Re: [CAD_CAM_EDM_DRO] Re: stewart platform

Graham,

I don't measure the error. I manually "fly" the platform with a
joystick to the position where it should be, and then record the
length of the actuators that puts it at that place. To set the Pitch
and Roll, I've used an electronic level with 0.01 (I think, maybe
0.1) degree resolution. To set the Yaw (my W axis), I attach a
straight indicator to the platform, and align it visually with the
base coordinate system.

I then use simulated annealing to estimate the error variable values
that are needed to make the actuator lengths work out to be correct.

The simulated annealing is used to minimize the total error
contribution, ie., how much the recorder actuator length differ from
the calculated lengths. When all is right, the measured and
calculated lengths match.

Other approaches would work, but this has worked for me.

I had some problems attempting to use simulated annealing on ALL
error variables at once, but by controlling all six axes, the problem
decomposes into 6 separate estimation problems -- one for each
leg. My original plan was to estimate all errors at the same time,
and not control all six carteasian axes. I intended to have a
precision flat surface, and use a surface probe to record the leg
lengths at which the probe touched. The partial-control of the
carteasian coordinates coupled the actuator errors together
(necessitating the all-variables estimation), so I changed to the
strategy I outlined.

-- Carl


At 08:56 PM 8/29/2006, you wrote:

>--- In
><mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>CAD_CAM_EDM_DRO@yahoogroups.com,
>Carl Mikkelsen
><c.mikkelsen@...> wrote:
>
> > 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.
>
>How do you go about measuring the error? Say you move 100mm in X, how
>do you measure the error created in one of the rotational axes?
>
>Graham
>
>

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