Stepped sine testing using recursive estimation

Abstract

Measuring frequency response functions (FRFs) of structures using stepped sine excitation is an accurate but time-consuming process. At each frequency step an adequate time must be allowed for transients to die away before the force and acceleration signals are sampled and processed. Most stepped sine analysers require the user to specify a global time delay and sampling time for all frequencies at the start of a test. This paper describes how the time required to perform a stepped sine test may be reduced by recursively estimating the FRF value at a given excitation frequency. Once this value has converged the next frequency may be tested. Thus the time taken to measure the FRF at each frequency is not fixed, but is the time required to predict the FRF value to the selected accuracy. The critical choice in the recursive algorithms is the rate at which the initial transient signal is forgotten. This must be a compromise between speed of convergence and noise rejection properties. The technique is demonstrated using simulated and experimental data.