What for the "D" in a PID control loop?

[George Herold]

I’ve built several PID control loops, mostly for temperature control,
but sometimes for other things. I’ve always used the all three PID
parameters. I use to think that the D term was there to help reduce
the ringing. (As stated in the wiki PID article.) And was needed to
get critical damping of the loop.

But this misunderstanding was corrected a few years ago by a
colleague. For thermal loops you’ve got one pole in the thermal time
constant of the system and can add another with PI control and with
proper tuning can have critical damping of the loop with just P and I
control.

Hence my question, What’s the D for? Is it helping or only making
things worse?

George H.

 

[Tim Wescott]

On Fri, 19 Feb 2010 10:41:28 -0800, George Herold wrote:

I’ve built several PID control loops, mostly for temperature control,
but sometimes for other things. I’ve always used the all three PID
parameters. I use to think that the D term was there to help reduce the
ringing. (As stated in the wiki PID article.) And was needed to get
critical damping of the loop.

But this misunderstanding was corrected a few years ago by a colleague.
For thermal loops you’ve got one pole in the thermal time constant of
the system and can add another with PI control and with proper tuning
can have critical damping of the loop with just P and I control.

Hence my question, What’s the D for? Is it helping or only making
things worse?

In the hands of the inexperienced, it is raising management's hopes,
destroying production's peace of mind, and generating potential revenue
for me.

The use of the differentiator in a PID controller is problematical. In
general, the differentiator acts as a 'predictor' to let the controller
anticipate future behavior of the plant. This lets you get snappier
response out of the system, because the controller knows in advance when
to 'put on the brakes'. But its action as a predictor is imperfect, so
if you depend on it too much you just get oscillation at a much higher
frequency than you could with proportional control alone.

So it _does_ help to damp the system, but only when you were pushing your
proportional gain a bit too hard.

Differential action predicts, but it does so by looking at the past. If
you plot the action of a differentiator as response vs. frequency, the
amplitude of the response goes up and up and up with frequency. This is
an undesirable trait, as just about any plant's behavior gets less and
less predictable as the frequency gets higher. Push your derivative gain
too hard, and you'll find your plant ringing like a bell, or outright
singing.

If you use a differentiator you really want to band-limit its response --
but to do this right you really need to understand more about control
theory than just cookbook PID implementations.

Here's an article that has some examples of tuning systems with and
without differential action:
http://www.wescottdesign.com/articles/Sampling/pidwophd.html.

If you _really_ want to go overboard, particularly if you're implementing
your controllers in software, you'll be interested in my book:
http://www.wescottdesign.com/actfes/actfes.html.

HTH

--
www.wescottdesign.com

 

[Tim Wescott]

On Fri, 19 Feb 2010 13:21:50 -0800, George Herold wrote:

without differential
action:http://www.wescottdesign.com/articles/Sampling/pidwophd.html.

If you _really_ want to go overboard, particularly if you're
implementing your controllers in software, you'll be interested in my
book:http://www.wescottdesign.com/actfes/actfes.html.

HTH

--www.wescottdesign.com

Hi Tim, Thanks for the response.

I'm not sure I get your 'predictor' analogy. But if you want to say
that the D gives you more gain at higher frequencies then I agree. I
guess I tend to look at PID controllers from the frequency domain. P
has a flat response at all frequencies, I gives you lots of gain at DC
but then goes away, and D is all at the high frequency end. (Oh and I
do always roll the D back off.. typically at frequencies that are about
ten times greater than were it 'turns on'.)

I didn't know how much you knew, so I was using the time-domain analogy
that's easier to grasp if you don't understand frequency domain stuff.
The differential term predicts because it measures velocity. It's the
difference between being parked in front of a brick wall, and hurtling
toward that same brick wall at 60 miles an hour -- if you're not moving
(D term = 0), one foot away from the wall is merely 'close'. If you're
moving fast (D term = +60MPH), 50 feet away is 'oh my gawd!!!'. Why?
Because the controller in your brain is calculating speed and distance
vectors, and predicting a noisy and maybe painful crash.

