Re: [Emc-users] Tuning a servo

2020-12-09 Thread Gene Heskett 
On Wednesday 09 December 2020 13:58:14 Todd Zuercher wrote:

> -Original Message-
> From: dave engvall 
> Sent: Wednesday, December 09, 2020 1:10 PM
> To: emc-users@lists.sourceforge.net
> Subject: Re: [Emc-users] Tuning a servo
>
> On 12/9/20 8:21 AM, Jon Elson wrote:
> > On 12/08/2020 10:53 PM, Gene Heskett wrote:
> >> What does that 22mv mean in folllowing error since I have that set
> >> for about half an inch at the moment?
> >
> > It is not milli VOLTS, it is milli-units.  Generally inches or mm,
> > depending on your preference.
> > so 22m means 0.022" assuming that's what you use.  Or, if this is a
> > rotary, then it could be
> > 0.022 degrees.
> >
> > My method of tuning is to first increase MIN_FERROR to prevent
> > following error trips, then reduce P to a small value, just enough
> > that error doesn't creep up. Adjust
> > FF1 until error is small at moderate speed, less than 50 encoder
> > counts, perhaps.  Then, assuming a real axis with encoder feedback,
> > add a LITTLE FF2 to reduce the error spikes on accel/decel.  (Don't
> > bother on stepper-simulated axes.) Then, increase P to drive the
> > error down to a small fraction, add a little D to improve stability.
> > It takes practice and experience to know what to adjust based on the
> > Halscope trace.
> >
> > Jon
>
> Tuning seems to be a highly individualized endeavor. Hopefully a
> damped (in following error) search. :-) I start with a classic ZN ...
> increase P until you get un-dampened oscillation.  Make starting P .6
> of that value. Set FF1 equal to one and then adjust as necessary to
> minimize following error. Adjust FF2 to minimize spikes on accel and
> decel. Adjust FF0 if needed for zero crossing. Try small amounts of D
> and I although I usually leave them a zero. Following error is < 1e-3
> and can get down as good as 5e-4. The values in PID, etc do seem to
> interact so going thru the loop a few times can't hurt. As always
> YMMV. The major error on my machine is backlash. Adding a glass scale
> to X and Y might help. (future project).
>
> I suspect that having tach feedback makes tuning much easier. I've
> never had much success with torque mode.
>
> If I had the skill in linuxcnc internals I'd would try a well
> constrained Monte Carlo. No chance of my  doing that. ;-(
>
>
> That’s all well and good if the command input of the amp is velocity,
> but I don't think that is the case for this one.  In torque/current
> mode the FF1 and FF2 have very different affects than with a velocity
> drive.  (and I never really completely figured it out, before I gave
> up and set up two nested PID loops.)  For a single loop you'll need as
> much P and D as you can make stable, and lots of I.  I've also found
> that setting the max error I limit pin to something that prevents
> windup can allow for very quick response from the I term without the
> negative side effects. (I've had good luck with setting the pid
> maxerrorI = 10/I).  I've also noticed some strangeness, where adding I
> actually seemed to made the steady state following error worse up
> until it was large, then the error started getting better until it
> finally came back in line and more I started to help a lot.  I did
> this searching for the point where more I caused instability, which
> I'm not sure if I ever did find.  (This was done before I tried
> limiting the max error for I.)
>
> One of the main reasons I gave up on the single loop, was that the
> tune seemed to be very changeable and touchy, every few weeks I was
> tweaking on it again trying to get rid of following errors,
> instability, or overheating the drives.  I've not had to mess with it
> since switching to the double loop.

When I get it all assembled, which is slowly getting closer, I may do 
this, and pick your brain via PM for help. But as I just posted, I have 
major problems understanding the SCALE setting because its obviously 
different for steppers than for servo's, and until that is sorted, and 
its assembled, I think I'm barking at an empty tree.

Thanks Todd.

