Well, some progress figuring out what is going on.
I just ran some more tests using Ubuntu 16.04 with a freshly built
preempt-rt kernel and freshly built Git master. No surprise, it still
shows the fault. I took a Halscope log from the original setup. The
encoder velocity is rather noisy but no big spikes. On the new install
the encoder velocity is not as noisy but it has occasional spikes. The
encoders are wired to a 7i29.
This log is from the old setup <www.sheetcam.com/lathe/old.log>
This log is from the new setup <www.sheetcam.com/lathe/spike.log>
This lathe is fitted with high resolution handwheel encoders so I also
did something I should have done much sooner and looked at the handwheel
encoder velocity. Big difference - nice clean signals on both configs.
So it looks like something funky is going on when reading encoders
with the 7i29, resulting in noisy velocity estimation. I guess the
velocity estimation code has changed a bit over the years, giving the
different results. The fault has probably been there for a long time,
quite possibly since I built it. My immediate thought is noise on the
encoder signals but this machine never loses position. It will turn out
accurate parts all day. Peter, any idea how the velocity estimation can
be noisy while the overall counts seem to track accurately?
Some time back I was involved with developing flight controllers for RC
models. They used nested PID loops. The loops worked well even though
the accelerometer and gyro feedback signals were extremely noisy. I
think that explains why my old config seems to run smoothly. The signal
is noisy but consistent so the noise averages out. On the later config
the velocity estimation ends up with spikes which are noticeable in motion.
Les
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