all power fails, and could be considered for a fail-safe or backup
solution. Power consumption is 0W
4.3.2 Original
The connection used in Electrodynamic Tether
for DTUsat, was chosen for maximum safety.
It had both the Tether and the Gate connected
to the Chassis, ensuring that all external parts
were at equipotential (although not at plasma
potential).
This is not a very efficient combination
however, as the generator will supply 1mA at
100V i.e. 100mW. It is however more or less
guarantied to work.
S
T
G
E
Figure 10: Original
4.3.3 Efficient with Grounding
An efficiency optimisation of the original configuration is to have the voltage generator in
the Gate connection rather than the Emitter.
The gate is expected to draw 100 times less
current, thus lowering the power consumption
of the circuit dramatically to only 1mW. However, the Gate is no longer connected to the
Chassis. Having an external surface (the gate
electrode) is at a different potential from the
Chapter 4
Electrical Configuration
G
S
T
E
Figure 11: Efficient grounded
page 24
Richard Tøpholm
Tether Electronics for DTUsat
rest of the satellite, could be a potential source of problems. The tether is
however still grounded to the chassis, eliminating the greatest concerns.
4.3.4 Efficient using EMF
An even more efficient design is to decouple
the tether from the chassis. This allows the induced voltage to aid in generating the voltage
for the gate-emitter. The voltage generator is
still delivering the low current of the gate, but
now at a somewhat lower voltage, thus lowering the power consumption even more to
0.7mW. However, this is no safer than the first
“Simple Passive” connection.
G
S
T
E
Figure 12: Most Efficient
4.3.5 Floating Gate
A completely different approach, which
was explored by the DTUsat Tether team at
the University of Copenhagen, was to use
photo-emitting materials. While it proved
unpractical to generate enough photocurrent, to work as a primary emitter, they
could probably well be used to raise the Figure 13: Floating gate
potential of the gate. In effect, this will
multiply their photocurrent by a factor of 100. This assumes that they
will continue to emit, even while 100V above the plasma voltage. In
practice, a part of the emitted electrons will be deflected back by the negative plasma, depending on the energy transferred to the electrons. This is
something, which will require extra hardware (the photo-emitting material), and is unlikely to be made for DTUsat. However it will be shown later in this text, that the addition of this current source is very simple in the
electronics.
G
S
T
E
Power consumption is basically 0W, however the design could be combined with other elements, if performance was not sufficient.
Chapter 4
Electrical Configuration
page 25
Richard Tøpholm
Tether Electronics for DTUsat
4.3.6 Power Generating Design
A further development of the efficient designs would be to extract energy from the
tether current. This could be accomplished by
having a generator in the gate path, and in
the emitter path. The last would be polarized
opposite the configurations used above, such
that the tether current would run into the generator. The generator should thus draw power
from the tether current, and presumably de-
S
T
G
90V
E
20V
liver it to the satellite power management
system. In the case shown to the right, a Figure 14: Generating design
tether current of 1mA would generate
20mW of power, while the gate current would drawn only 0.9mW (assuming the FEAC has 70V+EMF as gate-emitter voltage, and a current
amplification of 100).
The net power supplied by this circuit would thus be ~19mW.
The generating designs are not explored further in this text. They
have been deemed possible but complicated, and of little theoretical value. I.e. this might be an incentive for commercial use of tethers, but it is
not something DTU will learn from doing.
4.4 Selection of Connections
There is a conflict of interests when choosing the configuration. While on
one hand, the Original configuration is the safest way, in that it will remove the possibility of short-circuits between tether and satellite, the Efficient configurations will allow higher performance.
The Simple Passive has another benefit: It will continue working,
should the electronics onboard the satellite fail – ensuring long-term operation, although the selection of a polar circuit all but made this configuration unusable.
Chapter 4
Electrical Configuration
page 26
Richard Tøpholm
Tether Electronics for DTUsat
As several of the circuits have connections in common, I have tried to
explore how many of them could be realized with simple ways of switching between them.
It was quickly seen that all had the emitter connected to the tether
(sometimes through a voltage supply), and all had the gate connected to
the chassis (sometimes through a voltage supply). Further, sometimes the
chassis was connected to the tether.
There is however, no simple way to switch a voltage generator so that
is can be used to either raise potential of the gate, or lower the emitter.
Although one connection is in both cases chassis, it is not the same pole
of he generator. It would thus be
necessary to have a doublethrow switch on each pole of the
generator to switch between the
original and the efficient configuration. As shown right.
G
S
The Figure show the generator used for boosting the gate
voltage (Efficient Configuration), however notice that the
tether is not connected to the
T
E
emitter, as it should. This will
Figure 14: Switched design.
require another two doublethrow switches (or one big Notice: Half the switches are missing.
quadruple-pole double-throw I.e. T and E are not connected here.
switch) to make the direct connection between S-G or T-E.
If it should further be desirable to use the passive connection, it must
be possible to disconnect the generator completely, i.e. at least one of the
switches in Figure 14 should have an off position too. When this is chosen, both S-G and T-E should be connected. The number of switches is
quickly becoming impractical. The complexity does not seem to be worth
the gain. However, if the figure is studied, it is seen that current always
Chapter 4
Electrical Configuration
page 27
Richard Tøpholm
Tether Electronics for DTUsat
flows in the same direction, in the same leads. I.e. the S-G connection
always conducts from S to G. The T-E always conducts from E to T.
This enables us to replace
some of the switches with diodes. For example, the switches
needed to connect S-G directly,
when the voltage supply is not
engaged there, can be replaced
by a diode. This is shown to the
right (now also, the switch to
connect S and T is shown).
G
S
T
When the voltage generator
is engaged, the diode is back
biased, and blocks. It will thus
no longer be part of the current path.
Using two voltage generators instead of one will do away with the
switch completely. This is shown
right. There is still a switch to connect the tether to the chassis.
Notice that a voltage supply
(which would need to be variable
anyway) need only be turned off, to
make the circuit change configuration.
E
G
S
T
E
This design has the benefit of
being able to use a combination of raising the gate, and lowering the
emitter voltage, in order to achieve the desired G-E voltage.
It further gives redundancy. If one of the voltage supplies were to
malfunction – by either short-circuiting or disconnecting - the circuit will
still work, as the diodes will allow the current to flow past the defect
voltage generator, and the other generator will be able to provide the
Chapter 4
Electrical Configuration
page 28
Richard Tøpholm
Tether Electronics for DTUsat
needed emitter gate bias. If both generators are turned off, and the S-T
switch is closed, the circuit will be passive, as everything is grounded,
and the gate-emitter voltage is (near) zero. This is virtually an OFF position.
Opening the switch will then make the circuit work according to the
passive simple configuration, with S-T and T-E connected directly
(through diodes as it is).
If the bias is insufficient to drive the emitter, the generator connected
to either the gate or the emitter (or both), may then be turned on. If shorts
between tether and satellite are a concern, the switch may in addition be
closed.
Depending on what is turned on the circuit will work as follows:
Switch
GateEmitterResulting Function
Generator Generator
Closed
OFF
OFF
OFF
Closed
ON
OFF
Efficient with Grounding (4.3.3)
Closed
OFF
ON
Original (4.3.2)
Closed
ON
ON
Combination of the above
Open
OFF
OFF
Simple Passive (4.3.1)
Open
ON
OFF
Efficient using EMF (4.3.4)
Open
OFF
ON
Hybrid configuration
Open
ON
ON
Hybrid configuration
The last configuration considered used the trick of raising the gate
voltage with a photocurrent. This is alluring, although it requires extra
hardware. From an electronic point of view, just permanently connecting
the photo-emitting plate to the gate, while still using other means to reach
Chapter 4
Electrical Configuration
page 29
Richard Tøpholm
the desired gate-emitter voltage,
should never have bad effects. It
will only aid in providing the needed voltage/current for the gate. Just
as a precaution, it could be connected by a diode to be sure it does never draw current away from the gate.
Other than the added photoemitting plate and the diode, this
diagram is the same as the preceding.
Tether Electronics for DTUsat
G
S
T
E
4.5 Overall Circuit
The circuit with two generators, shown in the preceding section, is chosen as suitable – flexible and without major drawbacks. It does have two
generators, which could be reduced to one, however that does pay in both
redundancy, and freedom to vary one more operational parameter: Now
gate-plasma voltage can (within limits), be adjusted independently of
gate-emitter bias.
The possibility to add a photo-emitting surface is gained simply by
adding one diode. This is not further explored in this report, as it would
be possible at any time, and would not influence the rest.
4.5.1 Measurements
As much as possible state information about the tether operation should
be recorded. This will help both provide useful data, and debugging information.
Chapter 4
Electrical Configuration
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Richard Tøpholm
Tether Electronics for DTUsat
First, the current in the tether is interesting. Next the current in the
gate. If the switch S-T is closed, the current in it might be interesting too.
If it is open, the voltage across it is surely interesting. The voltage added
is commanded, and so should be
known – though it wouldn’t hurt to
Current S-G
measure it too. The needed measurement points are shown in this
figure.
G
S
Remember that the Chassis S is
DC at electrical ground. This means
that measuring relative to it is simple. For this reason, two of the
measurement points are referenced
to S. The current in and voltage of
the Tether however, is not so simple.
Chapter 4
Electrical Configuration
Current S-T
Voltage T
T
Current E-T
E
Figure 15: Measurement points
page 31
Richard Tøpholm
Tether Electronics for DTUsat
Chapter 5
Special Challenges
5.1 High Voltage generation
The emitter must be supplied with a gate-cathode bias of typically 100V.
