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Auto Circle Target with Variable

rickfngp
After a year of using PC-DMIS on a vision CMM I’m still trying to adjust to the fact that measuring an Auto Circle requires knowing the size and location before it can be measured. I spent 13 years with another vision system that allowed double clicking on the edge of a circle and it just traced what it saw and then you had the size and location when it was finished.

PC-DMIS requires a defined Auto Circle target and can only look in this pre-defined location for the edge. The problem that I run into is the diameters are often not very round. The target boundaries are two concentric circles with a dashed nominal diameter circle midway between them. These are perfectly round circles. When the circle to be measured is out of round enough to leave this target area, no points are recorded. Expanding the Auto Circle target boundaries is often not an option, especially with top lighting involved. A lot of points get recorded that are not on the edge of the circle that is supposed to be measured.

There’s a solution to keeping the target area small when the circle is round and automatically expanding it to a controlled diameter that’s dependent on how much it’s out of round. This is based on a pre-measurement of the circle with a few points around the diameter for an estimate of the out of round condition, as well as giving the nominal diameter and location.

In the Auto Circle code there’s a line for HIT TARGET.

HIT TARGET=EA1,0,1,LOW, 1.659

The number at the end, 1.659 is the radial distance between the dashed nominal diameter, and the two target diameters. Changing this number to a variable that uses the amount of reported roundness in the pre-measured circle (LOC2.RN) controls the size of the hit target.

HIT TARGET=EA1,0,1,LOW, 0.2+(LOC2.RN.MEAS*0.8)
A study that lasted 6 hours gave me a variable that works pretty well.
The 0.2 is the size that will be used when the reported roundness is zero. As the (out of) roundness grows, the hit target limits grow proportionally.

Are there any comments, questions, or other solutions?
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  • Rick,

    I moved to PC-DMIS about 2 years ago after more than 6 years with other systems. I went from working completely without CAD, from the displayed image only, to never working from the displayed image and always using CAD. Naturally, CAD would prepopulate the code for the auto circle, but I understand it is not always available.

    You should be able to select the Auto Circle, then change the hit target type to "Manual". You can then click 3 points (iirc) and it will create the code for an auto circle at that location, as if you had created it from CAD. The points should show up as cross hairs with a circle around them. It may take a few clicks to get the pattern right.

    All of the auto features should behave this way when set to manual hit targets. I will be away from the machine for a few hours, but will reply here again if I find this is not the case.

    Edit: I thought better of it. You should be able to leave the feature in Automatic Hit Target, but select the 3 points from the camera view, then it will fill them in correctly.
  • When the circle is completely in the camera view and you are using the Automatic Hit Target, double click on the edge of the circle. It will detect the circle. You can do the same thing for lines.
  • Auto Circle Target with Variable

    Rick,

    Edit: I thought better of it. You should be able to leave the feature in Automatic Hit Target, but select the 3 points from the camera view, then it will fill them in correctly.

    Dom,
    Three points does not determine out of round condition. The post was about parts of the circle not being in the target area because of the out of round condition. The variable described is for the width of the Auto Circle target to automatically expand in proportion to the how much out of roundness exists.
  • +1! I experience similar problems; I'll have to try this!
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