Prediction of the dynamic behaviour of electromagnetic relays submitted to mechanical shocks

During space flights, pyrotechnic devices are widely used to separate structural subsystems, to unfold solar panels or to activate propellant valves. The firing of these pyrotechnic devices generates severe shock waves (so-called pyroshocks) with high inten- sity and wide frequency range, which can damage the surrounding electronic equipment. Common observed damages more especially concern relay chatter and transfer, as well as failure of magnetic components. There is a lack of failure criteria for electronic equipment as well as computational techniques able to predict the dynamic behaviour of complex structures subjected to high frequency shock waves. The pyrotechnic shock behaviour is checked experimentally: test specifi- cations imposed to embarked electronic devices are generally defined as a maximum limit imposed through the Shock Response Spectrum (SRS). Alcatel Alenia Space ETCA and the Faculte Polytechnique de Mons collaborate to develop a pyroshock test facility dedicated to the testing of electronic devices. This facility uses a resonant test fix- ture which can be either a simple or a double plate assembly, excited by a explosive charge (a non electric detonator mounted with a 5 or 12 g/m penthrite detonating cord ) or a mechanical impact (pneu- matic actuator). The magnitude and the shape of the resulting shock are controlled by tuning different parameters, in order to satisfy the specifications of the launchers or spacecrafts. Experimental results obtained when checking the perturbations in- duced by shocks on the electrical behaviour of some relays, such as the latching GP250 relay are also presented. Microswitches levels have been correlated with the magnitude and shape of different Shock Response Spectra. A finite element model has also been developed to simulate the elec- trical perturbation and the mechanical behaviour of relays submitted to severe mechanical shocks. Numerical results have been correlated with experimental results obtained with harmonic and impulse shaker excitations as well as with pyroshocks applied to a less compact relay such as the PED PXC-1203 relay. The pertinence of using SRS limit levels to check electric perturbation of a relay is discussed.