This should further be floating with respect to the satellite electronics.
The output voltage should be variable, and able to turn off. 40 – 120V
is desired, and the emitter current is expected to reach at least 1mA, perhaps more.
The gate current is expected to be 100 times less than the emitter current. While we don’t expect currents much beyond 1mA emitted, the cost
of allowing more than 10µA gate current is virtually zero, as all components considered can supply this. It has therefore been decided that the
gate-generator should be able to supply the full emitter bias voltage of at
least 120V, with at least 100µA. This will allow us to attempt to emit up
to 10mA, although without the use of the emitter generator. When using
the emitter generator, it is well acceptable to limit the emitted current to
1mA. Another reason for the 1mA limit on the emitter-generator is that
the power supply might not be able to supply that power to us for extended periods (1mA at 120V correspond to 40mA at 3.3V, but at an expected efficiency which could be low, say 25%, it becomes 120mA,
which is more than we can expect continuously).
EMCO Highvoltage’s Q-series DC/DC converters were identified (se
the components chapter). They produce an output proportional to the input, and come in a plethora of version. They turn on at 0.7V input and
have a max input of 5,12 or 24V, depending on version.
Chapter 5
Special Challenges
page 32
Richard Tøpholm
Tether Electronics for DTUsat
Output at max input is 100V - 20,000V depending on version.
5.2 Isolation
One of the major problems with the design of the tether electronics is that
the currents to be measured are not at potentials inside the supply rails.
Even the currents, which are running close to ground, are contaminated with radio-noise. This can be handled by decoupling and highimpedance ADC interfaces.
That still leaves the two major problems of measuring the tether current at –200V, and of switching the gate-current at +200V, which could
be less than 1µA.
5.3 Isolated Low Current Measurements
The Current G and ST can easily be found by running the current through
a sense resistor. This resistor should be small enough that it does not affect the operation, yet large enough to give a usable signal.
Since the voltages are in the order 100V, a voltage drop of a volt or
two across the resistors is not a problem. As the current runs from S to
G/T, the voltages measured are going to be negative (a current will drop
across the resistors, from S, which is at zero volt DC). The measurement
itself should be high-impedance, and filtered to avoid AC components.
As the ADC is not going to tolerate input voltages outside the supply
rails (which are 0-3.3V), the voltage must be inverted before measurements.
Measuring the current from T to E is bothersome, as the potential can
be as low as –200V relative to our supplies. The usual approach would be
an isolation amplifier, but for reasons detailed in 7.2.1 Isolation Amplifiers p. 51, this was not practical. Instead, I looked to magnetic hall sensing
and optically coupled solutions.
A Hall-effect sensor, can measure the magnetic field of a DC current
in an inductor (which might even be just a wire). It has problems in our
Chapter 5
Special Challenges
page 33
Richard Tøpholm
Tether Electronics for DTUsat
case, however, as the field produced must be large enough that the Earths
field does not influence measurements, and small enough that the field
does not influence the ACDS sub-system measurements of the Earths
field. This is not as impossible as it sounds, as the field being measured
drops rapidly with distance. Unfortunately, the solution would still require manual manufacturing of the sensing head, as no such are available
for measuring in the desired current range (10µA to 10mA).
Instead, an optical method was focused on. Optocouplers in general
are very non-linear, i.e. their current transfer ratio (CTR) varies with
(among other things) the current.
To avoid this problem, usually the measurement is made on the isolated (high voltage) side, and transferred in pulse trains (current to frequency) or by using a linear optocoupler. Neither is in usual possible,
without power available on the high-voltage side.
Linear optocouplers consist of an
optocoupler with two output photodiodes as shown to the right. The current
IF in the input LED, will make a current
run in both of the output photodiodes IPD1
and IPD2.
The transfer gain K3 = IPD1 / IPD2 is
very constant over the input range, and Figure 16: Linear Optocoupler
temperature, hence the linearity. Notice
that the gain K1 = IPD1 / IF is not constant.
Optocouplers with photodiode outputs do not act like the normal
optocouplers with transistor outputs. While a transistor opens to allow a
current to flow, photodiodes generate a current, which is forced to flow.
This is similar in principle to a solar cell.
Notice that if this current is not allowed to run, the photodiodes will
increase forward bias until either the current runs, or the biasing exceed
~1V – 1.5V, at which point the current will run through the diodes them-
Chapter 5
Special Challenges
page 34
Richard Tøpholm
Tether Electronics for DTUsat
selves, i.e. they can short-circuit themselves. If on the other hand, they
are reverse biased too much, it will negatively affect linearity.
Usually linear optocouplers are employed as with a op-amp on the input side, the output of which will provide the input for the LED, adjusting it to achieve the current to be transferred at IPD1. This current can then
be “read” isolated on the other side, at IPD2. This set-up is shown below.
Figure 17: Typical isolated amplifier, using a linear optocoupler
The op-amp on the left makes sure that the current IF is maintained so
the current to be measured is always running in IPD1, In effect a servo
loop is closed around the optocoupler on the input side. This is characteristic of all generic linear optocoupler circuits. Unfortunately, there is no
supply to power an op-amp on the input side in our application.
It might be possible to design a circuit, where an op-amp was driven
by the tether current, and but it would not have enough current to drive
the input LED of the linear optocoupler.
A novel design was discovered in an article by S. Argirò et al., Nuclear Instruments and Methods in Physics Research section A 435
(1999): 484-489. In particular, see section 3, on page 485 to 488. This
text is included in Appendix B Monitoring DC anode current of a
grounded-cathode photomultiplier tube on page 73.
The principle is that the loop is closed on the output side of the linear
amplifier, but as the error signal is present on the input side, the error
signal has to be transmitted to the output side by a standard optocoupler.
Chapter 5
Special Challenges
page 35
Richard Tøpholm
Tether Electronics for DTUsat
This optocoupler will be powered directly by the error-current, it thus
becomes important to find an optocoupler that will turn on at very low
input currents. What really make this circuit shine is that it consists only
of the two optocouplers, and one transistor! In fact, if the two optocouplers have enough CTR to give the desired open look gain, the transistor
might not even be necessary. Most linear optocouplers have low CTR,
though, so in practice it’s probably needed. The mirrored current is simply the output IPD2.
I have build and measured this optical mirror in my prototype, and the
results are found in 8.2.2 Tether current measurement, on page 66. The
linearity looks excellent, and the offset is only a few µA.
The accuracy and simplicity of this solution, made the choice easy.
Only a fully integrated isolated amplifier, working off 3.3V could have
been easier, had it existed.
Chapter 5
Special Challenges
page 36
Richard Tøpholm
Tether Electronics for DTUsat
Chapter 6
Design Details
2
2
2
2
2
b1
a1
1
b1
a1
1
b1
a1
b1
b1
a1
a1
SPST
SSR
1
1
2
SPST
SSR
1
2
b1
2
SPST
SSR
1
a1
b1
a1
2
2
SPST
SSR
1
b1
b1
a1
2
b1
b1
a1
a1
1
1
1
SPST
SSR
1
Vcc
Vadj1
C+
VL+
a1
b1
VH+
a1
High Voltage
Generator
V+
1
2
An overview of the diagram so far is shown below. A larger version can
be found in Appendix C on page 75.
SPST
SSR
GND
H
VV out
VLC
-MAX350
1
2
220k
-1.5V DC
470k
3
4
1
~0V DC
1
33k
2
3
4
Satellite
Chasis
a1
a2
a3
b1
SPI Ctrl
b2
b3
a4
b4
a1
b1
a1
b1
a2 8 x SPST b2
a3 /w common b3
a4
b4
5
6
7
8
2
5
6
Electron
emission to
ionosphere
7
8
1k
-1.5V
220k
470k
1
2
Vcc
Induced EMF
0-200V
Typ 50V
3
4
a1
FEAC
emitter
a2 4 Channel
ADC
a3
a4
Vref
200k
700m
Tether
20M
High Voltage
Generator
V+
VH+
C+
VL+
Vadj2
GND
H
VV
out
VLC
--
Vcc
Figure 18 Simple diagram
The Tether-Emitter and Chassis-Gate current paths are shown bold.
The Tether and Chassis connections are on the left (together with a
dashed connection representing the plasma-path, and the induced voltage.
The connections start with a CL filter, to filter out radio-noise. This is not
100% effective, however all the interfaces to the rest of the electronics
are either isolated or high impedance.
Chapter 6
Design Details
page 37
Richard Tøpholm
Tether Electronics for DTUsat
The optically coupled current mirror is shown next in the tether current path. The current is mirrored and converted to a voltage by the opamp.
In both current paths, there is a high-voltage generator (shown as a
stylised transformer), parallel with a diode, which conducts when the
voltage generator is off. The generator has an adjustable low-voltage
source as input.
The Chassis-Tether connection is closed by a optocoupler.
Additionally two currents are measured, the Gate and the ChassisTether current. They are measured across two sense resistors, and the
voltages are inverted.
In the upper right corner of the diagram, is the optocouplers, which
select which parts of the emitter’s gate-areas are active.
The optocouplers are driven by a SPI controlled switch.
In total, there are four measured values: three currents, and one voltage. The voltage (which is negative by 0-200V) is brought into range by
voltage division between the actual voltage and the reference voltage.
They are converted by an ADC, which outputs data to the onboard computer by SPI.
