application of frequency-domain system identification techniques in ...

3.2. Non-stationary data 55some of the patches.In (Mevel et al., 2002), the idea was explored to re-scale and merge the datafrom the different patches before performing the system identification step. Forthis purpose, the subspace-based output-only identification technique was adaptedto handle multi-patch measurement setups.In this chapter, similar techniques, for use with multi-patch non-stationary outputonlydata, will be presented in the frequency-domain. Two different methods arepresented for obtaining correctly re-scaled mode shape estimates from a single estimationprocedure applied to the complete set of data gathered by all patches.The unwanted non-stationary effects are removed respectively before and after thesystem identification step.Apart from being a lot faster than the classic approach, when a high numberof patches is used, a single set of natural frequencies and damping estimates is obtainedfrom the data in the different patches. Moreover, since all data is processedtogether in a single estimation procedure, no mode pairing approach is requiredfor the re-scaling of the mode shapes.The discussed re-scaling approaches were tested on two applications where measurementsare often, if not always, performed in patches: Scanning Laser Vibrometry(SLV) and ambient civil engineering testing. First, SLV results were usedfrom experiments performed on a clamped plate. Both situations with controlledacoustic excitation and uncontrolled ambient traffic excitation were considered forvalidation purposes. Next, the results of multi-patch ambient vibration testing, onthe stairway case study structure introduced in Chapter 2, were compared to theinput-output forced excitation test results described in that same chapter. Lastbut not least, the re-scaling approaches were tested on the Z24 bridge bench markdata. These results can be found in Chapter 5 which is partly dedicated to theZ24 bridge benchmark.Throughout this Chapter, a frequency-domain output-only ML identification techniquewill be used for the modal parameter estimation. However, it should be notedthat the discussed techniques are not restricted to the ML estimator and that thepresented re-scaling techniques can be used with other system identification techniques.3.2 Non-stationary dataDifferent sorts of non-stationarity can be distinguished. A first type to be consideredis excitation level instability, where the level of excitation becomes a functionof time. In this case, re-scaling methods that are simply based on the computation