Got it all tuned, for the most part.  X and Y calibrated and Z was really close.  So I started to experiment with how fast I could go.  Got it up to 1.5 meters per second.  I was happy as a clam, took a nice video and was ready to post it.  Uh... wait.  Watching the video, it was reading 1.5 meters per second but just timing it over the distance traveled... no.  It also had a strangely even tone for a printer using tripod kinematics.  it should sound like cats fighting at that speed. 

I know what you are thinking.  Scale issues, microstep issues... losing steps.  Nope.  It was 'warping' the paths, rounding corners to make as close to the requested speed as it could.

Now I know the next thing you are thinking.  It wasn't the trajectory planner Precision or Quality' setting They are set to an excessively low 0.01 and 0.02.

This also only happens while going fast.  And the strangest part of all of this is when it came to a stop it was always exactly where it should be.

I thought perhaps I was at the physical limit for how fast you can twiddle the pins of a DRV8825, but I reduced those values below what they should be, and it did the same thing, at the same speed, so that's not it.

I tried altering the trajectory planner period, the base period, and the real-time thread period also, with no change.

So a couple hours of tuning this and that, and I've come to the realization I'm hitting some limit I haven't hit before.  Actual top speed before it starts to cut corners is around 700mm/sec, but I'm barely at half current for my steppers.  I had set them low for setup so I could run into endstops and such and not break things, but I never turned them back up!  So... I should be able to go a whole lot faster if I can figure this out.  Then again, that is actually pretty fast.

Writing this out just made me think of something.  Perhaps the PRU has its own base period I need to muck with... Hmmm..

Well, I'm moving on for now and considering this low hanging fruit.  700mm/sec is fast enough.

I have now decided I want to move my endstops out more, which undoes some of my tuning.  Working it.

I'll remake the video, with a little less giddiness, when I have it tuned again.