6.1 Filtering
The Tether is a big antenna. For this reason it is important that it is decoupled effectively. First off, it will be connected with a though-hole capacitor, as in enters the satellite body. This will ensure that any large AC
current will run in the body of the satellite, not in the electronics inside.
This filter is part of the mechanical structure, and does not figure on the
print layout.
Once inside the body the tether current will run through an inductor,
which has been chosen to have maximum reactance at 435MHz
(~0,1μH). This will probably be followed by a high-voltage 100pF capacitor to ground.
Chapter 6
Design Details
page 38
Richard Tøpholm
Tether Electronics for DTUsat
The connection to the chassis and the emitter-cathodes follow the
same rule, although there are no capacitors before the inductor. The emitter-gates are very low current (typ. 10µA), so using a resistor instead of
the inductor is reasonable.
6.2 Measurements
I will in the following subsections run through the measuring methods,
ranges, filtering in detail.
6.2.1 Current S-G
The current from Chassis to gate should be measured in order to know
(together with the S-T current) the total current drawn from the chassis.
It’s also interesting, as it will allow performance data for the emitters to
be collected.
The current is lead from S through a sense resistor before it passes to
the gate-generator. This sense resistor will have one terminal at S, which
is at ground potential (DC wise). As the current flows TO the gates, the
other terminal will be at negative potential (relative to ground).
In order to measure the current, the negative voltage at the sense resistor should be inverted, and AC components filtered out. This is accomplished by an op-amp, which samples the signal through a 220kΩ
resistor, and inverts it using a resistor parallel with a capacitor in the
feedback path.
The gate current is assumed to be 100 times less than the emitter current. The max emitter current using the emitter-generator is 1.6mA.
Without the generator, 10mA might be possible, but 4mA is probably realistic. That corresponds to 40µA gate-current.
Using a sense resistor of 33kΩ gives -1V at 30µA. This is further enhanced by the op-amp, which amplifies the signal by a ratio of –
470kΩ/220kΩ. The resultant sensitivity is 14.2µA/V, for a range of 035µA with a 2.5V reference, 0-43µA with a 3V reference.
Chapter 6
Design Details
page 39
Richard Tøpholm
Tether Electronics for DTUsat
If the input higher than the range, the op-amp will give an output beyond the reference voltage. This should give max-reading from the ADC,
and as the op-amp cannot drive its outputs beyond the supply, it will not
be able to damage the ADC.
The 470kΩ in the op-amp feedback path is parallel to a capacitor, in
order to smooth the results (and remove any RF noise). Measurements
are probably going to take place every second, so a filtering time constant
of 10mSec is chosen, yielding 22nF for the cap.
6.2.2 Current S-T
The measurement of the chassis to tether current, allows us to know the
total current from the chassis. This is the current from collected ion’s on
the satellite surface, and so holds information about the plasma environment.
Measuring this current is similar to the previous (S-G). A sense resistor will have one terminal at S (~GND), and the other terminal will be
negative. We do not know for sure how large this current can be, but I
assume that steady-state maximum will be around 1 mA. Using the same
op-amp coupling as in the S-G measurement, gives a sense-resistor of
1kΩ.
Another function of the resistor here is to limit the in-rush current,
when the switch closes. As the S-T potential can be up to 200V (although
not in our orbit), the current through a 1kΩ resistor will start out as
200mA. From the datasheet of the AQV214S, it is found to have a max
continuous DC-current of 120mA (coupled with the two MOS in parallel), and a peak current of 300mA. This should be more than enough, especially as the inductors on the tether and chassis connections will limit
the inrush current.
A diode is connected in reverse across the measurement + switch, in
order to assure that S will never be at lower potential than T. This could
happen otherwise if the tether induced EMF voltage reversed (when the
field lines reverse. This happens in a near-polar circuit), or when the
Chapter 6
Design Details
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Richard Tøpholm
Tether Electronics for DTUsat
switch was open, if the gates draw more current than the chassis can collect.
6.2.3 Current E-T
As mentioned in section 5.3, this current will be measured through a current mirror. The current to be measured flows through the linear optocoupler, which correspondingly sets limits on the range of measurements.
The device used in the prototype (IL300 from Vishay), has a typical K1
of only 0.007. With a recommended max input current IF of 20mA, it is
only able to mirror currents up to 140µA. This may not be a problem if
there is a op-amp on the input side to provide a suitable input range
matching, but this op-amp is hard to supply in our set-up. The whole
point of the design was after all, to eliminate an isolated supply.
As the IL300 was the component ready for the prototype board, the 0140µA is the range of the E-T measurement. This current is translated
into the 0-2.5V range of the ADC by an op-amp on the output side, passing the current through a 15kΩ, giving an input range of 167µA, exceeding the IL300 range, allowing us to measure as the IL300 goes into saturation.
As detailed in the components section, a better alternative has been
found in Agilent HCNR201. With a K1 of typically 48%, minimum 36%
and a max-recommended avg. input current of 20mA, this will allow us
to measure up to typically 9.6mA and minimum 7.2mA of tether current.
A proper choice of resister to measure the current over would be
250Ω. It should be noticed that the currently used op-amps would not be
2.2K
able to supply this current. A current divider on the output op-amp, letting a large
100E
percentage of the current from the photo10E
diode dissipate in a resistor would make
this possible though. This principle is
shown to the right. At 11mA photocurrent, the circuit shown would have
0.1V forward bias across the photodiode, which should be insignificant.
10mA will run in the 10Ω sense resistor, while only 1mA will run
Chapter 6
Design Details
page 41
Richard Tøpholm
Tether Electronics for DTUsat
sourced from the op-amp, thus reducing it’s power consumption dramatically. The 2.2kΩ output resistor will give 2.2V out in this case.
6.2.4 Voltage T
The potential of the tether relative to the satellite, can be measured when
the switch between S and T is open. This allows us to measure the EMF
when no current is running, and the change when a current runs can give
information of the tether behaviour. The EMF can also be calculated
from the B-field measurements, so the measurement of the satellite-tether
voltage could tell if the length of the tether has changed or the satellite is
not near plasma potential.
The voltage can have values from –200V up to maybe +30V. If positive, it will be clamped to +0.5V, by the diode across the S-T switch.
In order to get the voltage into range, it is divided by a simple resistor
voltage divider, between the tether voltage and Vref.
Assuming a 2.5V reference, a 470k to 10M division will give a measurement range from –53 to +2.5V. This will be enough for the typically
encountered voltages in our orbit. The range is 56V, and assuming 10bit
effective resolution, will give accuracy better than 0.06V.
From the ADC input to ground is a capacitor, in order to filter out
noise on the signal. In the prototype, this was chosen as 100nF, which
yields a time constant of 1 second. This might be excessive depending on
the use of the measurement.
If the voltage is below –53, the division would result in a input to the
ADC of less than 0 volt. However, the ADC has internal protection diodes, clamping the inputs to the supply rails ±0.3V. According to the
datasheet, they are not to be loaded by more than 2mA, or the conversion
accuracy will suffer. -200V on the tether and 0V on the input, would give
a current drain from the ADC of 200V/10MΩ – 2.5V/470kΩ = 15µA.
This should be quite safe.
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Design Details
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Richard Tøpholm
Tether Electronics for DTUsat
6.3 Power Supply
It is necessary to monitor the current consumptions of the circuit for indications of latchup. As some of the devices require very little current, they
might be able to latch up, without the total current of the circuit being
higher than a normal situation. For this reason, the circuit has been segmented into 4 parts. 1 part for each DC/DC regulator + highvoltage generator. 1 part for driving the LED’s in the PhotoMOS. And 1 part for all
the electronics. Each segment is supplied by a MAX890L, which monitors the current to it.
The high voltage generators can draw up to 100mA. The DC/DC regulators should have efficiencies of 80-95% when supplying 100mA. Using 75% as worst case, the total current draw is 133mA. In order to avoid
false alarms from the current spikes of the DC/DC regulator, a limit of
200mA for each would probably be sensible. That would mean a 6.9kΩ
set-resistor for the MAX890L. The closes available at the time of prototype-manufacture, was a 5.6k, yielding 246mA max current.
The PhotoMOS are supplied through 220Ω resistors. The PhotoMOS
LED’s have a typical VF=1.14V. Assuming minimum 1V, the max current draw is (3.3V-1V)/220Ω = 10.5mA. Typical current will be affected
by the SPI switch’s on-resistance of typ. 60Ω. (3.3V1.14V)/(220Ω+60Ω)= 7.7mA. This is reasonable, as the PhotoMOS have
recommended IF=5mA, and should probably be derated after radiation
damage.
For all 7 switches active (a nonsense situation by the way), the maximum current is 73.5mA. This is not likely to be exceeded much even
shortly. The chosen set-resistor for the prototype is 15kΩ, yielding 92mA
max current
The last group is all the “electronics”. This is Voltage reference,
ADC, DAC, SPI-SW, 2 double op-amps and the two optocouplers. A
quick estimate of their power-requirements follows:
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Design Details
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Richard Tøpholm
Tether Electronics for DTUsat
The voltage reference consumes 40µA typical, 75µA max at –40+85°C (+150µA during power up), plus the current is sources at Vref.
The ADC consumes typically <1µA when powered down between
conversions, and up to max 1.3mA while doing the conversion. It draws
up to 250µA from the reference during conversion.
