Graduation Design Thesis of Plc —— Talking about the Application of PLC
Programmable controller is a digitally operated electronic system, which is specially designed for application in industrial environment. Programmable controller adopts programmable memory, which is used to execute operating instructions such as logic operation, sequence control, timing, counting and arithmetic operation, and control various types of machinery or production processes through digital and analog input or output. The position of programmable controller in industrial automation control, especially in sequence control, is irreplaceable in the foreseeable future.
Programmable controller; analog quantity
Programmable controller is the abbreviation of programmable controller, usually referred to as PC for short. But it's not a personal computer. Not only (but also) early programmable logic controller (PLC), programmable sequence controller (PSC) and programmable matrix controller (PMC).
Programmable controller and its related equipment should be designed according to the principle that it is easy to form a whole with industrial control and expand its functions. At present, PLC has been widely used in steel, petroleum, chemical industry, electric power, building materials, machinery manufacturing, automobile, textile, transportation, environmental protection, culture and entertainment industries at home and abroad, and its uses are mainly divided into the following categories:
1. Switch logic control
Replacing the traditional relay circuit with logic control and sequence control can be used not only for single equipment control, but also for multi-machine group control and automatic production line. Such as injection molding machine, printing machine, stapler, modular machine tool, grinder, packaging production line, electroplating production line, etc.
2. Industrial process control
In the process of industrial production, there are some continuously changing quantities (namely analog quantities), such as temperature, pressure, flow, liquid level and speed. PLC adopts corresponding A/D, D/A conversion modules and various control algorithm programs to process analog signals and complete closed-loop control. PID regulation is a widely used regulation method in general closed-loop control system. Process control is widely used in metallurgy, chemical industry, heat treatment, boiler control and other occasions.
3. Motion control
Programmable controller can be used to control circular motion or linear motion. Generally, special motion control modules are used, such as single-axis or multi-axis position control modules that can drive stepping motors or servo motors, which are widely used in various machinery, machine tools, robots, elevators and other occasions.
4. Data processing
Programmable controller has the functions of mathematical operation (including matrix operation, function operation and logic operation), data transmission, data conversion, sorting, table lookup and bit operation, and can complete data acquisition, analysis and processing. Data processing is generally used in some large-scale control systems, such as papermaking, metallurgy, food industry and so on.
5. Communications and networks
The communication of programmable controllers includes the communication between programmable controllers and the communication between programmable controllers and other intelligent devices. With the development of factory automation network, now PLC has communication interface, and communication is very convenient.
Programmable controller is a kind of equipment used for automatic control of industrial production, which can be used directly in industrial environment without any measures. However, despite the above-mentioned high reliability and strong anti-interference ability, when the production environment is too harsh, the electromagnetic interference is particularly strong, or improper installation and use, it may cause program errors or operation errors, resulting in input errors and output errors, leading to out-of-control and misoperation of equipment, thus failing to guarantee the normal operation of the programmable controller. In order to improve the reliability of PLC control system, on the one hand, PLC manufacturers are required to improve the anti-interference ability of equipment; On the other hand, we should attach great importance to the design, installation, use and maintenance, and multi-party cooperation can improve the problem and effectively enhance the anti-interference performance of the system.
Today's era is an era of automation, and the equipment in many industries, such as traffic light control, is closely related to computers. Therefore, a good traffic light control system will make technological innovation for road congestion and illegal control. With the rapid development of large-scale integrated circuits and computer technology, and the wide application of artificial intelligence in control technology, intelligent devices have made great progress and become the mainstream direction of modern science and technology development. This paper introduces the design of an intelligent traffic signal lamp system. The intelligent traffic light control system can realize the following functions: monitoring four main traffic intersections in the urban area; Each intersection has a fixed working cycle, and the central control center can change the cycle when the road is crowded; It can immediately take photos of illegal motor vehicles at intersections and extract license plates. In the world, an information revolution with microelectronics, computer and communication technology as the forerunner and information technology and information industry as the center is in the ascendant. How to combine computer technology with practical application more effectively and play its role effectively is the hottest topic in the scientific community, and it is also an unprecedented active field of computer application today. This paper mainly realizes the intelligent management of traffic lights at intersections by controlling the normal operation of passing vehicles with single chip microcomputer.
With the development of city and economy, traffic lights play an increasingly important role. It is precisely because of the traffic lights that the traffic flow and people flow are standardized, and the probability of traffic accidents is also reduced. However, the unreasonable use or setting of traffic lights will also affect the smooth flow of traffic.
