Project Controller!

[Iawia]

Hi All,

I am trying to build a motor controller and would like some advice. I have a knob (rheostat), that I would like to use as my speed control point.

I am using a PI controller made of op amps, and a power amp to feed the motor. I coupled a feedback motor to the plant motor. Measuring the back emf from the feedback motor will provide me with enough info about the motor's live speed and will feedback into the controller.

my question is: how do I supply a positive and negative voltage using a knob (rheostat) to change directions of the motor?

 

[ADWSystems]

Wow. Since you asked the question, I'm double thinking myself. A pot has three terminals, either side of the resistor and the wiper. Using a pot with a resistance value high enough it won't burn out, connect the resistor between V+ and V- and the wiper will show you a positive and negative voltage relative to ground. The key concept is relative to ground (0V). So make sure you have ground as well as V+ and V- through out the circuit or it's not goung to work.

 

[misterT]

I am using a PI controller

Motor controller should be a PD controller. Maybe just a small integrator constant. Motors have an inherent integrator in their transfer function.
PI controller might be ok for your application though.. You did not mention what exactly are you doing. Is it a servo control or a "steady speed regulator"?
ADWsystems answered pretty well to your original question. I have no idea why he is "double thinking". The answer was good. Maybe he is confused because you are making this fancy motor controller and measuring back emf etc... all these sophisticated things. But then you are asking how a potentiometer works.. that is strange.

 

[Iawia]

Hi ADWSystems,

Thanks for your answer. That worked perfectly.

And misterT, you are correct that there is inherent integration, but only if it is a position controller, which this is not. it is a speed controller. and not all people who use this forum are ee trained, like myself, hence the seemingly dumb question. I am a mech eng college student who really doesn't know how a pot works ... but now I do! thanks.

 

[misterT]

And misterT, you are correct that there is inherent integration, but only if it is a position controller, which this is not. it is a speed controller. and not all people who use this forum are ee trained, like myself, hence the seemingly dumb question. I am a mech eng college student who really doesn't know how a pot works ... but now I do! thanks.

I understand you.. I did not mean to be rude. I just wasn't sure if the problem was really that simple There are no stupid questions, just.. well anyway, you are right about the difference between position vs. speed controller.