Mechanical Systems and Signal Processing www.elsevier.com/locate/jnlabr/ymssp Mechanical Systems and Signal Processing 18 (2004) 727–738 Excitation design for FRF measurements in the presence of non-linear distortions $ J. Schoukens a, *, J. Swevers b , R. Pintelon a , H. Van der Auweraer c a Department of ELEC, Vrije Universiteit Brussel, Pleinlaan 2, B1050 Brussels, Belgium b Department of PMA, Katholieke Universiteit Leuven, Belgium c LMS International, Belgium Received 3 October 2002; received in revised form 19 June 2003; accepted 24 June 2003 Abstract In this paper, we discuss optimised strategies to measure the frequency response function in the presence of(non-linear)distortions.Todosowewillcomparethreeclassesofexcitationsignals.Thesesignalswillbe used in an optimised measurement strategy, reducing the leakage effects to acceptable (user-defined) levels, allowing to separate the disturbing noise influence from the impact of non-linear contributions, and resulting in the ‘best linear approximation’ of the system. r 2003 Elsevier Ltd. All rights reserved. 1. Introduction The measurement of the frequency response function (FRF) of a system is a major activity in modal analysis, and it characterises the linear behaviour of the system. However, in practice, many systems are not perfectly linear, they are disturbed by non-linear distortions. Nevertheless, linear approximations are often used in practice because they offer important advantages: * They result in useful models that give the user a lot of intuitive insight in her/his problem. * Many design techniques cannot easily be generalised to non-linear models. * Non-linear model building is mostly difficult and time consuming. * No general framework is available for non-linear systems as it is for linear systems. Often dedicated models are needed, complicating the development/use of general software packages. ARTICLE IN PRESS $ This work was supported by the FWO-Vlaanderen, the Flemish Community (Concerted action ILiNoS), and the Belgian Government (IUAP-5/22). *Corresponding author. E-mail address: johan.schoukens@vub.ac.be (J. Schoukens). 0888-3270/$-see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0888-3270(03)00084-0