Design and Simulation of the Attitude Determination and Control System for LightSail-1 Matt Nehrenz

LightSail-1 Overview LightSail-1 is made up of a 3U CubeSat 10 x 10 x 34 cm, <5kg LightSail-1 will deploy a 32m^2 aluminized Mylar sail via 4m booms LightSail-1 will demonstrate controlled flight by Light Avionics Module Sail Storage Section Boom Deployer Payload Section LightSail-1 with deployed sail

ADCS Introduction Sensors: Magnetometers Sun Sensors Gyroscopes Actuators: Torque Rods Momentum Wheel Key Requirements: Point at sun with 10º accuracy Accomplish orbit raising Sun Sensors Magnetometers Momentum Wheel Gyros Torque Rods

ADCS Sensors Magnetometers Sun Sensors ELMOS Semiconductor E910.86 2 axis 150º field of view 2.7º resolution +/- 5º accuracy 4 x 4 mm Gyroscopes Analog Devices ADIS16135 1 axis Temp calibrated 0.0017 º/sec bias stability 0.012 º/sec resolution 44 x 36 x 14 mm Honeywell HMC1051 1 axis -6 to +6 Gauss range 120 μGauss resolution 2 x 2 x 7 mm

ADCS Actuators Torque Rods Momentum Wheel Sinclair microsatellite reaction wheel Nominal momentum: 0.050 Nms Dimensions: 75 mm x 65 mm x 38 mm Mass: 225 g Stras Space Torque Rod 1 Am2 magnetic dipole Dimensions: 90 mm long x 22 mm diameter Mass: 150 g Power consumption: 500 mW

ADCS Torque Comparison Want an orbit altitude where aerodynamic force is ~10x less than the solar force With 822 km orbit, worst case aero torque is half of solar torque Graph assumes 3 cm CP/CG offset

ADCS Modes B-dot detumble Momentum wheel turn-on Sun-pointing Orbit raising (thrust on/thrust off) Assumptions for MatLab simulations: Rigid body Body axes are the principal axes IGRF-10 magnetic field model Disturbance torques: gravity gradient, solar, and aerodynamic torque Sensor noise included, but sensor misalignment excluded Momentum wheel spins at constant rate

ADCS Modes: B-dot Detumble Only sensors used are magnetometers B-dot algorithm applies a magnetic dipole that minimizes the change in the magnetic field System Cycle Time: 10 seconds

ADCS Modes: Wheel Spin-Up Sinclair microsatellite reaction wheel Nominal momentum: 0.050 Nms When spun up, an angular velocity of 2.5 deg/sec will be imparted on the spacecraft which will require a second detumble mode. Wheel is needed for orbit raising mode

ADCS Modes: Sun-Pointing +Z axis points towards sun Sensors used: Magnetometers Sun sensors Rate gyroscopes Full hemispherical coverage without any reflection off the solar panels or solar sail +Z

ADCS Modes: Sun-Pointing Calculate a requested torque from Control Law θ - a vector of angle measures that is a function of the sun vector in body coordinates and the desired sun-pointing axis (+Z) Solve for magnetic dipole needed to achieve requested torque Disturbance Torques Control Torque Total Torque Control Law Spacecraft Dynamics Feedback

ADCS Modes: Sun-Pointing Parameter Value Spacecraft mass 5 kg Deployed inertias (kg·m2) Ixx = 1.4 Iyy = 1.4 Izz = 2.8 Orbit 822 km sun-sync System cycle 10 sec Wheel momentum 0.050 N·m·sec Max magnetic dipole 1 A·m2 Max power consumption < 3 watts Sun-pointing error over ~4 orbits (shaded areas are eclipses)

ADCS Modes: Orbit Raising Orbit Direction Thrust Off Solar Pressure 90 degree pitch maneuver Thrust On Orbit Direction

ADCS Modes: Orbit Raising Nominal wheel speed is changed 1000 RPM (20%) 90º maneuver takes 4 minutes Wheel is the only active actuator during maneuver Slew rate and time verified by hand calculations

ADCS Hardware Failure Scenarios System can be commanded into various settings due to: Critical hardware failure Convergence issues System will autonomously try to recognize failures and resolve the issue automatically Ground will have the ability to: Restart nominal control sequence Control wheel speed Change control gains for detumble mode Change control gains for sun-pointing mode Decide which torquers to use in case of torquer failure Torquer Failure Torquer failure is evident by a short or by zero current flow Stop torquing Report problem and wait for ground command Simulation shows B-dot is feasible with a single torquer out

ADCS Hardware Failure Scenarios Sun sensor failure When sun is in view of multiple sensors, system will take an average and compare readings If comparison of multiple readings show that a sensor is bad Log data point Don’t use that sensor for that point and move on If there are 3 consecutive bad readings from single sensor Discontinue use of bad sensor Report sensor failure to ground Magnetometer failure If magnetometer reading magnitude is out of bounds or in disagreement with running average Skip that iteration of the control algorithm and try again

ADCS Hardware Failure Scenarios Gyro failure If gyro reading is inconsistent with running average Log data point Skip that iteration of the control algorithm and try again If there are 3 consecutive bad readings from single gyro Exit sun-pointing mode Run B-dot Report problem and wait for ground command Momentum wheel failure If wheel speed feedback is inconsistent with commanded speed, or if adverse health status Turn off wheel Report Problem and wait for ground command If wheel is off, gains will be changed so that B-dot and sun-pointing will be possible

ADCS – Test/Analysis Status Test gyro capability Take stationary gyro readings over long time periods to characterize bias stability Spin table tests Processor in the Loop Tests Interface MatLab simulation with actual flight processor and flight code System dynamics, environment, attitude knowledge, and control actuation simulated in MatLab Processor reads in simulated sun sensor, gyro, and magnetometer data and calculate a dipole for the torquers to produce

Questions?