Servo setup with integrated PID

Im having a few issues which I believe are related to each other.

First the setup Im using is a Mesa 6I25 with a 7I77 daughter card which I am using to drive Mitsubishi MR-J3 -* servo amps running matched brushless AC servos. This is currently set up to run in velocity mode.

Now the main issue that I am having is that while under LCNC control the servos are very spongy while at idle. I can easily get a few degrees of shaft deflection just twisting the servo shaft with my hand. I have tried adjusting the P values and FF1 values, this makes the servo more responsive to movement commands but so far has had no affect while idle. The MR-J3 servo amps have their own PID loop and hold the servos rock solid on their own.

Ive tried searching here but so far have not come up with much other than to use P and FF1 when dealing with an integrated PID device. Im still Pretty green with using LCNC so any advice would be helpful.

Jim

You might post your config files.
also what version of linuxcnc are you using.

You need to tune the PID loop - including I and D

P is an adjustment based on error - so there must be an error before it responds (read spongy/laggy response)

FF1 helps because it does not require error. You can think of it as stepping on the cars gas pedal half way when you want to go 100 kmh - you know you must press at least this hard to get there (then the PID has less error to deal with)

I is an adjustment based on error and time - if there is error over a time period it tries to correct it. This makes the response stiffer.

d dampens the response usually allowing more P which minimises error again

ff2 apparently can help on the initial acceleration and final deceleration

all the settings overlap somewhat - PID tuning is a bit of a black art.

I'm sure someone will pipe in with more experienced advice but sounds to me you need some I and d.

i see our official docs are very lacking in PID tuning info - doesn't even mention ff1 and ff2 (nor tuning current mode amps)
Here is JT's
gnipsel.com/linuxcnc/tuning/servo.html

Also if its really easy to move the motor shaft in velocity mode,
it suggest that the drives velocity mode gain is too low

Ok, after a bit of playing here is what I came up with. While tweaking the settings I noticed that the position display was not counting during a move command even though Hal Meter was showing the encoder count incrementing. I reran Pncconf and overwrote the old configuration. I now have a working position display and the servo is responding to PID tuning. Im not sure exactly what, but the old configuration was buggered somehow.

Cmorley, thanks for the link to the servo tuning info It will be a big help when I get down to seriously tuning the system. For the moment everything is just servos sitting on the bench while I try to get everything playing nice together.