introduction
Since the 1990s, digital technology has achieved rapid development. Early programmable controllers are called programmable logic controllers (Programmable Logic Controller, PLC), which are mainly used to replace relays to achieve logic control. With the development of technology, the function of this industrial control device using microcomputer technology has greatly exceeded the scope of logic control. Therefore, today this device is called a programmable controller, or PC for short. However, in order to avoid confusion with the abbreviation of personal computer (Personal Computer), the programmable controller is abbreviated as PLC, plc has been developed by the American Data Equipment Corporation (DEC) since 1966. The current programmable controllers in the United States, Japan, and Germany Excellent quality and powerful functions.
At present, the latest application trend is to integrate the PLC control system with the existing GPRS wireless communication network technology. Through the GPRS network communication technology, ultra-remote and reliable data transmission across the country or even the world can be achieved, thereby achieving remote data analysis processing and remote control purpose.
PLC power supply plays a very important role in the entire system. Without a good and reliable power supply system, it will not work properly, so PLC manufacturers also attach great importance to the design and manufacture of power supplies. Generally, the AC voltage fluctuation is in the range of + 10% (+ 15%), you can connect the PLC directly to the AC grid without taking other measures
Advantages of GPRS + PLC solution
Short for General Packet Radio Service (General Packet Radio Service), it is a mobile data service available to GSM mobile phone users. GPRS can be said to be the continuation of GSM. GPRS is different from the previous continuous channel transmission method. It is transmitted in the form of a packet. Therefore, the cost borne by the user is calculated based on the unit of transmission data. It is not cheaper to use the entire channel. GPRS transmission rate can be increased to 56 or even 114Kbps
China Mobile GPRS system can provide wide area wireless IP connection. Building a distributed PLC control system on the mobile communication company's GPRS service platform to realize the PLC's wireless data transmission has the advantages of making full use of the existing network, shortening the construction cycle and reducing construction costs, and the equipment is easy to install and maintain. After comparative analysis, we chose China Mobile's GPRS system as the data communication platform of the distributed PLC control system.
GPRS is often described as "2.5G", which means that this technology is located between the second generation (2G) and third generation (3G) mobile communication technologies. It provides medium-speed data transfer by using unused TDMA channels in the GSM network. GPRS breaks through the GSM network can only provide the thinking of circuit switching, only by adding corresponding functional entities and partial transformation of the existing base station system to achieve packet switching, this transformation investment is relatively small, but the The user data rate is quite impressive. Moreover, because the intermediary converters required by current wireless applications are no longer needed, connection and transmission will be more convenient and easier. In this way, users can connect online, participate in interactive communication such as video conferences, and users on the same video network (VRN) can even continue to connect to the network without dialing up the Internet. GPRS packet-switched communication method In the packet-switched communication method, data is divided into packets (packets) of a certain length, and there is a packet header in front of each packet (where the address mark indicates where the packet is sent).
The GPRS wireless communication system has the following characteristics:
1. High reliability:
Compared with SMS short message mode, GPRS DTU uses connection-oriented TCP protocol communication to avoid the loss of data packets and ensure reliable data transmission. The center can simultaneously transmit data with multiple monitoring points without interfering with each other. The GPRS network itself has a perfect frequency division multiplexing mechanism, and has a strong anti-jamming performance, completely avoiding the multi-machine frequency band "collision" of traditional digital radio stations.
2. Strong real-time performance:
GPRS has the characteristics of real-time online, the data transmission delay is small, and supports multi-point simultaneous transmission, so the GPRS monitoring data center can quickly and real-time two-way communication between multiple monitoring points, which satisfies the system for data collection and transmission. Real-time requirements. At present, the actual data transmission rate of GPRS is about 30Kbps, which can fully meet the requirements of the system data transmission rate (≥10Kbps).
3. Wide monitoring range:
The GPRS network has achieved nationwide coverage, unlimited capacity expansion, and unlimited access locations, which can meet the access requirements of mountainous areas, towns, and trans-regions. For PLC control systems distributed throughout the city / province, or nationwide, GPRS network is the ideal choice.
4. Low system construction cost:
Since the GPRS public network platform is adopted, there is no need to construct a network, and only equipment needs to be installed, so the construction cost is low; network maintenance costs are also eliminated.
5. Low system operating cost:
Adopt GPRS public network communication, billing at a uniform rate nationwide, eliminating expensive roaming charges, GPRS network can be charged according to the actual communication flow of data, (1 minute-3 minutes / 1K byte), or by The monthly subscription is not limited to traffic charges, thus achieving low-cost communication of the system.
6. The system's transmission capacity and capacity expansion performance are good:
The monitoring center shall realize real-time connection with each PLC control site. The system requirements can meet the needs of sudden data transmission, and GPRS technology can well meet the needs of transmitting sudden data; because the system uses a mature TCP / IP communication architecture and has good expansion performance, a monitoring center can easily Support thousands of on-site PLC data communication.
7. Low GPRS transmission power consumption, suitable for field power supply environment:
Although two-way communication with a data center thousands of miles away, GPRS data transmission equipment only needs to communicate with nearby mobile base stations when working, its overall power consumption is equivalent to an ordinary GSM mobile phone, and the average power consumption is only 200 milliseconds. Watts are smaller than traditional digital radio stations. Therefore, the GPRS transmission method is very suitable for use in the field using solar power or battery power.
System Components
1. On-site control / collection point:
Input sampling stage In the input sampling stage, the PLC sequentially reads all input states and data in a scanning manner and stores them in the corresponding unit in the I / O image area. After the input sampling is completed, it shifts to the user program execution and output refresh phase. In these two stages, even if the input status and data change, the status and data of the corresponding unit in the I / O map area will not change. Therefore, if the input is a pulse signal, the width of the pulse signal must be greater than one scan period to ensure that the input can be read in any case.
The on-site monitoring point is automatically controlled by the PLC and collects relevant information. It is connected to the GPRS DTU terminal through the RS232 or RS485 interface. The device information collected by the PLC processes the data through the GPRS DTU terminal, encapsulates the protocol and sends it to the GPRS wireless network.
2. Monitoring center:
a) Public network access solution
The server is connected to the Internet through a public network, such as ADSL dial-up / telecom dedicated line broadband Internet access, etc., applying for a fixed IP address on the public network; it can realize the application of small and medium-capacity systems.
b) Private network access solution
The server uses the DDN special line provided by the provincial mobile communication company, applies for a fixed IP address, and is connected to the GPRS network. Since the DDN dedicated line can provide higher bandwidth, when the number of on-site PLCs increases, the center can meet the demand without expanding the capacity, and can realize large-capacity system applications.
After receiving the data from the GPRS network, the RADIUS server of the monitoring center first performs AAA authentication, and then transmits it to the computer host of the monitoring center. The system software is used to restore and display the data and perform data processing, which further enhances the security performance of the system data communication.
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