Download PWM Speed Controller

More ?’s I need to research.
PWM with current control – I think this is right. Can this circuit be changed to work on
an electrolytic water (or whatever) tank?
What is the difference between a motor and an electrolytic tank? And I also ran across
terms about different loads, like restive or capacitive or active or passive or… or… or…
What is the difference between all these different loads? I imagine the big difference is
what circuitry is best for each type of load.
This next section is about a motor control circuit. This might be close to what I want.
Maybe I should ask the forum guys, they are smart.
http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html
Speed Controllers
10. Current limiting
Current limiting is absolutely essential. If the motor is stalled, it can take huge currents which would destroy the
MOSFET’s very quickly. The form of current limiting presented here is to measure the current that the motor is
taking, and if it is above a preset threshold, turn the MOSFET’s in the bridge off. If you have a microcontroller
on board which generates the PWM ratio, it would be an advantage if the software could detect the over- current
status, and reduce the PWM ratio by, say, 10%.
A circuit to perform this function is shown below.
This circuit shows just the upper MOSFET’s of the bridge being driven for simplicity. The lower MOSFET’s
are not turned off during a current limit. There is only one sense resistor required for each motor, and that
should be connected immediately from the battery positive terminal.
Circuit description
The voltage dropped across the sense resistor is amplified by U1A, which is connected in a differential
amplifier circuit. The gain of this is 480k / 1k which is 480. This is a very large gain because the voltage
dropped across the sense resistor will be very small. The output of the differential amplifier is then heavily low
pass filtered by RxCx. This is because there will be a lot of noise coming from the motor, and we do not want to
limit the current if we don't need to. D13 is present to make sure that no negative spikes can affect the following
circuitry. U2B compares the filtered signal with a preset value (represented here by V5), and if the current is too
high (i.e. the signal is greater than V5), U2B will turn Q1 and Q2 on which clamps the PWM signals from the
PWM generator. This will force the MOSFET driver to turn the MOSFET off. Q1 must be repeated four times,
one for each of the MOSFET driver channels, but all four transistors can be driven from U2B. D11, R14 and C4
make sure that the MOSFET doesn't turn back on straight away, but takes a few milliseconds. This stops the
MOSFET being rapidly turned on and off.
10.1. The shunt resistor
The shunt resistor R7 in the circuit must be a very low value if we want large currents to be able to flow, up to
100 Amps for example. It must not drop too much voltage, thereby robbing power from the motor, and it must
be capable of dissipating the power without burning out when large currents are passed through. Some suitable
resistors are available from Farnell, code 156-267. These are still too large a resistance (and too low power), so
we can place eight in parallel. The power handling capability is then increased eightfold, and the resistance
decreased eightfold.
An alternative is to use a piece of wire of an appropriate thickness and length. This can be calculated using the
data on this web site - http://www.uslink.net/~cybercir/cir11.htm (link not valid now – it may just be off line
right now).
A simulation of the current limiting part of this circuit is shown in the diagram below. The V5 threshold voltage
was chosen to set a current limit of 30 Amps. The square wave is the PWM voltage (MOSFET gate voltage),
and the sloppy waveform is the drain (motor) current. The spiky bits at the top of the sloppy waveform are
when the current limiting is switching in and out.
There is an in-depth document here - http://homepages.which.net/~paul.hills/SpeedControl/SenseResistors.html
- which describes sense resistors in detail. (the body of text of thiis link is copied in the next section.
Some circuits you may see sample the current going through the main power MOSFET by placing a much
lower power MOSFET in parallel with it. There is a circuit on the 4QD site which does this here. This works
OK, but the problem is the actual limiting current is dependant on the value of Rds(on) of the MOSFET. If
Rds(on) was only half the value we were expecting it to be, then twice as much current would flow before the
limiting circuit took effect. Also the Rds(on) value depends very much on the current that is passing through the
MOSFET, and on the temperature. Any variation in Rds(on) will change the limiting current.
