The comparison of PLC and DCS

The design principle of difference between DCS and PLC is larger, the PLC controller is developed from relay control principle, it to store the execution logic operation, sequence control, timing, counting and arithmetic operations such as instruction; And through the digital input and output operations, to control various types of machinery or production process. User control program expressed the production process requirement, and advance in the PLC controller user program memory. Run time according to the content of the stored procedure to perform a detailed, to complete the process requirements of operation. PLC controller CPU contains instructions step program memory address of the program counter, in the process of program is running, perform step this counter is automatically add 1, from initial step ( Step number zero) Executed in sequence to the final step ( Usually the end instruction) Step, and then return to the starting cycle operation. PLC every time required to complete a cycle operation is called a scan cycle. Different models of PLC, loop scan cycle between 1 millisecond to a few microseconds. The program counter that cycle operation, this is the DCS. This is also the PLC redundancy as the cause of the DCS. DCS is developed on the basis of the operational amplifier. All the function and the relationship between the process variables are function block ( Some of the DCS system called puffing block) 。 Performance of DCS and PLC are the main differences in the switch quantity logic decoding and analog operation, even if later some infiltration between the two, but there are still differences. In addition to the logic operation, after the 80 s, PLC control circuit with the algorithm function has been greatly strengthened, but the PLC ladder diagram programming, analog computing in programming is not very intuitive, programming more troublesome. But in terms of solving logic, showed the advantages of quick, in microseconds level, solution 1 k logic program is less than 1 millisecond. It had all the input as a switch, and 16 ( There are also a 32-bit) As an analog. And DCS as all input analog, one is the switch quantity. Calculating a logic is in the hundreds of microseconds to several milliseconds scale. For PLC calculating a pid arithmetic in tens of milliseconds, with the DCS operation time. In terms of grounding resistance, the PLC may request is not high, but the DCS must be below a few ohms ( Usually under 4 ohms) 。 Analog isolation is also very important. The same I/o points system, PLC is much simpler to use than DCS, its cost is lower, Can save about 40% or so) 。 PLC no special operation station, it USES software and hardware is common, so the maintenance cost is much lower than DCS. If the object is mainly equipment chain, loop relatively few, PLC is more appropriate. If mainly analog control, and functional operation is very much, had better use DCS. DCS in the controller, I/o board, redundant communication network and so on, the special requirements of some advanced computing, industry is much better than PLC. PLC with general monitoring software, in terms of design management information system of enterprise, easier.
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