The mechanical pre-stressing in ultrasonic piezotransducers

Magnetic prestressing for a d32-mode single crystal ultrasonic transducer

This work describes a 35.9 kHz ultrasonic transducer that incorporates a magnetic arrangement to apply a static-compressive prestress to a d₃₂-mode relaxor ferroelectric single crystal drive-element. The magnetic arrangement produces a 22.5 N static-compressive force, inducing a static compression of ∼630 nm on the drive-element. Operating in air with a continuous-wave 10 V peak drive at ∼35.9 kHz, the measured resonant peak displacement of the transducers head-mass was 127 nm. This is well within the predicted static compression, thus, the drive-element is protected from damaging tensile stress. Under the same drive conditions and at an axial distance of 10 mm from the face of the head-mass, the measured acoustic pressure was ∼12 Pa. Analytical and finite element model predictions and the measured behaviour of a prototype device are presented and show good correlation, demonstrating that magnetic prestressing of the drive-element can be a viable alternative to the traditional bolt-clamp.

Stepped-plate ultrasonic transducer used as a source of harmonic radiation force optimized by genetic algorithm

Circular stepped-plates are often chosen to generate continuous waves in air due to their transmission efficiency, directive radiation patterns and capability to generate high-intensity airwave emission. Such features can be suitable for an acoustic radiator intended for the generation of harmonic radiation force, if the narrow bandwidth of the piezoelectric power transducer is not an obstacle to the application. This force has been used as the excitation source in noncontact modal analysis of mechanical devices, since it offers some advantages over the traditional impact method and the use of shakers. In this study, a directional stepped-plate ultrasonic device driven by a bolt-clamped Langevin-type transducer is used to excite the first flexural mode of a clamped-free beam by means of harmonic radiation force. The harmonic component was generated using an amplitude-modulated electrical input signal. The radiator design was done by solving a parametric optimization problem of finding a plate with the desired vibrational behavior using the genetic algorithm. Finite element analysis was used to obtain the dynamic behavior of the device and the airwave propagation was modeled using linear acoustic theory. Modal and harmonic analyses were conducted to obtain some of the electroacoustic parameters of the transducer. The acoustic field generated by the prototype is estimated by solving the Rayleigh integral and the obtained results are compared to experimental measurements.

Limits and Opportunities for Miniaturizing Ultrasonic Surgical Devices Based on a Langevin Transducer

Minimally invasive surgery offers opportunities for reduced morbidities, faster postoperative recovery, and reduced costs, and is a major focus of surgical device innovation. For ultrasonic surgical devices, which offer benefits of high precision, low force, and tissue selectivity in surgical procedures, there exist laparoscopic ultrasonic shears for minimally invasive surgeries that combine tissue cutting with vessel hemostasis and sealing functions. Another approach to laparoscopy that could enable new procedures, and increase the sites of surgeries that could be reached by an ultrasonic device, involves integrating a miniature ultrasonic tool with a flexible surgical robot. However, miniaturization presents challenges in delivering the ultrasonic vibrational energy required to cut hard and soft tissues, partly due to the concomitant small volume of piezoelectric material. This article aims to provide insights into the trade-offs between transducer size, volume of piezoceramic material, resonance frequency, and the achievable displacement amplitude of devices that, consistent with current ultrasonic surgical tools, are based on a bolted Langevin transducer (BLT) and tip. Different configurations of BLTs are studied, including a cascaded version, simple bar versions, and BLTs with different front mass geometries. Results show that a BLT with a larger number of piezoceramic rings exhibits a higher coupling coefficient [Formula: see text] but with the compromise of a lower mechanical Q and stronger nonlinear response at increasing excitation levels. Displacement amplitude is reduced considerably when a BLT is excited at a higher harmonic, where the PZT rings are maintained at a nodal plane, and the resonance frequency shift at increasing excitation levels increases significantly. The electromechanical and dynamic characteristics of a cascaded transducer excited in its third longitudinal mode (L3) are almost equivalent to a much shorter version of a BLT driven at the same frequency but in its first longitudinal mode (L1), showing that a cascaded BLT can be a realistic proxy for studying the dynamics of small BLT devices. A new figure of merit is proposed that is the product of Q , [Formula: see text], and gain, which [Formula: see text] accounts for the gain of cylindrical BLTs which is shown not to be unity. It also proves effective as it incorporates the key factors affecting the achievable displacement amplitude of a BLT, including for BLTs with gain profiles in the front mass. The order of highest to lowest amplitude of a series of six gain-profile BLTs matches the order estimated by the figure of merit. It is shown that a BLT with a stepped profile front mass can achieve displacement that has the potential to cut hard or soft tissue and exhibits the smallest shifts in resonance frequency at increasing excitation levels.

