The Laser Tracker is considered the most accurate large scale metrology device available for industrial measurements. These devices were developed in the early 1990’s and are based on using angle encoders and a laser interferometer to record angle and distance measurements. Using Horizontal and Vertical angle readings combined with the highly accurate distance measurement of the interferometer 3D coordinates can be quickly calculated.
The Laser Tracker derives its name from the Laser interferometer and from its ability to “track” an optical Cornercube (often called a Spherical Mounted Reflector – SMR). In order to “track” the Cornercube (SMR) the laser interferometer must be “locked on” to the Cornercube and a known distance established. All Laser Trackers utilize a Tracker mounted reset location (often called a birdbath due to its shape) where the Cornercube is placed and the commanded to turn to the known Horizontal and Vertical angles of this index, and then the Interferometer distance is set based on previous calibration. In recent years technology has advanced to what is commonly referred to as an “Absolute Distance Meter” or ADM these ADM’s are calibrated to the Interferometer and allow the operator to “catch the beam” and set the distance of the Interferometer WITHOUT the use of a “birdbath”. In 2011, trackers are sold almost exclusively with an ADM of some variety.
As you have probably already realized there are many mechanical and electrical components in use in a Laser Tracker so understanding if the Laser Tracker is measuring accurately is of the utmost importance when taking precision measurements.
Having spent the last 17 years operating Laser Trackers of all varieties the mis-conceptions that follow these devices and how to determine if they are accurate have been far and wide. When Laser Trackers were first introduced we viewed them as a faster more accurate form of the Computer Aided Theodolite System, these systems were completely geometry based so this view of the new technology was not accurate. Since Theodolites only measured angles this was the only instrument parameter that was ever of any concern, this could easily be checked using the “Plunge and Reverse” method of measurement to evaluate the angle encoders. With the Laser Trackers this view of the world has continued in many organizations world-wide with the phrase “all you have to do is check a backsight (akaTwo Face) to determine if the Tracker is accurate.”Angles and Distances
Unlike the Theodolite the Laser Tracker measures a distance via the Laser Interferometer, the Laser Interferometer is the most accurate distance measurement device available to date. The Laser interferometer can also contribute the largest amount of measurement error if it is not functioning correctly or out of calibration. The angle encoders of the Laser Tracker while extremely accurate do not affect the Laser Tracker measurements in the same magnitude of the Laser Interferometer.
For many years the Laser Tracker community has depended too much on evaluating if a Laser Tracker’s angle measurements are accurate, while not evaluating the entire Laser Tracker system, simply as I view it, that it is “too complicated” to evaluate the entire system; I disagree.
Having had many “opportunities” to trouble shoot Laser Tracker measurement errors the one thing that I learned was just because the tracker measures accurately in that “one area” does not mean it is accurate throughout the Laser Trackers entire measurement envelope. I have seen many Trackers look good for all their manufacturers Field Check specifications but not repeat at all during large surveys and in particular large multiple station surveys. The following is a simple method I like to use in a shop environment to quickly and easily determine how well the Laser Tracker is measuring; of course good metrology skills must be used; temperature, air pressure are accurate, instrument and points are very stable etc.Simple Tracker Accuracy Test
Typically, a tracker is checked for accuracy using the Backsight/ Two Face Test and while this verifies the angular accuracy of the tracker, it does not address the distance accuracy of the tracker. An Interferometer distance check confirms the “birdbath” distance, but does not show distance repeatability throughout the entire “measurement envelope”.
The accuracy of Tracker measurements may be verified by measuring point-to-point distances throughout the measurement envelope. A properly calibrated tracker will repeat all distance measurements within +/-0.0020” .
The method uses two points that will not move during the measurement process, these two points will be measured in five different orientations of the tracker. The first measurements should be done inline at approximately the same height as the tracker, and repeated twice, check the point-to-point distance on these points and average them, this will be your baseline distance. All further points will be compared to this baseline, with the total range being the difference between the longest and shortest point-to-point distance.
The tracker will then be moved to the side and an equilateral triangle formed between the two points and the tracker. Measure the points again as different point names, rotate the tracker 90 degrees, measure the two points again, do this until the points have been measured in all quadrants.
Check, the distances between all these measurements, total range should be within 0.0020”, if not, a Calibration should be done to correct the distance measurement errors.
Using this method should help you quickly identify the accuracy of the Laser Tracker in your Shop environment. If you can perform these tests within a controlled environment, with a recently calibrated Laser Tracker your results should be less than 0.0015″.
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