Hello,
My probe is at the CNC Workshop in Illinois this week so I cannot do any serious testing until I get it back. However, I have given calibration some thought and I did do a few simple experiments during the brief time that I had it.
The present calibration scheme requires a square bar of known dimensions to be placed on the table parallel to the Y axis, and the camera and laser in the YZ plane and the target scan in the XY plane. No motion is used in the calibration so data is captured in a single position.
This static scheme does not allow for:
misorientation of the probe,
misorientation of the calibration bar,
inaccurate calibration bar dimensions,
radial distortion due to camera lens,
rectangular distortion due to non-square detector pixel layout,
keystone distortion due to laser and camera axes not perpendicular,
preferring a different arrangement of probe and target.
I would propose using a single point as a target. This could be the pointy end of a nail or pin or it could be the apex of a cone. If it is a cone then the apex angle would have to be sharp enough that the camera cannot see where the laser hits the cone except at the point. For example, in my setup at
http://www.machsupport.com/forum/index.php/topic,5023.msg42913.html#msg42913, with the laser 30 degrees off the Z axis, the calibration target cone point would need to be less than 60 degrees.
I would jog until the point shows up near one corner of the screen then save the (X,Y,Z) coordinate from the machine axes and the (H,V) pixel horizontal and vertical coordinate from the CCD array where the center of the bright spot appears. I would do this for three corners to get three points as far apart as possible. This would create the data set:
(X1, Y1, Z1, H1, V1)
(X2, Y2, Z2, H2, V2)
(X3, Y3, Z3, H3, V3)
Three points are sufficient to define a plane so this is sufficient to define (H,V) points on the CCD plane in terms of the (X,Y,Z) points of the machine axes.
More than three points would permit a least squares fit which would be more accurate. Many more points, spread over the array, would allow for correction of distortion.
This scheme allows the probe to be installed in any orientation, even off axis orientations to get better data from odd shaped targets.
The probe could even be installed on the Z axis and pointed toward the front of the machine rather than down at the table. For this arrangement the point used for calibration would have to be setup so that it is pointed at the Z column. Then scanning the side of something would be possible.
Or side scan a target on a rotary table.
Or hang the probe on an articulating arm any way you wish.
Tom Hubin
thubin@earthlink.net 
Author