DC motor as a generator as a torque limiter?

Question

Is it possible to use a DC motor as a generator to regulate applied mechanical torque? A hypothetical situation:

I have a DC motor acting as a generator attached to the ceiling. The motor shaft has a pulley with a cord wound round it. The free end of the cord is attached to a weight. How can I regulate the speed at which the weight will fall to the floor? How can I adjust the system for differing weights?

My initial thoughts are that I could simply attach a potentiometer across the leads of the motor. Varying resistance will limit the output current of the generator and thus limit the speed at which it unwinds. Is there anything wrong with this assumption?

Thanks in advance!

Answer 1

I think your basic idea is OK, but the pot idea is not very good. The problem is that you have to take the position signal and differentiate it to get speed, and noise will be a big problem.

What you need is a rotary rate sensor, and there are a variety of these. An obvious choice would be a tachometer. DC motors can be found with built-in tachometers specifically to make regulating rotation rate simple. Encoders, particularly optical encoders, are cheap these days, and your job is made easier by the fact that you don't need an absolute encoder - incremental will work just fine for this application. There are other, more exotic sensors such as synchros and resolvers, as well.

What you do is to make a feedback loop that compares the rotational velocity to a desired value and adjusts the motor voltage accordingly. Depending on how ambitious you are for your loop design, you can adjust for a wide range of speeds and loads with a single circuit. And since you're interested in controlling speed rather than position, the feedback loop can be quite simple and still give good performance.