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Solar Panels Data Management using Multi-Processing System
for Nanosatellite Applications
M. Ferrante, F. Bolotti
M. Bordin, M. Gianetti, L. Mencuccini, R. Sabatano, F. Affinito
VITROCISET Space Division Via Tiburtina 1020, 00156 Rome, Italy
Abstract
This article proposes a real-time multi-processing system for the solar panels data
management of nanosatellite using two redundant microprocessors. A first
microprocessor is devoted to the acquisition of currents, temperatures and voltages,
measured from the solar panels, managing directly theirs analogical-digital
conversions and memory storage. A second microprocessor reads data saved by the
first one and estimates the sun direction, providing the results to the on board main
microprocessor. This method allows to free the on board main microprocessor from
the control functions of the solar panels data, so increasing the main 3 DYDLODEOH
time.
Introduction
In this paper, a sturdy system for the solar panels data management (SPDM) of a
nanosatellite is proposed, using two redundant microprocessors. The first one
microprocessor is dedicated to the acquisition of currents, temperatures and
voltages, measured from solar panels, managing directly theirs analogical-digital
conversions and memory storage. The second one reads data on the first
microprocessor and estimates the sun direction on the basis of an algorithm focused
on an accurate solar panels mathematical model. In case of the partial / total failure
of one FRQWUROOHUWKHRWKHURQHWDNHVRYHUFRQWURODQGSURYLGHWKHQHHGHGIXQFWLRQV
with reduced, but sufficient accuracy. Moreover, this system is also capable of
XSJUDGLQJ WKH WZR 3¶V VRIWZDUH GXH WR D JURXQG VWDWLRQ UHTXHVW 7KH SURSRVHG
method has been implemented on “IRECIN”, a modular “pocket” satellite designed
and developed in Italy, currently in the manufacturing and testing phase.
IRECIN Satellite Architecture
IRECIN nanosatellite is a prism constituted by sixteen external sides, 22 cm in
width and 9.7 cm in height, weighting less than 1 kg and composed of 3 internal
Aluminium plates (Figure 1). The solar panels, made of silicon solar cells, are body
mounted on all external faces. The power supply subsystem uses NiMH batteries.
Attitude is determined by two redundant three axis magnetometers and the solar
panels data. Control is provided by an active magnetic control system (magnetic
coils). The spacecraft is spin stabilised with the spin axis normal to the orbit.
Omni antennas are distributed to augment communication capabilities during the
mission.
TX–RX
Telemetry
Power Control
Magnetometers
SPDM
I2 C bus
Main
&
Coils Control
Batteries
Figure 1 – IRECIN Architecture and Subsystems Block Diagram
The solar panels necessary for power generation are also used as a sensing system
for attitude determination, eliminating the need for sun sensor usually employed in
spinning spacecraft attitude determination. The price paid for the achieved
reduction in weight and power consumption is the increased computation and
memory storage for the attitude determination advantageously managed by the
proposed multi-processing system.
The algorithm to evaluate the sun direction is based on a mathematical model of the
solar panels including temperature, space radiation degradation effects on the
electrical characteristics and sun angle effects.
Three internal plates are foreseen to host the following satellite subsystem:
œ
œ
œ
œ
œ
Receiver–Transmitter–TNC (RX–TX). This subsystem performs data-link with
the ground stations. The channel bit rate is 9.6 Kbps.
Power Control Unit (PU). It includes the solar panels, the batteries and the
necessary electronic to generate and distribute the power supply (5 volt) to all
subsystems. Rechargeable NiMH batteries ensure very high energy density,
reducing power system volume and weight. These batteries are characterised
also by a wide temperature range, enabling a simpler thermal design.
Main Microprocessors. This subsystem coordinates all activities of the satellite.
It communicates with subsystems through an I2C bus. It is able to turn off/on
each subsystem in order to manage its power absorption, and to communicate
with Ground Station.
Telemetry. It retrieves all physical values from IRECIN’s sensor.
