Characterizing dynamic test fixtures through the modal projection error

Abstract

Across many industries and engineering disciplines, systems of components are designed and deployed into their operational environments. It is the desire of the engineer to be able to predict if the component or system will survive its operational environment or if the component will fail due to mechanical stresses. One method to determine if the component will survive the operational environment is to expose the component to a simulation of the environment in a laboratory. One difficulty in executing such a test is that the component may not have the same boundary condition in both the laboratory and operational configurations. This paper presents a novel method of quantifying the error in the modal domain that occurs from the impedance difference between the laboratory test fixture and the operational configuration. The error is calculated from the projection from one mode shape space to the other, and the error is in terms of each mode of the operational configuration. The error provides insight into the effectiveness of the test fixture with respect to the ability to recreate the individual mode shapes of the operational configuration. A case study is presented to show the error in the modal projection between two configurations is a lower limit for the error that can be achieved by a laboratory test.