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ECE 5320 MechatronicsAssignment #1

Torque Coils/Rods and Reaction WheelsKedrick Black

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Outline

• Sensor/Actuator/Operation Interface• Explanation of Attitude Control• External Disturbances• Reaction Wheels• Torque coils/rods• Design optimization• Momentum dumping• Places to buy/cost

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References

• Space Mission Analysis And Design• http://uasat.arizona.edu/http://uasat.arizona.edu/• http://fuse.pha.jhu.edu/educ/RW_FAQ.htmhttp://fuse.pha.jhu.edu/educ/RW_FAQ.htm• www.mae.usu.edu/faculty/tmosher/classeswww.mae.usu.edu/faculty/tmosher/classes

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To Explore Further(web pointers)

• http://staff.ee.sun.ac.za/whsteyn/Papers/Magsat.pdf• http://ocw.mit.edu/NR/rdonlyres/Aeronautics-and-Astronautics/ 16-358JSystem-SafetySpring2003/B169A368-9DCC-4916-8897-12B0DE4DC855/ 0/hete.pdf• http://ocw.mit.edu/NR/rdonlyres/Mechanical-Engineering/2-141Fall-2002/4706270A- 4F69-4020-958A-F5172054F35F/0/dcpmm_basics.pdf

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Credit: www.ocw.mit.edu

Sensor/Actuator/Operation Interface

Credit: ECE 5320 class slides

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Attitude Control

• The system that turns and maintains a spacecraft in the required direction as indicated by its sensors.• Spacecraft subsystem capable of pointing the spacecraft toward a selected target. • Stabilizing a satellite's attitude (direction) in its orbit. Attitude control can be done by spinning the satellite, or by having it remain stabilized in three axes using an internal mechanism such as reaction wheels and/or torque coils/rods.

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Attitude Control Parameters

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External Disturbance Torques

• Gravity-Gradient: Constant for earth oriented vehicle, cyclic for inertially oriented vehicle Tg = 3μ/(2R3)*|Iz – Iy|*sin(2Θ)• Solar Radiation: Cyclic on earth oriented vehicle, constant for earth oriented Tsp = F*(cps – cg)• Magnetic Field: Cyclic Tm = D*B• Aerodynamic: Constant for earth oriented vehicle, variable for inertially oriented vehicle Ta = F*(cpa – cg) = FL

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Reaction Wheels

• Used to hold the spacecraft steady or to move from one pointing direction to another• spinning flywheel mounted on a central bearing whose rate of rotation can be adjusted as necessary by an electric motor to apply a force and move the spacecraft• any change in the rate of spin of these wheels creates a predictable force that nudges the satellite's pointing direction • Once in position, very tiny changes in spin rates work to keep us pointing at the object of interest

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Reaction Wheel sizing equationsand performance ranges

• Momentum Storage (h): h = TD*(Orbital Period/4)*(0.707)• Slew Torque: Θ/2 = (1/2)*(T/I)*(t/2)2 • 0.4 to 400 Nms @ 1200 to 1500 rpm• 2 to 20kg• 10 to 110W

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-3

10 200 400 600 800 1000 1200

-1

0

1x 10

0 200 400 600 800 1000 1200-1

0

0 200 400 600 800 1000 1200-1

0

1

0 200 400 600 800 1000 1200-1

0

1

Time (s)

Rea

ctio

n W

heel

Tor

ques

(N

m)

Reaction wheel torques

Credit: University of Arizona

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0 200 400 600 800 1000 12000

100

200

300

400

500

600

700

800

900

1000

Time (s)

Rea

ctio

n W

heel

Spe

eds

(rpm

)

Reaction wheel speeds

Credit: University of Arizona

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• Light Weight - assemblies as light as 0.3 lbs • Anomaly Free Operation • Retainerless Bearing - eliminates most failure modes • Operates in a Vacuum - no seals, no pressure housing • High Speed Capable - up to 100,000 rpm • Energy - up to 400,000 ft.lbs • Diameter - 0.2 to 15.5 inches and above • Thermal Changes - zero • Radial Runout - less than 1 millionth of an inch • Bearing Life -226 years for microsats

VFCT Reaction Wheel

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Torque coils vs. Rods

•Coils - Simpler than torque rodsSimpler than torque rods - Least costly option- Least costly option - Linear response to input current simplifies - Linear response to input current simplifies control requirementscontrol requirements - Possible issues with stray magnetic fields- Possible issues with stray magnetic fields•RodsRods - Most efficient option- Most efficient option - Easier to implement- Easier to implement - Hard to find appropriate core material- Hard to find appropriate core material

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0a

NR

0aNM

222

AM

mRiP

Aa

mV

0

AM

ami

0

0Naa

432

• Power, current, voltage and resistance:

• Mass and wire size:

• Moment equation:

Coil Design Formulas

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mas

s [k

g]

Power [watts]

Total mass vs.Power consumption

Design Optimization

Specifications -Dipole moment of 5 Am2 -Power consumption of

0.3 W (16 mA at 20 V) -Uses 32 gauge square magnet wire -Total mass of 3 kg

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Credit: University of Arizona

Mounting Possibilities

•Two ideas -Designed to fit within side beam -Wrapped into groove on exterior of satellite•Three coils form mutually perpendicular axes

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Momentum Dumping

• Presence of torque on spacecraft at all times causes wheels to spin at all times• Momentum builds up and needs to be dumped• The approach is to consider both the need and efficiency of dumping at a particular time.• Torque coils/rods are a good solution

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Places to buy/Cost

Reaction Wheels• Bendix• Honeywell• cost approx. $5000.00

Torque Coils• Ithaco• Hughes• Lockheed• McDonnell Douglas• cost approx. $2000.00