While FCS is inextricably linked to PLC and DCS, there are also fundamental differences between them.
DCS (Distributed Control System), also known as a distributed control system, is a type of computer control system that is developed and evolved from centralized control systems.
FCS (Fieldbus Control System) is an open, interoperable network that interconnects various controllers, instruments, and equipment in the field to form a fieldbus control system. It also decentralizes control functions to the field, reducing installation and maintenance costs.
PLC (Program Logic Control) is a programmable logic controller.
Detailed comparison of FCS and DCS
FCS evolved from DCS and PLC, possessing the characteristics of both while representing a revolutionary step forward. Currently, both new DCS and PLC technologies tend to converge towards each other. Newer DCS systems offer strong sequential control capabilities, while newer PLCs excel in closed-loop control. Furthermore, both can form large-scale networks, leading to significant overlap in their applicability.
Communication is key to a DCS system. The data bus can be considered the spine of a DCS system. Since its task is to provide a communication network between all components of the system, the design of the data bus itself determines the overall flexibility and security. The data bus medium can be a twisted pair of wires, coaxial cable, or fiber optic cable. By understanding the design parameters of the data bus, one can essentially grasp the relative advantages and disadvantages of a specific DCS system.
To ensure communication integrity, most DCS manufacturers provide redundant data buses. To guarantee system security, complex communication protocols and error detection technologies are used. A communication protocol is a set of rules used to ensure that transmitted data is received and understood as if it were sent. Currently, two types of communication methods are generally used in DCS systems: synchronous and asynchronous. Synchronous communication relies on a clock signal to regulate data transmission and reception, while asynchronous networks use a reporting system without a clock.
Traditional method: The connection between field devices and the controller uses a one-to-one I/O connection.
Fieldbus technology: Connecting all field devices using a single cable.
PLC and DCS
PLC:
The evolution from switch control to sequential control and transport processing involves multiple functions such as bottom-up continuous PID control, with the PID controller located in the interrupt station.
One PC can be used as the master station, and multiple PLCs of the same type can be used as slave stations.
Alternatively, one PLC can be used as the master station, and multiple PLCs of the same type can be used as slave stations to form a PLC network. The advantage of this over using a PC as the master station is that when programming, users do not need to know the communication protocol; they only need to write the code according to the instruction manual format.
A PLC grid can function as either an independent DCS or a subsystem of a DCS.
PLCs are mainly used for sequential control in industrial processes, while newer PLCs also have closed-loop control functions.
DCS:
Distributed Control System (DCS) is a monitoring technology that integrates 4C (Communication, Computer, Control, and CRT) technologies.
In a large, top-down tree-like topology system, communication is key.
In the interrupt station, the connection between the computer and the field instruments and control devices is a tree topology and a parallel continuous link structure. There are also a large number of cables running in parallel from the relay station to the field instruments.
Analog signals, A/D to D/A, and microprocessor-based hybrid signals.
One instrument has a pair of wires connected to the I/O, which is then connected to the local area network (LAN) from the control station.
DCS is a three-tiered architecture consisting of control (engineering station), operation (operator station), and field instruments (field measurement and control station). It is used for large-scale continuous process control, such as in petrochemical industries.
Industry insiders explain the differences between PLC and DCS
PLCs and DCSs cannot be directly compared. A PLC is a controller, an isolated product, while a DCS is a system. However, a PLC can be compared to a DCS control station. A PLC's cycle time is around 10 milliseconds, while a DCS control station's is around 500 milliseconds. PLCs are more open and have a stronger ability to work independently as a product.
—— OMRON
DCS is a system that includes host software, network and controller, while PLC is just a controller. To form a complete system, a host SCADA system and a network connected to it are also required.
For PID loop control, Mitsubishi's process controllers can now be programmed using FBD, similar to SAMA configuration; DCS systems are larger, control more loops, and have more control methods and algorithms, enabling them to handle more complex inter-loop control. Hardware reliability is roughly the same. DCS can achieve I/O redundancy, while PLCs cannot. Relatively speaking, PLC-based systems are less expensive.
— Manager of Automation and Precision Control Department, Mitsubishi Electric
DCS is a "distributed control system". In terms of hardware, it includes field controllers, operator station computers, engineer station computers, and the network system that connects them. In terms of software, DCS is a holistic solution that solves all the technical problems of a system, and the various parts of the system are closely integrated.
A PLC is a device, which in hardware is equivalent to a field controller in a DCS; in software, it is a local solution with loose organization between stations.
— Technical Manager at B&R responsible for APROL product application development
The key differences between DCS and PLC lie in two points: first, DCS is distributed control with a global database; second, PLC uses a sequential scanning mechanism, while DCS is time-based control. Our system meets the first point; for example, a change in an I/O tag is simultaneously reflected in the HMI.
