The Best-Fit Transformation Dialog
In this dialog you can review the fit results, apply tolerances, restrict degrees of freedom, edit point constraints, and more. Any time one or more changes is made, the current fit is no longer valid (indicated by a dark blue background on the fit button and the fit results) and needs to be re-solved.
The dialog is divided into three main parts: the controls (top left corner), the fit results (top right corner), and the point list (bottom). The top left corner of the best fit dialog contains the fit controls.
Degrees of Freedom
The top left corner of the best fit dialog contains the fit controls.
The degrees of freedom checkboxes in this section of the best fit dialog allow you to specify the transformation that may be permitted by restricting degrees of freedom relative to the working frame. You can lock any combination of X/Y/Z translation, X/Y/Z rotation, and scale. Any degree of freedom that is checked is unlocked—unchecking a degree of freedom locks it. Keep in mind that these degree of freedom restrictions are applied with respect to the working coordinate frame, therefore rotations are applied about the working frame origin.
As an example, suppose you have a bolt pattern measurement on a flange and you’d like to fit this measured pattern to a mating, nominal pattern. In this case, you would likely align the face of the measured points to the nominal flange face using perhaps a relationship fit, then you might wish to use the nominal bolt pattern to spin the measured flange about its axis until the bolt holes are aligned.
By locking all degrees of freedom except for rotation about the Z axis—and by placing the working frame at the center of the flange— the fit would simply clock the measured pattern to the nominals.
Notice that you can also set the instrument to a specified scale value for the fit, which allows you to apply temperature compensation to the measurements.
Fit Controls
Below the degrees of freedom controls are the tolerance coloring zones. You can apply tolerance coloring to the deviations in the point list by setting the tolerances in this section.
Below the tolerance coloring zones are a series of buttons controlling the fit.
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Re-fit button. Recalculates the best-fit transformation based on the current settings. A blue background on this button indicates that the current fit is not valid based on the current settings and a re-fit is needed.
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Edit Selected Items. Brings up the Best-Fit Item dialog for any selected items. More than one point can be selected at one time in the point list. This button allows you to change all of their properties at once. Alternatively you can double click on a point row directly within the point list.
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Quick Tips. Displays a series of tips related to the dialog.
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Advanced Settings. Displays the Advanced Best-Fit Options dialog, discussed below.
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Advanced View. Toggles to a simplified version of the interface, hiding less frequently-used parameters. This option reduces the best-fit dialog to a display as shown below:
Advanced Settings
The Advanced Best-Fit Options dialog provides some advanced, less commonly-used settings for best fit transformations.
■ Advanced Settings. Displays the Advanced Best-Fit Options dialog, discussed below.
Descriptions of these parameters follow:
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Reset transform and scale before each re-fit. The transformation is solved from the same initial starting position and orientation each time the fit is performed. This is the default and is applicable to most fitting operations.
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Apply transformations successively. Each fit operation solves using the previous calculated transformation as a starting point. This allows you to apply iterative best-fit operations with different degree of freedom settings for each. For instance, you might wish to first solve the best-fit with some degrees of freedom locked, then solve again from that position with a different set of locked degrees of freedom.
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Reset Transformation and Scale Now. Resets the current transformation back to the “starting point”.
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Store weight settings with points. Fit weights can be stored with nominal or actual points such that if the points are used in another best-fit operation, the weights are recalled.
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Make a new group when fit is finished. Makes a copy of the reference (non-moving) points and puts them into a separate group with the specified name at the completion of the operation. If nominal coordinates were changed in the Best Fit Item dialog, this group will use those updated coordinates. This is not a common need.
Reporting Options
Export to CSV. The Export to CSV button will export the fit results to a comma-separated .CSV file and import it into Excel (if installed).
Auto Vectors. The Auto Vector check box offers an option to directly construct a vector group to display the deviation between the included points.
Automatic Relationship Construction. By default a group to group relationship will be built as part of a best fit operation. This provides a means to both monitor the best fit status and repeat the alignment as needed. This relationship construction process can be turned on and off through the User Options on the Automation Tab.
Fit Weights
Double-clicking a point in the list will bring up the Best-Fit Item dialog.
This dialog provides another way to remove the point from the fit (Use in Fit), and conveniently allows you to change the reference point’s coordinates, as well as the per-component fit weight for the point. The 1 and 0 buttons are simply shortcuts for set- ting the corresponding fit weight value.
Fit weights give you control over defining whether a certain component of a point has any influence in the fit process. A value of zero indicates that the component has no influence in the fit, whereas a value of 1 indicates the component has full influence. Any intermediate value may be chosen. If a component is weighted to zero, it is displayed in gray in the point list. If an intermediate value is chosen, it is displayed in blue, and if the weight is set to 1, it is displayed in black.
One reason for reducing the weight of a specific component of a point is to reduce directional errors. Suppose you have a measured point that was performed on a floor that was flexing in the z-direction during measurement. It is reasonable to assume that the measured point has significant error in the z direction when compared to the x or y direction, therefore reducing the weight of the z component may be a wise choice. Another reason is when using discrete points to control specific degrees of freedom, an advanced technique that, while do- able using best-fit, is much easier to perform using relationship fits.
You’ll notice that you have a significant amount of control in the fit. Access to changing the fit parameters is provided via the controls in the upper-left hand corner of the dialog.