RESULTADOS

2-D field profiles at the level of the centre of each element in the array were measured to confirm that each element in the array was functional and to allow comparison to the simulation result for a single element. Each profile was measured in the acoustic tank after the array was aligned (following the procedure outlined in section 5.3) such that the profiles included the central axis of each element under test. The elements were driven by a 40 V peak-to-peak square wave using the multi-channel drive electronics (described in section 5.2) in order to replicate the drive conditions

that would be used when operating all fifteen elements as a phased array. When measuring the field from an individual element, that element was driven alone, with all other elements in the array left inactive.

Figure 6.1a illustrates the simulated 2-D intensity profile for an idealised single element of radius 2 mm, where the element is vibrating as an ideal piston at 1 MHz. The theoretical position of the last axial maximum is ata2_el{λ= 2.7 mm, where ael is the element radius. The near-field region of each element is therefore

small relative to the intended focal depth of the array. Since a minimum separation of 8 mm between the hydrophone and transducer was required to prevent damage to the hydrophone, all the experimental measurements were made in the far-field region of an individual element’s ultrasonic field.

The experimental measurements showed that nine of the fifteen elements in the array matched the simulation profile reasonably well, while the other elements showed poor agreement. Figure 6.1a shows the simulated intensity profile from a single element, while figure 6.1b shows a typical experimental profile of an element having good agreement with the simulated profile. Figure 6.1c shows an example profile of an element having poor agreement with the simulated profile, where the intensity was skewed to the side. Each of the six elements with poor agreement to the simulated profile showed similar asymmetric intensity profiles to that seen in figure 6.1c.

Despite the variation seen in the shape of the experimental 2-D intensity profiles, the 1-D profiles along the central axis of each element all had a similar shape, although the intensities varied by up to a factor of 4 at the intended focal depth of 60 to 100 mm. Figure 6.1d compares the experimental 1-D profiles along the central axis of each of the fifteen elements to the simulated profile, showing the difference in the intensities produced by the fifteen elements. This variation was still seen when the elements showing poor agreement with the 2-D simulated profiles were excluded.

The skewed intensity profiles observed in figure 6.1b,c may be a result of the array’s construction process. The epoxy used for the matching layer was of very

0 5 10 15 20 25 30 35 Z direction / mm X direction / mm 0 10 20 30 40 50 60 70 80 0 5 10 15 0 5 10 15 20 25 30 35 Z direction / mm X direction / mm 0 10 20 30 40 50 60 70 80 −8 −6 −4 −2 0 2 4 6 8 (a) (b) 0 2 4 6 8 10 Z direction / mm X direction / mm 0 10 20 30 40 50 60 70 80 −8 −6 −4 −2 0 2 4 6 8 0 20 40 60 80 100 10−1 100 101 102 Z direction / mm Intensity / mWcm −2 Simulation Experiment (c) (d)

Figure 6.1: (a) Simulated 2-D intensity profiles of a 2 mm radius single element. (b) Experimental 2-D intensity profile of a typical element showing good agreement with simulation. (c) Experimental 2-D intensity profile of a typical element showing poor agreement with simulation. (d) 1-D simulation and experimental intensity profiles along the z axis of each of the 15 elements in the array. The absence of experimental data close to the array surface is due to the minimum separation of 8 mm maintained between the hydrophone and transducer. The colours in plots a,b,c indicate the intensity in mWcm2

low viscosity, such that it seeped between the sides of the PZT and the perspex frame. However, air bubbles were occasionally trapped between the PZT and the frame, with the result that certain elements were not bound symmetrically to the frame, potentially causing the asymmetric field profiles that were observed for cer- tain elements in the array. Further construction trials were carried out on single element transducers and it was found that skewed profiles were absent in transduc- ers where the diameter of the hole in the perspex frame was increased slightly (by approximately 1 mm), thereby eliminating the formation of air bubbles between the PZT and the frame, and ensuring symmetrical bonding of each element. However, clamping of the PZT edge by the epoxy bonding is likely to restrict movement at the edge and promote non piston-like vibrational modes, thereby affecting the geometry of the field [Dekker et al., 1974]. It was not possible to verify this in the acoustic tank, since the entire near-field region for a single element lay within the region that was too close to the transducer surface to be measured without risking damage to the hydrophone by knocking it against the transducer surface.

A number of factors could be contributing to produce the large variation in the outputs of the individual elements in the array seen in figure 6.1d. According to the specifications for the grade of PZT used in the array (listed in table 4.1) the PZT parameters may vary quite considerably between elements in the array. The specifications for Navy type III PZT allow the piezoelectric coefficientd33to vary by

15%, which will impact on the amplitude of the pressure wave produced by each element. Since intensity is proportional to the square of the pressure amplitude, the intensity output by an element could therefore vary by32% due to this parameter alone. In addition, the elements in the array were driven at 1 MHz, a frequency close to their specified resonant frequency. According to the PZT specifications in table 4.1, the frequency constant (and hence the resonant frequency) may vary by

8% between each of the elements in the array, and this could potentially have a large impact on the output of each element. In addition to this inherent variation in the PZT parameters, differences in the mounting of each element within the array and variation in the performance of each output channel from the drive electronics

will affect the ultrasonic output of each element.

The existence of this large range in the output of each element is therefore reasonable given the large number of contributory factors. To minimise the variation in output between elements would require drive electronics with a programmable drive voltage amplitude for each channel, which was outwith the scope of the present work.