The Rds(on) figure is quoted as a maximum value on the datasheet, but it is not a design-safe parameter. This
means that it is not within defined limits which are published on the datasheet. For example, CMOS digital
logic guarantees that the output voltage, Vo, will be between Vcc-0.5v and Vcc, and that figure can be used to
design circuits which rely on that figure. However, with Rds(on), we only know that it will be between 0 and
the quoted value. We cannot rely on a minimum value of it, yet it is the minimum value which controls the
current limit. Therefore, using a separate shunt resistor is a much safer method.
One problem with the circuit presented above is that you may want to provide a larger current during
acceleration, or in emergencies. This can be solved by disabling the current limiting using a separate line from
any onboard microcontroller, or by adding a circuit which allows an over-current condition for just a short time.
The amount of time that this is allowed must be carefully calculated to prevent damage to the MOSFET’s, and
must take into account the cooling system that you have provided.
An alternative to using the op-amp differential amplifier circuit used above is to use an integrated current sense
monitor IC. Several companies make these; I have used the Zetex ZXCT1010. Zetex's range of current monitors
can be found here - http://www.diodes.com/zetex/?ztx=3.0/3-3-2b@rid~63 - Bipolars for driving MOSFETs
and IGBTs >> ZXCT1008/1009/1010/1011/1012 - Current output current monitors.
10.2. Current limited torque speed characteristics
If a DC motor is being driven by a speed controller with current limiting active, what happens to the torque
speed characteristic graph?
The DC Motors page describes the normal motor torque speed graph, and how the torque of a permanent
magnet DC motor is proportional to the current. If the current is limited however, the torque must also be
limited, at the value coincident with the limited current on the torque-current graph. The effect that this has on
the torque speed graph is shown below:
As the load torque increases, the speed drops - we are following the line in the torque speed characteristic from
the left hand side towards the right, drooping down. This is the same as the uncontrolled motor. The motor
torque always equals the load torque when the motor is running at constant speed (this follows from Newton's
first law - "An object in motion tends to stay in motion with the same speed and in the same direction unless
acted upon by an unbalanced force." The motor torque and load torque must be balanced out if the speed is not
changing).
Let's call the current limit value iL and the equivalent torque value on the torque-current graph at this current is
TL. When the load torque exceeds TL, the motor can no longer create an equal and opposite torque, and so the
load will push the motor backwards in the opposite direction - we are now following the line as it drops
downwards into negative speed.
Let's take an example; an opponent's robot is more powerful than ours (or his current limit is set higher), and we
are in a pushing match. As each pushes harder, our speed controller reaches its current limit first. Our robot is
now pushing at a constant force (since the motor torque is now constant at its highest value). As the opponent
pushes harder, our wheels start to rotate backwards, and the pair of robots accelerates backwards at a rate given
by Newton's second law:
F=ma or a=F/m
Where F is the difference between the forces of the two robots pushing and m is the total mass of the two
robots.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+++++++++++++++++++++
Making Sense of Sense Resistors
1. Temperature Effects
by Dennis Feucht
Some components are relatively unimportant in their effect on overall circuit behavior. Sense resistors are not one of
them. It may only be a resistor, but it's a very important resistor with subtleties that can impede design progress.
Temperature Coefficient
Sense resistors are usually power resistors in that they dissipate non-negligible amounts of power and are typically rated
at 0.5 to 5 W. At the same time, they must hold their resistance value, for it contributes proportionally to the current
measurement. Temperature coefficient (TC) of resistance is consequently of high design importance. Using a clipped
length of copper wire will demonstrate this point. In a motor drive, the torque will fall off as the motor operates due to the
increasing resistance of the copper sense resistor, as it heats. Wire made of alloys with low TCs are available for use
instead.
But sometimes a copper sense resistor, whether as a discrete wire resistor or a circuit-board trace, can be used to
advantage. The TC of copper is fairly constant over a wide temperature range, and its positive TC of about +0.4 %/°C can
be used to compensate for the characteristically negative TC (–2.2 mV/°C) of a semiconductor junction elsewhere in the
current control loop.
