High Frequency Flexural Ultrasonic Transducers (HiFFUT) - a new class of transducer

Flexural transducer currently are only designed for operation in ambient atmospheric conditions, at frequencies of up to approximately 50 kHz, with a long wavelengths in fluids and therefore reduced measurement resolution in many cases. If we could find a way to increase the frequency range of operation of these devices, whilst at the same time creating new designs that could withstand high pressures and temperatures, a plethora of new applications will open up, in some cases enabling measurements to be made that could not otherwise be taken - that is what this project will do, establishing a world lead in this field of research of High Frequency Flexural Transducers. Techniques will be created that used the HiFFUTs for the non-destructive testing of low acoustic impedance materials such as aerospace composites, flow measurements and metrology in hostile environments.

Flexural ultrasonic transducers (sometimes referred to as uni-morphs) operate through the action of the bending / flexing of a piezoelectric material that is attached to a passive material. This is exactly how an ultrasonic car parking sensor operates, and these devices operating at twice the maximum audible frequency of humans, of around 40kHz, have had a tremendous impact, particularly on the automotive sector. The key to the success of flexural transducers used in parking sensors lies in the fact that they are extremely sensitive and efficient, whilst at the same time they are relatively simple to construct and are extremely robust. Imagine the typical environment that these sensors have to survive in; high vibration, large fluctuations in operating temperature, corrosive, dirty and wet conditions - whilst operating at a low power with a high sensitivity. So what makes these flexural transducers attractive to the automotive sector, where there is high pressure to keep sensor costs low at the same time as the sensors being very reliable? The two key factors are that (1) the piezoelectric element is bonded to the inside of a metal cap and the rear of the cap is hermetically sealed, and (2) the flexing of the metal cap and thin piezoelectric element, either from piezoelectric excitation or the arrival of a pressure wave requires relatively little energy. There is currently a surprising lack of any published, rigorous scientific study on these types of small flexural transducers, even at low frequencies and nothing appears to have been attempted using these types of transducers in liquids or for non-destructive evaluation.

The vibration characteristics of a HiFFUT are dependent on the combined response and interaction of all the sensor's components with the medium it operates within or upon. Usually the mechanical response of these transducers is dominated by the vibration behaviour of the passive flexing membrane of the transducer housing to which the piezoelectric is attached, rather than the thickness or diameter of the piezoelectric element bonded to the housing. There are related examples of MEMs based transducers that operate by a flexural membrane at higher frequencies such as Capacitive Micro-machined Ultrasonic Transducers and Piezoelectric Micro-machined Ultrasonic Transducers and whilst these are clearly elegant devices, there are clearly a number of significant advantages to the use of HiFFUTs in many industrial applications.

The most useful modes of operation are probably the axisymmetric modes, which will generate axisymmetric wave fields and work will mainly focus on these, but there may be instances where an anisotropic wave field provides an advantage. Flexural transducers or HiFFUTs can also be driven at a number of axisymmetric harmonic modes or frequencies - using one transducer to cover a wide bandwidth, with each mode having a different directivity pattern will dramatically increase the depth and breadth of information that can be obtained. These transducers are going to find applications in a wide range of industrial application

The results of the research into HiFFUTs will impact across a number of applications, including non-destructive testing, ultrasonic measurement of flow, process control and ultrasonic metrology such as range finding. HiFFUTs or more generally flexural transducers have several key advantages over existing transducers; high sensitivity and low voltage requirements making them suitable for low power applications and intrinsically safe, they can be mechanically and environmentally robust in high pressure, high temperature and corrosive environments and they can be tuned to operate at specific frequencies.

Knowledge - despite their extensive use in low frequency range finding applications, there is a severe lack of any published rigorous scientific research on this type of flexural transducers, let alone High Frequency Flexural Transducers (HiFFUTs). To date, this this lack of peer reviewed research has prevented the application of the technology into areas where it will provide a step change in the quality and depth of information that can be provided. We shall provide a UK based, world lead in flexural transducer research through publication and dissemination at international meetings and online and through engagement with industry and the general public. In addition to industrial applications outlined in the pathways to impact document, HiFUTTs will also be used in research, where their increased sensitivity and ability to work at several discrete frequencies will open up new areas of research and new methods in fields such as the air coupled inspection of complex composite materials, immersion testing of large or thick samples or measurement through or in attenuative fluids.

Economy - the UK already has a strong research and commercial lead in areas of non-destructive testing and flow measurement: HiFUTTs will add significant value to these, creating new applications and products. Our work will establish HiFFUTs as a new sensor technology that has application over a broad range of technologically important areas. HiFFUTs will undergo some industrial trials during the course of the Fellowship, and a spin-out company will be set up to help transfer this technology to industry and consumers, and arising intellectual property will be licensed as appropriate. Whichever route is taken, it will be done with a view to maximizing the impact and adoption of the technology arising from the research. My experience with my existing spin-out company Sonemat Ltd (set up in 2005) has given me experience and insight as to how to help technology transfer of scientific research.

Society - the safety and reliability of components will improve as measurement resolution and reliability improves, with new procedures becoming integrated into relevant standards. Thus, we anticipate that HiFUTTs will influence policy and standards for applications in NDT and flow measurement and these improved standards will benefit society. Flow measurement will be possible in new areas, leading to improved process control, metering, fluid process and utility management, and will boost industrial process efficiency. HiFFUTs are also a strong candidate technology for residential smart metering, where the combination of a high frequency, robust and efficient sensor will lend them to good measurement resolution with safe and practical battery operation.

People - the PDRAs, PhD/EngD students and undergraduate project students that will work on HiFFUTs together with industrial partners and PI and CI on future projects will benefit from training and obtaining new knowledge. Researchers outside the group will also develop new skills through the dissemination activities, and the UK will lead a new technology area, leading to the creation of wealth and jobs. I will lead a new research team, setting the agenda for flexural transducer research, working closely with industry to take developments from low to high TRL efficiently in a relatively short time scale.