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DINO-ADCS-TCM-DESIGN
ADCS Technical Manual
Written By: Stephen Stankevich
Date: August 4, 2003
Revision: A
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Description
First draft presented by ADCS team
Date
8/4/2003
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Abstract
This document discusses the design and function of the attitude determination and control
subsystem, ADCS. The document will discuss the primary requirements of ADCS to keep attitude
knowledge of 2° and attitude control of +/-10° in each axis. The document brings forth the
mechanical design of the system including the torque rod design, the integration of the sensors to the
interface board, and software design. The document also discusses the functioning of the system
from data collection to actuation of the torque rods.
Table of Contents
1
2
Overview ..................................................................................................................... 5
Sensors ........................................................................................................................ 5
2.1
Honeywell HMC2003 Magnetometer................................................................. 5
2.2
Analog Devices ADXRS150 Rate Gyroscope .................................................... 6
3 Actuators ..................................................................................................................... 6
3.1
Magnetic Torque Rod ......................................................................................... 7
4 Attitude Board ............................................................................................................. 7
4.1
Rate Gyro Design ................................................................................................ 7
4.2
Torque Rod Current Control ............................................................................... 8
5 Software ...................................................................................................................... 9
5.1
Attitude Determination ..................................................................................... 10
5.2
Attitude Control ................................................................................................ 10
5.3
Simulations ....................................................................................................... 10
Appendix A – List of Acronyms ....................................................................................... 11
Appendix B – Honeywell HMC2003 Magnetometer ....................................................... 12
Appendix C – Analog Devices ADXRS150 Rate Gyroscope .......................................... 13
1 Overview
Attitude determination and control has the primary objective of maintaining the spacecraft’s
orientation in order to obtain scientific data defined by the mission objectives. For the
Deployable and Intelligent Nanosatellite Operations, DINO, mission the primary mission
objectives are mechanical deployments and stereoscopic imaging of cloud formations. For these
objectives to be met an ADC system has been developed. The system is designed to provide a
nominal orientation for deployment of first a gravity-gradient tethered boom followed by
deployments of FITS and aerofins, which are secondary solar arrays and not attitude control
devices. The system will then provide passive control in the roll and pitch axis through gravitygradient stabilization and yaw control through a feedback control system. While the system will
only be used for yaw control in nominal operations it has been designed to provide periodic
three-axis control for s/c detumbling after mission start and prior to the deployment of the
gravity-gradient tether.
The ADC system has been designed to provide a simple and effective means of controlling
s/c attitude to meet mission objectives. The system will consist of a magnetometer for
measurement of the local magnetic field and three single-axis rate gyroscopes for measurement
of the s/c rotational rates. These devices will be used with an onboard orbit propagator and
magnetic field model yielding the expected local magnetic field. The attitude will be determined
through software algorithms comparing the local sensed magnetic field and expected magnetic
field and directly incorporating the s/c rotation rates. The attitude will then be controlled by
magnetic torque rods exerting a magnetic dipole moment that interacts with the Earth’s magnetic
field to provide a torque on the s/c. The torque rods will be controlled by electronics on the
ADCS interface board which in turn are controlled by the software control algorithm.
2 Sensors
The sensors associated with the attitude determination are the Honeywell HMC2003
Magnetometer and the Analog Devices ADXRS150 Rate Gyroscope.
2.1 Honeywell HMC2003 Magnetometer
The Honeywell magnetometer provides local magnetic field sensing data. The output of the
magnetometer is three 0.5 – 4.5V analog signals for the x,y, and z components of the magnetic
field. These components will be used in attitude determination and also to provide the optimal
control of the magnetic torque rods for interaction with Earth’s magnetic field sensed by the
magnetometer. All other specifications for the Honeywell HMC2003 Magnetometer are shown
in Appendix B.
Due to the magnetometer’s sensitivity to local magnetic fields it shall be isolated from other
electronic equipment. For this reason it will not be located on the attitude board with the rest of
the electronic components of the system. Figure 1 shows the electrical interface with the
magnetometer that shall be built onto a small circuit board.
12V DC
HMC2003
1
2
3
4
Xout
5
Yout
SR-
Zoff+
Xoff-
Zoff-
Ztrim
Xoff+
Xtrim
Yoff-
6
Zout
SR+
Yoff+
Zout
Ytrim
Xout
Yout
Vr ef
Vbias
GND
Vbridge
V+
Vsense
7
9
10
19
100k
18
17
16
15
22u
22u
ZTX605
Reset In
14
8
GNDout
20
10k
13
12
11
10V DC
Figure 1 - Schematic of basic magnetomer circuit.
2.2 Analog Devices ADXRS150 Rate Gyroscope
The Analog Devices rate gyroscope provides rotation rate measurements along the z-axis of
the IC chip. Therefore, three of these single-axis rate gyros are placed in an orthogonal
alignment to provide full three-axis rate information. These sensors shall be placed on the
attitude board discussed in detail later in the manual. The resolution of the gyro has been tested
up to 0.004°/s by the manufacturer. The output of each rate gyro chip is a 0.25V – 4.75V
analog output that is converted to digital data on the interface board. The rate data is directly
used by the attitude determination algorithms as s/c rates. All other specifications for the
Analog Devices ADXRS150 Rate Gyroscope are provided in Appendix C.
3 Actuators
The only actuators associated with the ADC system are three single-axis magnetic torque rods.
The magnetic torque rods provide continuous two-axis control and periodic three-axis control. A
full discussion of the magnetic torque rods is included in this section.
