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

弯曲传感器目前仅设计用于在环境大气条件下工作，频率高达约50 kHz，在流体中具有长波长，因此在许多情况下降低了测量分辨率。如果我们能找到一种方法来增加这些设备的工作频率范围，同时创造出能够承受高压和高温的新设计，那么将会出现大量的新应用，在某些情况下，可以进行无法进行的测量-这就是这个项目将要做的，在高频弯曲传感器的研究领域建立了世界领先地位。将创建使用HiFFUTs进行低声阻抗材料无损检测的技术，如航空航天复合材料，恶劣环境中的流量测量和计量。弯曲超声换能器（有时称为uni-morphs）通过连接到被动材料的压电材料的弯曲/弯曲动作进行操作。这正是超声波停车传感器的工作原理，这些设备的工作频率是人类最大可听频率的两倍，约为40 kHz，产生了巨大的影响，特别是在汽车行业。在停车传感器中使用的弯曲传感器成功的关键在于它们非常灵敏和高效，同时它们构造相对简单并且非常坚固。想象一下这些传感器必须生存的典型环境：高振动、工作温度大幅波动、腐蚀性、肮脏和潮湿的条件-同时以低功耗和高灵敏度运行。那么，是什么让这些弯曲传感器对汽车行业具有吸引力呢？汽车行业在保持传感器成本低的同时，传感器也非常可靠。两个关键因素是：（1）压电元件结合到金属盖的内部，并且盖的后部是气密密封的，以及（2）金属盖和薄压电元件的弯曲，无论是来自压电激励还是压力波的到达，都需要相对较少的能量。目前令人惊讶的是，即使在低频下，也没有任何关于这些类型的小型弯曲换能器的已发表的严格科学研究，并且似乎没有尝试在液体中使用这些类型的换能器或进行非破坏性评估。HiFFUT的振动特性取决于所有传感器组件与其内部或其上运行的介质的组合响应和相互作用。通常，这些换能器的机械响应由压电元件所附接的换能器壳体的被动挠曲膜的振动行为而不是结合到壳体的压电元件的厚度或直径来支配。存在基于MEMS的换能器的相关示例，其通过柔性膜在较高频率下操作，诸如电容式微加工超声换能器和压电式微加工超声换能器，并且虽然这些显然是优雅的设备，但是在许多工业应用中使用HiFFUT显然具有许多显著的优点。最有用的操作模式可能是轴对称模式，其将产生轴对称波场，并且工作将主要集中在这些模式上，但是可能存在各向异性波场提供优势的情况。弯曲换能器或HiFFUT也可以在多个轴对称谐波模式或频率下驱动-使用一个换能器来覆盖宽带宽，其中每个模式具有不同的方向性图案将显著增加可以获得的信息的深度和广度。这些传感器将在广泛的工业应用中得到应用

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