Re: [CAD_CAM_EDM_DRO] stewart platform

I've had what seems like good success calibrating a home-brew parallel robot.

The general method is:

1) add a bunch of error variables into your kinematics equations to
represent the major error contributors. This are typically the x, y,
&z positions of the base and platform joints, the minimum leg length
(at "0" position reference), and possible the actuator scale error.

2) move the platform to a series of known orientations, including all
6 axes being varied. I'll do something like a gird measuring every 6
to 12 inches, with Pitch, Roll, and Yaw at -30, 0, +30 for many of the points.

3) use simulated annealing to adjust (hill climb) the error variables
for the closest fit.

4) look for measurements whose final error falls 3 standard
deviations away from the mean, and either remeasure those points or
discard them from the data set.

5) repeat steps 3 & 4 as necessary until the process converges.

Steps 4 & 5 are necessary only if you (as I do) sometimes mess up the
measurements.

-- Carl

At 01:20 PM 8/29/2006, Anders Wallin wrote:

>did you check out
><http://robofac.sourceforge.net/>http://robofac.sourceforge.net/
>one of the few DIY hexapods I have seen on the net. click screenshots
>and download a video at the bottom of the page.
>there's a cnczone thread about the machine at:
><http://www.cnczone.com/forums/archive/index.php/t-7467.html>http://www.cnczone.com/forums/archive/index.php/t-7467.html
>
>If you just find the kinematics interesting and want to play around, you
>could suspend the moving platform from strings. I remembers seeing
>pictures of a couple of these kind of machines but I can't find the
>links now...
>
>If someone can come up with a smart way of calibrating home made
>parallell robots, then I'm very interested too - until then I'm sticking
>with a simple cartesian mill...
>
> > I'm curious about the Stewart platform, and what kind of actuators might be
> > salvaged to create a small one - like meter or less in dia? I suppose just
> > some salvage steppers with threaded rod, but if others might have better
> > ideas, I'd love to hear. One of these "sandtables" would be cool to build.
> >
> >
>
>

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