| The MEMS inertia microswitch can realize both the sensing and the actuating functions.The working principle of the inertia microswitch is that the movable electrode(usually the suspend proof mass moves toward the stationary electrode and contact it when the switch is accelerated up to the threshold level,it can realize the switch on/off of the external circuit.In most cases,MEMS inertia microswitch can replace the complex detection system which is composed of accelerometer,control unit and actuation unit.And it can make up for the shortcomings of the traditional accelerometer,such as the spurious trigger caused by electromagnetic interference,the complex signal processing circuit and so on.MEMS inertia switch is regarded as a passive device with the sensing and the actuating functions,which has many advantages such as small size,light weight,fast response speed and low manufacturing cost.Therefore,the MEMS inertia switch is widely used in detonator systems,safe airbag of the vehicle and vibration monitoring of products and storage.In recent years,it has been paid much attention by more and more researchers from MEMS field.Especially,with the development of the internet of things(IoT)technology,MEMS inertia switches are widely used for detecting the vibration shock and then sending corresponding overload signal in remote areas or difficultly recharging areas where the power supply is limited.Because it has the advantages of zero power consumption.Contact time is one of the key indicators whether the MEMS inertia switch can effectively realize switch-on/off of the external circuit.However,the stationary electrode and movable electrode are rigid structures in the conventional MEMS inertia switches,and the rigid collision between the two electrodes is easy to damage the device.Moreover,the serious bouncing behavior between their two electrodes happens and it will result in an extremely short contact time(often less than10?s).Therefore,it will increase the difficulty of the signal processing when these switches are integrated on the circuit.Two design of the contact-enhanced vertically-drive and horizontally-driven MEMS inertia switch based on surface micromachining technology have been proposed in the paper considering disadvantages above-mentioned.Scheme I,one cross beam with holes have been used as the stationary electrode in the vertically-driven switch,the cross beam can elastically deform after the sensitive proof mass collides with it,which can prolong the contact time between the two electrodes.In order to further enhance the contact effect,the"mouth"type cross beam with holes is used as the stationary electrode,which can further reduce the stiffness of the stationary electrode.Numerical simulation and structural optimization of the designed inertia switch have been completed by ANSYS software.The effect of the shape,width,length and thickness of the stationary electrode on the stiffness and contact time was studied.The simulated results show that it is a convenient method to enhance the contact duration by reducing the thickness of the fixed electrode in order to realize miniaturization of the device.Furthermore,the contact effect of the MEMS inertia switch was better when the"mouth"type cross beam with holes is used as the fixed electrode.Scheme II,the vertically-driven MEMS inertia switch with the synchronous follow-up compliant electrodes is designed.The stationary electrode and movable electrode both are optimized,which makes the contact mechanism of the two electrodes turn into flexible contact to extend the contact time.The contact dynamic process of synchronous follow-up compliant electrodes is analyzed by using the basic physical model of inertia switch.Meanwhile,the dynamic contact process of the model is simulated by using ANSYS software.The simulation results show that the flexible designed stationary electrode can elastically deform in the contact process,and firstly the two electrodes both move in the sensitive direction.In particular,the movable and fixed electrodes can both rebound in the sensitive reverse direction,so the two electrodes can maintain a long contact to realize the longer contact time of the inertia switch.The dynamic response process of the inertia switch with L-shaped elastic cantilever beam as a movable electrode is simulated by ANSYS.It shows that the optimized design structure not only can extend the contact time as expected,but also the optimized design structure can avoid the deformation stress concentration under the high-g acceleration.At the same time,the maximum stress is correspondingly reduced,the L-shaped cantilever beam is not easy to fracture,and the reliability of the device is improved.In addition,the parameters of the inertia switch are designed and simulated reasonably in order to improve the shock-resistibility of the inertia microswitch.The simulated results show that the shock-resistibility of the inertia switch decreases with the increase of the mass weight,the gap between the proof mass and reverse constraint blocks,but increases with the whole system stiffness.Finally,the limited structure layer can improve the shock-resistibility of the inertia switch and prevent spurious trigger occurring.In this paper,the single technology(such as the baking glue,electroplating,lithography,release and so on)was deeply studied.Different kinds of MEMS inertia switch with high aspect ratio structure were successfully fabricated by multi-layers electroplating technology based on non-silicon surface micromachining after integrating related compatible technologies and determining appropriate parameters.The volume of the fabricated device is 1.2×1.2×0.12mm~3,the miniaturization goal and the advantages of MEMS surface micromachining are fully realized.The dynamic response properties(including threshold acceleration,contact time,contact resistance,and shock-resistibility,etc.)of the fabricated inertia microswitch under half-sine shock acceleration with~1ms pulse width was tested by a drop hammer system.The test results show that the contact time of the inertia switch with the multi-holes crossbeam as the fixed electrode decreases with the increase of the width and thickness of the beam,but increases with the length of the beam.The test contact time of horizontally-driven the inertia switch with the optimized movable electrode under the acceleration 288g is 150?s,and the threshold acceleration and the test contact time increase with the acceleration pulse width.In order to further extend the contact time,the two electrodes both are optimized.The contact time of the inertia switch with synchronous follow-up electrodes is approximately 390?s,which is longer than the one of the switch with only one flexible electrode.The test reverse threshold acceleration decreases with the gap between the proof mass and reverse constraint blocks.The test results are generally consistent with the simulation ones.The deviation between the testing and simulating is mainly because the half-sine wave shock acceleration applied to the micro-switch in the test is not ideal as in the simulation.In addition,the electroplated Ni as the device structure generally has a smaller Young's modulus than the block Ni used in the simulation. |
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