Re: [CAD_CAM_EDM_DRO] Re: A "simple" Hexapod project - fear factor

Graham,

There isn't anything that needs to be fear inducing -- at all.

The first step is to make a mechanism. You can measure the joint positions
(or build accurately to your drawings), and you'll have quite interesting
behavior. I mean interesting in a good sense, not a back-handed
sense. Depending on how accurately you've build things, you'll have
more-or-less accurate performance.

The process of "calibration" is just to take you to the next level.

I did NOT have any kind of model of my machine before I built it. None. I
made some rough design calculations assuming that I wanted something like
equilateral triangles at the center of the operating space. I "thought"
that I'd make the base about 4'x5', mostly because it seemed like a good size.

When I got into actually making something large, I found that I didn't have
the range I expected. It wasn't a big problem, bit it made me want to be
more careful for the next one.

The "model" I'm talking about is really just fundged versions of the code
for the first machine.

There is virtually NO mathematics required to operate the inverted Stewart
platform. There only one matrix equation required. This equation
transforms the moving joint positions into the coordinate system of the
base, for a particular pose (x,y,z,P,R,W). With the transformed locations,
you change the leg lengths to be equal to tbe distance between the two
ends. Make the legs be that length, and you've put the tool where you want it.

I have code for this, which I could release under GPL or something like
it. It is checked into my CVS server, which I could open up to limited
public access.

I have friends who are much better modelers than I am. They will carefully
analyze every aspect of a problem before they begin. This is admirable is
many ways, but I need the reality of seeing things work. What I do is just
move ahead with partial solutions, learning as I go, and developing better
solutions as I move forward. There is nothing more real that seeing your
milling bit crash into the table to help you realize a programming error.

Tonight was a good night. I got to spend some time working on the new
machine. For the first machine, I bent up some 1/8" x 1/2" stock, and
these became the gimbals for the platform joints. For the second, the
platform joints will be (it's not done yet) made up of four welded bars,
some 3/8" x 1/2", some 1/4" x 1/2", each cut accurate to maybe .010" with a
chop saw. I don't know if they will work better, but they'll be able to
handle many times the load of the first ones. I'll need many times the
load, since I intend to be cutting steel with this version. TIme will tell
if I succeed.

It is nearly impossible to fail at building a Stewart platform. When
you're done, you may not be completely happy with the results, but you will
have results which will be far more valuable that protracted consideration.

There isn't anything to fear. Follow safe procedures. Wear safety classes
and gloves. Use a respirator when making dust or fumes. Be safe.

Success will follow.

-- Carl


At 10:33 PM 6/15/2003 +0000, you wrote:

>Thanks again Carl, I don't think I realised how tricky the process
>might be when I first started thinking about the problem. It is all
>rather alien, we are not used to having virtual models of machines
>just to make them move. I don't mind the maths or the programming
>but is does induce fear!
>
>Graham
>
>
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