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.
Development of Swept-sine Excitation Control Method to Minimize the FRF Measurement Error
Shaker excitation in FRF (Frequency Response Function) measurement of a testing system can be controlled by using swept-sine signal source in a signal generator and it is called with swept-sine excitation. FRF’s magnitude error of the system which is obtained from the FRF measurement using swept-sine excitation depends on swept function of swept-sine signal. In this paper, swept-sinesignals using linear and S535 swept functions have been simulated to controlling swept-sine excitation in the FRF measurement of SDOF (Single Degree of Freedom) system. Linear swept is swept function of swept-sine signal which is often used in the FRF measurement and S535swept is a swept function has been developed in this paper. Based on simulation results, the FRF’s magnitude error at system’s resonant frequency which was obtained from the FRF measurement using linear swept-sine excitation can be minimized by redoing the FRF measurement using S535 swept-sine excitation.
Improved Driving Point Measurements with a Coincident Mechanical Impedance Head Sensor
With the increasing popularity and reliance on experimental modal analysis as a product design and testing tool, test engineers are continually demanding a more accurate estimation of the modal parameters. Consequently, there has been greater emphasis placed on obtaining high quality, highly reliable data. One facet related to this task involves accurately determining the driving point measurement. (The driving point measurement requires the input force and response acceleration to be measured at the same point on the test structure.) This measurement contains useful information for estimating data quality and is required for deciphering the modal parameters during data processing. Currently, the driving point measurement is often acquired by using separate force and acceleration sensors placed in close proximity. The obvious problem with this method is that the information is not being taken at the same location. Thus, the coupling between the two sensors greatly affects how well the measurement was estimated. Previous attempts to rectify this situation have resulted in devices which include force and acceleration sensors in the same housing. Commonly known as mechanical impedance head sensors, these devices have traditionally suffered from poor performance characteristics and have been unable to fill the role required by today's advanced testing laboratories. To rectify the aforementioned issues, this paper documents the design, development and test results from an improved coincident mechanical impedance head sensor. This new device features force and acceleration sensing elements that are co-located, thereby, insuring a true driving point measurement. Furthermore, advancements in the microelectronics industry and improvements in piezoelectric design technology have resulted in additional benefits such as low dynamic mass, compact packaging, high stiffness and superior signal to noise ratio.