Development Of The Local Geomagnetic Field Offset System

In the weak magnetic field measurement, the presence of the geom-agnetic field can not be ignored.It is the premise of the work of measuri-ng weak magnetic field to offset the geomagnetic field. With the continu-ous development of sensor's technology, the magnetic field measurement techniques are rapidly changing. It has practical significance to develop geomagnetic field offset system. In this work, it had made discussion of the method to offset the geomagnetic field, and further, developed the g-eomagnetic field offset system.In this paper, it introduced the development of the geomagnetic field offset system in three aspects: the principle, the hardware design and the software design.In the principle part of the system,it had made analysis and d-iscussion of the characteristics of the centeral magnetic field of the Hel-mholtz coils. The uniformity and the identity are two important aspects of the magnetic field. Therefore, it carried out the analysis of the relative deviation from the direction and the magnitude of the magnetic field from software simulation and mathematical calculation. In the center of the cube range of Helmholtz coil (-0.1R≤x≤0.1R,-0.1R≤y≤0.1R), it ca-n be considered that the magnetic field has uniformity in direction and i-dentity in magnitude. So, geomagnetic field offset system could obtain an adjustable offset magnetic field by using Helmholtz coils.The overall structure of the system's hardware is divided into e-ight parts: the control core CPU, A/D conversion, current measure-ment, magnitude measurement, excitation power supply control circ-uit, power control circuit, LCD display, and keyboard. AT89C52 was chosen to be the contrl core for its feature-rich and simple operat-ion. In order to meet the system's measurement requirements of re- solution and data transfer capabilities, AD conversion unit used 16Bit dual-channel ADC chip MX7705. To deal with three-dimensional magnetic field signal and current signal, it designed three-dimensio-nal signal selection circuit formed by dual-channel analog switches CD4052 and coupler isolation device 6N137, TLP521.The system u-sed the sensor reset circuit to ensure the measurement resolution before measruing different magnetic field. To achieve automated te-sting and compensation, it designed a parameter storage circuit, u-sing multiple high-speed read and write nonvolatile memory X24C44.At the end of the offset process or a sudden power-down or oth-er emergency situations, the parameters were stored automatically. Thereby,it enhanced the degree of the system's automation.In the system's software design, monitoring program was desig-ned by using assembly language. In the monitoring program, it had completed system initialization, reading and writing parameter memory, data acquisition, displaying measurement results, handling the key-board interrupts and calling subroutines of functions. Monitoring pro-gram responded to the control commands from keyboard inputs. So that, it could achieve the system's various functions.The working process of offseting the geomagnetic field could bedescribed as follows: when the system begins to work, monitoring program compeles the initialization of all the functional parts and se-ts the oprating parameters. After that, it responds to the commands from the keyboard inputs to complete the security settings, magne-tic field measurements, current measurements, LCD display, the m-agnetic field intensity adjustment, parameter storage, etc.If the mag-netic field in the coils'center is not zero after measured, the syst-em will constantly adjust the magnetic field in three directions until the magnetic field is zero, and then, it has compeleted the geom-agnetic field offset work.