Standard Closed-Loop Feedback Controllers

On Thu, 31 Dec 2009 13:11:53 -0800, ssylee wrote:


Yes, in theory.  In the real world there are a host of factors that must
be considered; primarily the fact that if you build that system your G(s)
won't necessarily match the plants, the fact that the scheme doesn't work
for a metastable (i.e. integrating) or unstable G(s), and the fact that
for a G(s) with poles close to the stability boundary (i.e. if it has
resonances or if it responds very slowly) the disturbance rejection of
the system will be poor.

Do a web search on "pole-zero cancellation".


That's a loaded question.  Typically one considers the disturbances that
may make a difference for the system at hand, and that can cover a lot of
ground.


By changing the plant.  The most valuable lesson I ever learned as a
control system engineer is that there's only so much performance
improvement that you can squeeze out of any given plant, even with the
best controller.  Once you've reached that, you're done until you change
the plant.

Note, too, that their last bullet point is incorrect, in a way that could
be considered to be wildly optimistic.  Sensitivity does not _typically_
increase in one frequency range when you reduce it in another.  In this
fallen and imperfect world we live in, sensitivity _must_ increase in one
frequency range when you reduce it in another, unless you're fixing a
(very) badly implemented controller.  Look up the "Bode Sensitivity
Integral" for more information.

You _cannot_ reduce a systems sensitivity to disturbance across the
board; all you can do is shove that sensitivity to frequency ranges that
you (or your customer) cares less about, from ones that you care more
about.

--
www.wescottdesign.com

I'll strengthen that, control engineers often have the job of applying
controls to work around problems caused by poorly designed plant. Many times
I've made that point to plant designers, then acceeded to their request
under sufferance and implemented a configuration that may or may not have
ameliorated the situation. The solution then is to keep lobbying, so that
next time there are funds available such as during a major revamp, the core
problem can be resolved.

On Fri, 01 Jan 2010 11:11:33 +0800, Bruce Varley wrote:


(snip)

Me, too.  You get the same situation in product design, by the way -- the
mechanical design gets finalized, it works marginally, but no one wants
to change it because that opens up a huge can of worms.  I seem to end up
getting called in to help at that stage a lot.  I do what I can, and I
can often get significant improvement for a customer, but it's often
frustrating all around.  You just can't get silk out of a sow's ear, even
if you're a control system designer.

Some of the most valuable work that I've done (IMHO) wasn't diddling with
a control loop -- rather, it was working with the mechanical design team
from Day 1 on a project, to make sure that the plant's own inherent
disturbance rejection, stemming from its mechanical design, was as good
as possible.

Or maybe it was making it clear that the attention needed to be paid up
front in the first place.

--
www.wescottdesign.com

Tim Wescott wrote:

A typical scenario: The mechanical guys have finished their machine.
Motors are mounted (often steppers where analog servos would be better,
or vice-versa) and the structural parts are anodized. I'm invited to a
meeting to tell them how long my group will need to "wire it up". I ask
where the limit switches will go, how the wires will get from here to
there, and how the connectors will attach. You know the reaction: "We
can't drill it now! it's all anodized! (or plated, ir irridited,
whatever.) At that point, I say, "Here's the list of what I need to go
where. You figure out how to arrange for it. If you don't want holes,
decide where the cables will wrap around the outside. Next time, call me
when it's still on paper. Really, fellas, I can read a print." I know
that next time will be exactly like this time.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

On Sat, 02 Jan 2010 13:01:31 -0500, Jerry Avins wrote:


I spent a whole project once asking "Where are the cables going to go?  
Where are the cables going to go?".

When it came time to put the two major assemblies together -- assemblies
that had to move freely with respect to each other for the machine to
perform -- guess who got to route the cables?

They still use essentially the same sheet metal brackets that I came up
with, just laser cut and bent on a brake instead of hacked out with
snips, cleaned with a file, bent on the edge of a desk and held in with
double-stick tape.

I still contract with that company part-time -- now they actually discuss
cable routing in conceptual-level design reviews.

--
www.wescottdesign.com

Thanks for your in-depth response. I'm clearer on the responses to the
first two questions. While the third question has a ridiculously
optimistic assumption to it, I'm just going to take its word with
respect to this course. Your conversation with other community members
on this newsgroup gave me perspective on how control system design
functions in the real world. Looking forward to talk to you more about
controls in the near future.

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Noise it is regarded to have to accept. But one can have a least square
approximation of an process transfer function.

* http://home.arcor.de/janch/janch/_control/20100103-compensations/

Page 1



Disturbances can be eliminated/compensated by known true process transfer
functions.

Page 2: without compensations and with PID control
Page 3: with compensations and without PID control

See also MathModel


--
Regards JCH

Why not simply use feed forwards?

If OLTF(s)=K/(tau*s+1) then
the feed forwards are 1/OLTF(s) or 1/K and tau/K
Start with the feed forwards and add the closed loop control.


What is an output disturbance?  A log falling on a actuator is not an
output disturbance.  Perhaps we are using different names for the same
thing.


I never have used your type of control.  I don't see why your model
would be much different from any other form of control.  I look at
feedback quantizing errors first but that is because feedback in
motion control systems is usually pretty clean otherwise.

My usual case is a log dropping on a log carriage.  The actuators will
move but will recover quickly.
I don't do temperature systems but if I were controlling an oven I
would consider any rate of change in product going through the oven to
be a disturbance.   The higher the rate of going through the oven the
more heat required.   If I can control the conveyor or can detect the
product going into the furnace I should be able to work out a feed
forward to estimate the load.  If the amount product in the oven
changes the time constant I should be able to compute that too.  The
speed of the conveyor might affect the dead time.  If I can do system
identification on this adjust my feed forwards and closed loop gains
to compensate for the changes.   If the dead time is severe then a
Smith Predictor may be necessary.  All of this requires a bit of work
for average joe but I were making ovens I would have it all worked
out.

I don't think of this as reducing disturbances but rather compensating
for them.  Reducing disturbance is  limiting the rate of change in
product in the oven or not letting the logs drop to far before they
hit the carriage knees.  Now you are getting into performance
limitations or the mechanical design issues discussed above.

Peter Nachtwey