Re: need your opinion!

["Patrick Riedlinger" <patr-at-precidesign.com>]

Hello Hassan:

----- Original Message -----


>
>  hello all,
> am running the maxnc, i have a problem with the stepprs
> first i have a question: are the setting for the stepper motors in the ini
> file have effect on the performance of the stepper morts actaully on the
> machine????
> i can't get the velocity of the machine to go beyond 15 percent of
> 2inch/minute.....the feed rate is 13...if i go more with these
> settings...the motors will just vibrate with a buzzering noise and will
not
> move??? so can any one tell me  what might be the problem??
>

Please give us more information, i.e. what drivers are you using to drive
the steppers, what torque the steppers are rated at, Bipolar or Unipolar,
etc.

Initially looking at your message, I can only think of 2 possibilities
(there could be more...)

1) Your steppers are under-powered, and have stalled (why I asked for above
information)

2) Your ini file is not properly set up so the steppers are "tuned"


I have not yet hooked my EMC to steppers, I am waiting to get other parts
(like ballscrews), but I have read the documentation (several times...and I
will probably need to read it several more!)

I understand there is a certain amount of "tweaking" you will have to do to
the following variables in the ini:

P, I, D

Below the line is a snippit from Ray's PID tuning experience. There are more
examples on the website handbook, under PID Tuning.

I hope this helps.

_________________________________________________________

Ray's Experience

All three variables (6+ really) look at what is happening between commanded
and actual position in the emc. My comments below may be a bit unsatisfying
for those who are accustomed to very precise things but they come from a
number of years of twiddling with servo controls.

P - process variable.
---------------------
This is the gain control. It is a bit like the volume on a music system. If
it is set too low you can't follow the lyrics. If it's set too high the
windows rattle and the neighbors fuss.

Some authors refer to P as proportional band. Think of it as a pair of
lines, one ahead of the commanded position and one behind it. Actual
position should be somewhere between the lines. If the actual position is
farther behind then that below line the controller will run the axis flat
out to reach the slower line. Conversely, if the actual position is ahead of
the above line the controller will do all it can to bring actual position
down to that upper bound.

As long as the actual position is between the lines, the controller will
ramp gain up and down so that actual approaches commanded.

The larger the P number the narrower the space between these lines. Set P
too low and your axis works like a dedicated couch potato during super bowl
or world soccer finals -- even a fire may not rouse it. This condition is
rather easy to spot because the axis is sluggish.

Set P too high and your axis will develop palsy. On most machines you can
hear this condition by putting your ear to the motor. You may want to do
this when others aren't watching -- or use a stethoscope or a long socket
extension -- or perhaps the graph function.

Palsy will sound like a hum or grind when the motor is sitting still or
moving very slowly. You can also feel palsy if you wrap your hand around the
ball screw or grip the drive belt or pulley. (disclaimer -- This
demonstration is done by a professional with only three remaining fingers,
don't try this at home)

Deadband may mask too much P when a motor is sitting still so you may want
to move the axis very slowly and listen to the sounds around each step. If
the axis keeps up with commanded position at high speed and during
acceleration and there is not a lot of ringing, grinding, jumping at very
low speed, then you are real close.



I - Integral variable.
----------------------

Integral works a bit like a shock absorber. Any change in either actual or
commanded position gets rounded off or averaged in so that
acceleration/deceleration brought about by P is absorbed and released more
slowly over time.

No integral and you get the full P effect of change in commanded position.
Too much integral and the axis seems to wander off on it's own without much
regard for P. A little integral may smooth out some of the frequency jumps
when a stepper is running right near one of those troublesome rates.



D - Derivative variable
-----------------------

Derivative works like passing gear for acceleration or a jake-brake for
stopping. Whenever commanded position changes rapidly, d will really kick
the amp/motor in the *** to follow the rate-of-change of the axis command
rather than the difference between commanded and actual position.

Derivative works against inertia so if you've got lots of iron to start or
stop dial some in. But derivative will increase palsy so you have to balance
it against gain.


FF1-3  Feed Forward variables
------

I know very little about the effect of these variables.  My first experience
with them is while using EMC.  I need to include an idea from Jon Elson and
his work with servos and his bridgeport.  He has used a value up to 8.0 for
FF1 with a somewhat reduced acceleration to very successfully minimize
following error.  The relevant portions of his ini file are:

  DEFAULT_VELOCITY =      0.75
  MAX_VELOCITY =          1.5
  DEFAULT_ACCELERATION =  2.0
  MAX_ACCELERATION =      2.0
  MAX_VELOCITY =          1.2
  P =                   100.000
  I =                     0.000
  D =                     0.000
  FF0 =                   0.000
  FF1 =                   7.500
  FF2 =                   0.000

Your results will no doubt vary.

T - Test
--------

The final proof of tuning is in the cutting. So after your best guess with
all the watching, hearing, feeling done to each axis, get out a chunk of
soft aluminum, a small end or ball mill, and begin to mill circles or arcs
that pass 90 degrees between each pair of axis. I like outside circles
because you can hold them up to the light and see how the finish looks near
the quadrants.

Digital systems will give you some steps as one axis approaches zero and the
other approaches the set feedrate so don't expect a perfect mirror arc
finish. Backlash and backlash compensation also affect the appearance here.
Servo drives will work better than steppers. (digital vs analog) But the
smoother the saw teeth the better.

Be prepared to spend some metal on this! And make a list of your settings,
changes, and a better/worse judgment about the result of each change. On
occasion I've spent pages of paper and made piles of swarf to get a stubborn
machine to where I wanted it.

Good luck and may the electromotive force be with you.

Ray <rehenry-at-up.net>