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Stepper Motor Current as a Measure of Break Away Torque

I am hoping to determine if a stepper motor driven flapper valve assembly was properly built. The mechanical design guy says it should open (break away) from it home position with 50 grams of mass applied to one side of the flapper.

The electrical guy I am taking over for says that he cannot detect a change in current between a "50 gram" and a "100 gram" assembly. He said it may have something to do with the stepper motor driver's "smart" PWM capabilities making it didficult to pick out something conclusive on an oscilloscope trace of current.

Anybody in the group used current to characterize torque in a stepper motor?

I am thinking maybe running the flapper full travel 10 times and averaging the current.

Maybe use a non PWM driver and capture a "cleaner" current waveform.

Of course this is a last minute problem that has to be done without spending any money in the next two weeks. ;-)

Thanks in advance for any advice, Ed V.

Reply to
EdV
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Stepper coil current isn't reflective of shaft torque at low step rates. Most smart PWM drivers poke constant current into the coils. Most dumb drivers do, too.

You could try reducing drive current until the valve doesn't work, and see how much margin you have. Or add some load to the shaft, ditto.

John

Reply to
John Larkin

Many of the better drivers will reduce the motor current after a time of inactivity. This provides a holding torque without burning the power of full current.

JAM

Reply to
Leo Marx

All of the stepper drivers that I know of operate in open loop -- so the current will be the same no matter what the motor is doing.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply to
Tim Wescott

That's the key. The driver used for normal operation won't give you the information you need. If you bypass it and provide a driver capable of controlling the current (or the pulse width), you should be able to ramp it up and see the threshold at which the valve moves.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
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Reply to
Paul Hovnanian P.E.

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