Pyroshock identification using deconvolution methods in the frequency and time domains

During their operational life, the electronic equipments embarked inside space shuttles may be exposed to severe vibratory environments often produced by the activation of pyrotechnic devices. Consequently, some failures in electronic and magnetic components can appear, as for example the contact losses of electromagnetic relays or the appearance of cracks in the ferrite cores. Thales Alenia Space Etca industry (Belgium) develops test facilities allowing reproducing in laboratory the vibratory environment that the equipments will undergo during their space flight. The test facility is generally a resonant fixture that can be excited by a dropping mass, a pneumatic actuator or by a detonating charge. The resonant fixture can be a beam, a simple plate, a double plate assembly or a more complex structure. The Device Under Test (DUT) is screwed to the resonant fixture and subjected to either shock wave and the resonant response of the structure simulating the required shock. For several years, the Faculté polytechnique deMons and Thales collaborate to develop numerical tools in order to predict the vibration levels generated by pyroshocks and to simulate the dynamic behaviour of embarked equipments to external mechanical vibrations. The computer modelling of pyroshocks requires an accurate dynamic model of the test facility as well as an accurate mathematical description of the excitation source which is, in the case of pyroshocks, unknown because it cannot be directly measured. The impact force must then be determined by an inverse method from the Finite Element model of the system and its transient response. Two classical inverse methods using the deconvolution theory have been tested in this paper. The first one is based on the theory of the Wiener's filters in which the frequency spectrum of the unknown force is estimated from the one of the measured transient response. The second one uses a wavelet analysis and consists in establishing the relation between the Discrete Wavelet Transforms (DWT) of the measured transient response and the ones of the impact force. These two identification procedures have been performed to identify the excitation sources generated by pyrotechnic shock on a simple plate structure, for different amounts of explosive. The approach byWiener's filters is very sensitive to the noise level and its application to the identification of pyroshocks does not lead to conclusive results. The wavelet deconvolution method provides better results: it allows to estimate the characteristics of the shock (amplitude and duration) and to reproduce the measured transient responses with accuracy comparable with the specification tolerances that are generally admitted by the equipment manufacturers. The numerical results have also been compared with the ones obtained with an approach by EquivalentMechanical Shock (EMS). The EMS method consists in replacing the actual excitation by a force located at the centre of the explosive device with a triangular time evolution. The amplitude and duration of triangular force are tuned so as to generate equivalent acceleration fields. The EMS approach allows eliminating in the identification procedure the harmful effects of the noise. Besides, it leads to results comparable to the ones obtained by the wavelet deconvolution but with a better accuracy. Nevertheless, EMS approach is less general than the classical inverse methods because it requires to impose the force profile.