Modelling of sprayer boom dynamics by means of maximum likelihood identification techniques, part 1: A comparison of input-output and output-only modal testing

It is a well-known fact that the linear dynamic behaviour of mechanical structures can be studied by modelling the relation between force(s) [input(s)], acting on the structure, and their resulting structural vibration response(s) [output(s)]. For industrial structures, in their real in-operation conditions, it often becomes hard (or impossible) to experimentally measure the excitation. For this reason, attention has been paid to the development of system identification techniques that work on a basis of response data only. The use of such techniques allows the identification of modal models for structures excited by unknown ambient noise and vibration. In this contribution, controlled vibration experiments were conducted on a sprayer boom that was mounted on a tractor. A comparison was made between the results of a classic input-output and output-only modal analysis based on maximum likelihood system identification techniques. Due to the interaction between the dynamic artificial excitation device and the test structure, the output-only approach modelled the excitation device together with the test structure. The identified output-only modal parameters were validated by direct comparison to the identification results obtained on a classic input-output transfer function data set between the generator input signal and the structural responses.