Abstract: The wireless remote monitoring system is a new type of monitoring and control system formed on the basis of the original embedded products and the current booming wireless communication technology. This article systematically discusses some of the core technologies used in the design of wireless remote monitoring systems, including hardware circuit design, chip selection, embedded operating system selection, real-time software design, wireless communication network formation, and control center application software design. Wait.
The wireless remote monitoring system is a new type of measurement and control system developed based on the traditional monitoring and monitoring system, combined with the current wireless communication technology and information processing technology.
Generally speaking, the existing wireless remote monitoring systems mostly conform to the construction mode of "control center-monitoring station". The control center is the core of the operation of the entire system and is responsible for collecting monitoring information uploaded by each monitoring station and sending various operation commands to control the industry of the monitoring station. The monitoring station is deployed at each monitoring point far away from the control center and is responsible for collecting information and responding to control commands issued by the control center. The control center can be realized by ordinary microcomputer, workstation or industrial control computer, and the software development is reliable based on the existing Windows or Unix operating system. The design and implementation of the monitoring station can adopt specific technical forms according to different application purposes and application environments, such as single-chip computers, DSPs, or Intel X86 series microprocessors. The networking mode of the wireless remote monitoring system is also very flexible. It can use the existing wireless communication network, such as GSM / GPRS network, CDMA mobile network, etc., and can also build a special wireless local area network separately. The following systematically discusses some of the core technologies involved in the design and development of wireless remote monitoring systems, including three aspects: the design and development of monitoring stations, the establishment of wireless networks, and the software design of control centers.
1 Design and implementation of monitoring station
The design and implementation of the monitoring station is the focus of the development of the entire wireless remote monitoring system. The ability and accuracy of the monitoring station to process information data will affect the final performance of the entire system. In the whole development process, the design of the monitoring station is the part with the largest workload and the longest time required. The monitoring station is at the work site, and only completes data collection, processing, and control. The task is relatively single and fixed, and there is no need to use a large desktop computer. Considering energy saving and convenient deployment, most monitoring stations are embedded systems. According to the functions to be realized by the entire wireless remote monitoring system, and the requirements for data processing and sensor control capabilities, the complexity of the design of the monitoring station and the specific technology used are different.
1.1 Design and implementation based on single chip microcomputer
Using single chip microcomputer is the first choice for most embedded system design. Due to the rich peripherals integrated on the chip and good control capabilities, the single chip microcomputer is tailor-made for embedded systems and occupies the largest share in the embedded market. 
The design scheme based on the single-chip microcomputer is generally suitable for remote monitoring systems that have low requirements for data processing and little calculation. According to needs, the single-chip microcomputer can choose a lower-end 4-bit machine or 8-bit machine, such as 8051, etc., or a special chip with stronger function, such as MSP430FE42X series. The single chip microcomputer is mainly used for system control at the monitoring station side. The off-chip memory is generally RAM, EEPROM, Flash and other memories; I / O devices are generally keyboard, LCD and other human-computer interaction interfaces for design and debugging; sensors are generally Microphones, cameras, speakers, and servo motors. Wireless communication interface implementation is relatively complicated. The codec is optional, and is generally not necessary for low-rate data. According to the system's processing tasks and the type of information, the codec can use different cores, such as CMX639 (for audio) or LD9320, etc., and can also be implemented by programming logic devices. The software of the monitoring station can be realized directly through 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 microcomputers, the control capability is low, the system is simple, and the method of directly writing control programs is generally adopted. For systems with more powerful functions and complex interactions between various devices, most of them use the operating system for task management and device interaction, and the application software only completes upper-level data processing and other tasks.
1.2 DSP-based design and implementation
As we all know, DSP has strong digital processing capabilities, the technology is very mature, 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 type of design is generally suitable for monitoring systems that require a large amount of data processing operations, high real-time requirements, and relatively low control capabilities. Unlike the monitoring system based on single chip microcomputer, DSP can also be used for data calculation and encoding / decoding in addition to being a controller. For the more complex encoding / decoding and compression and decompression operations (such as the processing of image and video data), whether it is still completed by the DSP must be considered comprehensively. If the DSP is too heavy in system control and implementation of the transmission protocol, this part of the operation needs to be completed by a special processing chip; if the system control and transmission protocol is relatively simple, or does not reach the upper protocol stack at all, this part of the complex operation can be DSP completed.
1.3 Design and implementation based on MCU + DSP
Obviously, this design method absorbs the advantages of the single chip microcomputer and DSP: the characteristics of the single chip microcomputer determine that it is good at control, and the internal structure of the DSP guarantees strong data processing capabilities. The combination of the two can achieve some quite complex system functions, but because two processors are used in the system, the information interaction between them is a problem that must be considered when designing such monitoring stations. Only when the SCM and the DSP work well together can their respective advantages be fully utilized; otherwise, the coordination between the two consumes a lot of resources, and the overall performance may not be higher than the system using a single processor. The common method to realize communication coordination between the single chip and DSP is to use dual-port RAM.
