The design and implementation of monitoring station is the focus of the whole wireless remote monitoring system, and the ability and accuracy of monitoring station to process information and data will affect the final performance of the whole system. In the whole development process, the design of monitoring station is the part with the largest workload and the longest time. The monitoring station is located in the work site and only completes data acquisition, processing and control. The task is relatively simple and fixed, and it doesn't need a big desktop to complete it. Considering energy saving and convenient deployment, monitoring stations are mostly embedded systems. According to the function of the whole wireless remote monitoring system and the requirements for data processing and sensor control ability, the complexity of monitoring station design and the specific technology adopted are different.
Design based on single chip microcomputer
Single chip microcomputer is the first choice for most embedded systems. Due to the rich peripherals and good control ability on the chip, the single chip microcomputer is naturally tailored for embedded systems and occupies the largest share in the embedded market.
The design scheme based on single chip microcomputer is generally suitable for remote monitoring system with low data processing requirements and small calculation. The microcontroller can choose a low-end 4-bit computer or 8-bit computer, such as 805 1, or a special chip with strong functions, such as MSP430FE42X series. Single chip microcomputer is mainly used for system control of monitoring station. Off-chip memory is generally RAM, EEPROM and Flash. I/O equipment is generally a man-machine interface for design and debugging, such as keyboard and LCD. Sensors are generally microphones, cameras, speakers and servo motors. The implementation of wireless communication interface is relatively complicated. Codecs are optional and are usually unnecessary for low-rate data. According to the processing task of the system and the type of information, the codec can choose different cores, such as CMX639 (for audio) or LD9320, or it can be realized by programmable logic devices. The monitoring station software can be directly implemented in C or assembly language, and the application software can also be developed on the real-time operating system. For low-grade 4-bit or 8-bit single chip microcomputer, the control ability is low and the system is simple, so the method of directly writing control program is generally adopted.
Design based on DSP
As we all know, DSP has strong digital processing ability and mature technology, and there are many general-purpose and special-purpose chips that can handle various operations. The monitoring station designed and developed with DSP as the core can complete high-speed data processing and ensure the real-time requirements of the system.
This design scheme is generally suitable for monitoring systems with large amount of data processing and calculation, high real-time requirements and relatively low control ability requirements. Different from the monitoring system based on single chip microcomputer, DSP can be used for data calculation, coding/decoding besides being a controller. Whether it is more complex encoding/decoding and compression/decompression operations (such as image and video data processing, etc. Whether it is completed by DSP should be considered comprehensively. If the burden of DSP in system control and transmission protocol implementation is too heavy, this part of the operation needs to be completed by a special processing chip; If the system control and transmission protocol is simple, or there is no upper protocol stack at all, this part of complex operation can be completed by DSP.
Design of DSP based on MCU
Obviously, this design method absorbs the advantages of single chip microcomputer and DSP: the characteristics of single chip microcomputer determine that it is good at control, and the internal structure of DSP ensures strong data processing ability. The combination of the two can realize some quite complex system functions, but because two processors are used in the system, the information interaction between them is an important issue to be considered when designing this kind of monitoring station. Single chip microcomputer and DSP can only play their respective advantages if they cooperate well; Otherwise, due to the coordination between them, a lot of resources are consumed, and the overall performance may not be higher than that of a single processor system. The common method to realize communication coordination between single chip microcomputer and DSP is to use dual-port RAM.
In order to expand the application scope of the chip, some DSP or single chip manufacturers have expanded the original chip and integrated their own characteristics, so that the same chip has good data processing and control performance at the same time. For example, dsPIC introduced by microchip is convenient for customers to transfer the functions of single chip microcomputer to DSP, and the products introduced are dsPIC30FXXX series. Because DSP and MCU are implemented on the same chip, the reliability of the system is improved, the design difficulty of the monitoring station is reduced and the printed board space is saved. This kind of chip is favored by users.
Design based on microprocessor
Another choice to design embedded products is to adopt the design method based on microprocessor. Compared with industrial control computer, embedded microprocessor has the advantages of small size, light weight, low cost and high reliability. At the same time, the technology in this field is mature, there are many types of products, and the choice space is large. Processors that meet various performance requirements are relatively easy to obtain. With the appearance of high-performance MPU with RISC architecture (such as processor chip with ARM architecture, etc.). ), MPU's position in the embedded field is enduring. However, when designing the monitoring station, the circuit board must include ROM, RAM, Flash, bus interface and various peripherals, so that the reliability of the system will be reduced, the technical confidentiality will be poor and the implementation will be more difficult.
