K.J. Cross

The evaluation of arc erosion on electrical contacts using three-dimensional surface profiles

Three-dimensional surface profiling is a useful technique for the evaluation of electrical contact erosion. A review is given of the various methods by which a 3-D surface profile can be measured, and numerical techniques are described which can be used to analyze electrical contacts to define erosion in terms of volume and height changes. Experimental data on the rate of change of mass of Ag-CdO contacts are used to compare with volume and height changes over a number of switching cycles. Emphasis is given to the evaluation of 3-D surface profile in the particular condition where the change of mass of the contacts is nearly zero after a large number of break only switching cycles, and it is shown that in this condition the contacts exhibit volume changes both above and below the datum surface of a new contact.

A method for the selection of algorithms for form characterization of nominally spherical surfaces

This paper presents a method for the selection of algorithms for the form characterization of nominally spherical surfaces. The form characterization is applied to discrete data obtained from a three-dimensional (3-D) high-precision measurement system. In this paper, five sphere-fitting algorithms are outlined and compared. The fitting algorithms under investigation are: linear and nonlinear least-squares sphere fit, minimum zone sphere fit, four-point sphere fit, and sphere fit by error curve analysis. A method for selection of the best-fit algorithm, based on a spectral analysis of the surface irregularity is suggested and applied to the surface measurement of worn electrical switching contacts.

The measurement and analysis of the three-dimensional form of curved surfaces

In an attempt to reduce manufacturing costs, contact lenses are manufactured using a cast moulding technique. If quality is to be maintained, it is essential that the mould components are manufactured to very high standards, with particular emphasis being placed on the curvature of the optical surfaces. If these surfaces are to be: accurately measured and described, then they must be considered in three dimensions. Several different :measurement instruments have been considered for this purpose, and three-dimensional numerical algorithms developed for assessment of the data from these instruments.

Optical fiber imaging for high speed plasma motion diagnostics: Applied to low voltage circuit breakers

An integrated portable measurement system is described for the study of high speed and high temperature unsteady plasma flows such as those found in the vicinity of high current switching arcs. An array of optical fibers allows the formation of low spatial resolution images, with a maximum capture rate of 1 x 10(6) images per second (1 MHz), with 8 bit intensity resolution. Novel software techniques are reported to allow imaging of the arc; and to measure arc trajectories. Results are presented on high current (2 kA) discharge events in a model test fixture and on the application to a commercial low voltage circuit breaker.

3-D laser metrology for the assessment of curved surface

Measurement instruments are identified which are suitable for the assessment of curved samples typically used in the manufacture of lens technology. The advantages and limitations of each instrument are discussed, together with their suitability for measuring surfaces and producing data that can be analysed for sphericity information. A number of numerical sphere- fitting algorithms are suggested for radius of curvature analysis, and a Contour Analysis technique is developed for the assessment of errors of Form without the need to sphere-fit to the data. Contour Analysis is shown as a useful visualisation tool, allowing the user to clearly see surface distortions, such as astigmatism, without the need to interpret the deviation from sphere plots.

A reference workpiece for voxel size correction in x-ray computed tomography

X-ray computed tomography (CT) is increasingly used for dimensional metrology, allowing the inspection of both interior and exterior features impossible to observe using traditional optical and tactile measurement techniques. X-ray CT offers many benefits over traditional instruments as a visual inspection tool, however, extracting dimensional information from the reconstructed data-sets must be approached with caution due to error sources that can propagate through the image reconstruction processes. One error source originates from values of the source-to-object and source-to-detector distances; these are critical inputs as they define the voxel size, a global scalar directly influencing all dimensions extracted from the data. To reduce voxel size errors a reference workpiece can be scanned using the same measurement settings as the actual workpiece. By reconstructing the reference workpiece a reference dimension can be evaluated and this then used to adjust the voxel size of the actual workpiece. This reference dimension must be threshold independent, namely it is determined without the influence of edge detection thresholds. This paper offers a reference workpiece designed for measurement in an X-ray CT system, a coordinate measuring machine (CMM), and an optical profiler. Repeated measurements are made of the reference workpiece using all three instruments and

Studies of high current arcs using an optical fiber array based imaging system

A portable arc imaging system is presented for the study of high speed and high temperature unsteady plasma flows such as those found in the vicinity of high current switching arcs. The system permits direct measurement of arc light emission images with a capture rate of 1 million images per second (1MHz), and 8 bit intensity resolution. Novel software techniques are reported to visualise the arc motion and to measure arc trajectories. Results are presented on high current (2kA) discharge events where the electrode and arc runner surfaces are investigated using optical surface scanning methods; such that the position of the arc roots on the runner can be correlated to the measured trajectories. The results show evidence of the cathode arc root stepping along the arc runners.