Feedback Type Fiber-optic Current Sensor Using Ferromagnetic Collector And BGO Crystal

Comparing with the traditional electromagnetic type current sensor,magnetic Optical Current Sensors(MOCS) have the following advantages:simple structure, good reliability,high accuracy,high sensitivity and relative immunity to electromagnetic interference,so it is attractive especially for power system applications.However,as the sensing head of MOCS,Verdet constant of magneto-optic material and the intrinsic linear birefringence are greatly sensitive to the external environmental condition(such as temperature,shock),the long-time stability of MOCS can't be sure in many atrocious environment.Accordingly,a closed-loop fiber-optic current sensor with ferromagnetic collector was designed in this paper.It is a new kind of MOCS with a ferromagnetic collector to produce a compensation coils.The negative feedback coil,introducing an opposite direction magnetic field is added to the one generated by the measured current,therefore,the primary magnetic field was weakened.In this way,the sensor operated at a linear range near zero field strength,so the frequency response and temperature stability were improved.In this paper,the following works are developed based on the thought upwards. The research of the fiber-optic current sensor is presented.The theory of the fiber-optic current sensor upon Faraday rotation is inferred,and the theory why a ferromagnetic collector can collect the magnetic field,as well as the negative feedback circuit's principle of operation is analyzed.The schematic diagram of the feedback fiber-optic current sensor with ferromagnetic collector is presented in detail. The characteristic of BGO crystal is researched and analyzed,and the crystal shows stronger magneto-optic effect and smaller temperature coefficient when it was exposed by light,which was produced by a semiconductor laser and had a working wavelength of 655nm.The sensing head of the fiber-optic current sensor was designed and made,and the working process was illustrated.The operating principle of components on every part of this sensor and the basis why it was chosen are analyzed.Among them,the ferromagnetic collector set up on the sensing head is used to wind the feedback circuit besides the effect of collect magnetic field.The signal conditioning circuit of this sensor was designed,and linearity of every electric circuit was measured as well.Three kinds of experimental methods were used to present the sensor's performance index,(measuring magnetic field produced by permanent-magnet with the open-loop fiber-optic current sensor,measuring direct current with the open-loop fiber-optic current sensor,and measuring direct current with feedback fiber-optic current sensor)" By comparing and analyzing the experimental result,we know that the performance of feedback fiber-optic current sensor is stable,and many kinds index are better than the open-loop fiber-optic current sensor.In a word,this sensor's long-time stability is good,and it is a new test for this kind of fiber-optic current sensor.