Magnetic Induction Servos - RC Groups
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Dec 08, 2010, 01:06 AM
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flyzwell's Avatar

Magnetic Induction Servos

I got this in my email today from Hobby King.

Mi Servos finally hit the market!
Until now servos have always relied on a potentiometer, however 'Pots'
have always been the root cause of many issues in servos,
from poor accuracy to eventual failure from wear.
Now there is a new type of servo that uses a Magnetic Rotary Encoder
instead of the old-school potentiometer to control servo arm position.

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Dec 08, 2010, 11:18 AM
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Hmm... I wonder how they work... Is the servo case shielded or something? Magnetic induction... makes me think that magnetic hatches or other sources might mess with them.... anyone have any definitive info on these things??

Dec 08, 2010, 11:20 AM
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It sounds like a system that makes a digital servo out of an analog radio signal, kinda like having some sort of module on a brushless motor that would allow a brushed motor controller to control it. That's what I think, but it's pretty much a guess.
Dec 08, 2010, 12:14 PM
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Well, I don't think that that is the case.... from what I've read it seems most-likely that we're talking about a magnet on an output shaft that senses the rotational movement of the motor or the servo arm (motor more than likely). This sensing of the servo position was/is in typical servos done with a variable resistor/potentiometer... like a volume knob on a stereo... it senses when you twist, and what way you twist, and how far you twisted... and adjusts the volume accordingly....

The magnetic sensor would be much like the crankshaft position sensor, or crank angle sensor, in an automobile engine... The flywheel, or camshaft, or crank (all = motor output) has a toothed gear on it and a little magnetic (hall effect) sensor on it that tells how fast the motor is spinning by the magnetic pulses (caused by the metal in the gear being closer then farther as the gear teeth spin around.. this makes a pulse wave) that are transfered into the coil inside the sensor... these pulses are amplified and interpreted by an IC to read the motors RPM, position, acceleration, etc....

All fine and dandy, assuming that the little magnetic sensor inside these servos is magnetically shielded so that magnetic interferece doesn't screw up the signals.... I would think that they'd've thought of this at the factory, but who knows....

I would like to see a review on these servos, maybe some centering/travel tests with common potential interference scenarios... (i.e. - magnet near the servo, a running motor near the servo, spark plug/ignition running near the servo.... anything RFI or magnetic...)

I'll keep reading on these little guys maybe someone will make a review...

Dec 08, 2010, 12:41 PM
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GeorgeC525's Avatar
Just got the email too, and was thinking about them. Unless they offer a noticable improvement over standard servos, I don't see much point in rushing out and buying them now, Although I did say the same about 2.4ghz when that came out, and now THAT's pretty much mandatory!
Dec 08, 2010, 05:38 PM
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Interesting... Does this introduce "steps" into the movement which a regular servo digital or otherwise does not have?

Dec 08, 2010, 06:14 PM
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gouch's Avatar
all this is is a non contact version of a pot.

Industrial servo motors use them, From what I understand they are technically called resolvers.

advantages are no contact, so no dirty pot issues or worn sliders.

Are these servos any good? who knows, time will tell. The system can be made to be extremely accurate, but like anything there are expensive versions, and cheap versions but I am guessing as this kind of feedback system has no wearing parts whatsoever, so it should be as accurate when it is old as it was when it was new, mechanical tolerances and wear on the rest of the servo notwithstanding.

note, as I know not much about this kind of thing, I asked my brother in law who is a motion control guru (industrial robotics etc)

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