Wireless sensor network in intelligent transportation system applications

Intelligent transportation systems (ITS) for urban transit primarily microscopic traffic information acquisition, traffic control and guidance, etc., by increasing the effective use of traffic information and management to improve the efficiency of transport systems, mainly from information gathering input, policy implementation, control, output subsystems data transfer and communication and other subsystems.

Information acquisition subsystem through the sensor acquisition vehicles and road information policy control subsystem according to the targets set (such as traffic volume maximum, or average waiting time, etc.) using the calculation method (for example, fuzzy control, genetic algorithms, etc) to figure out the best solution, and output control signals to the subsystem (normally a traffic signal controller) to guide and control the movement of the vehicle reaches the preset goals.

Wireless sensor network is a fusion of short-range wireless communication technologies, micro-electronic sensors, new technology for embedded systems, gradually being used for intelligent transportation systems require data collection and testing of related areas.

Based on the IEEE 802.15.4 ZigBee technology specifications, have good characteristics: ① power consumption low, 5 2 section ordinary batteries can support a node work 6 ~ 24 months; ② networking ability, the network can be up to a node, and support for the tree, Star, mesh, and other network; ③ transmission distance, the two nodes outdoor transmission distance of up to several hundred meters, while increasing the transmitter power can be up to several kilometers; ④ high reliability, with multi-level security mode; ⑤ low cost, open ZigBee protocol stack of simplified, working in the license-exempt 2.4GHz ISM band.

Wireless sensor network with excellent characteristics for intelligent transportation systems information collection to provide an effective means of monitoring crossing all directions of traffic, according to the monitoring results, improved simplification, improved signal control algorithm, traffic efficiency.

Wireless sensor networks can be applied to the implementation of control subsystem in the subsystem and boot subsystem. For example, you can apply this technique to improve signal controller that implements Intelligent public transport systems for transit priority function.

Used in ITS construction of wireless sensor networks

As shown in Figure 1, in wireless sensor networks, installing road on both sides of the aggregation nodes form a self-organization of multi-hop mesh Mesh based network architecture, traffic information collection dedicated sensor terminal node and each adjacent pool nodes star network address, the final data will be gathered to the gateway.

Gateway points can be used as a module in the intersection of traffic signal controller, via the signal controller proprietary network, will the collected data is sent to the traffic control centre for further processing.

In wireless sensor network deployment, nodes can be installed on the side of the column, a dash and other transportation facilities, network key points that can be integrated and cross-road traffic signal controller, special sensor terminal node to landfill in pavement or installed on a roadside in moving vehicles on the road or you can install the sensor nodes dynamically join a sensor network.

　　

　　

Figure 1 is used for intelligent traffic information acquisition of wireless sensor network architecture

Use of wireless sensor network traffic information acquisition

In the traffic information collection, Terminal node can adopt non-contact magnetic sensor to regularly collect and sensing range vehicles of speed, distance, and other information.

When the vehicle enters the sensor monitoring range, the terminal nodes through magnetic sensor to collect vehicle speed, and other important information, and information to the next scheduled wake up node. The next node induction to the vehicle, the combination of vehicles in the two sensor nodes of travel time estimates, we can estimate the average speed of vehicles. Multiple terminal node is their collection and preliminary processing of information gathered through aggregation node to gateway, data fusion, access to the road traffic and vehicle information such as the exercise of the speed, thus crossing traffic signal control provides accurate input information. Through to the terminal node installation temperature and humidity, illumination, gas detection, sensor, can also be used for pavement conditions, visibility, vehicle exhaust pollution detection.

　　

　　

Figure 2 is used for traffic data collection for the deployment of wireless sensor networks

Wireless sensor network in ITS application in

Realization of the intelligent transportation system bus priority function requires an existing traffic signal controller.

Add sensor and other auxiliary equipment, traffic signal controller can estimate bus arrived at the intersection of time (travel time), to calculate the buses in junctions need to be given priority (you can select the number of passengers as a priority emphasis), and then select the appropriate priority control strategy, by adjusting split to give priority to the release of public transport vehicles. Traffic signal controller innovation include:

① car wireless communication terminal node;

② intersection traffic signal controller integrated wireless gateway;

③ used public transportation vehicle location of terminal nodes;

④ by building on ZigBee wireless sensor network can implement these features.

When to approaches intersection, on-board ZigBee wireless terminal node with transit vehicle information broadcasting, roadside deployment of wireless sensor networks for information, transit vehicle location of terminal nodes on the track together to obtain the information and wireless sensor network gateway, through the internal connection last information transmitted to the traffic signal controller, make the appropriate priority.

In ITS wireless sensor network design, network nodes according to the function, you need to design respectively.