Actual Sense Resistors
One simple approach to sense resistors is to make your own out of a low TC conductive material. Manganin wire, an alloy
of copper, manganese, and nickel, has a low TC of within 15 ppm/°C from 0 to 80 °C. AWG # 18 manganin wire has a
resistivity of 0.361 /m. Smaller diameters are available and the wire can be bought by the roll. Cut the wire to length for
the desired resistance, tin the ends, and solder into the board. A manganin resistor is shown in Figure 1 (left). By keeping
the half-loop area small, inductance is minimized. The one shown has a resistance of 25 m .
Figure 1: Homemade and Commercial Sense Resistors
If you do not want to make your own, the resistors shown on the right on Figure 1 (front and rear) are commercially
available power resistors made of a low-TC metal foil on an anodized aluminum substrate, and in TO-220 or TO-247
packages. Several companies, such as Caddock Electronics, now supply these accurate, low TC, power resistors at an
attractive price for power-circuit design.
Another commonly-used low-TC material is nickel-chromium, or nichrome. Its resistivity of 133
-cm requires less wire
length than mangagin's 43
-cm, which can reduce inductance for very low-value resistors. Manganin, however, is
superior to nichrome in TC and long-term stability of resistance value.
If future circuit-board fabrication technology allows a wider range of substrate materials (more than copper), thin-film
power resistors can be integrated onto the board during layout. With clever circuit design, even copper traces can be TCcompensated with BJT junctions.
Kelvin Sensing Resistors
Series parasitic resistance is also a nuisance for low-value resistors. It is necessary to sense across the actual resistance
of specified value and not some of the component lead resistance in addition. To solve this problem, 4-wire, or Kelvin,
sensing is made possible with 4-wire resistors, such as those shown on Figure 2. The leads are attached internally to the
desired resistance and are brought out of the package in pairs.
Figure 2: Kelvin Sensing Resistors
Kelvin-sensing resistors are also available in foil-on-substrate form, as shown on Figure 3. (The quarter is for size
comparison.) The smaller, inner leads are 4-wire Kelvin sense leads while the wider, outer leads are the drive leads.
Figure 3: Kelvin-sensing Resistors
in foil-on-substrate form
Closure
Current-sense resistors for power electronics must be of low TC, relatively high power rating, and accurate at low
resistance values. Such resistors are commercially available, in multiple packages, and at moderate prices. If prices are
too high for a given application, consider making instead of buying sense resistors out of manganin wire, or for the lowest
cost, use a copper circuit-board trace compensated by a silicon junction.
2. Parasitic Series Inductance—Frequency Sweep Measurement
A sense resistor is not only a resistor. A better model includes series inductance. The terminal leads (or terminal traces for
surface-mount resistors) contribute an inductive element. In most resistor applications, this inductance is of no
consequence, for it forms a time constant that is very small. But when the resistance is also small, the time constant, τ =
L/R, becomes large enough so that the frequency 1/τ lies too close to the loop bandwidth of the power circuit.
It is not difficult to encounter parasitic inductances in the 50 nH to 200 nH range. This is too small a value to measure
accurately on common RLC meters (or "bridges"), but can be measured conveniently on the lab bench by different
methods (covered below). A 100 mΩ sense resistor with a series inductance of 100 nH has a time constant of 1 µs. And a
10 mΩ resistor will have a time constant of 10 µs, or a break frequency of about 16 kHz, within the bandwidth of many
power-circuit feedback loops.
The series RL combination has an impedance of
,
where s = jΩ for frequency-response analysis (and Ω = 2πf, where f is the frequency in Hertz). The additional zero at a
radian frequency of 1/(Ls/Rs) introduces an additional pole in a current-amplifier feedback loop if it is in the feedback path.
Consequently, the zero cannot be ignored, and some estimate of the parasitic inductance becomes worthwhile.
