Iterative Alignment Issues: Part Rotates On Final Iteration

[plopperzz]

Morning everyone, hopefully you are all doing well right now!

I have a part here about which PC-DMIS is giving me a very hard time.

So, in essence the part is three sides of a cube - two parallel faces and a face connecting them. The parallel sides each have a hole, datums B1 and B2, with planes A1, and A2 around each hole respectively and a point for datum C on the perpendicular side connecting the other two. Hopefully you are able to picture the part.

The main problem is that PC-DMIS will run through the first and second iterations just fine, then on the third, if it doesn't throw an "Iterative Alignment Error" it will rotate the part by nearly 90° about the X-Axis.

I set up the co-ordinate system matching the CMMs but centered in a hole on the faces in which datum C lies. What I am trying to do in the alignment is level to A1 and A2, rotate about the midpoint of B1 and B2 to datum C, then origin on the midpoint. Every time that I run this offline it works just fine but it doesn't work the way that I want it to on the machine.

I have tried changing the axis the iterative alignment uses and it doesn't seem to make a difference.

[plopperzz]

Here is the code.

STARTUP =ALIGNMENT/START,RECALL:USE_PART_SETUP,LIST=YES
ALIGNMENT/END
MODE/MANUAL
FORMAT/TEXT,OPTIONS, ,HEADINGS,SYMBOLS, ;NOM,TOL,MEAS,DEV,OUTTOL, ,
LOADPROBE/STAR_90
MOVESPEED/ 100
TOUCHSPEED/ 2
ACCELERATION/MAXACCELX=300, MAXACCELY=300, MAXACCELZ=300
PREHIT/1
RETRACT/1

TIP/T2A0B0, SHANKIJK=-1, 0, 0, ANGLE=0

COMMENT/OPER,NO,FULL SCREEN=NO,AUTO-CONTINUE=NO,
Place Tip T2A0B0 in the center of Datum B2.
-------------------------------------------------

READ_B2 =FEAT/POINT,CARTESIAN
  THEO/<24.685,53.345,39.223>,<0,0,1>
  ACTL/<385.035,-820.151,-517.101>,<0,0,1>
READPOINT/

TIP/T3A0B0, SHANKIJK=1, 0, 0, ANGLE=0

COMMENT/OPER,NO,FULL SCREEN=NO,AUTO-CONTINUE=NO,
Place tip T3A0B0 in the center of Datum B1.
------------------------------------------------

READ_B1 =FEAT/POINT,CARTESIAN
  THEO/<-24.614,53.461,39.11>,<1,0,0>
  ACTL/<334.318,-819.367,-517.202>,<1,0,0>
READPOINT/

LIN1 =FEAT/LINE,CARTESIAN,UNBOUNDED,NO
  THEO/<-24.614,53.461,39.11>,<0.9999946,-0.002353,0.0022921>
  ACTL/<334.318,-819.367,-517.202>,<0.9998786,-0.0154567,0.0019827>
  CONSTR/LINE,BF,3D,READ_B1,READ_B2,,
  OUTLIER_REMOVAL/OFF,3
FILTER/OFF,WAVELENGTH=0

ALN_MAN =ALIGNMENT/START,RECALL:STARTUP,LIST=YES
   ALIGNMENT/ROTATE,XPLUS,TO,LIN1,ABOUT,ZMINUS
   ALIGNMENT/TRANS,XAXIS,LIN1
   ALIGNMENT/TRANS,YAXIS,LIN1
   ALIGNMENT/TRANS,ZAXIS,LIN1
ALIGNMENT/END

