Why you can't ignore those vibration fixture resonances

Vibration fixtures, at times, have resonant frequencies that exist in the frequency range needed for conducting vibration tests. These resonant frequencies can cause significant problems when running vibration tests over ranges which include these resonances. A test engineer will attempt to control the shaker system through a feedback accelerometer which controls the level of vibration input into the test specimen. However, the control accelerometer can only control the level of vibration at that point. It cannot change the resonant behavior of the vibration fixture. Some problems involving this are described in this article along with some basic material about these phenomena. Often times, vibration tests, are performed to qualify or verify the acceptability of specific hardware for certain environments. We use shaker systems to generate forces or accelerations to replicate some known operating condition or to generate an input spectrum that envelops the actual environment. Usually, a specification mandates the particular input that is needed to satisfy the requirement. The interface between the shaker and the test article is the test fixture. As defined in this article, the test fixture includes the shaker armature, expander head (or slip plate) as well as the attachment fixture (i.e.,.the item designed to accommodate the test specimen). So, when we refer to the fixture, we mean everything between the shaker’s driver coil and the test specimen. (Just because we can’t see the armature, doesn’t mean that it’s not part of the fixture setup). Figure 1 shows a typical schematic of the elements of a fixture.