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

3. 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

4. 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
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

5. ADCS Actuators Torque Rods Momentum 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 Am2 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 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

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
Disturbance Torques
Control Torque
Total Torque
Control Law
Spacecraft Dynamics
Feedback

12. 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)

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

14. 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

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
Skip that iteration of the control algorithm and try again

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
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?