No description available.

No description available.

No description available.

No description available.

This thesis describes a theoretical and experimental investigation into compressional wave ultrasonic probes for use in high resolution non-destructive testing. In the theoretical section, the pulse performance of the ultrasonic probe is analysed in detail. The response of the piezoelectric transducer is initially determined by direct time domain analysis of electrical equivalent circuits and specially devised computer programs are then used ito predict the performance of various probe constructions The effects of intermediate layers lying between the transducer and the backing and between the transducer and the job are included. In the experimental section, the design, construction and subsequent performance of high resolution probes is described. Both contact and immersion probes are included, the former having a resonant frequency of 5MHz, and the latter having resonant frequencies of 5MHz and 10MHz. Whilst the computer analysis does not extend to contact probes, agreement between predicted pulse shapes and those obtained experimentally is excellent in the case of immersion probes. In addition to pulse shapes, full details of such aspects as repeatability of probe performance, suppression of acoustic waves in the backing and experimentally observed beam-profiles are also given. Finally, the performance of a single immersion probe having a resonant frequency of 20MHz is described and some suggestions for further work are given.

A simple stand-alone Scanning Tunnelling Microscope has been designed and constructed with the aim of providing first stage toward an in-situ instrument for study of thin-film growth. It is functionally capable of STM operation (constant current, constant height, line test) and STS operation (I - V and dI/dv - V). This thesis describes mechanical design, electronic design, computer interfacing in hardware and software, and computer graphics, and presents the results of testing and some applications. The STM head mechanical design features a helical spring/diaphragm coarse approach assembly and a concentric tube scanner. An analysis of the control system indicates that an integrator and single-pole low-pass filter provides near optimal behaviour of the control loop and deals with mechanical resonance in a particularly satisfactory manner. A discussion of hardware/software tradeoff and analog/digital tradeoff leads to a stand-alone design based on a PC, giving good performance with maximum use of standard commercial components and relatively simple special-purpose interface. Commercial graphical software package UNIRAS PC-DOS is employed for image display and hardcopy output. Results show excellent repeatability and stability of STM operation, with atomic resolution.

No description available.

No description available.

Underwater measurements of acoustic backscattering coefficients from gravel surfaces at oblique incidence were made in a laboratory tank. A side scan geometry was adopted in these measurements using a conventional, 300kHz, side scan transducer to insonify an area which is beam width controlled in one direction and a pulse length controlled in the other. As the pulse travels along the rough surface the variation of the backscattered signal with grazing angle was studied. In the second set of measurements a normal incidence geometry was adopted to investigate the dependence of the backscattering coefficient upon range from the rough surface insonified. Two different frequencies (1MHz and 250kHz) and two different rough surfaces (a pressure release and a gravel surface) were used in this experiment. The backscattered signal was measured by a small LC5-2 hydrophone which was inserted on the acoustic axis of the transmitter. The experimental results were compared with the theoretical values developed using the Helmholtz-Kirchhoff integral. In the oblique incidence case the Fraunhofer phase approximation was used to evaluate the scattering integral. The predicted values compared well with the experimental results measured for the gravel surfaces. The backscattering coefficient was found to vary rapidly with angle at low grazing angles while this variation was not so obvious at grazing angles between 30 to 60. In the normal incidence mode the scattering integral derived using the second order, Fresnel, approximation was used to predict the theoretical backscattering coefficient values. A good agreement with the experimental results was obtained even at ranges close to the rough surface. The normal incidence backscattering coefficient of a rough surface is shown to be dependent upon the surface properties alone, only in the farfield region of the scattering area. In the nearfield of the surface, the backscatterintg coefficient is shown to be depending on the surface reflection coefficient only. In the region between these two ranges the backscattering coefficient is shown to be depending upon both the surface statistics and the measurements geometry.

The purpose of this thesis is to show the application of modern digital techniques to produce a high speed computational system that has the capability of producing a faster than real-time model for the visual systems associated with flight simulator systems. A distributed/multiprocessing system was chosen due to the increased reliability, flexibility and throughput of the computational process. The system also has the advantage of providing increased modularity and ease of expansion. This research is firstly concerned with the development of a high resolution graphics system (providing a 1024 by 1024 point screen resolution), and the techniques for the production of moving visual scenes observed by a pilot when flying over a region of terrain in a flight simulator. In particular, this work is concerned with the real-time simulation of the aerodynamics of the aircraft and considers the problems of designing and accessing a data base which represents a model of the terrain. There are three major design considerations for the real-time CGI (Computer Generated Imagery) system of the type required. The first is to ensure that all processing for each picture frame is performed within a specified time limit. Secondly, the picture must have a realistic appearance and requires realism functions to be performed in real-time. The third design consideration is the cost.