William T. Estler

Large-Scale Metrology – An Update

Abstract Developments in large-scale engineering metrology since the 1978 report of Puttock are reviewed. Advances in optical technology and fast, low-cost computation have led to wide-spread use of laser trackers and digital photogrammetry for general-purpose coordinate metrology. Techniques for high-accuracy straightness measurement, precision leveling, and absolute distance metrology are described, together with approaches to compensation for the effects of atmospheric refraction. Methods for uncertainty evaluation are discussed and several illustrative examples are presented.

The estimation of measurement uncertainty of small circular features measured by coordinate measuring machines

Abstract This paper examines the measurement uncertainty of small circular features as a function of the sampling strategy; i.e., the number and distribution of measurement points. Specifically, we examine measuring a circular feature using a three-point sampling strategy in which the angular distance between the points varies from widely spaced, 120°, to closely grouped, a few degrees. Both theoretical and experimental results show that the measurement uncertainty is a strong function of the sampling strategy. The uncertainty is shown to vary by four orders of magnitude as a function of the angular distribution of the measurement points. A conceptual framework for theoretically estimating the measuring uncertainty is described, and good agreement with experiments is obtained when the measurements are consistent with the assumptions of the theoretical model. This paper is an expansion of a previous internal report 1 with additional material on analog probes and probe lobing models.

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.

Contribution of CIRP to the Development of Metrology and Surface Quality Evaluation during the last fifty years

Abstract An overview of the contribution of the members of the C.I.R.P. community to the progress of Metrology and Surface Roughness Quality Evaluation is given. The following items are included in the part on metrology: brief overview of the existing situation before 1950, contribution to the successive definitions of the unit of length and related reference length standards, traceability, preliminary work to standardization, thermal effects, design and construction of precision machine tools and measuring machines, CMM, large scale metrology. In the field of surface quality evaluation, the following items are reviewed: reference profiles and related definitions of parameters, filtering, surface and subsurface integrity, functional meaning of parameters, instrumentation, scanning probe microscopy, 3D surface evaluation. A comprehensive list of references is provided.

Displacement Uncertainty in Interferometric Radius Measurements

Abstract Interferometric radius measurements may be completed using a radius bench, where radius is defined as the displacement between the confocal and cats eye nulls (identified using a figure measuring interferometer). Measurements of a Zerodur sphere have been completed on the X-ray Optics Calibration interferometer (XCALIBIR) and a coordinate measuring machine. Larger recorded disagreements than indicated by the current uncertainty analysis call for an exploration of the analysis model. This paper details uncertainties associated with the use of multiple displacement measuring interferometers (DMIs) to record motion in a single axis by treating the specific case of displacement measurement on XCALIBIR using three DMIs equally spaced around the optical axis.

An Advanced Angle Metrology System

We describe the new Advanced Automated Master Angle Calibration System (AAMACS) at the U. S. National Institute of Standards and Technology. The heart of this system is a set of three stacked concentric indexing tables with 832, 729, and 625 teeth respectively. The system is fully error corrected and achieves a full-circle positioning accuracy of ± 0.02 arc-seconds with an angular positioning resolution of 0.0034 arc-seconds. System accuracy is limited by autocollimator noise. We will describe the error mapping process and the use of the system for high-accuracy autocollimator calibrations.

A Careful Consideration of the Calibration Concept

This paper presents a detailed discussion of the technical aspects of the calibration process with emphasis on the definition of the measurand, the conditions under which the calibration results are valid, and the subsequent use of the calibration results in measurement uncertainty statements. The concepts of measurement uncertainty, error, systematic error, and reproducibility are also addressed as they pertain to the calibration process.

Calculation of Measurement Uncertainty Using Prior Information

We describe the use of Bayesian inference to include prior information about the value of the measurand in the calculation of measurement uncertainty. Typical examples show this can, in effect, reduce the expanded uncertainty by up to 85 %. The application of the Bayesian approach to proving workpiece conformance to specification (as given by international standard ISO 14253-1) is presented and a procedure for increasing the conformance zone by modifying the expanded uncertainty guard bands is discussed.

Angle Metrology of Dispersion Prisms

Abstract We present a new technique for calibrating the apex angles of dispersion prisms, which are used in the measurement of the index of refraction of optical glasses. The new method requires only a phase measuring interferometer, together with an electronic autocollimator, and eliminates the need for a rotary or indexing table. The apex angles of a nominally equilateral prism of fused silica were measured and the results compare favorably with a traditional calibration using an indexing table comparator.

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.