Parasitic Inductance Measurement: Frequency Sweep Method
We will examine two possible methods to measure series inductance based on the frequency and time domains, in that
order.
The test circuit shown below can be built easily on a lab bench and used to measure the parasitic series inductance.
Typical function generators have 50 Ω outputs and can be used for this measurement.
Figure 1 - Test Circuit Used to Measure the Parasitic Series Inductance
Let the generator source resistance and its output terminating resistance (both 50 Ω) be combined (in parallel) to form the
equivalent series source resistance of Rg = 25 Ω. The transfer function of this circuit is
.
For Rg >> Rs, this is approximated by
.
By frequency-sweeping the generator, the frequency fa at which the amplitude increases by a times is then substituted
into
to produce the value of Ls. (Use a value of a >> 1 to avoid the knee of the frequency-response curve around the break
frequency.) For a = 5, sweep the generator upward in frequency until the measured amplitude is 5 times that of its
unchanging, low-frequency value. Then substitute a and this frequency, fa, into the above equation for Ls.
Several factors that limit the usefulness of this method are listed below.
1. Resistance (distinct from reactance) increases with frequency due to the skin effect, the self-induced eddy-current
effect within a conductor. Resistance increases above a frequency at which the effective depth of current
penetration into the conductive material is reduced. Thin-film resistors suffer this effect at relatively higher
frequencies than bulk resistors because the skin depth exceeds their thin conductive dimension at lower
frequencies, causing no change in their effective resistance.
2. The source cable, if not terminated into 50 Ω, will set up standing waves, which degrade the accuracy of the
amplitude measurement. If possible, set up the test at the generator output terminals as shown below, but
preferably not with a ×10 scope probe, as shown.
Figure 2 - Recommended Setup of Function Generator Output
The better approach is to use a 50 Ω terminated cable, as shown in Figure 3. Even better, especially for chip
resistors, is to use a GR line insertion unit to preserve the 50 Ω cable environment.
Figure 3 - Use of 50 Ω Terminated Cable with Function Generator
3. Parasitic Series Inductance—Pulse Response Measurement
Parasitic Inductance Measurement: Pulse Response Method
A better series-inductance measurement technique uses a pulse generator. In the time domain, Rs can be eliminated from
its effect on the measurement, a big advantage over the previous approach. Using the same setup as before, except with
a pulse generator as an open source, the generator voltage pulse rising transition is adjusted to be 10 V in 50 ns, or 200
V/µs in this case. Then the source — still considered to be a current source (because Rg >> Rs) — drives the sense
resistor with a current ramp of
.
(The ÷ 2 is due to the 50 Ω termination divider.) The inductance follows from the v-i relationship for inductance:
,
where vL is the constant inductor voltage due to the current ramp.
The following waveforms were observed with the inductance driven by a Tektronix PG508 pulse generator.
Figure 1
The top waveform (A) is the open-source voltage without a sense resistor; the ch 2 waveform is the response with a
resistor, though only approximately time-aligned with the (stored) A waveform. (Note the ×1 setting for ch 2, which is
terminated with 50 Ω.)
The voltage, vo, steps up to vL, a value of about 100 mV (on ch 2, at 20 mV/div). The current-ramp voltage drop across Rs
causes a ramp-up superimposed on vL, which is negligible and undiscernible amidst waveform ringing. The pulse flattens
on top to a constant voltage, leaving (on ch 2) a constant-current drive of Rs, producing about 6 mV (about ¼ div). The
value of Rs can be calculated from the voltage-divider formula, where Vg (= 12.5 V) drives Rg = 25 Ω in series with Rs to
produce about 6 mV across it:
.
For this measurement, the resulting value of Rs is about 24 mΩ, which is 4% less than the approximately 25 mΩ
measured with an RLC meter.
The inductance is calculated from the measured voltage:
,
which is a reasonable value based on the geometry.
Several factors that limit the usefulness of this method are listed below.