Cheers, Gene Heskett
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
If we desire respect for the law, we must first make the law respectable.
 - Louis D. Brandeis
Genes Web page <http://geneslinuxbox.net:6309/gene>


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Re: [Emc-users] Tuning a servo

2020-12-09 Thread Todd Zuercher 
-Original Message-
From: dave engvall  
Sent: Wednesday, December 09, 2020 1:10 PM
To: emc-users@lists.sourceforge.net
Subject: Re: [Emc-users] Tuning a servo

On 12/9/20 8:21 AM, Jon Elson wrote:
> On 12/08/2020 10:53 PM, Gene Heskett wrote:
>> What does that 22mv mean in folllowing error since I have that set 
>> for about half an inch at the moment?
> It is not milli VOLTS, it is milli-units.  Generally inches or mm, 
> depending on your preference.
> so 22m means 0.022" assuming that's what you use.  Or, if this is a 
> rotary, then it could be
> 0.022 degrees.
>
> My method of tuning is to first increase MIN_FERROR to prevent 
> following error trips, then reduce P to a small value, just enough 
> that error doesn't creep up. Adjust
> FF1 until error is small at moderate speed, less than 50 encoder 
> counts, perhaps.  Then, assuming a real axis with encoder feedback, 
> add a LITTLE FF2 to reduce the error spikes on accel/decel.  (Don't 
> bother on stepper-simulated axes.) Then, increase P to drive the error 
> down to a small fraction, add a little D to improve stability.
> It takes practice and experience to know what to adjust based on the 
> Halscope trace.
>
> Jon
Tuning seems to be a highly individualized endeavor. Hopefully a damped (in 
following error) search. :-) I start with a classic ZN ... increase P until you 
get un-dampened oscillation.  Make starting P .6 of that value.
Set FF1 equal to one and then adjust as necessary to minimize following error.
Adjust FF2 to minimize spikes on accel and decel. Adjust FF0 if needed for zero 
crossing. Try small amounts of D and I although I usually leave them a zero. 
Following error is < 1e-3 and can get down as good as 5e-4.
The values in PID, etc do seem to interact so going thru the loop a few times 
can't hurt. As always YMMV.
The major error on my machine is backlash. Adding a glass scale to X and Y 
might help. (future project).

I suspect that having tach feedback makes tuning much easier. I've never had 
much success with torque mode.

If I had the skill in linuxcnc internals I'd would try a well constrained Monte 
Carlo. No chance of my  doing that. ;-(


That’s all well and good if the command input of the amp is velocity, but I 
don't think that is the case for this one.  In torque/current mode the FF1 and 
FF2 have very different affects than with a velocity drive.  (and I never 
really completely figured it out, before I gave up and set up two nested PID 
loops.)  For a single loop you'll need as much P and D as you can make stable, 
and lots of I.  I've also found that setting the max error I limit pin to 
something that prevents windup can allow for very quick response from the I 
term without the negative side effects. (I've had good luck with setting the 
pid maxerrorI = 10/I).  I've also noticed some strangeness, where adding I 
actually seemed to made the steady state following error worse up until it was 
large, then the error started getting better until it finally came back in line 
and more I started to help a lot.  I did this searching for the point where 
more I caused instability, which I'm not sure if I ever did find.  (This was 
done before I tried limiting the max error for I.)

One of the main reasons I gave up on the single loop, was that the tune seemed 
to be very changeable and touchy, every few weeks I was tweaking on it again 
trying to get rid of following errors, instability, or overheating the drives.  
I've not had to mess with it since switching to the double loop.

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Re: [Emc-users] Tuning a servo

2020-12-09 Thread dave engvall 



On 12/9/20 8:21 AM, Jon Elson wrote:

On 12/08/2020 10:53 PM, Gene Heskett wrote:
What does that 22mv mean in folllowing error since I have that set 
for about half an inch at the moment?
It is not milli VOLTS, it is milli-units.  Generally inches or mm, 
depending on your preference.
so 22m means 0.022" assuming that's what you use.  Or, if this is a 
rotary, then it could be

0.022 degrees.