CLEARP/ZPLUS,30,ZPLUS,30,OFF
MODE/DCC

[plopperzz]

iter_start =label/

dat_b1 =feat/contact/circle/default,cartesian,in,least_sqr
theo/<-24.65,0,-0.056>,<0.9999974,0.0000031,0.002296>,8
actl/<-24.968,0.342,-0.166>,<0.9999639,0.0001793,-0.0084993>,8.074
targ/<-24.65,0,-0.056>,<0.9999974,0.0000031,0.002296>
start ang=0,end ang=360
angle vec=<0.0022945,0.0341411,-0.9994144>
direction=ccw
show feature parameters=no
show contact parameters=no

a1_1 =feat/contact/vector point/default,cartesian
theo/<-24.991,0,17.943>,<0.9999974,0.0000031,0.002296>
actl/<-24.819,0.036,17.956>,<0.9999629,0.0005643,-0.0085994>
targ/<-24.991,0,17.943>,<0.9999974,0.0000031,0.002296>
snap=no
show feature parameters=yes
surface=theo_thickness,-0.3
measure mode=nominals
rmeas=none,none,none
auto wrist=no
graphical analysis=no
feature locator=no,no,""
show contact parameters=yes
avoidance move=before,distance=5
show hits=no

a1_2 =feat/contact/vector point/default,cartesian
theo/<-24.95,-18,-0.057>,<0.9999974,0.0000031,0.002296>
actl/<-24.992,-17.846,-0.154>,<0.9999629,0.0005643,-0.0085994>
targ/<-24.95,-18,-0.057>,<0.9999974,0.0000031,0.002296>
snap=no
show feature parameters=yes
surface=theo_thickness,-0.3
measure mode=nominals
rmeas=none,none,none
auto wrist=no
graphical analysis=no
feature locator=no,no,""
show contact parameters=yes
avoidance move=no,distance=10
show hits=no

a1_3 =feat/contact/vector point/default,cartesian
theo/<-24.909,0,-18.057>,<0.9999974,0.0000031,0.002296>
actl/<-25.128,0.297,-18.032>,<0.9999629,0.0005643,-0.0085994>
targ/<-24.909,0,-18.057>,<0.9999974,0.0000031,0.002296>
snap=no
show feature parameters=yes
surface=theo_thickness,-0.3
measure mode=nominals
rmeas=none,none,none
auto wrist=no
graphical analysis=no
feature locator=no,no,""
show contact parameters=yes
avoidance move=no,distance=10
show hits=no

a1_4 =feat/contact/vector point/default,cartesian
theo/<-24.95,18,-0.057>,<0.9999974,0.0000031,0.002296>
actl/<-25.01,18.17,0.076>,<0.9999629,0.0005643,-0.0085994>
targ/<-24.95,18,-0.057>,<0.9999974,0.0000031,0.002296>
snap=no
show feature parameters=yes
surface=theo_thickness,-0.3
measure mode=nominals
rmeas=none,none,none
auto wrist=no
graphical analysis=no
feature locator=no,no,""
show contact parameters=yes
avoidance move=no,distance=10
show hits=no

tip/t2a0b0, shankijk=-0.424, -0.906, 0, angle=0

dat_b2 =feat/contact/circle/default,cartesian,in,least_sqr
theo/<24.65,0,0.057>,<-0.9999974,-0.0000031,-0.002296>,8
actl/<24.78,0.021,-0.344>,<-0.9999188,0.0067825,0.0107852>,8.076
targ/<24.65,0,0.057>,<-0.9999974,-0.0000031,-0.002296>
start ang=0,end ang=360
angle vec=<-0.0022945,-0.0341411,0.9994144>
direction=ccw
show feature parameters=no
show contact parameters=no

a2_1 =feat/contact/vector point/default,cartesian
theo/<24.909,0,18.058>,<-0.9999974,-0.0000031,-0.002296>
actl/<24.979,0.069,17.528>,<-0.9999634,-0.00049,0.0085377>
targ/<24.909,0,18.058>,<-0.9999974,-0.0000031,-0.002296>
snap=no
show feature parameters=yes
surface=theo_thickness,-0.3
measure mode=nominals
rmeas=none,none,none
auto wrist=no
graphical analysis=no
feature locator=no,no,""
show contact parameters=yes
avoidance move=before,distance=5
show hits=no

a2_2 =feat/contact/vector point/default,cartesian
theo/<24.95,-18,0.058>,<-0.9999974,-0.0000031,-0.002296>
actl/<24.679,-17.807,-0.587>,<-0.9999634,-0.00049,0.0085377>
targ/<24.95,-18,0.058>,<-0.9999974,-0.0000031,-0.002296>
snap=no
show feature parameters=yes
surface=theo_thickness,-0.3
measure mode=nominals
rmeas=none,none,none
auto wrist=no
graphical analysis=no
feature locator=no,no,""
show contact parameters=yes
avoidance move=no,distance=10
show hits=no

