Problem- 6t Servo tuning?

[Jerry Shorter]

Anyone out there have instructions on how to tune an axis servo on a 6T control?
Problem: The Z axis occillates when it reaches position. after a couple of seconds the occillation stops.

Jerry

[Dan Fritz]

Sounds like a gain problem. The first things to check are the tiny jumpers on the servo amplifier board. These jumpers are intended to compensate for the differences between the types of motors. If, for instance, you have a model 5 motor and you're jumpers are set for a model 10 motor, you're velocity loop gain will be off my a mile. If you've replaced a servo board and didn't pay attention to these jumpers, that might cause trouble.

You also have jumpers on the big 3-phase transformer that supplies power to the servos. The line voltage on the primary side is cut down to the correct voltage for the servos. When you move a machine from one shop to another, you may have to adjust these jumpers to adjust for your shop's line voltage.

Assuming that these jumpers are set correctly, you can then begin actually adjusting the servos. It's really necessary to have a small oscilloscope and a good digital voltmeter to do the job right. I suspect that your jumpers are off, so please post the info on your motor type (both X and Z). Then, measure the AC voltage on the terminal strip on the back of the servos. Measure AC voltage between the terminals "A", "1" and "2". This is the 3-phase from the transformer to the servo. Check between "A" and "1", then between "1" and "2", and then between "A" and "2". Post your results and maybe we can help you further.

[fanucman]

i think the only way to do it is to adjust the gain know on the top board of the servo drive..

[Jerry Shorter]

Thanks for the replies guys.
Ok here we go Dan.

Servo Motors: Type 0N-2500M, A06B-13-B-032 (I think)
no. D-071711 (X-Axis)

Type 0N-2500M, A06B-13-B-032 (I think)
no. 0N-2500M (Z-Axis)

Servo Unit: A06B-6045-H001/2
Mfg # T18

Servo PCB: A20B-0007-0360 / 03A (X-Axis)
A20B-0007-0361 / 05A (Z-Axis)

Voltage: 68 Vac A-1 (X and Z)
68 Vac 1-2 (X and Z)
68 Vac A-2 (X and Z)

The A06B Motor numbers were derived form the service records.

Thanks, Jerry

[Dan Fritz]

Jerry,

There should be two sets of 8-pin jumpers on each of the servo boards. According to my 6T-B maintenance manual, these jumpers are labeled from "S1" at the bottom to "S16" at the top. Each set of 8 jumpers is about the size of a regular IC socket, and the jumpers are small white plastic bars short togther two of the pins on opposite sides of the socket. You will also see some smaller individual jumpers that each plug into two tiny pins. These are labled "S17" through "S22" on the board. If you see that your jumpers are set differently from what I show here, write down your current settings before changing them, just in case your changes make things worse.

Since both your motors are type 0N, and you have pulse coders inside the motors (the "-2500" in the motor number means it has a 2500 pulse/rev. encoder), you need to have these jumpers in place:

S1 shorted
S2 open
S3 shorted
S4 open
S5 open
S6 shorted
S7 shorted
S8 shorted
S9 - S12 open
S13 - S19 shorted

The S20 jumper is only used to bypass the TGLS alarm, but the manual is unclear about whether it should be open or shorted for normal operation. Just don't change it , whatever it is.
The S21 jumper will be shorted on the board with the -361 part number, and it will be shorted on the -360 board. This is because both boards share the same power supply.

The small jumpers labeled "S23" to "S28" seem to be normally in the "Shorted" position

There are some potentiometers (the little blue adjustable resistors). The one marked "Gain", or RV1 should be set to about half-scale (50%). The one marked "Offset", or RV2, should also be about 50%. If they are much different from that, set them to 50% as a base setting.

Now, on the Fanuc 6T-B main board, there should be some 8-pin jumpers labeled "SPA6" and "SPA7". These are for setting the pulse coders in the motors to function as a tach-generator. For both your axes (X and Z) set them this way:

SPA6 and SPA7 (both the same):

pin 1 (bottom pin) - open
pin 2 open
pin 3 open
pin 4 shorted
pin 5 open
pin 6 shorted
pin 7 open
pin 8 (top pin) open

If your jumpers are different from these, write down what they're set to and change them.

Your voltage levels for the servos (68vac) is OK. there's no need to change it.

There are also some parameters in the Fanuc 6T control that can cause a gain problem. I can't tell you what these parameters SHOULD be set to in your case because they're different for each machine/motor/ballscrew pitch/pulse coder combination. The only think you can do at this point is check to see if these parameters are set as they were originally set from the factory. The parameters in questino are 086, 087, and parameter 090. Normally, parameter 90 is set to "30" for a gain setting of 1.666 sec-1, or 1.666 in/minute/mil. The parameters 086 and 087 are multipliers that compensate for the pulse coder and ballscrew pitch. These parameters MUST NOT BE ZERO, or you will have certain trouble with servo gain.

I hope this information is helpful.
Dan Fritz

[Jerry Shorter]

Thanks Dan for the detailed reply,
All of the jumper settings are as you described.
parameter 86 is 714 and 87 is 1734,
parameter 90 is set to 3000,
All are the same as in the factory parameter sheet.
I set RV1 and RV2 as you described, but I can't seem to get the motor to stop occillating.

Update: upon checking the current waveform with a scope.
The Z-axis signal (problem axis) jumps all over the place, positive-negative-positive, etc. All three phases are there
they are just not stable.
X-axis the signal is steady, all rounded peaks on the positive side.

Jerry