3.1 Magnetic Torque Rod
A separate document titled DINO-ADCS-RPT-TQSIZE outlines the methods used to design
the magnetic torque rods. A final design was selected based on a large trade study, material
availability, and analytical analysis. The magnetic torque rods will be built using a magnesium
zinc core with a magnetic permeability of 800. The density of the material is much less than
that of iron. The cores are wrapped in 24 guage wire. The torquers will simply have an input
of two power lines. Depending on the desired control direction one wire will be grounded
while the other is supplied a current.
The design calls for three torque in an orthogonal reference frame. The torque rods are placed
in the s/c at locations most convenient for the structural layout of the s/c and to isolate them as
much as possible from electrical equipment. They are placed in the s/c such that the ends of
the torque rods are at the sides of the s/c. This will force the largest magnetic fields emanating
from the torque rods outside of the s/c rather than into sensitive electronic equipment. The
actual dimensions of each individual torque rod are yet to be determined.
4 Attitude Board
The attitude board will be the primary center for electronics associated with the ADC system.
The ADC system contatins circuits for the magnetometer, rate gyros, and torque rod control. The
attitude board will contain all the electrical circuits and sensors except that of the magnetometer.
The magnetometer is isolated away from other electronics.
This section of the technical manual discusses the design details of the attitude board.
4.1 Rate Gyro Design
The analog devices rate gyros are supplied in an easy to integrate evaluation board package.
The package has 20 pins connected inline. Figure 2 shows the circuit design for connection of
the rate gyros.
Y-axis
Rate Gyro
X-axis
Rate Gyro
19
CP1
20
CP2
18
CP4
17
CP3
2
TEMP
2.5V
19
CP1
9
20
CP2
7
18
CP4
17
CP3
22n
11
ST 1
22n
3
RATEOUT
TEMP
2.5V
2
9
7
22n
AGND
CMID
12
8
4
5VDC
CMID
PDD
PGND
13
AVCC
AGND
1
AVCC
PDD
22n
10
SUMJ
ST 1
RATEOUT
14
ST 2
ST 2
22n
3
CP5
11
SUMJ
10
PGND
22n
47n
14
CP5
47n
13
12
8
4
100n
100n
1
5VDC
100n
100n
100n
100n
Z-axis
Rate Gyro
Xout
Yout
22n
10
11
ST 2
ST 1
Zout
22n
3
SUMJ
14
CP5
47n
GNDout
19
CP1
RATEOUT
2
20
CP2
TEMP
9
18
CP4
17
CP3
2.5V
7
22n
PDD
PGND
AGND
CMID
AVCC
13
12
8
4
1
5VDC
100n
100n
100n
Figure 2 - Schematic for rate gyro electrical interface.
4.2 Torque Rod Current Control
Several requirements are placed upon the attitude electronics by the torque rods.
1. The input current must be supplied in proportional control between 0-500mA.
2. The input current must be capable of being supplied in either direction through the
torque rod.
3. The amount and direction of current must be capable of being controlled by the flight
computer.
These requirements led to a design shown in Figure 3. The input to the control circuit is an
analog voltage and a TTL signal. The analog voltage shall be supplied through a D/A
converter. The TTL signal shall be supplied directly from the flight computer.
Torque Rod Control Preliminary Schematic
Direction-TTL
1
1
2
Vin-Analog
+
2
Rbias
Out+
In+
OUT
-
20
OPAMP
1
1
Torque Rod
2
Out-
In-
2
Figure 3-Schematic of torque rod control circuit.
The methodology of this circuit is fairly simple. The amount of current to the torque rod may
be controlled by varying the input current (V=IR). Since there is a set resistance in the Rbias
and the torque rod, I=V/R. Thus, a digital signal from the flight computer to a D/A can supply
an output voltage. The output voltage is amplified through the op-amp and thereby supplies
the desired current to the torque rods. The direction of current is controlled through the
switching MOSFET’s. By sending a TTL high/low one of two things occurs. Either both
switches labeled 1 are closed or both switches labeled 2 are closed. When switch 1 is closed
the current flows in a positive direction and when switch 2 is closed current flows in the
negative direction. The integration of the D/A converter with this circuit has not been
incorporated as of yet. It is not yet determined whether this task will be completed by the
ADCS team or C&DH team.
Which A/D converters we are using and why
The torque rod control circuits
Schematics of the board
5 Software
While several sensors and torque rods provide the data collection and control actuation the real
attitude determination and control is done through software running on the flight computer. This
section discusses the software architecture, design, development, testing and simulations of the
system.
5.1 Attitude Determination
The attitude determination software takes data from the sensors and provides an attitude error
from the nominal s/c alignment. The input to the software algorithm is the local magnetic field
vector and s/c rotational rates provided as digital data from the sensors and interface board.
The software also requires orbit data be uploaded periodically to maintain an accurate orbit
propagation for determining the expected magnetic field vector from the magnetic field model.
Block Diagrams and Explanations
5.2 Attitude Control
The attitude control software takes the attitude error and determines the desired magnetic
dipole moment of the s/c to torque the s/c towards the desired position. The software then
determines the specified current through each torque rod to obtain the desired s/c magnetic
dipole moment to interact with the Earth’s magnetic field thereby creating the desired torque.
The specified current is then used to control the interface board to provide the specified
current to the torque rods.
Block Diagrams and Explanations
5.3 Simulations
Appendix A – List of Acronyms
Appendix B – Honeywell HMC2003 Magnetometer
Attach printout of Magnetometer Specification Datasheet
Appendix C – Analog Devices ADXRS150 Rate Gyroscope
Attach printout of Rate Gyro Specification Datasheet