You could do it with a laser array, but it would need to be a dense array
and it would be planar.
It may be better to redesign your end effector to have several displacement
sensors, maybe by having 4 quadrant placed displacement sensors, attached
to thin wires that run the length of the effector. Maybe from these 4
sensors you could determine the minute length changes as the system bends?
You can't 'idiot proof' anything....every time you try, they just make
Just how precisely must the OP measure position? Are we talking inches,
thousandths, tenths of a thousandth, millionths? And what time scale is
acceptable for the measurement? How fast is the device moving? Is there
something on the manipulator that will impart torque? What is wrong with
simple modeling? How rigid is the device?
Somewhere along the line the OP will have to work through the marginal cost
of decimal places in measurement. What is the acceptable cost of this
Who do you expect to implement this? This kind of project isn't
Even if you know the answers what could you do? I agree with Jerry.
This project is probably too complicated for a newsgroup. Especially
when the OP won't or more probably doesn't know the required data.
This is much more than a student problem. Even if I had all the data
I wouldn't do anything unless I was being paid. It cost money to pull
That is what I would do. I don't think that it will be simple
though. Notice that the rest of the group members have not mentioned
modeling. From previous threads and posts I have come to the
conclusion that the group members don't believe in models or more
precisely, don't believe their models.
Do you think the OP knows? I bet the model must be determined
empirically. That will not be easy. This is much more difficult that
just a simple second order system. This system may be an under
damped second order upper arm attached to a under damped second order
lower arm. I have yet to set a piece of equipment that comes with a
transfer function. This system doesn't sound like one that was
designed. No one designing the mechanics AND doing the controls would
design a system that isn't rigid. If you can't tell by my posts the
last year, I have very little respect for most mechanical engineers.
Do you think the OP knows?
Do you think the OP knows? It has been two weeks now. If the OP was
serious at all then he would have asked me about the system integrator
that can do this job. Now I wouldn't provide the name because it
would just be a botther to the system integrator and I wouldn't want
to waste their time. We are wasting our time. I know this borders on
cynical but it is justified.
Actually, I assumed that a model was the first thing considered and
discounted because of variables that are unknown and cause to many
If you include me as one that does not "believe" in models, then you
are mistaken. I have faith in the great model in the sky. I believe!
Actually I believe models have their place and are a great asset when
used properly and with the understanding that a model is not perfect.
What I have seen here is not so much a discounting of modeling, but
the discounting of intuitive processes, which are models really.
When I join in this group I assumed it was populated by practicing
instrumentation and controls engineers, trading knowledge and
discussing the finer nuances of the field, not a cheat site for
students. I was somewhat puzzled as to why the members here were
willing to take the time to try and find a design solution to some of
the problems posed without thoughts of getting paid. It made more
sense when I realized that the majority of the question were coming
from students. I am willing to help those people, but what about the
guy who is out of school and now working for someone who is paying
them to solve problems like this. My tolerance goes down in these
cases. Consulting fees start coming to mind then.
But seriously, what is your beef and why hang around if that beef gets
you angry enough to make the kind of posts you do?
On Oct 15, 8:18 am, Paul M <PaulMatWiredogdotcom> wrote:
This groups lets too much trash pass for information.
Why do you think I am angry? I am just telling it like it is.
The OP is appears to be clueless because of the lack of informatio0n
The OP needs a system integrator that has done something like this
The OP has a system that will be very hard to control because it
It will cost a lot of money to control because the controller will
need to be custom made.
It will cost even more money if the system integrator has to spend two
weeks or more to get the required information about the system.
PaulM, what part do you want to dispute?
I understand your anger with the OP, but you posted about more than
just this thread. It was about the entire nature of the group. I want
to know more about that. I don't know the credentials of many of the
posters here. There regulars, yes, like you, Jerry and Tim. If someone
posts trash info in other groups I am involved in and continue to do
so, some of the regulars normally get together and post regular
disclaimer info about those people, filer out their posts and ignore
them. The nature of a open Usenet group is that they always have a
fringe element, Kooks if you want to use popular vernacular.