"http://www.wescottdesign.com/articles/Sampling/pidwophd.html." Thanks
for the link. Thanks for the link. I'm afraid I've got a PhD... can I
still read your article?? :^)

I guess I'm wondering if, given the simple thermal control loops I'm
typically using, if it would be better to throw away the D on the next
project. Saves an opamp and a few big caps.

Well, that depends. Some thermal plants tend to pile all of their phase
shift in one place, so there's only so much you can buy with differential
control. In that case there's certainly reason to think about a cost/
benefit tradeoff. If you can measure the frequency response of the plant
you can do all of this ahead of time, including tuning, before you commit
to a board. Or you can just prototype a controller, of course.

I'm not much of a computer
guy and make only analog PI(D) loops. I've always hated the dead band
you get with commercial Digital controllers, from say Omega or others.

Commercial digital controllers are, as near as I can tell, designed and
implemented by Oog and Og using methods and technology -- other than the
communications bells and whistles -- that was obsolete decades ago.

If you're controlling a temperature you can do the whole job of control
with an 8-pin microprocessor connected to an 8-pin 24-bit ADC; at worst
you'll need a level shifter and low pass filter before you go to your
drive electronics, and you may well use less board space than one of your
big caps. _And_ your controller's response time will be well within the
response time of your plant, so you won't see any discernible change in
performance.

But don't let that keep you from your appointed path.

--
www.wescottdesign.com

 

[George Herold]

On Feb 19, 3:46 pm, Tim Wescott <t...@seemywebsite.com> wrote:

On Fri, 19 Feb 2010 10:41:28 -0800, George Herold wrote:
I’ve built several PID control loops, mostly for temperature control,
but sometimes for other things.  I’ve always used  the all three PID
parameters.  I use to think that the D term was there to help reduce the
ringing.  (As stated in the wiki PID article.)  And was needed to get
critical damping of the loop.

But this misunderstanding was corrected a few years ago by a colleague.
For thermal loops you’ve got one pole in the thermal time constant of
the system and can add another with  PI control and with proper tuning
can have critical damping of the loop with just P and I control.

Hence my question, What’s the D for?  Is it helping or only making
things worse?

In the hands of the inexperienced, it is raising management's hopes,
destroying production's peace of mind, and generating potential revenue
for me.

The use of the differentiator in a PID controller is problematical.  In
general, the differentiator acts as a 'predictor' to let the controller
anticipate future behavior of the plant.  This lets you get snappier
response out of the system, because the controller knows in advance when
to 'put on the brakes'.  But its action as a predictor is imperfect, so
if you depend on it too much you just get oscillation at a much higher
frequency than you could with proportional control alone.

So it _does_ help to damp the system, but only when you were pushing your
proportional gain a bit too hard.

Differential action predicts, but it does so by looking at the past.  If
you plot the action of a differentiator as response vs. frequency, the
amplitude of the response goes up and up and up with frequency.  This is
an undesirable trait, as just about any plant's behavior gets less and
less predictable as the frequency gets higher.  Push your derivative gain
too hard, and you'll find your plant ringing like a bell, or outright
singing.

If you use a differentiator you really want to band-limit its response --
but to do this right you really need to understand more about control
theory than just cookbook PID implementations.

Here's an article that has some examples of tuning systems with and
without differential action:http://www.wescottdesign.com/articles/Sampling/pidwophd.html.

If you _really_ want to go overboard, particularly if you're implementing
your controllers in software, you'll be interested in my book:http://www.wescottdesign.com/actfes/actfes.html.

HTH

--www.wescottdesign.com

Hi Tim, Thanks for the response.

I'm not sure I get your 'predictor' analogy. But if you want to say
that the D gives you more gain at higher frequencies then I agree. I
guess I tend to look at PID controllers from the frequency domain. P
has a flat response at all frequencies, I gives you lots of gain at DC
but then goes away, and D is all at the high frequency end. (Oh and I
do always roll the D back off.. typically at frequencies that are
about ten times greater than were it 'turns on'.)

"http://www.wescottdesign.com/articles/Sampling/pidwophd.html."
Thanks for the link. Thanks for the link. I'm afraid I've got a
PhD... can I still read your article?? :^)

I guess I'm wondering if, given the simple thermal control loops I'm
typically using, if it would be better to throw away the D on the next
project. Saves an opamp and a few big caps. I'm not much of a
computer guy and make only analog PI(D) loops. I've always hated the
dead band you get with commercial Digital controllers, from say Omega
or others.

