PRACTICAL ASPECTS OF SHAKER MEASUREMENTS FOR MODAL TESTING
Shaker testing is commonly used as a method for measuring forced input in experimental modal analysis. The setup of conventional shakers, stingers and transducers is often the source of test difficulties and avoidable measurement errors. This paper reviews the basic design of shakers as beneficial to modal testing, and the common problems associated with setup issues and resulting measurement errors. These include shaker selection, shaker alignment, stinger selection, connections, reciprocity assumptions and other common mishaps and misunderstandings.
Important aspects of precise driving point FRF measurements using a mechanical impedance head sensor
The advance of experimental modal analysis as a design and testing tool has led to a demand for more accurate estimation of modal parameters. To help meet this demand, mechanical impedance head sensors have been developed to more accurately determine driving point measurements. Initial validation of a newly developed impedance head showed that the driving point frequency response measurements were affected by the mounting stiffness of the sensor and the compliance of the test structure [1] . In this paper, a lumped parameter model of a simple test structure is used to show the effects of test structure compliance and sensor mounting stiffness on driving point measurements. Comparisons are made to driving point measurements made with an adjacent accelerometer to show variations in these two methods.
Part 4: What’s shakin’, dude? effective use of modal shakers
ConclusionsThere are several significant modal testing challenges to overcome when striving for the goal of an “adequate linear estimate” of the structural dynamics model. Shaker excitation often has an advantage over impact excitation in meeting this goal. In many cases, shaker testing can reduce the effects of nonlinear response, provide better signal to noise ratio, eliminate overload problems associated with out-of-band force input, provide more consistent data, and provide faster testing by utilizing MIMO acquisition. There are tradeoffs between FRF linearization, FRF distortion, signal to noise ratio, and testing time depending on the shaker forcing function. Some of the major tradeoffs have been addressed for random, pseudorandom, periodic random, burst random, chirp, and stepped sine testing. Shaker mounting and fixturing requires special considerations to mitigate undesirable frequency and damping effects of the shaker and its hardware upon the test article and to minimize side loads and moments on the force gage. Certain steps can be taken to ensure the shaker fixturing is adequate, and many diagnostics are available to ensure the goal of obtaining an “adequate linear estimate” of the structural dynamics model.