3D Finite Element Analysis of the Glass Encapsulated Magnetic Reed Switches under Pyroshock Environment

Since their invention in the late 1930 by the Bell Labs, the unique features that characterize the reed switches made them the component of choice for such diverse applications as telecommunications, automotive, appliances, test, military and aerospace. This paper deals with the application of glass encapsulated magnetic reed switches as the telemetry indicator in space mechanisms, specifically after they have been exposed to pyroshocks environment. The reed switch as a telemetry indicator in space mechanisms application has to survive a mechanical shocks environment that exceeds the standard allowable. Reed switches are usually rated to survive, without false operation of the contacts, mechanical shocks levels of up to 100 g for SPST type and up to 50 g for SPDT type. In space applications, the mechanical shock levels are much higher, with peak accelerations of 2,000g to 3,000g and the frequency content much more complex (Pyroshocks are usually expressed in terms of the Shock Response Spectrum-SRS with frequency content up to 10,000 Hz). The reed switch has to be protected against these high loads by adequate mounting impedance of the printed circuit board (PCB) and device housing to the base plate. This requires an accurate 3D FEM Analysis of the reed switch, PCB and device housing. In this work, two methods were used to derive the loads acting on the reed switch during Pyroshocks: the first method employed a Shocks Response Analysis consisting from a Modal Analysis, using IDEAS 12.0 and Model Solution solver to obtain the modal solutions of the reed switch-PCB-Housing assembly, followed by the Response Analysis where the equations of motion are written in terms of the modal Degrees-of-Freedom (DOF), and the physical responses are recovered from modal responses. Because the peak acceleration information only is retained via the SRS, the Shock Response Analysis has to employ a rule to combine the peak loads (accelerations, stresses, elemental forces, etc) generated for each of the dynamic modes. The software offers 5 choices of such rules: the absolute value rule ABS; the square root of the sum of squares rule - SRSS; the Naval Research Lab rule - NRL; the complete quadratic combination rule - CQC and the Nuclear Regulatory Commission rule - NRC. All rules have been used and compared. The second method is based on Transient Analysis and it was used to derive the loads on the reed switch during Pyroshocks for a time domain realization of the SRS spectrum. This method can be used post test to analyze the stress induced in the reed switch contacts, leads and glass capsule by a certain Pyroshocks event or prior to the test by generating analytically a time domain realization of the SRS spectrum. The FEM results are compared with experimental data in order to predict the main failure modes of the reed switch: contact surface damage resulting in contact resistance deterioration, contact reset that modifies the pull-in/drop-out points and, in extreme cases, change of the state of the contacts, micro cracks in the glass seal, broken leads, etc.

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