Design of train collision avoidance system based on GPS/dead reckoning integrated navigation

The collision avoidance problem is an important research problem in the field of modern transportation and is closely related to people's lives. There are many studies in the fields of aviation, navigation, road and rail transportation, such as the Traffic Warning and Collision Avoidance System/Automatic Decision-making and Broadcasting System TCAS/ADS-B in the aviation field; the Automatic Identification System AIS in the navigation field; the Car-to-Car Collision Avoidance System C2C ​​in the road traffic field, etc.

Road traffic, aviation, and navigation have all had relatively mature anti-collision warning systems. In contrast, the research on anti-collision warning systems in the field of rail transportation started late and is more difficult to develop. This paper mainly focuses on the operation of low-speed freight trains under complex track geographical conditions (tunnels, forests, hills, etc.), and reasonably uses single-chip microcomputer technology, GPS satellite positioning technology, and dead reckoning navigation algorithms to design a low-cost railway train anti-collision system.

1 System Overview

The train collision warning system consists of a positioning system, a wireless communication mechanism, a decision-making system, and an alarm device. The system structure diagram is shown in Figure 1.

After the system is started, it first broadcasts its own information (real-time position, real-time speed, etc.) to the nearby area, and at the same time receives broadcast information from other trains in the area that are equipped with the system. The train can fully understand the current traffic conditions nearby through the information received from other trains. If there is a possibility of danger, it will immediately provide warnings and suggestions to the conductor to avoid collision accidents. Therefore, the anti-collision warning system has three main functions: ① Acquisition of relevant information such as position and speed; ② Broadcasting and receiving these relevant information; ③ Processing this information and detecting whether there is a risk of collision, and issuing an alarm signal if there is a risk of collision.

2 Integrated navigation system

The combined navigation system mainly provides reliable location information for the train collision warning system. The real-time positioning information of the train plays a vital role in the train control and system collision detection.

2.1 GPS Global Satellite Positioning System

The basic principle of GPS (Global Position System) positioning is to use the instantaneous position of the high-speed moving satellite as the known starting data, and use the principle of spatial distance intersection to determine the position of the point to be measured. The mature automatic block system developed on the railway has confirmed that GPS is suitable for train positioning. GPS positioning does not rely on other trackside equipment, and only a high-precision GPS terminal receiver is needed to achieve conventional positioning. However, GPS has the disadvantages of poor dynamic response capability, susceptibility to electronic interference, and easy signal obstruction. If the GPS signal cannot be restored in time for a long time, the system error will inevitably accumulate over time. When the train is running in a tunnel or forest with poor GPS signal, pure satellite positioning will not meet the system requirements.

2.2 DR dead reckoning positioning system

DR (Dead Reckoning) is a navigation and positioning method that uses the known initial position of the carrier to calculate the coordinate position of the next moment in real time according to the heading, speed and navigation time of the moving carrier at that point. It is an autonomous positioning, and its positioning accuracy will not be affected by external factors such as electromagnetic interference and occlusion. However, the dead reckoning system does not have long-term stability and error correction must be performed at regular intervals. The dead reckoning system used in this system is similar to an on-board odometer, and its structure and principle are roughly the same. It is composed of a magnetoelectric sensor and a group of magnetic sheets attached to the wheels. For each rotation of the wheel, the magnetoelectric sensor generates a certain number of pulses. By counting these pulses, the distance traveled by the train during this period can be known. The train running distance at time is

Where n(t) is the number of pulses output at time t; n(t)-n(t-1) is the number of output pulses in this cycle; M is the number of pulses that should be output per rotation of the wheel; and D is the diameter of the wheel.

According to the train running distance at time t, the running speed of the train is

In the formula, τ is the counting period. When the counting period is small, the speed can approximately describe the instantaneous speed of the train.

2.3 Integrated navigation algorithm

When the train is running in tunnels and other places where the satellite signal is poor, the system will automatically record the last valid coordinate output by the GPS. At the same time, the DR system uses the geographic coordinate system with this point as the origin of the coordinates (generally the east, north, and sky coordinate system, which satisfies the right-hand rule) as the reference coordinate system for dead reckoning, and takes this point as the starting point for its dead reckoning position. The heading sensor and the DR system can determine the position of the vehicle at each moment:

In the formula, x(t), y(t) is the position of the train in the reference coordinate system at time t; x(t-1), y(t-1) is the position of the train in the reference coordinate system at time t-1; θ is the angle between the direction finding velocity and the north direction of the reference coordinate system. Here, we make a reasonable simplification of the tunnel. In general, we consider the tunnel to be a straight tunnel. Therefore, the track can be calculated based on the valid coordinates of the last GPS output recorded by the system combined with the real-time position of the train:

Where λ(t) and L(t) are the real-time longitude and latitude of the train during the dead reckoning process; λ0 and L0 are the longitude and latitude of the starting point, respectively.

