On 07/26/2015 09:31 PM, Brian wrote:
> I have had an idea lingering in my head for quite some time now. How about
> moving the screw/backlash comp out of motion and implement it with a HAL
> module. The purpose would be for two goals. One it would make it more
> logical to implement more sophisticated joint compensation, such as thermal
> expansion, deflection, and belt stretch. Second, it would set the stage to
> be more friendly for dual scale/motor feedback, because it would give control
> over where the compensation is added in the feedback loop.
I agree with Andy, backlash compensation belongs in Motion, since that's
where the trajectory is planned.
Motion can insert a backlash compensation "move" when a joint changes
direction, and compensate for the time that the backlash move takes by
not moving the other joints (except that possible backlash moves on
other joints may happen at the same time).
An external hal component that tried to do that would have to capture
and buffer all the joint waypoints that Motion was generating during the
backlash move, then when the backlash move is finished either try to
catch up to Motion (which would violate the accel & vel that Motion had
planned), or stay behind Motion from then on out (which would introduce
lag on things that immediately affect the trajectory, like Feed Override).
While backlash compensation surely belongs in Motion, slow,
small-magnitude, long-term offset changes like thermal compensation can
probably be implemented safely as a HAL modification to the waypoints
commanded by Motion. See Dewey's "moveoff" demo for an example of how
this can be implemented (sim/axis/moveoff), with some video here:
https://www.youtube.com/watch?v=KY6hx7WBkO8
https://www.youtube.com/watch?v=XGDq6620fPQ
--
Sebastian Kuzminsky
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