Non-invasive Passive Current Sensing For Diverse Currents And Diverse Multiline Cables

With the advent of the 5G era and the Internet of Things(Io T)era,Wireless Sensor Network(WSN)is gaining interest as one of the core technologies in Io T.Since WSN enables low-cost,low-power and high-flexibility monitoring,it has great value in applications such as smart grid and smart factory.To ensure the stable operation of equipment and cables in various environments,health monitoring of equipment and cables is particularly important.And just as human pulse signals reflect the health of human body,current signals reflect the health of equipment.If current sensors for health monitoring form a huge network,users will be able to monitor the operation state of each equipment everytime and everywhere.Thus,the development of wireless current sensor nodes is necessary for Io T.In this study,a non-invasive,non-contact current sensor for passive measuring diverse multiline cables(e.g.single-wire cables,two-wire cables,three-phase four-wire cables,three-phase five-wire cables,etc.)with diverse currents(e.g.DC electric current,single-phase AC electric current,three-phase AC electric current,etc.),is proposed for desirable application of WSN in Io T.The sensor is mainly comprised of a piezoelectric cantilever,a magnet,an analyzer and a photodiode.The basic idea is to achieve a synchronous vibration of both the analyzer and the cantilever to modulate the natural light signal incident on the photodiode.In this study,the magnet,fixed at the end of the cantilever,is used to passively convert the applied current induced magnetic force into a cantilever vibration,which is further converted to a polarization angle variation via the above synchronous vibration.The natural light signal is accordingly modulated and a varied voltage,as a function of the applied current,is thus output from the photodiode.Although the piezoelectric material on the cantilever is integrated for energy harvesting,it can also convert the above cantilever vibration to an output voltage as a function of the applied current via the positive piezoelectric effect.In this way,the output voltage of the current sensor is related to the measured current.From the output voltage of the photodiode or the piezoelectric material,the amplitude of the measured current can be derived by an exact mathematical model.A theoretical model describing the relationship between the sensor’s output voltage and the measured current is developed.The model is applicable to diverse currents(e.g.DC electric current,single-phase AC electric current,three-phase AC electric current,etc.)and diverse multiline cables(e.g.two-wire cables,three-phase four-wire cables,etc.).The model also shows that the amplitude of the sensor’s output voltage(including photoelectric output and the piezoelectric output)is proportional to the amplitude of the measured current(including DC electric current and AC electric current).For further enhancing of the sensor’s sensitivity,accuracy and resolution,obtaining a more accurate data set in the following experiment,and achieving high practicality in WSN,various analytical studies are carried out to improve the sensor’s output voltage.First,the magnetic field distribution around two-wire DC current,single-phase twowire AC current,and three-phase four-wire AC current is studied.By adjusting the position of the permanent magnet,the center and the magnetization direction of the permanent magnet are placed in the direction of the maximum magnetic field gradient.Second,the horizontal and vertical magnetic forces,which are applied on the permanent magnet,are studied to decrease the measurement error of the sensor.The permanent magnet is adjusted to the optimal position,where the interference of the horizontal magnetic force is the lowest.A prototype is fabricated for experimental verification of the proposed methodology.Two-wire DC electric current and three-phase four-wire AC electric current are used to verify the feasibility of the proposed methodology.In the experiments,the measured current can be adjusted by the power supply or the load to obtain the performance of the sensor under different conditions,and the output voltage of the photodiode and the piezoelectric material is then recorded in the oscilloscope.Results show that at each position around the cable,the amplitude of the prototype’s output voltage always has a linear relationship with the amplitude of the measured current.The closer the distance between the magnet and the cable,the higher the sensitivity of the prototype.For example,in the measurement of three-phase four-wire unbalanced current,when the distance between the magnet and the cable is 1 mm,the sensor has the highest sensitivity,which is 31.15 m V/A.Some discussions on the theoretical model and the experimental results are carried out to further improve the practicality of the proposed current sensor.First,the decoupling and reconstructing of the multiphase current is studied for sensing and monitoring of unbalanced multiphase systems,including a theoretical model and a preliminary scheme using a cantilever array.Second,several ways to improve the performance of the sensor are proposed,including decreasing the measurement error by calibration and adjusting the sensor’s linear range by changing a parameter in the calibration model.Third,the interference of harmonic currents in the experiment is discussed,and the influence of harmonic currents on the accuracy of the prototype is analyzed.Fourth,the misalignment error of the sensor is discussed,and a preliminary scheme is proposed to decrease the misalignment error by calibration.Finally,a schematic consideration for Io T applications is discussed.The influence of illumination variation,noise and harmonic currents on current monitoring is analyzed,to improve the feasibility of this consideration.