A New Method for Obtaining FRF of a Structure in Area Where Impact Hammer Cannot Reach

The Frequency Response Function (FRF) is a fundamental component to identifying the dynamic characteristics of a system. FRF's have a significant impact on modal analysis and root cause analysis of NVH issues. In most cases the FRF can be easily measured, but there are instances when the measurement is unobtainable due to spatial constraints. This paper outlines a simple experimental method for obtaining a high quality input-output FRF of a structure in areas where an impact hammer can not reach during impact testing. Traditionally, the FRF in such an area is obtained by using a load cell extender with a hammer impact excitation. A common problem with this device is a double hit, that yields unacceptable results. The new method provided in this paper is shown to be superior to the traditional method for several reasons: (i) it is simple and requires no new equipment, (ii) a double hit issue is avoided, (iii) the desired FRF is calculated using a closed form equation, provided in this paper, which results in a more accurate solution.