George H.

 

[George Herold]

On Feb 19, 4:39 pm, Tim Wescott <t...@seemywebsite.com> wrote:

On Fri, 19 Feb 2010 13:21:50 -0800, George Herold wrote:
without differential
action:http://www.wescottdesign.com/articles/Sampling/pidwophd.html.

If you _really_ want to go overboard, particularly if you're
implementing your controllers in software, you'll be interested in my
book:http://www.wescottdesign.com/actfes/actfes.html.

HTH

--www.wescottdesign.com

Hi Tim,  Thanks for the response.

I'm not sure I get your 'predictor' analogy.  But if you want to say
that the D gives you more gain at higher frequencies then I agree.  I
guess I tend to look at PID controllers from the frequency domain.  P
has a flat response at all frequencies, I gives you lots of gain at DC
but then goes away, and D is all at the high frequency end.  (Oh and I
do always roll the D back off.. typically at frequencies that are about
ten times greater than were it 'turns on'.)

I didn't know how much you knew, so I was using the time-domain analogy
that's easier to grasp if you don't understand frequency domain stuff.  
The differential term predicts because it measures velocity.  It's the
difference between being parked in front of a brick wall, and hurtling
toward that same brick wall at 60 miles an hour -- if you're not moving
(D term = 0), one foot away from the wall is merely 'close'.  If you're
moving fast (D term = +60MPH), 50 feet away is 'oh my gawd!!!'.  Why?  
Because the controller in your brain is calculating speed and distance
vectors, and predicting a noisy and maybe painful crash.

Hmm, Yeah most of the time the 'plant' is just sitting there at the
setpoint and the D is not doing anything. (At least in my
applications this is the case.) I'm not quite sure what a 'plant'
is. Most of the time I dragging little (100 gram) pieces of 'stuff'
around with ~10 Watts of power. (The last thermal project was
temperature control of a ~1kg permanent magnet used for NMR.)

"http://www.wescottdesign.com/articles/Sampling/pidwophd.html." Thanks
for the link.  Thanks for the link.  I'm afraid I've got a PhD... can I
still read your article?? :^)

I guess I'm wondering if, given the simple thermal control loops I'm
typically using, if it would be better to throw away the D on the next
project.  Saves an opamp and a few big caps.

Well, that depends.  Some thermal plants tend to pile all of their phase
shift in one place, so there's only so much you can buy with differential
control.  In that case there's certainly reason to think about a cost/
benefit tradeoff.  If you can measure the frequency response of the plant
you can do all of this ahead of time, including tuning, before you commit
to a board.  Or you can just prototype a controller, of course.

Hmm I'm not quite sure what the phase shift is in this case. Are you
talking about how distributed the thermal mass is?

I'm not much of a computer
guy and make only analog PI(D) loops.  I've always hated the dead band
you get with commercial Digital controllers, from say Omega or others.

Commercial digital controllers are, as near as I can tell, designed and
implemented by Oog and Og using methods and technology -- other than the
communications bells and whistles -- that was obsolete decades ago.

Yeah they suck! Someone should make something better then. We pay
almost $300 for these POS's.

If you're controlling a temperature you can do the whole job of control
with an 8-pin microprocessor connected to an 8-pin 24-bit ADC; at worst
you'll need a level shifter and low pass filter before you go to your
drive electronics, and you may well use less board space than one of your
big caps.  _And_ your controller's response time will be well within the
response time of your plant, so you won't see any discernible change in
performance.

Well it's the whole learning curve thing. Your micro and ADC are
replacing a few opamps and caps. I then just have to choose and tweak
some R's. Still I'd like learn more about micro's and all.

But don't let that keep you from your appointed path.

What appointed path. I think my life is more like the blind pig
finding an acorn.

--www.wescottdesign.com- Hide quoted text -

- Show quoted text -

But still I think you implied that simple 'approximately single pole'
thermal systems may not need the Derivative term. Hmm maybe I should
try doing some thermal modeling on LTspice ala John L.

George H.

