Design of railway track alarm information collection system based on magnetic field effect

1 Introduction

With the demand for railway transportation in social and economic life and the continuous development of science and technology, the capacity of railway transportation is increasing, and the requirements for railway transportation safety are also higher. However, with the implementation of the five-speed increase of the whole railway, the safety hazards of crossings have become more and more prominent, and have become the bottleneck of railway transportation safety and railway transportation capacity. Crossing monitoring and alarm devices have a very positive significance in reducing the accident rate of crossings and protecting the safety of crossings. However, from the current research results and practical applications, most of them adopt track circuit type and mechanical type, and a small part adopts Doppler radar type and acoustic receiving type. Each has its own advantages and disadvantages in performance, and the cost and volume are also different.

This paper uses magnetoresistive sensors and designs a railway track alarm information collection device based on the magnetic field effect, and gives the design method of its software and hardware.

2 System working principle

The phenomenon that the resistance value of a current-carrying conductor changes in a magnetic field is called magnetoresistance effect. For iron, cobalt, nickel and their alloys, if these metals are made into thin film strip conductors, when current passes through, their resistance changes. The magnitude of the change varies depending on the relative relationship between the combined magnetization direction of the internal and external magnetic fields and the direction of the current flow. If they tend to be in the same direction, the resistance increases; otherwise, it decreases [1]. As shown in Figure 1, four permalloys form a wheatstone bridge, and the change in resistance converts the external magnetic induction intensity into a differential voltage output [2].

Large ferromagnetic objects, such as trains, can be seen as a model composed of multiple north and south pole magnets. When a train passes by, it will cause disturbances in the geomagnetic field, and its combined effect is to cause distortion and distortion of the geomagnetic field lines. When the sensor is in this changing magnetic field, it can be seen from the magnetoresistance effect that a changing voltage will be generated at the differential output end of the sensor, and this is the theoretical basis for this system to detect trains.

Figure 1 Schematic diagram of magnetoresistive sensor

3 System Hardware Design

The hardware part of the railway track alarm information collection system is mainly composed of a data acquisition module and a data processing module. The data acquisition module is responsible for the collection of magnetic field signals, and its main core is the magnetoresistive sensor. When the train approaches the magnetoresistive sensor, the sensor will collect the magnetic field change signal, and after signal amplification and A/D conversion, it will be converted into a discrete digital signal and transmitted to the microcontroller (single-chip microcomputer). The main component of the data processing module is the single-chip microcomputer. The single-chip microcomputer is responsible for the timing control of each chip. At the same time, in order to improve the anti-interference ability of the system, the collected data must be filtered before it can be sent to the serial port and read by the communication device. The hardware design block diagram of this system is shown in Figure 2.

Figure 2 System hardware design block diagram

3.1 Data acquisition module

In this system, the magnetoresistive sensor used is the HMC1051Z single-axis magnetoresistive sensor produced by Honeywell. HMC1051Z has a wide angle range, a resolution of <0.07° within ±45°, a sensitivity of 1.0mV/V/Gauss, and a full-scale output of 120mV when powered by a 5V power supply. There are no moving parts inside, the inherent impedance is small, the anti-electromagnetic noise and interference ability is strong, and the built-in set/reset belt can reduce the temperature drift, nonlinear error and the impact on the output signal in a high magnetic field environment. The on-chip bias circuit can eliminate the influence of magnetic field distortion [3].

The LM358 op amp, with two 4.99kω, two 1.00mω resistors and a 150pF capacitor, can form an amplifier circuit with a low-pass filter. Its gain is 200 and the bandwidth is about 1kHz, which can achieve the amplification and hardware filtering of the sensor output signal.

The A/D converter uses an ADC0804 analog-to-digital converter with an 8-bit resolution to perform analog-to-digital conversion on the amplified signal.

3.2 Data processing module

Atmel's AT89C51 microcontroller is compatible with the instruction system and pins of MCS-51, and comes with a 4KB E2ROM. However, in order to meet the needs of future upgrades and functional expansion, the memory of AT89C51 is expanded here. The program memory AT28C64 and the data memory HM6264LP-70 are both 8KB in capacity, and P0.0~P0.7 and P2.0~P2.4 are used to provide 13-bit addresses, and 74LS373 latches its lower 8-bit address. P2.5 and P2.6 are used to line select the two memories.

Since the signal needs to be sent to the DB-9 serial port connector, MAX232 is used here to convert between TTL and RS-232 levels. Connect the T1in pin of MAX232 to the serial transmission pin TXD of AT89C51, and R1out to the serial reception pin RXD of AT89C51; the corresponding R1in and T1out are connected to the corresponding RXD (pin 2) and TXD (pin 3) of the 9-pin serial port connector (DB-9).

4 System software design

The software design part of this system is mainly the design of the A/D conversion subroutine and the data processing subroutine, and is developed using the Keil μVision2 development tool.

4.1 A/D conversion subroutine

The flow chart of the A/D conversion subroutine is shown in Figure 3. First, set (the design end has been grounded) to a low level, start ADC0804, query interrupt 0, and the analog-to-digital conversion is completed 100μs after the rising edge, and the result is stored in the data latch. When it is low, the data signal is sent to the P1 port.

Figure 3 A/D conversion subroutine flow chart

4.2 Data processing subroutine

The data processing subroutine of this system is to perform anti-interference processing on the collected data to improve the stability and reliability of the system. The flow chart is shown in Figure 4.

Figure 4 Data processing subroutine flow chart

The data processing subroutine uses a constant threshold combined with a dynamic base value algorithm to achieve the system's anti-interference ability, that is, the most recent collected data is subtracted from the base value, and the subtracted value is judged. If it is greater than the threshold, it is recorded. When a certain number of records are recorded, it can be determined that the train has arrived; the base value can be updated in real time according to the changes in the surrounding magnetic field. The program written according to this algorithm can achieve the purpose of system self-adaptation on the one hand, and on the other hand, it can freely set the anti-interference processing level (threshold size and base value sampling times) to facilitate different types of needs.

Using a magnet and a radio as the target objects, respectively, under the operating conditions of moving away from and approaching the system, observe and record the voltage effective value of the differential output terminal of the magnetoresistive sensor and the voltage value after the LM358 operational amplifier amplifies it 200 times. The two groups of experiments were conducted ten times respectively, and each time the target object cuts in at different angles and positions when moving away from and approaching the system. The experimental results are shown in Tables 1 and 2.

According to the experimental results, the magnetic field strength of the object and the position of the object relative to the sensor will cause a large change in the measured value, and the change is more obvious for objects with a large magnetic field strength. This characteristic can be used to judge the arrival of a train.

5 Conclusion

This paper describes the development process of a railway track alarm information collection system based on the magnetoresistance effect from the perspective of engineering application. The system has the advantages of low power consumption, low cost and stable performance. The system can be connected to wireless devices such as GPRS to form a remote alarm system, so it has certain practical value.