The process control level specifically implements distributed control functions such as signal input, transformation, calculation, and output. In different DCS systems, the control devices at the process control level vary, such as process control units, field control stations, and process interface units, but their structural forms are roughly the same and can be collectively referred to as field control units (FCUs). The process management level consists of engineer workstations, operator workstations, and management computers, which perform centralized monitoring and management of the process control level; these are usually called operator workstations. Both the hardware and software of a DCS are designed with a modular structure, so DCS development is essentially the process of combining the various basic modules provided by the system into a system according to actual needs; this process is called system configuration.
1. Field control unit
Field control units are typically located far from the control center and installed near the field. Their highly modular structure can be configured into process control units ranging in size from a few monitoring points to hundreds of monitoring points, depending on the needs of process monitoring and control.
The structure of the field control unit consists of many functionally distributed plug-in boards (or cards) installed in a plug-in box in a certain logical or physical order. Each field control unit and the control management level are connected by a bus to realize information exchange.
The hardware configuration of the field control unit needs to include the following:
The configuration of the plug-ins is based on the system requirements and control scale, including hardware devices such as host plug-ins (CPU plug-ins), power plug-ins, I/O plug-ins, and communication plug-ins.
Hardware redundancy configuration is an important means of improving the reliability of DCS. DCS can usually implement redundancy configuration for host modules, power supply modules, communication modules, network modules, and critical I/O modules.
Different DCS hardware installations have corresponding logical or physical order rules for the installation of various plug-ins in the plug-in box. In addition, field control units are generally divided into two types: basic and extended. The basic type means that all plug-ins are installed in a plug-in box, but more often an extended structure is required, that is, a field control unit also includes several digital input/output expansion units, which are interconnected by a bus.
In essence, the structure and configuration requirements of the field control unit are consistent with the hardware configuration of the modular PLC.
2. Operator station
The operator station is used to display and record process data from various control units, serving as the interface for human interaction with production process information. A typical operator station includes a host system, display devices, keyboard input devices, information storage devices, and printing output devices. Its main functions include powerful display capabilities (such as analog parameter display, system status display, and multiple screen displays), alarm functions, operation functions, report printing functions, configuration and programming functions, and more.
In addition, DCS operator stations are divided into operator stations and engineer stations. From a system function perspective, the former mainly performs general production operations and monitoring tasks, with functions such as data acquisition and processing, monitoring screen display, fault diagnosis, and alarms. The latter, in addition to the general functions of an operator station, should also have functions such as system configuration and modification of control objectives. From a hardware perspective, in most systems, the engineer station and operator station are combined, distinguished only by a single engineer keyboard.