 

[Tim Wescott]

On Fri, 19 Feb 2010 18:58:27 -0800, George Herold wrote:

On Feb 19, 4:39 pm, Tim Wescott <t...@seemywebsite.com> wrote:
On Fri, 19 Feb 2010 13:21:50 -0800, George Herold wrote:
without differential
action:http://www.wescottdesign.com/articles/Sampling/pidwophd.html.

If you _really_ want to go overboard, particularly if you're
implementing your controllers in software, you'll be interested in
my book:http://www.wescottdesign.com/actfes/actfes.html.

HTH

--www.wescottdesign.com

Hi Tim,  Thanks for the response.

I'm not sure I get your 'predictor' analogy.  But if you want to say
that the D gives you more gain at higher frequencies then I agree.  I
guess I tend to look at PID controllers from the frequency domain.  P
has a flat response at all frequencies, I gives you lots of gain at
DC but then goes away, and D is all at the high frequency end.  (Oh
and I do always roll the D back off.. typically at frequencies that
are about ten times greater than were it 'turns on'.)

I didn't know how much you knew, so I was using the time-domain analogy
that's easier to grasp if you don't understand frequency domain stuff.
The differential term predicts because it measures velocity.  It's the
difference between being parked in front of a brick wall, and hurtling
toward that same brick wall at 60 miles an hour -- if you're not moving
(D term = 0), one foot away from the wall is merely 'close'.  If you're
moving fast (D term = +60MPH), 50 feet away is 'oh my gawd!!!'.  Why?
Because the controller in your brain is calculating speed and distance
vectors, and predicting a noisy and maybe painful crash.


Hmm, Yeah most of the time the 'plant' is just sitting there at the
setpoint and the D is not doing anything. (At least in my applications
this is the case.) I'm not quite sure what a 'plant' is. Most of the
time I dragging little (100 gram) pieces of 'stuff' around with ~10
Watts of power. (The last thermal project was temperature control of a
~1kg permanent magnet used for NMR.)

"Plant" == "thing my controller is responsible for". It comes, I think,
from "steam plant" back in the days when control theory was being
invented.

I've heard lots of scary stories about how hard it is to control
temperature, but the few times I've done it things have acted like
straightforward single-order low pass filters.

"http://www.wescottdesign.com/articles/Sampling/pidwophd.html."
Thanks for the link.  Thanks for the link.  I'm afraid I've got a
PhD... can I still read your article?? :^)

I guess I'm wondering if, given the simple thermal control loops I'm
typically using, if it would be better to throw away the D on the
next project.  Saves an opamp and a few big caps.

Well, that depends.  Some thermal plants tend to pile all of their
phase shift in one place, so there's only so much you can buy with
differential control.  In that case there's certainly reason to think
about a cost/ benefit tradeoff.  If you can measure the frequency
response of the plant you can do all of this ahead of time, including
tuning, before you commit to a board.  Or you can just prototype a
controller, of course.

Hmm I'm not quite sure what the phase shift is in this case. Are you
talking about how distributed the thermal mass is?

Yes.

I'm not much of a computer
guy and make only analog PI(D) loops.  I've always hated the dead
band you get with commercial Digital controllers, from say Omega or
others.

Commercial digital controllers are, as near as I can tell, designed and
implemented by Oog and Og using methods and technology -- other than
the communications bells and whistles -- that was obsolete decades ago.

Yeah they suck! Someone should make something better then. We pay
almost $300 for these POS's.


If you're controlling a temperature you can do the whole job of control
with an 8-pin microprocessor connected to an 8-pin 24-bit ADC; at worst
you'll need a level shifter and low pass filter before you go to your
drive electronics, and you may well use less board space than one of
your big caps.  _And_ your controller's response time will be well
within the response time of your plant, so you won't see any
discernible change in performance.

Well it's the whole learning curve thing. Your micro and ADC are
replacing a few opamps and caps. I then just have to choose and tweak
some R's. Still I'd like learn more about micro's and all.

But don't let that keep you from your appointed path.

What appointed path. I think my life is more like the blind pig finding
an acorn.


--www.wescottdesign.com- Hide quoted text -

- Show quoted text -

But still I think you implied that simple 'approximately single pole'
thermal systems may not need the Derivative term. Hmm maybe I should
try doing some thermal modeling on LTspice ala John L.

