Laser Tracker Testing

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Laser trackers have become popular measuring tools for users needing to make precision measurements of items that are simply too large to be handled by Coordinate Measuring Machines (CMMs). Fundamentally, a laser tracker is a polar coordinate measuring system – it makes measurements in terms of two angles (horizontal and vertical) and a distance (radius).

As with any metrology tool, a laser tracker needs to be calibrated and its performance verifi ed. NPL is setting up a service for the calibration and verifi cation of laser trackers. This will include:• laser wavelength calibration• calibration/check of air sensors (pressure, temperature)• performance verifi cation to ASME B89.4.19 - 2006 or similar standards.

As well as the tests specifi ed in ASME B89, NPL also plans to conduct additional testing as suggested in a recent NIST publication [1] in order to fully test the vertical encoder eccentricities and horizontal and vertical encoder scale errors. In addition, NPL will also be performing Network Testing [2] which will be used to derive values for the residual tracker errors.

Laser tracker geometric alignment errors

Laser trackers are subject to alignment errors due to manufacturing tolerances and design constraints - the number and confi guration of the errors is dependant on the mechanical design. In a steered-mirror design, there can be up to nine alignment errors, as shown above, in addition to birdbath distance, and encoder scale errors. Incomplete compensation for these errors can lead to loss of measurement accuracy when operating the tracker. [3]

[1] J. Res. Natl. Inst. Stand. Technol. 114 21-35 (2009)[2] S. Sandwith, LVMC 2007[3] R. Loser & S. Kyle, Boeing Large Scale Optical Metrology Seminar, 1999

Testing laser trackers to B89

Specifi cation standard ASME B89.4.19-2006 Performance Evaluation of Laser-Based Spherical Coordinate Measurement Systems, prescribes a series of tests for evaluating the performance of laser trackers:

Range test

1D length up to 72 % of range – tests the interferometer and ADM accuracy.

Volumetric

System tests

Single length artefact measured at different distances/orientations (35 tests)

• Horizontal

• Vertical

• Right/Left diagonals

Two-face tests

Front and back face measurements using length artefacts (36 tests).

Additional tests

(NIST recommended)

Recent work by NIST has shown that the standard B89 tests are not sensitive to vertical encoder eccentricity in z, or encoder scale errors (vertical and horizontal). By repeating some of the B89 tests with the artefact placed asymmetrically, NPL will be able to detect these remaining error components.

Limitation

Overall, the B89 tests compare the performance of the laser tracker with the MPE specifi ed by the manufacturer. The result is a ‘Pass’ or ‘Fail’. No further information is reported as to the uncertainties of the instrument or the residual errors in the error mapping.

Testing laser trackers using a bundled networkAn alternative to the B89 tests is to measure a series of fi xed target locations using the laser tracker, with the tracker positioned sequentially at a number of stations. After measuring the targets from all stations, the resulting network of data is bundle adjusted,

effectively determining the relative tracker locations by constraining the set of measured target locations. If suffi cient tracker stations are used for the bundle, the system contains redundant data, which may then be used to assess tracker measurement uncertainties. By careful design of the target network locations, the network of data can be interrogated to solve for parameters of the tracker error model. In this way, the residual error parameters such as offset and tilt errors can be determined in addition to the angle and distance measurement uncertainties of the tracker.

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Transit axis offset, e

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Mirror offset, f

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Beam offset, Ox & Oy

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Mirror tilt, c

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Transit axis tilt, i

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Beam axis tilt, lx & ly

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Horizontal encoder eccentricity, Ex & Ey

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Vertical encoder eccentricity, Kx & Ky

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Vertical index offset, j

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