Ramblings about feedback control loops

In school I took the usual control classes. Nyquist , Bode, RHP etc. Pretty much did not understand any of it at the time. I just remember one graph that seems to get drilled into my brain. The ole 3dB of overshoot and 45 degree phase margin. Like , man , you may not know anything about control loops but that is the absolute nirvana of all control theory....Gotta have that 3 dB overshoot.....

So I slowly have to work through these concepts as I get assigned real world problems that require control loops. The idea of oscillations and BW start taking hold. Seeing how to close a loop using a bode diagram and phase margin takes hold.

But I still remember the glorious diagrams of 3 dB overshoot. This is How you do it! I am even understanding it now!

And time marches on and I do more and more loops and finally it dawns on me that I have been duped. The best control loop is the absolute most overdamped loop that you can possibly build that is still responsive enough to get the job done. Wow, they never taught me that in school

 

[Bill Sloman]

On Sunday, October 6, 2019 at 9:04:16 AM UTC+11, blo...@columbus.rr.com wrote:

In school I took the usual control classes. Nyquist , Bode, RHP etc. Pretty much did not understand any of it at the time. I just remember one graph that seems to get drilled into my brain. The ole 3dB of overshoot and 45 degree phase margin. Like , man , you may not know anything about control loops but that is the absolute nirvana of all control theory....Gotta have that 3 dB overshoot.....

So I slowly have to work through these concepts as I get assigned real world problems that require control loops. The idea of oscillations and BW start taking hold. Seeing how to close a loop using a bode diagram and phase margin takes hold.

But I still remember the glorious diagrams of 3 dB overshoot. This is How you do it! I am even understanding it now!

And time marches on and I do more and more loops and finally it dawns on me that I have been duped. The best control loop is the absolute most overdamped loop that you can possibly build that is still responsive enough to get the job done. Wow, they never taught me that in school

Depends on the problem. There is actually a second order loop that always gets closest to the target value at any time after the impulse.

The damping factor is 0.7, and the response overshoots by about 5%, but there's no ringing.

If you don't want any overshoot, you have to set the damping factor a bit higher - at 1.0 or above - but you are farther way from the target value for a bit longer, though there's not a lot in it.

http://faculty.mercer.edu/jenkins_he/documents/2ndorderresponseMSD.pdf

It sounds as if you lecturer didn't implant quite the right message.

--
Bill Sloman, Sydney

 

[Allan Herriman]

On Sat, 05 Oct 2019 15:04:11 -0700, blocher wrote:

In school I took the usual control classes. Nyquist , Bode, RHP etc.
Pretty much did not understand any of it at the time. I just remember
one graph that seems to get drilled into my brain. The ole 3dB of
overshoot and 45 degree phase margin. Like , man , you may not know
anything about control loops but that is the absolute nirvana of all
control theory....Gotta have that 3 dB overshoot.....

So I slowly have to work through these concepts as I get assigned real
world problems that require control loops. The idea of oscillations and
BW start taking hold. Seeing how to close a loop using a bode diagram
and phase margin takes hold.

But I still remember the glorious diagrams of 3 dB overshoot. This is
How you do it! I am even understanding it now!

And time marches on and I do more and more loops and finally it dawns on
me that I have been duped. The best control loop is the absolute most
overdamped loop that you can possibly build that is still responsive
enough to get the job done. Wow, they never taught me that in school

PLLs used for repeating network clocks (e.g. for SONET, synchronous
Ethernet, etc.) are required to have at most +0.10dB gain peaking. This
is to stop jitter buildup when a lot of PLLs (i.e. network nodes) are
cascaded.

It's been a while since I designed one from scratch (since in modern
products this is done by a single off-the-shelf IC (e.g. Si5347A, etc.)),
but when I did, I used to aim for +0.05dB gain peaking. This results in
a loop that's massively overdamped. It's also very tolerant to parameter
variations.

Regards,
Allan

 

[George Herold]

On Saturday, October 5, 2019 at 6:04:16 PM UTC-4, blo...@columbus.rr.com wrote:

In school I took the usual control classes. Nyquist , Bode, RHP etc. Pretty much did not understand any of it at the time. I just remember one graph that seems to get drilled into my brain. The ole 3dB of overshoot and 45 degree phase margin. Like , man , you may not know anything about control loops but that is the absolute nirvana of all control theory....Gotta have that 3 dB overshoot.....

So I slowly have to work through these concepts as I get assigned real world problems that require control loops. The idea of oscillations and BW start taking hold. Seeing how to close a loop using a bode diagram and phase margin takes hold.

But I still remember the glorious diagrams of 3 dB overshoot. This is How you do it! I am even understanding it now!

And time marches on and I do more and more loops and finally it dawns on me that I have been duped. The best control loop is the absolute most overdamped loop that you can possibly build that is still responsive enough to get the job done. Wow, they never taught me that in school

I never took a control class, (though I've read some books), so I'm not sure
exactly what you are referring to. Is the 3dB overshoot the response to
a change in the control parameter or a change in the load on the plant?

I use to make my control loops snappy.. quickest response with some over
shoot as you say. These days I make 'em slower. But TBH it's hard to tell
the difference.

I'd like to read a good control book. Any suggestions?
George H.

