Development Of Magnetic Bistable MEMS Electromagnetic Microrelay

As a kind of control component, relay is widely applied in automobile, communication systems, household electrical appliances, precise instruments and so on. With the increasing of the density of electromechanical components in the above equipments, the development of miniaturized relay with small dimension and low power consumption has been considered. However, conventional precision machining cannot satisfy the requirements of miniaturized device. Micro-electro-mechanical system (MEMS)technology based on the microelectronics technology makes it possible.Electromagnetic drive is always chosen in the classic conventional relay. While, the drive methods of miniaturized relays fabricated by MEMS technology are more diversified. Silicon-based electrostatic MEMS relay is a pioneer of miniaturized relay depending on mature integrated circuit process. Rapid response is the outstanding property of electrostatic MEMS relay. But it is only applied in low power devices owing to the low electrostatic drive capability. Electrothermal drive is able to make a larger force in a long distance compared with electrostatic drive. However, the response is too slow. It is not easy to be solved because the heat elimination is the essential problem. However, as to the electromagnetic drive mode, it is not a problem and the voltage for driving is lower than the one for electrostatic drive. So the electromagnetic MEMS relay is more prospective to be applied.The structure of electromagnetic MEMS relay is mostly the miniaturization of conventional prototype. The miniaturized electromagnetic system is made up of electric structure and magnetic structure. It is a little bit more complicated than the other drive methods. Especially, as the foundation for fabricating the miniaturized magnetic structure, the process of magnetic materials is not mature and needs to be developed. Until now, many jobs were focused on developing some key structures of miniaturized electromagnetic devices in order to increase the whole properties. But, the whole properties of the miniaturized devices were increased unobviously. Combining with the current research of classic MEMS electromagnetic microrelay, the main job is as follows:1,Design, simulation and optimized analysis of a novel overall structure of electromagnetic miniaturized relayThe theory for driving the performer is based on the drive theory of linear motor. The end-effect is decreased using the annular driving structure. The friction on the axis is decreased because the"sandwich"symmetric structure in the axial direction makes the magnetic force counteracted. The magnetic flux density in the working air gap is changed by embedded soft magnetic iron core in the planar coils. The difference around the iron core leads to a magnetic force to the iron core. But the planar coil in the stator is not move in the operation. So the electromagnetic force retroacts on the rotor. The contact system mounted on the armature executes the on/off control by oscillating movement. The contact mode is slide contact. The moving contact with arched surface is a kind of thin film structure that is helpful to increase the contact stability. The contact resistance can be reduced and the loading current can be increased because the contact surface can be increased under the contact pressure. In order to keep good performance in a long time, the contact system can be changed by the requirements. The whole can't be rejected owing to the failure of contact system as quick-wear parts.As we know, the magnetic flux prefers to pass the material with higher permeability, so when the soft magnetic iron core is embedded in the planar coil, the magnetic force is induced by the difference of magnetic flux density around the iron core. Because the iron core is fixed in the immovable stator, the magnetic force will counteract on the rotor and make the device keeping a certain state without other power consumption. It is difficult to analyze the magnetic field clearly owing to the complicated miniaturized electromagnetic structure. ANSYS software provides us a good way to solve this problem. The magnetic figure is shown and the structure of iron core is optimized using ANSYS. The theoretical arithmetic of electromagnetic drive, the parameters of every part and the confirmation of operation benefit from the analysis using ANSYS.2,Development of contact structure with an arched surfaceA novel electric contact structure with an arched surface is designed based on the operation properties of the electromagnetic driver. The strike to the breakback contact can be relieved by the novel contact structure. The moving contact will slide on the breakback contact in a certain distance and the bump phenomena can be avoided. The electric conductivity is increased because the oxidized contact surface can be destroyed by the slide contact mode. A novel fabrication of arched structure with large radius curvature is developed. The difficulty of using photoresist as sacrificial layer is solved by precision thermal control.3,Fabrication of Cu-based carbon nanofiber (CNF) composite as contactThe contact system is quick-wear part. So it is necessary to study the material of contact system for diversified requirements. Electroplating gold as contact material is a common method in the MEMS devices. The physical and chemical characteristics of gold are very stable for the electric contact, but it is easy to be abraded. CNF composite metal can be used as electric contact material because of the perfect mechanical and electric properties. The process for fabricating Cu-based CNF composite as electric contact material is developed by modifying the electroplating composition and environment. The properties of Cu-based CNF composite are increased and the fabrication method is compatible with another MEMS technologies. It makes a technical foundation for the application of CNF composite metal in other MEMS devices.4,Batch fabrication and measurement of the bistable electromagnetic microrelay The core components of the microrelay are batch fabricated based on the requirements for designing the whole device. They mainly include the planar winding coil with iron core, the electric brush with arched surface contacts and the cover board with breakback contact. The other parts are fabricated by precise mould punch forming technology which is kind of batch fabrication technology. Finally, all the components are assembled precisely to make a whole device. Combined with precision machining technology, the fabrication method makes batch fabricating complicated miniaturized devices more easily. The yield is increased and the encapsulation difficulty is decreased.The test results show that the response and contact resistance is related to the amplitude of oscillation of the armature. The minimum response time is up to 0.3 ms. DC pulse is adopted for driving the device because the response time is so short. Additionally, the drive power is zero when the device is in one stable state due to the magnetic locking structure. So the minimum power in one operation cycle is only about 1 mJ which is much less than the one of the products in the same category. The minimum contact is 120~200 m? when the contact material is fabricated by gold. The maximum loading current is up to 2A. The contact resistance is 200~300 m? when the contact material is Cu-based CNF composite film. Although the resistivity of the Cu-based CNF composite is a little bit more than the one of copper, the hardness is increased by 50.9%. Thus, kinds of electric contact structures can be adopted based on the diversified requirements of different application fields.