Ultrasonic Transducers: Piezo Ultrasound for Medical, Welding, Flow Measurement & Therapeutic Applications

Ultrasound has been used in nature for navigation above and under water. Bats and dolphins are but two species who perfected the transmission, reception, and processing over millions of years.

When an AC voltage is applied, piezo elements begin to oscillate. The extremely fast response in the microsecond range and below allow for ultrasound generation with frequencies of up to 20 MHz.

Ultrasonic applications can be classified in sensor applications and power ultrasound where the energy densities are higher and frequencies range from 20 to 800 kHz.

Power ultrasound can be used in medical applications, e.g. to crush kidney stones and for removing dental plaque, and industrial processes, such as cleaning and welding or bonding.

In addition to the material selection, different geometric variants and resonant frequencies can be realized to optimize the transducer for the respective application. While most common piezo transducers are based on PZT formulations, lead-free PIC700 material is now also available, suitable for ultrasonic transducers in the MHz range and for sonar and hydrophone applications; components such as disc- or plate-shaped transducers in thickness vibration mode,

The range of applications for piezoceramic components is diverse. The direct as well as the inverse piezo effect is used in ultrasonic measurement of propagation times. A typical application for propagation time measurement is the measurement of filling levels. Unlike the biological solutions where sender and receiver are separate – superfast muscles in the bat’s larynx create the sound while highly sensitive ears detect the reflection – a piezo transducer can work as both a transmitter and a receiver, almost at the same time. The ultrasonic pulse it transmits is reflected by the filling medium. The propagation time required is a measure of the distance traveled in the empty part of the container.

Piezo transducers have successfully been used in oilfield services industry. A recent paper by Goujon N, Hori H, Liang KK, Sinha BK illustrates examples of these versatile elements in “marine seismic survey, precision pressure measurement, sonic logging-while-drilling, and ultrasonic bore-hole imaging”.

An even more extreme application of piezo transducers can be found on the science lab onboard the Mars Rover Curiosity. Here, piezo transducers help prepare drill samples for the X-ray diffraction and fluorescence spectrometry system that will help scientists figure the possible existence of life on the red planet.

The inverse piezoelectric effect (shown above) comes about when an electric field is applied to a poled piezo material, it will result in a deformation that can be used to create a displacement or force. If the transducer is glued to a structure, it can be used to actively control the structure, damp vibrations, etc.

Using direct and inverse piezo effect (shown below) in a structural health monitoring system, one piezo transducer is electrically excited to induce vibration which can be detected by a multitude of piezo transducers operated in sensor mode. By evaluating the excitation signal and the various senor output signals, conclusions on the state of the structural health can be drawn.

Piezo elements and transducers have also become indispensable in medical engineering. In addition to propagation time measurements such as in air bubble detection (Air In-Line sensing), typical applications are in pumping and dosing. The dosing amounts range from the microliter and nanoliter range to the picoliter range. Here, piezo-based precision systems excel due to their minute dimensions, low energy consumption, and low costs. The same also applies to devices for aerosol generation, in which a piezo element excites a diaphragm to ultrasonic vibrations. The frequency is approx. 35 kHz. Due to the resulting pressure changes at the diaphragm, the fluid is pressed through the holes in the diaphragm and thereby aerosolized.

The challenge with micro-systems cleaning is to avoid damage to tiniest structures on the surface to be cleaned. High frequencies in the range of 700 kHz to 3 MHz provide the best results without damage. With these systems, contamination particles in the nanometer range can be removed reliably without damaging the sensitive surfaces by excessive pressures and temperatures. The functional principle is easy shown below.