Abstract
The network control system is a closed-loop feedback control system composed of controller, sensor and actuator that are connected together through the computer communication network. Aiming at the problem that colleges and universities demand more and more practices but lack relevant experimental facilities and fields, this paper puts forward a solution of building network control laboratory based on EPA. By using the generalized network control system, it can not only control and detect the control system itself, but also realize remote monitoring and management of the experimental process on the basis of the public computer network data transmission of network control system. Through TCP/IP communication mode and the web publishing of the Force Control, it realized the direct operation of the network control laboratory development. After testing, it can realize the operation of the network control laboratory, adjust the corresponding parameters, observe the results and verify the algorithm. The system has practical value and referential value.
Keywords
Introduction
The network control system (NCS) is a kind of closed-loop feedback control system composed of sensor, controller and actuator which are connected together by computer communication network [1]. As a complex control system that integrates control, calculation and communication, the network control system involves control theory, computer science, information theory, mechanical electronic engineering, software engineering, reliability and other disciplines. Therefore, the network control system is also called the integrated communication and control system (ICCS).
The real-time Ethernet named Ethernet for Plant Automation (EPA) is a new open real-time Ethernet standard for industrial field equipment. The EPA applies a lot of mature technology to industrial control system. Using efficient, stable and standard Ethernet and UDP/IP protocol deterministic communication scheduling strategy, it establishes a new standard for real-time work for field equipment [2]. The control system based on EPA standard is distributed network control system [3]. and a hotspot for the research of network control application. It was an open distributed system based on FCS system model, capable of complete digital computation and digital communication, In addition, it completely transfers control function to site, which has improved the reliability and flexibility of the system.
With the rapid development of higher education in China, practices are in greater demands now. Conseqently, university laboratory is facing many problems, such as the shortage of laboratory fund resources and increasingly higher laboratory management cost. In the background of college enrollment expansion, the existing experimental buildings can hardly hold more students. As a result, the limited experimental fields will certainly have an impact on students’ experimental results and their enthusiasm. However, the shortage of experimental equipment has become a universal phenomenon [4, 5, 6, 7], which affects the development of experimental teaching and the cultivation of students’ practical innovation ability to some extent.
Hardware structure of network control laboratory.
Shortage of laboratory construction funds, lack of experimental rooms, high laboratory management costs, the conflict between limited available experimental equipment and increasing number of students in need, and the conflict between limited equipment using time and students’ sufficient spare time, all the problems result in poor practical results of experiment without helpful contributions to the cultivation of students’ practical ability. In view of the above problems, the the construction of EPA-based network control laboratory is proposed. Using the generalized network control system, it can not only control and detect the control system itself, but also realize remote monitoring and management of the experimental process on the basis of the public computer network data transmission of network control system. Through TCP/IP communication mode and the web publishing of the Force Control, it realized the direct operation of the network control laboratory development. It can cut down the laboratory construction investment, save space and resources, reduce management costs, enable resources sharing, avoid repeated investment, and improve the use efficiency of experimental equipment and students’ spare time. What’s more, students can carried out experiments from a long distance, which can rise students’ enthusiasm and combine the theoretical learning with the practice. All these will contribute high teaching quality and the cultivation of talents in colleges and universities.
The network control laboratory based on EPA is constructed by the technologies of computer network, EPA, VPN and so on. It can not only control and detect the control system itself, but also implement remote monitoring and management of the experimental process. Remote users can get the experimental experience as real as a local experimenter have. The provided experiments are based on a real system, instead of simple computer simulation and virtual reality technology. It can realize the function that can’t be realized under the full software mode, and effectively use the existing equipment.
The structure of network control laboratory based on EPA is divided into five layers in its design, namely client layer, network layer, server layer, controlling and monitoring layer, device layer, as shown in Fig. 1. The client layer includes the remote client side [8] connected to the VPN server, directly facing the users and processing man-machine interface. The network layer uses TCP/IP as communication protocol and http as the application communication protocol. The server layer includes VNP server, management computer, data server and Ethernet switch. It provides communication services, system management services, database services, etc. The control layer includes programming computers, monitoring computers and various controllers with network function which constitute the engineer station, operator station and automatic station respectively to realize the configuration and monitoring of the control system. The device layer is the control object, exchange the information of sensor and actuator with that of controller. Both the controller and the control object are configured according to the needs of the automation course or the existing field equipment.
System structure diagram.
Software development platform
The configuration software whose functions have become more and more powerful provides a powerful and flexible development platform for automation network remote laboratory. The network control experimental control system uses Force Control configuration software as the development platform. Force Control monitoring configuration software is special software for the collection and process control of production data. It has real-time open database system that can communicate with many I/O devices, provide alarms, events, animation connection, and safe protection mechanism, etc. It can communicate with the industrial control equipment at home and abroad and also can provide the hardware and software interfaces conveniently to control layer and management layer. The controller epa8000 used in our network control laboratory has its own professional supporting software which applies the control strategies and implements the control for control object.
Composition and function
The whole system can be divided into 3 parts, namely, the main function of the student, the main function of the teacher, the main function of the administrator, as shown in Fig. 2. Students can make appointments about experiment time and project, set personal information, submit experimental report, etc. Teachers can set up and update experimental items, experimental requirements and correct the experimental reports. Administrators can maintain the system, share resources, and manage information, lab devices and labs. Through this way, the management of users, experimental teaching, equipment and laboratory are enabled.
Design of field monitoring software
The field monitoring computer and the I/O device are connected. In the force-control I/O manager equipment, Modbus (TCP) is found, the link controller is configured. After configuring all the parameters such as address and communication mode, we enter the DB-Manager to configure the database to realize the connection between the database and the control object I/O. The corresponding I/O of the corresponding area is configured and the basic parameters and data connection modes are described. The bias of the I/O point should be checked in the state table of the control software to ensure the hardware and software matching.