Design and Implementation of Wireless Communication
Compared with the monitoring station, the design of wireless communication is relatively simple, and many existing products and communication systems can be used. The key point is to make the best choice from various implementation methods.
Common ways to achieve this are: using existing communication networks (GSM/GPRS, CDMA mobile networks, etc. ) and corresponding wireless communication products; Through wireless transceiver devices, such as wireless modems, wireless bridges and other private wireless local area networks; At the monitoring station, the wireless communication between the circuit board level and the monitoring center is realized by using the transceiver integrated chip.
Using network to realize wireless communication
There are many existing communication networks, and building networks according to services is the characteristic of communication networks before 3G, and wireless networks are no exception. The wireless networks that can be used for reference in the design of wireless remote monitoring system mainly include: Global Digital Mobile Phone System (GSM), General Packet Radio Service (GPRS), mobile networks adopting code division multiple access (CDMA) technology, and cellular digital packet data (CDPD) system.
GSM(Globem System for Mobile) is the most important 2G standard in the world, which can provide high communication quality with low service cost and low terminal cost. As far as its services are concerned, GSM is a mobile ISDN (Integrated Services Digital Network), which can provide a variety of services.
GPRS (General Packet Radio Service) is to add some hardware devices and software upgrades to the existing GSM network to form a new network logical entity. It is based on packet switching technology and adopts IP data network protocol, which improves the data service transmission rate of the existing GSM network, and the highest rate can reach170kb/s/s. GPRS introduces packet switching technology into the existing GSM system, integrates mobile communication and data network, and has the characteristics of "extremely fast transmission speed", "always online" and "affordable price".
CDMA (Code Division Multiple Access) network adopts spread spectrum technology and various diversity receiving methods, which makes it have the characteristics of large capacity, good communication quality, high confidentiality and strong anti-interference ability.
CDPD (Cellular Digital Data) wireless mobile data communication is based on digital packet data communication technology, with cellular mobile communication as the networking form, which is the combination of data and mobile communication. This communication mode is based on TCP/IP, and the system structure is open, providing seamless connection of the same layer network and multi-protocol network services. CDPD network has the characteristics of high speed and high data security, and can be interconnected with public wired data networks, which is very suitable for transmitting real-time, sudden and online data.
In order to make the wireless communication between the monitoring center and the monitoring station use the existing network, the specific wireless network needs the corresponding access equipment. There are ready-made products of this kind of equipment on the market. Communication module connected to GSM network is Siemens TC35i, GPRS can be accessed through Siemens MC35GPRS module, CDMA network is accessed through Huali H 1 10CDMA module and AnyData CDMA modem (DTS-800/1800), and wireless modems following CDPD mode are OmniSky and NovatelMinstrel.
Use the existing network to establish a wireless remote monitoring system, and the network connection is shown in figure 1. Among them, wireless access module products are generally equipped with RS232 as external communication interface, and some of them have built-in antennas. Using the characteristics of the existing network, such as wide coverage and roaming, the location of monitoring stations and control centers is not limited by distance; However, due to the use of public network, security will be reduced.
Using Chip to Realize Wireless Communication
One of the characteristics of the first two networking methods is that the existing network systems and products are adopted, and the wireless communication part does not need special development and is easier to realize. However, because the purchased products are independent equipment, the whole system, especially one end of the monitoring station, is complex in structure and huge in volume, which often brings disadvantages when the system is popularized, and outsourcing products will increase the cost of the system. If the functions of outsourced products can be integrated with the monitoring station and realized at the circuit board level, the above disadvantages can be avoided; But this will increase the difficulty of system development and prolong the development cycle. We must weigh the pros and cons and make the most favorable choice according to the development strength of the project team and the system life cycle.
In this way, only the wireless communication interfaces in Figures 1, 2 and 3 are needed to design the monitoring station (see the full text of the online version of this article). The hardware real-time block diagram of this part and the relationship between processor and memory are roughly shown in Figure 4. Each sub-module has a variety of chips to choose from, such as ML275 1 and RTF6900 for RF front-end, ML2722 for modulation/demodulation, LD9002DX2 and Stel-2000A for spreading and despreading.
