Introduce DCS and PLC differences and applications using the simplest basis

The topic of DCS and PLC has been debated in the industry for at least 40 years. To understand that everyone is arguing, the instrumentation industry must understand the difference between DCS and PLC, objectively understand these fundamental differences, and rationally choose according to products and needs. Apply DCS and PLC.

1. DCS focuses on the overall control system, while PLC focuses on local logic control. Who is this prescribed? Why is this divided? This is because DCS is developed from analog meters, mainly based on analog quantities. DCS bias process control can realize complex control such as PID, feedforward, cascade, multi-stage, selective control, etc., emphasizing the precision of continuous process control; PLC is developed from electrical relays, mainly digital, PLC oriented to general industry Control field, strong versatility. The general PLC only has the PID function, and its control accuracy is not as high as DCS.

2, DCS and PLC in the difference in communication DCS uses the international common TCP / IP protocol, which is what we often say Internet interface, has a very good versatility and scalability; and PLC communication is varied (such as PPI, Internet, Device net, modbus, Profibus, USB, etc.), it is often difficult to increase and reduce the operator station after the PLC is built. In terms of network security, a large number of technicians in the Internet field are engaged in network security work, and PLC communication security currently has no good protection measures. The DCS adopts the overall consideration scheme, and the operator station also has the function of the engineering station. The station and the station are closely combined after being installed in the running program. Each station, each function, each controlled device The system is interlocked and coordinated, and the system consisting of PLCs connected to each other is loosely connected between the PLC and the PLC, and the function of coordinated control cannot be performed.

3, DCS and PLC reliability differences DCS is dual redundancy, can achieve bumpless switching, DCS I / O modules are equipped with CPU, can achieve acquisition and output signal quality judgment and scalar conversion, module failure Charged plugging and random replacement; most of the PLCs are not redundant, and the cost of the two-way PLC will be higher. The PLC module is a simple electrical conversion unit, no smart chip, and the corresponding units are all defective after the module fails. At present, PLC has been able to achieve redundancy, hot standby and live plugging, and has been able to complete most functions of DCS. DCS and PLC have their own advantages, and the two are also infiltrating each other, so now it is no longer a good idea to talk about DCS and PLC. For example, Siemens' SIMATIC PCS7 system is a new system combining PLC and DCS.

4, DCS and PLC in the number of points difference DCS points a lot, generally more than 500 points; PLC points are small, generally around 100, more than 500 points basically do not use all the control system connected by PLC, PLC system cost will also Very high. In the DCS update control scheme, the engineer executes the downloaded control command on the engineer station to execute the download command. The program download process is automatically completed by the system and does not affect the operation of the original control scheme. PLC change control scheme, first of all need to know which PLC, compiled with the compiler and passed to the corresponding PLC, the debugging time is high, which is not conducive to future maintenance.

5. Differences in software The DCS system has a powerful software package. It can also develop software according to process parameter configuration and control algorithm requirements. It has a recipe function, and the software aspect DCS is more convenient than PLC application. Should the industrial site choose DCS or PLC? The choice between the two is mainly determined by the investment situation and where it is used. For example, PLC burner logic control and protection PLC is more suitable than DCS, and some turbines such as steam turbine control system have many analog probes, DCS is more suitable, do not blindly worship DCS and ignore PLC!