probe digitizing algorithms

[Doug Fortune <pentam-at-home.com>]

Ray wrote:

> Yes the EMC does support probing.  There is a working probe routine under
> the tkemc scripts menu.  If you are using steppers, you can access the
> probe pins on the parallel port.  They are settable in the script.
>
> But this may not answer your question because one kind of real digitizing
> requires the repeated probing of many points on some kind of user definable
> grid.  Such a script could be written.  I've got a real crude one here.
>
> So that said, I'm curious to know what your suggestions are?  Several
> others on this list have expressed interest also.

This post concerns finding a way to maximize the density (number) of 3D
probes within a certain time constraint, by eliminating slow movement
of the probe through empty space.

To preface my comments, I am mostly interested in digitizing rather
large smooth objects (fuselages, propeller blades, hulls, etc),  with a
machine much much larger than a metalworking table.

With that in mind (but also applicable for digitizing smoothish smaller
items on a mill table),  my approach is to have something like a "roughing"
cycle, whose main purpose is really to quickly find where the object is NOT!

Once that is done, the detailed pass with tiny XY & Z increments can be
done, but it is optimized because (now that you mostly know where the
object isn't) you can have smaller vertical retractions between probes
(while scanning an XY grid pattern).

For this roughing cycle,  I propose two alternate strategies:

First the easiest to do in EMC today:
- make a probe by attaching some large diameter ball on the spindle (say a
    hollow hard plastic ball about 3" to 5" in diameter).
- manually jog the machine through the air some distance above the object
    (at a comfortable distance thats not too much work in the Z direction, but
    nevertheless close enough to approximate the surface contour ) and
    run through a rough XY grid.  With a 3" diameter ball, that should only take
    2 to 5 passes.   EMC remembers this, and using the radius of the ball, now
     knows where not to look (this should speed things up vs having no knowledge
     of the part).   As the human has good "look-ahead"  obstacles like vertical
      walls can be anticipated and compensated for.

Second method, some new hardware is needed:
A .5" wide,  3" diameter rollerblade wheel, a vertical sliding axis (maybe 5" travel) ,
and a potentiometer  (which goes to an A/D converter) which roughly measures
the vertical movement of the roller as it rolls over the object, going up and down.

As an XY grid search pattern commences at high speed and high elevation, the
controller is noticing that the wheel is at full extension (hanging down) and so
continues the search at high speed.

As the wheel intermittantly contacts the surface, speed is slowed somewhat,
and the rough surface co-ordinates are calculated by the XY position and the
approximate potentiometer deflection.   The rollerblade wheel has the ideal
characteristics of stickiness (causing it to roll vs skid),  and hardness (quite
stiff so compression doesn't affect the measurement much, but also soft enough
not to marr the object).   Perhaps a secondard set of grid passes at right angles
to the first pass would improve the results.

This whole process would take a few minutes, and would drastically reduce
the search space (you would know the surface within about 1/4"), and knowing
this would optimize the path to be calculated for the "detailed pass".

What do you think?

Doug Fortune