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Toroidal Torquers

Toroidal Torquers. M.Lampton Feb 2003 Augmented April 2007. Two components. Stator: toroidal electromagnet whose current “I” is supplied by user; it gives an external B field proportional to I. The case considered here is two poles (one North and one South).

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Toroidal Torquers

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  1. Toroidal Torquers M.Lampton Feb 2003 Augmented April 2007

  2. Two components • Stator: toroidal electromagnet whose current “I” is supplied by user; it gives an external B field proportional to I. The case considered here is two poles (one North and one South). • Rotor: permanent magnet whose built-in magnetic moment M = H*V which is magnetization * volume of magnet • Torque is cross product M x B

  3. Stator Design • Field in gap is • Example: if gap length is 0.04 meter and N=300turns on each leg (upper side and lower side in the figure) then B=0.01 tesla per ampere • This neglects fringing and core reluctance. Fringing will reduce the field to 1/2 or 1/3 of the above prediction.

  4. Rotor Designhttp://www.magnetcore.com/soft%20fe_spe/ndfeb___magnets.htm • Rotor magnetic moment M=HV where H is the rotor magnetization and V=core volume • Nd-Fe-B is an example of a high coercivity permanent magnet material: H~1E6 A/m • Example: V=(0.03m)^3=3E-5m3 volume; the magnetic moment M=30 A-m2.

  5. Torquer Example • Torque = M x B = MB sin(angle) • M = 30 amp-m2 • B = 0.003 tesla including fringing • MB = 0.1n-m/amp • not too shabby! • peak torque. Remember sin(angle) • Losses: size example wire length 0.1m/turn, choose (say) #28 wire (0.3mm diam) which is 0.2 ohms/meter, 300turns x 2legs = 60m = 12 ohms • 12 ohms and one amp = 12 watts peak power

  6. More Poles?J Bercovitz 2003

  7. Pancake Torquershttp://www.aeroflex.com/products/motioncontrol/torque-intro.cfm

  8. Detent Action? • Important to have stable position when power is off • Don’t want shutter to wander into the light! • Mechanical detents • slider on notched wheel? No: friction; lubrication; wear • roller on notched wheel? No: lubrication • Magnetic detents • stationary permanent magnet attracts sprocket iron tooth • no friction, no wear, no lube, no particulates generated • two stationary magnets and one iron tooth allows independent adjustment of the two positions “open” and “shut” • alternative is two stationary iron pole pieces (separately adjustable) and one permanent magnet tooth on the wheel

  9. Magnetic Detent Sketch magnet wheel N keeper 1 S shaft keeper 2

  10. Linear Solenoids?www.guardian-electric.comhttp://www.magneticsensorsystems.com/solenoid/solenoidcatalog.asp • Self-shielded tubular type has acceptably small external magnetic field • Tubular types also are not prone to magnetically picking up loose stray hardware items on orbit • Widely available, long life, simple to drive • typical pulse 28volts 2 amperes 100 milliseconds • Two solenoid actuators in opposition could drive a single shaft via bellcrank: “open” and “shut” actions

  11. Linear solenoid lifetime?http://www.kgs-america.com/sdc_8r.html Saia-Burgess STA series tubular solenoids: >25 million operations

  12. Rotary Solenoids?www.ledex.com http://www.magneticsensorsystems.com/solenoid/solenoidcatalog.asp http://www.solenoids.com/rotary_solenoids.html • Spring return type is unsuitable: would need power all the time shutter is open. Therefore would require bidirectional latching action. • Angular travel on stock items is typ 30 deg or 45 deg, not the 90-100 deg we require. • Custom product could probably be built to yield bidirectional latching action with 100 deg stroke.

  13. Two Methods of Calculationignore: fringing; core reluctance; cos(theta)assume: toroid inner Diam = Liron + Lmagnet + Lgap • Lampton: Magnetic moment method, torque = M x B • Sholl: Linear force method, torque = 2 Fpole Radius

  14. Design Calculation: Two StepsAgain ignore: fringing; core reluctance; cos(theta)And assume: square wire; square turns for simplicity First Step: adopt “torque per root watt” as Figure of Merit. Then: Second Step: pick Rdc to match available max voltage and current. Then:

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