Terminal node, node and network convergence joint point software functionality as shown in Figure 3. Terminal node installation different sensors for motion vehicle information acquisition and road information. Its functionalityImplementation in accordance with the streamlined functionality of devices (RFD, Reduced Function Device) standard. Terminal node and aggregation nodes according to the star network, at a fixed point in time to wake up from sleep and aggregation node active communications. Information routing is handed over to the parent (convergence) node and network with the routing function of Coordinator and router, reduced power consumption and the software node of complexity. Aggregation node is a terminal node software functional extensions for extended network and routing messages, allowing more focused nodes access network. May, in accordance with the full function device (FFD, Full Function Device) standards for design.

　　

　　

Figure 3 wireless sensor network node software features

Gateway is a network of the Coordinator, is responsible for starting the members of the network, configuring the network address, network maintenance, maintenance of node bindings table, etc., is also responsible for the collection of data on the initial deal and deliver the traffic signal controller transmitted to the information centre, need more storage space, computing and communications capabilities.

Network node hardware feature design

Existing more wireless sensor network solution, including all chip manufacturers introduced SCM external RF chip and integrated single-chip RF, microprocessor, etc.

The design of the node is often used for ZigBee RF chip with Atmel's AT86RF230, TI's the MC1319x CC2420, Freescale and the MRF24J40 MC1320x, Microchip, etc. In addition, chip manufacturers introduced a single-chip solution, such as TI CC2430 by extending the schema for the CC2420 chip, in a single chip to integrate ZigBee RF front-end, memory, and the micro-controller; Freescale MC1321x/MC1322x and Jennic's JN5121/JN513x-chip solution, etc.

● Based on Atmel's AT86RF230 radio frequency chip and AVR MCU design

A typical Terminal node and aggregation node design shown in Figure 4, using the Atmel 8-bit RISC architecture and low-power MCU as system control ATMegal1281V core.

With 512 KB of AT45DB041D as an external program memory. RF module with Atmel's support for ZigBee Protocol AT86RF230, RF power reached 3dBm, outdoor transmission distance of up to 300 meters above the node extension interface can be connected to the analog input and digital I/O, SPI and I2C, UART interface, these extended interfaces make it easy and sensors and other peripherals connected, such as the external luminosity, temperature, pressure, sound, geomagnetic and acceleration sensor.

　　

　　

Figure 4 sensor node design

● Based on TI's CC2420 chip and arm design

In the design of a wireless sensor network gateway, you need strong data processing capacity to implement complex routing protocols and information processing, etc.

As shown in Figure 5 the Crossbow imote2 node uses the Marvell PXA271 high-performance, low-power processor. This processor uses dynamic voltage scaling technology, frequency range 13MHz ~ 416MHz, can work at a low voltage (0.85V) low frequency (13MHz) mode, with excellent dynamic power management technology. In addition, the processor package integrated three chip 256KB 32MB FLASH and SRAM, SDRAM, 32MB reduced volume. By providing multiple I/O, the flexibility of supporting different types of sensors. The processor also supports an MMX coprocessor, multimedia processing power can be used for wireless multimedia sensor network for voice and image processing. Use the TI Imote2 CC2420 ZigBee RF chip supporting 2.4GHz, 16 channel 250kb/s data transfer, sending power-24 ~ 0dBm. Effective communication distance is 30 m, SMA interface external antenna to increase transmission distance.

　　

　　

Figure 5 Imote2 system structure

● Node design other considerations

In intelligent transportation system dedicated wireless sensor network node needs to consider at design time as follows:

① nodes low power design.

Terminal node is battery (available solar battery) power.

② low cost to the node.

In a massive traffic information acquisition, deployment, the node will be the key to the project costs.

③ node's data processing and storage capacity.

Some nodes require high speed data acquisition and run recognition algorithm, so the need for data processing. You also need to consider in the limited space within stored procedures, data, and support code online updates, and other functions.

④ In addition, according to the needs of different applications, wireless sensor nodes to have different sensor interfaces, can add a different sensor.

Where energy consumption management should as a priority.

In particular, using a 32-bit ARM processor external RF chip solution, you need to effectively reduce energy consumption, the node system-level software on further improvement of energy management, such as optimizing the TinyOS or embedded Linux power management features.

Conclusion

Wireless sensor network technology and research to get more attention.

This article is a combination of intelligent transport systems applicationsTo discuss the design of wireless sensor networks. As the technology development and maturity, wireless sensor network technologies in intelligent transportation system more crucial occasions are applied, such as electronic toll collection, traffic safety and automatic driving, parking management, traffic guidance systems, etc., to further promote the development of intelligent transport systems.