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A low-g triggered micro-electromechanical system (MEMS) resonant acceleration switch is designed, fabricated and tested in this paper for near-zero power wake-up applications. The switch is actuated by ambient low-g vibration, consuming zero power while waiting for vibration at its resonant frequency. A cantilever beam and proof mass structure is adopted in the switch. The patterns of spiral cantilever beams are designed for low resonant frequency and threshold. Once the vibration with resonant frequency exceeds the acceleration threshold of the switch, the movable electrode becomes sufficiently displaced to contact the fixed electrodes and causes them to trigger. The dynamic responses of the switch are tested on a piezoelectric stack. The experimental results show that the switch closes under vibration at a frequency as low as 39.3 Hz and at an acceleration threshold of 0.074 g. A wake-up sensor node connected to the switch can awaken when the switch is under vibration as an intended characteristics. [ABSTRACT FROM AUTHOR]

This paper presents a review of the recent advances on micromachined inertial switches/triggers. The review focuses on their advantages and disadvantages, sensitive directions, mechanisms of contact-enhancement, threshold accuracy, and the tunability of the acceleration threshold. Several applications of these sensors are highlighted including in healthcare, structural health monitoring, internet of things, and military. Recent contemporary research directions are also discussed, such as multi-directions/axis, multi-threshold sensors, and machine learning implementation. The article concludes with discussion on future development trends and performance improvements of inertial switches. [ABSTRACT FROM AUTHOR]

A low-g triggered micro-electromechanical system (MEMS) resonant acceleration switch is designed, fabricated and tested in this paper for near-zero power wake-up applications. The switch is actuated by ambient low-g vibration, consuming zero power while waiting for vibration at its resonant frequency. A cantilever beam and proof mass structure is adopted in the switch. The patterns of spiral cantilever beams are designed for low resonant frequency and threshold. Once the vibration with resonant frequency exceeds the acceleration threshold of the switch, the movable electrode becomes sufficiently displaced to contact the fixed electrodes and causes them to trigger. The dynamic responses of the switch are tested on a piezoelectric stack. The experimental results show that the switch closes under vibration at a frequency as low as 39.3 Hz and at an acceleration threshold of 0.074 g. A wake-up sensor node connected to the switch can awaken when the switch is under vibration as an intended characteristics.

Aiming at the problem of poor reliability of MEMS devices, regarding MEMS acceleration switch as the research object, ANSYS finite element analysis of ware was used, and the main part of the MEMS acceleration switch was the structure of the"block torsion beam-cantilever beam-quality". The finite element model of the switch was established, and the finite element analysis of the drop impact in the three directions of the MEMS acceleration switch was carried out. Through simulation calculation and analysis, the maximum magnitude of the device under the impact load was obtained, and the failure mode of acceleration switch was analyzed. The impact test was verified by the Marshall hammer test platform, the limit magnitude of the acceleration switch impact failure was obtained, and the main failure mode of the MEMS acceleration switch was fracture failure. The results indicate that the simulation analysis results of the main failure modes of the MEMS acceleration switch are reliable and consistent with the test results. [ABSTRACT FROM AUTHOR]

Contact time is one of the most important properties for inertial micro-switches. However, it is usually less than 20 µs for the switch with rigid electrode, which is difficult for the external circuit to recognize. This issue is traditionally addressed by designing the switch with a keep-close function or flexible electrode. However, the switch with keep-close function requires an additional operation to re-open itself, causing inconvenience for some applications wherein repeated monitoring is needed. The switch with a flexible electrode is usually fabricated by electroplating technology, and it is difficult to realize low-g switches (<50 g) due to inherent fabrication errors. This paper reports a contact enhancement using squeeze-film damping effect for low-g switches. A vertically driven switch with large proof mass and flexible springs was designed based on silicon micromachining, in order to achieve a damping ratio of 2 and a threshold value of 10 g. The proposed contact enhancement was investigated by theoretical and experimental studies. The results show that the damping effect can not only prolong the contact time for the dynamic acceleration load, but also reduce the contact bounce for the quasi-static acceleration load. The contact time under dynamic and quasi-static loads was 40 μs and 570 μs, respectively. [ABSTRACT FROM AUTHOR]

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. 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. The impact of the squeeze-film damping on the threshold value was theoretically studied. 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 design strategy was verified by FEM (finite-element-method) simulation and an experimental test. 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 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 measured contact time was 50 μs which is also in good agreement with our previous work.

This paper reports an analysis on the relationship between the tilt of the proof mass in MEMS switch and the chattering phenomena. Low-g MEMS acceleration switch developed by Kim’s group was modelled in 2D and displacements of each end of the proof mass were analysed using RK 4th method. Some elementary assumptions were made to ease the modelling and analysis. The chattering time of the MEMS switch gets longer as the tilt of the proof mass increases. The reason is that the elongated travel distance of one end of proof mass increases the impact velocity and lengthens the bouncing back time. From the results, we found that the chattering phenomena can last very long even if the tilt of the proof mass is very small.