My method of tuning is to first increase MIN_FERROR to prevent 
following error trips, then reduce
P to a small value, just enough that error doesn't creep up. Adjust 
FF1 until error is small at moderate speed, less than 50 encoder 
counts, perhaps.  Then, assuming a real axis with encoder feedback, 
add a LITTLE FF2 to reduce the error spikes on accel/decel.  (Don't 
bother on stepper-simulated axes.)
Then, increase P to drive the error down to a small fraction, add a 
little D to improve stability.
It takes practice and experience to know what to adjust based on the 
Halscope trace.


Jon
Tuning seems to be a highly individualized endeavor. Hopefully a damped 
(in following error) search. :-)
I start with a classic ZN ... increase P until you get un-dampened 
oscillation.  Make starting P .6 of that value.
Set FF1 equal to one and then adjust as necessary to minimize following 
error.
Adjust FF2 to minimize spikes on accel and decel. Adjust FF0 if needed 
for zero crossing. Try small amounts of D and I although I usually leave 
them a zero. Following error is < 1e-3 and can get down as good as 5e-4.
The values in PID, etc do seem to interact so going thru the loop a few 
times can't hurt. As always YMMV.
The major error on my machine is backlash. Adding a glass scale to X and 
Y might help. (future project).


I suspect that having tach feedback makes tuning much easier. I've never 
had much success with torque mode.


If I had the skill in linuxcnc internals I'd would try a well 
constrained Monte Carlo. No chance of my  doing that. ;-(


Dave
Be careful, stay safe.



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Re: [Emc-users] Tuning a servo

2020-12-09 Thread Chris Albertson 
The bottom lie with controllers is that we trade simplicity for performance
and at some point just have to say "good enough".

You really do have to know what you are measuring and what you are
outputting.   It is this I think you are measuring position (not velocity)
 So the "error" is distance or degrees.  What or outputting is a PWM value
which in the end works out to "acceleration."   So this PID controller is
controlling position by adjusting acceleration (force).  What is "D"?
  It is the rate of change in what is being measured or the rate of change
of position, we call that "velocity" in English.   And what is "I"?  That
would be like a car's odometer reading, total distance traveled.

Note that this PID controller is very different from one that controls
spindle speed. In that case we measure velocity and control
acceleration and "D" is the rate of change of velocity.  We call this
"acceleration"  And what is "I"?  That would be velocity integrated over
time, we call that "position". So both "I" and "D" are very
different here then in the above PID.

Back to the case of using PID to control to position of a massive axis that
has low friction
Using only "P" is like drivng a fully loaded semi truck using only the gas
pedal.  When you get to the parking spot and take you foot off the gas the
truck just keeps moving unless you have brakes.  With just "P" there are no
brakes.  You need something negative to oppose forward motion are you never
stop .

PID is not the best kind of controller for this.  PID is purely reactive
and never plans ahead and will never give optimal performance in the case
where you use acceleration to control position.The smarter truck driver
knows how effective his brakes are at different speeds and loaded gross
weights and can see the distance to the parking space.  He fully calculates
a good-enough trajectory plan and then only makes tiny adjustments to his
plan as he drives and if he is good the truck stops right on the mark.   We
can do this with computerized motion control but "MPC" controllers are
needed which are perhaps 200X more complex than PID controllers.  So we
stick with PID and learn to accept the performance hit.

While getting a perfect plan using MPC is hard, sometimes getting a good
plan is good enough and people sometimes use a nested PID for this.   The
first one controls motor velocity and works well because velocity is so
much easier to control.   Then a second PID controller is used to provide
velocity set points to control position.   The nested PID is only 2X more
complex than a single PID, not 200X like MPC.

My understanding is that LCNC does use nested controllers and does have
some form of look-ahead planning.  I know that I don't know how to use that.


