compass-1 picosatellite
COMPASS-1 PICOSATELLITE:
MAGNETIC COILS FOR ATTITUDE CONTROL
Ali Aydinlioglu, Marco Hammer
Astronautical Department
University of Applied Sciences Aachen
Hohenstaufenallee 6, 52064 Aachen, Germany
Phone: +49 241 6009-1542, Fax: +49 241 6009-1542
[email protected], [email protected], [email protected]
ABSTRACT
The COMPASS-1 Cubesat is an active controlled picosatellite by using magnetic coils as means of attitude
control. The Attitude Control System (ADCS) is featured with a three-axis stabilizing capability. The ADCS was
developed to stabilize the spacecraft against disturbances. Magnetic coils are mounted to generate control torque
in the roll and yaw axes. The attitude sensors use magnetometer for measurements of the local magnetic field
vector, GPS System for determine the position and sun sensors. The paper describes the required hardware
components and the design and development of the electromagnetic torquers.
INTRODUCTION
COMPASS-1 is the name of the first picosatellite
being developed at the University of Applied
Sciences Aachen, Germany [1]. Since the projects’
initiation in September 2003 it is being managed
and carried out by students of different engineering
departments, with a majority being undergraduate
students
from
the
Astronautical
Department. Currently the team counts
fourteen students with an increasing
number of new participants. The project
focuses on a number of goals. Mainly the
students will gain essential practical
experience in realizing a research and
development project from start to end.
Moreover, an adequate infrastructure shall
be created that enables more space engineering
activities to take place at our university in the
future. And definitively not least, a fully functional
picosatellite is going to be built and finally
launched into orbit!
OVERVIEW
ADCS System Overview
The Attitude Determination and Control System
(ADCS) is to stabilize the spacecraft against
attitude disturbing influences resulting from the
environment in the earth orbit in order to orients it
in the desired fixed nadir pointing.
For the design layout of the ADCS,
firstly a mathematical concept had to be
established. Then, the hardware is being
built around the mathematical equations.
In no other subsystem design it becomes
more obvious than in dynamics related
spacecraft systems like the ADCS: the
hardware is the carrier of the software, which is the
theoretical concepts translated into a numerical
solution.
The satellite is being built according to the CubeSat
specification documents [2] published by Stanford
and Calpoly University, which define a cubical
structure with 10cm edges and a mass of not more
than 1kg. Powered by solar cells, such a satellite
will have an average of 1.5W for operation.
Attempting to develop a spacecraft within the
stringent constraints mentioned above becomes
reasonable when considering the satellite being
stored inside a container (P-POD) for simultaneous
launch with other CubeSats, which in turn helps
decreasing launch costs significantly.
The launch date of COMPASS-1 is not yet
determined. Nevertheless it is planned to conclude
the development and have the spacecraft ready for
launch acceptance testing by end 2005.
Fig. 1: ADCS Hardware Interfaces
Figure 1 summarizes the interfaces between the
ADCS Printed Circuit Board (PCB) and the
adjacent components and subsystems and aids in
obtaining an overall perspective on the ADCS
hardware configuration.
The ADCS subsystem board is mounted
perpendicular at the Command and Data Handling
System (CDHS) board as indicated in figure 2. It is
exchanges information with the CDHS (shaded
green) and receives the commands to switch modes.
The communication is done via I²C system bus.
There are two voltage levels available for the
ADCS via the connectors; those are 3.3V and the
unregulated battery voltage (around 3.7V).
The components that define the interface between
the Coils and the ADCS PCB are the coils driver,
the H-Bridge, the low pass and the current sensor.
GPS System
For the computation of the reference vectors for
attitude determination, the information of the
current position of the spacecraft is important. In
order to enable the autonomy of the system
operation, a GPS will be incorporated. GPS system
is made up by two prime components:
Fig. 4: The Phoenix GPS Receiver and the San Jose
F-19 active patch antenna
The antenna receives the GPS signal and transmits
it to the GPS receiver.
Fig. 2: The ADCS placed on the CDHS board
ADCS Hardware Components
The ADCS Hardware is separated into two units:
- one unit determines the spacecrafts attitude
and position and
- the other unit stabilizes the spacecraft.
The determination unit comprises the SunSensors,
the magnetometers and the GPS system.
The GPS receiver is the device that receives a
signal from the antenna, conditions and processes
the signal into an location information, and sends
the data, with a time stamp, to the ADCS unit. The
GPS board contains two duplex serial ports that
operate at high voltage levels.
Magnetometer
The measurement of the local magnetic field vector
for
attitude
determination
requires
the
implementation of magnetometers on the
subsystem. The selected hardware is the HMC6352
digital compass developed and distributed by
Honeywell.
The stabilizing unit consists of magnetic coils (also
referred to as magnetic torquer) as the only devices
for actuation on board the satellite.
Magnetic Torquer
The coils are mounted close to the panels of the
satellite. In dependence of the current, sent through
the turns, a magnetic field is established. An
interaction, between these produced magnetic field
vectors and the local geomagnetic field vector, is
producing a control torque. This control torque will
rotate the spacecraft to the desired position.
Fig. 3: One magnetic coil and its integration onto a
panel
Fig. 5: The HMC6352 digital magnetometer
The magnetometer communicates via two-wire I²C
bus system as a slave device.
Sun Sensor
The objective of the sun sensors is to measure the
relative position of the sun in order to aid the
attitude determination procedure. The sun sensors
used on COMPASS-1 have been developed by the
Micro Electronics Institute (MIC) of the Denmark
Technical University (DTU) in MOEMS (MicroOpto-Electro-Mechanical System) technology.
