How to get the position of a manipulator's end

workaholic wrote:

What will you measure? Where will your reference be?

Jerry
--
Engineering is the art of making what you want from things you can get.
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On Mon, 01 Oct 2007 07:42:23 +0000, workaholic wrote:


There are any number of choices, depending on how precise you need to be,
how much money you have to spend, what sort of an environment you're
working in -- the possibilities are almost endless.

I assume that by "calculate the formulas" you mean that you feel that
measuring the angle of each joint and calculating the end-affector's
position isn't sufficient.  Is this correct?

So you're looking at a way to figure out the end-affector's position
in an independent, global way?  One method that I've seen mooted about is
a sort of "sonic LORAN", where a number of transmitters send out
ultrasonic pulses, and a receiver detects their times of arrival to figure
out where it is.  You could augment this with some accelerometers and
gyros for some inertial nav effect, but by the time you'd increased your
accuracy by any significant degree you may have spent enough money to just
hire a full professor & equip him with a tape measure and a level (I don't
know about this -- you may be able to get significant accuracy quite
cheaply this way).

At worst, you could always use three string pots.  This wouldn't be at all
good for anything resembling a harsh environment, but then you haven't
told us anything at all about yours.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
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Tim Wescott wrote:


A professor...with a tape measure and a level?

That is sooooo not real time

Fulliautomatix wrote:

The OP didn't specify any time requirements, real or otherwise.

--

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

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Tim Wescott wrote:
  > The OP didn't specify any time requirements, real or otherwise.

The lag man...the lag...U'll kill us all!!!!!!

Fulliautomatix wrote:

How about three non-coaxial cameras? If telecentric lenses with fields
as large as the desired motions are unavailable, perspective correction
will be needed. The camera angles needn't be mutually orthogonal. Other
computations can resolve the positions along orthogonal internal axes.
Those would need to be recursed with the perspective correction until a
stable solution is found. Modern computers can probably manage that
within one scan time of a standard CCTV camera. Simple, really.

Tell me again what's wrong with calculating position from measured joint
angles?

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

thanks, but I am worry about the heaviness of the equipment added,
cameras may work but I think sometimes the end of the manipulator may
be overlapped, so the cameras can't see it. And of course, the
computing time must be considered.

Can you explain more about the ultrasonic sender and receiver? I am
afraid the end of manipulator may turn to any directions,so where to
put the receiver is a big problem.

workaholic wrote:

Before we dig too deeply into possible solutions, consider the recent
thread "Quality qualificatif" and give us a hint about why calculating
the position from limb lengths and joint angles is inadequate.

I feel that a sonic approach will not be robust in the presence of
inevitable reflections. If you try that way anyway, put the sender on
the moving member and use three or more fixed receivers.

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

On Wed, 03 Oct 2007 14:24:14 -0400, Jerry Avins wrote:


I have never, ever actually used the sonic approach -- I've just seen it
mentioned.

I would also like the OP to give us some idea of why he can't measure
joint deflections.  I could see needing a _really_ precise measurement
that wouldn't fly by using joint deflections, but if you need to get that
tight you have to expect to sweat bullets for weeks or months to solve
your problems.

Is there a robotics newsgroup?  This seems like a problem that should have
been solved by now...

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

Tim Wescott wrote:

There must be a robotics newsgroup, but I don't know it.

The position of the end of the arm is seldom of interest in itself. The
position *and orientation* of some point on what the arm is holding is
more usually needed. Fitting tab A into slot A requires either a learned
sequence or a calculation even if the location of a point on the arm is
known precisely.

With heavy loads, members can flex and joints be displaced. A learned
sequence accounts for that directly if the loading is consistent. Strain
gauges can measure the deflections and allow software to account for
them. There's often more than is first imagined.

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

My manipulators are designed to do experiments and not intended to a
certain industry process, and it is a flexible one, so we can't
calculate precisely where the position of the end,only based on the
joints' information.

Since the trajectory is constantly changed for several experiments, I
have to install a more general detection system for it.

workaholic wrote:

How flexible? How do you flex it? How can you tell the orientation of
the end? Even the position of a snake is determined by the sum of the
angles of its vertebrae.

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

proclaimed to the world:


What about designing a system that uses three or more sonic sources at
known locations at the limits of the manipulator work area. Drive
synchronized pulse trains from these sound sources and receive these
pulses with a receiver mounted on the manipulator arm. You should be
able to derive location from the phase shifts that result from the
variable distances from the source signals.

I have also looked at some coordinate measuring systems that were able
to resolve a tip location to within microns. I am not for sure what
they used for sensing, but the arm itself did not support any load,
you simply touched to tip to the material being measured.

"Paul M" <PaulMatWiredogdotcom> wrote in message

It would be better to have one sonic source at a reference point on the arm,
and three sensors positioned around the device.  The sensors should measure
flight time of the sound.  My guess is that you will want the transducer to
send out some patterned sound, so that the sensors can get a better idea of
what time each wave left.

Bear in mind that sound velocity may change ever so slightly if one side is
warmer than the other, or dustier, or whatever.  Also, sound tends to be
blocked.  I don't know the size of your equipment, so I don't know if it is
likely that someone will walk around.  Also, if it lifts any material then
that will act to deflect sound from one direction or another.

Michael

On Oct 5, 10:44 pm, "Herman Family"

I second the doubt about using sound.  I have never heard of finding
an end effector's position by sound before.  However, I know people
that have done this with cameras.  I believe they used two cameras and
a lot of math.  Maybe even a model or two ( ooooh ).  The purpose of
the system was to grab or connect to something in the ocean.  It had
to be able to keep up with the wave action.

I can provide the name of the system integrator that did this and the
name of the person on the project if interested.

Peter Nachtwey

On Sat, 06 Oct 2007 08:21:07 -0700, pnachtwey wrote:


In spite of sonar having been my suggestion, this sounds better.

OP, have you done a web search?  Surely this is a solved problem?

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

My question is the static tip position of the manipulator is easy to
obtain, but when it is moving, it oscillates a lot, and my task is to
control its very trajectory in an acceptable error, so how to test the
actual real-time trajectory is very important to form a closed loop.

And until now I can not find anyone doing this.

Cameras do work in detecting the static tip position, but it doesn't
fit in this question.

Sure does.   Read this.

"We want to build a closed loop to control the trajectory of the end
of
a manipulator, how to detect the precise position of it rather than
just calculate the formulas? Which kind of sensor should I choose?
thanks! "

9 days have past.

Peter Nachwey

I am very sorry, for my lacking of skills to put it clearly and
effectively....