hexagon logo
Register Sign in
  • Replies 20 replies
  • Subscribers 222 subscribers
  • Views 205 views
  • Users 0 members are here
Options
Related

line measurement inconsistancies depending on workpiece orientation

Daniel P
Dear CMM gurus!

You might or might not be aware of a nasty behavior of PC-DMIS when measuring planes. If the actual plane you're measuring has a normal vector that's pointing into a direction more then 90° different from the plane's theoretical normal vector, the measured normal vector flips such that it's pointing into the workpiece instead of out of it. (I should probably add here that we're not using PC-DMIS for inspection purposes per se, but rather for precision adjustments of components, which is why it is very common that our workpieces are located and oriented a lot differently than the corresponding features' theoretical values.)

With lines, however, I have not yet encountered a similar problem----until last week. According to my tests, the following alignment procedure for a cuboid-shaped workpiece will yield consistent results:
  1. measure a plane (theoretical normal vector <0,0,1>, actual normal vector anything as long as its Z component is positive)
  2. level and Z-translate to the plane
  3. measure a line on the workpiece's side (side perpendicular to the plane measured before,workplane = ZPLUS, apart from that no restrictions to how much the side surface's normal vector may differ from the actual one)
  4. rotate XPLUS and Y-translate to the projected line
  5. measure a point on the third side (side perpendicular to the previous two with a consistent approach vector)
  6. X-translate to the projected point
In particular, with this strategy the line's directional vector has consistently been pointing in the correct direction no matter which way the workpiece was oriented. However, I did run into a problem recently under the following circumstances:
  1. start a subroutine
  2. reload an external alignment of which the Z axis and the origin's Z component are to be left unchanged
  3. measure a line under the exact same conditions as above
  4. rotate XPLUS and Y-translate to the projected line
  5. measure a point under the exact same conditions as above
  6. X-translate to the projected point
I would have expected this to behave exactly the same as the scenario listed before, but this is not the case. Here, the line all of a sudden becomes dependent on the orientation of the workpiece. If the actual line vector is pointing into a direction more than 90° different from the theoretical one, the actual line vector flips and is pointing into the opposite (wrong) direction. Furthermore, I noticed that the line and the point used for the alignment become red in the alignment's edit dialog. However, I did confirm that they are getting measured by outputting the measured directional vector through an operator comment----including the directional flipping.

Can you give me an explanation for this weird behavior and the red features in the alignment?

Thanks a lot in advance! 

STARTUP =ALIGNMENT/START,RECALL:USE_PART_SETUP,LIST=YES
ALIGNMENT/END
MODE/MANUAL
PREHIT/1
RETRACT/1
MOVESPEED/ 100
MANRETRACT/1
FORMAT/TEXT,OPTIONS, ,HEADINGS,SYMBOLS, ;NOM,TOL,MEAS,DEV,OUTTOL, ,
LOADPROBE/KU_5X50
TIP/T1A0B0, SHANKIJK=0, 0, 1, ANGLE=0
SUBROUTINE/BS_LISA22_FULL,
=
MODE/MANUAL
RECALL/ALIGNMENT,EXTERNAL,BASE_FINE
AM_LIN =FEAT/LINE,CARTESIAN,UNBOUNDED
THEO/<18,0,3.5>,<-1,0,0>
ACTL/<18,0,3.5>,<-1,0,0>
MEAS/LINE,2,ZPLUS
HIT/BASIC,NORMAL,<18,0,3.5>,<0,-1,0>,<18.032,0,3.494>,USE THEO=YES
HIT/BASIC,NORMAL,<4,0,3.5>,<0,-1,0>,<3.842,0,3.494>,USE THEO=YES
ENDMEAS/
AM_LIN_P =FEAT/LINE,CARTESIAN,UNBOUNDED,NO
THEO/<18,0,0>,<-1,0,0>
ACTL/<18,0,0>,<-1,0,0>
CONSTR/LINE,PROJ,AM_LIN,,14
COMMENT/OPER,NO,FULL SCREEN=NO,AUTO-CONTINUE=NO,
"AM_LIN: " + AM_LIN.IJK
"AM_LIN_P: " + AM_LIN_P.IJK
AM1 =ALIGNMENT/START,RECALL:BASE_FINE,LIST=YES
ALIGNMENT/ROTATE,XMINUS,TO,AM_LIN_P,ABOUT,ZPLUS
ALIGNMENT/TRANS,YAXIS,AM_LIN_P
ALIGNMENT/END
AM_PNT =FEAT/POINT,CARTESIAN
THEO/<0,1,28>,<-1,0,0>
ACTL/<0,1,28>,<-1,0,0>
MEAS/POINT,1,WORKPLANE
HIT/BASIC,NORMAL,<0,1,28>,<-1,0,0>,<0,1.745,27.279>,USE THEO=YES
ENDMEAS/
AM_PNT_P =FEAT/POINT,CARTESIAN,NO
THEO/<0,1,0>,<0,0,1>
ACTL/<0,1,0>,<0,0,1>
CONSTR/POINT,PROJ,AM_PNT,
AM =ALIGNMENT/START,RECALL:AM1,LIST=YES
ALIGNMENT/TRANS,XAXIS,AM_PNT_P
ALIGNMENT/END
COMMENT/OPER,NO,FULL SCREEN=NO,AUTO-CONTINUE=NO,
BS: after AM
SAVE/ALIGNMENT,BS_manual.aln,MACHINETOPARTS
...
ENDSUB/
​
Parents
  • When you are recalling the external alignment, is the origin ever in between the hits of the new line?

