Stupid Question-- Why not Open-loop?

On 3 Nov 2005 22:20:30 -0800, the renowned "sunnewton"


The transfer function of the plant is not often known accurately
enough for the desired control accuracy, and it's not in reality
linear nor is it time-invariant. It's also impractical to account for
all the disturbances.

Could you you adequately control the speed of your car merely from the
accelerator pedal position, without feedback from the speedometer?



Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
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Hello Spehro,


In some countries they can. It's either full up or full down. Avoiding
too many dents is handled in a similar fashion, mostly by using the horn
which is a digital device. Using the brakes a lot is for sissies...

Regards, Joerg

http://www.analogconsultants.com

On Fri, 04 Nov 2005 02:01:02 -0500, Spehro Pefhany


This is a good analogy. When I was a kid I used to watch people drive,
observing their foot on the pedal and the speed. I noted how some
would not react until the speed had dropped after mounting a hill.
Others would anticipate the hill and speed up for it. My mom always
had a cycle. She would accelerate to speed, then coast, then do it all
over again. She hated for me to point this out.

Have you ever noticed that cruise controlled cars and manually
controlled cars do not interact well. People tend to react slower than
the machine which never wavers from it's attention on the task.

I've noted here how many tend to define a problem mathematically and
have a lesser intuitive understanding of the process while I tend to
be the opposite. I've decided that this has to do with how you learn
this field. I started doing it, learning my the seat of my pants and
the math came afterwards. People that go to school for controls learn
the math first and the intuitive part comes later after doing it over
and over. Both are important. It's just interesting to observe.

This does make me want to ask a question. How many of us had an early
fascination with controls?

Be well,

HoP

The preceding message represents personal opinions
and/or advice that may prove incorrect or harmful. But then maybe not.
Feel free to disregard.

 ------- Words have no Warranty ------
 ------- No View without Merit  ------

With me it supposedly started when I began migrating from the horizontal
floor crawl to vertical surfaces. Like the TV or the coal stove. So ...
what's this little thingie do if I wiggle it ... usually followed by a
loud scream from mom "Nooooo!".

One control where I realized early on that it wasn't supposed to be
messed with was the 110V/220V selector, if you are in a 220V country.
Phfffssssst ... POOF.

Regards, Joerg

http://www.analogconsultants.com

On Sat, 05 Nov 2005 02:16:25 GMT, Joerg


I had a cousin that came over and took disassembled all my toys
leaving them for me to figure out how to reassemble them. Perhaps this
contributed a little for me.

A common service call I get now is to fix some factory's machine that
has already been picked apart by a half dozen others in an attempt to
fix it. Deja vu?

Be well,

HoP

The preceding message represents personal opinions
and/or advice that may prove incorrect or harmful. But then maybe not.
Feel free to disregard.

 ------- Words have no Warranty ------
 ------- No View without Merit  ------

napisa³:


Consider the structure of your C' compensator. It's much more complicated
then C (consists of both C and P dynamic). It means that your compensator
can be more expensive then your plant. So maybe build a better plant? It  
would be simplier.
Consider also that you can't control unstable plant in this way (serial  
connection  C'P).
Most significant is that you have no idea what is happening on the plant  
output
because you have no sensor. How to set plant output exactly in required  
state
if you can't check it? No sensor means no feedback.
However stepping motors are sometimes control without feedback :). But it  
is another story.

--
Mikolaj

Mikolaj wrote:


The key word here is **sometimes**. There are, however, situations where
the stepper motor control can continue to provide step pulses while the
motor has jammed stationary. Without some form of feedback you are totally
lost about what is happening on plant.

--
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Dnia Fri, 04 Nov 2005 12:54:23 +0100, Paul E. Bennett  


I know about that of course. It was kind of curious tale.

--
Mikolaj

In the real world we never know the transfer function for a process.
Any knowledge of the process is an approximation, and includes
variables that can't be measured.

It is more typical to have almost no knowledge of the transfer function
of the plant when designing the controls.


sunnewton wrote:

sunnewton wrote:

You know the way from home to work as well as you know anything-- you've
done it so many times. To gain a deep appreciation of feedback, try
traveling the route with your eyes closed.

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

Interestingly, there are some very precise open loop physiological control
systems, because for some things, feedback would be much too slow, and
introduce oscillations.

Very often, though, such systems have monitoring for long-term errors, and
the parameters of the open loop control system are adjusted if errors
persist.

--
Scott
Reverse name to reply

Scott Seidman wrote:

Indeed: that's what reflexes are about. "Reflex" literally means "bend
back" and a reflex is a pathway from afferent (sensory) nerve to spinal
column to efferent (motor) nerve, without traveling the round to and
through the brain. As far as I and my internist know, I deduced one that
  can be traces but hadn't been noted before.

Given the speed of nerve impulses, one jumping from a six-foot with
knees slightly bent to avoid a jarring impact would knee himself in the
chin if the brain had to signal when to stiffen the leg muscles relying
on the sensation of contact for input. Instead, a reflex is primed by
the ankle being bent downward and an inhibitory signal being removed
from the priming synapse. The ankle straightens when contact is made,
and the signal from it goes no further than the base of the spine. The
reflex there briefly tenses the leg muscles open loop until the brain
has time to take over.

Sometimes the pathway is inappropriately activated, and the result is
usually a calf cramp (sometimes thigh). Forcing the ankle to a right
angle by standing immediately releases a cramp of this origin.

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

Of course, sometimes it is harder to see what the feedback mechanism
actuially is. It comes in a lot of guises.

An example:- Signal into amplifier, amplifier to loudspeaker audio to ear.
This may seem like a very open loop system but there are a number of
feedback paths that are attempting to keep matters under some sort of
control.

Each amplifier stage has its own feedback loop to provide just the level of
gain set by the "gain demand" controls (so that is at lerast two). Then
there is the loop that includes the listener. He will adjust the level to a
point that he desires to hear the signal. Beyond that there may be the
neighbours who begin to bang on the walls, ceiling or floor when the level
of sound becomes too much for them to bear. So, as you see, you may have to
consider expanding the boundaries of the system to see what may be the
overall feedback mechanisms in play.

Naturally, you will select a system boundary so that you can proiperly
consider the peice of the overall system you are really interested in.

--
********************************************************************
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ....EBA. http://www.electric-boat-association.org.uk/
********************************************************************

Jerry Avins wrote:

Be careful what you suggest. I was driving home tonight and the person
in the lane next to me apparently read your post and was trying it. :-}

John

John Shaw wrote:

Ouch!

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

funny I had the exact same question way back in my college years. Aside from
inaccurate P(s) others have mentioned, inject some disturbances into the
system and you will appreciate that open-loop will not be equivalent to
closed-loop.

hh_ca wrote:

In fact, where you have a plant that's well behaved enough to drive
open-loop you do, and you avoid all the stability problems with
closed-loop control.  For instance this is why stepper motors are so
popular, even though the generally suck in every other way when compared
to DC motors.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

In processes there are, on occassion, loops that are sufficiently self
regulating, and with no requirements for tight control, so no feedback
is needed. Sometime a manual loader will allow the operator to position
the valve, or in some cases no valve is needed.

However, these are rare. Most of the time feedback and closed loop
control is required

John

If there is perfect and complete knowledge of the plant and any
disturbances at all times, you could use open loop control.
Unfortunately, that isn't a realistic situation.  So you need some form
of feedback.  It's reality that gets in the way.