How to get the position of a manipulator's end

workaholic wrote:


You are asking for the solution to a very difficult problem. In addition to the problem of location, you want to servo out the vibration that inevitably accompanies a flimsy mechanical design.
Jerry
--
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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?
--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
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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 project?
Michael
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On Oct 14, 8:45 pm, "Herman Family"

Who do you expect to implement this? This kind of project isn't trivial.

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 teeth.

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.
Peter Nachtwey
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proclaimed to the world:

Actually, I assumed that a model was the first thing considered and discounted because of variables that are unknown and cause to many errors.
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?
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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 provided.. The OP needs a system integrator that has done something like this before. The OP has a system that will be very hard to control because it flexes. 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?
Peter Nachtwey
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proclaimed to the world:

On subjects other that models and PID?

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 than you.

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 not constructive.
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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.
http://groups.google.com/group/sci.engr.control/browse_frm/thread/d11d0963eaa46b65/43d61036dda8ded8?lnk=gst&q=tuned&rnum=4&hl=en#43d61036dda8ded8
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 stay.

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 effort.

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 wrong? 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 engineer?
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 Nachtwey
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proclaimed to the world:

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.
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On 10 15 , 11 45 , "Herman Family"

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.
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workaholic wrote:

What the good of knowing where it is if you don't know which way it's pointing?
Jerry
--
Engineering is the art of making what you want from things you can get.
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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 resume.
--
Tim Wescott
Control systems and communications consulting
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proclaimed to the world:

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 decision.
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Paul M wrote:

A million US dollars would have been cheap if the machine had included a flexible manipulator arm.
...
Jerry
--
Engineering is the art of making what you want from things you can get.
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proclaimed to the world:

Just model the thing as if it were a couple of big springs and be done with it! Avoid the temptation of getting mesmerized by the XYZ indicators that measure in microns.
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Paul M wrote:

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 analysis fruitful.
Jerry
--
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proclaimed to the world:

This was all my idea of humor btw.
Peter has already voiced this opinion in his charming manner. :-)
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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 experiments.
Michael
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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.
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workaholic wrote:
...

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.
Jerry _________________________________________ * 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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