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Toroidal Torquers M.Lampton Feb 2003 Augmented April 2007

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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

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4 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.

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5 Rotor Design 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.

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6 Torquer Example Torque = M x B = MB sin(angle) –M = 30 amp-m2 –B = 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

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7 More Poles? J Bercovitz 2003

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8 Pancake Torquers

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9 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

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10 Magnetic Detent Sketch N S wheelmagnet keeper 1 keeper 2 shaft

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11 Linear Solenoids? 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

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12 Linear solenoid lifetime? Saia-Burgess STA series tubular solenoids: >25 million operations

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13 Rotary Solenoids? 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 deg we require. Custom product could probably be built to yield bidirectional latching action with 100 deg stroke.

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14 Two Methods of Calculation ignore: fringing; core reluctance; cos(theta) assume: toroid inner Diam = L iron + L magnet + L gap Lampton: Magnetic moment method, torque = M x B Sholl: Linear force method, torque = 2 Fpole Radius

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15 Design Calculation: Two Steps Again 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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