PLC full name is Programmable Logic Controller, according to the name of its role is mainly used to achieve industrial logic control.
Early automation is divided into industrial electric automation, automatic control automation and production process automation, only the industrial automation of this course to learn PLC this course, it can be seen that the PLC focuses on the application of electrical control system.
PLC was originally invented to replace the relay, control logic
Earlier, when PLCs were not available or were expensive, logic control was basically done using relays and contactors. Relay or contactor is a coil plus a few normally open or normally closed contacts, the use of coils and contacts between the self-protection and interlock to combine a variety of complex industrial control logic, such as machine tool control, to start the motor forward and reverse, to achieve the various axes with each other, to turn on the cold some liquid circulation pumps, to make the drive start and stop, all with a large number of relays. Logic is divided into combinational logic and sequential logic, relays to achieve combinational logic is easy, but the realization of sequential logic is a little difficult, after all, the time relay is more crude.
Because the relay and contactor volume is relatively large, so that the logic is complex, the design is more laborious to consider a lot of line circuits, a little inattention to get it wrong; installation is also troublesome to use a very large cabinet, the layout is not easy; debugging is also a painful thing to modify the line to death, because it is easy to change the line is wrong; maintenance is more worrying, the line is much more, so that you are dazzled by the relays have a certain Use cycle, maintenance electricians often face problems such as poor contact.
Electronic technology has developed, single-board computer appeared, and then people try to use single-board computer to replace the relay circuit, the results are very complex logic, as long as the integration in a board can meet the functional requirements. So industrial electrical systems can realize more complex logic control, and small size, maintenance is much easier than the relay.
But the reliability of the single-board computer can not, industrial places need to be very strong anti-interference, dust and vibration is also very strong, and the use of assembly language programming, the general electricians will not learn. After the microcontroller came out, the function of the single board machine is basically integrated into a chip, so there is a company called Modicon designed the PLC this product out, the hardware takes into account the requirements of industrial sites, processing is more stable and reliable, and then the software according to the habits of mind of electricians, to get out of a set of ladder diagrams programmed to the system, and the line of relay circuits are almost the same, so that through the computer on the "Wiring", with symbols to replace the physical, you can realize the 'soft' method to modify the logic circuit.
And PLC is designed with counters and timers, so as to avoid the use of troublesome physical timers and time relays, modifying the counting and timing values is very fast and convenient, and the traditional relays are difficult to achieve the timing logic control done very accurately. Even some bosses worry about customers do not give money, the use of perpetual calendar to set a clock, running to a certain year, a certain month, a certain day, a certain time, a certain minute, a certain second, automatically will stop part of the function to.
In the field of industrial control, as long as the need to use logic control equipment, can be realized with PLC. For example, metallurgy, plastics, woodworking, water treatment, chemical, petroleum, building materials, electricity, home appliances, etc., almost all areas, as long as there are automation equipment occasions, will use the PLC, and some civilian areas, such as security, transportation, smart home, etc., will also be used to PLC. PLC strengthened data processing and communication, equivalent to a small computer.
Early PLC can only complete the logic control only, but the chip and electronic technology development is very fast, PLC increased analog processing capabilities, can directly read the sensor analog data, so that can be used to do some data acquisition and monitoring, if these data are used to control the inverter and other devices, and can achieve a number of closed-loop target control.
PLC increased high-speed counting function, you can achieve some length and other parameters such as measurement, while there is a high-speed pulse output, can be used to control servo and stepper controller positioning.
PLC computing speed, you can write some of their own PID or fuzzy algorithm function, can be used to do some complex loop control, and even some PLC itself with a PID module, directly call on it.
PLC also integrates a variety of communication interfaces such as 485, 232 and RJ45, which can be realized and inverter, touch screen, temperature control module, industrial control machine and other products for communication networking.
PLC increased these functions, many aspects of the process control field has replaced the controller, and PLC only need to add a function module, you can easily achieve some of these traditional controller functions, some of the large-scale PLC to achieve the function, not much worse than the DCS system.