The DAC consumes max 205µA, plus max 47µA from the reference,
plus whatever current is sourced on the output. The MAX1820 DC/DC
regulator will only draw 1µA from the DAC.
The SPI switch draws max 30µA.
The op-amps draw <1µA each, plus what they source through the
feedback path. As one end of the feedback path is on virtual ground in
the circuits used, the max current they can source, is for VO=3.3V. Two
of them have 470kΩ in the feedback path giving 3.3V/470kΩ = 7µA.
The last op-amp sources the current for the current mirror. Assuming the
HCBR201, the max-recommended avg. input current IF is 20mA. The
mirrored current is 72% (max CTR K1) of this, or 14.4mA. It should be
noted however that the current op-amp are not able to source more than
11mA! The max total current draw from the op-amps is thus 3*1µA +
2*7µA = 17µA, plus 14.4mA for the current mirror.
The optocoupler uses max 1mA per channel. The linear optocoupler
HCBR201 has a max-recommended avg. input current of 20mA, which
should be enforced by the load resistor.
The total current not including the linear optocoupler, is:
75µA+1.3mA+250µA+205µA+47µA+1µA+30µA+17µA+1µA
= 1.926mA
Add to that the 20mA to supply the input of the linear optocoupler,
and 14.4mA for the output, and the max total current draw becomes
36.3mA.
Setting the current limit to 40mA might make the circuit not notice a
latchup in one of the components with very low normal power consumption.
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Design Details
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From this it seems that it might be logic to move the supply of the
linear optocoupler to the optocoupler group – however while moving the
input (the 20mA) would be easy, moving the output entails moving one
of the op-amps to the optocoupler group too, which might not be wanted.
An option could be to use a transistor on the output of the op-amp, to
drive the current to the linear optocoupler output. That would make it
easy to move the current drawn to the optocoupler group. This would also overcome the problem, that the op-amp is not presently able to drive
the full current of the optocoupler.
Another way to go is to limit the current of the current-mirror. It
could simply be decided, that the max tether-emitter current was 4mA.
Then the input to the linear optocoupler could be limited to 4mA / 36% =
11mA (36% is the minimum CTR K1). The output current should be
4mA for full range, i.e. at 2.5V output from the op-amp (assuming a
2.5Vref). The max would then be (4mA/2.5V)*3.3V = 5.28mA.
In total this would limit the linear optocoupler current consumption
from 34.4mA to 16.4mA.
For the prototype print, the current was limited to 14mA, but the
MAX890L limiters appear to be unable to control currents that small.
The solution is to use MAX892L or even better a MAX4373 current
sense amplifier.
6.4 DC/DC Regulators
The input to the high voltage generators should be variable, to allow the
regulation of the emitter bias. This would be easy done by a DAC, if it
wasn’t that the high voltage generators use quite a lot of current
(100mA). A driver on the DAC output would help – it would in practice
mean a linear voltage regulator supplying the input for the generators.
This is problematic, because the generators may use full current, at
low input voltages, resulting in a large power-loss in the drivers/regulators supplying them. This would both be a waste of scarce
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Design Details
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Richard Tøpholm
Tether Electronics for DTUsat
power, and a heat-dissipation concern. To avoid this, I decided to use
DC/DC switch mode buck regulators to power the high voltage generators.
The MAX1820 chosen has a REF input, to control the voltage. This is
connected to directly to the output of the DAC, which means it can be set
to any voltage from 0V to Vref. The REF to Vout gain is 1.76, which
means that the maximum output voltage of the MAX1820: 3.4V is
reached at 1.93V input. The high voltage generators are connected directly to Vout.
This output is of course dependent on the unregulated bus being at
least the commanded voltage, as the MAX1820 only steps down. If the
unregulated bus is below the desired voltage, the load is connected fulltime to the input, (through the inductor, which is less than 1Ω).
In case the unregulated bus drops below a threshold of typically
2.35V, the MAX1820 will shut down and stop all current drain, until the
input voltage rises again.
The MAX1820 has a skip function, enabling it to skip cycles (it otherwise switches at 1MHz, from an internal clock), to lower switching
losses when load is low (less than approximately 65mA), which is likely
to happen often in our application. This is enabled (SKIP is connected to
GND).
The external components are calculated from the recommendations in
the datasheet. First off, there is no schotky rectifier needed, as it features
synchronous rectification, with an internal MOSFET.
The compensation network has been populated with a 15kΩ resistor
and 22nF capacitor. These values correspond to a converter bandwidth of
around 10kHz for the emitter generator, and 50kHz for the gate generator, however this depends on the load impedance of the high voltage generators, which is not well known.
If assuming that the generators will draw max 100mA and 20mA respectively, and using half the suggested converter bandwidth fC of
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Design Details
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Richard Tøpholm
Tether Electronics for DTUsat
100kHz, and a 2µF output capacitor, I find the following values for the
compensation components. Emitter regulator: 4.1nF and 16.6kΩ. Gate
regulator: 20nF and 16.6kΩ. The second compensation capacitor C2 is
not needed (~0.5pF). The problem is that all the theory assumes that the
load is resistive – which it is not.
22nF and 15kΩ was used in the prototype, along with much larger
output capacitors (4.7µF tantalum parallel with 2 ceramic 1µF). This was
probably overkill.
The size of the inductor is very debateable. Using an expected
Vin=4.2V, Vout=2V and 30% p.p. current ripple at Imax=100mA, the ideal
value is 35uH. This is inversely proportional to Imax, however, and the
selected values mean that the current will become discontinuous below
85mA, which is almost always. The internal synchronous rectifier disconnects the coil from ground, when the current reverses, and generally
handles this as a normal mode of operation. Using 50mA for the max
current (which is very fair for the gate regulator), gives 70uH. In practice,
the max current might well be even smaller. In the prototype, 68uH was
used.
Physical size of the inductor is debatable as well, the bigger an inductor that can be accommodated, the smaller internal resistance can be had,
and thus higher efficiencies. Space however is a premium on the flight
prints, so we will probably have to compromise.
6.5 High voltage switching
When the tether is operating in a self-powered (passive) mode, the induced voltage in the tether biases the emitter. The one problem with this
approach is that it is not controlled. Control is wanted, both to limit the
rate, if out of hand, and to allow dampening of oscillations in the tether.
A way to overcome this would be to have a variable ballast resistor in
the emitter current path. This would introduce unwanted heat dissipation
in the resistor, though. The approach taken is to segment the emitter, so
that a smaller subsection of it can be activated. This segmentation is done
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Design Details
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Richard Tøpholm
Tether Electronics for DTUsat
at the gate, which is divided into 5 parts. The cathode is unbroken across
the whole chip, but only emits, where a gate-bias is present. The first
gate-part covers half the emitting area, the next covers one quarter, the
third covers one eight, the fourth one sixteenth, and the last half that.
There remains an area equal in size to the smallest gate, which is unused.
We are blessed with two emitters, and the second was planned used in
one big area, as gate number zero, covering one whole emitter. This allows the areas to be turned on, and in a 6-bit fashion determine the effective area.
Later it was considered, that it would be smarter to connect the five
gate-areas of the two emitters in parallel, and switch of the cathodes of
the two emitters. This would allow the same range of operations, although with 1 bit less resolution, when both emitters are on (i.e. at half to
full power). This has the advantage of more redundancy, as there are two
cathode connections. It also adds the ability to compare equal areas (different from 100%) of the two chips.
Switching the gate or cathode currents is complicated by their potential being up to 200V away from the supplies. For this reason, a series of
PhotoMOS (optocouplers with MOSFET output) was chosen to switch
them. They easily provide the needed isolation. There is however a problem with leakage currents. The gate current, especially to the smallest
gate-areas, can significant, while still well below 1 µA. This means that
if the switches leak 1 µA, they might as well not be there. This problem
is written about at greater length in the components section.
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Design Details
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Richard Tøpholm
Chapter 7
Tether Electronics for DTUsat
Component Selection
In designing the electronics, the choice of components has probably been
one of the most time-consuming parts. The list of considered components, where some specification in the data-sheet wasn’t good enough, is
very long. I will go through the selected components in this chapter, but
not thoroughly through those not used. The reason for this is primarily
the scope. The datasheets for most of the components will be found on
accompanying CD media. It counts more than 2000 pages, which is why
it is not attached as paper. Should anyone wish to know more detailed
about some part of the components selection process, please do not hesitate to contact me. A thermal calculation, which can be found in Appendix A on page 71, shows that we will probably not need components that
work outside –10°C to 60°C. But it doesn’t hurt either. I have usually
tried to find components with operational range –40°C to 85°C.
7.1 High voltage converters
The major concern here was size. The requirements are for a variable
output voltage, isolated from the input. The output range should be variable through at least 40-120V (and off!). For the emitter-generator, the
design goal has been 1mA. For the gate-generator, 0.1mA (see section
5.1).
A list of high-voltage converters was considered, but most 120+V
converters do not run on 3.3V input. Many do not have easily variable
output voltages, or require external regulation circuits.
And all others were both bigger, and used more power than the
EMCO Highvoltage Q-series DC/DC converters, which was chosen for
the design.
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Richard Tøpholm
Tether Electronics for DTUsat
They turn on at 0.7V input and have a max input of 5, 12 or 24V dependant on type. The output is proportional to the input in the operational
range, with a maximum (at max input) of 100V to 20,000V, for different
types, which are seen on the specification sheet listing all the models, in
appendix.
