Dual PID loops and appropriate pins for feedback to the Trajectory Planner et al

hey guys

just wanted to report back on this dual feedback thread

I have a large cnc with 0.1mm backlash on the z axis.  so i put a scale on it and the results are awesome

i have accuracy within 0.002mm (compared to the scale accuracy at least assuming that is  good)

i tried both 1 PID loop and 2 PID loops that have the outputs summed together.

with the single loop i had the velocity PID input coming from the motor rotary encoder and the final position from the linear encoder.  and it worked but was really rough and kept bouncing back and forth with the 0.1mm backlash  

the two summed loops are really smooth and awesome  the trick is the first loop has the high resolution encoder on servo drive and that has really good performance and of course no backlash.  and I can change all the normal PID values except Integral  and the 2nd PID loop only has the linear encoder input.  and only PID setting is Integral.  so i get nice and smooth motor but still backlash from first loop and then once the axis gets close the Integral moves the last 0.1mm over a second or so.
so by the time i am cutting the machine is perfectly where it should be.  this is probably not so good for high speed surfacing stuff for that you need to remove the physical backlash.  but for a slightly slower machine its awesome.

have a look at this thread where i show some more photos of it.  its working most days now and running hard in production.


forum.linuxcnc.org/show-your-stuff/58689...ing-running-linuxcnc

cheers