Re: [CAD_CAM_EDM_DRO] Feb 25

Ok, I got it working. The problem was in the OUTPUT_SCALE it came set at
1.000 0.0 and it needed to be 1000.0 0.0.
I thought I read that you can set the acceleration separately for each axis
but I only find one setting.
Darrell

----- Original Message -----
From: Tim Goldstein <timg@...>
To: <CAD_CAM_EDM_DRO@onelist.com>
Sent: Friday, March 03, 2000 4:34 PM
Subject: Re: [CAD_CAM_EDM_DRO] Feb 25


> 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
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.
>
>
> 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 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
10
> 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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