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Old 5th Dec 2017, 4:42 am   #13
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Mini-drill; Speed Controller Problem

Quote:
Originally Posted by Bazz4CQJ View Post
Experimenting by just rubbing a thumb against the chuck to load the drill and putting 15 volts in to the controller, the setting of the 50k pre-set is fairly tight; the "boost" is better turning the trimmer one way, but it tends to start "chugging" (pulsing?), but turning it the other way to get perfectly stable running seems to diminish the boost. Wonder if a little capacitance might be useful somewhere in there?
The chugging effect won't be cured by adding additional capacitance, though you might slow down its frequency. It comes about because as the motor is loaded, the sensed motor current drives the motor harder (speeds it up) so its simply oscillation from the positive feedback nature of the circuit. When there is just the right amount of feedback any friction/drag on the motor is just compensated for by increase in motor drive. The "chugging rate" represents the cumulative delays in the whole circuit.

There is a simpler version of this circuit, with a single OP amp driving a darlington output stage used as an emitter follower, the motor is in the emitter (or other transistor output circuit) and in series with the motor's wire to ground is a copper coil used as a current sensing resistor. The signal from that is filtered with an RC network and a proportion of it fed back to the OP amp's positive input.

It was a common technique used in very early JVC VHS video machines in the servos. So if friction slowed the head drum for example the positive feedback would compensate for that, its a technique of "artificial torque". A copper wire resistor was used to better match the temperature coefficient of the wire in the motor.

The other way to help stabilize the speed of a motor, if it has an attached frequency generator is to make a frequency to voltage converter and put that into a heavy negative feedback loop to control the motor speed, that way when the motor is forced slow, the feedback diminishes and it attempts to compensate. But most small motors don't have that generator attached, so the positive feedback method from a current sensing resistor is often much easier.

Actually I just remembered, I think I have a circuit at home, it was designed to speed and phase control a DC motor in a film camera, to lock it to video vertical rate (to avoid a rolling bar when the film camera filmed a TV screen/video monitor). It used an LM3900 Norton OP amps and the current sensing resistor trick for the speed control/artificial torque. I could find it and post if you are interested?

Last edited by Argus25; 5th Dec 2017 at 4:54 am. Reason: ask question.
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