Closure
It is difficult to measure the series inductance of small-value resistors. An approximate value can be measured, however,
using a very simple setup with a pulse generator and an oscilloscope. This approach will yield the small values not
measurable with most RLC meters. A network analyzer can provide more accurate measurements at a much higher price.
Copyright © Dennis L. Feucht, 2000
4. Example problem
Question:
We are looking for the following shunt resistors.
Current
(A)
Voltage Drop
(mV)
TCR
(ppm)
40
100
10
1
100
10
Can you kindly suggest a manufacturer?
Answer:
Current sense resistors are sometimes called shunt resistors. While many people think of a shunt as "a piece of wire with
low resistance," there are actually a number of considerations.
Small Values—First, a current sense resistor must be large enough so that a reasonable voltage drop develops across it.
On the other hand, the resistance must be kept small, otherwise it dissipates too much power. To obtain low resistance
values, one has to use special metal alloys. Commercially, one can purchase current shunt/sense resistors with values in
the range 0.0003–100 Ω, designed for currents in the range 1–30,000 A.
Temperature Coefficient of Resistance (TCR)—Clearly the questioner is concerned how the shunt resistance will
change with temperature, since this will directly impact the accuracy of any measurements made. A quick examination of
manufacturer's catalogs show that typical TCRs of current shunt/sense resistors are in the range 100–700 ppm/°C, or
equivalently, 0.01–0.07%/°C. A TCR of 10 ppm/°C (0.001%/°C) is a stringent requirement by any measure. Most pure
metals have much higher TCRs, but there are a few alloys such as Manganin that have TCRs below 20 ppm/°C.
Inductance—If the shunt will be used to measure AC current, then inductance may become an issue. Normally, one
could safely ignore the inductance of a piece of wire 0.5–1.0 inch long, even at relatively low frequencies. However, since
shunt/current sense resistors have values in the milliohm range, even inductances in the order of 100 nH can affect
measurements. ChipCenter's Dennis Feucht explored the inductance of shunt/sense resistors in a series of articles (see
the References below).
Thermal EMV—A current shunt/sense resistor normally implies at least three different metals:
(a) copper wire or PCB cladding,
(b) lead/tin solder, and
(c) the special low TCR alloy of the current shunt/sense resistor.
Whenever dissimilar metals are in contact, it is almost inevitable that small EMVs are generated. This will be in the range
of ± 2 µV/°C for the metal-metal junction in current shunt/sense resistors.
Returning to the question, the table below shows that the questioner wants:
(a) 2.5 mΩ, 4 W, and
(b) 100 mΩ, 0.1 W resistors.
Current
(A)
Voltage Drop
(mV)
TCR
(ppm)
Resistance
(mΩ)
Power
(W)
40
100
10
2.5
4
1
100
10
100
0.1
It is not clear whether inductance is an issue.
One possibility would be the A-H type of current-sense resistors from Istotek that are available in resistances of 0.001–
100 Ω, are made from Manganin foil with low (< 10 ppm) TCR, and are rated at 10 W. Isotek offers online ordering as
well. I list a few other suppliers in the references below.
References
1. Caddock manufacturers a diverse range of precision resistors, including current-sense resistors.
2. Dennis Feucht, Making Sense of Sense Resistors — Part 1: Temperature Effects, ChipCenter.
3. Dennis Feucht, Making Sense of Sense Resistors — Part 2: Parasitic Series Inductance–Frequency Sweep
Measurement, ChipCenter.
4. Dennis Feucht, Making Sense of Sense Resistors — Part 3: Parasitic Series Inductance–Pulse Response
Measurement, ChipCenter.
5. Empro Manufacturing Company markets a wide range of Manganin (remember: TCR for Manganin ~ 20 ppm/°C)
DC current shunts.
6. Isotek Corporation markets specialized resistors, including current-sense resistors.
7. Ohmite is a well-known manufacturer of a wide variety of resistors. Some of their resistors may be used for
current sensing applications.
8. There are laboratory test-equipment current shunts available, where the user programs a resistance value in.