Kedrick Black1 ECE 5320 Mechatronics Assignment #1 Torque Coils/Rods and Reaction Wheels Kedrick Black

Kedrick Black2 Outline Sensor/Actuator/Operation Interface Explanation of Attitude Control External Disturbances Reaction Wheels Torque coils/rods Design optimization Momentum dumping Places to buy/cost

Kedrick Black3 References Space Mission Analysis And Design

Kedrick Black4 To Explore Further (web pointers) JSystem-SafetySpring2003/B169A368-9DCC B0DE4DC855/ 0/hete.pdf 4F A-F F35F/0/dcpmm_basics.pdf

Kedrick Black5 Credit: Sensor/Actuator/Operation Interface Credit: ECE 5320 class slides

Kedrick Black6 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.

Kedrick Black7 Attitude Control Parameters

Kedrick Black8 External Disturbance Torques Gravity-Gradient: Constant for earth oriented vehicle, cyclic for inertially oriented vehicle T g = 3μ/(2R 3 )*|I z – I y |*sin(2Θ) Solar Radiation: Cyclic on earth oriented vehicle, constant for earth oriented T sp = F*(c ps – cg) Magnetic Field: Cyclic T m = D*B Aerodynamic: Constant for earth oriented vehicle, variable for inertially oriented vehicle T a = F*(c pa – cg) = FL

Kedrick Black9 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

Kedrick Black10 Reaction Wheel sizing equations and performance ranges Momentum Storage (h): h = T D *(Orbital Period/4)*(0.707) Slew Torque: Θ/2 = (1/2)*(T/I)*(t/2) to to 1500 rpm 2 to 20kg 10 to 110W

Kedrick Black x Time (s) Reaction Wheel Torques (Nm) Reaction wheel torques Credit: University of Arizona

Kedrick Black Time (s) Reaction Wheel Speeds (rpm) Reaction wheel speeds Credit: University of Arizona

Kedrick Black13 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 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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Kedrick Black15 Torque coils vs. Rods Coils Simpler than torque rods - Simpler than torque rods - Least costly option - Least costly option - Linear response to input current simplifies - Linear response to input current simplifies control requirements control 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

Kedrick Black16 Power, current, voltage and resistance: Mass and wire size: Moment equation: Coil Design Formulas

Kedrick Black17 mass [kg] 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

Kedrick Black18 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

Kedrick Black19 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

Kedrick Black20 Places to buy/Cost Reaction Wheels Bendix Honeywell cost approx. $ Torque Coils Ithaco Hughes Lockheed McDonnell Douglas cost approx. $