Frontsights and Backsights Measurements Information for the Laser Tracker

Background

The FARO^® Laser Tracker measures the position of a Spherically Mounted Retro-Reflector (SMR) by locking on with a helium neon laser. The tracker reads the azimuth and zenith angles and the distance to the target. The readings are compensated through the use of a kinematic model. The model has parameters for the laser’s four degrees of freedom (two rotational and two translational), and two parameters for the gimbal (axis offset and axis non-squareness).

Tracker accuracy can be verified through the use of backsight checks. These checks compare a point reading taken in frontsight mode with one taken in backsight mode. The resulting deviation reports twice the worst-case error for a point measured at the range and position of the backsight reading. The error in a backsight reading is effectively error that was not compensated for by the kinematic model.

Although the kinematic model is highly effective in minimizing tracker measurement error, there are still many factors that are not accounted for by the model. Target quality, beam retrace, servo tuning, position sensing detector accuracy, encoder accuracy, atmospheric induced errors, thermal expansion and hysteresis are some of the errors that are not addressed by the model. Most of these errors are taken into account in formulating the specification for the tracker.

Frontsight/Backsight Measurement

In an effort to increase the accuracy of the laser tracker beyond its specifications, some users have implemented the use of frontsight/backsight averaging (also referred to as two-faced measurements). This method requires that all points be measured in frontsight and in backsight mode. Features (planes, lines, circle, etc.) cannot be measured in this way, so this strategy is limited to point-only measurement jobs.

In theory, the frontsight/backsight measurement eliminates the systematic error in the laser, which includes four of the kinematic model parameters. The problem with this theory is that it becomes most effective when the tracker is out of compensation. With a well-compensated tracker, the parameters are already optimized, so averaging the frontsight/backsight readings loses its effectiveness.

Pointing CompIT is designed to compensate for the two rotational and two translational parameters. The axis-offset and axis non-squareness are factory set. With these parameters set, the remaining error that is seen in a backsight check can be due to many other factors outside of the kinematic model. Using the frontsight/backsight averaging method adds an averaging component to data collection, but it does not improve accuracy on a compensated tracker.

Measuring with frontsight and backsight on all points can add heat to the tracker, as it continuously spins around for each point. This added heat can add drift to a measurement job, and reduce accuracy. The method also doubles the time of measurement, which also can add to drift.

Recommendation

FARO recommends frontsight only measurement with a well-compensated tracker to obtain optimal measurement accuracy. For the most critical measurement tasks, Pointing CompIT can be run even if the tracker passes the interim test, to make the tracker as accurate as possible. Pointing CompIT consists of measuring six points and takes less than five minutes.

Click here to download: Frontsight-Backsight Theory

 

Keywords: Front sight Back sight frontsight backsight accuracy