PID tuning once again

Hi,

I have servo system with absolute linear scales. Control is analog +-10V. I did some servo tuning today. And I used halscope as an aid. After getting closer to my goal. I noticed that there was saw blade form in following error. Is this normal ? Is it possible to get rid of this ? I can get this following error centered by adjusting FF1. And I can get accel/decel decent with FF2. But no matter what I do it seems this saw blade on ferror remains. Also I noted that there is moaning noise(like train horn) always when moving this axis to +direction when moving to -direction this noise is not present. Video here:




Tero

I would first try to isolate the problem to feedback or motor drive/analog signal related

If you set the P,I,D terms to 0 you will have nothing but feedforward terms (feedback is ignored)
In this case if FF1 is correct, motion should be approximately correct ( a very small P term say .01 will keep it from drifting too far)
if you still get the sawtooth, then its likely a motor drive issue (velocity loop gain too high in drive?)
or possible a analog signal issue

I will try these ASAP.I have connected analog signal to 7i83 by shielded twisted pair wire shielding connected to 7i83 side only. I have similar connection on Y-axis and it does not have this sawing effect. Are mechanical issues possible? Ie. binding bearing ?

Tero

Yes, could be mechanical, motor drive tuning, LinuxCNC tuning

(you can eliminate LinuxCNC tuning or encoder issues as a suspect by zeroing PID as I mentioned before)

Ok,

Here is a pic of scope after I tried my best to tune:

And here is a screenshot wityh P=0.01 I=0.0 D=0.0 AND FF1=0.055

Tero

As a guess, it looks like a drive tuning or mechanical issue (not LinuxCNC tuning)

Maybe really bad stiction?

Ok,

I will try to locate the problem tomorrow. Ball screw&nut combination has backlash of about 0.1mm. But that should only have an affect when changing direction. Since position feedback is with linear scale. But I will be wiser tomorrow(maybe)

Tero