Integration and application of three systems: DCS, FCS and CIPS

1 Introduction

As production develops from a simple process to a large-scale, continuous, integrated and complex process, the corresponding control technology, control theory, control system and control tools have undergone great changes. That is, control technology has developed from single-variable control technology to multi-variable coordinated control technology; control theory has developed from classical control theory (based on frequency domain method) to modern control theory (based on time domain method) and intelligent control theory (based on rules and knowledge); control systems have developed from simple control systems (such as PID control, cascade, ratio, feedforward, Smith, etc.) to advanced control systems (such as prediction, inference, fuzzy, neural, robust control, etc.) and integrated automation systems (integration of control, management and decision-making); control tools have developed from conventional instruments to computers and their networks.

The development of computer technology has laid a material foundation for the realization of advanced control, and the application of control technology has promoted the development of computer technology. The two are closely related. The computer-based control system has gone through four development stages: DDC (Direct Digital Control-single machine control system), DCS (Distributed Control System-distributed control system), FCS (Field Bus Control System-field bus control system) and CIPS (Computer Integrated Process System-computer integrated process control system).

DDC has centralized control and centralized risk. It generally adopts a dual-machine/multi-machine system redundant structure to ensure the reliability of the control system. In order to further improve the reliability of the control system, the DCS system was launched in the mid-1970s. The mid-1980s to early 1990s was the peak period of DCS development. DCS was widely used and the technology was mature. Domestic enterprises have introduced various complete sets of DCS products, such as Honywell's TDC-3000/S9000, WestingHouse's WDPFⅡ/Ⅲ, Bailey's INFI-90, Foxboro's I/AS system, Yokogawa's Centum-XL/μXL, etc. However, the DCS of each manufacturer is a closed communication system, which makes it difficult to achieve the integration and synthesis of various DCS systems within the enterprise.

The new FCS that appeared in the 1990s broke through the defect of DCS system that communication was realized by the closed system of dedicated network, and turned the closed and dedicated solution into an open and standardized solution. FCS and its application are the hot topics of current research.

The fieldbus/ DCS -based integrated automation system CIPS integrates process control and integrated management (business decision-making, management, planning, scheduling, etc.), making it easier for enterprises to quickly adapt to the changing market and enhance their competitiveness. The realization of CIPS is a future goal.

2 Architecture and characteristics of DCS , FCS and CIPS

2.1 DCS

The architecture of a typical DCS system is generally three-layer, namely, management level, monitoring level (engineer station/operator station) and process control level. The connection between the DCS process controller and the field transmitter and actuator adopts a one-to-one device connection method, which consumes a large number of connection cables and is time-consuming and labor-intensive to install and maintain, as shown in Figure 2-1. DCS is a "semi-digital" system, and it also requires a large number of intermediate templates such as AI/AO, DI/DO and PI/PO to complete the information conversion between analog and digital quantities.

2.2 FCS

The FCS system has the following advantages due to the use of intelligent field devices and fieldbus:

①Use one-to-N device connection method, low cost

FCS is a "pure digital" system that does not require AI/AO, DI/DO and PI/PO intermediate templates. The field bus-based field equipment and operation station are in a fully digital, serial, bidirectional, multi-station communication mode, and the system reliability is high. Moreover, digital signals are used to replace analog signal transmission, and multiple field devices can be connected on a pair of twisted pairs or a cable, saving hardware quantity and investment, saving installation costs, and low system cost, as shown in Figure 2-2.

② Achieved complete decentralized control

The control functions in the control room of the original DCS system can be placed in the field equipment and completed directly on site, that is, local information collection, local processing, and local control. The host computer mainly performs overall supervision, coordination, optimization control and management, realizing complete decentralized control.

③Openness and interoperability simplify system integration . The biggest feature of fieldbus is the use of a unified protocol standard, which makes it open and interoperable. Field devices from different manufacturers can be easily connected to the same network and can access each other, simplifying system integration .

④ Comprehensive information and flexible configuration

By digitally transmitting field data, FCS can obtain various status and diagnostic information of field devices and realize real-time system monitoring and management. FCS introduces the concept of function blocks, which makes configuration very convenient and flexible. Function blocks in different field devices can form a complete control loop; configuration can be performed on the host computer and then downloaded to the field device after completion; users specify a device based on the identifier without considering the device address, storage address, etc.

In short, the fieldbus is fully decentralized, fully digital, intelligent, bidirectional, multi-variable, multi-point, multi-station, and interoperable. It is the basis for the implementation of process control systems and the starting point of the data information flow of the integrated automation system CIPS .

2.3 CIPS

CIPS, an integrated automation system that integrates conventional control, advanced control, online optimization, production scheduling, enterprise management, and business decision-making, is the current trend and hot spot in the development of automation. CIPS is supported by computer communication networks and distributed databases to achieve information and function integration , comprehensive management and decision-making, and ultimately form a globally optimal, high-quality, highly flexible, and highly efficient intelligent production system that can adapt to the uncertainty of the production environment and the variability of market demand.

