[SOLVED] strange PID behavior. Closed loop Servo drive system. EXE impedance

Hello community,
I am currently tuning my axis and ran into a strange behavior.

I am tuning my PID and used I/D/FF0/FF1/FF2=0 and started with the P value.
I increased the P value and could see the following behavior:
P=1    follow error = -5
P=2    follow error = -4.5
P=4    follow error = -3.3
P=6    follow error = -2.2
P=10    follow error = -1.8
P=15     follow error = -0.8
and finally
P=20     follow error = -0.22
When I arrived at P=20 the axis started to accelerate and deccelerate strongly.
I first thought that the encoders are giving strange feedback, but the position graph in the hal scope looks flawless.

 
 
 

I checked the PID inputs and I realized that the encoder uses velocity not position.
I assumed velocity and position are linked by the first derivate, but the new screen shot shows, that position looks good but velocity goes to 0 when the strange behavior occures.
This seems to be the problem.
Can anybody tell me how this can happen?
Computing power?
I run my glas scales with Heidenhain EXE 612 and 10 fold interpolation.
so 4000 pulses per mm.
 

Hi Andy,
I did some further investigation. Indeed the position value stays constant for the time in question.
Am I correct assuming that even with 10 fold interpolation of the encoders the input frequency should not be a problem for my MESA card?
I ran my tests with 1000mm/min so 16.6 mm/s and 66.6 kHz input frequency to the encoder input.
I will run test with 5 fold and check if the powersupply is sufficient. I use very thin cables that are shielded, even for 5V. Maybe this leads to a reset of the EXE. I will report.

It really looks like the encoder signals just stopped...

The 7I97 encoder inputs should be good to about
400 KHz with default settings (filter on and default sample
rate) and up to a few MHz if adjusted for maximum count rate.

I swapped the EXE of the Y and Z axis with the X axis. Same behavior.
I can go down to 50mm/min and the behavior still persists.
No other axis shows this behavior.
I assume it must be some problem with the glas scale.
The strange thing is that there was no problem with the glas scale on the old Grundig Dialog 11, but I do not if that is a thing to consider.
I probably have to take the glas scale out and clean it.
Anything else I should test before?

Could just be a fuzzball dragged along by the read head,
or just a dirty spot on the scale.

I once had a rotary encoder that would only count in one direction.
It turned out that it had a hair the got pulled in front of the read head
when turned in one direction but swept out of the way in the other
direction...

I do think you are right, something is with my glas scale. I tried some less Interpolation but the error stays the same.
Tomorrow I will try to get the glas scale out and clean it.
Plan B is to fit a cheap china scale until next week.
We are racing in 8 days and I need to mill tripple claps and brake adapters .

Thanks so far. I will post an update here.

Hello. Good news. I found the problem.
First I took the glas scale out and cleaned it. It was a little dirty but nothing that would alarm me.
After cleaning I installed the scale and the error still excisted.
The solution was that the EXE 612 have 2 settings for the output impedance.
A High and a Low setting. The X axis was on high.
When I switch it to low the error was gone.
I could tune all my axis and I can start milling now.

I attached the EXE manual with all the needed information.

Thanks and greetings
A.