Low speed motor setup/tuning


I am attempting to drive a shaft at pretty slow speeds (1 turn/s ish), through a 3:1 belt reduction, using a 420kV T-motor U7 (I realise probably not the best motor for the job, but what I had on hand).

I’ve run through the getting started process and the reccomended tuning, giving the following gains:

pos_gain: 8
vel_gain: 0.07
vel_integrator: 0.015

This seems ok(ish) during the tuning process, however settling time is amazingly slow - It’ll reach the setpoint in a second or so, then maybe 15-20 seconds for the steady state error to more or less disappear.

More concerningly, however, is after going through this tuning process, the motor no longer behaves correctly at all - twisting the shaft by hand in closed loop mode will often not return to the setpoint at all, but will drive in the wrong direction (triggering a deadline missed error) - if it doesn’t trigger an error, it just won’t even attempt to move at all. Iq_measured usually gives 1-2A, so it’s not like it’s trying to drive anywhere. Changing input_pos does nothing to convince it to start moving.

I assume these symptoms are all due to bad tuning, but I’m starting to run out of expertise in how to remedy it - vibrations are quite extreme with the standard vel_gain of 0.16.

Mechanically the system is fairly loose, there’s some inertia to it, but I can spin it a full 360 degree arc with a prod from one finger, so not much resistance.

An interesting extra point is that it behaves very well during the calibration sequence, it carries out those movements at about the right speed and settling time for my final application.

In super typical fashion, after several days of tweaking and finally deciding to post a question, I got it to a decent state some 15 minutes later. Gains are now 200, 0.05 and 0.001. Presumably that pos_gain was where most of the problems were. Still very open to any other suggestions/advice for this type of motor though!

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What’s your encoder?
If it’s too low resolution then you will have trouble at low speeds. Halls are particularly bad for this.

Its the amt-102v, so ‘reasonably’ high resolution.