Choosing Test Positions for Laser Tracker Evaluation and Future Standards Development
Carson Rooms 3 and 4, enter at Carson 3
Bala Muralikrisnan - Representing NIST
A laser tracker task force within the ISO TC 213 committee is currently working on a draft document for an international Standard on laser trackers. One major topic of discussion is the number and type of tests to be included in the Standard. There are at least two excellent documents that serve as starting points for this purpose, the ASME B89.4.19 Standard and the draft VDI/VDE 2617 part 10.
Each of these documents has its own testing philosophy. The B89.4.19 attempts to test each of the three measurement axes (the ranging axis and the two angular axes) by choosing test positions that are sensitive to the different tracker error sources. In this respect, it is similar to the ISO 10360-2 Standard for CMMs where lengths are placed along the principal axes of the CMM and along its diagonals. The draft VDI/VDE 2617 part 10 proposes testing the tracker in a large volume by measuring different lengths placed in different orientations. In recommending these system level tests, that draft document treats the tracker as a black box without attempting to isolate errors in each axis. That draft is also, in a sense, similar to the ISO 10360-2 Standard in that the limiting value of the length measurement error is a single quantity (a constant) or a simple formula (of the form A+BL) and must be complied with for any length over the entire volume.
Both the B89.4.19 and the draft VDI/VDE 2617 have strengths and weaknesses. The objective of this paper is to propose a new set of tests that combine the strengths of these two documents while eliminating the weaknesses and redundancies. This paper describes a set of proposed tests that should be considered for the testing of laser trackers. Our proposal contains a core set of tests that exhibit sensitivity to different geometric/optical misalignments in laser trackers. This core set was determined from a sensitivity analysis performed using mathematical error models for different tracker designs. In addition to the core set, the proposal contains an additional set of tests that may be either user defined or selected from tests described in the B89.4.19 Standard or draft VDI/VDE 2617 part 10. The overall number of proposed tests is 105, to be consistent with the number of tests required in the ISO 10360 series.