Magnetometers/Magnetic coils. Two redundant three-axis magnetometers are
added on board to measure the earth magnetic field direction. The estimated
sun direction and the earth magnetic field direction are then used to evaluate the
body axis orientation, using the classic cone intersection algorithm. Spacecraft
control is provided by an active magnetic control system through the interaction
between the earth magnetic field and the on board magnetic dipole generated
by the current pulsed on the coils rolled between internal and external
spacecraft sides.
SPDM System Description
This sub-system (Figure 2) is very important because it is capable to manage the
available power supply in a smart way. It is able to estimate panels’ status, sun
direction, panels’ efficiency, panels’ temperature and satellite’s illumination status
(phase sun/ phase shadow).
I2 C
Bus Communication
between C A and
CB
CA
Electronic Acquisition
Main
C
Sensors/Panels Interface
CA
Sensors
SPDM System
Figure 2 – SDPM Block Diagram
This “ smart” approach allows keeping under control the discharge capacity of
IRECIN’s batteries and the absorption due to the different sub systems. For this
reason SPDM the sub-system has been designed with a redundant architecture,
using two microprocessors. Therefore, in case of a partial/total failure of one
microcontroller, the system SPDM is still capable to carry out its tasks correctly.
Standard systems based on two microprocessors actually work using only one of
them; being the second one just used as back up system. On the other hand, SPDM
uses two micro-controllers performing different functions and controlling each
other. If one of the two micro-controllers fails, the other takes control on subsystem’s operations, executing its functions and the functions of the previous failed
micro-controller, using different algorithms. In order to control the correct
functionality of the SPMD the following procedure is adopted: A, the first
microprocessor, interrogates periodically (every 500 ms) B, the second one. If B
doesn’t answer, then A resets B, waits one period and then verifies again the correct
operation of B. If B doesn’t work correctly then A takes the control and switch to
safety status.
Whenever B doesn’ t receive a new call from A within 500ms, then B executes a
reset of A, waits one period and then verifies again the correct operation of A. If A
doesn’ t work correctly then B takes over control and go in safety status. The “ safety
status” condition is promptly notified to the Main mC.
The system SPMD will provide the following main functionalities:
œ
œ
œ
To calculate sun position
To calculate spin rate
To acquire telemetry and status
data from solar panels (efficiency,
phase sun/shadow)
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To store the most recent telemetry
data
To upgrade the software of the 2
mC.
To communicate with & 0$,1
,QQRUPDORSHUDWLRQVWDWXVHDFK &H[HFXWHVRQHWDVNFRPSRVHGE\PRUHIXQFWLRQV
All functions, run by the mCs, are implemented using two different algorithms, one
more complex that requires more resources [memory, time, computational work
load], the other less complex that takes less resources. Given the “ function 1” , F1
uses the complex algorithm to execute the function in a complex way, while f1
executes a simpler version of the function. In the same way, there are two
implementation of “ function 2” (F2, f2). Assumed that, in the operative status,
“ function 1” (F1) is executed by µCA and “ function 2” (F2) by µCB, in safety
status (only one &LVRSHUDWLYH—&$—&%VKRXOGEHDEOHWRH[HFXWHEHVLGHVWKH
F1 (F2), also f2 (f1) (in case of low resources is foreseen the use of both f1 and f2
simplified algorithms).
SPDM Detailed Description
To understand how this subsystem works is sufficienW WR DQDO\]H 63'0 DQG &
MAIN. The others smart sub-systems that composed IRECIN are based using a
VLPLODUDUFKLWHFWXUH(DFKVXEV\VWHPFRPPXQLFDWHVZLWK &0DLQE\,2C bus.
63'0DQG &0$,1FDQH[FKDQJHWKHIROORZLQJLQIRUPDWLRQ
œ
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63'0¶VVWDWXVERWK &$ and
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Status and telemetry of solar
panels. (current, voltage,
temperature)
Sun position, spin rate
Instantaneous and average
power from solar panels
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œ
œ
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Request telemetry values.
