Peer Review ADCS

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Attitude Determination and Control System: 

Attitude Determination and Control System Peer Review December 2003

Introduction: 

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.

Requirements: 

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.

Imposed System Requirements: 

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.

Imposed System Requirements: 

Imposed System Requirements

Slide6: 

Torque Rods (3) 3/4’’ x 10’’ @ max 150mA nominal Flight Computer Att. Det. IGRF Magnetic Model Orbit Propagator Compare Expected And actual B Fields Damp rates 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 Sensor Analog Inputs Sensor ADC ADCS Electronics POWER 12V and 5V supply to board Magnetometer Honeywell HMC2003 12V@20mA w/ 40μG resolution Rate Gyros 3 single-axis MEMS gyros +5V input @ 6mA Current Control Circuit (3) 0-5V analog (3) 0-5V analog 0-300mA

Magnetometer: 

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

Rate Gyros: 

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

Magnetic Torque Rods: 

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

Design: 

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

Internal Placement: 

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

Sizing: 

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

Sizing - Mass: 

Sizing - Mass

Sizing - Power: 

Sizing - Power

Electronics Design: 

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

Sensor Circuits: 

Sensor Circuits Rate Gyro Circuit Magnetometer Circuit

Torquer Control Circuit: 

Torquer Control Circuit

Software Design: 

Software Design Control system design Hardware control functions Functional test software

Software Design Overview: 

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

Software Design Overview: 

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.

Control Design: 

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.

Hardware Control Functions: 

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.

Prototype Report: 

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

Commands: 

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.

Commands: 

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.

Test Plans-Hardware: 

Test Plans-Hardware 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: 

Test Plans-Hardware 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 - Software: 

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.