Table of Contents
Advertisement
8
Calibration and Firmware
8.1
Checking the Calibration of the Polaris Vicra System
The Position Sensor is calibrated at NDI, using the methodology described in
time, it is possible for the Position Sensor to lose calibration. A periodic calibration check should be
performed on the Position Sensor. The frequency of the calibration check depends on the specific
application and environment in which the Position Sensor is used.
If the Position Sensor begins to lose calibration, it may lose the ability to track some tools before
others. This is due to the various constraints used by the Polaris Vicra System, which make certain
tool designs more sensitive to loss of calibration than others. For example, consider a tool that has
several similar segment lengths or similar angles between segments, or has segment lengths similar
to those of another tool. An out-of-calibration Position Sensor may not be able to determine which
markers belong to which tool, because the segment lengths will be measured less accurately. In this
case, the system will report the tools as missing. (See
page 28
and the "Polaris Tool Design Guide" for details on segment lengths and angles.)
NDI's Accuracy Assessment Kit can be used as an aid in determining whether a Position Sensor is
performing acceptably for the user's application. For all calibration procedures, return the Position
Sensor to NDI. This practice ensures that all calibrations are conducted in accordance with
procedures established specifically for the Polaris Vicra Position Sensor. See
page 77
for instructions on returning equipment to NDI.
Line Separation and Calibration
The Position Sensor line separation (described on page 28) can sometimes be an indication that a
Position Sensor is out of calibration, but is not necessarily the best indicator and cannot always
detect calibration loss. For example, suppose one of the sensors was to shift towards the centre of the
Position Sensor while maintaining its horizontal alignment. In this case, the Position Sensor would
be out of calibration without any change in line separation, and the reported position of the tool
would be shifted.
If the Position Sensor is measuring high line separation values, it may be an indication of loss of
calibration. Line separation alone should not, however, be used as a test for calibration loss.
Bump Sensor
The bump sensor detects when the Position Sensor has been bumped above a shock limit broadly
equivalent to a 200 mm to 254 mm drop onto a vinyl tiled concrete surface. A bump can cause the
system to lose calibration; however, it is not the only cause of calibration loss. Checking the bump
sensor status should be part of your regular calibration check.
You can use the GET command to check the bump sensor status. The bump status is indicated in bit
0x00200000 of the "Info.Status.Alerts" parameter. You can also check the bump sensor status using
NDI ToolBox.
Because a bump can affect the calibration of the Position Sensor, the system will not report
transformations if the bump sensor has detected a bump, unless:
•
the bump sensor has been cleared, or
•
reply option 0x0800 is used with the TX or BX command.
Polaris Vicra User Guide - Revision 2
Calibration and Firmware
Appendix
"Marker Detection and Tool Tracking" on
"Return Procedure" on
A. Over
57
Advertisement
Table of Contents
