A time delay ultrasonic imaging system

Davies, Colin J. S. (1982). A time delay ultrasonic imaging system. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f93c

Abstract

The design and construction of a time delay ultrasonic imaging system is described. After describing previous and existing methods of ultrasonic imaging, the probable advantages of relative simplicity in comparison with phased arrays, avoidance of side lobes, improved resolution and the disadvantages of ambiguity and distortion of the time delay system are discussed. Polyvinylidene fluoride (PVF2) is used as a transducer material because its high damping enables one to produce single cycle ultrasonic pulses in water and its flexibility enables it to be formed into a curved surface. The properties of PVF2 as a transducer are examined both by library search and by practical experiment. Schlieren images are shown of the ultrasonic wave front emanating from PVF2.

A system for imaging under water is designed that comprises one transmitting and four receiving transducers. These are made from PVF2 film on a cylindrical nylon backing, and construction methods are given. The system is controlled by a Rockwell AIM 65 desk top computer and the image is shown on a television screen. The design of both hardware and software is discussed, and methods are suggested for later improvement. The imaging system is demonstrated using four receiving transducers, and the improvement that can be achieved using more transducers is shown by simulating eight receivers. It is shown that, while the time delay system suffers from spurious images caused by ambiguities, it is free from the effect of side lobes because of the short pulse used. The design of a sectioned transducer that should reduce the number of ambiguities is outlined. In the system demonstrated, the image repetition period is two minutes, but with improved computation, this could be reduced to ten seconds or less. The problems envisaged in imaging in metal are discussed and methods of overcoming them are suggested.