Re: [CAD_CAM_EDM_DRO] Hexapod question

Chuck,

You don't need to have three triangles. You do need to have legs with
sufficiently different orientations so they share the stresses. I drive my
kinematics from a "joints.txt" file that describes the geometry.

Are there pictures or diagrams available?

Jon,

For calibration -- the process of determining where the joints really are
-- I use a process similar to what you are suggestion. I move the platform
to "known" positions in the work volume (in 6 axis), and record the leg
lengths. I then use simulated annealing to make the best fit of a large
set of error variables, representing the error in joint positions for both
ends of each leg. This can also model systematic errors in the actuators,
such as screw pitch errors. The simulated annealing takes several minutes
to run. Like many statistical processes, I check to see if there are any
particular measurements which are disproportionately in error with the
adjusted parameters, and remeasure them. Usually, I've found that I made a
measurement error, such as not having the rotational axes aligned properly.

The adjusted joint positions become the new joint positions for subsequent
operation.

-- Carl

At 12:38 PM 12/10/2003 -0600, you wrote:

> >
> >
> >Can this setup be made to work? Why doesn't my hexapod lock the platform
> >in place?
> >
>
>If I understand the dynamics of these things, they need to have 6 legs.
> BUT, the legs need to form 3 triangles...yours doesn't do this. You'll
>notice that you have space between them on your platform, and
>consequently have created a 4-sided structure. 4-sided constructs
>aren't stable...and in addition to general "wiggle" in your system, I
>believe this is the source of your problem.
>
>If you view it, not as 6 legs, but as 3 triangles with adjustable length
>struts, then you can start to see how it would form an octahedron with a
>variable geometry. A very elegant system, really.
>
> -- Chuck Knight
>
>
>
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