Reference Point System (RPS) Alignment
Much like a Best Fit, Reference Point System (RPS) alignments require a nominal and measured set of points for alignment. RPS is unique in that it provides the means to use a set of point reducible features (such as circle centers) as well as surface points to achieve an alignment. It then expects the input points to be used to constrain a single axis or set of axes for the alignment.
Actions
A full list of available functions for adding and removing features from the RPS alignment is listed under the Actions menu. The primary functions are also available through the icon buttons, and include the following:
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Add Feature. This command initiates the addition of a feature to the inspection list. By selecting Point, Projection Plane, Cir- cle, Slot, or Sphere, a built from CAD prompt will be initiated. It also provides access to selecting from Existing Features.
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Delete Feature. Removes the selected feature from the RPS list. This does not delete the feature from the tree.
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Clear All Features. Removes all features from the RPS list of features.
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Run Fit. Performs a fit computation, populating the summary table.
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Apply Alignment. Transforms the Active instrument. If no instrument is marked as Active it will prompt for objects and then instruments to move. This then provides the opportunity to align multiple instruments at once.
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Undo Alignment. Returns the instrument and objects moved to their starting location.
Features List
Each point reducible feature included in the RPS alignment is listed in the Features List as well as check marks for the dX, dY, and dZ components to include in the computation. What makes RPS unique is that it provides the ability to specify individually if a feature will contribute to the alignment in individual XY or Z components of the working frame. So if you specify a surface point and only use the Z axis then the measured point will influence the alignment only along that Z axis and will not influence the alignment along either X or Y. An example alignment then could include a set of 3 surface points to define a Z axis (or a top planar direction) and a set of two holes each constraining X and Y to lock the position of a part in space.
Additional information about each feature can be found by hovering over it, including projection plane assignment and additional tolerance information.
Projection Planes
Projection planes are not required but are recommended for features where the depth within a hole feature is not contained. A circle measured free-hand within the hole will have some degree of mis-orientation which will influence the fit and the resulting nominal comparison. However, projection planes are not part of RPS alignment. Surface points are expected to constrain a planar surface along an axis. When a projection plane is defined all features defined afterward that can take a projection plane as an input with include it.
View Menu
The display of the Summary and Advanced sections of the dialog can be controlled through the view menu. Each or both can be hidden if desired to save graphic space.
Summary Section
Features that are out of tolerance will be identified in the dialog and marked in red in response to the numerically value entered in the tolerance field in the summary section. Relationships with additional tolerances set on them directly and who’s additional tolerances are out of specification will be marked in Blue. When the RPS alignment computation is performed the Max, RMS, and Average will be displayed and reflect the individual component values.
The alignment Status , Tolerance and feature count are included in the second column of the Summary section. The Tolerance field controls the colorization of the relationships within the feature list.
Advanced Section
The Advanced section provides a set of more advanced controls not necessary for most fits. These include the ability to add scale to the fit which in turn will scale the moving instrument to pro- duce the best possible solution. It also provides a set of weight controls which will expose additional fields to adjust weights within the feature list.
Surface Normal Considerations
Building a surface point through the RPS dialog will result in a point to point relationship being built in the tree. This relationship will include the nominal point, the measured point, and the surface normal from the selected surface. Its important to realize, however, that the alignment is still a point to point alignment. If surface points are built from curved surfaces it is very important to measure the actual point very closet to the design location or some cosine error can result due to the change in curvature of the surface.
Performing a Basic RPS Alignment
Much like a best fit operation an RPS alignment is intended to provide a means to align point to their nominal locations. The only difference is that these points are the center locations of features or surface points and they are only used to adjust the fit along a single axis or set of axes. A basic fit to the NRK Demo part may go like this:
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Add an RPS alignment using the RPS option from the Alignment tab
and import the CAD model into your job.
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Build a set of surface points on the top of the CAD model to define the Z axis (these should be built from planar surfaces normal to the Z axis of the working frame but do not need to be co-planar).
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Define a projection plane using the projection plane icon
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Add 2 circles by first selecting the circle button and then click- ing on the inner cylindrical surface of the CAD model to define nominal geometry relationships.
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Double check that the surface point feature only have the dZ component checked and the circles have both the dX and dY components check but not the dZ component.
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Add an instrument, connect and measure these features.
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Press the Run Alignment Icon and check that everything is within the specified tolerance (set within the Summary Section).
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Press the Apply Alignment Button to locate the instrument.