Abstract:
The aim of this investigation was to make a flexible array of
pulse-echo ultrasound transducers by etching two orthogonal linear
arrays of conducting elements into the metallisation of either side of
a sheet of PVdF. These would then be multiplexed under computer
control in an X-Y raster, thereby forming an image of subsurface
defects in a material specimen.
A potential source model was used to predict the sensitivity
of a single element air-backed transducer far from resonance. Initial
investigations confirmed the predictions, and reaffirmed the results
of previous workers.
In making a prototype array, it was found necessary to use a
bi-laminar arrangement with a central ground plane, due to
difficulties with crosstalk and charge leakage into the specimen
materials. The radiation pattern of this array was tested and found
to agree with the predictions for Fraunhofer (Far-Field) radiation.
A 10 MHz analogue to digital converter was constructed to
interface with the IBM-PC clone as a transient recorder, through a
data capture program written in 'C'. However, the electrical noise
generated by the PC was found to interfere strongly with the signal
from the array transducer.
A wide-band amplifier and full-wave rectifier was then added
to the multiplexer and A/D converter, and the system enclosed in an
electrically isolated environment, which made it possible to obtain
clear signal data from the transducer.
Non-linear regression was implemented in the software, to
smooth the data and locate echo peaks, and the most frequently
occurring peak separation was used to indicate sample thickness at
that location in a false-colour mapping on the screen of the PC.
