Iterative Alignment Rules

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Iterative Alignment Rules

There are some general rules when doing an iterative alignment  :

PC-DMIS needs both the measured values and theoretical values  for each of the elements in the sets. The normal  vectors for the first set of elements must be approximately parallel. The one  exception to this rule is if only three features are being used in the set.

If measured points (VECTOR, EDGE, or SURFACE) are being used,  then all three sets of elements are needed (three features to Level, two  features to Rotate, and one feature to Set Origin) to define the alignment. Any  feature type can be used, but three-dimensional elements are better defined  elements and therefore improve the accuracy. Some of the possible 3D elements  are sheet metal circle, slot, cylinder, sphere, or a corner point.

  Note: The  sheet metal circle, slot, and cylinder need at least three sample  hits.


The difficulty of using measured points lies in not knowing  where to take the measurement until after the alignment. This poses a problem as  the points must be measured before the alignment. Three dimensional elements, by  definition for this use, are elements that can be measured precisely the first  time.

Further, if measured points (VECTORS, EDGE, or SURFACE) are  being used, the normal vectors of the features in the ROTATE set must have  normal vectors that are approximately perpendicular to the vectors of the  features in the LEVEL set. The features in the ORIGIN set must have a normal  vector that is approximately perpendicular to both the vectors from the LEVEL  set and the ROTATE set.

If measured points (VECTORS, EDGE or SURFACE) are being used  as part of the set, PC-DMIS may ask that they be re-measured if they were taken  too far from the nominal location. PC-DMIS first "best fits" the measured data  to the nominal data. Next, PC-DMIS checks to see how far off each measured point  is. If the distance is greater than the amount specified in the Point Target Radius box, PC-DMIS will request that the  point be re-measured. PC-DMIS in effect puts a cylindrical tolerance zone around  the theoretical location of each vector, surface, or edge point. The radius of  this tolerance zone is the point tolerance specified in the dialog. PC-DMIS will  continue to re-measure point features until all the measured points fall into  'tolerance'. The tolerance zone only affects measured points.

A special capability of PC-DMIS allows the center point of a  slot to slide up or down the axis as needed. For this reason, an iterative  alignment cannot converge when a slot is used as part of the ORIGIN set. It is  possible to use a slot as part of the ORIGIN set by first constructing a point  from the slot and then using that constructed point in the ORIGIN set.

  A slot is not recommended as part of the ORIGIN set of an  iterative alignment.


     Type of Feature used:

  Minimum # of Features needed:

   Circle

  3 circles:

  This method uses 3 DCC circles for the  alignment.

   Line

  This feature type is not recommended.

   Point

  6 points:

  The points are used as a 3-2-1 alignment.

   Slot

  This feature type is not recommended as part of the ORIGIN  set.

   Sphere

  3 spheres:

  This method uses 3 set spheres for the  alignment.