3 Hardware Design

3.1 Integrated navigation module

GPS positioning has long-term stability, but the positioning is discontinuous. The dead reckoning system has good short-term stability, but the positioning error must be corrected at regular intervals. It can be seen that GPS positioning and DR are highly complementary. This system uses a GPS/DR combined positioning system. Through DR positioning error compensation, it ensures that the train can still effectively determine its real-time position when the GPS signal is lost. The structure diagram of the combined positioning system is shown in Figure 2.

3.2 Wireless Communication Module

The communication mechanism completes the communication function between modules in the system and realizes data transmission. The mutual communication between trains is to transmit real-time position information to adjacent trains, calculate the distance to adjacent trains, and judge whether there is a possibility of collision. According to the judgment result, the corresponding warning signal is given to realize the safety warning of the train.

Wireless data transmission is the process of using radio waves as a transmission medium, encoding and modulating data information onto a carrier frequency, sending it to the RF front end, transmitting it through an antenna, or decoding and modulating the received radio wave signal to restore the original data. Wireless data transmission technology has the characteristics of not requiring preset network facilities and autonomous networking. It is mainly used to solve the communication problems of small wireless networks that are difficult to lay cables but require data collection and transmission. Since it is inconvenient to lay communication lines along the railway and the workload of laying lines is relatively large, it is more reasonable to choose wireless data transmission as the communication method between various train collision avoidance systems.

Due to the large inertia of the train, its braking distance is long, which may reach several kilometers. Therefore, there are requirements for the communication range between trains. The minimum requirement stipulated in this project is 400m, but considering the safety factor, a safe distance of at least 1km is more appropriate.

Through comprehensive comparison, the system communication module selects the YL-5001W medium-power wireless data transmission module. YL-5001W is a wireless transparent data transceiver module that uses GFSK modulation with high stability, low power consumption and high cost performance. Compared with other communication modules, this module has the characteristics of small size, high sensitivity, long transmission distance, high communication rate, automatic internal communication protocol conversion and data transceiver control. It can complete the wireless data transmission function without changing any data and protocol of the user. The working frequency is 433MHz and the serial port baud rate is 9.6kpbs, which can meet the communication requirements between systems.

3.3 Controller and other circuits

The main control chip is the STM32 series ARM chip from STMicroelectronics. This series of chips uses the Cortex-M3 core and an 8M crystal oscillator. The clock frequency can reach up to 72MHz after multiplication by a phase-locked loop. The 32-bit STM32F103RCT6 selected in this system is a low-power, high-speed single-chip microcomputer with rich peripheral resources. The chip integrates hardware interfaces such as SPI, I2C, USB and A/D.

Serial communication is adopted between modules of this system, mainly using the four serial ports of the single-chip microcomputer. Serial port one is used to download programs, serial port two communicates with the navigation system to obtain the positioning information of the train, serial port three exchanges data with the wireless communication module, and serial port four is connected to the touch screen to display the required information on the touch screen.

4 Software Design

The system software is designed using the programming environment Keil and is mainly written in C language. It consists of the main program main(), the trajectory calculation processing subroutine GuiJi-Calcaulate(), the touch screen display subroutine Show(), the interrupt processing subroutine, the communication subroutine Communication(), and the error processing subroutine Error-Processing(). All parts of the software are programmed according to the structured programming concept, which is easy to debug and maintain. The main program flow chart is shown in Figure 3.

5 System Testing

According to the system design plan, the hardware modules were connected and the prototype was assembled. In the experiment, we used an electric motorcycle to simulate a low-speed train, removed the GPS antenna to simulate the situation where the train entered a tunnel, a forest, and lost GPS lock, and conducted the experiment with the two prototypes about 500 meters apart. The system interface is shown in Figure 4. The experimental results show that when the GPS signal is good, the error is less than or equal to 10m, and when the satellite is lost, the error does not exceed 30m.

After comprehensive analysis, this system meets the project requirements and can meet the actual operation of the train.

6 Conclusion

This system makes reasonable use of GPS satellite positioning technology and dead reckoning navigation algorithm to achieve stable, reliable and low-cost combined navigation. The use of high-power radio stations greatly increases the wireless data transmission distance and the safety warning distance. The touch screen display interface is friendly and can directly obtain information such as the real-time position of the train, the source of navigation information, heading, positioning error, etc. The experimental results show that the system works stably and reliably and has certain application prospects and reference value in the railway system.