Research On Passive Non-Contact Current Sensing Mechanism And Measurement Method

With the rapid development of IOT（Internet of things）,the IOT production value of the manufacturing industry will reach 2.5 trillion US dollars in 2025 and the number of IOT devices will reach 20.8 billion.In addition to the performance requirements in the IOT use and promotion process,the sensor also has some new requirements in terms of functions.In the process of applying and using current monitoring of a large number of nodes in smart grids,smart cities and smart homes,some problems have been exposed,which have caused widespread concern of relevant scientific research institutions and enterprises.Most of the existing current sensors are designed with external power supply or built-in battery,in order to ensure the raw measurement of the current,signal processing and wired or wireless transmission.But the two forms of power supply have some problems more or less.Therefore the problem of the current sensor’s power supply will limit the pace of its application and large-scale promotion to some extent.This thesis focuses on the passive sensing field of IOT nodes and explores the passive and miniaturized MEMS current sensing nodes.The aim has three aspects.Firstly,propose the basic theory and method of current sensing based on a cantilever beam with piezoelectric materials.Secondly,further expand the response law of piezoelectric current sensor to various forms of current detection.Thirdly,explore a basic standard and process of current sensor design and process that meets the requirements of IOT sensor nodes.The main research contents and methods are summarized as follows:Two basic configurations of passive non-contact dual-core current sensors are proposed.The piezoelectric and surface acoustic wave cantilever beam sensing structures are comprehensively considered into factors such as ease of processing and cost.The piezoelectric cantilever structure with micro-magnets is finally determined as the ultimate solution of the passive current sensor.The distribution law of the magnetic field and magnetic field gradient of the single-core and double-core current-carrying wires is analyzed.The force of micro-magnet affected by the magnetic field is analyzed and studied to obtain the distribution state of the magnetic field force.Also the optimal positional relationship between the cantilever beam structure with the micro-magnet and the wire is determined.The arrangement of the current sensor electrodes is studied when the piezoelectric material is working in the mode of d₃₁1 and d₃₃.The d₃₁1 working mode with large output charge is selected with concerning the electrode distribution.The electromechanical coupling model of the current sensor is given.The influence of the neutral layer position on the charge output of the multilayer piezoelectric structure is also studied.Furthermore,the design criterion of the piezoelectric layer is determined.The static and dynamic output models under the influence of magnetic field force is constructed.In addition,the offsetting effect of the current sensor’s mode shape on the output charge is studied.It is found that the current sensor have a maximum output of charge in the 1^stt vibration mode.The typical signal forms and laws of household appliances are analyzed,and the characteristic regions of typical currents are determined.The method of measuring DC,rectangular square wave and triangle wave current signals is proposed.The limitation that the piezoelectric current sensors can only measure AC current has been broken through.The range of the measurement of other current forms by the piezoelectric passive current sensor is greatly expanded.And the measurement of the direct current is realized by measuring the response of the step signal of the DC current loading section.The theoretical model of current sensor response under DC load based on Duhamei integral of unit impulse response is given,for the first time,theoretically explaining the characteristics of DC measured waveform as well as providing the method of solving DC current value.The characteristics of rectangular square wave current signal are studied.It is found that the rectangular square wave signal is the superposition form of two reverse step signals at different trigger points.The response general solution under the square wave current load is obtained.The law of the output response of the cantilever beam under the square wave signal is also found.Therefore the method of measuring square wave signal is proposed.What is more,the influence law of the parameters such as damping on the signal measurement response is studied and the model of the triangular wave current signal is constructed.The general solution of the response can be obtained which is related to multiple parameters.However,through simulation analysis,it is found that when the load is loaded with a certain slope,the output response of the sensor has a linear variation law,which is applicable to the measurement of the triangular wave signal.When studying the AC current excitation,the solution indicates the response under the general solution.With simulated analysis,it is found that there are three superposition responses in the initial response phase of the sensor.These responses include the oscillation response caused by the initial excitation and current excitation,the steady-state response caused by current excitation,and the accurate current response that the current sensor can measure only under steady-state response conditions.It has been found that increasing the damping can make the sensor enter the steady-state response quickly but reduce the sensitivity of the sensor.A method for improving the sensitivity by segmenting a piezoelectric film is proposed.The method is designed to segment a whole piece of piezoelectric film into a plurality of pieces and to connect them in series and in parallel.When those pieces of the piezoelectric film are connected in series,the theoretical model shows that the output voltage of connected piezoelectric pieces can be increased by n times compared with that of the undivided piezoelectric film.When connected in parallel,the theoretical model shows that the output voltage before and after the segmentation does not change.The high-sensitivity current sensor structure of ten piezoelectric sheets is designed.The relationship between the thickness of the piezoelectric layer and the output voltage is studied.The current sensor prototype is fabricated and a set of MEMS technological process of piezoelectric cantilever with micro-magnet is designed according to the structural characteristics of the sensor.The thickness of each layer of the cantilever beam is tested in turn.The polarization effects of the segmented piezoelectric pieces are tested and analyzed,and the effects of different process flows on the polarization are also tested.