Study On A Non-invasive Resonant Current Sensor With A High Applicability

With the rapid development of wireless sensor network(WSN)related technologies,WSN has extremely high application value in the fields of Internet of vehicles,smart factories and smart power grids.The health monitoring of equipment or power transmission systems in these fields is particularly important,and the power in these fields also requires efficient energy management systems and strategies,such as improving the efficiency of electric vehicle(EV)batteries when charging and managing the power output to various power systems when discharging,as well as the power distribution management in smart factories and smart grids.Health monitoring technology,energy management technology need to acquire current signals.If a current sensor can detect the cable current in all the above scenarios and guarantee the accuracy,and then compose a wireless current sensor network and apply it to the above technology,it will be more conducive to the application of the above technology.This paper develops a high-applicability non-invasive current sensor,which can detect a variety of currents and cables.The sensor uses piezoresistive layer external circuit for output,and uses piezoelectric layer for energy acquisition.When it is applied to the radio transmission sensing node,The energy acquisition and signal output mode has more advantages than the piezoelectric layer and other signal output modes.The sensing unit is a cantilever beam structure,which is mainly composed of magnetic materials,PMMA substrate,piezoresistive layer,piezoelectric layer and single bridge module.The magnetic materials are used to obtain the space magnetic field excited by the measured cable,which causes the static deformation or bending vibration of the cantilever beam,so that the piezoresistive layer is connected to the external circuit or piezoelectric layer output voltage.The input-output relationship can be established and the information of measured current can be backderived through the above physical quantity conversion.In this paper,a theoretical model of sensing is established for different currents and cables respectively.Theoretical analysis shows that under ideal conditions,the current amplitude of the measured cable has a linear relationship with the output voltage amplitude of the piezoelectric layer and piezoresistive layer external bridge circuit.The design of sensing unit "T" cantilever beam structure,to improve the sensitivity.In order to improve the detection accuracy of the sensor,this paper analyzes the errors existing in the practical application of the non-invasive cantilever current sensor.According to its principle characteristics,the errors caused by the assumption of infinite length straight wire,position errors and nonlinear errors are quantified.It is concluded that the error caused by the assumption of infinite length straight wire can be ignored by finding a reasonable installation position.Focusing on the position error and nonlinear error,the sensing theoretical models of different cables are normalized and the error correction sensing theoretical model is established.The model includes the position error parameters and the characteristics of nonlinear vibration.The theoretical model is analyzed and the position error is solved and the nonlinear error is corrected by multi-point calibration.This method does not increase the cost,and does not require multiple cantilever beams to form an array.The solution is faster and the practical application is more simple and convenient.In order to carry out practical application of the model,the relevant parameters of the error correction sensing theoretical model were calibrated through experiments,and the outputs of sensors at different installation positions were simulated and analyzed,and the optimal installation position was determined.The position error was simulated and analyzed.The degree of freedom of position error was sorted according to the variation range of error and the degree of influence of the same range on the accuracy.It was concluded that the influence of two degrees of freedom was very small compared with the other degrees of freedom.By analyzing the vibration mode of the sensor,it is concluded that the torsional and oscillating vibration of the sensor caused by the position error and nonlinear error is very weak,that is,the sensor has a very strong ability to resist the oscillation and torsional interference.In this paper,single-phase AC,three-phase AC and DC detection experiments are carried out on the sensor.For single-phase AC current of 20 A,the sensor has an output of nearly 1.3V;for three-phase AC current of 6.56 A,the sensor has an output of nearly 0.7V;for DC current of 18 A,the sensor has an output of nearly 60 m V.At the same time,the existence of nonlinear error is found and proved and the correctness of the sensing theory is preliminarily proved.The existence of the position error is proved by the current detection experiment with the change of position parameters and the comparison with the calibrated model,and the sensitivity of the position error in different directions at different positions is different,which further explains the correctness of the sensing theoretical model.The different methods of converting the output voltage to the measured current are compared and analyzed.The minimum relative error is nearly 40% and the maximum is nearly 350% in the range of 0-20 A.Error correction was carried out by multi-point calibration method to solve the experimental results of single-phase AC and three-phase AC.The absolute relative error between the restored measured current value and the real current value was less than 9%.It is shown that most of the errors of the cantilever beam current sensor come from the synthesis error and can be corrected without sacrificing the sensitivity and range by using the correction method proposed in this paper.For the application of wireless sensor nodes,this chapter puts forward a simplified correction method according to the application scenario,analyzes the advantages of using piezoresistive layer external circuit output voltage and piezoelectric layer auxiliary energy acquisition,so as to make the application of the sensor more feasible.