Cylindrical Piezoelectric Elements

Traditional piezoelectric transducer arrays are constructed by mechanically dicing a block of piezoelectric material of a thickness which corresponds to the desired frequency, in order to form the individual elements. For low frequency arrays this method can pose some issues, as a typical PZT material with a through thickness mode at 150 kHz will typically be of the order of 10 mm thick, compared to the sub-millimetre thicknesses which relate to the high frequencies commonly found in NDT imaging and medical arrays. Dicing such a thick material can be difficult. In an attempt to avoid this issue and simplify the manufacture of such an array, an alternative method of driving the waveguide elements was investigated, using discrete piezoelectric elements.

Rather than using a single, diced, piezoelectric, each waveguide element in the array would instead be driven using a separate cylindrical piezoelectric element. The cylindrical element would be positioned such that it is aligned tangentially to the end of the waveguide strip, forming a line contact with the 1 mm x 10 mm face of the waveguide, as shown in figure 8.3. The piezoelectric element would then be excited in the thickness mode, generating ultrasonic waves in the waveguide with

Figure 8.3: A schematic diagram of the individual elements in the strip array, con- sisting of a stainless steel waveguide strip and a cylindrical piezoelectric element.

the resulting radial motion of the piezoelectric rod.

A FE simulation was carried out to allow the use of a cylindrical piezoelectric element to be compared to standard a thickness mode piezoelectric element. In the first of the models, a stainless steel waveguide (1 mm x 10 mm 300 mm) was driven with a block of PZT5H, with the same cross-section as the waveguide and a thickness of 9 mm mounted at one end. In this model the direction of polarisation of the piezoelectric element is aligned with the length of the strip, giving an optimal geometry for generating ultrasonic waves in the waveguide strip to use as a reference. In the second model, an identical waveguide strip was driven by a 3 mm diameter, 10 mm length PZT5H cylinder, aligned as shown in figure 8.3. The two types of piezoelectric elements were designed such that they have a resonance which occurs around 150 kHz. These elements were driven with identical input voltages (a single cycle sine wave with a frequency of 150 kHz) and the displacement of the front face

0 50 100 150 200 250 300 Time / µs -1 -0.5 0 0.5 1

Normalised Displacement Amplitude

0 50 100 150 200 250 300 Time / µs -1 -0.5 0 0.5 1

Figure 8.4: The displacement of the front face of the waveguide strip, driven with a thickness mode of a piezoelectric element, top, and the radial displacement of a cylindrical piezoelectric element, bottom.

was recorded. These displacement measurements are shown in figure 8.4, with the displacement from the strip driven by the through thickness PZT shown at the top, and the strip with the cylindrical PZT at the bottom.

The amplitudes in figure 8.4 are normalised relative to the amplitude of the maximum displacement measured on the waveguide driven with the standard through thickness piezoelectric element to assist in comparing the data. Clearly the use of the cylindrical piezoelectric element changes the shape of the transmitted pulse, with a longer, decaying, tail on the wave packet. However, the amplitude of both of the signals remain comparable, with approximately two thirds of the maximum amplitude retained when the waveguide is driven with a radial motion of the cylindrical PZT. The frequency content of the first wave packet of each of the two signals can be seen in figure 8.5. From this figure it can be seen that when driven using the cylindrical PZT the bandwidth of the transducer is reduced, with the

0 100 200 300 400 0 0.5 1 Frequency / kHz Normalised Amplitude 0 100 200 300 400 0 0.5 1 Frequency / kHz

Figure 8.5: The frequency content of the displacement signals when the strip waveg- uide is driven with a thickness mode piezoelectric element, top, and when driven with the radial motion of a cylindrical piezoelectric element, bottom.

FWHM decreasing from approximately 44 kHz to 15 kHz, with a slight shift in the centre frequency, to 140 kHz. These effects, along with the reduction in amplitude should not pose an issue in a practical application. The loss of amplitude could be offset by driving the PZT with a higher voltage, which is often limited as a safety precaution. For some fluids, such as natural gas, the voltage driving a transducer is often kept low to reduce the ignition risk. However, with the addition of the waveguide strips this voltage would be much further from the fluid, reducing the risk of ignition, and potentially allowing the use of higher driving voltages. Additionally, the loss of bandwidth should not effect the operation of the transducer in most circumstances. The main advantage of this method of driving the transducer in this manner is the relative ease of manufacture and the increased electrical isolation

DAC FPGA TxGEN board Tx/Rx module DAC Tx/Rx module DAC Tx/Rx module 16 channels total Serial RS232 PC

Digital pulse signals Analogue pulse amplitude control

Transducers

Figure 8.6: A simplified block diagram showing the operation of the LF-PAC. A PC is used to program the waveform generated by the FPGA on the TxGEN board. The FPGA has 16 channels of output. Each of these channels includes a pair of differential signals and a separate analogue voltage signal.

between the individual elements, which is beneficial for array operation.