Design Of Magnetometer Based On Magnetic Resistance

The applications of precision measurement technology of magnetic field particularly weak magnetic field (such as the earth magnetic field), have stretched into industry, agriculture, national defense, as well as biology, iatrology, aerospace, space research, and other departments. Various magnetometers have emerged, mainly are the fluxgate magnetometer, proton precession magnetometer, optically pumped magnetometer, the superconducting magnetometer, and in the research and testing phase of the solid electron spin resonance magnetometer, atomic magnetometer. Although the accuracy of these magnetometer relatively high, but they are high cost, large size, complex composition, poor reliability, unable to meet low-cost, low-power, miniaturized and the need for a simple and reliable design. Those magnetometer are generally one-dimensional magnetometer, if we carry out the three-dimensional magnetic field measurement, needs at least three (groups) of the probe, leading to the formation of more complex, larger, higher price.As semiconductor technology develops rapidly, anisotropy magnetic resistance (AMR) sensor came into being. The sensor is small in size, low power consumption, high sensitivity, high reliability, strong anti-jamming capability, temperature stability, and strong resistance to harsh environment, the wide frequency bandwidth, easy in adaption to digital circuit and easy to install. We design and product a magnetometer based on the 3-D AMR sensor.In the hardware design, using AMR HMC1001/1002 to form a triaxial magnetoresistive sensor for measuring magnetic field strength, its output has good linearity within±0.2mT, and measurement bandwidth can be up to 5 MHz. Biaxial angle inclinometer SCA100T is used to measure the magnetometer's roll angle and pitch angle in order to achieve the magnetometer coordinate transformation and functional expansion. Take into account such as system's requirements of accuracy and precision, a 16-bitsΣ-ΔA/D converter, AD7798, is chosen to convert the output voltage of AMRs. Another AD7798 is chosen to convert the output voltage of angle sensor SCA100T. Micro Control Unit (MCU) MSP430F149 is used in this design, which has a 16 R1SC structure. With the 16 CPU registers and the constant generator, the code can be made to the highest efficiency. And it also integrates the JTAG interface, which can directly program the Flash memory. A liquid crystal display and a keyboard are used as Human Computer Interface (HCI) to achieve the magnetometer's settings and operation. Measurement results are kept in the expanded FLASH memory. A USB chip is selected to communicate with PC to transmit the measurement results. The software in PC can get the measurement results into an Excel table directly, easy to deal with them. Furthermore, in order to meet the power supply voltage requirements of different modules, specifically power module is designed to output 3.3 V, 5V,±12V.In software design, the single-chip microcomputer's software is a design of the multi-task which based onμC/OS-II, a Real-Time Operating System (RTOS).μC/OS-II is a source code open, ROM-based operating, reductable, preemptive real-time multi-task kernel with a high degree of portability, high reliability, robustness and security, particularly suited to the microprocessor and microcontrollers. It can perform as well as many commercial real-time operating systems.μC/OS-II is the first background process program that the MCU runs after it stared. It works as the main management control procedure to manage the resources of the system and schedule various tasks. And user's application that run on it are called task. In accordance with the needs of the various tasks,μC/OS-II undertakes the task such as resources management, task scheduling, information management, exception handling and so on. As each task has a unique priority, therefore, according to the priority and state of each task,μC/OS-II dynamically switchs between the various tasks, in order to ensure real-time. Independent preparation of the various tasks, inter-task communication and synchronization mechanisms which ensure coordination between the mission, reduce the complexity of the procedures the workload of the program design to a large extent, but also reduces the possibility of error, make procedures more reliable, easier maintain. PC software design using MFC programming under Visual C++6.0 and the measurement results stored in the FLASH can be transmitted into an Excel spreadsheet through COM interface.The system overall test shows that the magnetometer can complete the magnetic field acquisition and get high precision measurements through software processing such as error compensation. Besides, it also shows that the magnetometer can store, display, transmit the measurement results and with the feature of simple composition, low power, low cost, small size, safe and reliable.So we can say the design has achieved the expected objective.