Considering the hierarchical structure of DCS, the process control level and control management level are the two most basic links of DCS. The process control level realizes the decentralized control functions of signal input, transformation, operation and output. In different DCS, the control devices at the process control level are different, such as the process control unit, the field control station, the process interface unit, etc., but their structural forms are roughly the same, which can be collectively referred to as the field control unit FCU. The process management level is composed of engineer station, operator station, management computer, etc., which completes the centralized monitoring and management of the process control level. It is usually called the operation station. The hardware and software of DCS are designed according to the modular structure, so the development of DCS is actually to combine various basic modules provided by the system into a system according to actual needs. This process is called system configuration.

(1) Field control unit

The field control unit is generally far away from the control center and installed near the site. Its highly modular structure can be configured into process control units with different scales from several monitoring points to hundreds of monitoring points according to the needs of process monitoring and control.

The structure of the field control unit is that many plug-in boards (or cards) with scattered functions are installed in the plug-in box according to a certain logical or physical order. The field control units and the control management level are connected by bus to realize information exchange.

The hardware configuration of the field control unit needs to complete the following contents:

Plug-in configuration Configure the host plug-in (CPU plug-in), power plug-in, I/O plug-in, communication plug-in and other hardware devices according to the system requirements and control scale;

Hardware redundancy configuration Redundancy configuration of key equipment is an important means to improve the reliability of DCS. DCS can generally achieve redundancy configuration for host plug-in, power plug-in, communication plug-in and network, and key I/O plug-in.

For DCS with different hardware installations, the installation of various plug-ins in the plug-in box will be specified in logical or physical order. In addition, the field control unit is usually divided into two types: basic type and extended type. The so-called basic type is that all kinds of plug-ins are installed in a plug-in box, but more often, the expandable structure is required, that is, a field control unit also includes several digital input/output expansion units, which are connected with each other by bus.

In essence, the structural form and configuration requirements of the field control unit are consistent with the hardware configuration of the modular PLC.

(2) Operation station

The operation station displays and records the process data from each control unit, which is the operation interface for the interaction between human and production process information. Typical operation stations include host system, display equipment, keyboard input equipment, information storage equipment, print output equipment, etc., mainly realizing powerful display functions (such as analog parameter display, system status display, multiple screen display, etc.), alarm function, operation function, report printing function, configuration and programming function, etc.

In addition, the DCS operation station is also divided into operator station and engineer station. From the perspective of system functions, the former mainly realizes the general production operation and monitoring tasks, and has the functions of data acquisition and processing, monitoring screen display, fault diagnosis and alarm. In addition to the general functions of the operator station, the latter should also have the functions of system configuration, modification of control objectives, etc. From the perspective of hardware equipment, the engineer station and operator station of most systems are combined, and only one engineer keyboard is used to distinguish them.