Investigation of mechanisms to control the longitudinal and torsional mode vibration response of an ultrasonic transducer

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Combining longitudinal (L) and torsional (T) vibration motion is common in ultrasonic devices. The combined motion deliver LT vibrations at the output face or tip of the device. Examples of such devices that have been the basis of research programmes include ultrasonic drills for planetary exploration, ultrasonic needles for bone biopsy, and ultrasonic welding devices. Commercial examples include ultrasonic surgical shears and phacoemulsification devices. In these examples, the transducer is based on a conventional Langevin configuration. There are then a number of different approaches to exciting the LT motion. One is through L-wave degeneration that creates a non-uniform section, by cutting and twisting a number of slots or a helix along the path of the L-wave such that part of the wave converts into a T wave whilst the remaining part propagates unchanged through the section. A second approach is to couple the L mode and a T mode by tuning the two modes to be excited in resonance at the same frequency. Another mode coupling approach additionally uses two set of piezoelectric elements, where one set generates L vibration whilst the second set generates T vibration. LT devices can also be based on delivering LT motion in the transducer itself or, in the case of mode degeneration devices, by introducing the geometrical features in an ultrasonic horn attached to the output face of a conventional Langevin transducer. Where LT motion is delivered in the transducer itself, this can be via two sets of differently poled piezoelectric rings (one exciting L and the other exciting T motion) or by incorporating a helical or slotted structure in the front mass. In all cases, however, there is often little control of the torsionality of the device (defined as the ratio of tangential to axial vibrational displacement of the tip). There is also a commercialised phacoemulsification device that relies only on T mode vibrations of the tip. A gap in current offerings is a switchable-mode transducer, where L and T motions can be excited independently in one device at the same resonance frequency. This PhD will explore a range of configurations that can deliver such a switchable-mode ultrasonic transducer, including through the incorporation of differently poled piezoceramic elements.The main tasks to be carried out include:1) A review of the currently available LT devices developed commercially or published in the literature2) Research the potential applications of a switchable-mode ultrasonic transducer3) Support the design and testing of torsional piezoelectric rings and investigate their incorporation into designs for a switchable-mode transducer4) Develop FEA models in OnScale to design potential configurations of a switchable-mode ultrasonic transducer and to simulate their performance5) Design, prototype, characterise and test a number of configurations with potential for controlled switchable responses, and validate the FEA models6) Establish a range of performance tests to indicate the most promising transducer