Christopher J. Blackburn

ASME B89.4.19 Performance Evaluation Tests and Geometric Misalignments in Laser Trackers.

Small and unintended offsets, tilts, and eccentricity of the mechanical and optical components in laser trackers introduce systematic errors in the measured spherical coordinates (angles and range readings) and possibly in the calculated lengths of reference artifacts. It is desirable that the tests described in the ASME B89.4.19 Standard [1] be sensitive to these geometric misalignments so that any resulting systematic errors are identified during performance evaluation. In this paper, we present some analysis, using error models and numerical simulation, of the sensitivity of the length measurement system tests and two-face system tests in the B89.4.19 Standard to misalignments in laser trackers. We highlight key attributes of the testing strategy adopted in the Standard and propose new length measurement system tests that demonstrate improved sensitivity to some misalignments. Experimental results with a tracker that is not properly error corrected for the effects of the misalignments validate claims regarding the proposed new length tests.

Measuring Scale Errors in a Laser Tracker's Horizontal Angle Encoder Through Simple Length Measurement and Two-Face System Tests.

We describe a method to estimate the scale errors in the horizontal angle encoder of a laser tracker in this paper. The method does not require expensive instrumentation such as a rotary stage or even a calibrated artifact. An uncalibrated but stable length is realized between two targets mounted on stands that are at tracker height. The tracker measures the distance between these two targets from different azimuthal positions (say, in intervals of 20° over 360°). Each target is measured in both front face and back face. Low order harmonic scale errors can be estimated from this data and may then be used to correct the encoder’s error map to improve the tracker’s angle measurement accuracy. We have demonstrated this for the second order harmonic in this paper. It is important to compensate for even order harmonics as their influence cannot be removed by averaging front face and back face measurements whereas odd orders can be removed by averaging. We tested six trackers from three different manufacturers. Two of those trackers are newer models introduced at the time of writing of this paper. For older trackers from two manufacturers, the length errors in a 7.75 m horizontal length placed 7 m away from a tracker were of the order of ± 65 μm before correcting the error map. They reduced to less than ± 25 μm after correcting the error map for second order scale errors. Newer trackers from the same manufacturers did not show this error. An older tracker from a third manufacturer also did not show this error.

Assessing ranging errors as a function of azimuth in laser trackers and tracers

Tilt and radial error motion of a laser tracker head as it spins about the two rotation axes result in small but measurable ranging and angle errors. The laser tracer, on the other hand, measures range with respect to the center of a high quality stationary sphere. It is therefore not expected to be influenced by the radial error motions of the carriage that carries the optics and the source, but the form error of the reference sphere and possibly the eccentricity in its placement with respect to the circular path traced by the carriage will be contributors to the ranging errors. In this paper, we describe experiments to assess the magnitude of these ranging errors as a function of the azimuth angle in different laser trackers and a laser tracer.

Performance evaluation of laser trackers

The American Society for Mechanical Engineers (ASME) recently released the ASME B89.4.19 Standard [1] on performance evaluation of spherical coordinate instruments such as laser trackers. At the National Institute of Standards and Technology (NIST), we can perform the complete set of tests described in the Standard, and have done so for a variety of laser trackers. We outline the tests described in the Standard, discuss our capabilities at the large-scale coordinate metrology group, and present results from B89.4.19 tests conducted on a few trackers. We also outline an analysis approach that may be used to evaluate the sensitivity of any measurement, including the tests described in the B89.4.19 Standard, to different geometric misalignments in trackers. We discuss how this approach may be useful in determining optimal placement of reference lengths to be most sensitive to different geometric misalignments.

In-situ Temperature Calibration Capability for Dimensional Metrology

Abstract: The Dimensional Metrology Group (DMG) at the National Institute of Standards and Technology (NIST) has developed a new in-situ temperature calibration system. This paper discusses the system components, an in-situ calibration procedure, and the uncertainty sources involved in the calibration process. It also presents an uncertainty budget, and examines it with a Monte Carlo simulation. This system enables the DMG to perform quicker in-situ temperature calibration, at frequent intervals with minimal downtime, and provides lower uncertainties for the dimensional measurements.

High Accuracy Measurements Using a Scanning System with a Single Point Triangulation Sensor

Abstract: The capabilities of non-contact laser spot triangulation sensors for high accuracy measurements have slowly increased over the past decade, and now have usable resolution below 0.1 μm. The Dimensional Metrology Group at the National Institute of Standards and Technology (NIST) has developed a simple scanning system to work with these sensors, and presents the system details and data from a variety of geometries. The features measured include screw threads, fuel cell channels, and computer tomographic (CT) scanner phantoms.