Programmable Logic Controller,also known as Programmable Logical Controller,or PLC for short.It is an electronic system device with digital computing operation based on a microprocessor,designed for industrial field applications.PLCs are involved in industries such as steel,mining,cement,petroleum,chemical industry,electricity,machinery manufacturing,automobiles,loading and unloading,papermaking,textiles,environmental protection,and even entertainment in our daily lives.In short,it is a controller commanded by a program written by electrical technicians according to the control requirements of the controlled machine.
Basic structure of programmable logic controller：
01.CPU module
The CPU module is also called the central processing unit or controller,which is mainly composed of a microprocessor(CPU)and memory.It is used to run user programs,monitor the status of input/output interfaces,make logical judgments,and perform data processing,that is,read input variables,complete various operations specified by user instructions,send results to the output end,and respond to requests from external devices(such as programmers,computers,printers,etc.)and perform various internal judgments.
02.I/O module
The I/O module is the eyes,ears,hands,and feet of the system,and is a bridge connecting the external field and the CPU module.The input module is used to receive and collect input signals.
03.Power supply
The PLC generally uses a 220V AC power supply.The DC regulated power supply inside the PLC provides DC voltage for the components in each module.
04.Editor
The programmer is an external programming device of the PLC.Users can input,check,modify,debug programs or monitor the working status of the PLC through the programmer.The PLC can also be connected to the computer through a dedicated programming cable,and the computer can be programmed and monitored using programming software.
05.Input/output expansion unit
The I/O expansion interface is used to connect the expansion unit that expands the number of external input/output terminals to the basic unit(i.e.,the host).
06.External device interface
This interface can connect external devices such as programmers,printers,barcode scanners,and inverters to the host to complete the corresponding operations.
Working principle of programmable logic controller：
The programmable controller has two basic working states,namely,the running(RUN)state and the stopping(STOP)state.
In the running state,the programmable controller implements the control function by executing the user program that reflects the control requirements.In order to make the output of the programmable controller respond to the input signal that may change at any time in time,the user program is not executed only once,but repeatedly executed until the programmable controller stops or switches to the STOP working state.
In addition to executing the user program,the programmable controller must complete internal processing,communication processing and other tasks during each cycle.One cycle can be divided into 5 stages.
In the internal processing stage,the programmable controller checks whether the CPU and the hardware inside the module are normal,resets the monitoring timer,and completes some other internal work.
In the communication service stage,the programmable controller communicates with other intelligent devices with microprocessors,responds to the commands typed by the programmer,and updates the display content of the programmer.
In the input processing stage,the programmable controller reads the on/off status of all external input circuits into the input image register.
In the program execution stage,even if the state of the external input signal changes,the state of the input image register will not change accordingly.The changed state of the input signal can only be read in the input processing stage of the next scan cycle.
In the output processing stage,the CPU transmits the on/off state of the output image register to the output latch.
Features of Programmable Logic Controller：
High reliability,strong anti-interference ability
General and flexible
Simple and convenient programming
Complete functions,strong expansion ability
Short design,construction and debugging cycle,easy maintenance
Due to the above characteristics of PLC,the application range of PLC is extremely wide.It can be said that as long as there is a factory and there is a control requirement,there will be PLC application.
