Medium frequency magnetic field measurement system based on Linux

1 Introduction

With the development of science and technology, embedded operating systems play an important role in more and more fields, and have become one of the symbols of product technology level. Among them, Linux is widely used in instrument measurement equipment because of its openness, multi-user, multi-tasking, good user interface, rich network functions, reliable system security and good portability. Traditional

magnetic field measurement equipment (chisler meter, gauss meter) generally has the disadvantages of low accuracy (typical measurement accuracy is 1.5%) and inconvenient operation. This paper proposes a medium-frequency magnetic field measurement system based on embedded Linux, which can not only meet the needs of current magnetic field measurement data acquisition, but also because it is embedded with the operating system Linux, it has the characteristics of good reliability and convenient upgrade, which not only improves the accuracy of magnetic field measurement, but also brings convenience to the function upgrade of the instrument. It can be applied to laboratory instruments, medical instruments, attitude control, safety detection and other fields that require magnetic field detection.

2 Hardware structure of magnetic field measurement system

The magnetic field measurement system uses ARM9 as the controller in the hardware structure, and the hardware such as signal amplification, rectification filtering, and program-controlled amplification constitute the entire magnetic field measurement system. In addition, the measurement system is equipped with USB, RS232, and Ethernet communication interfaces, and the system has strong communication capabilities and can realize network connection functions. Its hardware structure is shown in Figure 1.

The ARM9 embedded processor uses Samsung's $3C2410. The S3C2410 is a 16/32-bit RISC embedded microprocessor based on the ARM920T core, mainly for handheld devices and cost-effective, low-power applications. It uses a 5-stage pipeline and a full-performance MMU. At the same time, the chip integrates a large number of functional expansion units, such as LCD controller, I2C bus, touch screen interface, USB interface, etc. The powerful chip function simplifies the system design, not only reduces the system size, but also improves the system reliability.

USB, RS232 and Ethernet interfaces can provide different communication methods for the system, suitable for different measurement environments and conditions, and the Ethernet interface can also realize the system networking function.

In the magnetic field measurement system, the touch screen can be used to easily control the system to realize functions such as switching between different display modes, parameter setting and measurement data storage. In

the magnetic field measurement voltage signal part, the magnetic field sensor obtains a weak voltage signal, which is input into the control system after amplification and rectification.

The alarm output can realize the user's self-set alarm upper and lower limits, which is convenient for users to monitor the measurement site. [page]

3 Front-end signal processing module of magnetic field measurement system

The measurement method adopted by the front-end sensor of magnetic field measurement system is electromagnetic induction method. The electromagnetic induction method is to place the measuring coil in the alternating magnetic field. According to Faraday's law of electromagnetic induction, an induced electromotive force will be generated between the leads of the coil, and the magnitude of the induced electromotive force is proportional to the magnitude of the magnetic flux passing through the coil. The induced electromotive force e is:


The formula for the induced electromotive force e is

The structure of the front-end signal processing module of the measurement system is shown in Figure 2.

The structure of the front-end signal processing module of the measurement system

(1) In order to realize the signal input of multiple magnetic field sensors, the chip select chip 74HCl5 is used in the design to select the channel. By inputting different data combinations (00, 10, 01, 11) to the control terminals S0 and S1 of 74HCl53, the output terminals lY and 2Y can realize the selection of different input channels.

(2) In the circuit design of the entire system, the main function of the preamplifier circuit is to amplify the weak voltage signal output by the sensor, which is proportional to the magnetic field strength. According to its requirements, the designed preamplifier circuit adopts the differential amplification method, and the circuit is shown in Figure 3. It has the advantages of high common mode rejection ratio, high input impedance, low output impedance, small offset, small temperature drift, and good linearity. [page]