Marker Detection And Tool Tracking - Northern Digital Polaris Vicra User Manual

How the Polaris Vicra System Works
transformations of these tools using reply option 0x0800 with the BX or TX command. See the
"Polaris Application Program Interface Guide" for details.
A tool is flagged as partially out of volume if:
• less than the minimum number of markers (a parameter in the tool definition file) are inside
the characterized measurement volume, and
• at least one marker on the tool is inside the characterized measurement volume
For example, consider a five-marker tool, with three markers inside the characterized measurement
volume and two markers outside the volume. If the minimum number of markers is set to 3, the tool
is considered to be inside the volume. If the minimum number of markers is set to 4 or 5, the tool
will be flagged as partially out of volume.
3.4

Marker Detection and Tool Tracking

Detecting Markers
The Position Sensor detects active and passive markers using different methods. Active markers are
triggered by a 20 Hz "chirp" signal emitted by the Position Sensor illuminators, which causes the
active markers to emit IR for the integration time. (The chirp signal is only a trigger.)
To detect passive markers, the Position Sensor's illuminators flood the surrounding area with IR for
the whole integration time by flashing at 20 Hz (similar to the flash on a camera). The passive
sphere markers have a retro-reflective coating that reflects the IR directly back to the Position
Sensor instead of scattering it.
For both active and passive markers, the Position Sensor collects IR for a period of time called the
integration time. This acts like an electronic shutter. The system makes automatic adjustments to
the integration time so that the intensity of the brightest IR detected is set to a maximum value, and
the intensity of all other IR detected falls below this value. This process is called dynamic range
control.
The system distinguishes between potential marker data and background IR using a value called the
trigger level. The trigger level is the minimum IR intensity considered to be valid marker data.
Background IR that falls below the trigger level is rejected by the Position Sensor. The trigger level
generally increases with integration time; see
more details.
Acquiring and Tracking Tools
When the Polaris Vicra System first begins tracking a tool, or whenever a tool goes missing, it must
"acquire" the tool. (For information on why a tool goes missing, see
Transformations" on page
positions of all the visible markers.
IR light hits the Charge Coupled Devices (CCDs) in the Position Sensor. If the system is unable to
detect individual IR sources, or has detected more IR sources than it can process, it will report an
error, see "Tracking Errors and Flags" on page
of the IR sources.
28
"Setting the Infrared Light Sensitivity" on page 54
48.) To acquire tools, the Polaris Vicra System first measures the
49. Otherwise, the system will calculate the position
"Missing and Disabled
Polaris Vicra User Guide - Revision 2
for