Hi Dave,
Been thinking about your question. What you are looking for is not, I think, just a revision. It is pretty much a re-design & engineer job.
I think, and I could be wrong. But the system you have described so far leads me to believe that is a part on a motion control setup. It uses a stepper system in place of a servo. The simple replacement of the stepper driver will be insufficient.
In a stepper based system, a "move" command in generated within your processor. That command generally takes the the shape of "move this far and do it this way, increase velocity at this rate, to this speed, then decrease velocity at this rate, then stop. The processor then ouputs a series of pulses to the stepper driver, which then drives the stepper motor. The amount of pulse (square waves) will denote the distance that the stepper moves. The frequency of the pulses will control how fast, or slow, the stepper moves.
A very simplistic example is this;
(A) You have a stepper motor that takes 10 steps(pulses) per revolution.
(B) You want it to rotate 1 time.
(C) You want that revolution to take place in 10 seconds.
(D) you can't start or stop it instantly because of inertia. so, you must accelerate & decelerate at some rate. Say 3 second each.
(E) So, 3 seconds to get to speed, 4 seconds at speed, and 3 seconds to stop. 10 seconds tot
(F) The processor will then ouput a string of pulses at a frequency needed to get to speed in 3 second. This string of pulse will start at 0Hz (stopped) and increase to the frequency needed to drive the stepper at the desired speed for 4 seconds.
(G) The processor will maintain that frequency for the desired 4 seconds
(H) The processor will then start to decrease the frequency of the pulses back to 0 Hz. At a rate to achieve 0Hz in 3 seconds.
(I) End of Move
The "accel/maintain/decel" is called the "profile"
Now in almost all stepper applications, the processor, simply assumes the motion has taken place. It has output the required pulse train and doesn't care if the move has actually taken place, it will then move to the next command line. The exception to that, is a stepper system that has some kind of feedback device tied in to tell the processor that the move has actually taken place.
Now, the processor does some other things also. It sets direction of rotation, sets brake signal (in order to hold the motor shaft in position) and monitors for motor over currewnt, amoung others. it has buried within it all of the motor characteristics and the various algorithms needed to take that data and turn it into something useable.
That kind of takes us to what do you need to change motors?
1) You're gonna need to change that pulse train into some other kind of signal. An analog voltage or digital word.
2) Put that other signal into something that will turn it to power that can be used by an AC motor. A drive of some type
3) Reprogram your processor. Since it is no longer controlling a stepper and AC motors are a lot different breed of motor.
4) Introduce a feedback device into your system since that movement assumption is gone. More programing.
5) Install some kind of brake, if drift is a factor
6) Probably some other things that don't immediately come to mind.
In your orignal post,it looked as if your system was comprised of a processor simply turning on a transistor to supply DC to your motor. If that had been the case, the relatively simple insertion of a relay(s) would have worked fine. As you can see, the replacement of your stepper, is not a relatively simple thing.
Those do not make up simply a revision in my opinion.
You would be as well off to toss your motor control scheme out and start from scratch with an AC motor in mind
As I said, I could be wrong. I have been quite a bit over the years. There are a lot of very sharp, experienced folk in here that might be able to suggest some other course to you. And in the process correct me. I welcome any replies here. If I'm wrong or there is a simple way to change a stepper over to an AC motor. Please, I welcome the enlightenment.
BTW, being unemployed at the moment, I'd have loved to taken your offer.
