Stepper Motor Spontaneously Reversing Direction

[JasperBurtonIII]

Hey guys, sorry if this is in the wrong forum but I figured this was a bit of an engineering question. I have an 8'x12' multicam that has been retrofitted and frankensteined heavily that we use at work. A while back the motor on the x axis started acting up so I bought 3 identical motors to replace it. The old motor had been getting hot (170+ deg F) and I figured it was just old, so I replaced it and all was fine. After 6 months it started to get hot like the old one and when it reached 160F it would begin to reverse direction suddenly. So I replaced it one more time with the newest one, but this one doesnt even get warm before it begins to go in the opposite direction.
Initially all these motors would work properly after turning on the machine, and heck, some days I could cut for hours without anything going wrong, but lately it is just not wanting to work with me.
So my question for you engineers is what on earth could cause a stepper motor to reverse its direction so suddenly? I'm not an engineer and to be honest currents and magnetism were not my best subjects in school, though I do have an alright grasp on it, but this problem is far beyond my field of study. Any help is greatly appreciated.
Thanks
Jasper

[jalessi]

Jasper,

Most likely the direction wire is intermittent, possibly broken inside the cable. Wiggle the cable and check for loose connections.

Check the cable well in any area where it flexes.

Jeff...

[JasperBurtonIII]

Well I guess its time to get my hands dirty, its gonna be a lot of fun trying to run some new cabling but its probably the best direction to go. I was hoping someone was going to tell me to flip a switch and be done with it. So a broken wire could be causing it to go from moving in the proper direction, a sudden stop, and then cause it to go in the opposite direction or cause it to suddenly change direction?

[Andre' B]

I had a home built unipolar stepper motor driver that was doing that. Turned out to be one of the power transistors in the driver was not getting turned completely on and under the right conditions the motor would change direction.

[JasperBurtonIII]

Well, I replaced the wire leading from my driver to the motor, I don't know about power transistors and what not, but I would assume that on a factory built driver this wouldn't be a problem... am I wrong?
The machine is running a program right now so we will see how long this works...
Anyone care to explain what exactly happened, if there was a short in my wire, that would make the machine suddenly think left is right? When it does go haywire, it tends to keep its backwards thinking rather consistently...

[jalessi]

Actually it is very simple.

A high pulse "+5 volts" tells the motor to move in one direction, the absence of a high pulse "0 volts" tells it to move in the opposite direction.

Imagine that the X axis moves right when it sees a high pulse "+5 volts" and left with a low pulse "0 volts".

If the axis is moving to the right and the wire opens it will immediately change direction and move to the left.