Problem- Amada w/Fanuc 04PC ocsillation at stop

[Mister NCT]

We have an Amada Pega357 with Fanuc 04PC control and older style drives (look like the DC drives) that has an oscillation when it stops ONLY when moving in the -Y direction (away from the servo motor, going towards the turret). You can see the feedback oscillate in the 800 diagnostics, it starts at about plus to minus 30. It is worse at high speed and not noticeable at low speed. The operator said it developed over about a months time. Here is what I've checked so far:

Swapped the X and Y drives (identical)
Swapped the Y motor with another from another machine
Swapped the motion board
Checked the 8200 series parameters
Checked for mechanical resistance in the Y axis by disconnecting the ball nut from the yoke (takes 16 lbs to get it to move along the entire length and in either direction as compared to 25 lbs on an identical machine)
Found the Y ball screw had .012" lash so replaced ball screw, nut, and end bearings and adjusted to zero at each end with .002" sag in middle. Adjusted gear lash. No change.
Ball screw appears to deflect down at the mid point between the drive end and the ball nut but have no way to measure and not sure if it's an optical illusion.

This is a real puzzler and the boss is looking at me cross-eyed. Has anyone one seen this before? Any ideas? Is it time to sacrifice a goat and spread it's entrails across the machine? Any help would be gratefully appreciated!

[Dan Fritz]

Sounds like the tach signal is not stable, or some velocity loop gain (or position loop gain) setting is not right. Most of these Fanuc models used a small black hybrid circuit on the main board (not the servo boards) to generate an analog tach signal from the pulse coder signals.

Look on the main board of the CNC for two black square ICs labeled "A-PCO6". You should have one for each axis. One is for the X axis, the other is for the Y axis. The output from this IC should be a pretty "clean" analog signal that will be 0v when the motor is stopped. For the Fanuc model 0 and 5 motors, you will see +3v when the motor is running at 1000 rpm, and -3v when the motor is running at the other direction at 1000 rpm. If you have the larger motors (model 10, 20, or 30) the voltage will be 6v per 1000 rpm. You can see these same signals on the servo boards, possibly labeled "TSA" or "TACHO" on check-pin CH2

If the PC02 is bad, you may see the Y servo jump around a bit when you tap on the IC with your finger.

Another possible cause of this oscillation is a bad oldham coupling inside the motor. Since you say that you've swapped out the motors, that's probably not the problem.

Please post photos of servo boards and main board along with part numbers, and maybe I can help you more.

Dan Fritz

[Mister NCT]

Dan, thanks for your help. Here are the numbers:
Its a 1990 control.

Main bd: A20B-2000-0180
Drive unit: A06B-6057-H403
Velo Bd: A16B-1200-0670
Axis bd: A16B-2200-0360
Mtr: 10L A06B-0572-B003

I was not able to find the IC "A-PC06" on either the main bd. or the axis bd. The channels on the drive are:
CH1 - *PWMA
CH2 - *PWMB
CH3 - *PWMC
CH4 - *PWMD
CH5 - *PWME
CH6 - *PWMD
CH7 - IR
CH8 - IS
CH9 - 0V
CH10 - +5
CH11 - +15
CH12 - -15
CH13 - +24

BTW, I still have quite a few customers using PCDNC2 on their older tape machines - nice software!
Tom Eads