I want very much for this group to be useful to me. After working in
the field for 20+ years, due to some health problems, I have been
absent from the field during a time where computers have
revolutionized my profession...my passion. Keeping up with
developments is crucial to me. I don't need to read bullshit, more so
I don't dispute any of this. I think you are correct. It is you
approach that I question. Since we know little about the application
details, all we can do is throw out possible ways of approaching the
OPs problem. I can think of a dozen ways that might work, but this as
far as anyone can go with the info provided. This is what you stated
(and much more) earlier. I just think you could approach this in a
more constructive way. Being angry over how things are in itself is
On Oct 16, 10:55 am, Paul M <PaulMatWiredogdotcom> wrote:
I am not angry my actions are very cold and caluclated. I know I
don't have to respond or even read his posts. I just think the OP
should find a job as a game show host or something where keeping the
audience guessing is a good thing.
>but you posted about more than
Yes, I have deliberately stuck a thumb in the eye of this group. This
groups needs some negative feedback to keep from oscillating. It has
for many years. Don't argue. I have have a list of post and replies
that I call the sci.engr.control hall of shame. I can list them for
you. One is my all time favorites.
I can't believe that someone wanted to recommend Walter for some sort
of award for that.
I don't know where I was but I have a nice symbolic answer to Tom's
request. Do you care to see it?
Obviously, you are also right about the history. By now you should
know that it was Tim picking on the controlguru guys that finally set
me off after many year of seeing garbage on this group. I thought
Tim would straighten things and be part of the solution but he has
declined. At least the old priesthood of the PID is gone. Jerry may
be the last of that era..
What you want is a social club. Myspace or facebook may do. I know I
have sucked the life out of this group but the real engineers will
I am working on getting my doctorate from the school of hard knocks.
I don't know about the others.
That doesn't help. The damage is already done. Look at the link
above. The OP left sci.engr.control because he can see there was no
intelligent life at sci.egnr.control. Again, I don't know here I was
at the time.
They should be stomped out as soon as possible. You must not be
involved with tech support for a product. You can't believe all the
misinformation that people believe. It is easy for people to believe
the kooks and idiots because one doesn't need to understand the math
and physics to do so. To under stand the truth requires understanding
the math and physics which obviously takes a lot more education or
Do you understand all the formulas in the .pdf file I post links to?
So far now one has even asked a quesiton about them. That makes me
wonder if anyone even understands the first thing about controls.
Can you look at the formulas on JCH's web page and see were he went
You should know I was just playing around with JCH. I knew I could
come up with a PID response that what beat his any time. I couldn't
understand why he persisted. I don't know why the group let him
persist with his nonsense unless the group really didn't understand..
So why are you willing to put up with people that talk about infinite
gains and infinite rates of energy transfer? I was playing around
with JCH and during those weeks no one else made and effort to
discredit what he was positing. Yes, my posts were as much a thumb in
the eye to this group as it was to JCH because the group didn't
recognise what JCH was doing wrong.
If you think about it controlling energy flow is what we do. It
doesn't make that much difference whether t is motion or temperature.
Energy is added and energy is lost but hopefully, and with
calculations, what you are trying to control has just the right
amount of energy.
I saw your comments about me. That is OK. I even remember the commet
about 60 HZ hum too. I am a big boy. Now do you want to join the
kooks, the tweaker and coffee drinkers, or do you want to be a real
Yes, this is a thumb in the eye for the whole group. It is a challege
to the coffee drinkers and tweakers and the kooks and those that let
their bad information pass for good.. PaulM, were do you stand?
Meanwhile I hope that the controlguru guys, Doug Cooper and Bob Rice,
come back along with Fred Thompson and Dave Y. They have always
provided good information.
Peter, I am a controls engineer and have been for many years. I would
have given you some response one what I thought of the links you asked
me to look at and respond to, but your smartass comments have made me
decide to pass. I will say that I have never came across a PID system
I designed that I could not tune quickly without using any model other
than a scrap of paper and a calculator to get the values close. I've
been hired by many customers to come in behind an engineering firm and
modify a system so it will work. Maybe one of your job?
Tom's post you reference has a lot of good solid experience and
reasoning to it. Your reaction tells me a lot about you. I suspect
that control engineers that would have been of value to me have been
run off by you. You are an ass and if your behavior here carries over
to your job, you will be an unemployed ass sometime soon. "Real"
control engineers don't behave as you do.
BTW, I suggest you get yourself to a mental hospital. You exhibit
classic symtoms of a mood disorder. It will eventually destroy every
single relationship in your life if not land you in jail.
Have a nice day.
I am sorry, even now I don't know how much will the precison be
enough, for it is a research program, and we want establish a
experiment platform to do some experiments for flexible manipulator
control, so the more precise, the better.
