Attitude Determination and Control Preliminary Design Review Stephen

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

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. 2/25/2021 DINO PDR 2

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. 2/25/2021 DINO PDR 3

Imposed System Requirements • Placement of torque rods – Rods must lie in right hand orthogonal system – Preferably along the s/c body axis. • • Need an I 2 C line from C&DH. May need 12 -16 I/O lines from C&DH Need a 5 V and 12 V line from PWR. S/C c. g. shall be located along the z-axis (tether) axis. 2/25/2021 DINO PDR 4

Hardware Flow Diagram 12 V and 5 V supply to board Magnetometer ADCS Electronics (3) 0 -5 V analog X Analog Inputs A/D Conversions Torquer Analog Outputs D/A Conversions Honeywell HMC 2003 12 V@20 m. A w/ 40μG resolution Rate Gyro(s) (3) 0 -5 V analog 3 single-axis gyros +5 V input @ 6 m. A I 2 C or I/O lines Sun Sensors Flight Computer Possible donation by Ithaco 12 single axis Torque Rods 0 -300 m. A (3) 3/4’’ x 10’’ @ max 150 m. A nominal Controller Att. Det. Likely a P-D or LQR IGRF Magnetic Model Orbit Propagator Output cmds to turn rods on/off and current direction Power I 2 C data Standard Commands Possibly use multiple voltage levels requiring a D/A Converter Compare Expected And actual B Fields Damp rates ADCS software running at 1 – 10 Hz 2/25/2021 DINO PDR 5

Attitude Determination Overview Orbit data update Propagate orbit Obtain expected B field vector from model in orbit frame. Euler Angles and Rates Compare the s/c frame to the orbit frame Obtain B field and rates in s/c frame • The Euler angles and rates will provide and attitude error to the control algorithm. 2/25/2021 DINO PDR 6

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. 2/25/2021 DINO PDR 7

Magnetometer Honeywell HMC 2003 • 20 m. A @ 12 V • mass < 100 g • -40 to 85 C operating temp. • 40 μGauss Resolution • $200 • 3 Analog Outputs (Bx, By, Bz) • Set/Reset Capabilities 2/25/2021 DINO PDR 8

Rate Gyros Analog Devices ADXRS 150 – Single axis rate gyros provide the rotational rate of the s/c about the output axis – Microchip operating at 5 V and 6 m. A. – Single analog output – -40 to 85°C operating temp – $33 each 2/25/2021 DINO PDR 9

Attitude Control • The s/c magnetic dipole moment interacts with the Earth’s magnetic field to provide a torque on the s/c. (T = Mx. B) • 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. 2/25/2021 DINO PDR 10

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 m 2 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. 2/25/2021 DINO PDR 11

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) 2/25/2021 DINO PDR 12

Magnetic Torque Rods Minimum Requirements – Rod of length 10” and diameter 3/8” – Mass = 0. 4 kg each, total of 1. 2 kg – Power use of 200 m. W at max input of 300 m. A • After detumbling normal use should not exceed 150 m. A – – – 2/25/2021 Max output of 3. 0 Am 2 24 Gauge Wire Common ferrite material 33 with μ = 800. Complete manufacture under $100 each The bigger the better DINO PDR 13

ADC Electronics Board Mag Sensor Rate Gyros Resistor Bank Multiplexer Analog A/D Converter To Flight Computer I 2 C A/D Converter data 5 V from Power 2/25/2021 GND DINO PDR I/O from FC To Torque Rods 1, 2 and 3 12 V from Power 14

ADCS Electronics • The ADC system will contain an electronics board which houses the sensors and data converters. • Power will supply both a 5 V and 12 V 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. 2/25/2021 DINO PDR 15

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 m. A) 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 2/25/2021 DINO PDR 16

Parts and Materials Summary Part Mass (kg) Power (W) Cost Honeywell HMC 2003 Magnetometer 0. 1 0. 24 $200 Torque Rods (3) 1. 2 < 0. 6 $300 . 0005 0. 03 $100 A/D Converters (2 – 6) n/a $200 Totals 1. 56 0. 4 $800 Analog Devices ADXRS 150 Rate Gyro (3) 2/25/2021 DINO PDR 17

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. 2/25/2021 DINO PDR 18

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 2/25/2021 DINO PDR 19