 

On Sunday, October 6, 2019 at 9:22:00 PM UTC-4, George Herold wrote:

On Saturday, October 5, 2019 at 6:04:16 PM UTC-4, blo...@columbus.rr.com wrote:
In school I took the usual control classes. Nyquist , Bode, RHP etc. Pretty much did not understand any of it at the time. I just remember one graph that seems to get drilled into my brain. The ole 3dB of overshoot and 45 degree phase margin. Like , man , you may not know anything about control loops but that is the absolute nirvana of all control theory....Gotta have that 3 dB overshoot.....

So I slowly have to work through these concepts as I get assigned real world problems that require control loops. The idea of oscillations and BW start taking hold. Seeing how to close a loop using a bode diagram and phase margin takes hold.

But I still remember the glorious diagrams of 3 dB overshoot. This is How you do it! I am even understanding it now!

And time marches on and I do more and more loops and finally it dawns on me that I have been duped. The best control loop is the absolute most overdamped loop that you can possibly build that is still responsive enough to get the job done. Wow, they never taught me that in school

I never took a control class, (though I've read some books), so I'm not sure
exactly what you are referring to. Is the 3dB overshoot the response to
a change in the control parameter or a change in the load on the plant?

I use to make my control loops snappy.. quickest response with some over
shoot as you say. These days I make 'em slower. But TBH it's hard to tell
the difference.

I'd like to read a good control book. Any suggestions?
George H.

Overshoot is a time-domain idea. A critically-damped loop has no overshoot.
Both underdamped and overdamped loops have longer settling time. Peaking in the frequency domain is a related but distinct issue. IIRC critical damping in a second-order system occurs at a damping factor delta = sqrt(0.5), and there's no frequency response peaking below delta = 1.0.

Cheers

Phil Hobbs

 

[George Herold]

On Sunday, October 6, 2019 at 10:43:20 PM UTC-4, pcdh...@gmail.com wrote:

On Sunday, October 6, 2019 at 9:22:00 PM UTC-4, George Herold wrote:
On Saturday, October 5, 2019 at 6:04:16 PM UTC-4, blo...@columbus.rr.com wrote:
In school I took the usual control classes. Nyquist , Bode, RHP etc. Pretty much did not understand any of it at the time. I just remember one graph that seems to get drilled into my brain. The ole 3dB of overshoot and 45 degree phase margin. Like , man , you may not know anything about control loops but that is the absolute nirvana of all control theory....Gotta have that 3 dB overshoot.....

So I slowly have to work through these concepts as I get assigned real world problems that require control loops. The idea of oscillations and BW start taking hold. Seeing how to close a loop using a bode diagram and phase margin takes hold.

But I still remember the glorious diagrams of 3 dB overshoot. This is How you do it! I am even understanding it now!

And time marches on and I do more and more loops and finally it dawns on me that I have been duped. The best control loop is the absolute most overdamped loop that you can possibly build that is still responsive enough to get the job done. Wow, they never taught me that in school

I never took a control class, (though I've read some books), so I'm not sure
exactly what you are referring to. Is the 3dB overshoot the response to
a change in the control parameter or a change in the load on the plant?

I use to make my control loops snappy.. quickest response with some over
shoot as you say. These days I make 'em slower. But TBH it's hard to tell
the difference.

I'd like to read a good control book. Any suggestions?
George H.


Overshoot is a time-domain idea. A critically-damped loop has no overshoot.
Both underdamped and overdamped loops have longer settling time.
Right, snappy in that it responds fast, not that it settles faster.

Peaking in the frequency domain is a related but distinct issue. IIRC critical damping in a second-order system occurs at a damping factor delta = sqrt(0.5), and there's no frequency response peaking below delta = 1.0.
Hmm OK... (delta, (damping) = 1/(2*Q) ?) Is this different than 2nd order
filters? Where delta (or Q) = 0.707 (sqrt(0.5)), is flat in frequency
but has some bumpies in the time domain?

George H.

Cheers

Phil Hobbs

 

[Peter]

blocher@columbus.rr.com wrote

In school I took the usual control classes. Nyquist , Bode, RHP etc. Pretty much did not understand any of it at the time. I just remember one graph that seems to get drilled into my brain. The ole 3dB of overshoot and 45 degree phase margin. Like , man , you may not know anything about control loops but that is the absolute nirvana of all control theory....Gotta have that 3 dB overshoot.....

So I slowly have to work through these concepts as I get assigned real world problems that require control loops. The idea of oscillations and BW start taking hold. Seeing how to close a loop using a bode diagram and phase margin takes hold.

But I still remember the glorious diagrams of 3 dB overshoot. This is How you do it! I am even understanding it now!

And time marches on and I do more and more loops and finally it dawns on me that I have been duped. The best control loop is the absolute most overdamped loop that you can possibly build that is still responsive enough to get the job done. Wow, they never taught me that in school

That is certainly the safest way to design control systems.

Just got to make sure there is enough margin.

For example I have been making a product for 25 years which has a
switching PSU, outputting 100mA.

Now I need to get 500mA out of it. It does it easily, but over 200mA
it goes unstable. The more load, the worse it gets - as expected
because the output caps (which form the dominant pole) become less
effective with a lower load resistance in parallel with them.

So I had to increase the main feedback cap from 10nF to 100nF.
Actually 20nF was enough, so I went for 100

Exactly what you suggest.