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This is due to the collision and friction contact process of the two hook-shaped electrodes.ġ The data was not presented in the paper. , for instance, was first switched on when the applied acceleration was between 28 g and 43.7 g, and it was completely closed when a higher acceleration was applied. The latching switch with a 50.59 g designed threshold value in Ref. However, the keep-close function requires special constructions, such as the hook-shaped electrodes of the latching switches, the V-shaped beams of the bi-stable switches, and the valve-channel of the micro-fluidic switches, complicating the structure topology or working mechanism or fabrication method, thus reducing the threshold accuracy, as shown in Table 1. The switches with a keep-close function can keep it closed after the acceleration event is over, thus improving the contact reliability. The main methods to improve the contact effect of the inertial micro-switches are designing the switches with a keep-close function, or flexible electrodes. This is because the inherent issues of the methods employed to improve the contact effect usually lead to a low threshold accuracy. However, most of the switches reported in the past have been mainly designed to improve the contact effect and the threshold accuracy is rarely considered. Since the first inertial micro-switch was reported in 1972, a great number of inertial micro-switches based on various working mechanisms and manufacturing methods have been developed. As such, a high threshold accuracy is also essential for the inertial micro-switches. From the perspective of application convenience, since most of the switches are mass produced in the industry sector, a high degree of device-to-device threshold uniformity of the same production batch is needed. Thus, the inertial micro-switches require a reliable contact effect of the two electrodes, such that the turn-on signal can be recognized by the external circuit. At a pre-selected threshold acceleration, the moveable electrode moves toward the substrate and it comes into contact with the fixed electrode, turning on the switch and triggering the external circuit. The proof mass serves as a moveable electrode, and it is separated by a certain distance from a fixed electrode on the substrate.
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The inertial micro-switches are typically designed with a proof mass that is anchored to a substrate through flexible springs. Inertial micro-switches based on MEMS (micro-electro-mechanical system) technology have been widely used for acceleration sensing applications due to their small size, high integration level, and low or even no power consumption. The measured contact time was 50 μs which is also in good agreement with our previous work. The measured threshold values were 4.9–5.8 g and the device thicknesses were 18.2–22.5 μm, agreeing well with the simulation results. The simulation results show that the maximum threshold deviation was only 0.15 g, when the device thickness variation range was 16–24 μm, which is an adequately wide latitude for the current bulk silicon micromachining technology. The design strategy was verified by FEM (finite-element-method) simulation and an experimental test. The theoretical results show that the threshold variation from the designed value due to fabrication errors can be reduced by optimizing the device thickness (the thickness of the proof mass and springs) and then establishing a tradeoff between the damping and elastic forces, thus improving the threshold accuracy. The impact of the squeeze-film damping on the threshold value was theoretically studied. As an extended study toward its potential applications, the switch with a large proof mass suspended by four flexible serpentine springs was redesigned to achieve 5 g threshold value and enhanced threshold accuracy. Our previous report based on a 10 g (gravity) silicon-based inertial micro-switch showed that the contact effect between the two electrodes can be improved by squeeze-film damping.