Servo Tuning FF

I agree that for first time servo power-up you should disconnect the motor from the mechanics, This is because the first time you connect things you have a better than 50% chance of a runaway.

So _expect_ a runaway and verify that you can power things down or disable the servo quickly before starting. As John says , once you have feedback working and can jog things, then reconnect the motors to the mechanics for tuning.

I would also add that you should verify the drive enable circuitry before starting.
you want a ferror to instantly disable the drives. You will need to have larger values of FERROR and MIN_FERROR as you start to tune

Sounds like it could get real interesting then. I'm working on a cincinatti millacron 10cc from the 80's. The resolvers are mounted to the ball screws, which are belt driven from the motors, with no coupling in the ballscrew shaft to unhook to allow this.

Thanks guys


Rick

In your case your more or less stuck with a live test so make sure your E-Stop works and is handy and manually move the axis to the center of travel before powering up. As Peter said double check your drive enable.

When I tested my CHNC I made a simple box with a couple of switches, one for enable, and one for jog and the jog input was from a variable voltage power supply so I set it at like 1/10 of a volt just to make sure jog plus and jog minus went the correct way. I did find out that with the drive enabled and no velocity input there was some drift... YIKES. So the moral of that story is don't jumper out the enable from the rear of the machine unless you can see the axis.

John

It is possible to verify the feedback direction first. (and assuming analog servos here)

The very first thing is to verify that the position feedback works, is scaled properly and has the proper direction (prehaps by hand moving the motor shafts with the drives disabled and watching the Axis DRO)

to check feedback direction, you first determine which voltage input polarity drives the axis which direction This can be done with a custom hal file and manually setp-ing the analog output voltage or simply with a 1.5v battery connected to the drives analog input pins

Once you know which input polarity drives the axis in which direction write this down

Now you disable the servo drives(remove power), set a large FERROR and MIN_FERROR and start LinuxCNC with PID setup with a small P and all other terms = 0

Now when you hand crank the motor shaft the analog output voltage should change such that it would oppose the motion. That is if a positive voltage makes the axis go right, moving the axis by hand to the right should generate a negative voltage on the analog out pins, and moving to the left of the starting point should generate a positive voltage. If these are reversed you will need to reverse the PID output polarity

The other thing to check here is that the analog voltage varies smoothly from -10V to +10V as you rotare the motor shaft

LAIR82 wrote:

After reading these posts about servo tuning, I feel pretty good about tuning both servos on my turning center, but before I try that I want to make sure that I am at least close on my values in my "ini" file for my axis, before i throw power to the servos and have a major crash.

What I would do is set P to a low value, and ID and FFx all to zero, as you have done.
You will almost certainly have to increase MIN_FERROR, maybe to 0.1, assuming
inch units. make sure the encoder/resolver picks up the movement correctly
before enabling the servo. Then, attempt to jog. If it jumps and gets a following error,
you can reverse the motor leads, or the polarity of OUTPUT_SCALE. You may have
to increase P to get it to move at all, it depends on the system. Then you have to
get into tuning with Halscope and the calibration menu.

Jon

Everthing looks good now in regards to the readout/actual values on the display, we had it reading backwards and we were off about 0.100" per inch of travel on the display. I have encountered another problem though, I am not getting any command voltage out of my 7I49 pwmgen output. The enable is working properly on all 6 circuits on the card, and I am getting the proper command out of pwmgen.00 but nothing out of 01-05. I have the spindle on 00, Z on 02, and the X on 04. We did it this way because when we had X on 01 next the spindle we were getting noise between the two on the display.

I have read the thread "Mesa 7i49 Board configuration" from 4 months ago twice to try to remedy but cannot get this figured out. His situation is pretty identical to mine. I have gone through my hal file over and over again with a fine tooth comb and cannot find anything. Talking with my boss he was wondering if maybe the firmware is the problem.\

Thanks

Rick