Basic knowledge: PLC programmable controller

In late 1970s, with the rapid development of microelectronics and computer technology, programmable logic controller has more computer function, not only replaced the hardwired logic with logic programming, but also increases the computation, data transmission and processing and other functions, to truly become an electronic computer industrial control device, and achieve miniaturization and miniaturization. The function of industrial control device using microcomputer technology is far beyond the scope of logic control and sequence control, so it is called programmable controller, or PC (Programmablc Controller). But because PC is easily confused with personal computers (Personal Computer), people are still accustomed to using PLC as an abbreviation for programmable controllers.
 
The structure of programmable logic controller
 
Programmable controllers are virtually the same as microcomputers. The microprocessor (CPU), the memory (ROM, RAM), the input / output unit (I/O), the programmer, and the power supply. CPU is equivalent to the human brain, and memory is the storage file. Scan the file, print out the document, this is the function of I/O, quite with human features. Programmer for user programming, debugging, inspection and monitoring, but also through the keyboard to call and display some of the internal state of PLC and system parameters. The power supply is PLC powered.
Characteristics of programmable logic controller
 
(1) high reliability, strong anti-interference ability! (mean time to failure is generally up to 3— 50 thousand hours.
(2) convenient maintenance;
(3) flexible: general purpose
(4) perfect function;
(5) wiring is simple
(6) it is easy to program and easy to use.
The working principle of programmable logic controller
 
The working principle of PLC is infinite cycle scanning. The scanning process is initialization processing, dealing with the input signal phase, the program processing phase, and the output signal phase. The scan cycle process is T= (read a bit of time * points) + (speed of operation * program steps) + fault diagnosis time.
The performance of programmable logic controller
 
(1) working environment, general PLC work temperature is 0, facilities degree to 55 facilities degree, the highest is 60 facilities degree, storage temperature is -20, facility degree +85 facilities degree, relative temperature is 5%~95%. (air conditions can not be mixed with flammable, explosive and corrosive gases)
(2) vibration resistance, impact performance, general PLC can withstand vibration and shock frequency of 10~55HZ, the amplitude of 0.5mm, acceleration of 2G, impact of 10g
(3) cycle scanning, one cycle scanning time is about 10ms, so PLC fault rate is low, not easy to bad, high reliability.
Programmable logic controller, application situation
 
(1. switch logic control
(2. analog closed loop control
(3 digital intelligent control)
(4 data acquisition and monitoring
(5 communications, networking and collection control