Re: digitizing arm

Message: 12
Date: Thu, 16 Nov 2000 10:04:42 -0500
From: "Patrick Huss" <patrick@...>
Subject: digitising arm Digest Number 889

Has anyone here ever used microscribe? I have, and it was in the top ten
MOST frustrating things I have ever tried. It is capable, if I recall
correctly, of measuring to .011" accuracy. But you hold it in your hand,
which doesn't even come close to the kind of stability required to keep the
scribe steady. Using a probe on a cnc machine, or a laser range finder (not
interferometer), will always be IMO a more reasonable experience. As
Ballendo pointed out in his response re: steppers vs. servos - 'Accuracy is
a function of the system design.'
If your hand is an integral part of the system design, accuracy is
nonexistent. If anyone is seriously considering creating their own
digitising arm, I highly recommend that you use one to digitise an object,
you may have to fight a strong urge to throw the device out the window!

Patrick

[I hope you managed to resist! Actually I've got a Microscribe, and I've
come to like it (I even decided to start selling them). I went through some
of the learning curve you describe, until I got a lesson in how to use the
thing. While digitizing a stream of points may be fast, it has, as you
point out, the disadvantage of picking up every irregularity in your
movement. Instead of doing that, here's the correct procedure:

Draw a grid on the object to be digitized. These don't have to be perfectly
straight lines, as long as all the lines intersect all the lines going the
cross-direction. Put more lines in the areas with the most detail- smooth
areas need fewer lines; places with a lot of texture need more. With Rhino,
(other programs support the arms and may work differently, but this is the
one I know about) you need to either construct a grid of open curves (think
of a fringed rug) or a combination of open curves crossing closed ones
(like a teepee, with the "poles" overlapping the closed curves (the
"canvas") at the top and bottom). These configurations will make surfaces,
using the Surface from a Network of Curves function. The open/closed
combination will result in surfaces that have holes at the top and bottom
ends, but these can be patched over later on.

Once you've got it set up this way, then start making curves, following the
lines you've drawn. Use the Interpolated curve function, with all snaps and
constraints off. Don't try to digitize a stream of points, but instead
digitize POINT by POINT, paying particular attention to the intersections
of your drawn lines. While it's admittedly tedious to click the footpedal
for each point, you eliminate the problem of hand-shake, since the stylus
is at rest when a point is chosen. Adding more points along the lines will
increase detail, if you want it. Since fewer points are used this way, the
curves are much less dense, and the file-size won't overwhelm your
computer. Try it- you may even like it!]

Andrew Werby
http://www.computersculpture.com




Andrew Werby - United Artworks
Sculpture, Jewelry, and Other Art Stuff
http://unitedartworks.com