Hmm, looks like that was a red herring. The problem is back. I can't do
any more testing because the pixies inside the old computer finally
abandoned ship and it now no longer boots. Doesn't even post. My
priority now is going to be to find a box that will run my old EMC.
Les
On 27/06/17 19:50, Les Newell wrote:
150 uS?? YIKES! That is not generally good enough for even a servo
system
I have done a lot of work with PID loops in various non LinuxCNC
systems and in my general experience they generally seem to be very
tolerant to timing variation. After all we are dealing with motors
that have a bandwidth far lower than the PID loop speed. Anyway I have
two boxes here so I can run back-to-back tests at some point.
I think I may have found the problem. Pncconf doesn't connect
pid.n.feedback-deriv to the encoder velocity. The version of EMC I am
running doesn't have feedback-deriv so my config doesn't either. With
no connection, pid estimates the derivative and something funky is
going on with that estimation. Sometimes the machine twitches while
stationary which makes it a lot easier to see what is going on.
Watching feedback-deriv on halscope I can see it move at exactly the
same moment as the output jumps. A few milliseconds later the encoder
velocity moves in response.
After connecting the encoder velocity to feedback-deriv my tuning
values have to be very different but the twitching appears to have
gone away. The tuning at the moment is very rough with just P,D and FF
used. I need to experiment further and tune the axis properly before
saying the problem is definitely fixed. Now I need to try to figure
out why my jogwheels (one per axis) don't work.
Is there a reason why Pncconf doesn't connect the encoder velocity? By
the way Pncconf wouldn't let me set the maximum feed rate to more than
2000mm/min which is a bit annoying as the machine is capable of 4800.
Les
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