Design of bus fire rapid positioning system based on Zigbee technology

Bus fire has become a major hidden danger to public transportation safety. It threatens the safety of people's lives and property and reduces people's trust in bus safety. The prevention of bus fire is very important, and timely rescue after the fire is also extremely important. It takes a period of time from the occurrence of a fire to contacting rescue personnel. If this period can be shortened as much as possible, when the rescue personnel can arrive at the scene as soon as possible after the bus fire, it will reduce a lot of casualties and property losses. Therefore, rapid positioning and rapid alarm after the bus fire is particularly important. Based on this need, this paper proposes a bus fire rapid positioning system based on Zigbee wireless network. When the sensor located in the bus detects a fire, it can immediately transmit the signal to the fire center. At the same time, the positioning sensor also uploads the location of the bus to the fire center. The fire center can immediately arrange the fire rescue work for the bus on fire according to the location reported by the positioning system to minimize the loss of life and property. The Zigbee network has flexible and simple networking capabilities, low cost and power consumption, and can meet the requirements of networking detection and positioning on a large range of urban bus routes.

1 Overall system design

This bus fire rapid positioning system consists of a bottom-level data acquisition network, a gateway, and an upper-level management center. The bottom-level data acquisition network is responsible for collecting two types of information: fire signals and bus locations. The gateway is responsible for building, maintaining, and managing the Zigbee network, and transmitting bottom-level data to the upper-level computer.
The upper-level management computer is located in the fire center. When a fire alarm is received, the control personnel can immediately know and dispatch a fire truck, and contact medical staff for rescue.

The gateway selects the Zigbee chip CC2430 for design. CC2430 is a system-on-chip (SOC) that integrates an industrial-grade, compact, and efficient 8051 controller. The core of CC2430 is a high-performance 2.4 GHz DSSS (direct sequence spread spectrum) radio frequency transceiver. CC2430 assigns network addresses to routers and nodes in the Zigbee network and acts as a coordinator.

The bottom-level information acquisition network is mainly composed of sensor nodes and positioning nodes for detecting fires, as well as reference nodes that assist in positioning and act as routers. The fire detection node is composed of a smoke sensor and a radio frequency chip CC2430 based on the Zigbee standard. The positioning node is designed with a CC2431 chip. The CC2431 chip has a positioning engine inside. Figure 1 is a framework diagram of the bus fire positioning system. The collected on-site information is uploaded to the management center computer from bottom to top.


2 Zigbee wireless network

Zigbee has simple and flexible networking capabilities, and its power consumption and cost are superior to other wireless networks. Therefore, the Zigbee network is selected for data transmission in this design. Zig bee's wireless mesh network can realize signal propagation in a "multi-hop" manner and has self-organization and self-healing functions. The anti-interference and reliability of the Zigbee network meet the requirements, and its anti-interference performance has obvious advantages over Bluetooth and WLAN under the same circumstances. The advantages of Zigbee technology in anti-interference ability and communication reliability are due to the design of the RF physical layer and communication protocol. Experiments have shown that the bit error rate of IEEE 802.15.4/ZigBee can reach 10-9, especially when the signal-to-noise ratio is 4 dB; to achieve the same bit error rate, the signal-to-noise ratio of Bluetooth/802.15.1 must reach 16 dB, and 802.11b must reach 10 dB. It can be seen that ZigBee's anti-interference performance is significantly higher than Bluetooth and WLAN. [page]

3 Positioning principle

The Zigbee chip CC2431 has a positioning engine designed inside. The reference node in the positioning system is designed using CC2430. After the CC2431 chip is enabled, it will collect the signal strength values ​​when communicating with multiple reference nodes, and use the three parameters (A value, N value, RSSI value) values, combined with the coordinates of the corresponding reference nodes for positioning calculation. Among the three parameters, the A value is the absolute value of RSSI at 1 m away from the transmitter (CC2430/CC 2431); the N value is the absolute value of RSSI attenuation for every 1 m increase in the distance from the transmitter; RSSI is the signal strength of CC2430/CC2431, in dBm. The triangulation (trilateral) measurement method is used in the calculation.

The principle of triangulation (trilateral) measurement is to select a series of control points on the ground, connect them into several triangles, and form various net (lock)-shaped figures. By observing the inner angle (or side length) of the triangle, and then according to the coordinates of the known control points, the side length of the starting side and the coordinate azimuth, the side length and coordinate azimuth of each side of the triangle can be obtained by solving the triangle and the coordinate azimuth, and then the plane coordinates of the point to be determined are calculated by the rectangular coordinate forward calculation formula.
The calculation is performed by hardware, occupies very little CPU resources, has a fast calculation speed, and the positioning response time is less than 40μs. In order to ensure the accuracy of bus positioning, a certain number of reference points along the way should be guaranteed, and at least three reference nodes are required for one positioning.

4 System control flow

The software control flow of the system is shown in Figure 2. After the system is initialized, the system will continuously scan the status value of each bus fire signal. When the status is a fire, the system will immediately sound an alarm. On the electronic map covering the entire area of ​​the bus route, the location of the bus will be displayed with a flame symbol. At the same time, the system will send a text message to the relevant responsible personnel to inform them of the location of the bus where the fire occurred so that the rescue operation can be carried out in time.


5 Conclusion

The urban bus fire rapid positioning system in this article can shorten the response time of firefighters and medical staff to rescue the bus, which is of great significance to people's travel safety. The Zigbee network adopted is flexible and simple, with low cost and energy consumption, which is conducive to the system being put into practical application.

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