Application of OPC Technology in Monitoring and Control Systems

1 The Origin of OPC Technology
OPC (OLE for Process Control) technology is the application of object linking and embedded technology in process control, including a series of industrial automation interface specifications. This technology is proposed to solve the communication between application software and various device drivers. It separates hardware manufacturers and application software developers, greatly improving the work efficiency of both parties.
With the continuous development of industrial production, due to the expansion of production scale and the increase in process complexity, industrial control software design faces huge challenges, that is, to integrate the increasing number and types of field information. In traditional control systems, information sharing between intelligent devices and between intelligent devices and control system software is achieved through drivers. Devices from different manufacturers use different drivers, forcing industrial control software to include more and more underlying communication modules; in addition, since relatively specific application drivers generally do not support hardware changes, the upgrade and maintenance of industrial control software and hardware are extremely inconvenient. In addition, at the same time, two clients cannot read and write data to the same device because they have different and independent drivers. Operating the same device at the same time may cause access conflicts and even cause system crashes. The emergence of OPC technology has solved these problems well. OPC is based on OLE/COM/DCOM technology and adopts the client/server model. It provides a unified standard for object-oriented development of industrial automation software. The standard defines the method of exchanging automation real-time data between PC-based clients using the Microsoft operating system. After adopting this standard, hardware developers will replace software developers in developing unified OPC interface programs for their own hardware products, while software developers can be exempted from the work of developing drivers and devote more energy to the development of their core products. This not only avoids duplication of development, but also improves the openness and interoperability of the system.

2 Application of OPC Technology
OPC is a software interface standard that connects data sources (OPC servers) and data users (OPC applications). Data sources can be control devices such as PLC, DCS, barcode readers, etc. Depending on the structure of the control system, the OPC server as a data source can be a local OPC server running on the same computer as the OPC application, or a remote OPC server running on another computer. The OPC interface can be used to provide the original data of the lowest-level control device to the data user (OPC application) HMI (hardware monitoring interface)/SCADA (supervisory control and data acquisition), batch processing and other automation programs, and even higher-level historical databases and other applications through the network. It can also be used for direct connection between applications and physical devices. Therefore, the OPC interface is an interface standard with high thickness and flexibility that is suitable for many systems. The innovation of OPC technology is that users can combine any visualization or control system with any selected hardware (i.e. PC plug-in board) through OPC, as shown in Figure 1.

As can be seen from the figure, the OPC standard software bus enables the integration of multiple field line systems, such as PROFIBUS network, CANopen (Open Control Area Network) network, Device Net (Device Network), etc., which can also deeply reflect the standardized relationship between OPC and field bus: OPC provides important additional performance beyond field bus, and the main goal of standardization in the field of field bus is fast and reliable data transmission. OPC enables standard communication to such an extent that any OPC server and application software can be networked and run without any problems.

3. Solutions to the Problems of OPC Technology
OPC was proposed to standardize the software interfaces between devices and applications from different suppliers and to simplify the data exchange between them. As a result, users can be provided with process control software component products that are free to combine and use without relying on specific development languages ​​and development environments. The system using OPC consists of an OPC server that provides data acquisition services according to the requirements of the application (client program), an OPC interface required to use the OPC server, and an OPC application that receives the service. The OPC server is developed according to the hardware of each supplier, so it can absorb the differences in hardware and systems of each supplier, thereby realizing a system structure that is independent of hardware. At the same time, a data type called Variant is used to provide data formats according to the requirements of the application without relying on the same data type in the hardware. Using OPC to standardize the interface can form a system as shown in Figure 2. The typical OPC architecture is shown in Figure 3.

4 Application of OPC technology in a monitoring and control system
OPC has been widely used in the field of industrial control due to its characteristics and advantages. OPC technology is usually used to collect data in industrial process control. OPC data subscription and dynamic data binding provide data sources for all ActiveX controls with data binding functions, and users can easily observe real-time data from the OPC server. Figure 4 is an application example of OPC in a waste incineration system. The system is divided into two layers: production management monitoring level and field control level.

The management layer mainly includes industrial control machines or computers, video monitors and other equipment, all of which are installed in the central control room. In the management layer, through the Kingview industrial configuration software development and design software monitoring platform, the working status of the underlying equipment can be observed in real time, the system fault points can be promptly alarmed, and the report data can be saved in real time. At the same time, the algorithm program calculation is carried out in the same computer, and the operating platform realizes remote intelligent control of the underlying equipment. The control layer mainly includes various hardware devices, such as PLC controllers, various measuring instruments, etc. These hardware are installed at the working site of the waste incineration system to directly control the system. Among them, the underlying hardware equipment uses the commonly used Siemens S7 series PLC, including S7-200 and S7-300 series. At the same time, high-speed industrial Ethernet (TCP/IP protocol) is used to connect the management layer and the control layer to realize information and resource sharing, with perfect control capabilities, extremely high reliability and convenient and flexible expansion capabilities. The system design starts from the process flow to realize automatic control of the production process. The field control level accepts the scheduling of the production management level, but does not rely on the production management level to operate: if the monitored computer fails or is not put into use or the communication network fails, each field control station will still work normally and will not affect the entire process. When realizing data exchange between the control layer and the management layer, the industrial configuration software Kingview provides a way to establish an OPC server to realize data communication. Therefore, an OPC server is established between the management layer and the control layer, and its block diagram is shown in Figure 5.

In the figure, OPC connects to a widely used PLC system, and the underlying device information OPC server enters the upper-level human-machine interface. These systems exchange information with the top-level quality control software, production management software, and Internet application software OPC interface, so that information can be fully circulated between systems. If the system has the OPC application conditions, when the existing system needs to add a new device, it only needs to connect the new device to the system and install the OPC server used to access the device. The system can work normally after the expansion. Adding a new device does not affect the operation and use of other parts of the system.

5 Conclusion
Through the study of OPC technology, the realization of OPC server and client was discussed, and the study was applied to the control scheme of a garbage incineration system, which was remotely monitored by the client. The results show that OPC technology greatly improves the open performance of the data acquisition system, and can effectively avoid the duplication of the development process and the incompatibility of multiple software system integration. This technology has broad application prospects in industrial control systems and gradually realizes the standardization between software and hardware.