Help, Z axis runaway

[Nicholas]

Hi,
I have a 1987 comet mill with a trak cnc2 controller.
When trying to jog the z the servo runs away.
I have tried swapping drive boards and also tried using the y axis servo.
Same result. Is the controller at fault.
Any ideas
Thanks
Nick

[Torchhead]

Servo run away typically occurs from two sources: Bad (shorted) drive FET(s) or unplugged/defective encoder. Since you swapped drives and it still happens that leaves the encoder. For instant run away to occur you have to typically lose both encoder channels. If it starts up slow and accelerates to full speed it would indicate one channel is defective. If it instantly goes to full speed you have lost both channels.

The above conditions are for systems using standard TTL encoders and where the servo loop is closed in the motor drive. Not knowing your specific system I can only generalize.

tomCAUDLE
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[One of Many]

I'd start with the feedback loop including ringing out the cables for continuity. If there is a tach and encoder inside the servo end cap. Verify they do function and have proper output going to the boards inputs.

Maybe remove the belt to the Z leadscrew until it acts like control has returned. If there are home and over-travel limit switches, trigger them by hand for the time being if need be during testing.

Swapping servos may give a false indication if the 2 tachs were set up with opposite polarity. It can be the little gremlins like that, that just might have you chasing your tail.

DC

[Al_The_Man]

If you have tach feedback and they are DC servo's they are most likely velocity loop drives, (they look like SEM DC) in that case you have two control loops, the inner one from tach to drive (velocity) and the outer one from encoder to control (position).
To eliminate the control/encoder loop, remove the analogue control to the drive. two wire input.
If you leave this unconnected or preferably connect both terminals together to simulate zero command, then the servo should remain stationary and oppose any resistance to turn.
If it runs away, then it is tach/drive problem.
Al.

[Nicholas]

I have a nine pin connector on a shielded cable that goes to the servo.
Pulled the cover off the back of the motor and found three wires go to ground, one connector has four wires and the other connector has two.
Tom,
The motor runs full speed from the start about four inches of spindle travel then brakes suddenly. I removed the belt.
DC,
The wires ring out ok.
Al,
I'm dont know if I have tach feedback or not. Looks like I have some learning to do. I've been searching the archives here .
Would the two pin connector be for the tach output you mentioned.
I pulled the two pin connector and its the same. except the speed is much slower
The plate on the motor reads: dc motor 70v indiana general 4040D-38
I pulled the back off the Y motor and everything looks identical. The X and Y axis also have wheeled travel dial type encoders.
The Z axis DRO seems to work ok
Thanks
Nick