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nuckollsr(at)cox.net
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 Posted: Fri Dec 01, 2006 8:28 pm Post subject: Auto-pilots |
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At 01:43 PM 12/1/2006 -0700, you wrote:
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Stepper may not freewheel with the power removed from the driver circuit.
If the silicon is shorted, the stepper will plug brake. If you force the
stepper to spin too fast, you can blow out the silicon with over-voltage,
and perhaps cause it to short. The back-feed of voltage into the driver
circuit (on or off) from the spinning stepper can cause all sorts of
problems. If you simply spin the stepper itself too fast, you can possibly
arc over, then short the windings, causing it to plug brake.
Your auto-pilot system should be designed to be tolerant of all of
these fault scenarios.
Steppers don't really "freewheel", instead they "cog."
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Bill, can you cite me a part number and manufacturer of
a motor that exhibits these characteristics? I've done
about a dozen programs with steppers of various sizes
(20A machines with 400 oz-in of breakout torque) and
all would freewheel nicely when de-energize and generated
no back-emf.
I suppose one COULD build a stepper with a PM rotor
but it would be difficult to achieve the 1.8 degree stepping
angle rotors that have strong permanent magnet poles.
Bob . . .
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nuckollsr(at)cox.net
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| Posted: Fri Dec 01, 2006 8:32 pm Post subject: Auto-pilots |
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At 02:26 PM 12/1/2006 -0700, you wrote:
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Seems I've read of autopilots having a clutch of sorts. I would think
that an electromechanical clutch could be included which would allow
decoupling of the stepper motor from the control bellcrank. The clutch
would failsafe in the decoupled configuration..
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A VERY common feature in autopilot servos of yore and even today
with the large servos used to fly bizjets. But the little stepper
driving a single pass gear train to hold a heading is not
capable of producing breathtaking maneuvers and it's output
torque is sharply limited.
Bob . . .
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