firepick1
commented
9 years ago Neil, the latch backoff serves one purpose only. It is a small number designed to cancel optical limit overshoot caused when the interruptor slams into the opto-limit stop at high speed. In other words, a high speed trigger might be different than a low speed trigger. The backoff is just the amout to move away from the limit switch before probing slowly and carefully.
After getting a more accurate limit switch value, the code currently backs off again after the slow accurate move to prevent high speed overshoot into the limit switch on what might be a perfectly good delta move. For example, if you whip the arm to the limit coordinate, it should never trigger. That's what the backoff is intended for. It's not really a homing angle adjustment, although that's actually how it's implemented.
We have two choices:
1) keep as is and use small values (500 seems a bit large. I initially though 32 would be fine) 2) eliminate the second backoff and never do high speed moves close to home
WDYT
FlyingLotus1983
Hey Karl,
Posting this here rather than emailing you as this should be easier to track (it's a bug tracking system after all, haha).
Homing back-off doesn't work in the way that I would think it would. There isn't much documentation on what exactly backoff is used for, so it's hard to tell if this is a documentation problem or a code implementation problem. Either way it has caused confusion not just to myself, but some others have made the same incorrect assumption that I mad.
The behavior that I'm seeing from homing backoff is: Set homing backoff to "0". Then move to a given raw coordinate, say, 1500. Mark angle to where the arm traveled to. Now set backoff angle to some othe rnumber, say "500". Go to same raw coordinate (1500). Measure the arm angle. What we are seeing is that the arm is at a different position, depending on the backoff value.
The wiki / documentation doesn't say this explicitly, so I'm not sure if this is the intended behavior or not. What I would expect is that you could set the backoff to some value, but the raw-position-to-actual-posiiton correlation shouldn't change. Others that are helping me have came to the same assumption, correct or not.
Are my assumptions correct or incorrect? What is the intended behavior? And can the documentation be expanded to better explain what it does? I don't mind making the change but need to get clarification here before I get the scalpel out.