According to the requirements of continuous production process control, overall engineering optimization and information integration , the CIPS project can be composed of four subsystems, namely, production process control subsystem, enterprise comprehensive management subsystem, integration support subsystem, and human and organization subsystem, as well as corresponding lower-level subsystems, as shown in Figure 2-3.

3 Integration of DCS , FCS and CIPS

The development of computer network technology has solved the problem of office automation in enterprises, that is, the interconnection of computer equipment at the top of enterprises is easier to achieve. However, the equipment used in the process control level of enterprises may be products of different periods or manufacturers, which is a heterogeneous environment. Considering the continuity of control technology and production and the investment benefits of enterprises, it is impossible to completely abandon the original feasible system and adopt new technology and equipment. The enterprise field control network will inevitably have a situation where multiple buses coexist, multiple systems are integrated , and multiple technologies are integrated .

3.1 Integration of FCS and DCS

The integration of multiple systems refers to the integration of fieldbus control systems FCS and traditional control systems ( DCS , PLC, etc.) and the integration between various fieldbus control systems FCS (because there are still many fieldbus protocols, such as FF, Profibus, CAN, LonWorks and HART, etc.).

Integration of FCS and DCS Mainly reflected in the compatibility of field communication protocols and the unification of configuration, monitoring and operation software.

DCS is widely used and mature in technology, especially in advanced control. The functional modules of fieldbus can generally only solve basic control problems. Moreover, under the existing technical and market conditions, FCS cannot completely replace DCS . Instead , the integration of the two should be considered to combine the advantages of DCS and FCS . That is, FCS realizes the basic measurement and control loop, and DCS, as a higher-level management coordinator, realizes complex advanced control and optimization functions.

At present, DCS and FCS can be integrated at three levels , as shown in Figure 3-1a, Figure 3-1b, and Figure 3-1c;

a. Fieldbus Integration At the I/O level of the DCS , the fieldbus device appears as an I/O card in the DCS .

b. Fieldbus Integration On the DCS network, the fieldbus is connected to the DCS communication network through a fieldbus interface unit .

c. DCS and FCS work independently, and realize communication protocol conversion and information exchange through the gateway.

The above three integration approaches may encounter some practical difficulties in their application, because the design elements of DCS are different from those of FCS , and the closed nature of DCS limits the realization of the decentralized and networked features of the fieldbus. Different DCSs need to be integrated with different FCSs , and there will be a situation where multiple protocols coexist in the field control network.

3.2 CIPS based on field control network

CIPS is an integrated automation system that integrates management and control. It often adopts a multi-level hierarchical control system structure (basic control → advanced control → constraint control → local optimization → global optimization → integrated management and decision-making) and the method of "decomposition/integration, coordination". The field control network is the basis and key to transmitting process information and the starting point of CIPS data information flow. Only by obtaining a large amount of process information can CIPS perform system optimization, integrated management and decision-making.

Usually, a large CIPS network contains four types of networks: the top layer uses a wide area network (WAN), the upper layer uses an office automation local area network (TOP), the middle layer uses a manufacturing automation protocol MAP backbone network, and the lower layer uses a field control network (factory bottom control network). These four types of networks can be interconnected into a CIPS network or an Intranet network through bridges, routers, and gateways .

Large and medium-sized CIPS networks based on fieldbus or DCS networks include WAN/TOP/MAP/Fieldbus or DCS network structures and TOP/MAP/Fieldbus or DCS network structures, and small CIPS networks include LAN/Fieldbus or DCS network structures and LAN/ DCS + Fieldbus network structures (i.e., the field control network is an integration of DCS and FCS ).

In the past two years, many control system equipment suppliers have adopted Ethernet as the information transmission backbone of the factory's bottom-level control network to connect system monitoring equipment and advanced on-site intelligent equipment, and TCP/IP has entered the field, allowing factory management to directly enter the control field. The most typical application form in industrial fields is Ethernet + TCP/IP. Sensors and transmitters based on Ethernet + TCP/IP can directly become nodes of the network, and their control parameters and their status can be directly transmitted and shared within the enterprise information network, thus avoiding the situation where DCS and FCS are difficult to integrate due to the existence of multiple protocols .

4 Conclusion

DCS is widely used and has mature technology, but each manufacturer's DCS is a closed communication system, which makes it difficult to achieve the integration and synthesis of information within the entire enterprise.

FCS is characterized by being fully decentralized, fully digital, intelligent, bidirectional, multi-variable, multi-point, multi-station, and interoperable.

FCS can be seen as the result of DCS's full digital opening downward, and CIPS can be seen as the product of DCS 's upward expansion. FCS and DCS focus on process monitoring and management, while CIPS focuses on information integration , comprehensive management and decision-making within the enterprise. DCS and FCS are the starting points of CIPS data information flow. The key technology for the implementation of CIPS system is network technology, and the network is the lifeblood. Future automation systems will mainly present the characteristics of openness, integration , intelligence, informatization and networking.