Re: Lost steps

Ozzie@...

> Message: 17 [CAD_CAM_EDM_DRO] Digest Number 621
> Date: Sun, 13 Aug 2000 21:20:02 EDT
> From: JanRwl@...
> Subject: Re: Lost steps
>
> In a message dated 8/13/00 8:01:32 PM Central Daylight Time, Ozzie@...
> writes:
>
> << Any ideas, and how should I go about finding the problem???? >>
> Howz Harriet? I have fiddled with stepper-motors and home-brew machinery for
> 25 years, now, and, without seeing it, only reading what you say, I wonder if
> you might not be STEPPING at too-fast a rate, or, not "accelerating" and
> "decelerating" into/out-of your move? If the "book say" they will step OK
> at, say, 400 steps/sec. without acceleration into that rate, and you start
> "cold" at 400 steps/sec., the motors might-just "be surprised" before they
> get moving! Try reducing the stepping-rate 25 to 50 steps/second, and re-try!

You can easily adjust acceleration in the ini file that you are using.
The stock emc.ini uses a value of 20 which is pretty steep for some setups.
It looks like this:

; Trajectory planner section
--------------------------------------------------
[TRAJ]
...
DEFAULT_ACCELERATION = 20.0
MAX_ACCELERATION = 20.0

You can play with the numbers there. Try 2.0 and you should hear your
motors ramp up. That would take care of any missed steps do to accel.

You didn't say what your steps per inch are but a small part of what you
may be reading as error could be the difference between the commanded
position and the nearest step position. (You may also loose a little
accuracy half stepping. I don't remember how much and don't have the
url's for those articles at hand.)

The first thing I'd do is set backlash comp to 0.0, command one axis to
move in one direction so that you take up any real backlash. Set the dial
indicator to read zero and command a move nearly as large as the
indicator's travel. Compare the distance on the dial to the distance you
commanded to see if the number of steps per unit are what you expected.
If not, change the steps per unit in the ini file and try it again.

Now with the steps per unit set properly for each axis, compute the
real deadband - the distance between steps - and enter those values in the
ini file. As you work to minimize the backlash using compensation you
need to remember that your indicator readings will be off as much as +-
the deadband.

This explanation is speculation. If you do not set a deadband equal to the
size of your minimum step distance the EMC will oscillate. Since backlash
comp is fed in a little each time the motion stuff updates it may not be
done compensating when the next oscillation occurrs and it starts to feed
in the comp for the other direction.

IMO Backlash comp is iffy at best. If you try to take up very much you
will get some strange results that show up in surface finish even on the
best production machines with the latest controllers.

Ray