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How Does an Engineer Predict Step Motor Pull-Out Torque?

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cary2
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Iron
Empical Caveat
cary2   4/26/2014 2:03:08 PM
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    You're right of course that performance has to be developed empirically, but the published specs should be carefully considered too. Your performance envelope may exceed the 'intentional published spec.  No matter how well 'checked',  a difference in the two can come back to bite ya. Case in point.

I inherited a (HPLC) syringe pump design, and production had been going along OK for several years. Then one day, assembly/manufacturing observed that it was 'whining' more than usual at final test; it functioned fine, but it was just plain noisy. I went through ALL the checks as to build quality, and etc. and all was fine. A month later complaints came in from an install in Australia: really noisy, particularly against an older system in the same lab; ...should they try to repair it, or what, I was asked. I said no, pull it all. My homework on this issue had already been done, or so I thought. 

I called the U.S. (now defunct) motor maker, and spoke to their manufacturing manager. They had been a large manufacture for years, -and I know our original R&D had worked for 8 months at finding the best price/performance model they could. The OEM said:          "Look, although our name has been out there for decades, we've changed ownership three times since I've been here. Our recent owner had us check our published specs. against the actual performance, and yes, our motors were over-performing. Therefore, they had us 'adjust' our motors to (just) meet specification.' "IF you want the motor that use to have that part number, which over-performed its spec, then you will now have to buy model xxxx-xxxx (and pay more money)."

  Check the empirical torque specs against the published, and see where your performance window lies. With steppers, that's not a trivial task. Also, as your chart would indicate: EMI-EMF is at it's worst when the motors are in a 'hold postion' state.

 [This is also why Europe and Japan do not respect UL versus ISA, JIS blessings IMHO. Medical device and research equipment manufacturers need have contractual QA notification requirements regarding ANY changes that could impact performance, whether the changes remain 'within spec' or not. I've seen UL approval of transformers straining at 80% of its spec-load accompanied by smoke, and loud whistling... and the inspector observ: 'yep, I see no flames, -it's a pass!]'

  

William K.
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Platinum
The other method of determining pull out torque
William K.   3/28/2014 11:28:16 AM
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Back in the early seventies I did have an occasion to require knowledge of stepper pull out torque, in odrder to avoid damaging parts of a mechanism driven by the stepper motor. Fortunately we had a small dynamometer at that company, and so it was fairly simple to run the speed/ voltage/ pull out torque curves for the speed and running time in our application. The test procedure was fairly sandard.

Given the standard stepper drive systems of the time, the stepper was always run with a much higher voltage and a large series resistor in order to provide a means to overcome the large inductance and allow faster acceleration. What became clear quite quickly was that as th motor warmed up due to the current, the resistance would rise and both the torque and the acceleration would drop. The ultimate solution was to include a stall detection circuit that would see the change in the waveform when the motor stalled and instantly inhibit the drive pulses in that direction. A math model would have been handy, but accounting for thermal effects would have made getting useful results a challenge.

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