Brushless Servo Support

Features
8

Interesting. I’d be curious what chips you plan to use on the board you are designing.

I have spent a bunch of time on “stepper servo” support in the past. Both in prototyping of “mechaduino support” and in implementation of magnetic hall sensors for precise stepper axis position monitoring ( Configuration reference - Klipper documentation ).

Unfortunately, due to time constraints on my side, I don’t think I’ll be able to contribute to any new efforts in this area in the short term.

I can share a few high-level thoughts that may (or may not) help:

  • Consider reading through all the threads that have linked to Mechaduino experiment (as found immediately after the original post at that link). This topic has come up a few times before, and there have been several different ideas on the approach.
  • For what it is worth, I’m no longer convinced that a PID position/command feedback loop is a good idea. The concern is that if the motor over shoots a desired target then it will result in a print blemish, and commanding a return to the actual desired target will not undo that blemish, but is instead likely to make the blemish more noticeable. This, I believe, is what leads to the “salmon skin effect” that is a common complaint with past servo steppers implementations.
  • In contrast, it may very well be worthwhile to build some type of “overall feedback loop” so that, over extended time periods, the printer learns not to “overshoot the target in the first place”. That is a lot of work though.
  • If interested in utilizing velocity and/or acceleration in determining motor control, then the best way to do that may be to reuse the existing host kinematic system and calculate the desired velocity/acceleration from the existing queue_step commands that are queued in the micro-controller. That is, the Klipper host code already queues upcoming motion in the micro-controller 100+ms prior to it being used. It should not be difficult for micro-controller code to inspect that queued movement to obtain velocity and acceleration.

Cheers,
-Kevin