Modal analysis of ultrasonic block horns by ESPI

Abstract Slotted block horns are used in many high power ultrasonic tools, to achieve a uniform vibration amplitude across a wide output face. Finite element (FE) analysis is an established method for designing tuned block horns and for the prediction and design of horns with a well isolated tuned operating mode. Validation of FE models has relied on pointwise, non-contact, normal-to-surface vibration measurements, which have proved far from ideal in the analysis of horns tuned to deliver predominantly in-plane surface vibration responses in the longitudinal mode. This paper presents the results of a modal analysis of an ultrasonic block horn using electronic speckle pattern interferometry (ESPI) to measure three mutually orthogonal components of surface vibration response. By monitoring an electrical input signal from the ultrasonic transducer, frequency response function data is obtained by fringe processing of the raw, wholefield ESPI data, such that subsequent extraction of the modal parameters is achieved by utilising the facilities of commercial modal analysis software. Measurement of the surface in-plane and out-of-plane response of the block horn allows all modes to be identified successfully and enables correlation with FE models.