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DINO-ADCS-TCM-DESIGN ADCS Technical Manual Written By: Stephen Stankevich Date: August 4, 2003 Revision: A Revision Log Revision A Description First draft presented by ADCS team Date 8/4/2003 Document Verification and Checkout Author/Title Date QA/Title Date 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