The FARO Laser Tracker measures the three-dimensional position of an optical target with a laser beam and two angular encoders. If the tracker has an optional interferometer (IFM) it emits a Helium Neon (HeNe) laser that is reflected back from the target. ADM Only trackers have a laser diode that produces this red beam of the same wavelength. The tracker senses the position of the return beam and follows it. The three dimensional position of the target is calculated from the distance to the target and the azimuth and zenith angles of the angular encoders. This document describes the specifications, recommended applications, care and troubleshooting for laser tracker targets.
The tracker’s optical target is called a retroreflector. A retroreflector has three adjacent reflective panels that are perpendicular to each other. It is sometimes referred to as a corner cube, since the three panels look like the inside corner of a cube. A retroreflector reflects all light in a parallel path, so the tracker’s beam enters and exits the reflector in parallel, and returns to the tracker regardless of its orientation.
In order for a retroreflector to work with a tracker there are several requirements. Reflectivity, dihedral angle and polarization all have critical specifications. In addition, the retro-reflector must be precision mounted into a ball or probe assembly.
The primary target used with the FARO Laser Tracker is the 1 ½” Spherically Mount Retroreflector or SMR. There are three sizes of SMRs, as well as two Retro-Probes, Repeatability Targets and two sizes of the new Bronze SMR.
The FARO Laser Tracker uses two distance measurement systems with two different laser beams. The HeNe beam is used for tracking and Interferometer (IFM) distance measurement, and an Infrared beam is used for Absolute Distance Measurement (ADM). The coating on the target’s reflective surfaces must meet specification for both the HeNe and infrared laser beam wavelength.
The dihedral angle is the angle between each panel of the retroreflector. It is expressed as a deviation from 90 degrees. A perfect retroreflector would have a dihedral angle of zero. It is also important to evaluate the difference in dihedral angles for adjacent panels. The table below denotes angle and angle difference specifications.
Larger retroreflectors have panels that are matched for polarization. When a retroreflector is tilted away from the beam, mismatched panels can change the contrast that is necessary for the interferometer to count. Smaller retroreflectors do not require matching, since they cannot be tilted to angles where this effect occurs.
The vertex of the retroreflector must be centered in the ball or probe per the target specification. SMRs have a radial centering specification. RetroProbes have a lateral and radial specification.
Recommended Use and Care
To minimize the affect of centering error, the SMR should be held in the same orientation for all measurements. Verifying that the serial number is up is an easy way to improve repeatability and accuracy.
Aiming the SMR directly back at the tracker will minimize the effects of polarization mismatch and beam clipping.
Check the SMR and target nests for debris. The magnetic tooling that is used with the SMR can easily pick up steel, which can stick to the SMRs and tooling causing bad readings.
Keep SMRs and RetroProbes clean. Dust and debris should be blown off the reflective surfaces with pure canned air. If necessary the surfaces can be cleaned with a sterile cotton swab and water vapor. Carefully breathe on the surface and wipe it with the swab. Gently roll the swab as it wipes the surface. Do not re-use the swab. If water vapor cannot remove the mark, denatured alcohol can be used.
NOTE: The coatings on the reflective surfaces are critical for the performance of the target. Cleaning can degrade the reflectivity of the surface, and should only be done when necessary.
Tracker performance depends on the tracker and the target meeting specifications. The following table lists performance issues, and the related critical target specifications. Definitions for the performance issues are listed below.
Performance Issue Target Specification
Dynamic Repeatability Vertex Centering, Dihedral Angle
Backsight Error Dihedral Angle
IFM Dropout Polarization, Reflectivity
Point-to-Point Accuracy Vertex Centering, Dihedral Angle
ADM invalid Reflectivity
Dynamic repeatability is the repeatability of a target that has been removed and replaced.
Backsight error is the difference between a target position measured in frontsight and backsight mode. The reported backsight error value is twice the worst-case single point error at the range where it is measured.
The Interferometer (IFM) measures distance by counting ¼ wave increments of the HeNe laser. If the IFM misses a count, it goes invalid and reports a dropout. When a dropout occurs, the tracker reports this to the user with a blinking green light near the laser aperture, and the application software will not allow a measurement.
Point-to-point accuracy is the calculated distance between two measured points.
An ADM reading can be invalid if the infrared beam return intensity is too high or too low.
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