Traffic lights consist of red light, green light and yellow light. Red light means no traffic, green light means permission, and yellow light means warning. Traffic lights are divided into motor vehicle lights, non-motor vehicle lights, crosswalk lights, lane lights, direction lights, flashing warning lights, and road and railway intersection lights. Traffic lights are used at intersections. By giving instructions to vehicles and pedestrians to move or stop, the traffic flow of people and vehicles arriving at the same time can minimize mutual interference, thus improving the traffic capacity of intersections and ensuring the smoothness and safety of intersections.
The schematic diagram of traffic lights at intersections is as shown in Figure 1. From north to south, from east to west, there are red, green and yellow traffic lights in every direction. In order to ensure traffic safety, the requirements are as follows.
1) PLC is used to form the electrical control of the north-south and east-west traffic lights at the intersection. After the system is electrified, the traffic command signal control system is controlled by the three-position switch SA 1. SA 1 handle pointing to the left 45? At this time, the contact SA 1- 1 is turned on, and the traffic command system starts to work according to the normal control function and works repeatedly according to the working sequence shown in Figure 2. SA 1 handle pointing to middle 0? At this point, the contact SA 1-2 is turned on, and the traffic command system is always on, with a green light in the north-south direction and a red light in the east-west direction. SA 1 handle refers to the right 45? At that time, contact SA 1-3 to connect, and the east-west green light of the traffic command system is always on, and the north-south red light is always on.
2) During normal control
(1) When the east-west direction allows traffic (green light), the north-south direction should be closed (red light); Similarly, when the north-south direction allows traffic (green light), the east-west direction should be banned (red light). ② Before the green light signal is switched to the red light signal, in order to remind the driver to slow down and brake in advance, there should be obvious prompt signals: the green light flashes and the yellow light is on at the same time. (3) The signal light control system should be able to automatically cycle after starting.
As shown in Figure 2, the action sequence diagram of the signal lamp is drawn according to the two states of signal lamp setting 1 and setting 0, and setting 1 indicates that the signal lamp is on.
3) Input/output signal distribution
With the rapid development of microprocessor, network communication and man-machine interface technology, industrial automation technology is changing with each passing day, and various products are fiercely competitive, and new products are constantly emerging. PLC has also developed from being able to handle only switching values to being able to handle analog and data. Combined with DCS, pid regulator, industrial pc and other technologies, it is no longer a simple control device, and will certainly develop and survive with the continuous development of automatic control technology. The position of programmable controller in industrial automation control, especially in sequence control, is irreplaceable in the foreseeable future.
The second part of Plc graduation design thesis is the application analysis of PLC engineering.
According to the application characteristics of PLC in engineering development, this paper studies the hardware composition and software structure of PLC, analyzes the working process of PLC control, and finally discusses the PLC programming language sentences, which has certain guiding significance for the application of PLC in control system.
Keywords: PLC project; Hardware system; Software system; Programming language statements; Control system document ID: a
China Library ClassificationNo.: TP27 ArticleNo.:1009-2374 (2015) 34-0033-03 doi:10.13535/j.cnki./kloc-.
Programmable logic controller (PLC) is a new type of industrial automation equipment. The core of PLC is microprocessor, which integrates automation, communication and computer technology. The characteristics of PLC are simple and flexible programmability, strong compression resistance to harsh working environment and strong adaptability. PLC has the advantages of small size, low price and light weight, and is widely used in industrial control and automation engineering of thermal power plants.
Research on the structure of 1 PLC
Different types of programmable logic controllers have the same structure and basic principles, so learning programmable control principles should start with hardware structure and software development.
Hardware composition of 1. 1 PLC
The hardware system of PLC includes CPU board, input and output circuit, memory expansion interface, etc.
1. 1. 1 CPU board: The core system of PLC is CPU board, which consists of CPU, ROM, RAM, parallel interface and serial interface. The function of CPU board in PLC is to calculate and control programs, manage and control different logical operations, arithmetic operations and the whole components of the system. Random Access Memory (RAM) and Read Only Memory (ROM) are installed in the PLC program and have the function of storing various system programs. Parallel interface and serial interface realize information exchange between CPU and interface circuits.
1. 1.2 input/output circuit: the input circuit includes DC input and AC input. The input circuit can receive the control signal program prompted by the field input device. After receiving, the photoelectric coupler can isolate the control signal for program coding, which can be converted into the standard signal format used in PLC program, and then read the signal through CPU, which can be transmitted to the memory.
The main function of the output circuit in PLC is to realize the output signal. When the control signal in the PLC system is output, the output circuit is responsible for transmitting the control signal to other external output devices to realize the work of the output circuit. There are three kinds of output circuits: (1) relay output circuit, which controls the coil of relay to make the contacts of relay on and off, thus achieving the purpose of electrical isolation; (2) transistor output circuit, which uses photoelectric coupler to switch the transistor, thus controlling the output device; (3) SCR output type, with SCR as the medium to control the output equipment. When the SCR is triggered, the circuit can be opened and closed.
