Optimizing Piezoelectric Stack Preload Bolts in Ultrasonic Transducers

Abstract

The selection of the preload bolt is often an afterthought in the design of Langevin type "sandwich" transducers, but quite often it is the root cause of failure for power ultrasonic applications. The main role of the preload bolt is to provide a "prestress" in the piezo stack to prevent interface "gapping" or tension in glued joints which can result in delamination. But as an integral part of a highly tuned dynamic system, the resulting parasitic resonances in these preload bolts, such as bending or longitudinal modes, are often difficult to predict and control. This research investigates many aspects of preload bolt design for achieving optimal transducer performance, including basic size and strength determination based on drive amplitude, as well as ensuring adequate thread engagement to the mating horn. Other aspects such as rule-of-thumb configuration and length guidelines to reduce parasitic resonances are also investigated. Optimizing the uniformity of stress in the piezoceramics is also considered, which is affected by end mass length, counterbores and proximity to threading. The selection of the bolt material based on stiffness is also investigated as related to electromechanical coupling. The investigation focuses solely on Langevin type transducers used for semiconductor wire bonding, and which are comprised of the common Navy Types I and III (PZT4 and PZT8) piezoelectric materials. Several metrics are investigated such as impedance, displacement gain, and electromechanical coupling factor. The experimental and theoretical research methods include Bode plots, scanning laser vibrometry and finite element analysis.

Parametric Study of Bolt Clamping Effect on Resonance Characteristics of Langevin Transducers with Lumped Circuit Models

“…Along with the conversion ratio between the mechanical and electrical energy applied to or from the piezoelectric element in thickness mode, the electromechanical coupling coefficient k t is expressed in Equation (1), where f r is the resonance frequency and f a is the anti-resonance frequency of a piezoelectric part. Other studies already showed that such a compression of the piezoelement by a stiff metal component may influence the transducer's resonance feature, with k t being the result [11,12].…”

Simulation-Based Design and Optimization of Accelerometers Subject to High-Temperature and High-Impact Loads

“…After reviewing the literature, some papers relate to the simulation of bolt preloads and impacts [23,24,25,26]. Lee simulated a centrally compressed piezoelectric sensor, whose structure is similar to our accelerometer, but only sensitivity and resonance frequency are analyzed [4].…”

Design of Miniature Ultrasonic Surgical Devices

“…During fabrication, transducers were pre-stressed to compensate for the low tensile strength of the piezoceramic elements [7], while ensuring the applied pre-stress was not excessive as this will result in piezoceramic pronounces depoling, with unstable impedance and ageing [8]. The resulting torque used was 3.0 Nm, providing a pre-stress in the acceptable region of 15 MPa to 30 MPa [9] and avoiding overtightening.…”