Attitude Determination and Control System

1. Attitude Determinationand Control System Peer Review December 2003

2. Introduction • Main purpose of ADCS is controlling the orientation of the s/c for mission and science objectives. • Spacecraft face disturbance torques in space causing the s/c to spin. ADCS senses these disturbances and corrects the error in the attitude. • Includes the necessary sensors for determining the attitude of the s/c. • Includes necessary actuators for controlling the s/c. • Includes software for attitude determination from the sensors and a control algorithm for the actuators.

3. Requirements • Maintain nadir attitude for communication and imaging objectives. • Perform onboard attitude determination and control. • Maintain roll and pitch control using a gravity gradient boom. • Maintain attitude knowledge to 2° in every axis. • Maintain attitude control to +/- 10 ° in each axis. Mass 2.25 kg Power 4 W operating • These requirements constitute a fairly coarse ADC system thus the design driving requirements are the mass and power limitations.

4. Imposed System Requirements • Placement of torque rods • Rods must lie in right hand orthogonal system • Preferably along the s/c body axis. • The rods shall be placed such that the ends are at the edge of the s/c structure thereby eliminating strong fields effecting equipment. • Data conversion will be performed by C&DH. • Bx,By,Bz (.5V – 4.5V analog) input • phidot, thetadot, psidot (.25V – 4.75V) input • 0-5V analog output x 3 • TTL high/low analog output x 3 • Need a 5V and 12V line from PWR. • S/C c.g. shall be located along the z-axis (boom) axis. • Magnetometer shall be located as individual component outside of attitude interface board box. • Remaining electronics will be placed on the attitude board.

5. Imposed System Requirements

6. ADCS Electronics Flight Computer Magnetometer Honeywell HMC2003 12V@20mA w/ 40μG resolution Sensor Analog Inputs Sensor ADC (3) 0-5V analog Rate Gyros 3 single-axis MEMS gyros +5V input @ 6mA Controller Likely a P-D or LQR Output cmds to turn rods on/off and current direction Possibly use multiple voltage levels requiring a D/A Converter Att. Det. IGRF Magnetic Model Orbit Propagator Compare Expected And actual B Fields Damp rates (3) 0-5V analog Current Control Circuit 0-300mA Torque Rods (3) 3/4’’ x 10’’ @ max 150mA nominal POWER 12V and 5V supply to board

7. Magnetometer • Honeywell HMC2003 • 20mA @ 12V • mass < 100g • -40 to 85 C operating temp. • 40 μGauss Resolution • $200 • 3 Analog Outputs (Bx, By, Bz) • Set/Reset Capabilities

8. Rate Gyros Analog Devices ADXRS150EB • Single axis rate gyros provide the rotational rate of the s/c about the output axis • Microchip operating at 5V and 6mA. • Single analog output • -40 to 85°C operating temp • $50 each

9. Magnetic Torque Rods • Electrical current is passed through wire wound around a ferrous material creating a magnetic dipole moment. • Torquer dipole moment interacts with Earth’s magnetic field to create the desired torques. T = MxB • 3 orthogonal torque rods can produce torques perpendicular to magnetic field vector • Unbiased momentum

10. Design • Material • Ferromagnetic material • Magnesium Zinc Alloy • Approximate density: 5000 kg/m3 • Magnetic permeability • μ = 800 W/(A m) • Wire • 24 Gauge • Copper • Output • 3 Am2 @ 300mA input • Counteract max drag disturbance torque

11. Internal Placement • Magnetic torque rods create interference • Magnetic fields emanate from ends only • Rods sized to span entire length of satellite Possible configuration

12. Sizing • Design to obtain 5 Am2 @ 500mA • Counteract max drag torques • Provide detumbling capacity • Tradeoffs • Weight • Power • Output Moment

13. Sizing - Mass

14. Sizing - Power

15. Electronics Design • Magnetometer and rate gyros require basic circuit design • Torque Rod control requires more complicated circuit. • Control current input • Control current direction through torquer

16. Sensor Circuits Magnetometer Circuit Rate Gyro Circuit

17. Torquer Control Circuit

18. Software Design • Control system design • Hardware control functions • Functional test software

19. Software Design Overview Obtain expected B field vector from model in orbit frame. Orbit data update Propagate orbit Compare the s/c frame to the orbit frame Obtain B field and rates in s/c frame Euler Angles and Rates • The Euler angles and rates will provide and attitude error to the control algorithm.

20. Software Design Overview • Onboard magnetometer data and rotational rate information. • Software-based orbit propagation and magnetic field model. • Partial error analyses has been completed. • Sensitivity of the magnetometer provides negligible attitude knowledge errors on the order of 0.01°. • Tracking data must be uploaded periodically to correct propagation errors. • Bdot data derivations could also be used for comparison between s/c and orbital attitude frames.

21. Control Design • Based on research paper by Cornell University faculty member. • Simple control law/code • Analysis not yet performed. • Primary control design work to be completed during spring semester. • Control design, analysis, simulation, flight code development.

22. Hardware Control Functions • Control D/A converter for torque rod current control circuit input. • Control of TTL line for current flow direction through torque rod.

23. Prototype Report • The hardware has been purchased and prototyped with a circuit. • Rate gyro • Prototyped and functioning • Still needs to be tested to be sure output is correct • Magnetometer • Prototyped and functioning • Still needs to be tested to be sure output is correct • Torque Rods • Prototyped in house • Functionality not yet tested

24. Commands • Torquer On/Off • Inputs • X,Y, or Z rod • amount of current • Direction of current • Outputs • digital signal to D/A to vary output voltage of D/A. • TTL high/low for control of direction of current.

25. Commands • Control on/off • Either run the control system or do not. • Attitude Board on/off • This will provide/cut power to the attitude board • Attitude determination and control will be completely off • Read data • Need to read data from sensors and store to variables.

26. Magnetometer Successfully tested to be sure it functions Verify the output is correct with rated magnets Rate Gyros Successfully tested to be sure it functions Verify the output is correct by spinning each rate gyro up on spinning apparatus Compare angular rate output value to known angular rate value Test Plans-Hardware

27. Torque Rods Use magnetometer to test the amount of torque produced Graph relationship between current input into current driver circuit and amount of torque produced Electric Circuits Torque rod current driver Magnetometer reset circuit-functions with hardware Rate Gyro circuit-functions with hardware Test Plans-Hardware

28. Test Plans - Software • Verify orbit propagation vs. STK HPOP • Simulate feedback control loop • Provide input torque to simulator • Model will predict s/c reaction to torque • Control loop will provide response • Model can predict time domain responses to input torques based on control design.