| Low-level signal measurement is a new subject which focuses on how to effectively suppress various kinds of noise and improve signal-to-noise ratio.It has been widely used in many fields such as biomedical devices,new nanomaterials,electrochemical analysis and nuclear technology.In the field of nuclear power,lowlevel current detection technology is one of the key technologies for on-line neutron measurement of self-powered energy detector,and self-powered detector is an important component for monitoring neutron flux of reactor and controlling reactor power.The dynamic range of output signal of self-powered neutron flux detector for reactor is pA~uA,which needs to match the post low-level current measurement circuit.At present,the commonly used low-level current measuring instruments have complex structure,poor applicability and difficult maintenance.It is difficult to meet the needs of nuclear power site monitoring.In cooperation with Chengdu University of Technology,China Guangzhou Nuclear Research Institute has developed a signal acquisition and processing system for self-powered detectors to achieve high-precision monitoring of reactor neutron flux.The main contents of this paper are as follows:(1)Selection of low-level current measurement methods.The basic principle,advantages and disadvantages of low-level current measurement method are analyzed.A low-delay and high-precision low-level current measurement method is selected to reduce the response time as much as possible under the premise of ensuring the measurement accuracy.(2)Design of low-level current measuring device.The hardware circuit of lowlevel galvanometer is designed by trans-resistance conversion method and the experimental prototype is made.The low-level current meter switches the range by analog switch,uses the data acquisition circuit to convert the output signal of the amplifier circuit to analog-to-digital and reduce the noise,and communicates with the host computer through the bus.(3)Research on the influencing factors of measurement accuracy.In-depth study of the factors affecting the accuracy of low-level current measurement,special shielding design and circuit compensation are used to suppress various interference factors.(4)Performance test of measuring device.Standard signal source is used to test the accuracy,precision,linearity and zero stability of the measuring device.The following achievements have been achieved in this paper:(1)Hardware circuits for low-level current measurement are developed,including low ripple power supply,transconductance amplifier and range switching circuit,and high precision analog-to-digital conversion circuit.A test signal source for low-level current signal is designed,and a special electromagnetic shielding design is carried out.(2)A high-precision data acquisition and control module based on FPGA and high-resolution analog-to-digital is developed.Through the serial debugging assistant of PC,the on-line measurement of low-level current signal is preliminarily realized.(3)Aiming at the problem of self-excited oscillation of I/V circuit at high resistance,theoretical research and circuit test are carried out.The results show that the self-excited oscillation caused by 1G?~100G? resistance feedback can be effectively eliminated by parallel connection of 10pF~200pF capacitor with high resistance feedback resistance.(4)Using standard adjustable signal source to carry out measurement,the relative accuracy,precision,linearity and zero-stability of the system are tested in detail.The results show that the relative accuracy of the system can reach 5% and the precision is less than 0.25%.The linearity of the system is better than 0.9981 and the fluctuation of the data measured by 12 hours continuous is less than 0.29 ?V. |
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