Programmable controller is actually similar to microcomputer.frommicroprocessor（CPU），storage(ROM, RAM), input/output unit (I/O), programmer and power supply.CPU is equivalent to human brain, and memory stores files.It is an I/O function to scan documents and print them, which is similar to human facial features.The programmer is used for user program preparation, debugging, inspection and monitoring, and can also call and display some internal status and system parameters of PLC through the keyboard.The power supply is the power supply for PLC.
In the 1960s, due to the emergence of minicomputers, large-scale production and the development of multi computer group control, people tried to use minicomputers to achieve industrial control, replacing the traditional relay contactor control.However, industrial control with minicomputers is expensive, input and output circuits do not match, and programming technology is complex, so it has not been popularized and applied.
In the late 1960s, the American automobile manufacturing industry was highly competitive. In order to meet the needs of constantly updating production processes, General Motors Corporation of the United States first invited public bidding in 1968. The specific requirements for the control system were basically ① its relay control system had short design cycle, easy change, simple wiring and low cost; ②It can combine the functions of the computer withRelay control systemCombine.But programming is easier to learn and operate than computer; ③The system has strong versatility.
In 1969, according to the above requirements, the American Digital Equipment Company developed the first programmable controller in the world, which was successfully applied in the automobile production line of General Motors for the first time, realizing the automatic control of production.Later, Japan, Germany and other countries introduced programmable controllers, which developed rapidly.But in this period, it was mainly used for sequential control. Although it also adopted the design idea of computer, it could only carry out logical operations at that time, so it was called programmable logic controller (PLC) for short.
In the late 1970s, with theMicroelectronicsAnd the rapid development of computer technology,PLCIt has more computer functions. It not only replaces hardwired logic with logic programming, but also adds functions such as calculation, data transmission and processing. It has truly become an electronic computer industrial control device, and has achieved miniaturization and super miniaturization.The function of this industrial control device using microcomputer technology is far beyond the scope of logic control and sequence control, so it is called programmable controller (PC) for short.However, because PC is easily confused with Personal Computer, people are still accustomed to using PLC as the abbreviation of programmable controller.
Features of programmable controller
(1) High reliability and strong anti-interference capability (MTBF can reach 30000 to 50000 hours generally)
(2) Convenient maintenance
(3) Flexible and universal
(4) Perfect functions
(5) Simple wiring
Programmable controller
(6) Simple programming and convenient use
Operating principle of programmable controller
The working principle of PLC is infinite cycle scanning, and the scanning process is initialization processing;Processing input signal stage;Program processing stage;Processing output signal stage.The scanning cycle process is T=(time to read a point * number of points)+(operation speed * number of program steps)+fault diagnosis time.
Performance of programmable controller
(1) Working environment: generally, the working temperature of PLC is 0 ℃ to 55 ℃, the maximum is 60 ℃, and the storage temperature is - 20 ℃ to+85 ℃;The relative temperature is 5%~95%.(Under air conditions, combustible, explosive and corrosive gases shall not be mixed around)
(2) It is resistant to vibration and has strong impact performance. Generally, PLC can withstand vibration and impact with frequency of 10~55HZ, amplitude of 0.5mm, acceleration of 2g, and impact of 10g
(3) Cyclic scanning, one cycle scanning time is about 10ms, so PLC has low failure rate, is not easy to break, and has high reliability.
(5) Communication, networking and acquisition control
Classification of Programmable Controllers
Integral type: CPU unit, memory, I/O unit are installed in the same machine body to form the host.
Combined type: all units are distributed on modules, and different modules can realize different functions.
Small computer: about 100~500 control points.(Overall test)
Medium shaping machine: about 500~1000 control points.(Overall test)
Mainframe: more than 1000 control points (combined test).
design theory
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What is switching value?What is analog quantity?
There are only two opposite working states of switching value, such as high level and low level, power on and power off of relay coil, and connection and disconnection of electric shock. PLC can directly input and output switching value signals.Some PLCs (such as Siemens S7 series) refer to switching value as digital value.
Analog quantity is a continuously changing physical quantity, such as voltage, temperature, pressure, speed and other PLC, which can not process analog quantity directly, and needs to input analog quantity into the moduleA/D converterTo convert analog quantity into digital quantity proportional to the input signal.Digital quantity in PLC (e.gPID controllerOutput of the analog output moduleD/A converterConvert them into voltage or current proportional to corresponding figures for external actuators (such as electric regulating valve orFrequency converter）Use.
So what is switching value?What is analog quantity?
The switching value is either energized or de energized, or called 0 and 1. 0 means energized, and 1 means de energized.For example, buttons, switches,Time relayOvercurrent pressure relay, which belongs to the input and output of contactors, relays, solenoid valves, etc
Analog quantity is a continuously changing quantity, for example, the input is sensor, and the output is servo motor, solenoid valve, distance, speed and other control signals.
In other words, the switching value is a real object, while the analog value is a virtual object, such as drawing on a computer
Which programming languages are most commonly used?
Ladder diagram is the most widely used PLC graphic language.The ladder diagram is similar to the relay circuit and has the advantage of being intuitive and easy to understand. It is easy to be mastered by the electrical personnel who are familiar with relay control in the factory, and is especially suitable for switching value logic control.
The statement table is a text language similar to the assembly language of a microcomputer. Multiple statements form a program segment.The input of statement table is convenient and fast, and comments can be added after each statement to facilitate the reading and understanding of complex programs.It is recommended to use statement table when designing advanced application programs such as communication and digital operation. The statement table program is difficult to read, and its logical relationship is difficult to see at a glance. Ladder diagram language is best used when designing programs with complex electric shock control circuits.
The function block diagram is similar toAnd door、OR GateThe function block diagram is suitable for those who are familiar with digital circuits.Ladder diagram is used most in China, and instruction list language is also used more. Europeans prefer function block diagram.
Sequential function diagram is a graphic language on top of other programming, which is used to describeSwitching value control systemAnd a powerful tool for writing sequence control programs.
Structure text isIEC 61131-3A special high-level programming language created by the standard.
What are the characteristics of ladder diagrams?
(1) Some programming elements in PLC ladder diagram follow the name of relay, such as input relay, output relay auxiliary relay, etc., but they are not real physical relays (i.e. hardware relays), but programming elements used in software. Each programming element corresponds to a memory unit of the element image register in PLC memory.Take auxiliary relay M0 as an example, if the correspondingStorage unitIf it is in 0 state, the coil of M0 in the ladder is "powered off", its normally open electric shock is disconnected, and its normally closed electric shock is closed, which is called M0 as 0 state, or M0 as OFF.If the storage unit is in 1 state, and the coil of M0 is "powered on", its normally open contact is connected and its normally closed contact is disconnected, M0 is called OFF.If the storage unit is in 1 state, the coil of M0 is "energized" and its normally open contact is connected, and the normally closed contact is disconnected, it is called M0 as state, or M0 as ON.
(2) According to the logic relationship of the states of the contacts in the ladder diagram, the ON/OFF state of the programming element corresponding to the lines in the diagram is calculated, which is called the logic solution of the ladder diagram.The logic solution is carried out from top to bottom and from left to right in the ladder diagram.The result of the solution can be immediately used by the subsequent logical solution.The logic solution is based on the value in the input image register, rather than the state of the instantaneous external input contact.
(3) Programming the state of the normally open contact of the element.
(4) The status of the input relay depends solely on the on-off status of the corresponding external input circuit, so the coil of the input relay cannot appear in the ladder diagram.
