Ben Hughes

Laser tracker error determination using a network measurement

We report on a fast, easily implemented method to determine all the geometrical alignment errors of a laser tracker, to high precision. The technique requires no specialist equipment and can be performed in less than an hour. The technique is based on the determination of parameters of a geometric model of the laser tracker, using measurements of a set of fixed target locations, from multiple locations of the tracker. After fitting of the model parameters to the observed data, the model can be used to perform error correction of the raw laser tracker data or to derive correction parameters in the format of the tracker manufacturers internal error map. In addition to determination of the model parameters, the method also determines the uncertainties and correlations associated with the parameters. We have tested the technique on a commercial laser tracker in the following way. We disabled the trackers internal error compensation, and used a five-position, fifteen-target network to estimate all the geometric errors of the instrument. Using the error map generated from this network test, the tracker was able to pass a full performance validation test, conducted according to a recognized specification standard (ASME B89.4.19-2006). We conclude that the error correction determined from the network test is as effective as the manufacturers own error correction methodologies.

Multi-channel absolute distance measurement system with sub ppm-accuracy and 20 m range using frequency scanning interferometry and gas absorption cells.

We present an implementation of an absolute distance measurement system which uses frequency scanning interferometry (FSI). The technique, referred to as dynamic FSI, uses two frequency scanning lasers, a gas absorption cell and a reference interferometer to determine the unknown optical path length difference (OPD) of one or many measurement interferometers. The gas absorption cell is the length reference for the measurement system and is traceable to international standards through knowledge of the frequencies of its absorption features. The OPD of the measurement interferometers can vary during the measurement and the variation is measured at the sampling rate of the system (2.77 MHz in the system described here). The system is shown to measure distances from 0.2 m to 20 m with a combined relative uncertainty of 0.41 × 10⁻⁶ at the two sigma level (k = 2). It will be shown that within a scan the change in OPD of the measurement interferometer can be determined to a resolution of 40 nm.

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.

On challenges in the uncertainty evaluation for time-dependent measurements

The measurement of quantities with time-dependent values is a common task in many areas of metrology. Although well established techniques are available for the analysis of such measurements, serious scientific challenges remain to be solved to enable their routine use in metrology. In this paper we focus on the challenge of estimating a time-dependent measurand when the relationship between the value of the measurand and the indication is modeled by a convolution. Mathematically, deconvolution is an ill-posed inverse problem, requiring regularization to stabilize the inversion in the presence of noise. We present and discuss deconvolution in three practical applications: thrust-balance, ultra-fast sampling oscilloscopes and hydrophones. Each case study takes a different approach to modeling the convolution process and regularizing its inversion. Critically, all three examples lack the assignment of an uncertainty to the influence of the regularization on the estimation accuracy. This is a grand challenge for dynamic metrology, for which to date no generic solution exists. The case studies presented here cover a wide range of time scales and prior knowledge about the measurand, and they can thus serve as starting points for future developments in metrology. The aim of this work is to present the case studies and demonstrate the challenges they pose for metrology.

Development of a high-accuracy multi-sensor, multi-target coordinate metrology system using frequency scanning interferometry and multilateration

We present a novel coordinate measurement system based on a combination of frequency scanning interferometry and multilateration. The system comprises a number of sensors (minimum of four) that surround the measurement volume. Spherical glass retro-reflectors act as targets that are used to define the points in space to be measured. The sensors all measure the absolute distance to all targets simultaneously. The resulting distances are then used to compute the coordinates of the targets and other systematic parameters such as the sensor locations. Initial experimental comparison with a commercial laser tracker has shown that the proposed system is capable of achieving coordinate uncertainties of the order of 40 μm in a measurement volume of 10 m × 5 m × 2.5 m. The system is self-calibrating, inherently traceable to the international system of units (the SI) and computes rigorous coordinate uncertainty estimates.

GPS Style Position Measurement with Optical Wavelengths

GPS location services have benefited the lives of millions. But what if it was possible to perform similar measurements using optical instead of microwave wavelengths? We investigate the potential and challenges of an optical analogue to GPS.

Large-scale dimensional measurement in the space environment using multi-lateration techniques

Multi-lateration measurement techniques are described which are expected to lead to significant improvements in the accuracy with which large structures, such as optical and x-ray telescopes and radar arrays, could be precisely assembled in space. A high-accuracy system is described, the working volume of which could be significantly extended for use in such applications.