Abstract: The sequential control is widely applied in industry control .To desige sequential control system by PLC sequential control method is the paper propose . User’s control program is compiled by SFC(Sequential Function hart) and LD(Ladder Diagram) to realizing sequential control on singled eviceor production process. This paper expounds basic elements of sequential function chart，basic rule for implement of transition in sequential function chart. Three kinds of design method on PLC sequential control system are introduced：the first is the desige in state chart . the second is the design in set/reset instructions（S/R）. and the third is the design in Step Ladder instructions（STL）. At last this paper gives an example of the machine of press autocontrol with the help of PLC . The exnmple uses the design method in STL，provides systematic schematic circuit diagram、diagram of program structure、 function diagram of program`s sequence ，trapezoidal diagram of the program and the program repertoire，to explanat the design steps and the process of the PLC control system.And shows that sequential control design method is an advanced regular design method for design of PLC ladder diagram，and it is a powerful tool. It is not only enhance effective and save the time of desige，but also is convenience of debuging、revising and reading.
Keywords: PLC； Sequential control system； The machine of process autocontrol system
