Standard Closed-Loop Feedback Controllers
ssylee
folder=fa6643eb-b915-4efe-90d1-f51768319499&file=inversion.png),
there is a gain of high gain feedback implicitly generates the
inverse of G(s) without having to actually carry the inversion. I do
agree that if the controller H(s) very high (i.e. >> 1), the ratio
between U(s) and R(s) is 1/G(s). But is the reciprocal function of G
(s) equivalent to an inversion operation of G(s)?
2) I've seen a few different models where only measurement noise +
output disturbance, or input and output disturbance are considered.
Typically, what are the disturbances and erroneous inputs considered
in this type of a model?
3) Regarding control system sensitivity (http://files.engineering.com/
getfile.aspx?folder=c1565d82-0a85-4d20-83ce-
c1df3c723c21&file=specifications.png), there is a claim that says
reducing the sensitivity function and the input/output disturbances
would result in a close to perfect setpoint tracking. My question is
how are the disturbances usually reduced?
Thanks in advance.
Tim Wescott
> 1) As shown in the diagram (http://files.engineering.com/getfile.aspx?
> folder=fa6643eb-b915-4efe-90d1-f51768319499&file=inversion.png), there
> is a gain of high gain feedback implicitly generates the inverse of G(s)
> without having to actually carry the inversion. I do agree that if the
> controller H(s) very high (i.e. >> 1), the ratio between U(s) and R(s)
> is 1/G(s). But is the reciprocal function of G (s) equivalent to an
> inversion operation of G(s)?
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".
>
> 2) I've seen a few different models where only measurement noise +
> output disturbance, or input and output disturbance are considered.
> Typically, what are the disturbances and erroneous inputs considered in
> this type of a model?
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.
> 3) Regarding control system sensitivity (http://files.engineering.com/
> getfile.aspx?folder=c1565d82-0a85-4d20-83ce-
> c1df3c723c21&file=specifications.png), there is a claim that says
> reducing the sensitivity function and the input/output disturbances
> would result in a close to perfect setpoint tracking. My question is how
> are the disturbances usually reduced?
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.
Bruce Varley
"Tim Wescott" <t...@seemywebsite.com> wrote in message
news:rsednQOWhfIKpKDW...@web-ster.com...
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.
Tim Wescott
(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.
iman way
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Jerry Avins
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.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Tim Wescott
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.
JCH
"ssylee" <stani...@gmail.com> schrieb im Newsbeitrag
news:591bacc7-485b-47e0...@r24g2000yqd.googlegroups.com...
> 1) As shown in the diagram (http://files.engineering.com/getfile.aspx?
> folder=fa6643eb-b915-4efe-90d1-f51768319499&file=inversion.png),
> there is a gain of high gain feedback implicitly generates the
> inverse of G(s) without having to actually carry the inversion. I do
> agree that if the controller H(s) very high (i.e. >> 1), the ratio
> between U(s) and R(s) is 1/G(s). But is the reciprocal function of G
> (s) equivalent to an inversion operation of G(s)?
Yes.
> 2) I've seen a few different models where only measurement noise +
> output disturbance, or input and output disturbance are considered.
> Typically, what are the disturbances and erroneous inputs considered
> in this type of a model?
EXAMPLES
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
> 3) Regarding control system sensitivity (http://files.engineering.com/
> getfile.aspx?folder=c1565d82-0a85-4d20-83ce-
> c1df3c723c21&file=specifications.png), there is a claim that says
> reducing the sensitivity function and the input/output disturbances
> would result in a close to perfect setpoint tracking. My question is
> how are the disturbances usually reduced?
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
pnachtwey
> 1) As shown in the diagram (http://files.engineering.com/getfile.aspx?
> folder=fa6643eb-b915-4efe-90d1-f51768319499&file=inversion.png),
> there is a gain of high gain feedback implicitly generates the
> inverse of G(s) without having to actually carry the inversion. I do
> agree that if the controller H(s) very high (i.e. >> 1), the ratio
> between U(s) and R(s) is 1/G(s). But is the reciprocal function of G
> (s) equivalent to an inversion operation of G(s)?
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.
>
> 2) I've seen a few different models where only measurement noise +
> output disturbance, or input and output disturbance are considered.
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.
> Typically, what are the disturbances and erroneous inputs considered
> in this type of a model?
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.
>
> 3) Regarding control system sensitivity (http://files.engineering.com/
> getfile.aspx?folder=c1565d82-0a85-4d20-83ce-
> c1df3c723c21&file=specifications.png), there is a claim that says
> reducing the sensitivity function and the input/output disturbances
> would result in a close to perfect setpoint tracking. My question is
> how are the disturbances usually reduced?
>
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
ssylee
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.