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Bi-Directional Motor Speed Regulator
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This circuit shows the concept I used in the drill motor control that I submitted earlier.  I cam up with it in 1994 when I built the device.  My algebra and my patience are a bit too rusty at my age now to write an equation that describes the relationship between the BEMF of the motor and Vin.

I've seen circuits like this that use two opamps that separate the positive and negative feedback portions of the motor control signal.  They're easier to analyze mathematically.  Still, the concept is the same.  The motor get a voltage applied to it that is proportional to the control voltage, but with a sample of the motor current used as positive feedback to compensate for the motor resistance loss.

Unless you use power opamps, this is one a concept drawing.  The ration of Rb/Ra cannot be greater that the ration of Ri/Rs, or it will oscillate.  It should be slightly less.  Perfect speed control would require the ratios to be the same, but that is not realistically possible.  For one thing, brush resistance varies with wear and during motor rotation.  For precise control, you canpt use carbon brushes.  Not that cassette record motors have wire brushed in them.

If you amplifier supports it, this concept allows for bidirectional rotation.

More:  Control CircuitsMotor Control Circuits Regulator Circuits  

 

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