a2_3 =feat/contact/vector point/default,cartesian
theo/<24.991,0,-17.942>,<-0.9999974,-0.0000031,-0.002296>
actl/<24.579,0.33,-18.463>,<-0.9999634,-0.00049,0.0085377>
targ/<24.991,0,-17.942>,<-0.9999974,-0.0000031,-0.002296>
snap=no
show feature parameters=yes
surface=theo_thickness,-0.3
measure mode=nominals
rmeas=none,none,none
auto wrist=no
graphical analysis=no
feature locator=no,no,""
show contact parameters=yes
avoidance move=no,distance=10
show hits=no

a2_4 =feat/contact/vector point/default,cartesian
theo/<24.95,18,0.058>,<-0.9999974,-0.0000031,-0.002296>
actl/<24.94,18.21,-0.355>,<-0.9999634,-0.00049,0.0085377>
targ/<24.95,18,0.058>,<-0.9999974,-0.0000031,-0.002296>
snap=no
show feature parameters=yes
surface=theo_thickness,-0.3
measure mode=nominals
rmeas=none,none,none
auto wrist=no
graphical analysis=no
feature locator=no,no,""
show contact parameters=yes
avoidance move=no,distance=10
show hits=no

pnt_b =feat/point,cartesian,no
theo/<0,0,0>,<0.9210653,-0.0932596,0.3780759>
actl/<-0.094,0.181,-0.255>,<0.9222083,-0.0811819,0.3780759>
constr/point,mid,dat_b1,dat_b2

tip/t1a0b0, shankijk=0, 0, 1, angle=64.911

dat_c =feat/contact/vector point/default,cartesian
theo/<0.086,-65,-37.15>,<-0.002296,0.0013984,0.9999964>
actl/<-0.327,-64.558,-37.867>,<0.0086025,-0.0057263,0.9999466>
targ/<0.086,-65,-37.15>,<-0.002296,0.0013984,0.9999964>
snap=no
show feature parameters=yes
surface=thickness_none,0
measure mode=nominals
rmeas=none,none,none
auto wrist=no
graphical analysis=no
feature locator=no,no,""
show contact parameters=yes
avoidance move=both,distance=10
show hits=no

aln_iter =alignment/start,recall:aln_man,list=yes
alignment/iterate
pnt target rad=0.05,start label=iter_start,fixture tol=0.05,error label=iter_error
meas all feat=always,max iterations=2,level axis=xaxis,rotate axis=zaxis,origin axis=yaxis
level=a1_1,a1_2,a1_3,a1_4,a2_1,a2_2,a2_3,a2_4,
rotate=dat_c,pnt_b,,
origin=pnt_b,,
alignment/end
iter_error =label/

[plopperzz]

The theoreitcal offset is due to a rev change (that wasn't added to the CAD) of a 0.6mm increase in the distance between the faces of A1 and A2.

[NinjaBadger]

For iterative alignment all features in each set (Level, Rotate, Origin) must point in roughly the same direction.

You have points in both XPLUS 1,0,0 and XMINUS -1,0,0 for your Level.

You then use pnt_b in both the rotate and the origin sections, which makes no sense (as far as I can tell).

[Matthew D. Hoedeman]

I've never had to do it for the LEVEL features, but I've used opposite vectors for rotate and origin a LOT, no issue doing so. Imagine offset keys, I want/need the center of the key. Could do it with extra constructed points, but a point on each side of each key, not an issue.

[plopperzz]

So I did just that: created some generic points from the set of points that made up datum plane A2 and simply flipped the vectors direction and now it is working great. Thank you very much for pointing it out!

[NinjaBadger]

I don't know why it doesn't work, I just remember it being a 'rule' of doing iterative alignments.

Have a read of this thread I made which explains how to do an 'iterative alignment' without the iterative command.

It could also be used in a situation like this.

https://www.pcdmisforum.com/forum/pc-dmis-enterprise-metrology-software/pc-dmis-for-cmms/388452-long-hand-iterative-alignment-without-using-iterative-alignment-command