On Tue, Dec 8, 2020 at 8:56 PM Gene Heskett  wrote:

> On Tuesday 08 December 2020 19:56:13 Chris Albertson wrote:
>
> > If the output of the PID controller is force ad the measured feedback
> > is the position, a P only controller will always oscillate.  Friction
> > will damp it eventually.   A negative "D" will reduce the velocity and
> > help with overshoot and "I" is required if the final result has an
> > offset.
> >
> > The root cause is the force is a couple of steps removed from
> > the position.
> >
> > If you want the system to go full speed then slam on the brakes at the
> > last minute and hit the marks then you need a controller that has a
> > physical model of the system and use that to predict the future and
> > plan ahead.  PID is always reactive.  It can't look ahead.
> >
> > But PIS can come close.   After you get P tuned "close" try "D" and
> > depending on how it is wired may be negative
> >
> I'll give that a try tomorrow. Or maybe right now since I wasn't sleeping
> well.  And with a Dgain of -2.5 got a 22mv error at 101 degrees a minute
> with little, maybe 10% ringing on the rising edge, and a less than 10%
> peak "effort" in reverse to stop it, That is a major improvement and
> doesn't trip off the psu getting stopped.  What does that 22mv mean in
> folllowing error since I have that set for about half an inch at the
> moment?
>
> This I think might be good enough to go ahead and finish the mount
> machining and get it installed on the bs-1.  With all the excitement and
> sadness here, I've only been working on easily interruptible things.
>
> Thanks Chris.
>
> > On Tue, Dec 8, 2020 at 4:42 PM Gene Heskett 
> wrote:
> > > On Tuesday 08 December 2020 19:05:52 Chris Albertson wrote:
> > > > What next?  You have set P-gain but what about I and D gains?
> > >
> > > That has been tried, gently, but doesn't seem to have the desired
> > > effect.
> > >
> > > > On Tue, Dec 8, 2020 at 12:49 PM Gene Heskett
> > > > 
> > >
> > > wrote:
> > > > > Lo all;
> > > > >
> > > > > I'm back to playing with the servo again, following John
> > > > > Thorntons instructs from the wiki article.  And I note a couple
> > > > > things.
> > > > >
> > > > > 1. The pid_a.FF1 value

Re: [Emc-users] Tuning a servo

2020-12-09 Thread Jon Elson 

On 12/08/2020 10:53 PM, Gene Heskett wrote:
What does that 22mv mean in folllowing error since I have 
that set for about half an inch at the moment?
It is not milli VOLTS, it is milli-units.  Generally inches 
or mm, depending on your preference.
so 22m means 0.022" assuming that's what you use.  Or, if 
this is a rotary, then it could be

0.022 degrees.

My method of tuning is to first increase MIN_FERROR to 
prevent following error trips, then reduce
P to a small value, just enough that error doesn't creep 
up.  Adjust FF1 until error is small at moderate speed, less 
than 50 encoder counts, perhaps.  Then, assuming a real axis 
with encoder feedback, add a LITTLE FF2 to reduce the error 
spikes on accel/decel.  (Don't bother on stepper-simulated 
axes.)
Then, increase P to drive the error down to a small 
fraction, add a little D to improve stability.
It takes practice and experience to know what to adjust 
based on the Halscope trace.


Jon



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Re: [Emc-users] Tuning a servo

2020-12-08 Thread Gene Heskett 
On Tuesday 08 December 2020 19:56:13 Chris Albertson wrote:

> If the output of the PID controller is force ad the measured feedback
> is the position, a P only controller will always oscillate.  Friction
> will damp it eventually.   A negative "D" will reduce the velocity and
> help with overshoot and "I" is required if the final result has an
> offset.
>
> The root cause is the force is a couple of steps removed from
> the position.
>
> If you want the system to go full speed then slam on the brakes at the
> last minute and hit the marks then you need a controller that has a
> physical model of the system and use that to predict the future and
> plan ahead.  PID is always reactive.  It can't look ahead.
>
> But PIS can come close.   After you get P tuned "close" try "D" and
> depending on how it is wired may be negative
>
I'll give that a try tomorrow. Or maybe right now since I wasn't sleeping 
well.  And with a Dgain of -2.5 got a 22mv error at 101 degrees a minute 
with little, maybe 10% ringing on the rising edge, and a less than 10% 
peak "effort" in reverse to stop it, That is a major improvement and 
doesn't trip off the psu getting stopped.  What does that 22mv mean in 
folllowing error since I have that set for about half an inch at the 
moment?