1. 1.3 memory expansion interface: it is an expansion card box for read-only memory and random access memory. There are three kinds of common expansion card boxes: (1)COMS ROM, which can be powered by lithium battery on the motherboard. The advantage of this card box is to ensure that data and programs will not be lost in the case of power failure or power failure; (2) Erasable programmable ROM card box requires a special programmer to write data into the debugged ROM. When erasing, the visible internal chip is irradiated with ultraviolet rays to erase the data, and a certain programming voltage is required when writing, so it can be erased and programmed repeatedly; (3)EEPROM card box, electrically erasable programmable read-only memory, is a kind of memory that will not lose data when power is off, and can be realized by using a special programmer when programming and erasing operations are implemented.
1. 1.4 I/O expansion interface: CPU is connected with I/O expansion interface through bus connection, which can connect all expansion units, making the signal point scale more flexible. The input/output expansion interface can also be connected with other adapters such as analog quantity and high-speed pulse, thus expanding and enhancing the functions of PLC.
1. 1.5 Programmer and its interface: The function of the programmer in PLC is to debug and edit the input of data and information, and to detect the safety of the input data. In normal operation, PLC does not need a programmer to program data, so the programmer exists as an independent design in PLC components. There is usually a special programmer interface on PLC, which is suitable for connecting different types of programmers in order to complete the programming and debugging of PLC programs.
Research and Analysis of 1.2 Programmable Controller
A control system must realize its own control function with the help of corresponding control programs. Control procedures are divided into the following two types:
1.2. 1 fixed wiring program control. In the old relay, if you want to control all kinds of programs, the circuit connection of the relay needs to be in the form of wiring, and the role of input devices is to send control signals to the control system, such as button switches and sensors. The function of the output device is to control the actions of the controlled person. The control mode of the output control signal of the equipment is completed by connecting lines. After the wiring is completed, the control program is also determined. If you need to change the control program again, you need to rewire all the wiring of the original control program and make a new connection mode. In complex control systems, such programs are difficult to control and programming feasibility is not high.
1.2.2 programmable control. When the programmable controller is used to control the system, it is only necessary to use a special programmer to realize programming through the corresponding programming language, download the control program into the memory, and finally perform various operations on the programming with the help of the programmable controller. If you want to change the programmable system, you only need to change the program language in the program memory, and you can do it through the programmer without changing the circuit connection and rewiring. Generally speaking, it is to write program code in a specific software programming language to realize all kinds of action control of the controlled object.
2 the working principle of PLC engineering
The core electronic component of PLC is microprocessor, which can also be regarded as a comprehensive component of relay, timer and state machine. In PLC, the input relay is driven by an external switch, and the output relay is equipped with many contacts. The work of PLC is actually to execute the program. In the working state of PLC, CPU takes time-sharing operation as its working principle, and performs corresponding operations in one cycle, that is, the program scanning of CPU. The speed of CPU processing programs is very fast, so from a macro point of view, it seems that the program operation of CPU is completed in a short time. The process of PLC program execution is divided into the following three parts:
2. 1 input processing PLC uses repeated scanning to complete the program. Before execution, the CPU programs all input signals into the input memory according to the coding sequence appearing in the address, and then starts to execute the program. When the CPU executes the program, even if the input state changes, the data content in the input register will not change with the change of the input state, and the CPU will not re-read the input state until the end of the scanning cycle.
2.2 program execution
When PLC executes the program, it scans the user programs in sequence. After the execution of a program is completed, the required information will be read by the program through the register and participate in the program operation, and then the data result of the program execution will be programmed into the relevant register.
2.3 output processing
After the PLC executes all instructions, it will input all program results into the output latch register and finally transmit them to the program execution terminal.
3 PLC software system components
A complete PLC control system consists of hardware system and software, which combine to form a complex control function. In the PLC software system, it is divided into system program and user program.
The function of system program in PLC is to manage, serve and translate user programs, which can be regarded as a software platform. The quality of system program is directly related to the performance of PLC. If the system program quality is good, the PLC performance is strong, otherwise the performance is weak. The system software is fixed in the program and cannot be modified or accessed by itself. The user program is the application program, which is written by the user in programming language according to the requirements of the control system and stored in the storage location specified by the system program.
4 PLC programming language
Use sequence-oriented and process-oriented to control programs? Natural language? , is the programming language of PLC. There are many programming languages of PLC, such as ladder diagram, logic equation, glossary or Boolean algebra. The following introduces the commonly used PLC programming language.
