Laser-based Trackers

1. Laser-based Trackers Jayhyun Kwon

2. Purpose • The laser tracker measures coordinates by tracking a laser beam to a retroreflective target held in contact with the object of interest. • A laser tracker can measure object features up close and as far away as 35 meters, with an accuracy of 25 microns (0.001 inch) at 5 meters.

3. Products for measure 3D coordinates • Laser trackers: High accuracy, high speed, broad range • Theodolites: Good accuracy, low speed, broad range • Total station system: Good accuracy, low speed, broad range

4. How laser trackers works? • Laser tracker measures two angles and a distance • The tracker sends a laser beam to a retroreflective target held against the object to be measured • Light reflected off the target retraces its path, re-entering the tracker at the same position it left • As light re-enters the tracker, some of it goes to an interferometer that measures the distance from the tracker to the retroreflective target

5. How laser trackers works?

6. Accuracy • Laser tracker can measure objects that range in size from one to hundreds of meters with an accuracy of 25 microns (0.001 inches) at 5 meters. • The tracker follows the mirrored target over features, updating the position at a rate of 1,000 times per second • The trackers provide a radial interferometer resolution of 0.16 microns (.6 micro inches). The target speed, regardless of resolution, is 6 meters per second (20 feet per second). Encoder angles are resolved to .25 arc seconds.

7. Incremental distance measurement 1. The laser light splits into two beams. 2. One travels directly into the interferometer. 3. The other beam reflects off the retroreflective target and return. 4. Electronic circuitry counts the cyclic changes to determine the distance traveled. Distance measurement

8. Absolute distance measurement (ADM) 1. Infrared light reflects off the SMR and re-enters the tracker, where it’s converted into an electrical signal. 2. Electronic circuitry analyzes the signal to determine its time of flight, multiplying this value by the speed of light in air to determine the distance from the tracker to the retroreflective target Distance measurement

9. Application • Reverse engineering • Inspection • Dynamic measurement

10. Application • Alignment: process of bringing objects into proper orientation

11. Application

12. Position-sensitive Detectors • Quadrant photodiodes • Silicon position sensors • Image-dissector tubes • Vidicons

13. Quadrant photodiodes • Each segment has a separate connector for signal output • Position information is derived from the relative signal output from each segment

14. Silicon position sensor • A silicon position sensor consists of a segment of photo detective silicon with either two or four terminals for signal output and a terminal for application of back-bias voltage

15. Image-dissecting Photomultipliers

16. Image-dissecting Photomultipliers • Search mode: When the location of the image on the photocathode is not known, it is necessary to systematically search the face of the tube until the image has been located • Track mode: Only the small area of the photocathode immediately surrounding the image location has to be searched.

17. Requirements • The environment needs to stable enough to achieve the required accuracy for the job • The tracker requires only a stable 110 v.a.c. power source to operate the tracker and it's corresponding P.C • The standard mounting stand for the tracker takes a 3' square area, with a minimum height of 72". The P.C. can be linked remotely by cable to the tracker Control Unit