Design And Fabrication Of A Low-g MEMS Inertial Switch With Novel Radial Electrode

With the development of micro electro mechanical technology,metal microdevices with small volume,low cost and high reliability have been widely used in automobile,industrial safety control and transportation.In this paper,a low-g acceleration switch applied to the airbag system was studied and designed,which has several advantages,such as: a uniform omnidirectional sensitivity in XOY plane,rapid response,reliable contact and excellent anti-overload performance.The microswitch was fabricated based on UV-LIGA technology.Finally,the performance parameters of the switch,such as: acceleration threshold,response time,contact time and anti-overload protection,were obtained by centrifuge device and dropping hammer test.Frequency analysis was employed in a simplified physical model of inertial switch,which can determine the optimum natural frequency range in the condition of the given acceleration pulse width.Four symmetric Archimedes' spirals were adopted as support springs.They can not only realize the low frequency performance of a low-g inertial switch,but also achieve a uniform “mass-spring”system stiffness in XOY plane.The novel electrode consists of a spherical contact surface and a modified “S” spring.Compared with contact point structure,the spherical contact surface structure improves the uniformity of the radial gap between proof mass and electrodes.The modified “S” spring decreases the electrode stiffness,which is conducive to prolong contact time.Around electrodes lies the lateral limitation block,whose role is to protect the radial electrodes and improve the anti-overload ability of the switch.The designed switch was simulated by ANSYS software.Simulated results show the acceleration thresholds distribute in the range of 30.7g-34.4g,which means a good omnidirectional performance.The response time and contact time under application environment(an acceleration with 38 g amplitude and 5ms pulse width)are about 1.28ms-1.40 ms and 100?s-165?s,respectively.In an overload acceleration of 700 g,the radial electrode stress decreases from 1736.2MPa to 468 MPa with the help of lateral limitation block.UV-LIGA technology was adopted to fabricate the switch.In view of the deviation of threshold acceleration induced by fabrication errors of crucial dimensions,a new method of “thickness compensation” was proposed.By adjusting a certain dimensional value,the threshold deviation induced by the errors of the other several dimensions will be compensated so that the final threshold accuracy can be increased.In this paper,the thickness of proof mass was adjusted to compensate the deviation induced by spring width,spring thickness and radial gap between proof mass and electrode.The actual structural dimension is 3870?m×3870?m×210?m.The fabricated switch was packaged using ceramic package carrier and tested by centrifuge device(quasi-static test)and dropping hammer(dynamic test).The both test results show that the acceleration thresholds of the switch are between 35g-40 g.Thereinto,the response time in dynamic test is about 2.4ms,the contact time is about 100?s-120?s.What's more,the switch remains the same after the overload test.