stepper motor oscillation

never seen this before.....

simple 3 axis stepper motor setup for a mill.

X, y, z + analog vfd
no encoders, glass scales etc.

l was jogging the z axis upwards and when I released the keyboard pgup the z began oscillating back and forth in ever decreasing motion.

first question, how is this possible..... I understand how it could happen with a closed loop arrangement, but this is purely stepper motors, no feedback..... so I would presume linuxcnc would just output step and dir pulses.

it is via a Mesa 7i76E, and the stepper motors are big hefty nema34's, the velocity of the motion was not very fast at all (it was driving a 10mm pitch acme leadscrew)

just never expected to see oscillation without a feedback signal
(I set it up using pncconf and although I've seen the PID values in the hal/ini file, I just left them at default (I think 1000 or 10000 for P, and I and D are zero.... given no feedback signal.... didn't expect any ringing)

The hardware stepgen uses "local" feedback which must be set
up properly for the servo thread rate.

FF1 should be 1
P should be 1/Servo thread period (or 1000 for a 1 ms servo thread)

A P of 1/servo-thread-period means that any errors between the expected
position (from LinuxCNC) and actual position (from the stepgen hardware)
are corrected at the next servo thread.

If the P term is higher than 2/servo-thread-period you will get oscillation
with ever increasing magnitude.

thanks as always I'll take a look.

servo thread is 1000, so I think (will check) that p is 1000 as it was default as per pncconf.

the FFx I didn't change (I didn't change any of it really)..... didn't know what the FFx was for

FF1=1 basically sets the stepgen velocity correctly before any feedback
is applied.

I have seen pncconf set wrong P values if you ever changed the servo
period