Study On A Passive DC Current Sensing Mechanism With Inverted Microstrip Antenna Structure

With the rapid development of power electronic systems,the proportion of DC loads in society is increasing year by year,such as laptop computers,cell phones,LED lights and monitors.The rapid development and popularity of new energy vehicles has exacerbated this trend.Compared with AC distribution grid,DC distribution grid has the advantages of higher transmission efficiency,easier to fulfill DC loads.And the rise of DC distribution grid is an inevitable trend in the future.Ubiquitous power Internet of Things（UPIo T）is an intelligent service system formed around the power system based on information technology.And its construction task is particularly urgent.A wireless passive DC sensor is especially important for the construction of the UPIo T.Its feature of wireless transmission can greatly reduce the design and construction burden of the UPIo T.And its feature of passive sensing（no power supply）can reduce the maintenance cost of the UPIo T.For the construction of the UPIo T for the wireless passive DC sensor,a passive DC sensor is developed based on the structure of inverted microstrip antenna in this paper.Based on the wireless interrogation technology of microstrip antenna,the sensor in this paper can realize the passive detection and wireless transmission of DC current.In this paper,based on the variable resonant cavity of the inverted microstrip antenna,a novel sensing mechanism is proposed.And the validity of the sensing mechanism is initially verified by simulation.A current sensor is designed based on the structure of an inverted microstrip antenna.Where the sensing unit consists of a permanent magnet,a cantilever（ground plane）,two connectors,a radiation patch and a dielectric substrate.Among them,the cantilever assumes two functions at the same time,it forms the current sensing component in the sensing unit together with the permanent magnet;it also serves as the ground plane and forms the signal transmission component,the inverted microstrip antenna,with the radiation patch and the dielectric substrate.Therefore,the sensor studied in this paper is equipped with both current sensing and wireless signal transmission functions.In this paper,the magnetic field and magnetic field gradient distribution around the two-core electrical wire are analyzed through theory and simulation.Based on the comparison of the magnetic forces on the permanent magnet when the magnetization direction is along both the x-axis and y-axis,it is concluded that the cantilever deforms more when the magnetization direction is along the y-axis.The dielectric distribution of the inverted microstrip antenna is studied by using conformal mapping to convert the dielectric layer in the free-space coordinate plane into a dielectric model in the flux-potential coordinate plane.And based on this,the filling fractions of each dielectric are derived,and they are further modified.And,corrects the problem that the theoretical value of the filled fraction does not agree with the actual value in the limit case.Based on the approximate arrangement of the dielectric,a theoretical model of the effective dielectric constant is derived and used for the calculation of resonant frequency.Furthermore,the resonant frequency theoretical model derived based on a new radiation patch length correction scheme proposed in this paper is more accurate for the resonant frequency calculation of inverted microstrip antenna.According to the new resonant frequency theory model,the sensing theory applicable to the new sensing mechanism of inverted microstrip antenna is derived in this paper.And the current sensing theory is established.In order to optimize the sensor design parameters,the effects of the dielectric substrate material and design parameters of the sensor on the sensor performance（sensitivity and linearity）are investigated in this paper by theory and simulation,respectively.According to the results,it can be obtained that the relative permittivity of the dielectric substrate material has a large effect on the sensitivity,and the theoretical results and simulation results show good agreement.Based on the comparison of simulation and theory,it is clear that the impedance matching of the sensor also affects the sensor performance.In addition,the design parameters of the sensor are parametrically scanned by simulation.And their effects on resonant frequency and impedance matching are analyzed to optimize the design parameters.In order to verify the correctness of the current sensing theory,the experiment is divided into two parts in this paper.The results of experiment I demonstrate a linear relationship between the normalized resonant frequency shiftΔf_r/f_r and the deflection v _A（R²=0.996）.The results of experiment II demonstrate a linear relationship between the current I₀ and the deflection v _A（R²=0.994）.The correctness of the current sensing theory can be verified by combining the results of two experiments.According to the results,the sensitivity of the sensor is 967.3 ppm/A.And the relative error between experimental result and theoretical result（955.6 ppm/A）is only 1.22%.In addition,the effect of the distance between the sensor and the two wire appliance cord on the sensor performance was also experimentally investigated.In this paper,a passive DC sensor with inverted microstrip antenna structure is proposed to address the current situation that current sensors are not suitable to be used as passive wireless DC sensor nodes and the demand for wireless passive DC sensors in the UPIo T.Based on the wireless interrogation technology,the sensor can realize passive sensing of DC current and wireless signal transmission,and facilitate the process of developing passive wireless DC sensor nodes.