At present, in order to expand the scope of application of the chip, some DSP or single-chip manufacturers expand on the original basis to accommodate each other's characteristics, so that the same chip has better performance in data processing and control. For example, dsPIC introduced by Microchip Company enables customers to easily transfer the functions of single chip microcomputer to DSP. The products currently launched include dsPIC30FXXX series. Since the two functional modules of DSP and MCU are implemented in the same chip, the reliability of the system is improved, the design difficulty of the monitoring station is reduced, and the space of the printed board is saved. Such chips are favored by the majority of users.
1.4 Design and implementation based on MPU
Another option for designing embedded products is to use a microprocessor-based design. Compared with industrial control computers, embedded microprocessors have the advantages of small size, light weight, low cost, high reliability, etc. At the same time, in this field, the technology is mature, the product types are many, the choice space is large, and the processor meets various performance requirements. Relatively easy to obtain. With the emergence of high-performance MPUs that use the RISC system (such as processor chips using ARM architecture, etc.), MPUs have a long-standing position in the embedded field; , RAM, Flash, bus interface and various peripherals and other devices, the reliability of the system will be reduced, the technical confidentiality is poor, and the difficulty of implementation is also greater.
1.5 Real-time operating system selection and embedded real-time software development
At present, there are many types of real-time operating systems (RTOS) and different software structures, which can be applied to various environments with different degrees of complexity, including loop query systems, front and background systems, real-time multi-task systems and multi-processor systems. Specific examples include VxWorks, pSOS, QNX, Palm OS, Windows CE, lynx OS, and embedded Linux. The importance of choosing RTOS suitable for the monitoring station and even the entire wireless remote monitoring system is self-evident, it may be related to the success or failure of the development of the entire system. The selection process is complicated and requires patience: to understand the characteristics and scope of application of each RTOS and compare the differences between them, you can find the most suitable one. When choosing a comparison, the main factors to consider are:
â‘ Can RTOS support the language and microprocessor used in the project;
â‘¡ Can RTOS work with ICE, compiler, assembler, connector and source code modulator;
â‘¢ Does the RTOS support the services used in the design, such as message queues, timing, and semaphores;
â‘£ Can RTOS meet the performance requirements of application products, such as real-time requirements;
⑤ Can you obtain the necessary components for product development, such as protocol stack, Nengxin service, real-time database, Web service, etc .;
â‘¥Whether RTOS can provide device drivers for publicly sold hardware;
⑦ Is it free to use RTOS;
⑧ Can you obtain the target code;
⑨How much technical support do you get;
â‘©For RTOS that requires authorization, what is the authorization method? 
The development of embedded real-time software has many similarities with the development of traditional software, and inherits the development habits of many traditional software; but because of the special functions and operating environment of embedded real-time software, it is determined to be different from the development of traditional software. The development of embedded real-time software uses a cross-development approach. The so-called cross-development refers to the environment in which program code is implemented, compiled, and connected differently from the environment in which it is debugged and run. The former is based on a common microcomputer platform, and the latter is based on an embedded system hardware platform. The debugging process is mostly carried out in cooperation with the host machine and the target machine with a communication connection. After the development is completed, curing and curing tests are required. In addition, the development process also requires corresponding development tools, including cross compiler, cross debugger and some simulation software. Embedded application system takes tasks as the basic execution unit, replaces multiple modules of general software with multiple concurrent tasks, and defines the interface between application software tasks. Because the real-time performance of the entire wireless remote monitoring system is affected by RTOS and application software, the real-time requirements are fully considered during the software requirements analysis stage. In addition, embedded application software has strict requirements on stability, reliability, anti-jamming and other performance, so the development of embedded real-time software is more difficult.
2 Design and implementation of wireless communication
The design of wireless communication is relatively simple compared to the monitoring station. There are many existing products and communication systems that can be used. The focus is only on making the best choice from a variety of implementations.
Common implementation methods are: use of existing communication networks (GSM / GPRS, CDMA mobile network, etc.) and corresponding wireless communication products; through wireless transceiver devices, such as wireless modems, wireless bridges, and other specialized wireless local area networks; use of transceiver integration The chip realizes wireless communication between the board level and the monitoring center at the monitoring station.
2.1 Use existing network to realize wireless communication between monitoring station and monitoring center
There are many existing communication networks. Networking according to business is a feature of communication networks before 3G, and wireless networks are no exception. The wireless networks that can be borrowed for designing wireless remote monitoring systems are: Global Digital Mobile Phone System (GSM), General Packet Radio Service (GPRS), mobile networks using 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, and can provide high communication quality under the condition of low service cost and low terminal cost. In terms of its business, GSM is a mobile ISDN (Integrated Services Digital Network) that can provide multiple services.
