How to design the overspeed monitoring system of expressway vehicles?

The embedded network camera system designed in this paper is a real-time video network transmission system based on the Internet. The main design goal is to realize the compression and coding of real-time video information collected by remote terminal, and realize TCP/IP protocol by using embedded system module, and transmit it through the Internet, so that the receiving terminal system can receive real-time video from the network and show it to users again.

The hardware platform of the whole system is mainly composed of embedded processor module, video acquisition module, video AD conversion chip, video compression module, power clock module, Ethernet interface chip, Flash, SDRAM and so on.

1. 1 The embedded controller module of the embedded controller is the control and management core of the whole system, and its main function is to realize the control and scheduling management of the whole system. The main control chip we choose is S3C25 10A, which is a high performance-to-price ratio 16/32-bit RISC microcontroller based on Ethernet application system produced by Samsung, and contains a low power consumption and high performance ARM940T core designed by ARM. Built-in two 10/ 100Mbps Ethernet controllers; 6 DMA channels; 6 32-bit timers; 64 programmable I/O ports; 2 high-speed UART；; In addition, there are interrupt controller, DRAM/SDRAM controller, ROM/SRAM and FLASH controller, system bus arbiter and external memory controller.

1.2 video capture module video capture is realized by the front-end high-speed camera. This system adopts dis-822C integrated camera of discover series, which is a special color camera for monitoring. It has 22 times optical zoom and 220 times digital zoom, can work normally in low illumination, has strong autofocus ability, and also has the characteristics of electronic automatic shutter and white light balance. The high-speed camera sends the collected analog data to the video A/D conversion chip for video decoding. Finally, the obtained video digital signal is input into the video compression chip.

1.3 video compression module This module uses a special audio and video compression coding chip G07007SB to complete the audio and video compression function. G07007SB is a multi-format audio and video compression coding chip newly launched by WIS Company of the United States. G07007SB's unique encoding structure can flexibly encode audio and video conforming to ISO/ITU standards. The supported output formats include MPEG-4, MPEG-2, MPEG- 1, H.263 and user-defined formats.

1.4 video network sending module This module mainly packages the video streams processed by the audio and video compression module through S3C25 10A to generate UDP packets or RTP packets, and then sends them to the network through RJ45 interface. The network interface chip selected here is RTL820 1BL of Realtek Company, which is an Ethernet transceiver controller with l0M/ 100Mps adaptive function, and it is the most widely used network interface chip at present.

1.5 PTZ lens control module needs PTZ decoder to decode the control signals from embedded controller and remote client, and convert the control signals from software into actual action level signals.

1.6 electronic information board the electronic information board adopts LED light-emitting tubes, which can automatically adjust the brightness, and the dynamic sight distance reaches 200 meters. It can convey the speeding situation, road information and special weather conditions to drivers and passengers all the time, highlighting the service function of expressway.

2 Software design

Embedded operating system is an important part of embedded system, which provides a software platform for the development of application programs. Here we choose uClinux as the development platform of the whole system. The development resources based on uClinux are rich, which is helpful to build powerful and complex large-scale systems. Other functional modules are built on Linux operating system, including video acquisition and preprocessing module, MPEG-4-4 coding module, RTP/ RTCP protocol transmission module, Yuntai control module and so on. The following focuses on the design of video data transmission module and speed detection module.

2. 1 data transmission The MPEG-4 video transmission process based on RTP/RTCP is: on the server side, the MEPG-4 video stream is encapsulated with RTP header, TCP header and IP header respectively to form corresponding IP packets, and then the IP packets are sent to the receiving end through the Internet. After receiving the IP packet, the receiving end of the client extracts the RTP header and video data in reverse order, and puts the video data into the buffer for the decoder to decode and output according to the sequence number in the RTP header.

2. 1. 1 video data encapsulation To transmit video data in the network, it is necessary to encapsulate it and add information such as time and synchronization.

2. 1.2 video data transmission Linux system realizes data transmission through socket programming. There are three types of sockets: SOCK STREAM, SOCK DGRAM and primitive sockets. According to the network transmission model of the system browser/server, the monitoring socket and the control socket are established on the server side. The request socket and control socket of socket type are established at the client, which are encapsulated and transmitted through TCP protocol.

2.2 Speed detection is the only basis to judge whether the vehicle is speeding, so how to get the speed is the core of this system. At present, the speeding standard of expressway has different regulations on the speed limit of large cars and small cars. At present, there are many algorithms to judge the vehicle type, some are based on the color of the license plate and some are based on the length of the vehicle. Here, the latter is used to identify the vehicle type.

The system uses a loop coil vehicle detector. The coil vehicle detection system consists of a grounding loop and a coil detector. It embeds two coils in the front and back of each lane, in which the front coil is called "speed measuring coil" and the rear coil is called "capture coil". For coil speed measurement, the common method is to obtain the time when the vehicle enters the two coils respectively, subtract the time it takes for the vehicle to pass through the coils, and cooperate with the width and distance of the coils to get the vehicle speed. The software programming process is as follows: ① detect the state change of coil 1 and record the vehicle entry time dwStart. (2) detect the change of the coil and whether the vehicle leaves the sign, and record the time dwEnd 1 when the vehicle leaves the coil. ③ Detect the state change of the coil 2 and record the vehicle entry time dw seconds. (4) Calculate vehicle speed dbSpeed and vehicle length dbCarLength. ⑤ According to the type and speed of the vehicle, judge whether the vehicle is speeding, and take some photos. ⑥ The car leaves the second coil and takes panoramic photos.

3 Conclusion

Expressway video monitoring system comprehensively applies advanced information technology, data communication and transmission technology, electronic control technology and computer processing technology to expressway management system, and establishes a real-time, accurate, efficient and widely-used expressway comprehensive management system. This paper designs and implements a video monitoring system based on ARM and Linux. At present, the system has been applied to the monitoring of a highway section, which has met the design requirements and achieved good application results.