Yes, depending on how well they have to reject disturbances they may well
not need it.

--
www.wescottdesign.com

 

[Ross Herbert]

On Fri, 19 Feb 2010 10:41:28 -0800 (PST), George Herold <ggherold@gmail.com>
wrote:

:
:I’ve built several PID control loops, mostly for temperature control,
:but sometimes for other things. I’ve always used the all three PID
arameters. I use to think that the D term was there to help reduce
:the ringing. (As stated in the wiki PID article.) And was needed to
:get critical damping of the loop.
:
:But this misunderstanding was corrected a few years ago by a
:colleague. For thermal loops you’ve got one pole in the thermal time
:constant of the system and can add another with PI control and with
roper tuning can have critical damping of the loop with just P and I
:control.
:
:Hence my question, What’s the D for? Is it helping or only making
:things worse?
:
:George H.

See http://mcshaneinc.com/html/Library_UnderstandingPID.html

 

[Jamie]

Ross Herbert wrote:

On Fri, 19 Feb 2010 10:41:28 -0800 (PST), George Herold <ggherold@gmail.com
wrote:

:
:I’ve built several PID control loops, mostly for temperature control,
:but sometimes for other things. I’ve always used the all three PID
arameters. I use to think that the D term was there to help reduce
:the ringing. (As stated in the wiki PID article.) And was needed to
:get critical damping of the loop.
:
:But this misunderstanding was corrected a few years ago by a
:colleague. For thermal loops you’ve got one pole in the thermal time
:constant of the system and can add another with PI control and with
roper tuning can have critical damping of the loop with just P and I
:control.
:
:Hence my question, What’s the D for? Is it helping or only making
:things worse?
:
:George H.

See http://mcshaneinc.com/html/Library_UnderstandingPID.html
We always use a little "D" to slow down the approach to zero error,

other wise, uncontrolled over shoot normally takes place! Especially
when the
driving system is reaching max output.. This works especially well on
dancer and heat control systems.

I have found how ever, that some devices (design of) do not implement
the "D" the same way. So experience is needed to tune these devices.

Jamie.

 

[George Herold]

On Feb 20, 5:10 am, Ross Herbert <rherb...@bigpond.net.au> wrote:

On Fri, 19 Feb 2010 10:41:28 -0800 (PST), George Herold <ggher...@gmail.com
wrote:

:
:I’ve built several PID control loops, mostly for temperature control,
:but sometimes for other things.  I’ve always used  the all three PID
arameters.  I use to think that the D term was there to help reduce
:the ringing.  (As stated in the wiki PID article.)  And was needed to
:get critical damping of the loop.
:
:But this misunderstanding was corrected a few years ago by a
:colleague.  For thermal loops you’ve got one pole in the thermal time
:constant of the system and can add another with  PI control and with
roper tuning can have critical damping of the loop with just P and I
:control.
:
:Hence my question, What’s the D for?  Is it helping or only making
:things worse?
:
:George H.

Seehttp://mcshaneinc.com/html/Library_UnderstandingPID.html

Thanks Ross, I've never had any problems with the the PID's I've
built. Turn off the I and D. Crank up the gain till it oscillates,
find the critical point and use the #'s for the Nichols- Zeigler (or
some such name) tuning methdod, I then usually back the numbers off
to get a nice looking step response.

But this doesn't seem to give me any information on how to choose D.

George H.

 

[George Herold]

On Feb 20, 9:49 am, Jamie
<jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:

Ross Herbert wrote:
On Fri, 19 Feb 2010 10:41:28 -0800 (PST), George Herold <ggher...@gmail..com
wrote:

:
:I’ve built several PID control loops, mostly for temperature control,
:but sometimes for other things.  I’ve always used  the all three PID
arameters.  I use to think that the D term was there to help reduce
:the ringing.  (As stated in the wiki PID article.)  And was needed to
:get critical damping of the loop.
:
:But this misunderstanding was corrected a few years ago by a
:colleague.  For thermal loops you’ve got one pole in the thermal time
:constant of the system and can add another with  PI control and with
roper tuning can have critical damping of the loop with just P and I
:control.
:
:Hence my question, What’s the D for?  Is it helping or only making
:things worse?
:
:George H.

