Whenever a modal test is performed using shaker excitation, the shakers should be disconnected from the structure while data is not being acquired, whether it is between different test configurations or during periods of inactivity, for example over a lunch break or overnight. There are many reasons for this. During the set up of the test, there may be some shifting or settling of the test article. Commonly, airbag soft support systems are used to provide a free-free boundary condition during testing. These may lose air over time, shifting the structure. Additionally, an elastic shock cord (bungee) will creep in length over a period of time. In between tests, there may be some reconfiguration of the test article for multiple sets of test data. For instance, a vehicles gas tank may be empty in one test and then filled in another test. Because of this, there may be a general shifting or redistribution of mass in the system, which in turn causes shifting of the test article relative to the original alignment of the shakers to the structure.
If the shakers are attached during down time between tests while the structure undergoes necessary reconfigurations, there may be side loads applied to the stinger attaching the shaker to the structure and the alignment of the system may be disturbed. These side loads will potentially bend the stingers, or worse, cause damage to the shaker armature. In addition, it may become difficult to disassemble the stinger from the structure once the alignment has been disturbed.
If the shakers are disconnected during periods between test cycles, it will be obvious if any misalignment has occurred upon reattaching the shakers for the next set of tests. If the original shaker alignment is disturbed, then the shaker must be realigned in order to provide a proper attachment to the system.