Highlights: [•] We firstly propose a bi-directionally tunable MEMS acceleration switch. [•] Threshold decreased from 10.25g to 2.0g by applying tuning voltage. [•] Threshold increased from 10.25g to 17.25g by applying tuning voltage. [•] Additional functionality of safe/armed position convertibility is realized. [•] Additional functionality of mechanical latching is realized. [ABSTRACT FROM AUTHOR]

Abstract: We describe the design, modeling, fabrication, and testing of a MEMS inertial switch that can detect accelerations in the x, y, and z axes using a single mass/spring assembly (first reported in Ref. [1]). A spiral spring suspension is used which is compliant in all directions. The inertial switch approach offers zero power draw until an acceleration event occurs. Arrays of individual triaxial sensors are created on a single chip, with each switch designed to close at a different acceleration threshold between 50g and 250g. A resistor ladder connects each switch together in series such that the number of pinouts from the chip is substantially reduced. The response of the switch array to half-sine acceleration pulses of 5–8ms duration is reported, with multiple switches in the array closing and opening in sequence for higher magnitude acceleration pulses. The switch closure time under these conditions is consistently less than 200μs, and bounce is minimal for applied accelerations lower than 200g. [Copyright &y& Elsevier]

In order to fulfill the requirements of low energy consumption and two sensing directions, a novel bidirectional acceleration switch is proposed by utilizing the magnetic-fields-based tristable mechanism that can maintain the three stable states without input power and with high position accuracy and repeatability. The bidirectional acceleration switch mainly consists of an inertial mass supported by two parallel elastic beams, two metal contact points, and four permanent magnets with one imbedded in the inertial mass and the other three fixed in the case along the vertical direction. Based on the magnetic charge model, the nonlinear magnetic force is analyzed, and then, a static design model of the bidirectional switch is established by considering the elastic force, the magnetic force, the contact force, and the inertial force. To validate the feasibility of the design method, a miniature sample of the switch is fabricated. The results of the centrifugal experiment show that the threshold accelerations in two directions are 53.0 and -52.0 g, respectively, which are close to the design values of 55.0 and -50.0 g, correspondingly. In addition, the threshold values can be adjusted by changing the relative distances among the four magnets. [ABSTRACT FROM PUBLISHER]

Raised strips were introduced into the micromachined acceleration switch to avoid the stiction between the mass and the substrate. The system spring constant of the switch with serpentine springs was calculated. The influence of the introduced anti-stiction raised strips on squeeze-film damping of the switch was analyzed closed-form using reasonable boundary conditions. Based on the calculated systems parameters, a Simulink model was constructed to simulate the dynamic properties of the switch under different types of shock acceleration. It is indicated that the dynamic responses (threshold acceleration and response time) were almost not affected by those raised strips which are instead beneficial to reduce the oscillation of the moving mass and improve the yield of the switch. Dependence of dynamic properties on the distance between two electrodes, thickness of the mass and spring constant has also been simulated. The drop hammer experiment was conducted to characterize dynamic properties of the fabricated and packaged acceleration switch. Test threshold acceleration and response time are, respectively, about 41 g and 0.7 ms when the half-sine wave shock was applied, which are in accordance with simulated ones. The package strength of the switch was also evaluated and corresponding critical shear stress for failure is about 22 MPa. [ABSTRACT FROM PUBLISHER]

Abstract: An acceleration latching switch with independent multi-contacts is presented in this paper. All the contacts and their beams are independent to the proof-mass so as to prevent the contacts from the impact resulting from the rebound or vibration of the proof-mass once the switch is latched. Moreover, multiple contacts are used in order to get highly reliable contact, to lower the contact resistance and to increase the maximum allowable current. The switch was fabricated by low-cost two-mask silicon on glass process and tested. The latching shock is 4500G and the response time is less than 0.1ms. The contact resistance is no more than 5Ω while the isolation resistance is more than 200MΩ and the maximum allowable current is up to 100mA. [Copyright &y& Elsevier]

Abstract: Modeling, numerical, and experimental efforts undertaken to develop a fundamental understanding of latching in a MEMS shock sensor are presented. A two degree-of-freedom model is developed and numerical studies are conducted with this model. These studies, which help shed light on difficult to observe experimental aspects, are used to examine the interaction forces between the shock sensor mass and latch, bounce effects, and loss of contact between the mass and the latch. High-speed video images of the shock sensor motions collected during a latching event are shown, and these results are used to verify the model predictions. Parametric studies conducted to examine the sensitivity of the design to friction and the effects of the latch mass and stiffness properties on the latch bounce are presented and discussed. [Copyright &y& Elsevier]

Inertial micro-switches have great potential in the applications of acceleration sensing, due to the integrated advantages of a small size, high integration level, and low or even no power consumption. This paper presents an overview of the recent advancements made in research on the sensitive direction, threshold acceleration, contact effect, and threshold accuracy of inertial micro-switches. The reviewed switches were categorized according to the performance parameters, including multi-directional switches, multi-threshold switches, persistent closed switches, flexible-electrode switches, and low-g high-threshold-accuracy switches. The current challenges and prospects are also discussed. [ABSTRACT FROM AUTHOR]

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. 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. The impact of the squeeze-film damping on the threshold value was theoretically studied. 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 design strategy was verified by FEM (finite-element-method) simulation and an experimental test. 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 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 measured contact time was 50 μs which is also in good agreement with our previous work. [ABSTRACT FROM AUTHOR]