How to control AC motor with stopping points?

Question

Basically, I want to control an AC motor where the end user will only have to push three buttons. One button would be "lower", in which the motor would turn for a certain amount of time and stop at the desired time. The other button would be "raise", which would turn the motor in reverse for the same period of time and stop. And the third button would be "stop" to stop the motor at any point along the way.

To describe what I'm talking about, basically I'm making a electric projector screen. So the user hits "Lower" and the screen goes down to the exact point it's supposed to.

Thanks for any help!

Answer 1

That's going to be fairly tricky and expensive.

Why not just put a small 'microswitch' at the limit(s) of travel and use it to shut off the motor?

Lots of circuits out there to do that.

HTH,

Doug

Answer 2

A positional reference indicator via switch or other kind of feedback is required. This could be hardwired, where as when the switch indicating a given position and the desired position is selected is true then the motor would stop.

Or a programmable controller along with a device, such as a encoder mounted to the motor, could provide the necessary system you are asking about as well. The program can be more sophisticated and provide speed control, any continuous stop location and what action to take if the stop position is passed, possibly a fault

Answer 3

you really don't want to control this with timers. You want to use limit switches to stop it when it reaches the limit.

Otherwise it would stop at the wrong spot as the motor bearings aged, or if the line voltage were off a bit, or the track grease wore out.

And it you are going to design something this complicated, you need to think of all the contingencies, and more.