1. Attitude Determination and Control Preliminary Design Review
Stephen Stankevich
Jeff Parker
July 28, 2003

2. 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.
January 17, 2019
DINO PDR

3. Requirements
Maintain nadir attitude for communication and imaging objectives.
Perform onboard attitude determination and control.
Maintain roll and pitch control with gravity gradient tether.
Maintain attitude knowledge to 2° in every axis.
Maintain attitude control to +/- 10 ° in each axis.
Mass kg
Power 4 W operating
These requirements constitute a fairly coarse ADC system thus the design driving requirements are the mass and power limitations.
January 17, 2019
DINO PDR

4. Imposed System Requirements
Placement of torque rods
Rods must lie in right hand orthogonal system
Preferably along the s/c body axis.
Need an I2C line from C&DH.
May need I/O lines from C&DH
Need a 5V and 12V line from PWR.
S/C c.g. shall be located along the z-axis (tether) axis.
January 17, 2019
DINO PDR

5. Hardware Flow Diagram ADCS Electronics Magnetometer Rate Gyro(s)
12V and 5V supply to board
ADCS Electronics
Magnetometer
Honeywell HMC2003
w/ 40μG resolution
(3) 0-5V analog
I2C data
X Analog
Inputs
A/D
Conversions
Torquer Analog
Outputs
D/A Conversions
Rate Gyro(s)
3 single-axis gyros
+5V 6mA
(3) 0-5V analog
I2C or I/O lines
Sun Sensors
Possible donation by Ithaco
12 single axis
Flight Computer
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
Att. Det.
IGRF Magnetic Model
Orbit Propagator
Compare Expected
And actual
B Fields
Damp rates
Torque Rods
(3) 3/4’’ x max 150mA nominal
0-300mA
Power
Standard Commands
ADCS software running at 1 – 10 Hz
January 17, 2019
DINO PDR

6. Attitude Determination Overview
Obtain expected B field vector from model in orbit frame.
Orbit data update
Propagate orbit
Compare the s/c frame to the orbit frame
Obtain B field and rates in s/c frame
Euler Angles and Rates
The Euler angles and rates will provide and attitude error to the control algorithm.
January 17, 2019
DINO PDR

7. Attitude Knowledge
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.
Sun sensors may be used if suitable vendor is found. These are not needed for accurate attitude knowledge within requirements.
Bdot data derivations could also be used for comparison between s/c and orbital attitude frames.
January 17, 2019
DINO PDR

8. 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
January 17, 2019
DINO PDR

9. Rate Gyros Analog Devices ADXRS150
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
$33 each
January 17, 2019
DINO PDR

10. Attitude Control
The s/c magnetic dipole moment interacts with the Earth’s magnetic field to provide a torque on the s/c. (T = MxB)
The s/c magnetic dipole moment can be controlled by passing specified currents through magnetic torque rods. (M=INA)
Torque rods can provide three-axis stabilization for detumbling at less weight, power, and cost than reaction wheels.
Control algorithm will determine how much current to provide to each torque rod to produce the desired dipole moment for the necessary torque.
January 17, 2019
DINO PDR

11. Gravity Gradient
Gravity gradient provides a restoring torque when a disturbance torque causes a movement from local veritcal.
The torque produced is dependent upon the s/c moments of inertia. This shall be a design concern for placement of s/c components.
Maximum disturbance torque is Aerodynamic Drag
Assume ½ m2 cross-sectional area
TAD = 5.56 x 10-5 Nm
Solar Radiation Pressure
4.37 x 10-6 Nm
Magnetic disturbance torques are not considered as disturbances because active magnetic control will be utilized.
January 17, 2019
DINO PDR

12. Magnetic Torque Rods Ferrite Material wound with wire
Produces a dipole moment that interacts with Earth’s magnetic field.
Will be designed in-house (unless donated)
January 17, 2019
DINO PDR

13. Magnetic Torque Rods Minimum Requirements
Rod of length 10” and diameter 3/8”
Mass = 0.4kg each, total of 1.2kg
Power use of 200mW at max input of 300mA
After detumbling normal use should not exceed 150mA
Max output of 3.0 Am2
24 Gauge Wire
Common ferrite material 33 with μ = 800.
Complete manufacture under $100 each
The bigger the better
January 17, 2019
DINO PDR

14. ADC Electronics Board Mag Sensor Rate Gyros Resistor Bank Multiplexer
To Torque Rods 1,2 and 3
Mag Sensor
Rate Gyros
Resistor
Bank
Multiplexer
Analog
Resistor
Bank
Multiplexer
I/O from FC
A/D Converter
A/D Converter
Resistor
Bank
Multiplexer
To Flight Computer
I2C data
5V from Power
GND
12V from Power
January 17, 2019
DINO PDR

15. ADCS Electronics
The ADC system will contain an electronics board which houses the sensors and data converters.
Power will supply both a 5V and 12V line to the board.
The magnetometer and rate gyros are IC (integrated circuit) chips.
A/D Converters will be needed for the magnetometer and rate gyros.
D/A converter may be necessary for torque rods.
Other options include multiplexing to control switches relating to different resistors supplying different amounts of current.
The electronics board will be designed and prototyped by CDR.
January 17, 2019
DINO PDR

16. Subsystem Commands Command Description Hardware Power on/off
Turns power to the adcs electronics board on/off
Electronics board (mag, RGs, A/Ds)
Rod current (x mA)
Turns power to torque rods with x amount of current (control alg.)
Electronics board, torque rods.
Deploy_boom
Allow deployment of boom in correct orientation
None
January 17, 2019
DINO PDR

17. Parts and Materials Summary
Mass (kg)
Power (W)
Cost
Honeywell HMC2003 Magnetometer
0.1
0.24
$200
Torque Rods (3)
1.2
< 0.6
$300
Analog Devices ADXRS150 Rate Gyro (3)
.0005
0.03
$100
A/D Converters (2 – 6)
n/a
Totals
1.56
0.4
$800
January 17, 2019
DINO PDR

18. Test Plan
Control algorithm may be tested with an external power supply to electronics board, software running on a linux PC, and mock sensor inputs.
Attitude determination algorithm may also be tested with linux PC and mock sensor inputs.
Actual output from actuators can be measured and compared to simulations using the same mock sensor inputs.
Complete testing after s/c integration is more complicated because the torque rods will not rotate the s/c in a gravity environment.
January 17, 2019
DINO PDR

19. Design Issues and Concerns
Control of torquers may require rather fine current control.
Current to torque rods must be able to go both directions.
Detumbling the s/c
Torquers do not produce high output for fast rotational rates.
Higher input currents may be necessary.
S/C moments of inertia are key to controller development.
One rate gyro will need to be placed orthogonal to the others. It cannot be placed on the same electronics board.
How do we take magnetic field measurements with torquers on?
Turn off…take measurements…turn back on
Subtract out torquer field in software
January 17, 2019
DINO PDR