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

North

South

MN S

1 amp

B

TWO-POLE LIMITED-ANGLE TORQUER

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.

tesla B 0gap

NI

5

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.

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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

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More Poles?J Bercovitz 2003

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

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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Magnetic Detent Sketch

N

S

wheelmagnet

keeper 1

keeper 2

shaft

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Linear Solenoids?www.guardian-electric.com

http://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

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Linear solenoid lifetime?http://www.kgs-america.com/sdc_8r.html

Saia-Burgess STA series tubular solenoids: >25 million operations

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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.

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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

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ILBF

DiamFRF

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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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16