Toroidal Torquers

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Toroidal Torquers

• M.Lampton
• Feb 2003
• Augmented April 2007

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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 HV which is magnetization volume of
  magnet
• Torque is cross product M x B

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Stator Design

• Field in gap is
• Example if gap length is 0.04 meter and
  N300turns on each leg (upper side and lower side
  in the figure) then B0.01 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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Rotor Designhttp//www.magnetcore.com/soft20fe_s
pe/ndfeb___magnets.htm

• Rotor magnetic moment MHV where H is the rotor
  magnetization and Vcore volume
• Nd-Fe-B is an example of a high coercivity
  permanent magnet material H1E6 A/m
• Example V(0.03m)33E-5m3 volume the magnetic
  moment M30 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
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Detent Action?

• Important to have stable position when power is
  off
• Dont 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
wheel
magnet
N
keeper 1
S
shaft
keeper 2
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Linear Solenoids?www.guardian-electric.comhttp/
/www.magneticsensorsystems.com/solenoid/solenoidca
talog.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.c
om/sdc_8r.html
Saia-Burgess STA series tubular solenoids gt25
million operations
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Rotary Solenoids?www.ledex.com
http//www.magneticsensorsystems.com/solenoid/sole
noidcatalog.asp http//www.solenoids.com/rotary_so
lenoids.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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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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