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PC-DMIS 3D Line Straightness

Rondog
PC-DMIS appears to have issues calculating the proper straightness of a 3D line. The definition of straightness from the PC-DMIS online help is: This is the diameter of a cylinder that contains all the input points. A best-fitting routine determines the axis of the cylinder. If this is actually what PC-DMIS is using then the results are wrong using any method available in PC-DMIS. 

; test points and resulting line
PNT1 =FEAT/POINT,CARTESIAN,NO
THEO/<432.03501,-1213.3127,50.2008>,<0,0,1>
ACTL/<432.03501,-1213.3127,50.2008>,<0,0,1>
CONSTR/POINT,CAST,PT_46_1
PNT2 =FEAT/POINT,CARTESIAN,NO
THEO/<455.70566,-1213.79075,42.17132>,<0,0,1>
ACTL/<455.70566,-1213.79075,42.17132>,<0,0,1>
CONSTR/POINT,CAST,PT_46_2
PNT3 =FEAT/POINT,CARTESIAN,NO
THEO/<479.38146,-1214.19232,34.15272>,<0,0,1>
ACTL/<479.38146,-1214.19232,34.15272>,<0,0,1>
CONSTR/POINT,CAST,PT_46_3
PNT4 =FEAT/POINT,CARTESIAN,NO
THEO/<503.0449,-1214.60886,26.09845>,<0,0,1>
ACTL/<503.0449,-1214.60886,26.09845>,<0,0,1>
CONSTR/POINT,CAST,PT_46_4
PNT5 =FEAT/POINT,CARTESIAN,NO
THEO/<526.72087,-1215.06874,18.08364>,<0,0,1>
ACTL/<526.72087,-1215.06874,18.08364>,<0,0,1>
CONSTR/POINT,CAST,PT_46_5
LIN1 =FEAT/LINE,CARTESIAN,UNBOUNDED,NO
THEO/<432.0354,-1213.32863,50.20282>,<0.9468433,-0.0173208,-0.3212286>
ACTL/<432.0354,-1213.32863,50.20282>,<0.9468433,-0.0173208,-0.3212286>
CONSTR/LINE,BF,3D,PNT1,PNT2,PNT3,PNT4,PNT5,,
OUTLIER_REMOVAL/OFF,3
FILTER/OFF,WAVELENGTH=0

​; straightness results
DIM STRA3= STRAIGHTNESS OF LINE LIN1 UNITS=MM ,$
GRAPH=OFF TEXT=OFF MULT=10.00 ARROWDENSITY=100 OUTPUT=BOTH
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL
M 0.00000 0.01000 0.00000 0.04133 0.04133 0.03133 -------->

​FCFSTRA2 =GEOMETRIC_TOLERANCE/STANDARD=ASME Y14.5,SHOWEXPANDED=YES,
UNITS=MM,OUTPUT=BOTH,ARROWDENSITY=100,
SEGMENT_1,STRAIGHTNESS,DIAMETER,0.01,TOL_ZONE_MATH =DEFAULT,
TEXT=OFF,CADGRAPH=OFF,REPORTGRAPH=OFF,MULT=10,
MEASURED:
LIN1:0.04623,
ADD
FEATURES/LIN1,,

​FCFSTRA3 =GEOMETRIC_TOLERANCE/STANDARD=ASME Y14.5,SHOWEXPANDED=YES,
UNITS=MM,OUTPUT=BOTH,ARROWDENSITY=100,
SEGMENT_1,STRAIGHTNESS,DIAMETER,0.01,TOL_ZONE_MATH =LSQ,
TEXT=OFF,CADGRAPH=OFF,REPORTGRAPH=OFF,MULT=10,
MEASURED:
LIN1:0.05824,
ADD
FEATURES/LIN1,,​


First off, why three different (and wrong) answers for the same thing?

The correct result is 0.04936 mm using the above data. I was able to verify this in PC-DMIS by creating an alignment from the best fit line and finding the smallest diameter circle containing all the input points: 

A1 =ALIGNMENT/START,RECALL:MC_CSY,LIST=YES
ALIGNMENT/LEVEL,ZPLUS,LIN1
ALIGNMENT/TRANS,XAXIS,LIN1
ALIGNMENT/TRANS,YAXIS,LIN1
ALIGNMENT/END

CIR1 =FEAT/CIRCLE,CARTESIAN,IN,MIN_CIRCSC],NO
THEO/<-0.00715,0.00477,463.95901>,<0,0,1>,0.04936
ACTL/<-0.00715,0.00477,463.95901>,<0,0,1>,0.04936
CONSTR/CIRCLE,BF,2D,PNT1,PNT2,PNT3,PNT4,PNT5,,
OUTLIER_REMOVAL/OFF,3
FILTER/OFF,UPR=0


I did figure out the method PC-DMIS used to calculate the straightness with the GeoTol command when the TOL_ZONE_MATH is set to DEFAULT or LSQ (Least Squares is not a valid description by the way). Neither method results in the smallest containing cylinder. The legacy method is a mystery and clearly wrong.
​
Parents
  • , The cross product function in PC-DMIS is broken so you can't use it for a lot of things as a result. In one regard you could call it a bug since it is a math function that produces an incorrect (unexpected) result but on the other hand it could be a special function unique to PC-DMIS that happens to have the name CROSS() and was never intended to be a real cross product. The worst part of this is when the result of the cross product should have a vector length of zero PC-DMIS reports the default vector of <0,0,1> instead - bleh!.

    Whatever the reason for this it will not be changed so you have to deal with it some other way. If they did change it then programs that use that function might no longer work properly so I suspect it will not be changed (I hope not at least). Whenever I need to use a cross product in PC-DMIS I create my own inline function to do this and ignore the build-in one. I don't have a sample of it handy but it is a simple one line command and I suspect you know how to code this yourself.

Reply
  • , The cross product function in PC-DMIS is broken so you can't use it for a lot of things as a result. In one regard you could call it a bug since it is a math function that produces an incorrect (unexpected) result but on the other hand it could be a special function unique to PC-DMIS that happens to have the name CROSS() and was never intended to be a real cross product. The worst part of this is when the result of the cross product should have a vector length of zero PC-DMIS reports the default vector of <0,0,1> instead - bleh!.

    Whatever the reason for this it will not be changed so you have to deal with it some other way. If they did change it then programs that use that function might no longer work properly so I suspect it will not be changed (I hope not at least). Whenever I need to use a cross product in PC-DMIS I create my own inline function to do this and ignore the build-in one. I don't have a sample of it handy but it is a simple one line command and I suspect you know how to code this yourself.

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