The establishment of the real time database is accomplished by creating the point parameters, connecting the I/O equipment and the database configuration. It is responsible for real-time data processing, historical data storage, statistical data processing, alarm information processing, data service request processing, etc. [9].
Using the function of Force Control, the picture of control system of control object is drawn. Manual button is configured; switch guide and effective action are designed. Then the animation is connected, the picture can change with the change of database variables, the real-time operation of the system can be seen in animation.
Web implementation of laboratory
The web publishing of the f Force Control software supports the Webserver of Force Control. The web server provided by the Force Control software is a standard web server, completely complying to the standard http communication protocol. It also supports IIS publishing. According to the actual needs of the laboratory, we choose force control web publishing, configure web server, set up IE browser, define web home page, configure net server, configure and start web server, start force control. These configurations can implement the web publishing of force control.
In the remote experiment process, through the web function of Force Control, the clients can use IE browser to visit the engineering scene of force control from a long distance. The engineering image showed in the browser is exactly the same as that in the View of Force Control System, including all the data and animation. You can also remotely operate the manual button to control field control object.
The physical diagram of traffic lights at crossroads based on the EPA network.
Test
According to the design of the five - layer structure, the network architecture is formed, network configuration is carried out, and the hardware is connected. First of all, we do the Ethernet debugging of the system. After the module is electrically powered, the parameters are set and the communication is debugged. If communication is normal, the configuration settings are carried out, and the software programming controls the controlled objects to test the operation of the software, as well as the matching of the software and hardware. First, the local test is done in the lab. Teachers log in to the teacher management page, certify and upload student information [8, 9, 10]. After students log in to the experimental interface, the experiment can be started. The laboratory intersection traffic lights are taken as the control object for the test; corresponding experimental parameters are set; and the control program is written in the computer. The program is written to the controller. If it’s observed in the monitoring computer that the operation condition of the controlled object in the engineering image is consistent with the field operation, it can be known that the laboratory runs normally in the local computer.
Following the local test is the remote test. The remote client side is connected to the internet and then the laboratory server and becomes laboratory client side. When a user inputs the controller address in the browser and obtains the control right, he can manipulate the controller in the browser and modify the parameters to observe its operation. The remote web engineering scene is consistent with the operation of the on-site controlled object. The test shows that the control system have realized the acquisition and control of analog input and output of the field unit, the input and output of the digital quantity of the field unit, including temperature, displacement, and speed and so on. The communication response time is less than 1 ms, which supports IEC61131-3 configuration programming. The system realizes the network-controlled operation of traffic lights at crossroad, as shown in Fig. 3.
Analysis
With consideration to the comparison of EPA-based network experiment system and the traditional PLC system, the module of the field control points of the experimental system is installed nearby so that the equipment is more beautiful; the structure is more compact; the network can achieve wireless connection between the field modules; the cables in the field can be reduced; troubles will be less, and maintenance will be more convenient. The EPA ahs the innate advantage that the field controller can be taken as a site of the internet that can be connected directly through the internet. Therefore, the maintenance of the field equipment can be easily achieved for engineers which can save a lot of time and greatly reduce the costs.
The network control system based on EPA is a distributed network control system that applies all the control functions by the mutual operations among various functional blocks on the field device. The configuration of the EPA device is realized by configuration and scheduling of the function blocks of the equipment. The problems of NCS system such as control algorithm, reliability, real-time, interoperability, security and so on present new features. The NCS with EPA control network is taken as deterministic scheduling. EPA distributed network control system can implement the device macro cycle scheduling and meanwhile meet the requirements of deterministic scheduling and synchronization control.
The network experimental system based on EPA has the characteristics of simple structure, easy networking, good real-time performance, high reliability and simpleness of being mastered. It also has the advantages of expanding randomly, convenient maintenance, seamless connection with campus network and information resource sharing and the operating costs can be greatly reduced too.
Conclusion
The network control laboratory based on EPA is built by computer network technology, EPA, VPN technology and so on. The provided experiments are based on the real system, instead of merely computer simulation and virtual reality technology. It has the functions that are not acheivable for full software mode, and makes full use of the existing equipment. The system has realized network control using TCP/IP communication mode which can, proved by test, implement the operation of remote experiment, adjust the corresponding parameters, observe the results, verify the algorithm, etc. On the basis of the existing network and teaching platform, the system uses modern information technology combined with network and computer control technology to realize the students’ remote-control experiment, providing solution to the problems in the current teaching experiment.
Remote network laboratory is an inevitable trend of the development of experimental teaching, with which users will not be limited by time and space anymore, the flexibility, adaptability and efficiency of experimental teaching has been improved [11, 12] the teaching process has been optimized and teaching methods are extended. What’s more, students can complete the scheduled experimental project as they do in real environments, and xperimental equipment can be shared by more people so that the resources are savedand the use efficiency and accessibility of experimental facilities is improved, which is helpful for improving teaching conditions and teaching quality. Furthermore, with remote networlk laboratory, students can better understand the knowledge taught in the classroom, equip themselves with higher practical ability, and have better experience. Meanwhile, their interest and enthusiasm about experiments will be risen, and they will have opportunities to learn networking technology, fieldbus control technology and the use of configuration software, which are ways to understand advanced control technology [13], master the knowledge of network control. Apart from that, it’s also a data and verification platform for scientific researches.
Footnotes
Acknowledgments
The authors would like to thank The Applied Basic Research Project of Yunnan Province (2013FZ098) and The Project of Education Department of Yunnan Province (2015C076Y) for financial support.