This I think might be good enough to go ahead and finish the mount 
machining and get it installed on the bs-1.  With all the excitement and 
sadness here, I've only been working on easily interruptible things.

Thanks Chris.

> On Tue, Dec 8, 2020 at 4:42 PM Gene Heskett  
wrote:
> > On Tuesday 08 December 2020 19:05:52 Chris Albertson wrote:
> > > What next?  You have set P-gain but what about I and D gains?
> >
> > That has been tried, gently, but doesn't seem to have the desired
> > effect.
> >
> > > On Tue, Dec 8, 2020 at 12:49 PM Gene Heskett
> > > 
> >
> > wrote:
> > > > Lo all;
> > > >
> > > > I'm back to playing with the servo again, following John
> > > > Thorntons instructs from the wiki article.  And I note a couple
> > > > things.
> > > >
> > > > 1. The pid_a.FF1 value has little or no effect on the amplitude
> > > > of the error. So that probably needs addressed by educating me
> > > > on how its calculated.
> > > >
> > > > 2.pid_a.FF2 isn't working as that article says it should, a
> > > > little effect on the shape but zero effect that would lead to a
> > > > zero error when cruising
> > > >
> > > > 3. pid_a.Pgain effects the amplitude of the error signal, with
> > > > about 35 being the onset of a low level oscillation, about
> > > > 10% when cruising.  But its also the lowest error at 15 to 20
> > > > millivolts.
> > > >
> > > > 4. I am getting some improvement. but nothing like the pix on
> > > > that wiki article.
> > > >
> > > > It explains that pid_a.FF1 should be 10/velocity@10V (velocity
> > > > is in machine units per second).  Since I do not yet have the
> > > > motor actually moving the BS-1 clone, nor have I managed to get
> > > > a home switch set up so I can actually measure the scale, the
> > > > scale temporarily set is likely wrong. Since I've 2 worms in
> > > > series, for playing I've a high value set for scale, 5000. With
> > > > the encoder on the back of the motor it could well be higher
> > > > than that.
> > > >
> > > > But no combination of numbers results in a cruising error of
> > > > zero, its alway plus at cruise speed.
> > > >
> > > > And if I go at more than about 30% high pwm, motor inertia
> > > > causes the servo to use reverse to stop if I ask for more than
> > > > 60 degrees a second. This use of reverse causes the supply, a
> > > > 350 watt 24 volt switcher, to do a shutdown because it thinks is
> > > > been crowbarred, and the powerdown to recover is around 3
> > > > minutes.
> > > >
> > > > I think I might have a way that will fix that as this driver can
> > > > do crowbar the motor braking if the signals are re-arranged and
> > > > fast enough. If I setup a hardware timer in the form of a a
> > > > retriggerable ooneshot drive  from the pwm signal, which is
> > > > running at 4 kilohertz, or 250u-secs pulse is a 100% signal, but
> > > > the timer timesout in 200 microseconds, it will time out and
> > > > apply the brakes for whats left of the 250 microsecond pwm cycle
> > > > if the pwn drops below 20%. This should slow the motor fast
> > > > enough it will never actually use reverse to stop.  And that
> > > > mode will never effect the psu.
> > > >
> > > > But first, what can I do about the ineffectiveness of setting
> > > > FF1 anyplace between .5 and 25. I think its effecting the motors
> > > > cruising speed but has zero effect on the amplitude or shape of
> > > > the error if enough Pgain is used to make it follow well.
> > > >
> > > > With Pgain at 25 it cruises stably, with a reasonably flat
> > > > top to the error, about 20 millivolts of error, but FF2 seems to
> > > > have little effect on the initial overshoot, or the overshoot
> > > > when stopping, and its the overshoot when

Re: [Emc-users] Tuning a servo

2020-12-08 Thread Chris Albertson 
If the output of the PID controller is force ad the measured feedback is
the position, a P only controller will always oscillate.  Friction will
damp it eventually.   A negative "D" will reduce the velocity and help with
overshoot and "I" is required if the final result has an offset.

