Electronically focused ultrasonic transmitting arrays.
This work describes a thirty-two channel programmable transmitter
unit for driving an ultrasonic transmitting array using transducers
with thickness resonances of up to 2 MHz. It has been developed to
allow the performance of time delay focussed transmitting arrays to be
thoroughly investigated before their eventual use in an imaging system.
The unit will produce a pulse or continuous wave output which is
programmable using either a microprocessor or a computer, in both
amplitude and delay or phase. The unit's operation is discussed in
some detail and the experimental underwater 1 MHz transducer array used
for the functional tests is described. Results are presented showing
the performance of the transmitting unit when used with this array and
demonstrate that the system provides an effective tool by which a proper
assessment of time delay focussing may be made.
A computer prediction technique·has been introduced. The computer
prediction of the field in the region in front of a focussed ultrasonic
array has been obtained by the summation of the fields due to the
individual array elements. The shape of the short duration acoustic
pulses due to the individual elements is determined.by the electrical
drive and transducer characteristics. The prediction technique is
valid for any pulse shape which can be represented mathematically,
however the results presented here have been limited to the pulse shape
used in the experimental work. The algorithm used is suitable for a
wide range of array formations and the close agreement of the practical
and the simulation work shows the validity of the prediction technique.
The experimental array has been used as the basis of a detailed
investigation into the resolving power of focussed arrays and a number
of results have been derived from this investigation. These are used
to support extensive simulation studies and computer prediction
techniques. The validity of the simulation techniques is assessed and
the effect of considering each transducer in the array as a single
small but finite sized centrally placed element is compared with that
where each transducer is represented by a number of synchronously
driven Huygens radiators. The significance of these results with
respect to imaging is discussed. The effect on performance of system
error is investigated and an assessment of the tolerance of the time
delay technique to these errors is made.