Download new program from
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5HTXHVWRIFDOOIURPRQH &WR
&0DLn
Notification of anomalies from
&WR &0DLQ
The main effort has been to design a system to manage both HW and SW in order
to share the task. Both microcontrollers are capable of:
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Communicate contemporaneous with Main Controller, (this issue is solved
using I2 C bus),
Manage contemporaneous the electronic part that acquires the telemetry
value, making the conversion, and storing the information in RAM
memory. This task is executed by the sensor/panel interface subsystem
(Figure 2).
The following tasks are carried out by the &V:
œ
œ
œ
œ
œ
œ
To communicate with Main Controller
To execute a complex algorithm (F1) in order to estimate sun position
with high accuracy (about 1 degree).
To execute a simplified algorithm (f1) in order to estimate sun position
with low accuracy, but using few resources (time, memory)
To manage A/D converter, multiplex, counter to acquire and to memorize
the data.
To manage A/D converter, multiplex, to acquire in easy way the result of
the A/D.
To retrieve telemetric data from the other C.
Table 1 shows how tasks are divided between &
&$7DVNV
SPDM safety operation
To manage in easy way the electronic
To communicate with Main C
devices (A/D, MUX) f2
To execute complex algorithm F1
To execute a easy algorithm f1
To estimate spin rate
7RDFTXLUHGDWDIURP &%
7RFKHFNIXQFWLRQDOLW\RI &%
&%7DVNV
SPDM normal operation
SPDM safety operation
To manage the electronic devices in
To manage in easy way the electronic
complex way (A/D, memorize data,
devices (A/D, MUX) f2
MUX ) F2
7RVHQGGDWDWR &$
To execute a easy algorithm f1
7RFRPPXQLFDWHZLWK &PDLQZLWK
7RFKHFNIXQFWLRQDOLW\RI &$
few instructions.
SPDM normal operation
Table 1 – &V7DVNV
SPDM Hardware Architecture
SPDM is composed by the devices depicted in Figure 3:
Condizionators
Sensors
Data Bus
Sensor/Panel Interface
ADC
Analog MUX
1
Control Bus
Memory
RAM
r
Counte
Address Bus
Figure 3 – Electonic Acquisistion Subsystem and Solar panel equivalent circuit
& A, & B, PIC16F84 8 bit Microcontroller, 5MHz clock frequency, 1Kbyte
program memory (Flash memory), 68 byte data RAM, low power consumption less
2 mA, 5V power supply.
Interface: It allows controllers to communicate with sensor acquisition (Figure 3).
Electronics Acquisition: This part allows acquiring the value of the physician
values (I, V, T). This part is constituted by: A/D converter, analog multiplexer,
memory, and adapter for analog signal conditioning. A/D converter used is
Ltc1285: 8 pin, low power, 12 bit resolution. It SURYLGHV WR & LWV RZQ UHVXOWV RI
FRQYHUVLRQ LQ VHULDO ZD\ (LWKHU &$ RU &% PDQDJHV WKH DFTXLULQJ RI WKH GDWD
from A/D. Lm335 is the sensor used for acquiring the temperature. IRECIN is
composed by 21 temperature sensors. SPDM uses 20 sensors, 16 are set on the
inside face of the later panels. Maxim 471 is the component used as current sensor.
Numerical Simulation and Results
To evaluate the angle between the sun and a solar panel, an accurate mathematical
model of the solar panel itself is needed, including temperature, space radiation
degradation and sun angle effects on the solar panel i-v curve. A single diode solar
panel equivalent circuit can be assumed as in Figure 3. From this model, connecting
8 cells in series, the I–V curve shown in Figure 4 is obtained for the IRECIN solar
panels. The temperature effect on the solar panel’s i-v curve is shown in Figure 4,
where voltages and currents are those of the IRECIN solar panels. The open circuit
voltage linearly decreases with temperature, while the short circuit current increases
with the temperature logarithm. The rate of change with temperature can be
accurately predicted by solar cell theory and it has been confirmed by tests run in a
solar simulator for the IRECIN solar panels, including solar cell cover material
effect. The effect of sun angle on the spacecraft short circuit current and open
circuit voltage is:
I
sc
I
sc
cos
0
V
oc
V
oc
0
V log(cos
T
)
where s is the angle between the sun and the direction normal to the solar panel.