On Wed, 17 Oct 2007 06:27:44 -0700, workaholic wrote:
I have seen the phrase "the more (of some desirable quality) the better"
kill more promising projects and products than anything else. It's a cop
out. Quality costs money, so what you're really saying is "I want it to
cost more than any budget I can get".
Your grants are finite, and so is your time. Unless you're just working
on a Master's thesis where you can get by with some impressive flogging
that goes nowhere, you should actually do something productive with your
time and money -- if for no other reason than getting stuff done is how
you'll end up with more money and help.
I presume you're in an engineering department -- well, engineering is all
about getting the best bang for your buck. If you can't decide on what's
good enough, perhaps you should allocate a certain amount of money toward
measuring your end-effector's position and do your best with that.
I can guarantee you, from personal experience, that trying for infinite
performance on a finite budget just leaves you with a smoking hole in your
Control systems and communications consulting
Here is an example of this. The last job I accepted was to come in to
a factory and get a fabricating machine running that had been plagued
with problems. The company paid over a million US dollars for the one
of a kind line that made a very simple part made from a soft metal
stock strip bent into an oval with a tab spot welded on it. The part
is used in a exhaust hanger brackets. The machine do the entire
process, about 15 steps automatically. When running it produced one
every two seconds. It had never run more than around 4 hrs before
breaking down. I got some of the bugs out of it and charged for my
time. The company was livid that they had to pay me the amount I
charged. They were pouring money into getting this thing running.
Here is the rub. They used to make this part by hand on four stations.
They had orders for enough of them a year that they had two people
working full time at making them on a single shift. The new machine
did the entire year supply in 16 hrs of operation, yet they could not
get it to run that long. The payback for the cost of the high volume
machine was never going to be returned. One of the investor/owners had
gotten sold on buying this machine for no reason other than it was
neat. Even if it worked flawlessly, it was a stupid management
A couple of springs? How? The OP claims that there are no measurable
joints because that his arm is flexible. He hasn't addressed (or won't
discuss) the issue of orientation as distinct from position. I think
that the concept is too amorphous at this stage to make any in-depth
Engineering is the art of making what you want from things you can get.
ok, we are getting somewhere here. If you are looking at basic control
techniques, you are not going to need precision to the gnat's eyelash. You
can do very well with much less.
How about starting with some basic measurments. Just look at the various
angles of each elbow and the length of each section of the arm. That will
most closely coordinate with your controls and your actual position. Do
your basic experimentation with this level of sensing. Perhaps you might
use an lvdt on your arm to help it figure out how closely you are to a fixed
object to determine repeatabilty, etc.
After you have learned all you can with running the manipulator with this
level of measurement, then examine how much more precisely you need to go
for your next step. You will find that most of the controls you want to
develop will work very well at the lower level of precision, with much less
cost and headache than the super high precision possibilities.
There is a lot to learn for not all that much expense. You may find that
even with the simple physical model based positioning, that you are close
enough for many applications. After you prove your techniques, then go out
and get the money to do it better.
There is such a thing as too much precision. I made an instrument one time
which could measure the in situ thickness of a material under 80 tons per
square inch pressure to within a millionth of an inch. Unfortunately, it
also was able to measure the vibrations of the building during the
Thanks, but I don't know how much precision can I obtain by the
displacement sensors, and time delay of them, if too high, it still
can not be used.
For my systems is a general purpose experiment equipments, the more
precise, the better it will be, so it is not desirable to just add
some constraints on it or add other physical adjustment.
You need more than the Cartesian coordinates of the tip of the arm.
Without knowing the mechanics, its orientation in space remains unknown,
and that is critically important if the arm is to be useful.* The arm's
load will in any case obscure a sensor behind it in some orientations,
and may cause unwanted reflections from a sonic or laser sensor. My
imagination may not encompass what you actually propose, but you haven't
told us enough to let me imagine more.
You still haven't provided a clue about how you achieve flexibility. Are
there many joints, like the spine of a snake? Is there a central
compression column surrounded by three or four tension members? It is
futile to attempt to design instrumentation for an unknown machine.
* Although my own arms are made of rigid parts, the several joints --
rotary and flexural -- make them effectively flexible. In order to sew
or use a soldering iron, I need to know much more than the X-Y-Z
coordinates of my thumbs.
Engineering is the art of making what you want from things you can get.
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