Attitude Determination and Control System

Attitude Determinationand Control System

Peer ReviewDecember 2003

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.

Requirements

Maintain nadir attitude for communication and imaging objectives.

Perform onboard attitude determination and control. Maintain roll and pitch control using a gravity gradient boom. Maintain attitude knowledge to 2° in every axis. Maintain attitude control to +/- 10 ° in each axis.

Mass 2.25 kgPower 4 W operating

These requirements constitute a fairly coarse ADC system thus the design driving requirements are the mass and power limitations.

Imposed System Requirements Placement of torque rods

Rods must lie in right hand orthogonal system Preferably along the s/c body axis. The rods shall be placed such that the ends are at the edge of the s/c

structure thereby eliminating strong fields effecting equipment. Data conversion will be performed by C&DH.

Bx,By,Bz (.5V – 4.5V analog) input phidot, thetadot, psidot (.25V – 4.75V) input 0-5V analog output x 3 TTL high/low analog output x 3

Need a 5V and 12V line from PWR. S/C c.g. shall be located along the z-axis (boom) axis. Magnetometer shall be located as individual component outside of attitude

interface board box. Remaining electronics will be placed on the attitude board.

Imposed System Requirements

Component Dimensions(inches) Mass (kg)

Location ofMounting

Bolts

Standoffheight Sockets

Torque Rod X 15'' X 1/8'' Rod 0.1

Aluminumincasing,

HorseshoeMounting

1’’ – 2’’ fromall other

ComponentsTBD

Torque Rod Y 16'' X 1/8'' Rod 0.15 Same same TBD

Torque Rod Z 12'' X 1/4'' Rod 0.25 Same same TBD

ADC PCB 4’’ x 4’’ Corners TBD DB-9 or DB-15

Magnetometer PCB 2’’ x 2’’ Corners TBD DB-9

Torque Rods(3) 3/4’’ x 10’’ @ max 150mA nominal

Flight Computer

Att. Det.

IGRF Magnetic ModelOrbit Propagator

Compare ExpectedAnd actualB Fields

Damp rates

Controller

Likely a P-D or LQR

Output cmds to turn rods on/off and current direction

Possibly use multiple voltage levels requiring a D/A

Converter

Sensor AnalogInputs

SensorADC

ADCS Electronics

POWER12V and 5V supply to board

MagnetometerHoneywell HMC2003

12V@20mA w/ 40μG resolution

Rate Gyros3 single-axis MEMS gyros

+5V input @ 6mA

Current ControlCircuit

(3) 0-5V analog

(3) 0-5V analog

0-300mA

Magnetometer

Honeywell HMC2003

• 20mA @ 12V

• mass < 100g

• -40 to 85 C operating temp.

• 40 μGauss Resolution

• $200

• 3 Analog Outputs (Bx, By, Bz)

• Set/Reset Capabilities

Rate Gyros

Analog Devices ADXRS150EB

Single axis rate gyros provide the rotational rate of the s/c about the output axis

Microchip operating at 5V and 6mA. Single analog output -40 to 85°C operating temp $50 each

Magnetic Torque Rods

Electrical current is passed through wire wound around a ferrous material creating a magnetic dipole moment.

Torquer dipole moment interacts with Earth’s magnetic field to create the desired torques. T = MxB

3 orthogonal torque rods can produce torques perpendicular to magnetic field vector

Unbiased momentum

Design

Material Ferromagnetic material Magnesium Zinc Alloy Approximate density: 5000 kg/m3

Magnetic permeability μ = 800 W/(A m)

Wire 24 Gauge Copper

Output 3 Am2 @ 300mA input

Counteract max drag disturbance torque

Internal Placement

Magnetic torque rods create interference Magnetic fields emanate from ends only Rods sized to span entire length of

satellite

Possible configuration

Sizing

Design to obtain 5 Am2 @ 500mA Counteract max

drag torques Provide detumbling

capacity Tradeoffs

Weight Power Output Moment

Sizing - Mass

Budget Estimate1.5 kg .6kg

Sizing - Power

Budget Estimate2.5W 1W

Electronics Design

Magnetometer and rate gyros require basic circuit design

Torque Rod control requires more complicated circuit. Control current input Control current direction through torquer

