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I have a project that requires really accurate torque measurement of a motor or a motor + a belt drive. How do I do that?

I am guessing that measuring current through a stepper motor is is proportional to the torque? How about when there is a belt drive attached? There is some resistance in the belt and the output torque drops.

I accurate I can go? Is it possible to get into micro Newton meter (uNm) region. Assuming that I have a quality stepper motor, quality parts and I know what I am doing (well, I know a guy...).

An example of parts would be nice or commercial products or something to kickstart the project. This is related to this question:

How to achieve very accurate/fine rotation with motor

but with accurate torque measurement.

P.S. other drives than belt drives could work also. I have no experience on harmonic drives so I don't know if they are good or bad for this project.

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Juha
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  • Big question. Do you need the torque while the belt is moving at various speeds, or can you measure it when the belt and motor shaft are stationary? – WhatRoughBeast Oct 08 '15 at 20:30
  • @WhatRoughBeast I need the torque and angular velocity at the output. The output rpm is going to be really slow, 1 or 10 at maximum. My original idea was to run the motor at higher rpm and measure the torque from the motor and the use 1:10 or 1:100 belt drive to step it down. The problem is how to get loss from the belt drive. Now I am thinking or torque meters and harmonic drives: http://www.interfaceforce.com/index.php?T2-Ultra-Precision-Rotary-Torque&mod=product&show=59 and http://www.harmonicdrivegearhead.com/products/harmonic-drive/csf-gh/14/csf-14-100-gh + a standard stepper motor. – Juha Oct 09 '15 at 22:22

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Current measurement would be good for BLDC, for stepper it will not work. Use torque gauge. You put it on axis and just see the number. Also you may use dynamometer inside the belt to measure linear force.