Depending on the available cpu resources, two different algorithms F1 and f1, may
be used to determine sun direction. A spacecraft reference frame x-y-z, with z axis
along the spin axis, x (out of the first panel) and y axes in a plane parallel to
satellite’s bases is used. Elevation “el” is the angle between the solar unit vector
“S” and xy plane, while azimuth “ Az” is the angle between the projection of “ S” in
xy plane and x axis. Thus the relations between sun power measurements and sun
direction are:
I up
sin(el )
I up
0
Ii
cos(el ) cos( i )
Ii
0
[Az
i
(i 1) *
i 1 16
o
]
0
22.5
Figure 4 –Sun Relations
and I –V Characteristic
Iup, Ii measured currents on panels (top and ith) and Iup 0, Ii 0 maximum currents on
panels
Since IRECIN is an hexadecahedron and solar panels are mounted on all its 18
faces, the maximum number of illuminated panels is 8 (7 lateral). Both algorithms
exploits sun power measurements from these panels (included top/bottom ones) to
evaluate the azimuth and elevation angles. F1 algorithm, based on the least squares
method and using exact formulas, provides an accurate solution (<2° azimuth),
however it requires a great cpu workload. f1 algorithm, instead, based on linearized
and normalized equations, leads to a quite satisfactory, low cpu resources
consuming result, although less accurate (max error 10°). The following graphs
(Figure 5) show the results provided by the two algorithms, varying the ideal sun
position (both in azimuth and elevation) and considering a noise disturbance.
According to numerical simulation, the proposed sun position algorithm can
determine attitude within a few degrees of accuracy, which is often enough for
many small, very low cost, missions. Moreover, this system has showed to work
FRUUHFWO\ZKHQRQHRIWKHWZR &IDLOVREWDLQLQJDQHUURURIƒXVLQJRQO\RQH &
ameliorating its safety.
23
180
simplified algorithm
complex algorithm
actual
22.5
22
)
s
e
er
g
e
d(
el
g
n
a
n
oi
t
a
v
el
e
140
)
s
e
er
g
e
d(
el
g
n
a
ht
u
m
i
z
a
21.5
21
20.5
20
120
100
80
60
19.5
40
19
20
18.5
actual
complex algorithm
simplified algorithm
160
0
20
40
60
80
100
120
actual azimuth angle (degrees)
140
160
180
0
0
20
40
60
80
100
120
actual azimuth angle (degrees)
140
160
180
Figure 5 – Azimuth and Elevation Estimation using both Methods
References
[1] Graziani F., Ferrante M. The Microsatellite Program at Università di Roma La
Sapienza, Proceedings of the 48th IAA Congress, Turin, Italy, 1997
[2] Santoni F., Bolotti F., Attitude Determination of Small Spinning Spacecraft
Using Three Axis Magnetometer and Solar Panels Data, Proceedings of the
IEEE Aerospace Conference, Big Sky, USA, 2000
[3] Agneni A., Ferrante M., Romoli A., et al., UNISAT Solar Array Integration
and Testing, Proceedings of the 5th International Symposium on Small
Satellite Systems and Services, La Baule, Francia, 2000
[4] Wertz J., Spacecraft Attitude Determination and Control, Boston, D. Reidel
Publishing Company, 1995
[5] Wertz J. and Wiley J., Space Mission Analysis and Design, Torrance,
Microcosm Press and Kluwer Academic Publishers, 1999
[6] F.Graziani F, Ferrante M., et al., Mechanical Tests for Low-Cost
Microsatellite Programs, Proceedings of the 51th IAA Congress, Rio de
Janeiro, Brasil, 2000