On Thu, 4 Mar 2021, John Dammeyer wrote:
Date: Thu, 4 Mar 2021 12:40:34 -0800
From: John Dammeyer <jo...@autoartisans.com>
Reply-To: "Enhanced Machine Controller (EMC)"
<emc-users@lists.sourceforge.net>
To: "'Enhanced Machine Controller (EMC)'" <emc-users@lists.sourceforge.net>
Subject: Re: [Emc-users] Following Error Joint 2
From: Les Newell [mailto:les.new...@fastmail.co.uk]
I'm going to guess that low-level I/O error is a general
_Following_Error_Joint_2_?
No. It is a different error. I can't remember the exact wording but I
have seen it before.
The following error message is correct. With steppers you have two
acceleration settings. There is the joint acceleration which is used by
the trajectory planner and the stepgen acceleration. STEPGEN_MAXACCEL is
used to simulate the physical limits of the stepper and it's load. It's
a simulated equivalent to to a servo motor running out of torque. If the
trajectory planner commands a higher acceleration than STEPGEN_MAXACCEL
the stepgen will not be able to keep up and you get a following error in
exactly the same way you would on a servo system that ran out of torque.
Les
That doesn't explain why, if STEPGEN _MAXACCEL is set to 125% of
MAX_ACCELERATION that the trajectory planner or Stepgen tries to use a number
larger than that.
The trajectory planner has no notion of what the stepgen/physical hardware
limits are, it just obeys is maxaccel constraints (with the one exception:
if you have backlash compensation enabled, the maximum acceleration during
backlash compensation can be double the joint constraints)
If you set the stepgen maxaccel to 125% of the joint constraint, and you have
backlash compensation enabled, the stepgen constraint will likely cause a
following error during backlash compensation moves because the stepgen
cannot follow the requested motion.
It's like saying I know you have a 1 gallon pail but because I have 1 gallon
in my pail I'm going to pour it into yours. Too bad it overflows when I pour
two of my pails into one of yours. You should have had a 2 gallon pail. What
the heck is the point of a parameter like STEPGEN_MAXACCEL or MAX_ACCELRATION
if it can be broken so easily.
Well you cannot ask for more than the physical hardware can produce so there
need to be bounds. Stepgen Maxaccel is useful for a couple of reasons:
1. If the stepgen is used alone, it be used to bound acceleration
(say for step/dir controlled VFD spindle)
2. It can improve robustness with regard to host servo thread jitter
(that is, it can bound velocity corrections to a value the mechanics can follow)
Just confused here.
John
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