The root cause is the force is a couple of steps removed from
the position.

If you want the system to go full speed then slam on the brakes at the last
minute and hit the marks then you need a controller that has a physical
model of the system and use that to predict the future and plan ahead.  PID
is always reactive.  It can't look ahead.

But PIS can come close.   After you get P tuned "close" try "D" and
depending on how it is wired may be negative

On Tue, Dec 8, 2020 at 4:42 PM Gene Heskett  wrote:

> On Tuesday 08 December 2020 19:05:52 Chris Albertson wrote:
>
> > What next?  You have set P-gain but what about I and D gains?
> >
> That has been tried, gently, but doesn't seem to have the desired effect.
>
> > On Tue, Dec 8, 2020 at 12:49 PM Gene Heskett 
> wrote:
> > > Lo all;
> > >
> > > I'm back to playing with the servo again, following John Thorntons
> > > instructs from the wiki article.  And I note a couple things.
> > >
> > > 1. The pid_a.FF1 value has little or no effect on the amplitude of
> > > the error. So that probably needs addressed by educating me on how
> > > its calculated.
> > >
> > > 2.pid_a.FF2 isn't working as that article says it should, a little
> > > effect on the shape but zero effect that would lead to a zero error
> > > when cruising
> > >
> > > 3. pid_a.Pgain effects the amplitude of the error signal, with about
> > > 35 being the onset of a low level oscillation, about 10% when
> > > cruising.  But its also the lowest error at 15 to 20 millivolts.
> > >
> > > 4. I am getting some improvement. but nothing like the pix on that
> > > wiki article.
> > >
> > > It explains that pid_a.FF1 should be 10/velocity@10V (velocity is in
> > > machine units per second).  Since I do not yet have the motor
> > > actually moving the BS-1 clone, nor have I managed to get a home
> > > switch set up so I can actually measure the scale, the scale
> > > temporarily set is likely wrong. Since I've 2 worms in series, for
> > > playing I've a high value set for scale, 5000. With the encoder on
> > > the back of the motor it could well be higher than that.
> > >
> > > But no combination of numbers results in a cruising error of zero,
> > > its alway plus at cruise speed.
> > >
> > > And if I go at more than about 30% high pwm, motor inertia causes
> > > the servo to use reverse to stop if I ask for more than 60 degrees a
> > > second. This use of reverse causes the supply, a 350 watt 24 volt
> > > switcher, to do a shutdown because it thinks is been crowbarred, and
> > > the powerdown to recover is around 3 minutes.
> > >
> > > I think I might have a way that will fix that as this driver can do
> > > crowbar the motor braking if the signals are re-arranged and fast
> > > enough. If I setup a hardware timer in the form of a a retriggerable
> > > ooneshot drive  from the pwm signal, which is running at 4
> > > kilohertz, or 250u-secs pulse is a 100% signal, but the timer
> > > timesout in 200 microseconds, it will time out and apply the brakes
> > > for whats left of the 250 microsecond pwm cycle if the pwn drops
> > > below 20%. This should slow the motor fast enough it will never
> > > actually use reverse to stop.  And that mode will never effect the
> > > psu.
> > >
> > > But first, what can I do about the ineffectiveness of setting FF1
> > > anyplace between .5 and 25. I think its effecting the motors
> > > cruising speed but has zero effect on the amplitude or shape of the
> > > error if enough Pgain is used to make it follow well.
> > >
> > > With Pgain at 25 it cruises stably, with a reasonably flat top
> > > to the error, about 20 millivolts of error, but FF2 seems to have
> > > little effect on the initial overshoot, or the overshoot when
> > > stopping, and its the overshoot when stopping that is shutting down
> > > the supply unless I set max jog to below 60 so friction stops it.
> > > That 60 is about 1/3rd of what the motor can do in terms of output
> > > shaft rpms if fed its normal 24 volts.
> > >
> > > Anybody know what I should look at next?
> > >
> > > Thank you.
> > >
> > > Cheers, Gene Heskett
> > >
> > >
> > > --
> > > "There are four boxes to be used in defense of liberty:
> > >  soap, ballot, jury, and ammo. Please use in that order."
> > > -Ed Howdershelt (Author)
> > > If we desire respect for the law, we must first make the law
> > > respectable. - Louis D. Brandeis
> > > Genes Web page 
> > >
> > > ___
> > > Emc-users mailing list
> > > Emc-users@lists.sourceforge.net
> > > https://lists.sourceforge.net/lists/listinfo/emc-users
>
>
> Cheers, Gene Heskett
> --
> "There are four boxes to be used in

