Tool Tip Offset - Northern Digital Polaris Vicra User Manual

3.8

Tool Tip Offset

Do not use a tool with a tip without first performing a pivot procedure to verify the tip offset. Reliance on data
produced by a tool with an inaccurate tip offset may lead to inaccurate conclusions. If your application involves
personal safety, these inaccurate conclusions increase the possibility of personal injury.
Warning!
The origin of a tool is defined as part of the tool's local coordinate system in the tool definition file.
When the Position Sensor tracks a tool, it reports the transformations of the origin of the tool.
In certain circumstances, it is useful to track a point on the tool other than the tool's origin. In
particular, it is useful to track the location of the tip of a probe. It is possible to define the tool's
origin at the tip of the probe; however, if the tool is later bent, the origin will no longer be located at
the tip.
NDI recommends determining the tool tip offset of the tool, prior to each use. The tool tip offset is
the vector between the tip of the tool and the origin of the tool. Application software can apply the
tool transformations reported by the Polaris Vicra System to the tool tip offset, in order to determine
the location of the tool tip.
Note
The Polaris Vicra System always tracks the origin of the tool. It is the application software, not the Polaris Vicra
System, that calculates the location of the tool tip.
Determining the tool tip offset prior to each use ensures that the location of the tool tip is known as
accurately as possible.The tool tip offset can be determined either by using a calibrator, or by
performing a pivoting procedure.
Using a Calibrator
A calibrator is a rigid body that incorporates three or more markers and a clamping mechanism. The
clamping mechanism allows another tool (usually a probe) to be clamped into place. An example of
a calibrator is illustrated in
clamp the probe in place on the calibrator. The origin of the calibrator is defined at the point where
the tool tip will rest. The Polaris Vicra System can then measure the positions of the probe's origin
and the calibrator's origin. The application software compares these measurements to determine the
tool tip offset of the probe.
Polaris Vicra User Guide - Revision 2
Figure 3-7. To use a calibrator to determine the tool tip offset of a probe,
How the Polaris Vicra System Works
35