To obtain a minimal idle power use, the 12V versions are better than
the 5V (see the spec. sheets and application notes). Even if we can’t supply them with more than 4V, we’ll just have to select a model rated 3
times our need. On the other hand, selecting a too high output voltage
just lowers the maximum output current.
They function as transformers, chopping the input voltage to AC,
transforming the AC, then rectifying the resultant high voltage AC to the
output DC. The linearity from input to output comes directly from the
transformation; there is no closed loop regulation of the output. The 0.7V
minimum input is the required voltage to power up the chopper.
As they are supplied from the unregulated supply to save power, the
max input voltage can vary from 2.6 to 5.5V. However, if the battery is
down to 2.6, we are surely not going to run the emitters. The DC/DC
regulators supplying the input voltage, has a maximum output of 3.4V.
This can be maintained with almost no dropout (i.e. with supply voltages
down to 3.4V).
Realistic full output is therefore always at 3.4V input. The devices
have a max rated current (output) which is independent on supplied voltage:
Model
Current
Max Rated
Rated Output
0.7V – 3.4V
140% Output
at 6V Input
Q02-12
2.50mA
12 – 57
140
Q03-12
1.60mA
18 – 85
210
Q04-12
1.25mA
23 – 113
280
Q05-12
1.00mA
29 – 142
350
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Richard Tøpholm
Tether Electronics for DTUsat
The 140% output at 6V input is shown as a safety margin. The unregulated bus is clamped below 6V by a zener diode, so if there is a shortcircuit in the power-supply, this should be the max consequence.
As we don’t expect to need more than 100 – 120V, the Q04-12 or
Q05-12 seems ideal for our purpose. Q04-12 is selected for the prototype,
while the flight-models will use Q05-12.
For the emitter generator max current is important, so a Q03-12
would probably be an even better choice. If the voltage supplied is not
enough to reach the desired current, it’s always possible to use the gate
generator as well.
This choice of generators limits the emitted current to 1.60mA (which
it would probably be wise to further derate for space use), when using the
emitter generator. However, if the emitter generator is not used, the gategenerator will be able to supply the gate with >1mA, which translates
into more than 100mA of emitted current. This exceeds our goals.
7.2 Isolation
One of the major problems with the design of the tether electronics is that
the currents to be measured are not at potentials inside the supply rails.
Even the currents, which are running close to ground, are contaminated with radio-noise. This can be handled by decoupling and highimpedance ADC interfaces.
That still leaves the two major problems of measuring the tether current at –200V, and of switching the gate-current at +200V, which could
be less than 1µA.
7.2.1 Isolation Amplifiers
To measure currents not referenced to the supply, the std. Solution is to
use an isolation amplifier (ISA). They have decoupled measuring and
out-put parts, and often handle isolated supply of the measuring part internally.
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Unfortunately, every single (small) isolation amplifier I have been
able to find, require more than a single 3.3V supply as input. Typical requirements are ±15V supply. A few make do with a single 4.5V supply,
but then typically don’t supply the isolated voltage internally.
To generate ± 15V would require two additional DC/DC regulators.
4.5V could be done by a simple charge-pump, but if another 4.5V has to
be made available isolated, the complexity rises again.
In short, I have given up finding a suitable solution using a readymade isolation amplifier.
7.2.2 Optical Switches
Switches are needed in two places: To connect the Tether to the Chassis
terminal (for safety), and to reduce the gate-current.
The first switch should preferably default to an open-circuit, when
power fails. This is a Normally Open, or “form A” relay. The gateswitches should default to closed circuit when power fails (Normally
Closed = form B).
The choice of form A/B is done to make the tether operate, in case of
a power-system failure.
Mechanical switches (normal relays and reed relays) were dropped
because of concern over shock and vibration resistance and vacuum
compatibility.
Many manufacturers carry series of Optically actuated electronics
switches, typically called SSR (Solid State Relays), PhotoMOS or
OptoMOS, the latter two referring to the MOS transistor actually switching the output. The major problem is leakage current at high voltages and
temperatures. Almost all specify a MAX leakage current of 1µA at 200V,
85°C.
The ones chosen were PhotoMOS by Matsushita (sold under the
names NAIS and Aromat). Most of their series specify the same max
leakages, but they also specify 1nA typical leakage for most products,
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Richard Tøpholm
Tether Electronics for DTUsat
and actual leakages vs. voltage/temperature graphs seem to indicate that
1µA is very conservative. PhotoMOS further have the benefit of being
easily available from DKI. They have a low-leakage RF series, which
specify max 10nA leakage. Unfortunately, the RF series are not available
in Form B, which was wanted for the gate-switch.
The GU-S (General Usage, SOP6) series are used throughout (mostly
because of availability). A few RF (SOP6) switches are taken home for
testing (they boast 90pA typical, 10nA max leakage).
PhotoMOS by Matsushita (NAIS, Aromat)
Type
GU
GU/Limit
GU-E
RF/SOP
RF/low
RF/SOP4
HE
HE/LED
HE/soft
HF
HS
PD
Max mA/Ω
350/400V
120/50
120/25
120/50
50/50
40/100
70/50
50/100
80V: 50/35
250/4
150/16
150/16
250/4
300/4
180/8
120/50
650/1
350/3
200V
Leak
/nA
1000
1000
1000
10
10
10
1000
1000
1000
1000
1000
10000
LedON
Max /mA
3
3
3
3
3
3
3
3
2
5
0,5
3
Comments
Only Form
(Econony)
Only Form
Only Form
Only Form
A
A
A
Only
Only
Only
Only
Only
A
A
A
A
A
Form
Form
Form
Form
Form
A
7.2.3 Optocouplers
While the PhotoMOS are used to switch currents, optocouplers are needed to pass logic signals isolated. This is needed in the measurement of the
tether-current. The optocouplers in this set-up is used to pass the errorcurrent, in a closed loop measuring the emitter-current.
The problem is that optocouplers need a minimum current to switch
on at all. This means that a certain error-current (typically 100µA) will
go undetected and thus taint the measurements. To minimize this error,
an ultra-low-current optocouplers is needed. Actual amplification is only
needed insofar as the error-current should be detectable at the output,
without noise. It is then easy to amplify later.
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When looking at all major optocouplers suppliers, it is found that
even low-current optocouplers are rarely specified below 500µA input,
and often graphs show the CTR dwindling to zero at 100µA.
Fortunately, the Agilent HCPL–070A (–4701/–4731/–070A/–073A
depending on housing) stands out, with a specified minimum input of
40µA (min CTR 800% at 40µA, typically 3500%). Actually it has it’s
peak CTR at 40µA, and graphs in the data-sheet shows it dropping by
25%-33% at 10µA, depending on temperature. This very impressive
performance has been verified in the prototype.
7.2.4 Linear Optocouplers
In order to close the loop, a linear optocouplers is needed. The dominant
type is IL300, which is found in a host of copies from different manufacturers. Originally made by Siemens (since then changed into Infinion,
which (like many others) has been bought by Vishay) it unfortunately
only has a CTR from input to output (K2) of typically 0.7%. This means
that if all of the measured tether current should run through it’s output, it
will only be able to sustain a tether current of 0.007 times the max input
current. Max recommended input current is 40mA, leading to a max tether current of 280µA. Recommended input for best linearity is even lower
at 20mA, giving just 140µA. As the linearity of the IL300 is around
0.5%, this is probably advisable.
It is of course anticipated that this will be remedied by having an opamp at the output, which facilitates precisely splitting the current. However, as there is no isolated supply, this is not easy to achieve in our design. The low CTR further lowers the gain of the total feedback loop, necessitating extra amplification. The prototype has used the IL300 and a
transistor, with the resultant current limit, but this is not usable for the
flight version.
Instead, Agilent again saves the day, by making a drop-in replacement for the IL300, the HCNR201 (and 200), which features a CTR of
typically 48%, minimum 36%. With a max-recommended avg. input current of 20mA, this will allow us to measure up to typically 9.6mA and
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Richard Tøpholm
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minimum 7.2mA of tether current. This diode is further superior by having a far better linearity of typically 0.01%, max 0.07% (at –40°C –
85°C, but at low currents), which should be compared to the IL300 which
has typically 0.5% (at 0°C – 75°C), and no guarantied maximum.
7.2.5 Magnetic Sensors
Magnetic sensors for sensing current were long considered for the tether
current. A list of suppliers was investigated, but in the end, it was found
that there was no simple solution. The nearest was the Honeywell sensors, which feature an internal coil, which could be used as the input current – however they could not guarantee any high breakthrough voltage
between coil and hall-sensor, and vaguely estimated it to around 100V.
7.3 I/O devices
The interface between the payload electronics and the onboard computer
has been fixed as an SPI bus. All information (except tether release and
temperature sensing) will use this bus.
7.3.1 ADC
The AD converters are needed to transmit the measured currents and
voltages to the OBC. It was decided that 10bit would be sufficient, however 12 bits is implemented, to avoid problems.
The device chosen is a MAX1245 low power, 8 channel, 12 bit SPI
ADC. It is the only device chosen, which runs from 2.4-3.3V. All others
run from 2.7-5.5V. This may be a bad design choice, but fortunately, the
MAX1245 has a pin-compatible plug-in replacement, the MAX147,
which could be used just as well, if wanted. The only drawback is a
slightly higher current consumption, but as it is still <50µA, it’s not a
concern.
