Self-oscillating Fluxgate Technology For Precision Measurement Of DC High Current

As precision measurement and feedback elements for dc high current, fluxgate current transducers are widely used in the fields of industry, medical as well as precision test and measurment, and so on. Typical examples include the new energy electric vehicle, high-speed railway, smart grid, magnetic resonance imaging instrument, precision dc high current meter and precision dc high current source, etc. However, due to the foreign monopoly on the core technology, the widely used precision fluxgate current transducers are almost entirely dependent on imports. In recent years, the self-oscillating fluxgate technology is gradually being concerned due to the advantages of its simple circuit and its sensitivity being independent of the excitation frequency and magnetic core parameters, etc. This provides us an opportunity for the development of a new type of precision current tranducer with independent intellectual property rights to break through the blockade of foreign technology. Under the above background, this subject comes from the National Key Scientific Instrument and Equipment Development Projects of China under Grant 2011YQ090004 to explore new solutions for dc high current measurement based on the self-oscillating fluxgate technology. Based on new solutions, new current transducers with independent intellectual property rights will be developed to break the monopoly of the foreign for the precision fluxgate current transducer and to improve the capability of independent innovation and self-equipment of the domestic instrument. The main research contents of the dissertation are summarized as follows.(1) Based on the in-depth study of the existing average current model, a duty-cycle model, in other word, an approximate linear relation between the excitation voltage duty-cycle and the measured current of the self-oscillating fluxgate is proposed. The new model is proved respectively based on the B-H curve models of piecewise linear function and arctan function and verified by experiment. On this basis, the main influence factors and improvment measures of the linearity and stability of the average-current model and the duty-cycle model are analyzed, which provides an important reference for the optimization design of the linearity and stability of the self-oscillating fluxgate itself as a dc zero-flux detector. The proposed duty-cycle model establishes a theoretical basis for establishing a theoretical model of the induced modulation ripple of the closed-loop system, thus studying the suppression principle of the magnetic integrator for the induced modulation ripple.(2) To solve the problem that the measurement accuracy of the existing closed-loop measurement solution is difficult to improve due to the problems of not considering the linearity and stability of the self-oscillating fluxgate itself, lack of theoretical basis for system optimization design, not considering the measurement error introduced by modulation ripple, and so on, a new solution by combining the self-oscillating fluxgate technology with the magnetic integrator technology to constitute a new zero-flux closed-loop system for precision measurement of dc high current is proposed. The basic construction and working principle of the new solution are analyzed and verified by simulation. The transfer function of the ampere-turns balance control system is established based on the magnetic circuit method and a dc steady-state error model for practical engineering applications is derived, which establishes a theoretical basis for improving measurement accuracy and realizing optimization design of the system. To suppress the conductive modulation ripple caused by the demodulation circuit, suppression method s by adding a high-pass filter and simultaneously reducing the excitation current sample resistor and the excitation peak current are proposed, the effectiveness of which are verified. To suppress the induced modulation ripple casued by the transformer effect, suppression methods by using an optimized magnetic integrator and simultaneously reducing the excitation current frequency are proposed, which are respectively proved by theoretical caculation, circuit simulation and contrast experiments.(3) Based on the above achivements, as verification, a 600-A new precision current transducer is developed and its key performance is tested. The results show that, the measurement accuracy of the transducer reaches to 1.3ppm, and compared with the best result of 0.2% of the existing similar solution, three orders of magnitude are improved. The modulation ripple is decreased to 0.12 μA, and compared with the 10 μA of the existing similar solution, 83 times are decreased. Compared with the commercial precision fluxgate current transducers of the foreign, key specifications have reached or even exceeded the level of the similar products of the foreign.(4) To solve the problem that the long-term stability of the precision fluxgate current transducers cannot be evaluated under rated current due to the reason that the long-term stability of the existing constant current source cannot meet the requirement, a high stable constant current source with output 10 A and stability 9.8×10-8/24 h is developed, and based on the equivalent ampere-turns method, the long-term stability(24 hours) of the new precison current transducer under rated current is tested. The measurement results show that the long-term stability of the new precison current transducer under rated current of 600 A reaches to 1.7×10-7/24 h. To realize the high stability of the constant current source, a strict current equation of the MOSFET-based voltage controlled current source(VCCS) is derived based on the low-frequency equivalent circuit model, which establishes a theoretical basis for the selection of key components. Reasons for self-oscillation of the multi-MOSFETs-based VCCS are analyzed based on the high-frequency equivalent circuit model, and a corresponding frequency compensation method is proposed. On this basis, a voltage-drop control circuit for the regulator based on an adjustable linear voltage regulator is proposed, which solves the key problem of the output current drift caused by the fluctuation of the regulator voltage-drop of the constant current source.