Seehttp://mcshaneinc.com/html/Library_UnderstandingPID.html

We always use a little "D" to slow down the approach to zero error,
other wise, uncontrolled over shoot normally takes place! Especially
when the
driving system is reaching max output.. This works especially well on
dancer and heat control systems.

   I have found how ever, that some devices (design of) do not implement
the "D" the same way. So experience is needed to tune these devices.

Jamie.- Hide quoted text -

- Show quoted text -

Yeah, Thanks Jamie. I always throw in a bit of D and look at the step
response. I guess the next time I tune one up I'll have to play
around a bit and look at the difference.

As I understand the theory, If I've got a two pole system I should be
able to set things to have critical damping... or oscillation... and
thing in between. It's like a damped harmonic oscillator.


George H.

 

[George Herold]

On Feb 20, 12:39 am, Tim Wescott <t...@seemywebsite.com> wrote:

On Fri, 19 Feb 2010 18:58:27 -0800, George Herold wrote:
On Feb 19, 4:39 pm, Tim Wescott <t...@seemywebsite.com> wrote:
On Fri, 19 Feb 2010 13:21:50 -0800, George Herold wrote:
without differential
action:http://www.wescottdesign.com/articles/Sampling/pidwophd.html..

If you _really_ want to go overboard, particularly if you're
implementing your controllers in software, you'll be interested in
my book:http://www.wescottdesign.com/actfes/actfes.html.

HTH

--www.wescottdesign.com

Hi Tim,  Thanks for the response.

I'm not sure I get your 'predictor' analogy.  But if you want to say
that the D gives you more gain at higher frequencies then I agree.  I
guess I tend to look at PID controllers from the frequency domain.  P
has a flat response at all frequencies, I gives you lots of gain at
DC but then goes away, and D is all at the high frequency end.  (Oh
and I do always roll the D back off.. typically at frequencies that
are about ten times greater than were it 'turns on'.)

I didn't know how much you knew, so I was using the time-domain analogy
that's easier to grasp if you don't understand frequency domain stuff.
The differential term predicts because it measures velocity.  It's the
difference between being parked in front of a brick wall, and hurtling
toward that same brick wall at 60 miles an hour -- if you're not moving
(D term = 0), one foot away from the wall is merely 'close'.  If you're
moving fast (D term = +60MPH), 50 feet away is 'oh my gawd!!!'.  Why?
Because the controller in your brain is calculating speed and distance
vectors, and predicting a noisy and maybe painful crash.

Hmm, Yeah most of the time the 'plant' is just sitting there at the
setpoint and the D is not doing anything.  (At least in my applications
this is the case.)  I'm not quite sure what a 'plant' is.  Most of the
time I dragging little (100 gram) pieces of 'stuff' around with ~10
Watts of power.  (The last thermal project was temperature control of a
~1kg permanent magnet used for NMR.)

"Plant" == "thing my controller is responsible for".  It comes, I think,
from "steam plant" back in the days when control theory was being
invented.

I've heard lots of scary stories about how hard it is to control
temperature, but the few times I've done it things have acted like
straightforward single-order low pass filters.





"http://www.wescottdesign.com/articles/Sampling/pidwophd.html."
Thanks for the link.  Thanks for the link.  I'm afraid I've got a
PhD... can I still read your article?? :^)

I guess I'm wondering if, given the simple thermal control loops I'm
typically using, if it would be better to throw away the D on the
next project.  Saves an opamp and a few big caps.

Well, that depends.  Some thermal plants tend to pile all of their
phase shift in one place, so there's only so much you can buy with
differential control.  In that case there's certainly reason to think
about a cost/ benefit tradeoff.  If you can measure the frequency
response of the plant you can do all of this ahead of time, including
tuning, before you commit to a board.  Or you can just prototype a
controller, of course.

Hmm I'm not quite sure what the phase shift is in this case.  Are you
talking about how distributed the thermal mass is?

Yes.





I'm not much of a computer
guy and make only analog PI(D) loops.  I've always hated the dead
band you get with commercial Digital controllers, from say Omega or
others.

Commercial digital controllers are, as near as I can tell, designed and
implemented by Oog and Og using methods and technology -- other than
the communications bells and whistles -- that was obsolete decades ago..