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7.3.2 DAC
Two analogue outputs are needed to regulate the generators. They are
generated by a MAX5721 low power, 2 channel, 10bit SPI DAC. I decided that 10bit was more resolution than could be expected of the following DC/DC converters anyway. If 12bits were wanted, the MAX5722
is a 12bit drop-in compatible version.
Current consumption is as usual negligible (~100µA).
7.3.3 References
Both the DAC and ADC was chosen without reference, as most built-in
references are not accurate enough to maintain 12bit resolution over the
full temperature range of the satellite. Rather than calibrate and compensate in software, an external voltage reference is chosen: MAX6033A has
<10ppm/° temperature dependency, and an initial accuracy better than
400ppm at 25°C. This means that in the range –35° to +85°C, it’s total
accuracy will is better than 0,1%, or 10bit. This is excellent performance,
especially for the initial accuracy.
While the circuit is designed for a 3V reference (it has 200mV dropout), the ones procured are 2.5V. This has the effect changing some of
the gains in the measurements. It can be restored by proper choice of resistors. While this was not planned, neither is it a real problem.
The 2.5V reference can run on 2.7V supply voltage, which could be
useful – however is not needed in the current specification.
7.3.4 SPI Switches
In order drive the PhotoMOS switches, an analogue SPI switch is chosen.
This is an overkill, as it is actually made for switching analogue currents
like audio-streams and inputs to ADC’s, but it will do the job. I was unable to find any SPI controlled switches, which could drive 8 PhotoMOS,
except for some significantly larger display-drivers, and of course, devices made for 15V or ±15V supplies.
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Maxim actually does have an SPI “relay driver” (MAX4820) in their
catalogue, but it was not marketed yet as this choice was being made (I
have requested samples and a data-sheet as soon as it becomes available).
7.3.5 SPI Potentiometers
To regulate the low voltage DC/DC regulators, two options were considered. One was a DC/DC regulator made for this purpose with a dedicated
control input (which would be driven by a DAC). The other was a SPI
controllable Potentiometer in the feedback path of a ordinary DC/DC
regulator. The later would give a much larger selection of DC/DC regulators, however might give problems with capacitances of the feedback
path.
In the end, backorders on SPI potentiometers made the choice easy.
7.4 DC/DC regulators
If the final design, two DC/DC regulators with variable controllable voltage outputs are used. In earlier design iterations, several other DC/DC
regulators was investigated both for the high voltage generators, and to
supply split voltages and isolated voltages for the isolated tether current
measurements.
The DC/DC regulators chosen are of the type MAX1820. The
(forced) decision to not use a controllable potentiometer in the feedback
path, made the selection of DC/DC regulators very easy. Actually, they
are the only suitable ones located.
They are buck step-down regulators, designed for supplying the output amplifier of a mobile phone, with a voltage, which can be regulated
pending on the need for transmitted power.
They have internal 1MHz clocks, MOSFET and synchronous rectifying (meaning no external Schottky diode is needed, and improving efficiency). They can supply up to 600mA average, and 0.4 – 3.4V output
(up to 100% dutycycle, with dropout re. power-supply < 30mV).
Chapter 7
Component Selection
page 57
Richard Tøpholm
Tether Electronics for DTUsat
The efficiency is quite high, estimated at >80% when supplying more
than 2V.
7.5 Current Sense & Limit
In order to protect the circuit from latchup, it is necessary to monitor the
current consumption. If it becomes too high, it is a sign of a latchup,
which will quickly burn the IC in question. To avoid this, a signal is sent
to the power-supply, which instantly cuts power to the satellite. Power is
restored after a while, when the latchup is expected to have reset.
The prototype uses MAX890L, which was specified by the project. It
would have been smarter to use MAX892L, which is a similar device
(unfortunately not pin-compatible), designed for smaller currents.
In the flight design, it would be logic to replace MAX89#L with
MAX4373T, which does not have a switch buildin. The DC/DC
regulators which are turned of by switching the MAX890L in the current
design, should instead use the buildin shutdown feature, which would
result in a more robust circuit.
7.6 Other Active
While all “major” components have been covered, a few “standard”
components need still be selected. Of course in the end, when they have
to be chosen, they need as much attention and consideration as all the
other components. They are no more “standard” than the rest, simply
more easily available and have more variants available.
7.6.1 Linear Regulators
The power-supply makes a 3.6V bus available, which each sub-system
must regulate down to 3.3V. This is done to avoid more powerdissipation on the power-board, and to minimize electric noise on the
supply between the boards.
Chapter 7
Component Selection
page 58
Richard Tøpholm
Tether Electronics for DTUsat
The regulator to use has been externally decided: REG103 from Texas.
7.6.2 Op-Amps
The Op-Amps needed are for inverting, filtering and boosting the measured voltages and currents. One of them will be supplying the mirrored
current for the linear optocouplers, which could be up to 14.4mA. In the
present design, this current is sourced by the op-amp. It is not easy to invert this to a current, which is sunk. This is because when it’s sourced,
the output voltage is gnd-referenced, which is fine (assuming a good
ground). If it is sunk however, it should be referenced to Vref, meaning
the voltage reference would have to source this current. Actually, the present voltage reference is able to supply 15mA, but it’s not a nice design,
as it introduces “big” current draws on the reference, which will influence its value, deteriorating the other measurements.
The measurements are all done relative to a virtual ground of an opamp. In order to keep measurement errors well below the 10’th bit, the
op-amp should have a input offset voltage well below 1/1024’th of the
reference voltage. This is 2.5mV (for the 2.5V references), which is not
easy for an op-amp.
The op-amps used in the prototype are not good enough to meet the
above demands. They do meet offset, but only at “typically” at 25°C.
They are only able to source 11mA, and not at Vo=2.5V (however, this
might not be that important, as the tether current is not likely to be that
high). They were chosen for their extremely low power use (<1µA), in
case one was needed on the isolated side of the tether current measurement. 1µA could be supplied either by a photovoltaic IC (Vishay and a
few others makes those for use in “home-made” OptoMOS, i.e. to supply
isolated gate current to a MOS.), or supplied by the tether current. In the
last case, it was important that the op-amp did limit the minimum tether
current, which could be measured, more than the offset error from the
optocoupler.
Chapter 7
Component Selection
page 59
Richard Tøpholm
Tether Electronics for DTUsat
The op-amps used are MAX4471 (1.8V 1µA). An even better choice
for use supplied by the tether current would be the Texas Instruments
OPA349NA, which has similar specs, but further allow inputs 0.2V
beyond the rails, which is handy for that design.
7.6.3 Transistor
While the original design has no transistors, the use of the IL300 optocoupler necessitated amplification in the feedback path. This might be a
good idea on the flight-design as well. Further, a transistor is needed for
each of the current-measurement devices, to detect latchup in those IC’s
used to detect latchup in the rest of the circuit.
The DTUsat team has made a common choice of transistor for this
purpose, the 2N2222 from ON Semiconductor. This transistor is plenty
good for amplification in the current measurement circuit as well, so I
will us it there too.
7.7 Passives
The passives of the circuit were chosen last, assuming they would not be
a problem. This was not completely true. For instance acquiring a 1GΩ
resistor (for testing) has proved to be difficult (fortunately a member of
the DTUsat team could get them through friendly connections).
7.7.1 Power Inductors
There is one power-inductor for each DC/DC regulator. They are specified by the MAX1820 datasheet as R < 200MΩ and saturation current >
1A. This may be a bit more than needed, as they will only need to supply
100mA to the high voltage generator, where the MAX1820 is rated for
600mA continuous.
A host of inductors have been considered, and a very nice one from
MicroSpire, the SESI-14S, was chosen. Unfortunately it could not make
it in time for the prototype, so a Würth WE-PD series was used instead.
Chapter 7
Component Selection
page 60
Richard Tøpholm
Tether Electronics for DTUsat
2 different 68uH WE-PD coils was purchased. A large size with
120MΩ and 1.5A nominel current. And a medium size with 380MΩ and
0.69A nominel current. The efficiencies have not been compared yet,
although that was the intention (a SESI-14S has arrived in the mean time
too).
7.7.2 Diodes
The biggest problem in passives however, has been the diodes. There are
blocking diodes in several places in the circuit, which should have low
leakage (<1µA). This is not easy to find, in diodes rated for 200-400V,
over temperature.
Actually it is not possible, the best I have found is the MMBD150#A
from Fairchild. They are rated at max 1nA leakage current at 125V,
25°C, but at max 3µA at 125V, 150°C. This is ofcourse an extreme
temperature, and the max rating at 85°C should be < 100nA (although it
is not specified).
Another useful diode is the BAS45AL, from Philips. It is only rated at
125V however.
7.7.3 Resistor/Capacitor
Resistors and capacitors on the prototype are laid out in 0402 footprint,
except for those carrying high voltage, which have either larger footprint,
or are axial threaded resistors.
Most capacitors are used for decoupling, and space efficient Y5V dielectric can be used for this. However, for the op-amp filtering, and the
DC/DC compensation line, X7R should be used (or C0G, but then the
largest is 10nF in a 1206, 22nF in 1825, 47nF in 2220, and 100nF in
2225 package).
The large capacitors for the DC/DC regulators input and output must
be assured to have self-resonance above 1MHz. X7R capacitors in 0805
(up to 2.2µF at 10V) should be good.
Chapter 7
Component Selection
page 61
Richard Tøpholm
Tether Electronics for DTUsat
For RF decoupling C0G 82-100pF capacitors should be used. 200V
versions are found in 0805, and 500V-1500V versions can be had in 1206
(for the tether-potential, or gate voltages).
