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

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Presentation on theme: "Design and Simulation of the Attitude Determination and Control System for LightSail-1 Matt Nehrenz."— Presentation transcript:

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

2 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 LightSail-1 with deployed sail Avionics Module Sail Storage Section Boom Deployer Payload Section

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

4 ADCS Sensors MagnetometersSun Sensors ELMOS Semiconductor E axis 150º field of view 2.7º resolution +/- 5º accuracy 4 x 4 mm Gyroscopes Analog Devices ADIS axis Temp calibrated º/sec bias stability º/sec resolution 44 x 36 x 14 mm Honeywell HMC axis -6 to +6 Gauss range 120 μGauss resolution 2 x 2 x 7 mm

5 ADCS Actuators Torque RodsMomentum Wheel Sinclair microsatellite reaction wheel Nominal momentum: Nms Dimensions: 75 mm x 65 mm x 38 mm Mass: 225 g Stras Space Torque Rod 1 Am 2 magnetic dipole Dimensions: 90 mm long x 22 mm diameter Mass: 150 g Power consumption: 500 mW

6 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

7 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

8 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

9 ADCS Modes: Wheel Spin-Up Sinclair microsatellite reaction wheel Nominal momentum: 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

10 ADCS Modes: Sun-Pointing +Z +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

11 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 Total Torque Control Law Spacecraft Dynamics Disturbance Torques Control Torque Feedback

12 ADCS Modes: Sun-Pointing ParameterValue Spacecraft mass 5 kg Deployed inertias (kg·m 2 ) I xx = 1.4 I yy = 1.4 I zz = 2.8 Orbit822 km sun-sync System cycle10 sec Wheel momentum N·m·sec Max magnetic dipole 1 A·m 2 Max power consumption < 3 watts Sun-pointing error over ~4 orbits (shaded areas are eclipses)

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

14 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

15 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

16 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 Log data point Skip that iteration of the control algorithm and try again –If there are 3 consecutive bad readings from single sensor Discontinue use of bad sensor Report sensor failure to ground

17 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 Run B-dot 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

18 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

19 Questions?


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