| The fluxgate magnetometer(FGM) whose operating principle is based on the non-linear characteristic of soft magnetic material core is a vector device for dc or low-frequency ac magnetic fields measurement. It is widely used in many fields for its simpleness, low cost, high resolution, etc.Most of the FGMs work in feedback mode to improve their linearity, measurement range, precision and the working stability. The principle of fluxgate sensor and construction of magnetometer are introduced and feedback magnetometer systems are emphasized in this paper. One-dimensional magnetometer can measure one component of the magnetic field. To get more information about ambient magnetic field, a three-dimensional FGM is designed, which consists of three sensors, compact spherical coils(CSC) and corresponding circuits. The sensors are orthogonal to each other, and they are located in the center of CSC where the field is homogeneous. The currents through the feedback coils are controlled by the analog PID controllers,we can get three components of the ambient magnetic field by recording the feedback currents while the outputs of the sensors are all zero. The calibration results show that its resolution comes to 60 nT, measurement range is beyond the components of geomagnetic field.The system is small, light weight and convenient to move. It is hopeful to be used in dynamic magnetic field measurement and space physical field research. It also can take active role in earthquake observation and forecast. Certainly the technology index of the system has some certain distance to meet the needs of engineering application.
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