Re: [CAD_CAM_EDM_DRO] Digitizing arms??

If your only after 2 axis, use a tablet and trace.

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, June 23, 2004 9:09 PM
Subject: Re: [CAD_CAM_EDM_DRO] Digitizing arms??


>
>
> doug98105 wrote:
>
> >Group,
> >
> >Looking for some ideas on making a two axis digitizing arm.
> >
> >Back in the days of Apple II computers I saw a couple of crude
> >digitizing arms built using potentiometers and inputting through the
> >game controller input. Position was determined by the resistance
> >readings.
> >
> >With high resolution encoder devices it seems like a much more
> >accurate digitizer could be made.
> >
> >Any thoughts?
> >
> >
> Yeah, look at Faro for how they did it. The problem with these things
> is that certain arm positions lead to poor accuracy, because a very small
> movement of one joint makes a big change in position.
>
> No doubt a 1000 cycle/rev, or even 2500 cycle/rev encoder would be
> a HUGE improvement over the ancient Apple digitizer with pots.
> Unless they used special pots, you couldn't possibly get better than
> 1% angular accuracy, and that is 3.6 degrees! Yikes!
>
> I worked for a medical computer company in the WAY distant past,
> and they made "Rho - Theta" and "Phi-Theta" digitizing arms using
> very expensive mil-spec rotary and linear pots, and had all sorts of
> trouble getting enough resolution with a 12-bit ADC to digitize
> the location of a patient's tumors, etc. Linearity was no problem, as
> the software had cal tables built for the individual digitizing arm.
>
> Jon
>
>
>
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