Andrew Morris had a problem with a small rotary tool built in China. The tool fit nicely in the hand and was useful for precision cutting, drilling, and polishing. Yet for delicate work, the tool was in bad need of a speed regulator.
Andrew had developed an analog motor speed regulator back in the mid-1990s, but this time, he wanted the benefits of a digital regulator. The digital version was just as efficient, but it was less expensive to build and easier to assemble. The digital circuit also provided more torque.
Andrew Morris' microcontroller-based DC motor speed regulator brings control for delicate work.
Before this article was published, when I added R10, I sent the editor an updated pdf file and for some reason, the first pdf file was published. I have resent the corrected file twice and it has not been posted. R10 is a 22k resistor connected between Q2's drain and ground. Please email me if you would like the updated pdf file.
Mrdon, the motor controller had one section of a 556 generating a triangle wave from 1/3 Vcc to 2/3 Vcc, the other section used a potentiometer to vary the reference for the second section that is used as a comparator. The signal from the comparator feeds a small transistor that drives a TO-3 power transistor.
The circuit to generate the PWM signal came from an applications note. So it is really not anything that complex. I may be able to find a copy of the circuit someplace in my archives, but that may take a while.
OK, it was a variable speed drive. What we used for feedback was the effect of the pump delivery rate, which was controlled by the motor speed. It was not clear that any feedback was used with the tool speed controller, either. Small tools seem to work better if they do slow as the load increases.
The other thing is that a motor controlled with PWM may not have as much speed drop as the load increases, since PWM control does not increase the source impedance.
William K. Thanks for your response. Was your device a controller or a regulator? In other words, how did you sense the motor speed? I can build a DC motor controller with a 555 timer, a pot, a MOSFET and a few small components. Such a device would not be a speed regulator as it would have no feedback.
When it comes to learning PIC microcontrollers, Mike Predko is an excellent teacher. I have several PIC microcontroller books by Mike and his writing style make developing microcontroller applications a breeze. His books also provide solutions on debugging software errors as well.
In 1979 I designed an inexpensive PWM speed controller for use with a 12 volt DC motor. The control kept the torque at maximum because of not having a voltage drop, other than the Vce of the switching transistor. And the design was not patentable, so I was home-free there as well. It use a 556 dual timer chip with one section as an oscillato9r and the other section as a comparator for the ramp waveform and the setpoint voltage. One intermediate transistor buffer stage and it drove a 100 volt 5 amp switching transistor, which I don't recall the number. The BOM cost, except for the control pot, was just under $5. Probably all of the parts are still available, and there is probably a much better output transistor available today. Possibly even a FET of some type.
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