PLC, DCS, and SCADA
PLC vs DCS
Programmable logic controller (PLC) is widely used in industry. It is typically used in a small-scale system to control multiple devices in a centralized manner, i.e. one PLC controls multiple components. Distributed Control System (DCS), as implied by the name, implements a decentralized control architecture such that a failure in one sub-system does not fail the whole system. This is typically used in a large-scale system. However, now it is not uncommon that multiple sub-systems can also be controlled by using multiple PLCs in an arrangement such that a failure in one sub-system does not fail the whole system. In this case, the difference between PLC and DCS becomes blurred.
PLC is more suitable to control many discrete processes and limited analog processes, whereas DCS is more suitable to control many analog processes and limited discrete processes.
DCS commonly comes with many prescribed functional block diagrams; this will fasten a system development. This is usually not the case with PLC.
SCADA vs DCS
A system of PLCs and RTUs (remote terminal units) can be integrated into a Supervisory Control and Data Acquisition (SCADA) system. As indicated by the name, a SCADA system basically acquires data from the control units and supervises the process from a supervisory computer. A SCADA system typically consists of supervisory computer (which is the core of the SCADA system), human-machine interfaces (HMIs), control units to be supervised (PLCs and RTUs), and communication infrastructure (which connects all into a network). The supervisory computer is typically a PC equipped with a SCADA software. Hence, the most important part of a SCADA system is the SCADA software.
According to Wikipedia, the development of SCADA can be referred to four generations. The first generation of SCADA is monolithic, i.e. without centralization, as networking infrastructure was not present yet at that time. The second generation of SCADA is distributed, in which multiple stations are interconnected through LAN. The third generation of SCADA is networked, in which the system supervises more than one LAN network. Several distributed SCADAs are integrated in a single supervisor. This allows the supervision of a very large-scale system. The fourth generation of SCADA is web-based, which takes benefit of the web technology. It is worth-noted that a SCADA system can be remote (geographically separated) or local (confined).
The idea of DCS is basically the same to SCADA. People usually see DCS as an “off-the-shelf SCADA system”. In other words, a DCS is basically a SCADA system deployed by a specific vendor whereas a SCADA is a custom SCADA. A DCS is typically deployed by a vendor as a ready-to-use system for a specific control environment. As the result, the supervisory system as well as all the other control sub-systems are from the same vendor/manufacturer. This eliminates the need to get a separate supervisory software as it is already built in in the system from the beginning. The communication in a DCS is more proprietary. With all these characteristics, DCS offers more reliability, safety, and security, but at the same time it is less flexible or more tedious to be scaled. A DCS is also typically local (confined) and does not include remote systems.
On the other hand, a SCADA system is built from scratch, with the control sub-systems can be from various manufacturers. A supervisory software needs to be made and added to a current system. The communication can be adapted to the advance of the communication technologies, and hence offers more flexibility and adaptability. A new sub system can also be easily added and integrated to the system and hence SCADA offers more scalability. However, as SCADA was much criticized for its safety, some efforts have been made to make it safer, i.e. less prone to hack. It is also worth noted that a SCADA system can be geographically separated.