On 06/27/2017 10:31 AM, Les Newell wrote:
Same PC, same hardware, same settings. The only difference
between the two setups is the hard drive. Changing the PC
(AMD processor VS Intel on the original) had no effect on
the symptoms. I checked latency on the new PC and if I
remember correctly worst case after a couple of hours was
~150us. Not good for steppers but fine for servos.
150 uS?? YIKES! That is not generally good enough for even
a servo system, although Peter's PLL scheme probably makes
it usable, but not very good. With RTAI, I generally get
about 10 - 15 us on any machine I think is good enough to
use. On linux-preempt, it is more like 15-20 us. Note that
150 us is 15% of the 1 KHz servo cycle.
I haven't had time to try tracing back from PID. That is
probably my next job.
First thing to do is bring up the encoder velocity, and look
for physically impossible velocity spikes. If the encoder
is being misread, or there are bad latency hiccups, you can
see spikes in the velocity that can't be real, that's an
immediate sign of some kind of difficulty between the
encoder shaft and the CPU. If you see them, then it can
take some detective work to determine where they are coming
from. If no spikes, then you may just need to retune the
PID. There could have been subtle changes in the way PID
works that make your old numbers less than optimum.
Jon
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