Back-EMF speed detection in motors

Tim Wescott · 2013-03-22T17:51:29+00:00

I'm working on a project that involves replacing some existing electronics with some that I've designed. It's controlling the speed of DC brushed motors. Since the original equipment uses a knowledge of the applied voltage and the motor current to infer back EMF and therefor motor speed, that's what I've done, too. My problem is that my equipment is consistently sensing the speed as being faster than it really is, by a constant amount -- except when the motor is stalled, when the reading is correctly zero. So I end up servoing the motor speed to a figure that's a 20-30 RPM higher than desired. It's not just a mis-calibration of the motor torque constant or the resistance -- if that were the case, then the motor speed error would depend on the command speed or the applied torque. I can easily calibrate out any dependence on torque (by adjusting the calibrated motor resistance) and I can get a 1:1 correspondence between increments in the commanded speed and increments in the actual speed. But I'm left with this @#$% offset. Has anyone seen this? Anyone care to hazard a guess at what's going on? Thanks. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software

Mar 22, 2013 102 Replies

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George Herold 13 years ago

tic

That's too bad. I've been hoping you'd chime in one day and tell us you'd solved it. In my limited experience if you don't chase down that last unexplained bump or wiggle, then the law of Murphy guarantees that the wiggle will come back and bite you in the future.

(Knock on wood, I don't want to jinx you :^)

George H.

ttdesign.com- Hide quoted text -

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Jim Thompson 13 years ago

That's why I was hanging in here so intently... so far ;-) my motor drivers were essentially one-sided with a "reverse" switch. So this full-H problem intrigued me... I was hoping for a solution before I'm facing it all by myself ;-) ...Jim Thompson

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josephkk 13 years ago

the

immaterial

Now if i only believed that is where you are sampling. I think you are hitting near the peak. 'Nuff said.

?-)

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