Misalignment of the stinger is often a significant problem in modal testing. Given a large enough misalignment, the shaker armature coil windings will be damaged due to scraping. More commonly, misalignment results in distortion and poor quality measured frequency response functions, which are uncharacteristic representations of the true system. At times this will cause difficulties in estimating modal parameters from data that has been contaminated with poor shaker alignment. This type of measurement contamination is often overlooked because of other issues related to many inconsistencies in the test structure system due to noise, nonlinearities and other effects. It is all too easy to overlook this simple measurement issue due to other commonplace factors.
The main problem with shaker misalignment is that the force transducer or impedance head transmits transverse loads that do not align with the sensing axis of the transducer normal to the surface. This causes a distortion of the actual measured force that is applied to the structure. It is very important to make the best possible measurement and alignment is an important part of this process.
Another problem will result when the structure is too compliant at the point of shaker attachment. As a result, the shaker may not have enough stroke for the actual structure displacement observed during testing. While displacement is one effect, consider the shaker coils velocity limitations (5.2 ft/s, around 62 ips for the 2100E11) as well. In these cases the structure wants to deflect (especially at resonant frequencies) and the shaker cannot keep up with the actual displacement/velocity of the structure. This causes a force dropout in the input force spectrum, especially at resonant frequencies. Many times this will be referred to as impedance mismatch between the shaker and structure. In order to remedy this, typically the shaker will need to be moved to another suitable input location where the structure is not as compliant, yet still adequately excites all the modes of interest.