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

LightSail-1 with deployed sail

Avionics Module

Sail Storage Section

Boom Deployer

Payload Section

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 WheelGyros

Torque Rods

Sun Sensors

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 DevicesADIS16135

• 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

BKM

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

• +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

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

com

e

e

com

CT

Control Torque

DT

T

Feedback

eDePreq KKT

2

reqreq

B

TBM

ADCS Modes: Sun-Pointing

Parameter Value

Spacecraft mass

5 kg

Deployed inertias (kg·m2)

Ixx = 1.4Iyy = 1.4Izz = 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)

Solar Pressure

Orbit Direction

Orbit Direction

90 degree pitch

maneuver

Thrust On

Thrust Off

ADCS Modes: Orbit Raising

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• 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

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

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?