Yeah they suck!  Someone should make something better then.  We pay
almost $300 for these POS's.

If you're controlling a temperature you can do the whole job of control
with an 8-pin microprocessor connected to an 8-pin 24-bit ADC; at worst
you'll need a level shifter and low pass filter before you go to your
drive electronics, and you may well use less board space than one of
your big caps.  _And_ your controller's response time will be well
within the response time of your plant, so you won't see any
discernible change in performance.

Well it's the whole learning curve thing.  Your micro and ADC are
replacing a few opamps and caps.  I then just have to choose and tweak
some R's.  Still I'd like learn more about micro's and all.

But don't let that keep you from your appointed path.

What appointed path.  I think my life is more like the blind pig finding
an acorn.

--www.wescottdesign.com-Hide quoted text -

- Show quoted text -

But still I think you implied that simple 'approximately single pole'
thermal systems may not need the Derivative term.  Hmm maybe I should
try doing some thermal modeling on LTspice ala John L.

Yes, depending on how well they have to reject disturbances they may well
not need it.

--www.wescottdesign.com- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -

Thanks Tim, Yeah I've only got simple thermal control tasks to take
care of.

George H.

 

[Jamie]

George Herold wrote:

On Feb 20, 9:49 am, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:

Ross Herbert wrote:

On Fri, 19 Feb 2010 10:41:28 -0800 (PST), George Herold <ggher...@gmail.com
wrote:

:
:I’ve built several PID control loops, mostly for temperature control,
:but sometimes for other things. I’ve always used the all three PID
arameters. I use to think that the D term was there to help reduce
:the ringing. (As stated in the wiki PID article.) And was needed to
:get critical damping of the loop.
:
:But this misunderstanding was corrected a few years ago by a
:colleague. For thermal loops you’ve got one pole in the thermal time
:constant of the system and can add another with PI control and with
roper tuning can have critical damping of the loop with just P and I
:control.
:
:Hence my question, What’s the D for? Is it helping or only making
:things worse?
:
:George H.

Seehttp://mcshaneinc.com/html/Library_UnderstandingPID.html

We always use a little "D" to slow down the approach to zero error,
other wise, uncontrolled over shoot normally takes place! Especially
when the
driving system is reaching max output.. This works especially well on
dancer and heat control systems.

I have found how ever, that some devices (design of) do not implement
the "D" the same way. So experience is needed to tune these devices.

Jamie.- Hide quoted text -

- Show quoted text -


Yeah, Thanks Jamie. I always throw in a bit of D and look at the step
response. I guess the next time I tune one up I'll have to play
around a bit and look at the difference.

As I understand the theory, If I've got a two pole system I should be
able to set things to have critical damping... or oscillation... and
thing in between. It's like a damped harmonic oscillator.


George H.

Like I said before, "D" is implemented differently depending on the device..

For example, an Analog PID may label the "D" as a "LEAD" function,
which is really what's all about..

Basically, the faster the reference signal changes, the more charged
a capacitor gets to develop a lead (90 degrees) which then drives a
circuit that sums it's value to advance it early or even put it into a
negative state for retreat.

This Lead circuit adds to the final out put. So in short, the output
of the PID may see the desired results or even over shoot the desired
results before the input reference actually gets there. This can slow
things down and we have a nice soft cushion when the error reaches 0.
Because the nearer it gets to zero error, the smaller the "D"/Lead
value will be due to the fact that it has already been slowing down
before hand.

In digital systems, at least most of them, the value is set in time
which really means that its the time between samples. The Differential
of the error value is used to add to the sum to bring it closer to the
desire value.. If the time value is large, the input reference may
have a chance to produce a high DT and thus, the SUMMED value will be
high, this can lead to oscillation or it'll just over shoot on the PID
output
and start slowing things down. Keep this in mind that this value will
use the error value and creates a reference from the last error value
with the time indicated.

In some systems. The D value does not reference the error point, it
simply creates a DT from the time window and uses that to put the PID
output into a negative state. So if the PID output is at MAX due to the
input REF not being satisfied by a large margin, you'll see a negative
effect on the PID output and is simply measuring the value of the ReF DT
and not the error value. This method has been used many times to
create a slow crawl speed for a device, even if the error is at 100%
because the input REF will have a changing value as it goes along. 0..10
Volts for example.

