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Tom | 5 Dec 08:10
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servo chatter at rest is going to drive me nuts...

I have all 3 axes of the Kasuga converted to EMC2 control using 500 line
encoders (2000ppr)on brush servos and Advanced Motion PWM amps. 
The ballscrews are 5mm pitch. I have the ini set for 10160 encoder counts 
per inch.
While pid tuning I noticed that in order to get my ferror down to below 
.002" during the accel/decel ramps, I had to reduce the deadband. This was 
after optimizing p,i,d,ff0,ff1,ff2. This causes the servos to continually 
hunt about +/- .0002"(two tenths)while at rest which makes a godawful chatter. 
The symphony of clicks, moans, pops and chatter is going to drive me nuts
eventually. Of course, I can always turn the machine off in Axis to stop the
noise - but I want to leave the machine on while fixturing, setting tool & 
work offsets, etc.
Is there some secret to tuning this chatter out while keeping following 
error down?
Tom

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Peter C. Wallace | 5 Dec 18:54

Re: servo chatter at rest is going to drive me nuts...

On Fri, 5 Dec 2008, Tom wrote:

> Date: Fri, 5 Dec 2008 07:12:01 +0000 (UTC)
> From: Tom <kestreltom@...>
> Reply-To: "Enhanced Machine Controller (EMC)"
>     <emc-users@...>
> To: emc-users@...
> Subject: [Emc-users] servo chatter at rest is going to drive me nuts...
> 
> I have all 3 axes of the Kasuga converted to EMC2 control using 500 line
> encoders (2000ppr)on brush servos and Advanced Motion PWM amps.
> The ballscrews are 5mm pitch. I have the ini set for 10160 encoder counts
> per inch.
> While pid tuning I noticed that in order to get my ferror down to below
> .002" during the accel/decel ramps, I had to reduce the deadband. This was
> after optimizing p,i,d,ff0,ff1,ff2. This causes the servos to continually
> hunt about +/- .0002"(two tenths)while at rest which makes a godawful chatter.
> The symphony of clicks, moans, pops and chatter is going to drive me nuts
> eventually. Of course, I can always turn the machine off in Axis to stop the
> noise - but I want to leave the machine on while fixturing, setting tool &
> work offsets, etc.
> Is there some secret to tuning this chatter out while keeping following
> error down?

Is there anyway to change the tuning parameters dynamically? Just bumping up 
the deadband when idle would probably solve the problem.

Peter Wallace
Mesa Electronics
(Continue reading)

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Tom | 6 Dec 07:40
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Re: servo chatter at rest is going to drive me nuts...


> 
> Is there anyway to change the tuning parameters dynamically? Just bumping up 
> the deadband when idle would probably solve the problem.
> 
> Peter Wallace
> Mesa Electronics
> 
> 

Peter, 
That's a good idea.
I wonder if a hal component could sense when motion is idle, then swap in a
larger value for deadband until motion is called for. I don't know if deadband
can be dynamically changed while the motion threads are active.

Tom

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Jon Elson | 5 Dec 17:41

Re: servo chatter at rest is going to drive me nuts...

Tom wrote:
> I have all 3 axes of the Kasuga converted to EMC2 control using 500 line
> encoders (2000ppr)on brush servos and Advanced Motion PWM amps. 
> The ballscrews are 5mm pitch. I have the ini set for 10160 encoder counts 
> per inch.
> While pid tuning I noticed that in order to get my ferror down to below 
> .002" during the accel/decel ramps, I had to reduce the deadband. This was 
> after optimizing p,i,d,ff0,ff1,ff2. This causes the servos to continually 
> hunt about +/- .0002"(two tenths)while at rest which makes a godawful chatter. 
> The symphony of clicks, moans, pops and chatter is going to drive me nuts
> eventually. Of course, I can always turn the machine off in Axis to stop the
> noise - but I want to leave the machine on while fixturing, setting tool & 
> work offsets, etc.
>   
Strange.  I have two servo-controlled mills here, and they both hunt at 
times.  The one with velocity servo amps and DC tachs is usually very 
quiet, with hunting a a several Hz rate which is not audible.  (Once in 
a while one particular axis breaks into a 500 Hz while which is quite 
annoying.  Touching the tach for a second damps it out.)  The other one 
is a PWM driver controlled only by EMC, and it grumbles and clatters now 
and then, but  usually settles after a couple second.

I have found setting the D term is quite tricky, and too much D causes a 
lot of oscillation.
I usually have stationary following error of about .0002" on the 
velocity servo machine, and .00005" on the PWM one.  The velocity servo 
machine has encoder resolution of 20000 counts/inch, the PWM machine is 
128,000 counts/inch.  If you see an association between following error 
and encoder resolution, you are right.  I think with proper amounts of 
FF2 (it takes VERY little) you can get the error down during the
(Continue reading)

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Tom | 6 Dec 08:03
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Re: servo chatter at rest is going to drive me nuts...

Jon,

I will be able to see the chatter/hunting in the pid.error signal in Halscope, so I will try to reduce it with
less d-term and try adjusting FF2 to compensate for f-error. 
Right now my values are roughly p-term=8500, i-term=375, d-term=45 - 

I am using the amplifiers in velocity  mode with tach feedback, so if what you say is true, then I have a lot of
hope of getting this right. I agree that higher count encoders will reduce the amplitude of the
oscillations and make the chatter quieter and higher pitched. 

I will try it tomorrow and give a report. 

Thanks,
Tom

--- On Fri, 12/5/08, Jon Elson <elson@...> wrote:
From: Jon Elson <elson@...>
Subject: Re: [Emc-users] servo chatter at rest is going to drive me nuts...
To: "Enhanced Machine Controller (EMC)" <emc-users@...>
Date: Friday, December 5, 2008, 9:41 AM

Tom wrote:
> I have all 3 axes of the Kasuga converted to EMC2 control using 500 line
> encoders (2000ppr)on brush servos and Advanced Motion PWM amps. 
> The ballscrews are 5mm pitch. I have the ini set for 10160 encoder counts 
> per inch.
> While pid tuning I noticed that in order to get my ferror down to below 
> .002" during the accel/decel ramps, I had to reduce the deadband.
This was 
> after optimizing p,i,d,ff0,ff1,ff2. This causes the servos to continually
(Continue reading)

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Gmane