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From: "Tim Goldstein" <timg@...>

You are using the right parts. I think your problem is your P value is set
too low. If it is too low the axis actual position will fall behind the
commanded position and you will get a follow error. It has nothing to do
with looking for an encoder input as in emcfreqmot the encoder input is
virtual. I find that on my system 150 is about the minimum value I can use
for P and that at 1000 the motors are getting driven faster than they can
respond and my max speed suffers.

Ray Henry posted a great description of the PID parameters written in
laymans terms on the EMC mailing list. It follows for the benefit of this
group:
Tim and other new freqmod/smdromod users.

This is a rather long post about P but this axis tuning stuff becomes
critical for stepper users with these new tools. This is an IMHO view of
P, I, D. (with flame suit in place) BTW I've seen some excellent stuff on
the web regarding PID tuning but don't have specific links handy. There
are also a bunch of auto tuning and computational tuning procedures but I
know nothing about any of them.

All three variables (6+ really) look at what is happening between

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

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

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.

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

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

regard for P. A little integral may smooth out some of the frequency

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.


T - Test
--------

The final proof of tuning is in the cutting. So after your best guess

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

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

changes, and a better/worse judgement 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 force be with you.

Ray
--- END OF REPOST----

I hope that helps. I have found that emcfreqmot does require you to fiddle
witht he settings a lot more, but once you get them right it is just
impressive in the speed it offers. There is also a setting in emcfreqmot.c
(located in /usr/local/nist/emc/src/emcmot if I remember correctly) that
controls the base period of the frequency. Setting this value to a smaller
value will improve the top speed you can achieve. I find it by searching
for:
period =
(yes, there is a single space between period and =)
The default is 20 and on an AMD K6 2D 500 mHz I am able to use a value of

and Matt Shaver is using a value of 12 on a PII 350 if memory serves me
right. After you edit this value you must recompile by entering:
make PLAT=rtlinux_09J all
from in the directory the file is in.

Tim
[Denver, CO]


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From: "Matt Shaver" <mshaver@...>

From: "Darrell" <dgehlsen@...>
I thought I read that you can set the acceleration separately for each

but I only find one setting.

It's not exactly the same thing, but you can vary the P gain with similar
effect.

Matt


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-----Original Message-----
From: "Darrell" <dgehlsen@...>

Does the P gain affect stepper motion?
Darrell