Re: [CAD_CAM_EDM_DRO] Re: A "simple" Hexapod project - calibration

For calibrating, I had good success using Simulated Annealing.

The problem is to determine where the joints are, and how long the legs
are. Of course, if you built everything perfectly, there wouldn't be a
problem. But, if you are like me, you won't build everything perfectly,
and you can easily be off by +/- 1/4" on the joint positions over a
4'x5'x5' frame.

You may also want to correct for actuator screw error. I didn't do this on
the last calibration, but I'll try it on the next.

Simulated Annealing (SA) is referred to in many places on the web. It is
often used now as a technique for determining how proteins fold, but it is
suited to any situation where you need to estimate parameters.

To cut through all the theory and such, you model the hexapod including
variables for everything that may not be fixed. This includes the
locations of the joints, the minimum leg length, the screw pitchs. You
carefully measure the extension of the legs at several (many)
positions. The challenge with a hexapod is to be sure you have measured or
fixed all six degrees of freedom. You now have a set of actuator lengths
required to reach a number of fixed positions, distributed through the
range of the machine.

The SA code, with the model function you wrote, attempts to find the best
fit of the unknown to the measured data. It tried moving the joints a
little, it adjusts the notion of leg lengths. It tried tens or hundreds of
thousands of slight variations, each time checking it against the observed
data, trying to find the best fit.

After a few minutes of computer time (depending on CPU speed), it prints
the best fit it could find for the model. This best fit becomes the input
to the kinematics you need to run the Stewart platform.

I downloaded some commonly used SA code form the net, and modified it for
calibration. I could share this, but it isn't terribly useful without some
programming skill.

It runs on Linux.

-- Carl

At 08:33 AM 6/14/2003 +0000, you wrote:

>Zeroing the hexapod could be a challenge as I'd think that you'd want
>to have all the struts at relatively equal tension / compression to
>start with. If you used high-lead ballscrews or belts or cables then
>powering off the motors and zeroing by hand might be simple and
>effective. You'd also need to be able to accurately measure the
>relative positions of all the joints and initial strut lengths in
>order to model the hexapod in software and probably devise a means
>for calibrating.
>If I could put together the hardware for a reasonable cost then I'd
>probably build a hexapod just for the fun although I'm currently
>working on a hybrid design using 3 actuators to tilt the tool
>platform in conjunction with 3 linear axes to position it, early
>stages but has potential advantages over the 2 rotary axes I'm
>currently using on my router.
>If you get the hardware together then by all means use my CNC Toolkit
>for the software side. It might not be very user friendly but that
>could be fixed with some decent documentation and some work on the
>user interface.
>
>Regards,
>Rab
>
>
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