Sensor Circuits

RATEOUT

TEMP

2.5V

AVCC

CP5 ST

2

ST1

SUM

J

CP3

CP4

CP2

CP1

AGN

D

CM

ID

PGN

D

PDD

9

1

19 2

20

7

11

17

18

14 10 3

4813 12

RATEOUT

TEMP

2.5V

AVCC

CP5 ST

1

ST2

SUM

J

CP3

CP4

CP2

CP1

CM

ID

PGN

D

AGN

D

PDD

9

19 2

20

7

1

11

17

18

10 3

48

14

13 12

47n

22n

22n

100n

100n 100n

22n

5VDC

47n

22n

22n

100n

100n 100n

22n

5VDC

RATEOUT

2.5V

TEMP

AVCC

CP5 ST

2

ST1

SUM

JCP3

CP2

CP4

CP1

CM

ID

AGN

D

PDD

PGN

D

47n

22n

22n

100n

100n 100n

9

219

1

7

17

18

20

311

8

14 10

12 413

22n

5VDC

X-axisRate Gyro

Z-axisRate Gyro

Y-axisRate Gyro

XoutYoutZout

GNDout

Vbridge

SR-

Vsense

Xout Yout

Vref Vbias

SR+

Xoff+

Xoff-

Yoff+Yoff-

Zoff-

V+

GND

Xtrim

Ytrim

Ztrim

HMC2003

12V DC

Zout6

5

4

3

2

1

10

9

8

7

Zoff+20

18

19

16

17

14

15

12

13

11

22u

22u

100k

10k

ZTX605

Xout

Yout

Zout

Reset In

10V DC

GNDout

Rate Gyro Circuit Magnetometer Circuit

Torquer Control Circuit

In+Out+

In-Out-

Torque Rod Control Preliminary Schematic

Direction-TTL

1 2

Vin-Analog

OPAMP

+

-

OUT

Rbias

20

1

2

1

2

Torque Rod

1

2

Software Design

Control system design Hardware control functions Functional test software

Software Design OverviewOrbit data update Propagate orbit

Obtain expected B field vector from model in orbit frame.

Obtain B field and rates in s/c

frame

Compare the s/c frame to the orbit

frame

Euler Angles and Rates

• The Euler angles and rates will provide and attitude error to the control algorithm.

Software Design Overview 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.

Bdot data derivations could also be used for comparison between s/c and orbital attitude frames.

Control Design

Based on research paper by Cornell University faculty member.

Simple control law/code Analysis not yet performed.

Primary control design work to be completed during spring semester.

Control design, analysis, simulation, flight code development.

Hardware Control Functions

Control D/A converter for torque rod current control circuit input.

Control of TTL line for current flow direction through torque rod.

Prototype Report

The hardware has been purchased and prototyped with a circuit. Rate gyro

Prototyped and functioning Still needs to be tested to

be sure output is correct Magnetometer

Prototyped and functioning Still needs to be tested to

be sure output is correct Torque Rods

Prototyped in house Functionality not yet tested

Commands

Torquer On/Off Inputs

X,Y, or Z rod amount of current Direction of current

Outputs digital signal to D/A to vary output voltage of D/A. TTL high/low for control of direction of current.

Commands

Control on/off Either run the control system or do not.

Attitude Board on/off This will provide/cut power to the attitude board

Attitude determination and control will be completely off

Read data Need to read data from sensors and store to

variables.

Test Plans-Hardware

Magnetometer Successfully tested to

be sure it functions Verify the output is

correct with rated magnets

Rate Gyros Successfully tested to

be sure it functions Verify the output is

correct by spinning each rate gyro up on spinning apparatus

Compare angular rate output value to known angular rate value

Test Plans-Hardware

Torque Rods Use magnetometer to

test the amount of torque produced

Graph relationship between current input into current driver circuit and amount of torque produced

Electric Circuits Torque rod current

driver Magnetometer reset

circuit-functions with hardware

Rate Gyro circuit-functions with hardware

Test Plans - Software

Verify orbit propagation vs. STK HPOP Simulate feedback control loop

Provide input torque to simulatorModel will predict s/c reaction to torqueControl loop will provide responseModel can predict time domain responses to

input torques based on control design.