Detailed analysis of PLC and DCS

PLC and DCS can not be compared, PLC is the controller, is the isolated product, and DCS is the system. However, the PLC can be compared with the control station of the DCS. The cycle time of the PLC is about 10 milliseconds, and the DCS control station is about 500 milliseconds. The openness of the PLC is better, and the ability to work independently as a product is stronger.

DCS is a system including upper software, network and controller, and PLC is just a controller. To form a system, a higher SCADA system and a network connected to it are required.

For PID loop control, Mitsubishi's process controller can now implement the same FBD programming as SAMA configuration; DCS system is larger, the number of controlled loops is more, and there are more control and algorithms, which can complete more complicated loops. control. Hardware reliability is similar. DCS can do I/O redundancy, and PLC can't. In contrast, PLCs constitute a lower system cost.

DCS is a kind of “decentralized control system”. The hardware includes the field controller, the operator station computer, the engineer station computer, and the network system that connects them. The DCS software is a holistic solution that solves all the technologies of a system. The problem is that the various parts of the system are tightly integrated.

The PLC is a device that is equivalent to the field controller in the DCS on the hardware; the software is a partial solution, and the station and the station are loosely organized.

Analysis of the difference between DCS and PLC, the most critical is two points, one is DCS is distributed control, has a global database; the second is PLC is a sequential scanning mechanism, DCS is time-based control. Our system conforms to the first point, such as the modification of an I/O tag, which can also be synchronized in hmi.

PLC: From the development of switching control to sequential control and transportation processing, it is a multi-function continuous PID control from bottom to top, and the PID is in the interrupt station. One PC can be used as the master station, and multiple PLCs of the same type are slave stations. It is also possible to have one PLC as the main station and multiple PLCs of the same type as slave stations to form a PLC network. This is more convenient than using a PC as the main station: when there is user programming, you do not need to know the communication protocol, just write it according to the format of the manual. The PLC grid can be used as a stand-alone DCS or as a subsystem of the DCS. PLC is mainly used for sequential control in industrial processes, and the new PLC also has closed-loop control functions.

DCS: Distributed control system DCS integrates 4C (Communication, Computer, Control, CRT) technology in one monitoring technology. From the top to the bottom of the tree topology system, where communication is the key. The PID is in the interrupt station. The interrupt station is connected to the computer and the field instrumentation and control device is a tree topology and a parallel continuous link structure. There are also a large number of cables from the relay station in parallel to the field instrumentation. Analog signal, A/D-D/A, mixed with microprocessor. A pair of instruments is connected to the I/O by a pair of wires, and the control station hangs to the LAN of the local area network. The DCS is a three-level structure for control (engineering station), operation (operator station), and field instrument (site monitoring station). Used for large-scale continuous process control, such as petrochemicals.