Re: Probing

["Dean L. Hedin" <dhedin-at-vsi.net>]

On Sunday 13 January 2002 12:17 am, you wrote:
> Dean,
>
> I have a probe tip on my Stewart Platform 6-axis machine.  Could we
> consider adding rotation axes as well?  P (pitch, about the X axis), R
> (roll, about the Y axis), and W (yaW, about the Z axis).
>
> So far, I'm not using the probe tip to sense forms, but instead to set the
> Z-axis reference for accurately carving into warped wood.  One day, though,
> I hope to get involved in creating "point clouds", and fitting surfaces to
> them.
>
> -- Carl

I am now sitting here imagining a small statue inside a hexapod getting 
probed in cylindrical coordinates.  Is this what you have in mind?

The tcltk interface provides us with:
emc_probe_move: <x> <y> <z>

So I assume that your current Y,P, & R stays constant as the tool head moves
towards the probe coordinate.

The problem we have here is defining regions with 6 axis along with
an appropriate probing "path".  

In the 3-axis case I am only considering that Z will be the direction of 
interest. 

A simple case could be achieved with the hexapod where the Y,R & P are fixed
thoughout a probing  "XYZ" region.  But I am not convinced that this would be
of any value.

Let me ask you the following: With the hexapod & EMC's interperter, can you
establish a temporary coordinate system where the Z axis is along the bit's 
current axis (assuming your tool head is at some arbitrary Y,R &P) ?

Such a translation would be quite useful (even not considering probing).
If such a coordinate translation is possible on a hexapod using EMC,
then it would be easy to accomodate in the file format proposed (as well
as the probing algorythm)

We simply tilt the tool head to the desired Y,R&P and then move
to a starting coordinate, then follow the region definition previously 
described.

FYI, Probing will always be imperfect. Consider the following little thought 
experiment:

We are on a 3-axis machine and I take a small cylinder and I cut it 
longitudelly in half and lay it on the table flat side down. I probe this 3d 
surface with a probe shaped like a normal cylidrical router bit.  I then 
create some g-code from this point cloud.

I then machine a block of material using this g-code with a bit that matched
the probe.  I end up with little ridges along the raising & falling arcs of 
the surface but no ridges along the top.  If I take very small cuts I can 
minimize these ridges but I can't get rid of them.

So I say! Ahah! I'll put it on a hexapod and I'll probe it in cylidrical 
coordinates!  That will get rid of the ridges by keeping the bit perpendicular
at all times!

But this required that you had some prior knowledge of the object being 
probed!  The whole idea of probing  is that we do not know how to easily
write a tool path (or a probe path) for some arbitary 3d object.


BTW, I put the pics up of my home-brew optical probe switch if anybody is 
interested:

http://web.starlinx.com/dhedin/dcp00468.jpg
http://web.starlinx.com/dhedin/dcp00469.jpg
http://web.starlinx.com/dhedin/dcp00470.jpg
http://web.starlinx.com/dhedin/dcp00471.jpg