Chapter 7
Component Selection
page 62
Richard Tøpholm
Tether Electronics for DTUsat
Chapter 8
Prototype
8.1 Layout
The prototype was laid out in Protel on a 100 by 100mm double sided
PCB. Components were only mounted on one side, and a ground plane
was used where there were no lines on the solder side (not shown).
Figure 19: Layout of prototype.
Chapter 8
Prototype
page 63
Richard Tøpholm
Tether Electronics for DTUsat
The high voltage part was laid out with 60mil between lines, while the
rest was minimum 8mil (except for high-current parts). 60mil was chosen
because that was the approximate distance between the pads on the PhotoMOS switches.
Care was taken to keep the DC/DC regulators layout as close as possible, and to minimize most ground loops.
8.2 Measurements
The function of the prototype board and its components has been verified, by several measurements. The OBC I/O has not been tested however, except for looking at the ADC output in an oscilloscope.
8.2.1 Switch leakage
The PhotoMOS switches should have a very low leakage, in order to ensure that a gate is actually switched off, and is not being held active by
the leakage current.
This means leakages of less than 1µA. The used switches “features a
very small off state leakage current of only 100 pA even at the rated load
voltage of 400 V” according to the description in the data sheets (see the
appendix). The data in the same sheet only specify “maximum 1µA”,
however. This seems paradoxical, so I decided to measure it myself.
The problem measuring this, is that a 1nA leakage current at 400V,
corresponds to 400GΩ. To measure this resistance RS, the switch is put in
series with a resistor R and supplied with a voltage V. The resistor is
chosen as high as can be found (1 GΩ), to facilitate getting a signal. The
voltage over the switch (VS) is then measured. Ideally, VS/V =
RS/(R+RS), which reduces to RS = R*VS/(V-VS). Then the practical problems set in: Trying to measure Vs will yield 0V. Trying to measure the
voltage over the resistor gives 0V too, although measuring over both the
switch and the resistor yields V, the voltage supply. This is because most
voltmeters have an internal resistance of 10MΩ, and will practically act
as a shortcut here.
Chapter 8
Prototype
page 64
Richard Tøpholm
Tether Electronics for DTUsat
The way to solve this is to use a “whetstone
bridge”, in which the switch to be measured is
V
placed as shown in red on the diagram to the
right. The voltmeter is then used across the two
centre poles, and the variable resistor is turned,
until the voltmeter shows zero volts. Then the
impedance of the red unknown component can
be simply determined, when the three other are Figure 20
known. In this way, the internal resistance of the
voltmeter is no longer of consequence, as there are zero volts across it,
and hence, no current will be flowing in it. The left hand side of the
whetstone bridge is actually the same as in the preceding set-up mentioned, but the voltmeter measuring VS no longer measures relative to
ground, but relative to a voltage that can be changed, until the reading is
zero, at which point the voltage VS is mirrored on the right hand side.
The beauty of this set-up is that it is insensitive to the quality of both the
voltmeter, and the power supply.
The whetstone bridge was a bit bothersome to implement, so I used
instead a hybrid, replacing the right hand side with a variable voltage
supply, which could be tweaked until the voltmeter showed zero. While
this set-up is sensitive to noise on the power-supply, it is still quite insensitive to the voltmeters internal resistance.
Vin
The switch measured was a
Vcc
220E
AQV414S, and the set-up is shown to
the right. Notice that it is a NC switch,
so it must be activated, in order to
close. This set-up was measured for
two LED currents, and two input voltages (Vin):
Vcc-Vled
1,878 V
1,878 V
0,821 V
0,821 V
Vled
1,152 V
1,152 V
1,116 V
1,116 V
Chapter 8
Prototype
I LED
8,54 mA
8,54 mA
3,73 mA
3,73 mA
Vin
13,40 V
30,34 V
13,06 V
31,24 V
Vout
11,22 V
28,03 V
10,73 V
28,89 V
V
< ±4 mV
< ±4 mV
< ±5 mV
< ±5 mV
VLED
AQV414S
1G
VM
Vout
V
RL=10M
I leak Rleak min max
2,18 nA 5,1 G 4,2 G 6,7 G
2,31 nA 12,1 G 9,9 G 15,7 G
2,33 nA 4,6 G 3,6 G 6,3 G
2,35 nA 12,3 G 9,7 G 16,7 G
page 65
Richard Tøpholm
Tether Electronics for DTUsat
V is the measurement to be zeroed, and the ±mV values specify the
best values obtainable. The min and max values are calculated, using the
uncertainties for V. Notice that the gab (from min to max) becomes quite
wide, because of the very significant current running in the voltmeter,
even at 5mV (5mV/10MΩ = 0.5nA), compared to the leakage current.
The total effect of this is actually quite difficult to calculate.
The leakage current seems largely unaffected by both the LED current, and the applied voltage, although the 30V I had available might be a
bit low for this measurement. A leakage current of 2nA is on one hand
very nice and low for our purpose, on the other hand it is quite high compared to the promised 100pA from the data-sheet.
As long as this result is close to what we will se in orbit, the result is
fine. Unfortunately, the leakage may rise sharply with temperature; but as
the gate-current will be measurable in orbit with an accuracy of better
than 40nA (40µA scale, 10bit), any big change in leakage will be observable.
Chassis
8.2.2 Tether current measurement
To verify the tether current measurement through the optical mirror, a series of measurements was performed.
A negative voltage between –2V
and –30V was applied to the tether,
while the chassis was kept at zero volt.
The negative voltage was applied
through a 39kΩ resistor, which allowed measuring the current. At the
same time, the output voltage from the
op-amp (on the output side of the current mirror) was measured, together
with the current running in it (the voltage over it’s 100Ω ballast resistor).
Chapter 8
Prototype
99K1
Vcc
Iout
15K42
B
Vcc
22K
C
100E
D
38K9
A
-2V to -30V is
applied to the
Tether here
page 66
Richard Tøpholm
Tether Electronics for DTUsat
The total current consumption of the menagerie was recorded for reference.
The tether “tip-voltage”, was called A. The tether at the satellite (after
the resister) is D, so the voltage across the 38.9kΩ resistor is A-D.
The op-amp output voltage is across a 15.42Ω resistor.
In the first attempt, the proportionality was sadly missing. Fortunately
the total current consumption of the board (which rose dramatically),
alerted me to the fact that the IL300 was actually drawing a lot of LED
current, although not enough to make the outputs stir much. This made
me aware of the very low CTR of the IL300, which I had been ignorant
of.
For this reason a discrete PNP was added on the output of the HCPL73A optocoupler, sinking current from the IL300 LED, into a 100Ω ballast resistor to ground. A pull 22k pull-up was added, to ensure that the
transistor did not see any leakage current from the optocoupler. The total
diagram is shown left, with the main tether current path in bold. The
measurement points are shown as well; notice that the tether current, the
measured tether current and the LED current can be measured. A spreadsheet with the measurements is found in Appendix E on page 79, showing graphs of actual vs. measured current.
The result indicated an actual CTR of the IL300 (K1) of 0.3-0.5%
over the linear range. This limited the range of the measurements with
linear results to 4-100µA. The K3 gain (input to output photodiode gain)
was found to be 1.26 by linear regression over the results with input currents up to 80µA. The same regression gave an offset of only 1.8µA! It is
quite impressive that the HCPL-073A was able to even turn on at all for
an input as low as this.
In all measurement in the 4-80µA input range, when correcting for
K3 and offset, the observed measurement error was less than 0.12µA!
When not correcting for the offset, the error is still less than 2.2µA up
to 100µA input. This exceeded my expectations a lot. The linearity is
Chapter 8
Prototype
page 67
Richard Tøpholm
K3=
1,260
measured
A
2,01 µA
A
7,61 µA
A 12,96 µA
A 18,76 µA
A 33,46 µA
A 47,60 µA
A 62,45 µA
A 76,85 µA
A 88,20 µA
A 93,71 µA
A 103,57 µA
A 112,84 µA
meas.* K3
2,53 µA
9,58 µA
16,32 µA
23,63 µA
42,15 µA
59,96 µA
78,66 µA
96,80 µA
111,09 µA
118,03 µA
130,45 µA
142,13 µA
Tether Electronics for DTUsat
about the same as the result reported by the article in Appendix B , although they have
higher
We
areadobted
limited
Yellowavalues
are range.
the K3 and
offset
for by the transistor not becalculations in other columns (written above),
ing able to supply more than
around 12mA, because
of the chosen ballast
Linear Regression
Offset= 1,821 µA
(on all
above
the line)
Meas.
( * K3 + Offset
)
Circuit
resistor, and theError
fact that
theresults
LED
need
1.25VCorrected
and two
transistors
(the
LED
K1
Using K3
K3
Offset End Pt Err Result
Error
Err % dissipation
output
NPN, and
the external PNP) need around
753
µA
0,0027
1,71 µA
4,35 0.5mA
µA -0,11base-emitter
µA
-2,56% 1,44 mA
2014 µA
0,0038
1,81 µA
1,277
1,67 µA
0,00 µA 11,40 µA -0,01 µA
-0,12% 1,71 mA
drop: (3.3V-1.25V-0.5V-0.5V)/100Ω
= 10.5mA. Using 0,03
a lower
value
3116 µA
0,0042
1,85 µA
1,273
1,69 µA -0,01 µA 18,14 µA
µA
0,19% 1,93 mA
4261
µA
0,0044
1,92
µA
1,272
1,70
µA
-0,01
µA
25,45
µA
0,10
µA
0,40%
2,19 mA
resister would probably extend our linear range, until the IL300 burns.
