Research On Influence Of Elastic Layer On Performance Of Electret Based Capacitive Microvibration Sensor

Vibration phenomenon is happened in the running of various instruments and equipments generally and inevitably.Vibration detection and analysis measurement is becoming more and more important with the increasing requirements on the accuracy,stability,safety of instruments and equipment.So vibration sensor is widely concerned because it is the key of vibration detection and analysis measurement.There are many methods for vibration detection and analysis measurement.Capacitive vibration measurement is most widely used.However,the weak output signal of capacitive vibration sensor limits its development in the field of vibration measurement.Recent studies have shown that the introduction of electret in structure of capacitive vibration sensor is an effective way to solve these problems.The stable electrostatic field of electret material generates charge on the capacitor electrodes by electrostatic induction and constructs a variable capacitor.It can be used in vibration detection and analysis measurement to improve the sensitivity of capacitive vibration sensor effectively.Thus,the weaker vibration signal can be measured.On the other hand,electronic devices are developing toward miniaturization and non-source.The use of electret material makes the capacitive vibration sensor get rid of the dependence on power supply.The sensor can be used in some special,complex,high risk and dangerous environments where it is difficult to change the power.So it is meaningful to introduce electret into capacitive vibration sensor.In this thesis,a capacitive micro vibration sensor based on bipolar electret material is designed and fabricated on the basis of the previous research of micro vibration energy harvester.Its variable capacitance structure is composed of "metal electrode/elastic layer/electret/elastic layer/metal electrode".Induced by electret electric field,induced charge will be generated in metal electrode.Under the excitation of external vibration,the thickness of elastic layer changes.The change of the thickness causes the change of the induced charge on the metal electrode.The current or voltage will change on the external circuit By measuring the output current or voltage,the corresponding vibration information can be obtained.Because of the introduction of bipolar electret,there are two variable capacitance structures in this kind of sensor.This kind of sensor can output stronger signal and improve sensitivity.According to the basic formula of capacitance:C=ε0εrS/d,increasing the relative permittivity of elastic layer in capacitor structure can effectively enhance its output performance.In order to achieve this goal,we propose to use polydimethylsiloxane(PDMS)film as elastic layer material,and improve the dielectric constant of PDMS film by doping carbon nanotubes(CNT).And using CNT@PDMS elastic layer to enhance the output performance of the sensor.The work and achievements of this thesis are as follows:1.In this thesis,the theoretical analysis of capacitive micro vibration sensor based on electret is carried out,and the physical mechanism of the sensor converting mechanical signal into electrical signal is clarified.Firstly,the vibration is composed of free damping vibration caused and constant amplitude vibration caused by forced vibration.It is confirmed that the existence of free damping vibration leads to the error between the output frequency and the excitation frequency,and the method to reduce the error is given in theory.Secondly,this thesis analyzes the changes of capacitance,voltage and charge in the dynamic and static electric fields,and deduces that the output of sinusoidal excitation will also output in the form of sinusoidal.The output amplitude function A is given,and the influence of each physical parameter in the sensor on the amplitude is determined.The principle of electrostatic induction introduced by electret to improve the output performance,such as the positive correlation between the amplitude and the surface potential and the relative permittivity of electret is expounded.2.A capacitive micro vibration sensor with CNT@PDMS dielectric elastic layer and electret material providing electrostatic induction was designed.The structure of the device is optimized to reduce the influence of air layer on the output performance of the device.A variety of sensors with an area of 3-20 cm2,a thickness of 50-500μm and a CNT content of 0-2wt%were fabricated in the experiment.3.The influence of CNT doping on the properties of PDMS was studied,including the influence of relative dielectric constant,elastic modulus and volume resistivity.When the mass fraction of CNT increases from 0 to 2.0,the relative permittivity of PDMS increases from 2.4 to 40.0.And the elastic modulus was kept at 1.3 MPa.The volume resistivity remains at a high level,reaching 1.2MΩ/m.The dielectric constant is increased while the elastic modulus and bulk resistivity are not affected.At the same time,the reason why CNT affects the dielectric properties of PDMS is analyzed from the perspective of percolation theory.in this thesis.According to the experimental results,the parameters in the percolation formula were determined,and the dielectric constant of CNT@PDMS elastic layer was accurately controlled.4 In this thesis,a set of preparation technique of CNT@PDMS elastic layer with high flatness is proposed to optimize the output performance of micro vibration sensor.Taking 300 μm thick elastic layer as an example,the difference between the thickest part and the thinnest part is about 20 μm.5 The output performance of the device is measured in this thesis.The influence of the relative dielectric constant(εg)of the elastic layer,the elastic modulus(k),the area(S)and the thickness(Lg)of the elastic layer on the output performance is obtained by experiments,and the correctness of the theory is verified.Combined with the output amplitude and frequency,the range of each physical parameter is determined when the sensor is in the optimal performance.If the cumulative error of output frequency is controlled below 4%,The main physical parameters should be controlled in the following range:the mass fraction of CNT should be greater than 1.5 wt%,the area of elastic layer should be greater than 12cm2,the thickness of elastic layer should be about 200 μm.These results verify the correctness of the theory proposed in conclusion 1 too.