Identification of Free and Fixed Normal Modes by Base Excitation.

Abstract

Most of the commonly used modal testing methods are based on multiple point excitation. The interface conditions of the test structure are usually either fixed or free. However, both interface conditions cannot be completely realized in the experiment. An alternative to multiple-point excitation is driven-base testing using a multi-axis shaker table. Up to now, only the fixed interface normal modes could be extracted from the measured structural responses. This paper presents a method of identifying the free interface normal modes by means of driven base tests. Therefore, the interface forces between the test structure and the shaker table have to be measured. A test example as well as a suitable test rig containing a force measurement device and a multi-axis vibration simulator is described in this paper. Typical measurement data are shown and extracted modal data for both fixed and free interface normal modes are included as an example. Finally, the advantages and disadantages of modal parameter identification by base excitation are discussed.