Hope that cleared some things up..

 

[Tim Wescott]

George Herold wrote:

On Feb 20, 5:10 am, Ross Herbert <rherb...@bigpond.net.au> wrote:
On Fri, 19 Feb 2010 10:41:28 -0800 (PST), George Herold <ggher...@gmail.com
wrote:

:
:I’ve built several PID control loops, mostly for temperature control,
:but sometimes for other things. I’ve always used the all three PID
arameters. I use to think that the D term was there to help reduce
:the ringing. (As stated in the wiki PID article.) And was needed to
:get critical damping of the loop.
:
:But this misunderstanding was corrected a few years ago by a
:colleague. For thermal loops you’ve got one pole in the thermal time
:constant of the system and can add another with PI control and with
roper tuning can have critical damping of the loop with just P and I
:control.
:
:Hence my question, What’s the D for? Is it helping or only making
:things worse?
:
:George H.

Seehttp://mcshaneinc.com/html/Library_UnderstandingPID.html

Thanks Ross, I've never had any problems with the the PID's I've
built. Turn off the I and D. Crank up the gain till it oscillates,
find the critical point and use the #'s for the Nichols- Zeigler (or
some such name) tuning methdod, I then usually back the numbers off
to get a nice looking step response.

But this doesn't seem to give me any information on how to choose D.

George H.

The structured way to do this is to model the plant, refine the model
with measurements, then design a controller to the model.

You can use frequency sweeps (this is time consuming with a thermal
plant, but you can maybe still do it), or you can use step responses and
fit an s-domain model. Then you design a robust controller to your
model, and hope that it is more robust than your model is erroneous.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

 

[George Herold]

On Feb 22, 12:34 pm, Tim Wescott <t...@seemywebsite.now> wrote:

George Herold wrote:
On Feb 20, 5:10 am, Ross Herbert <rherb...@bigpond.net.au> wrote:
On Fri, 19 Feb 2010 10:41:28 -0800 (PST), George Herold <ggher...@gmail.com
wrote:

:
:I’ve built several PID control loops, mostly for temperature control,
:but sometimes for other things.  I’ve always used  the all three PID
arameters.  I use to think that the D term was there to help reduce
:the ringing.  (As stated in the wiki PID article.)  And was needed to
:get critical damping of the loop.
:
:But this misunderstanding was corrected a few years ago by a
:colleague.  For thermal loops you’ve got one pole in the thermal time
:constant of the system and can add another with  PI control and with
roper tuning can have critical damping of the loop with just P and I
:control.
:
:Hence my question, What’s the D for?  Is it helping or only making
:things worse?
:
:George H.

Seehttp://mcshaneinc.com/html/Library_UnderstandingPID.html

Thanks Ross, I've never had any problems with the the PID's I've
built.  Turn off the I and D.  Crank up the gain till it oscillates,
find the critical point and use the #'s for the Nichols- Zeigler (or
some such name) tuning methdod,  I then usually back the numbers off
to get a nice looking step response.

But this doesn't seem to give me any information on how to choose D.

George H.

The structured way to do this is to model the plant, refine the model
with measurements, then design a controller to the model.

Yup, I've got 'RC' schematics of my last thermal project. The various
time constants seemed to work out pretty well. (The 'worst' part was
the steel yoke on the magnet with like 1 hour time constant.) But
when I close the loop to control it I've just been following some
'cook book' recipe for choosing D.

You can use frequency sweeps (this is time consuming with a thermal
plant, but you can maybe still do it), or you can use step responses and
fit an s-domain model.  Then you design a robust controller to your
model, and hope that it is more robust than your model is erroneous.

--
Tim Wescott
Control system and signal processing consultingwww.wescottdesign.com- Hide quoted text -

- Show quoted text -

I love looking at the step response. The pulse response is 'cool'
too. Though I've never tried it with a thermal system.

Hey, if you want to measure the frequency response of the plant all at
once you can hit it with white noise and look at the response.

Still, I figure the next 'plant' I tune I'll try just P and I first
and then add some D.

Thanx again,
George H.