They are extremely lightweight, small and power
saving [3].
Minimum
temperature
Nominal
temperature
Maximum
temperature
T min
-20
°C
T Nominal
20
°C
T max
40
°C
Fig. 6: MIC sun sensor chip mounted on the PCB
HARDWARE DEVELOPMENT
The interface between the sun sensor and the ADCS
PBC are the analog-to-digital converter, the
operation amplifier and the digital thermometer.
Except of the SunSensors and the coils, the ADCS
is purely built up with commercial off-the-shelf
(COTS) products.
The coils will require a significant fraction of the
critical budgets (mass and power). Hence, sufficient
analysis and optimization was spent on the coil
design in the development phase. During this
current definition phase an efficient tool has been
developed which enables a fast design output for a
given set of input parameters.
COIL DESIGN
Designing a coil for a satellite differs from
commercial coils, because the coil operates in the
vacuum of the space. It is therefore necessary to
pay attention to a range of constraints and
requirements explained in the following.
Coil Design
Due to the fact that the satellite should be a three
axis stabilized satellite it will require a minimum of
three torquers to dump momentum in every
direction. The magnetic moment is given from the
simulation with MATLAB. The torque requirement
can be calculated from the definition as 1.0 E-06
Nm.
The power budget for the coils, are limited with
750mW. It is reasonable to design a torquers with a
third up to a half of the given power consumption
or less and still be able to produce the required
moment. The coils are not used constantly; due to
the limited available power.
The maximum weight for the coils was limited to
around 20g for each coil.
The bigger the included surface of the coils is the
better is their efficiency. We intend therefore to
make square coils that follow the edges of the side
plates and which will be mounted on them.
Design Parameter
Parameter
Maximum width
Maximum height
Maximum cross
sectional breadth
Maximum cross
sectional height
Face of coil
Cross sectional
area
average Amount
Total Mass limit
Mass of coils
each axes
Symbol
b
h
Value
74
83
Unit
mm
mm
d
2,1
mm
sh
5
mm
A
6142
mm^2
Acmax
10,5
mm^2
C
Mges
296
60
mm
g
Mc
20
g
Coil Calculation
Considering the coil requirements an Excel
program was developed for the calculation. The
necessary parameters, the wire diameter and the
number of turns are the outputs of the Excel
program.
Parameter
Number of Turns
Bare wire diameter
Mass of one coil
Current through coil
Magnetic dipole
moment
Needed cross section
area
Power consumption
Coil resistance @ -20°C
Coil resistance @ 0°C
Coil resistance @ 20°C
Coil resistance @ 40°C
Symbol
n
dw
Mc
I (20°C)
Md
Value
396
0.15
19,008
38,703
Unit
mm
g
mA
Ac
9,73E-02
9,508
A*m^2
mm^2
P
R-20
R0
R20
R40
226,26
106,07
115,05
124.02
133,00
mW
Ω
Ω
Ω
Ω
It should be noted that the design is not only
dependent of the required mechanical torque but
also of commercially available wire properties, i.e.
diameters and material, as well as the selected
number of turns. The production of coils has shown
to be quite expensive, and for the convenience of
producing multiple prototypes of coils, a coil
winding machine is being designed and produced.
For the design of the coil mould and winding
machine, CATIA V5 R13 was used. The software
allows a full development of the winding tool
including the drafting features.
Fig. 7: The magnetic coil mould
Coil Winding Tool Design
TESTING
Testing methods
The tests are the most important aspect at the
design of magnetic torquer coils. The tests will
approve the design calculations. The goal of the test
will have a term which describes the real
dependency between the current, sent through the
coils, and the resulting magnetic dipole moment.
Test Hardware
Fig. 8: Coil winding machine
The majority of the coil tool design is developed on
commercial off-the-shelf (COTS) products. The
winding tool is guaranties flexibility in the
production of the coils itself and allows producing
coils with different winding methods. This gives the
liberty to produce coils with any input tension. The
production of the hardware parts are produced in
the workshop of our home university. The drive
mechanism itself and the track sensible drive
mechanism are solved with stepping motors. A
force measuring unit was installed, which helps to
secure the coils against a wire break.
Connection
The connection of the every wire string into an
none detachably conductor is achieved with the
method of “current bonding” [4].
To measure the magnetic dipole moment different
test hardware are subsist. The first variety consists
of using 3 axis magnetometer witch are already a
hardware component of the mostly ADCS
subsystem for pico - satellites.
The second variety is to measure with different
available magnetometer in the industry, witch is not
so easy to find.
SUMMARY
COMPASS ONE uses only magnetic torquer coils
for orient the spacecraft along nadir with an
accuracy of 8°.
The paper shows the design and development
aspects of the Coils based on the calculation.
For the generation of a magnetic moment vector m,
simple vector arithmetic’s have been used, i.e. each
coil produces one component of the vector m.
possibly the case is more complex.
So far the coils are produced perform the
requirements and builds the basis for the tests.
REFERENCES
[1]
The Compass-1 Picosatellite Project at the FH
Aachen. www.raumfahrt.fh-aachen.de
[2]
Calpoly and Stanford University. (2003). CUBESAT
Design Specifications Document, Revision VIII.
http://cubesat.calpoly.edu/
[3]
Pederson, M. et al., Linear Two-Axis MOEMS Sun
Sensor and the need for MEMS in space,
International Astronautical Congress, Bremen,
Germany, 2003
[4]
Bonding methods ( Verbackungsmethoden)
www.electrisola.com
Fig. 9: Coil prototype
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