Surface evaluation by the signal processing of ultrasonic pulses
The development of a surface texture evaluation technique for the study of roughnesses of the order of a few microns using the signal processing of ultrasonic pulse-echo signals is described. The technique of extracting surface information by means of deconvolution is introduced. Strictly, a solution to the deconvolution problem normally does not exist or is not unique. The chosen method of approaching a solution is by the nonlinear Maximum Entropy Method (MEM), which offers superior image quality over many other filters. The algorithm is described and translated into a standalone computer programme-the development of this software is described in detail. The performance of the algorithm in the field of ultrasonics is assessed by means of the study of simulations involving images similar to those obtainable in a real application. Comparison with the linear Wiener-Hopf filter is provided particularly in instances where the comparison shows weaknesses of either technique. Also examined is the frequency restoration property of the algorithm (not shown by the Wiener-Hopf filter)-potential applications of this property are also described. The final part of the study of the MEM is an examination of the effect on performance of some of the algorithm's parameters and on computer system dependencies. A brief overview of some of the surface metrology techniques currently used is given. The aim is an introduction to surface metrology and an assessment of where the technique described here fits into the general surface metrology field. The experimental system, which of course is essential to practical applications, is considered in some detail. Also considered is a wide range of ultrasonic transducers available for the research. These show a considerable variety of characteristics. Some assessment is carried out using the Maximum Entropy Method with simulated and real data to try and establish the properties of a transducer best suited to the application intended. Finally, results from grating-type test surfaces and more general rough surfaces are presented. The former are intended as a means of establishing the potential performance of the technique; the latter build on the grating results to analyse real surfaces as made by a variety of engineering techniques. Results are compared with those obtained by a stylus instrument. Generally good agreement is found, with roughnesses of around 2 microns being accurately assessed. With the accuracy of these results being less than a micron, it is concluded that this technique has a valuable contribution to the surface metrology field.