Advanced shaker excitation signals for aerospace testing Bart Peeters 1 , Alex Carrella 2 , Jenny Lau 1 , Mauro Gatto 3 , Giuliano Coppotelli 3 1 LMS International, Interleuvenlaan 68, B-3001 Leuven, Belgium 2 University of Bristol, Department of Aerospace Engineering, Queens Building, Bristol BS8 1TR, UK 3 Università di Roma “La Sapienza”, Dipartimento di Ingegneria Aerospaziale e Astronautica, Via Eudossiana 18, 00184 Roma, Italy ABSTRACT The need to reduce testing time without diminishing the quality of the data is an important driver for innovation in the aerospace testing industry. In this paper, the use of advanced, flexible shaker excitation signals will be investigated with the aim (1) to obtain improved Frequency Response Function (FRF) estimations and (2) to assess the non-linearities of the excited system / structure. Pseudo-random and more general multisine signals, rather than the more traditional pure or burst random signals, will be used to increase the accuracy of the FRF estimate. Moreover, special multisine data acquisition and processing methods to identify the level of non-linearity will be illustrated by means of Ground Vibration Testing data of an F-16 aircraft. The presented methods allow assessing the non-linearities at a single excitation level, which is in contrast to the more traditional method of repeating the test at multiple excitation levels and observing the FRF differences. In addition, a new perspective will be given on the post-processing of stepped sine FRFs. Stepped sine shaker excitation signals are traditionally used to highlight and study non-linear behaviour. In this paper, a curve-fitting method based on FRF data at fixed response levels is applied to identify and quantify the non-linearities of the structure. Again, the approach will be illustrated by means of F-16 aircraft data. 1. INTRODUCTION It is clear that an improvement of the accuracy of the estimate of the frequency response functions (FRFs) and a better exploitation of the available test data, e.g. by including non-linearity assessment without increasing the test time, will contribute to the efficiency increase of the whole testing phase. This is particularly true when dealing with actual systems disturbed by high noise levels and strongly affected by non-linear contributions that could be referenced, at least partially, as additional noise [1][2]. Typical non-linear behaviour could originate from control chains, friction, or free-plays of junctions [2]. In traditional modal testing, (burst) random noise sources are sent to the different shakers attached to the structure. Some recent developments allow the use of more sophisticated signals which offer much more flexibility and control during a vibration test. In this paper, frequency response functions will be estimated from pseudo random, user defined amplitude and random phase, excitation signals. These excitation signals are periodic and if the signals are synchronously acquired, the leakage distortions are limited to the initial transients [3][4]. Also, the whole time block is used to excite the structure, reducing the required testing time or increasing the Signal-to-Noise Ratio (SNR). In [5], both numerical simulations of a nonlinear system and laboratory experimental investigations are carried out in order to validate the proposed techniques. The non-linearities considered in the numerical model are introduced as quadratic and cubic stiffness terms, whereas the experimental data are gained from tests carried out on an aircraft scale model. This paper complements the work in [5] in the sense that multisine excitation is applied to a full-scale F-16 aircraft in order to obtain improved FRF estimates and to assess the influence of non-linearities on the FRF estimates. In addition, stepped sine measurements are acquired on the same aircraft and an alternative non-linear identification method is applied in Section 4. 2. THEORETICAL BACKGROUND This section provides the theoretical background. An overview will be given of shaker excitation signals aiming to get improved FRF estimates. The advantages of multisine and pseudo random signals compared to pure and burst random will be discussed. The Schröder multisine can be used to achieve a higher SNR. Furthermore, a special sequence of pseudo random excitation can be used for both FRF estimation and non-linearity assessment. The odd-odd multisine is useful in non-linearity detection [1].