I have a BP VMC 800/22 with a DX-32 control and Siemens Simodrive611 drives. The problem is that during a tool change the spindle seems to be searching for the proper orient position but never finds it. The spindle will go both CW and CCW while searching for the proper position. No alarms are set. Sometimes the spindle keeps searching even during the tool change. (tested without any tool loaded) In a possibly related observation, the spindle seems to search for which direction to spin at start-up, changing direction once or twice before going to full speed in the proper rotation.

I don't use the machine everyday but it had been working fine. Then the PC would not boot which turned out to be a bad motherboard. I changed that and got the machine back into service. Now this issue.

What I have done so far:
1) Removed the spindle encoder and tested it on the bench. All signals are fine and work according to the spec, at least the best I could tell.
2) Ohm-ed out the harness from the spindle encoder connector to the DB-50 connector on the BMDC(main embedded controller) card and all lines appeared to be fine.
3) Swapped the BMDC with one from my other mill. Tested by commanding a spindle speed and then a tool change, the operation of the spindle is exactly the same.
4) I ran the spindle at 1500rpm and the speed feedback was right on as displayed on the user screen.
5) I attempted a tool change with the spindle encoder disconnected which resulted it the motor going to full speed, which I stopped asap.
6) I put a scope on the analog speed control lines to the Siemens controller and this what I get during a tool change:


7) I tacked wires on the BMDC connector to look at the encoder signals while running and everything looks normal. I confirmed all six signals are present and accounted for.


So, what am I missing? What else could cause the BMDC to control the spindle in such a manner? It looks like the control loop is "broken" and it just keeps over shooting the target. It is seeing the encoder or the speed feedback wouldn't be accurate. And it must be seeing it or it wouldn't be driving the spindle that way during the tool change. It has the ability to stop the spindle since I can turn off the spindle from 1500 rpm and it stops almost instantly.

There are only two controls from the BMDC to the Siemens controller, the differential analog speed control and a control relay that disables the spindle and applies the brake. It appears that the BMDC just can't figure out where to stop the spindle. I did go into the maintenance program and found the parameter for the orient position which looked fine, the same as it was when I got the machine.

Any ideas?

Thanks for any help provided.