counting pulses of a dc motor

Hey guys,

i have a 12v linear actuator that is ip67 rated that will be used underwater. I want to keep track of it's position. problem is i can't compromise the unit at all. I contacted the supplier and they said they figure out the distance traveled based on the dc motor's pulses. I did some searching on the forums but got flooded with pwm posts so i'm looking for some advice from you guys. How would i count the pulses of a dc motor?

Do you have any information like a datasheet? Or a picture? Nr of wires?

It is difficult to see from here ;)

sorry about that. http://www.lencomarine.com/actuator.html

it's just a dc motor, 2 wires.

By reading the back EMF from the coils? I think this is how HDD controllers read the RPM on three-phase motors. I've been meaning to try this as an alternative to using hall-effect sensors on a POV clock.

hmm, i was planning on using relays to control this instead of a pwm h-brdige. after researching it seems you need to read the voltage during a no duty phase. i dont think that is possible with a relay. I'd need some sort of high amp h-bridge for back emf measuring, right?

is it possible to glue a hall ic to the outside of the case and read the differences of the motor as the coil spins?

Yeah, I'm not really sure.. All the documents I've read pertain to multi-coil motors where you can read induced current in off-duty coils even while other coils are powered. For a high-amperage motor, that would get even more difficult, I imagine. Though, you might still be able to do it with a high-power FET. Did the manufacturer not have any further info on their technique?

no they were pretty tight lipped as to how they accomplished it. the operator just said that they counted the pulses.

scott_fx: they figure out the distance traveled based on the dc motor's pulses. I did some searching on the forums but got flooded with pwm posts so i'm looking for some advice from you guys. How would i count the pulses of a dc motor?

This would make sense if it was a stepper motor, but if it's a two-wire motor presumably it's not a stepper.

The only approach I can see is to monitor the voltage or current to detect fluctuations as the coils switch over. To see whether that was viable, you'd need to put a scope on the motor to see if eiter signal was clear enough to be recognised. I tried to do this a few decades ago on a small motor and the signal I was looking for was completely lost in the noise. Perhaps you'll be luckier here.

thanks guys for the feedback. seems like this is already getting over my head and skillset. I think i'm going to try to retrofit a ip67 rated potentiometer to the pivot point. it only travels 90 degrees so it shouldn't be an issue. I think creating a mount that will hold up to the conditions will be easier then trying to figure this out on the software side.