    HIT1 ----------------------------------------recalled ORIGIN------------------------------------------Hit2

    if so, that's a very common error. if your workpiece is rotated positively to existing rotation, it will produce correct vector, if your workpiece is rotated negatively to the existing rotation, it will flip your line vector 180 on you.

    First of all, thanks so much for your detailed reply!!

    You might be correct with your assumption about the location of the origin----still need to double-check though. OK, so let me get this straight:
    1. By "origin in between the two hits" you mean that the origin lies between two planes with normal vectors collinear to the line's directional vector and each of the planes going through one of the two hits, right?
    2. What do you mean by "positive and negative rotation"? Do you mean a right-handed rotation around the workplane's normal vector of less than 180° will produce the correct directional vector for the measured line whereas a left-handed one----however small it may be----will result in a directional vector flipped by 180°? (That would be a very unstable situation even for common inspection-type CMM tasks, which is why I would have assumed the problem only ever arises if the rotation is more than +90° or less than -90°.)
    3. In your first sentence, you explicitly mention my recalling an external alignment. Do you mean to say there would be no problem if I had recalled an internal one or none at all (which of course isn't feasible in my subroutine)? If the answer to either one of these options is yes, then what is so special about recalling alignments?
    Why this flipping business of the directional vector could ever be considered a desired behavior is beyond me----just like the problem with planes I briefly described in my first post. Speaking of which, could this normal vector flipping in planes also be influenced by where the origin lies?

    For me, it would be very tempting to just test in my subroutine if the origin lies in between the two hits and, if so, to flip the line's directional vector. But the question then arises what will happen if the origin lies on one of the two planes going through the hits. I'm also curious to know what would happen if I create a new line out of the two hits of the first. You definitely gave me enough food for thought, but I'm a bit afraid of missing more problems like these. So I'm not just trying to get around the problems but to really understand their root causes and any additional explanations would be much appreciated. Thanks!!
Reply
  • When you are recalling the external alignment, is the origin ever in between the hits of the new line?

    HIT1 ----------------------------------------recalled ORIGIN------------------------------------------Hit2

    if so, that's a very common error. if your workpiece is rotated positively to existing rotation, it will produce correct vector, if your workpiece is rotated negatively to the existing rotation, it will flip your line vector 180 on you.

    First of all, thanks so much for your detailed reply!!

    You might be correct with your assumption about the location of the origin----still need to double-check though. OK, so let me get this straight:
    1. By "origin in between the two hits" you mean that the origin lies between two planes with normal vectors collinear to the line's directional vector and each of the planes going through one of the two hits, right?
    2. What do you mean by "positive and negative rotation"? Do you mean a right-handed rotation around the workplane's normal vector of less than 180° will produce the correct directional vector for the measured line whereas a left-handed one----however small it may be----will result in a directional vector flipped by 180°? (That would be a very unstable situation even for common inspection-type CMM tasks, which is why I would have assumed the problem only ever arises if the rotation is more than +90° or less than -90°.)
    3. In your first sentence, you explicitly mention my recalling an external alignment. Do you mean to say there would be no problem if I had recalled an internal one or none at all (which of course isn't feasible in my subroutine)? If the answer to either one of these options is yes, then what is so special about recalling alignments?
    Why this flipping business of the directional vector could ever be considered a desired behavior is beyond me----just like the problem with planes I briefly described in my first post. Speaking of which, could this normal vector flipping in planes also be influenced by where the origin lies?

    For me, it would be very tempting to just test in my subroutine if the origin lies in between the two hits and, if so, to flip the line's directional vector. But the question then arises what will happen if the origin lies on one of the two planes going through the hits. I'm also curious to know what would happen if I create a new line out of the two hits of the first. You definitely gave me enough food for thought, but I'm a bit afraid of missing more problems like these. So I'm not just trying to get around the problems but to really understand their root causes and any additional explanations would be much appreciated. Thanks!!
Children
No Data
© 2024 Hexagon AB and/or its subsidiaries. Privacy Policy | Cloud Services Agreement