Mechanical impedance techniques for evaluating the dynamic characteristics of biological materials

Abstract

Driving point mechanical impedance, which is the complex ratio of force to velocity at the driving point, is a useful tool for characterizing the dynamic response of a non-rigid distributed system. Being a complex quantity it has a magnitude and a phase angle for each frequency of excitation. The necessary apparatus includes a shaker, force and acceleration transducers and means for recording the transducer outputs. To simplify analysis of results the shaker should have a sinusoidal output which is variable over the frequency range of interest. Although vibrational intensity is usually detected as acceleration and integrated to get velocity it could be sensed directly. In order to eliminate manual treatment of the transducer outputs to get the magnitude and phase angle there are instruments available which compute these values automatically as excitation frequency is scanned. Observing the dynamic characterization of biological system by means of mechanical impedance techniques is particularly appropriate because the parameters of these systems are usually distributed. The method has the advantage of not requiring that transducers be attached to the material being tested; that is, the technique is an input-output measurement. The method is analogous to impedance evaluation of electrical circuits in alternating current theory. The method has been successfully used in the mathematical modelling and subsequent construction of a dynamic stimulator describing the responses of a seated subject to low frequency whole-body vibration. Work is underway on the medium to high frequency responses of the hand-arm system. Potential applications range from evaluation of structures to testing of fruits, vegetables, eggs and other animal products as well as intact plant systems.