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Digitizer-tablet for XY-measurement ...

Posted by VDX 
Digitizer-tablet for XY-measurement ...
September 27, 2007 06:09AM
Hi folks,

... an idea for easy reading the absolute coordinates of the XY-motors or of the tool-head absolute position in general ...

If someone has an old digitizer-tablet (best an inductive with crosshair-mouse) laying around or can buy it cheap at ebay or so, then it's worth trying, if it could be fixed under the reprap and the digitizer-mouse mechanically syncronized to the moving reprap-head (with a moving rail and two steel-wire-loops, like in oldstyle Computagraph-penplotters for example).

The better tablets are very fine, so it should fit for positioning feedback or error-correction of the stepper-motors.

As i know, the output of the tablet must be serial XY-coordinates as ASCII-string, so it should be no problem to read it out and use the information as close-looped servo-driving ...

Re: Digitizer-tablet for XY-measurement ...
September 27, 2007 03:47PM
... at home i checked the tecspecs of my old 'Summasketch III Professional'-tablet:
- active area = 12 x 18" or 305 x 457 mm
- resolution = 2540 lines per inch or 100 lines per millimeter (=10 micron)
- accuracy = 0,010 In or 0,254 mm ???

The tablet has a stylus and a 4-button-mouse, which would do better.
(I found an Aiptec-HyperPen-tablet too, but the active area is only 10 x 12,5 mm and the stylus is battery-driven, so the Summasketch is much better adapteble and for larger scales)

The resolution is good enough for servo-driving, so i hope to adapt the mouse to the toolhead and use the tablet as absolute reading.

The main problem would be the software to read the tablet-output in the reprap-software ...

If the overall accuracy should be a problem, there could be a mechanical upscaling of XY-movement to the digitizer-mouse 2- or 4-fold, so that the measured area for the reprap-toolhead would be smaller, but with a higher accuracy ...

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