Analysis of a Cascaded Piezoelectric Ultrasonic Transducer with Three Sets of Piezoelectric Ceramic Stacks

To increase the ultrasonic intensity and power of a piezoelectric transducer, a cascaded piezoelectric ultrasonic transducer with the three sets of piezoelectric ceramic stacks is analyzed. The cascaded piezoelectric ultrasonic transducer consists of four metal cylinders and three sets of piezoelectric ceramic stacks in the longitudinal direction. In analysis, the electromechanical equivalent circuit of the cascaded piezoelectric ultrasonic transducer is obtained, as well as the resonance/anti-resonance frequencies equations. By means of an analytical method, when the position of piezoelectric ceramic stacks PZT-2/PZT-3 changes, the resonance/anti-resonance frequencies and the effective electromechanical coupling coefficient of the cascaded piezoelectric ultrasonic transducer have certain characteristics. Several prototypes of the cascaded piezoelectric ultrasonic transducer are manufactured. The experimentally measured resonance frequencies are in good agreement with the theoretical and simulated results. The cascaded piezoelectric ultrasonic transducer with three sets of piezoelectric ceramic stacks presented in this paper is expected to be used in the field of high power ultrasound.

Dynamic Analytical Solution of a Piezoelectric Stack Utilized in an Actuator and a Generator

This paper presents the dynamic analytical solution of a piezoelectric stack utilized in an actuator and a generator based on the linear piezo-elasticity theory. The solutions for two different kinds of piezoelectric stacks under external load were obtained using the displacement method. The effects of load frequency and load amplitude on the dynamic characteristics of the stacks were discussed. The analytical solutions were validated using the available experimental results in special cases. The proposed model is able not only to predict the output properties of the devices, but also to reflect the inner electrical and mechanical components, which is helpful for designing piezoelectric actuators and generators in a comprehensive manner.

Study on tangentially polarized composite cylindrical piezoelectric transducer with high electro-mechanical coupling coefficient

In this work, we proposed an effective way to fabricate a tangentially polarized composite cylindrical transducer with high electro-mechanical coupling coefficient (EMCC) by radially connecting an inner tangential polarized piezoelectric tube with an outer metal ring. The resonance frequency and EMCC of the proposed transducer are calculated according to frequency equations, which are obtained from the equivalent circuit of the transducer, and the results indicate that EMCC of the tangentially polarized cylindrical transducer is much higher than that of the cylindrical transducer polarized in radial direction. Furthermore, the Finite Element Method (FEM) model of the transducer is established and used for numerical simulation of the vibration models and the optimum configuration parameters. On the basis of those theoretical results, serials of prototype transducers are manufactured with an inner tangential polarized piezoelectric tube connecting with different outer metal cylindrical shells. The admittance characteristics of the fabricated transducers measured by Impedance Analyzer clearly demonstrate that the resonance frequencies of the transducers are in good agreement with those of simulation results, and the effective EMCC of transducers varies with the material of metal cylindrical shell, in which aluminum metal shell possesses the highest EMCC.

The influence of the thickness of non-piezoelectric pieces on pre-stressed piezotransducers

The mechanical pre-stress applied in piezotransducers used to generate high power ultrasound is needed to avoid ceramics fracture on traction cycle. Pre-stress levels inferior to 50 MPa can yield resonance shifting due to effectiveness of acoustic coupling between transducer pieces. Symmetrical transducers with different thickness of passive parts were submitted to axial mechanical pre-stress up to 50 MPa and their resonances were measured. The experimental results show the increasing of the resonances frequencies with the level of applied pre-stress. Similar effect is verified in simulations by using a model based on Mason's equivalent electric circuit. Due to the similarity of these effects, a relation between applied pre-stress and pieces coupling was proposed for the transducer assembled. In addition, the dependence of the thickness of non-piezoelectric pieces on the coupling effectiveness between them is discussed. The results show that transducers with small thickness present more expressive shifting resonance ratio.