Attitude Determination and Control Preliminary Design Review Stephen Stankevich Jeff Parker July 28, 2003

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. January 17, 2019 DINO PDR

Requirements Maintain nadir attitude for communication and imaging objectives. Perform onboard attitude determination and control. Maintain roll and pitch control with gravity gradient tether. 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. January 17, 2019 DINO PDR

Imposed System Requirements Placement of torque rods Rods must lie in right hand orthogonal system Preferably along the s/c body axis. Need an I2C line from C&DH. May need 12-16 I/O lines from C&DH Need a 5V and 12V line from PWR. S/C c.g. shall be located along the z-axis (tether) axis. January 17, 2019 DINO PDR

Hardware Flow Diagram ADCS Electronics Magnetometer Rate Gyro(s) 12V and 5V supply to board ADCS Electronics Magnetometer Honeywell HMC2003 12V@20mA w/ 40μG resolution (3) 0-5V analog I2C data X Analog Inputs A/D Conversions Torquer Analog Outputs D/A Conversions Rate Gyro(s) 3 single-axis gyros +5V input @ 6mA (3) 0-5V analog I2C or I/O lines Sun Sensors Possible donation by Ithaco 12 single axis Flight Computer 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 Torque Rods (3) 3/4’’ x 10’’ @ max 150mA nominal 0-300mA Power Standard Commands ADCS software running at 1 – 10 Hz January 17, 2019 DINO PDR

Attitude Determination 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. January 17, 2019 DINO PDR

Attitude Knowledge 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. Sun sensors may be used if suitable vendor is found. These are not needed for accurate attitude knowledge within requirements. Bdot data derivations could also be used for comparison between s/c and orbital attitude frames. January 17, 2019 DINO PDR

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 January 17, 2019 DINO PDR

Rate Gyros Analog Devices ADXRS150 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 $33 each January 17, 2019 DINO PDR

Attitude Control The s/c magnetic dipole moment interacts with the Earth’s magnetic field to provide a torque on the s/c. (T = MxB) The s/c magnetic dipole moment can be controlled by passing specified currents through magnetic torque rods. (M=INA) Torque rods can provide three-axis stabilization for detumbling at less weight, power, and cost than reaction wheels. Control algorithm will determine how much current to provide to each torque rod to produce the desired dipole moment for the necessary torque. January 17, 2019 DINO PDR

Gravity Gradient Gravity gradient provides a restoring torque when a disturbance torque causes a movement from local veritcal. The torque produced is dependent upon the s/c moments of inertia. This shall be a design concern for placement of s/c components. Maximum disturbance torque is Aerodynamic Drag Assume ½ m2 cross-sectional area TAD = 5.56 x 10-5 Nm Solar Radiation Pressure 4.37 x 10-6 Nm Magnetic disturbance torques are not considered as disturbances because active magnetic control will be utilized. January 17, 2019 DINO PDR

Magnetic Torque Rods Ferrite Material wound with wire Produces a dipole moment that interacts with Earth’s magnetic field. Will be designed in-house (unless donated) January 17, 2019 DINO PDR

Magnetic Torque Rods Minimum Requirements Rod of length 10” and diameter 3/8” Mass = 0.4kg each, total of 1.2kg Power use of 200mW at max input of 300mA After detumbling normal use should not exceed 150mA Max output of 3.0 Am2 24 Gauge Wire Common ferrite material 33 with μ = 800. Complete manufacture under $100 each The bigger the better January 17, 2019 DINO PDR

ADC Electronics Board Mag Sensor Rate Gyros Resistor Bank Multiplexer To Torque Rods 1,2 and 3 Mag Sensor Rate Gyros Resistor Bank Multiplexer Analog Resistor Bank Multiplexer I/O from FC A/D Converter A/D Converter Resistor Bank Multiplexer To Flight Computer I2C data 5V from Power GND 12V from Power January 17, 2019 DINO PDR

ADCS Electronics The ADC system will contain an electronics board which houses the sensors and data converters. Power will supply both a 5V and 12V line to the board. The magnetometer and rate gyros are IC (integrated circuit) chips. A/D Converters will be needed for the magnetometer and rate gyros. D/A converter may be necessary for torque rods. Other options include multiplexing to control switches relating to different resistors supplying different amounts of current. The electronics board will be designed and prototyped by CDR. January 17, 2019 DINO PDR

Subsystem Commands Command Description Hardware Power on/off Turns power to the adcs electronics board on/off Electronics board (mag, RGs, A/Ds) Rod current (x mA) Turns power to torque rods with x amount of current (control alg.) Electronics board, torque rods. Deploy_boom Allow deployment of boom in correct orientation None January 17, 2019 DINO PDR

Parts and Materials Summary Mass (kg) Power (W) Cost Honeywell HMC2003 Magnetometer 0.1 0.24 $200 Torque Rods (3) 1.2 < 0.6 $300 Analog Devices ADXRS150 Rate Gyro (3) .0005 0.03 $100 A/D Converters (2 – 6) n/a Totals 1.56 0.4 $800 January 17, 2019 DINO PDR

Test Plan Control algorithm may be tested with an external power supply to electronics board, software running on a linux PC, and mock sensor inputs. Attitude determination algorithm may also be tested with linux PC and mock sensor inputs. Actual output from actuators can be measured and compared to simulations using the same mock sensor inputs. Complete testing after s/c integration is more complicated because the torque rods will not rotate the s/c in a gravity environment. January 17, 2019 DINO PDR

Design Issues and Concerns Control of torquers may require rather fine current control. Current to torque rods must be able to go both directions. Detumbling the s/c Torquers do not produce high output for fast rotational rates. Higher input currents may be necessary. S/C moments of inertia are key to controller development. One rate gyro will need to be placed orthogonal to the others. It cannot be placed on the same electronics board. How do we take magnetic field measurements with torquers on? Turn off…take measurements…turn back on Subtract out torquer field in software January 17, 2019 DINO PDR