fanuc servo motor moving without command

[mtech]

Machine: Leblonde Makino/4 axis horizontal mill/ Fanuc controlled with Proffessional 3 software.

Has anyone ever had a problem with a servo motor moving by itself without any command. The 4th axis (table motor) is moving itself and then faulting out. It's been giving drive fault error 416. I dont remember the description of that alarm but I think it said something not being detectable. Sounds kind of like an encodor problem or a wrong drive parameter perhaps. But when it does this, the encoder must be working because I can see the positioning data changing. Crazy isn't it. The motor is brand new. the old one was faulting on 414(excess error). There are definately no buttons stuck or anything that would tell it to move. It even moves when it's not in any of the manual traversing modes (ie jog,rapid, or handle) It will simply move if the servos are on. But it always alarms out after a second or two with the 416 fault. I play with it a while and finally it will reference out and then it will run. I have had the speed of the motor slowed down to 1000. Any help on this subject will be greatly appreciated.

[HuFlungDung]

Hi Mtech,

I know it can be a lot of trouble to do, but the way I try to isolate such a problem is to swap devices until I can narrow down which item the fault seems to "travel with".

Eg: swap motors with a different axis to see if the problem moves with the motor.

Swap drive amps.

Swap cables.

The controller could also have a fault, but if none of the above swapping seems to indicate any device as the culprit, then suspect something in one of your cards. The factory should be able to advise you once you have completed all these tests.

Shot in the dark: possibly retune the offending axis. You might have to check the documentation for your drive amps to locate if there is a balance pot on each amp. This setting can be tweaked to find a spot where the motor does not drift either direction. If I recall correctly (IIRC), this setting is done with the amp gain initially turned way down, and the command signal (+-10V) temporarily disconnected. The motor should not rotate without the command signal, and the balance is what sets this. The gain can then be turned back up to see if the setting holds. This test might be most meaningful if the motor is not attached to the load, to help remove friction from the equation.

Then there is the retuning procedure, but AFAIK, none of the PID settings actually will cause a motor to drift.

After this, then you should carry out the PID (proportional, integral and derivative gains) retuning procedures. Compare the current parameters for this motor's PID with the settings being used for the other motors. Similar motors driving similar loads should have similar PID parameters.

Just going from memory here (I'm at home right now):
Integral gain is very touchy, and should have a conservative setting from 0 to 5.

Proportional gain is not so touchy, and could have a range somewhere from 25 to 100.

Differential gain is least touchy, and could have a range from 500 to 2000

[wms]

Mtech,
You said in your post that the servo is brand new and that the old one was alarming out with a 414 error. That said, I would suspect either a bad cable or a bad driver card. I say this because you changed the servo and still get a positioning error.
Did the new servo come with the encoder, or did you install the old encoder on the new motor? If it is a new encoder then I would look at cables or drive cards. If you used the old encoder then that might be the problem.
Also you didn't state what model Fanuc. Some models the servo drives are on the mother board. Some the drives are seperate.
If they are on the main board you can swap conectors to trouble shoot.

As HU said I would swap drive cards to see if the problem follows the card. Then take a hard look at the cable to see if it is damaged. Look for wear on the outer sheath and look for cracks that could let chips and coolant into the inner part. Also look to see if the cable is bent around somthing that could cause the inner wires to be broken and shorting out. I have seen a broken wire in a cable assembly do some strange things, over load alarms, following errors, run aways, ect...

[mtech]

Thanks for the info. I will try these ideas Monday when I go back to work. One thing I forgot to mention is that when the motor moves on its on, its like it is being jogged. It's not just drifting off slowly. Its actually moving considerably and at jog pace. I dont think these drives have a gain pot on them. These are digital servo drives. I've seen the drift problem on our other machines which have siemens drives. They actually have a gain pot (Kp) which I ocassionally will adjust. I dont know how much and how fast they can make a motor move but in my experience it is rather small.

[SRT]

Mtech,
Be careful, will the E-Stop Buton "stop rotation" when pressed, or does the run away continue. It sounds as if in your description, it is moving at rapid G00 speed, is that correct??

[JIMMY]

I had the same problem with a cnc jig grinder with fanuc control. I called the company that handled problems for that machine and they told me that is the servo board out of adjustment. They told me how to do it over the phone and I adjusted it right in, they are very touchy. I hope this might help and lead you in the right direction.