Depending on the functionality and complexity of the control action, industrial control systems are differentiated into several types, but most commonly and widely used control systems are of three types:
- Programmable Logic Controllers (PLCs)
- Distributed Control Systems (DCS)
- Supervisory control and Data Acquisition (SCADA)
Programmable Logic Controllers (PLCs)
As an industrial computer, Programmable Logic controller (PLC) is capable of being programmed to perform various industrial control applications. PLC is built with modules like CPU, power supply, I/O modules (both digital and analog) and communication modules.
These PLCs can be modular or integrated types. A modular type PLC is fixed and compact one with limited I/O functionalities whereas an integrated PLC is capable to extend I/O modules based on the required I/O functionalities.
The sensors are connected to the input module and output devices or actuators to the output module of the PLC. The CPU of the PLC continuously reads the inputs from the sensors and according to the program, it produces the outputs to operate the actuators.
Distributed Control Systems (DCS)
Distributed Control System is a specially designed industrial control system which is used to control complex and geographically distributed applications. As the name implies, these controllers are distributed in the entire plant area.
The discrete field devices such as actuators and sensors are connected to these controllers and also maintains the continuous communications with operating PCs through a high speed communication network or bus.
The basic elements in the DCS control systems is an engineering PC controls all the distributed controllers, which controls the all the field devices connected to it.
The HMI operating station used to monitor the field parameters graphically and to log the data and communication media to establish data transfer between the controllers and operating stations. DCS facilitates the human machine interface (HMI), trend display and face plates for the effective monitoring of industrial processes.
Supervisory Control and Data Acquisition (SCADA)
A SCADA system is a type of industrial process automation system that collects data from various instruments which are located in plant area or even at remote sites. This data is further processed at a central location for monitoring and controlling purpose.
Based on the information from various remote stations, automated supervisory commands are sent back to these remote stations to control the field devices. Thus the remote control operation of equipments is possible with this SCADA.
SCADA gathers data using both hardware and software systems. It consists of Remote Terminal Unit (RTU) and Communication Terminal Unit (CTU) or Master Terminal Unit as hardware devices. The RTU is nothing but a programmable logic controller which collects the data from various field instruments and sends it to the CTU to transfer to remote location as shown in the figure.
At receiving side, CTU with SCADA HMI software displays all these parameters statuses and allows the user to send control signals to plant area. Apart from the industrial applications, SCADA is also popularly used in power transmission systems.
Hence the industrial control system is a multi-discipline system, which deals with disciplines like control systems, communication, instrumentation, electronics and electrical systems. Apart from the discussed control strategies some more advanced-control strategies are also used like cascade and ratio-control schemes. For any further discussions on this topic, you can leave a reply.