Re: [Emc-users] Tuning a servo

2020-12-08 Thread Gene Heskett 
On Tuesday 08 December 2020 19:05:52 Chris Albertson wrote:

> What next?  You have set P-gain but what about I and D gains?
>
That has been tried, gently, but doesn't seem to have the desired effect.

> On Tue, Dec 8, 2020 at 12:49 PM Gene Heskett  
wrote:
> > Lo all;
> >
> > I'm back to playing with the servo again, following John Thorntons
> > instructs from the wiki article.  And I note a couple things.
> >
> > 1. The pid_a.FF1 value has little or no effect on the amplitude of
> > the error. So that probably needs addressed by educating me on how
> > its calculated.
> >
> > 2.pid_a.FF2 isn't working as that article says it should, a little
> > effect on the shape but zero effect that would lead to a zero error
> > when cruising
> >
> > 3. pid_a.Pgain effects the amplitude of the error signal, with about
> > 35 being the onset of a low level oscillation, about 10% when
> > cruising.  But its also the lowest error at 15 to 20 millivolts.
> >
> > 4. I am getting some improvement. but nothing like the pix on that
> > wiki article.
> >
> > It explains that pid_a.FF1 should be 10/velocity@10V (velocity is in
> > machine units per second).  Since I do not yet have the motor
> > actually moving the BS-1 clone, nor have I managed to get a home
> > switch set up so I can actually measure the scale, the scale
> > temporarily set is likely wrong. Since I've 2 worms in series, for
> > playing I've a high value set for scale, 5000. With the encoder on
> > the back of the motor it could well be higher than that.
> >
> > But no combination of numbers results in a cruising error of zero,
> > its alway plus at cruise speed.
> >
> > And if I go at more than about 30% high pwm, motor inertia causes
> > the servo to use reverse to stop if I ask for more than 60 degrees a
> > second. This use of reverse causes the supply, a 350 watt 24 volt
> > switcher, to do a shutdown because it thinks is been crowbarred, and
> > the powerdown to recover is around 3 minutes.
> >
> > I think I might have a way that will fix that as this driver can do
> > crowbar the motor braking if the signals are re-arranged and fast
> > enough. If I setup a hardware timer in the form of a a retriggerable
> > ooneshot drive  from the pwm signal, which is running at 4
> > kilohertz, or 250u-secs pulse is a 100% signal, but the timer
> > timesout in 200 microseconds, it will time out and apply the brakes
> > for whats left of the 250 microsecond pwm cycle if the pwn drops
> > below 20%. This should slow the motor fast enough it will never
> > actually use reverse to stop.  And that mode will never effect the
> > psu.
> >
> > But first, what can I do about the ineffectiveness of setting FF1
> > anyplace between .5 and 25. I think its effecting the motors
> > cruising speed but has zero effect on the amplitude or shape of the
> > error if enough Pgain is used to make it follow well.
> >
> > With Pgain at 25 it cruises stably, with a reasonably flat top
> > to the error, about 20 millivolts of error, but FF2 seems to have
> > little effect on the initial overshoot, or the overshoot when
> > stopping, and its the overshoot when stopping that is shutting down
> > the supply unless I set max jog to below 60 so friction stops it. 
> > That 60 is about 1/3rd of what the motor can do in terms of output
> > shaft rpms if fed its normal 24 volts.
> >
> > Anybody know what I should look at next?
> >
> > Thank you.
> >
> > Cheers, Gene Heskett
> >
> >
> > --
> > "There are four boxes to be used in defense of liberty:
> >  soap, ballot, jury, and ammo. Please use in that order."
> > -Ed Howdershelt (Author)
> > If we desire respect for the law, we must first make the law
> > respectable. - Louis D. Brandeis
> > Genes Web page 
> >
> > ___
> > Emc-users mailing list
> > Emc-users@lists.sourceforge.net
> > https://lists.sourceforge.net/lists/listinfo/emc-users


