| During the construction and use of materials,devices,and facilities,plastic deformation or fracture occurs under the action of external or internal forces,and strain energy is released in the form of stress waves,a phenomenon known as acoustic emission.The use of sensors that can collect acoustic emission signals and data analysis instruments that analyze acoustic emission signals to detect the location of the source of acoustic emission and damage has become one of the important means of nondestructive testing.Acoustic emission sensor is the core part of the defect detection,acoustic emission source released energy in the form of waves through the medium to the acoustic emission sensor,acoustic emission sensor through the sensitive elements of the vibration signal(acoustic emission signal)into an electrical signal.Unlike piezoelectric and piezoresistive sensors,the sensitive element of a capacitive acoustic emission sensor does not depend on the material properties,but mainly on the design of the sensitive structure.The capacitive acoustic emission sensor consists of an array of sensing microelements,and the characteristics of the sensing microelements determine the characteristics of the sensor: the thin-film capacitive sensing structure is used to collect off-plane information in the acoustic emission signal,and the comb-tooth capacitive sensing structure is used to collect inplane information;the sensing microelements have the same or similar sensitive frequencies,and the sensor is resonant,and the sensitive frequencies and number of sensing microelements are selected and arranged in a wide frequency range.If the sensitive frequencies and the number of sensing elements are selected and arranged in a wide frequency range,the sensor is a wide-band type.In this paper,two types of comb-tooth microelements for in-plane sensing are established for the current research status of comb-tooth capacitive sensing structures and the application requirements of resonant sensors,and the related working mechanism and optimization theory are explored to design a new structure for in-plane sensing microelements that can effectively reduce the damping of the squeeze film,in which the main research contents of the paper are as follows:(1)For the sensing of in-plane information component of acoustic emission signal,two types of comb-tooth sensing microelements of variable pitch type and variable area type are designed respectively.The working principle and basic characteristics of the two types of sensing microelements in capacitive in-plane acoustic emission sensor are studied by establishing analytical models,including capacitance change mechanism model,microspring calculation model,air damping model and response current model,and the relationship between structural parameters and basic characteristics,such as characteristic frequency,elasticity coefficient,squeeze film damping,slip film damping,response displacement and current,are established to provide theoretical basis and design ideas for subsequent finite element geometry modeling and multiphysics field simulation calculation.(2)Finite element simulation models of variable-area and variable-spacing sensing microelements are established by giving specific structural and material parameters;the operating characteristics of both are analyzed separately: firstly,the eigenmode separation of the microelements is evaluated by eigenfrequency and eigenmode analysis,the displacement response obtained by exciting the frequency domain signal in different directions,the excitation mode of various vibration modes is determined,and the sensing microelements’ In addition,the accuracy of the acoustoelectric conversion is evaluated by analyzing the specific vibration behavior of the fundamental frequency modes in the frequency and time domains.In addition,the accuracy of the acousto-electric conversion is evaluated by analyzing the specific vibration behavior of the fundamental frequency modes in the frequency and time domains.In other words,a sensing structure with similar performance to the current type of sensing micro-element and a detailed performance validation simulation model are designed,and a detailed validation scheme and performance comparison are provided for the subsequent structural optimization of the variable area structure for the eigen-mode separation problem and a new optimized design for the poor acousto-electric conversion of the two types of sensing micro-element.(3)To address the problem of eigenmode separation of variable-area sensing microelements and the complex vibration behavior of fundamental frequency modes,specific optimization parameters are identified by capturing the common characteristics of its first three orders of eigenmodes,i.e.,the existence of certain torsional behavior of all three modes: the torsional stiffness of the microspring and the rotational inertia of the mass block containing the fork fingers.The eigenmode separation capability and singularized fundamental frequency mode vibration behavior are improved by increasing the polar moment of inertia of the microspring and shortening the length of the fork finger along the sensitive direction;additional discussion of the influence of the structural parameters of the mass block on the performance of the variable-area sensing microelements provides guidance for the design and layout of microelements in resonant capacitive in-plane acoustic emission sensors.(4)A novel design of fixed electrodes with etched grooves is proposed in conjunction with the mechanism of squeeze film damping generation to improve the acousto-electric conversion capability of variable pitch and variable area sensing microelements by reducing the squeeze film damping.The effectiveness of the new design in reducing the compression film damping and improving the electro-acoustic conversion capability of the acoustic emission sensor is verified by finite element multi-physics field simulations;in addition,the stability of the new structure is evaluated.New ideas and methods are provided for the design of in-plane sensing microelements.The research in this paper effectively solves the eigenmode separation problem of variablearea acoustic emission sensing microelements,and reduces the squeeze film damping existing in the operating state of two types of in-plane acoustic emission sensing microelements through a new electrode design,which improves the acoustic-electric conversion capability of microelements and provides new ideas and methods for the further design and development of capacitive in-plane sensors. |
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