Design implementation 2
Compared with the design and development of monitoring station, the design of control center is relatively simple and the hardware design is less. In addition to ordinary microcomputers (or workstations, industrial computers), network access devices are also needed (if self-designed modules are used to realize wireless communication, special wireless network cards must be developed and inserted into the bus slots reserved on the motherboard of microcomputers). The design and development of control center mainly focuses on the design and development of application software, which is generally based on common operating systems such as Windows and Unix. At present, there are many powerful tools to start and debug this kind of software, which brings convenience to the design of control center software.
As far as the form of software implementation is concerned, except the interface module, other functional modules can be designed as dynamic link library files (. dll)。 Man-machine interface module can be customized for the practical application of wireless remote monitoring system to meet the special requirements of users in terms of beautiful interface and convenient operation.
Using C/C language to design such system software in VC development environment involves many technologies, including memory management, network communication, multi-thread management, database programming and even ActiveX.
Wireless LAN scheme
Wireless network monitoring based on microwave spread spectrum technology and MPEG4-4 coding technology mainly adopts integrated wireless network video server and common gun/ball machine. Integrated wireless network video server integrates 2.4G/5.8Ghz wireless bridge, MPEG4-4 encoder and 18dbi high gain antenna. The integrated equipment is easy to install, and the whole installation and construction can be completed in a short time. This product is outdoor waterproof equipment with long transmission distance, strong anti-interference ability and clear image. Suitable for ports, docks, oil fields, factories, residential areas, construction sites and other areas with complex environment.
Wireless AP coverage scheme
Wireless AP coverage monitoring scheme mainly adopts wireless AP and wireless network camera. The IP network signal of the wireless network camera is transmitted to the computer of the monitoring center through the WiFi network covered by the wireless AP. The PC in the monitoring center realizes monitoring through software.
CDMA wireless scheme
The CDMA wireless video monitoring system is mainly composed of a CDMA wireless network video server and a common camera. The analog signal of the camera is converted into IP digital signal by CDMA video server, and then transmitted to the monitoring center through Unicom CDMA network. The monitoring center needs a PC and a fixed IP address. The monitoring center can control the rotation of the front camera.
Analog wireless scheme
Analog wireless video monitoring is a traditional wireless video monitoring method, which consists of an analog video transmitter and a pan-tilt control signal transmitter. It belongs to one-on-one communication.
Power carrier scheme
Power carrier video monitoring system mainly adopts power carrier technology, and the IP signal of network camera is transmitted to the receiving end through power carrier. The monitoring image is decoded by software on the receiving computer. The effective distance of common power carrier transmission is120 ~140m. This solution is suitable for wireless monitoring of buildings and villas.
remote system
The remote monitoring system consists of four parts: monitoring front-end subsystem, image transmission subsystem, central control subsystem and remote image user system. Image remote monitoring system is a set of digital video remote monitoring system based on network and designed with B/S structure, which is the highest level remote monitoring system in the industry at present.
Remote monitoring front-end subsystem consists of network camera or ordinary camera and decoder. Network cameras can directly convert images into IP signals, without the need for MPEG-4/IP converter in the transmission part. According to the needs of the site, infrared camera and alarm equipment can be installed in the front end to meet the needs of special experiments. The image transmission subsystem consists of MPEG-4/IP converter and campus broadband, and can also directly access the Internet by using ADSL and other devices. MPEG-4/IP converter converts the images received by ordinary cameras into IP packets, which are transmitted to the server through various networks. In this way, the existing campus broadband network can be used without laying video cables, and the transmission of images is not limited by distance.
The control subsystem of remote monitoring center is composed of digital video monitoring service software and PC server, which provides remote release of video images and user management functions. The desktop control system consists of users' computers, and no software needs to be installed. Just use the browser and enter the corresponding user name and password to access various functions of the system.
Performance and characteristics of remote monitoring system
Image format and network traffic: This system adopts MPEG4-4 coding, and the resolution is as high as 704×576(PAL)25 frames per second, which can provide high-quality video images with various qualities from 28.8kbps modems to 3Mbps. Control function: The remote monitoring system can control the aperture, focal length and depth of field of the lens. You can control pan/tilt in all directions. The system can remotely control translation/tilt rotation in all directions.
Scalability: The system adopts B/S mode and three-tier distributed design. By deploying multiple video servers to increase the number of monitoring points supported by the system, the system can be easily expanded.
Usability: The laboratory network video monitoring system adopts "B/S structure", and the client interface runs on the Web browser, so users can conveniently log in to the system remotely and use all the functions of the system. Reasonable system division, optimized functional layout, full Chinese operation interface and flexible mouse control of monitoring screen all provide users with powerful system functions.
Interoperability: The system provides a standard development interface.