唐 山 学 院 毕 业 设 计
40
唐 山 学 院 毕 业 设 计
41
唐 山 学 院 毕 业 设 计
42
唐 山 学 院 毕 业 设 计
外文资料
What is a PLC?
Programmable Logic Controllers (PLCs) consist of input modules or points , a Central Processing Unit(CPU) , and output modules or points . It is actually a control device that consists of a programmable microprocessor , and is programmed using a specialized computer language . A PLC monitors inputs , makes decisions based on its program , and controls output to automate process or machine .
An input accepts a variety of digital or analog signals from various field devices (sensors) and converts them into a logic signal that can be used by the CPU. The CPU makes decisions and executes control instructions based on program instructions from the CPU into a digital or analog signal that can be used to control various field devices(actuators).A programming device is used to input the desired instructions.These instructions determine what the PLC will do for a specific input.An operator interface device allows process information to be displayed and new control parameters to ve entered. Pushbuttons(sensors),in this simple examole,connecting to PLC input ,can be used to start and stop a motor which is in connection with a PLC through a motor starter(actuator).
Before,a programmable logic controller would have been programmed in ladder logic, which is similar to a schematic of relay logic.A modern programmable logic controller is usually programmed in any one of several languages,ranging from ladder logic to Basic or C.Typically, the program is written in a development environment on a personal computer(PC),and the is downloaded onto the programmable logic controller directly through a cable connection.The program is stored in the programmable logic controller in non-volatile memory.
PLCs are used in commercial and industrial fieds.They have made significant contributions to factory automation.Earlier automation systems had to use thousands of individual relays and cam timers,but all of the relays and timers within a factory system can often be replaced with a single programmable logic controller.Today, programmable logic controllers deliver a wide range of functions, including basic relay control,as well as being used in distributed control systems.
Siemens makes several PLC product lines in the SIMATIC(r)S7 family.They are:S7-200,S7-300,and S7-400.
(1) S7-200
The S7-200 is called as a micro-PLC because of its small size.The S7-200 can be
43