[lesnesman]

At work I use a Jet knee mill with a prototrak age 3 control.
The z axis was throwing a ton of errors and we decided to crack open the motor and see what was going on.

We took off the cover. The brushes were severly worn and the case was full of carbon dust. We dumped out the dust and replaced the brushes. We didn't turn the armature but there was enough diameter to do so.

So we put it back together and started up the control. We were expecting great results. But when we hit "go" the z axis would go full speed ahead until it hitt a stop then the control had an error.

Someone told me it sounded like a "runaway condition" and that there was probably a short in the motor. There is an encoder on the quil and an encoder in the motor housing.

The documentation with the control says nothing about troubleshooting and has no schematics for testing the motor with a voltmeter. Southwestern Industries suggests that we order a rebuilt motor for $700 and replace the whole thing. I would really like to know what happened.

Anybody have a similar experience? Anyone have any additional info about the magnetec dc motor that is used on the prototrak retrofits? Thanks -Les

[Al_The_Man]

Does the motor have a tach? if so spin the motor and the tach output should be a linear DC voltage, either -ve or +ve depending on direction.
Most modern controls detect when there is a large positioning error as in a runaway condition and will shut the drive off before hitting a stop.
BTW.The motor will also have a linear voltage output with rpm, if you spin it somehow.
AL.

[lesnesman]

there is no tachometer on the control or on the motor. I did look inside the other end of the moter case where the encoder is housed. There is a plastic wheel with lines and a sensor. Also there was a circuit board. I blew out the dust in that part of the housing then cleaned the wheel with alchohol. Put it back together and it changed nothing.

The z axis motor is very easy to unplug from the control. It's just a big military style connector. The machinist suggested plugging the z axis motor into the control x or y output to count out the control as the problem. I didn't want to crash anything so I discouraged it.

If I new the pin out then I could send power to different pins and see if I get the desired effect. This is uncharted territory for me but may be a good way of learning the basics of motor control. any suggestions are appreciated. -les

[HuFlungDung]

The runaway condition means two things to me: the encoder signal is missing or the phases of the encoder got accidentally reversed relative to the motor direction. I would disconnect the Z axis motor coupling to prevent running the axis to the end of travel, and then run another test, by reversing the leads to the motor and starting up the control again. If it holds position, then most likely improper hookup was the problem.

Of course, the motor needs to run in the correct direction, so then put the motor leads back the way they were, and interchange the A and B encoder connections.

This wiring mixup would only happen if you actually disconnected the leads and got them mixed up when reassembling everything. Perhaps you got some wires mixed up inside the motor when you replaced the brushes? If you disconnected nothing, and had this problem, then I would suspect the encoder is not putting out any signal (ie., it is dead for some reason).

[lesnesman]

The connection for the motor and the encoder that is on the quill was never touched and I verified that the connectors were in the right position on the back of the control box. It would be great to know what the pins are on the connectors for the motor.

I figured that the encoder that is on the motor was biffed too. That's why I went through the trouble to clean it up. The only way I can figure out if that encoder is not working is to give sensor power (on the right pins) and look at the ouput (on the right pins) and manually turn the output shaft. Unfortunately I don't know what pins are for what and I don't want to screw things up more than what they are now. Too bad southwestern industries won't help a little bit if someone wants to troubleshoot themselves.

After seeing how much carbon was in the z axis it makes me wonder how bad the x and y axis are. But now I'm afraid I'll frig those up too if I try to clean them out.

Makes me think that an AC brushless design is a good thing for a cnc control. Thanks for the reply. -Les

[HuFlungDung]

I wonder, if the encoder is sort of 'open construction' as you mentioned cleaning the optical wheel with alcohol, that perhaps the LED bulb and sensor might also have gotten coated with dust. With the power completely shut off to the motor and encoder, it might be best to wash those parts of the encoder off with an aerosol electrical contact cleaner. I say wash off, because the sensors will be so close to the wheel that you will not be able to clean them any other way.