Cheers, Gene Heskett
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
If we desire respect for the law, we must first make the law respectable.
 - Louis D. Brandeis
Genes Web page 


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Re: [Emc-users] Tuning a servo

2020-12-08 Thread Chris Albertson 
What next?  You have set P-gain but what about I and D gains?




On Tue, Dec 8, 2020 at 12:49 PM Gene Heskett  wrote:

> Lo all;
>
> I'm back to playing with the servo again, following John Thorntons
> instructs from the wiki article.  And I note a couple things.
>
> 1. The pid_a.FF1 value has little or no effect on the amplitude of the
> error. So that probably needs addressed by educating me on how its
> calculated.
>
> 2.pid_a.FF2 isn't working as that article says it should, a little effect
> on the shape but zero effect that would lead to a zero error when cruising
>
> 3. pid_a.Pgain effects the amplitude of the error signal, with about
> 35 being the onset of a low level oscillation, about 10% when
> cruising.  But its also the lowest error at 15 to 20 millivolts.
>
> 4. I am getting some improvement. but nothing like the pix on that wiki
> article.
>
> It explains that pid_a.FF1 should be 10/velocity@10V (velocity is in
> machine units per second).  Since I do not yet have the motor actually
> moving the BS-1 clone, nor have I managed to get a home switch set up so I
> can actually measure the scale, the scale temporarily set is likely wrong.
> Since I've 2 worms in series, for playing I've a high value set for scale,
> 5000. With the encoder on the back of the motor it could well be higher
> than that.
>
> But no combination of numbers results in a cruising error of zero, its
> alway plus at cruise speed.
>
> And if I go at more than about 30% high pwm, motor inertia causes the
> servo to use reverse to stop if I ask for more than 60 degrees a second.
> This use of reverse causes the supply, a 350 watt 24 volt switcher, to do a
> shutdown because it thinks is been crowbarred, and the powerdown to recover
> is around 3 minutes.
>
> I think I might have a way that will fix that as this driver can do
> crowbar the motor braking if the signals are re-arranged and fast enough.
> If I setup a hardware timer in the form of a a retriggerable ooneshot
> drive  from the pwm signal, which is running at 4 kilohertz, or 250u-secs
> pulse is a 100% signal, but the timer timesout in 200 microseconds, it will
> time out and apply the brakes for whats left of the 250 microsecond pwm
> cycle if the pwn drops below 20%. This should slow the motor fast enough it
> will never actually use reverse to stop.  And that mode will never effect
> the psu.
>
> But first, what can I do about the ineffectiveness of setting FF1 anyplace
> between .5 and 25. I think its effecting the motors cruising speed but has
> zero effect on the amplitude or shape of the error if enough Pgain is used
> to make it follow well.
>
> With Pgain at 25 it cruises stably, with a reasonably flat top to the
> error, about 20 millivolts of error, but FF2 seems to have little effect on
> the initial overshoot, or the overshoot when stopping, and its the
> overshoot when stopping that is shutting down the supply unless I set max
> jog to below 60 so friction stops it.  That 60 is about 1/3rd of what the
> motor can do in terms of output shaft rpms if fed its normal 24 volts.
>
> Anybody know what I should look at next?
>
> Thank you.
>
> Cheers, Gene Heskett
>
>
> --
> "There are four boxes to be used in defense of liberty:
>  soap, ballot, jury, and ammo. Please use in that order."
> -Ed Howdershelt (Author)
> If we desire respect for the law, we must first make the law respectable.
>  - Louis D. Brandeis
> Genes Web page 
>
> ___
> Emc-users mailing list
> Emc-users@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/emc-users
>


-- 

Chris Albertson
Redondo Beach, California

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