7088 µA
0,0047
1,91 µA
1,266
1,75 µA -0,04 µA 43,97 µA
0,09 µA
0,21% 2,75 mA
9780 µA
0,0049
1,74 µA
1,260
1,81 µA -0,10 µA 61,78 µA -0,08 µA
-0,13% 3,29 mA
The0,0049
offset reported
in1,260
the article
is 58µA, where our design -0,03%
boasts 3,83 mA
12675 µA
1,80 µA
1,82 µA -0,02 µA 80,48 µA -0,02 µA
15317
µA than
0,0050
2,18 µA
1,77 µA to 0,18
98,62optocoupler.
µA
0,35 µA
0,36% 4,36 mA
less
2µA. This
must 1,263
be attributed
the µA
Agilent
17390 µA
0,0051
5,88 µA
1,287
1,23 µA
2,20 µA 112,91 µA
4,05 µA
3,59% 5,01 mA
17621 µA
0,0053 16,93 µA
1,356 -0,50 µA
8,40 µA 119,85 µA 15,11 µA
12,60% 5,88 mA
When
the HCNR201 1,516
is fitted,
it should improve in both range
and
17630 µA
0,0059 41,53 µA
-5,57 µA 20,50 µA 132,27 µA 39,71 µA
30,02% 8,27 mA
17531
µA
0,0064
66,09from
µA
1,707
-12,50 µA is
28,10
µA 143,95
µA 64,27 µA
44,65% 10,46 mA
linearity.
A graph
the
spreadsheet
shown
below.
[µA]
Linearity
2,0%
7 uA
Error [%] when corrected for K3 and offset. Left axis.
1,5%
Error [µA] when using only K3, showing the offset. Right
axis.
6 uA
Actual-Corrected
Corrected
1,0%
5 uA
0,5%
4 uA
0,0%
3 uA
-0,5%
2 uA
-1,0%
-1,5%
150
200
-2,0%
0,00 µA
1 uA
The offset, fittet
from 4-80µA
20,00 µA
40,00 µA
60,00 µA
80,00 µA
100,00 µA
0 uA
120,00 µA
8.2.3 ADC
The ADC was connected to an oscilloscope, and the SPI input to it connected to the battery, giving the effect of sending continuous $FF com-
Chapter 8
Prototype
page 68
Richard Tøpholm
Tether Electronics for DTUsat
mands, which read out channel 7. The output was observed visually on
the scope, and seen to represent the input voltage (a division of the supply) consistently, and without noise, even in the least significant bit.
Changing the input (supply voltage), would cause smooth transitions in
the binary representation.
The circuit has yet to be connected to OBC, so the other channels,
and the DAC have yet to be tested.
Chapter 8
Prototype
page 69
Richard Tøpholm
Tether Electronics for DTUsat
Chapter 9
Conclusion
An electrical design for the DTUSat tether payload has been developed,
which gives maximum flexibility, while keeping the design safe and reasonably simple. In summarising, the following key points have been covered in the project:

The possible emitter configurations have been
found and considered.

The relevant measurements have been identified

Problems relating to measuring the high-voltage
tether circuit has been solved

Detailed design has been completed, along with
component selection

A prototype has been manufactured and tested
Chapter 9
Conclusion
page 70
Richard Tøpholm
Appendix A
Tether Electronics for DTUsat
Thermal Considerations
A simple model for the temperature of DTUsat has been produced in Excel. This was necessary to know what temperature ranges components
would need to operate in.
The results for a few surface finishes are shown on the following pages.
The actual spreadsheet is found on the accompanying CD-ROM.
Appendix A
Thermal Considerations
page 71
Richard Tøpholm
Tether Electronics for DTUsat
Appendix A
Thermal Considerations
page 72
Richard Tøpholm
Appendix B
Tether Electronics for DTUsat
Monitoring DC anode current of
a grounded-cathode photomultiplier tube
The article by S.Argiò et al., Nucl.Instrum.Meth.A 435 (1999): 484-489,
found at http://topserver.mi.infn.it/auger/documents/NIM_1999.pdf that inspired the T-S current measurement method.
Appendix B
Monitoring DC anode current of a grounded-cathode photomultiplier
tube
page 73
Richard Tøpholm
Tether Electronics for DTUsat
Appendix B
Monitoring DC anode current of a grounded-cathode photomultiplier
tube
page 74
Richard Tøpholm
Appendix C
Tether Electronics for DTUsat
Diagram
On the following pages are the simple diagram and the full Protel diagram used for the layout.
Appendix C
Diagram
page 75
Richard Tøpholm
Tether Electronics for DTUsat
Appendix C
Diagram
page 76
Richard Tøpholm
Appendix D
Tether Electronics for DTUsat
Prototype Layout
Notice that the ground plane on the solder side (blue) is not shown.
Appendix D
Prototype Layout
page 77
Richard Tøpholm
Tether Electronics for DTUsat
Appendix D
Prototype Layout
page 78
Richard Tøpholm
Appendix E
Tether Electronics for DTUsat
Tether current measurements on
Prototype
The actual spreadsheet is found on the accompanying CD-ROM.
Appendix E
Tether current measurements on Prototype
page 79
Richard Tøpholm
Tether Electronics for DTUsat
Appendix E
Tether current measurements on Prototype
page 80
Richard Tøpholm
Appendix F
Tether Electronics for DTUsat
Tether Parameters
Gravitational constant
G
66,7e-12 m³/kg/s²
10km orbitchange in 1 year (0,17uN)
Geophysical constants
Mass
6,0e+24 kg
Radius
6,4e+06 m, mean
μ = G*M
399e+12 m³/s²
Sat. Mass
1,0e+0 kg
Orbit
height
900,0e+3 m
inclination
98 °
B-felt-tvær
4,0e-6 T
0,04 gauss
----------------------------------------------------velocity
7,4e+3 m/s Orbit velocity
Energy
-27,4e+6 J
Orbit energy
ohmega
1,0e-3 rad/s angular speed
period
6170,25 s
102,84 minutes
Goal, Lowering orbit
change
-10,0e+3 m
10,0 km
time
2,6e+6 s
30,0 days
----------------------------------------------------Efinal
-27,4e+6 J
Final orbit energy
DeltaE
-37,7e+3 J
Change in energy
P
-14,6e-3 W
Power needed
F
-2,0e-6 N
Force needed
Length
Inclination Induced
Goal
20m
20m
100m
100m
700m
700m
1500m
1500m
60°
97°
60°
97°
60°
97°
60°
97°
0,85mA
2,40mA
0,17mA
0,48mA
0,02mA
0,06mA
0,01mA
0,03mA
1,5V
0,5V
7,5V
2,7V
52V
22V
113V
40V
(60°=>0,099G, 98°=>-0,041G)
ang.mom.
53,8E+9 N*m*s
Goal, changing declination
change
17,5E-3 rad
1 grader
time
31,5E+6 s
1,00 år
----------------------------------------------------DeltaL
940E+6 N*m*s
torq=dL/dt 29,8E+0 N*m
F
4,1E-6 N
Avg. Force needed
F*pi/2
6,4E-6 N
Amplitude of
sinusoidal force
Tether
Længde
700,0e+0 m
0,70 km
It takes a force
3,3 times higher to
Masse
60,0e-3 kg
60,00 g
change the inclination
1 degee, than to
resistivity
30,5e-9 Ω·m (alu)
lower the orbit by
10,0 km!! (In equal time)
densitet
2,7e+3 kg/m³ (alu)
This is not going to happen!!
tensile str
300,0e+6 Pa
(al7)
levetid
8,58 år
----------------------------------------------------Tape width
1,0e-3 m
1 mm
Radius
100,5e-6 m
0,10 mm
Crosssec
31,7e-9 m²
Cross-section of tether
Surface
442,1e-3 m²
Surface area of tether
Frontarea
140,7e-3 m²
Frontal area (i.e. For aerodynamics)
Resistance 672,5e+0 Ω
0,67 kΩ
Strength
9,5e+0 N
0,97 kg carrying capability
Vind
20,7e+0 V
Induced Voltage
Imax
30,8e-3 A
Max current (rest-voltage= 0V)
Pmax
639,1e-3 W
Max power dissipated in tether (rest-voltate= 0V)
Pmax/m²
1,4e+0 W/m² Max power dissipated per surface area
Psun
190,0e+0 W
Incident solar power
Igoal
702,5e-6 A
Current, goal (average)
Vt,goal
472,5e-3 V
Resistive voltage-drop over tether
Vrest,goal
20,3e+0 V
Rest voltage, Anode+Cathode+Plasma
Icritical1
23,3e-3
First critical current
angle,goal
88,3e+0
Tether angle to horizontal, at Igoal
angle,max Unstable@Imax Tether angle to horizontal, at Imax
Appendix F
Tether Parameters
page 81
Richard Tøpholm
Tether Electronics for DTUsat
Appendix F
Tether Parameters
page 82
Richard Tøpholm
Appendix G
Tether Electronics for DTUsat
Selected Component Data sheets
The datasheets of all used components are found in this appendix. Most
considered components have datasheets on CD-ROM.
Some of the used components have larger datasheet and/or application
notes on the CD-ROM too.
Appendix G
Selected Component Data sheets
page 83