GPRS (General Packet Packet Radio Service) adds some hardware equipment and software upgrades to the existing GSM network to form a new network logical entity. It is based on packet switching technology and uses the IP data network protocol to increase the data service transmission rate of the existing GSM network, up to 170kb / s. GPRS introduces packet switching technology into the existing GSM system, making mobile communication and data networks into one, with the characteristics of "extreme speed transmission", "always online", "affordable" and so on.
CDMA (Code Division MulTIple Access) network uses spread spectrum technology and uses a variety of diversity reception methods to make 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, and is a combination of data and mobile communication. This communication method is based on TCP / IP and the system structure is open, providing seamless connection of the same layer network and multi-protocol network services. The CDPD network has the characteristics of fast speed and high data security, and can be interconnected with the public wired data network, which is very suitable for transmitting real-time, bursty and online data.
To enable the wireless communication between the monitoring center and the monitoring station to use the existing network, corresponding access equipment is required for a specific wireless network. There are ready-made products on the market for this type of equipment. The communication module for access to the GSM network is Siemens ’SIEMENS TC35i. For access to GPRS, Siemens’ MC35GPRS module is available. For access to the CDMA network, there is the Holly H110 CDMA module and AnyDATA ’s CDMA Modem (DTS-800 / 1800). The wireless modems (Modem) are OmniSky and NovatelMinstrel.
Using the existing network to build a wireless remote monitoring system, the network connection is shown in Figure 1. Among them, wireless access module products generally provide RS232 as an external communication interface, and some antennas are built-in. The wide coverage and roaming characteristics of the existing network are used to make the location of the monitoring station and the control center not limited by the distance; but due to the use of the public network, the security will be reduced.
2.2 Establish a wireless local area network through a dedicated wireless transceiver device
This design implementation method has a simple structure and does not require payment to the network operator; the use of a private network provides high security. Wireless transmission uses microwave as the transmission medium. According to different modulation methods, it can be divided into two types: spread spectrum and narrow-band modulation. The spread spectrum mode system has high anti-interference ability and safety, and little interference to other electronic equipment. The narrow-band modulation method occupies fewer frequency bands and has a higher frequency band utilization rate. Usually, a dedicated frequency band is selected and application is required. The influence between adjacent channels is large, and the communication quality and communication reliability cannot be guaranteed.
The topological structure of establishing a wireless local area network using dedicated wireless transceiver equipment is shown in Figure 2. Wireless transceiver equipment includes wireless modems and wireless bridges. RS232 communication is used between wireless Modem and monitoring station and control center. If a network bridge is used to build equipment for the network, the network topology will be more flexible, as shown in Figure 3. Among them, the wired network at both ends of the wireless network is optional, and can be a local area network such as Ethernet, token ring network or point-to-point network. It can also be a metropolitan area network, or even the Internet, but when using the public network, security and cost issues must be considered. 
2.3 Wireless communication realized by the integrated transceiver chip at the monitoring station
One of the characteristics of the first two networking methods is the use of existing network systems and products. The wireless communication part does not need to be specially developed and is relatively easy to implement. However, because the purchased products are independent devices, the entire system, especially the monitoring station, has a complicated structure and a large volume, which often brings disadvantages when the system is promoted, and outsourced products 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 will be avoided; but this will increase the difficulty of system development and extend 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.
The only part that needs to be realized by using this method to design a monitoring station is the wireless communication interface in Figures 1, 2 and 3 (please refer to the full text of the online version of this article). This part of the hardware real-time block diagram and the relationship between the processor and memory is roughly shown in Figure 4. Each sub-module has a variety of chips to choose from, such as ML2751 and RTF6900 for the RF front end, ML2722 for modulation / demodulation, LD9002DX2 and Stel-2000A for spreading and despreading.
3 Design and implementation of the control center
The design of the control center is relatively simple compared to the design and development of the monitoring station, and the hardware design is less. In addition to ordinary microcomputers (or workstations and industrial control computers), network access equipment is also required (if wireless communication is implemented by a self-designed module, then Must develop a dedicated wireless network card inserted into the reserved bus slot of the computer motherboard). The design and development of the control center is mainly focused 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 tools for starting and debugging such software, and their functions are powerful, which brings convenience to the software design of the control center.
As far as the implementation form of the software is concerned, in addition to the interface module, all other functional modules can be designed as dynamic link library files (.dll). The man-machine interface interface module can be customized for the practical application of the wireless remote monitoring system to meet the special requirements of the user in terms of beautiful interface and convenient operation.
Using C / C ++ language to design such system software in VC ++ development environment involves more technologies, including memory management, network communication, multi-thread management and database programming, and even AcTIveX.
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