The basic instructions of PLC (such as Mitsubishi FX2 series) are as follows:
4. 1 logic and output (LD/LDI/ output) instruction
The LD/LDI command is used to connect normally open contacts/normally closed contacts to the bus. In addition, at the beginning of the branch, these instructions are used in combination with the following ANB (block AND) instructions; The OUT command is used to drive the coils of output relay, auxiliary relay, state machine, timer and counter, but it cannot be used to drive the coils of input relay. For the coils of timer and counter, appropriate constants must be set after the instruction (OUT) is output.
4.2 Description of contact series
AND (AND), ANI (not) instruction, and is normally open contact in series, ANI is normally closed contact in series, and ANI and ANI can both be used for electric shock in series and operate logically at the same time. There is no limit to the number of series contacts, and it is a reusable program instruction.
4.3 Contact Parallel Instruction
OR (or), ORI (or not) instructions, or normally open contacts in parallel, ORI normally closed contacts in parallel, can be used for contact parallel or logical operation. The number of parallel contacts is unlimited, and it is a reusable program instruction. When the series circuit blocks of two or more contacts are connected in parallel, the ORB (Block or) instruction described later should be used.
4.4 Parallel Instruction (ORB) Block of Serial Circuit Block
Series circuit block refers to two or more contact circuits connected in series. Generally, a series circuit block is a line branch. When series circuit blocks are connected in parallel, LD or LDI instructions are used at the beginning of each branch and ORB instructions are used at the end of each branch. In multiple parallel circuits, if ORB instruction is used at the end of each series circuit block, the number of parallel series circuit blocks is not limited. ORB instruction, like ANB instruction described later, is an independent instruction without operating component number.
4.5 Series Description of Parallel Circuit Blocks
ANB (block and) parallel circuit blocks are called parallel circuit blocks, and circuits with more than two contacts in series are usually called branches. When the parallel circuit blocks are connected in series, LD or LDI instruction is used at the beginning of each branch, and ANB instruction is used after the parallel circuit blocks are finished, so as to realize the series connection with the previous circuits.
ANB instruction, like ORB instruction mentioned above, is an independent instruction without operating component number. If multiple parallel circuit blocks are connected in series with the previous circuit in turn, the number of ANB instructions used is not limited.
4.6 Main Contact Description
MC (master control), MCR (master control reset), the starting point of MC master control circuit block, and the end point of MCR master control circuit block.
In the process of programming, it is often encountered that several logic lines are controlled by a contact or a group of contacts at the same time and by a common condition. This situation is called master control. At this point, you can use the MC/MCR command to edit. When the master control conditions are met, the instructions between MC and MCR are executed. After executing the MC instruction, the bus moves behind the MC main control contact, and after executing the MCR instruction, the bus returns to its original position. MC and MCR instructions must be used in pairs.
4.7 Setup and Reset Instructions
SET (set), RST (reset), set the holding element to open, close the holding element and clear the data register. When the SET instruction is executed, the corresponding operation element (y, m, s) is set, and it has a self-holding function. When the RST instruction is executed, the corresponding operation elements (y, m, s) are set, which has self-protection function. The RST instruction can also clear the data register d and the index registers v and z. ..
4.8 End (program end) instruction
End the I/O handler and return to the first one? 0? One step.
5 conclusion
When using PLC system design, a large number of input points are needed. Especially for thermal power plant systems that need multi-position and multi-point control, the requirements for input points are more prominent. Therefore, it can effectively reduce the input points of the system and effectively reduce the cost of PLC. In the design of PLC control system, we need to master the following skills and points: (1) In the design, we should design according to different control functions of the software. If it is a ladder diagram, the design method should be modular; (2) When cyclic scanning is used, the time sequence relationship between instructions and modules should remain unchanged, so that the program can run normally on the basis of designed functions; (3) Separate the parameters that need to be adjusted, such as automatic door closing, speed change and automatic switching time, from the program. Therefore, when the parameters need to be adjusted, there is no need to change the program, which is convenient, fast and easy to debug, and at the same time, the reliability of the software can be effectively improved; (4) For series switches and interlocking switches, such as interlocking switches between landing doors and between car roof and car, they can be set as input points; (5) Switch signals with the same functions, such as the switch of the safety touch panel and the gate switch, can be input into PLC(6 in parallel (6) and the combined button input method is adopted. When this method is applied, two input points should be used to combine keys, and then the program will automatically identify and recover the combined signal; (7) Coding input: adopting binary coding, inputting the identification signal in the button switch, and then automatically switching to the PLC program for recovery and identification can effectively reduce the number of PLC input points.
refer to
[1] Zhu shanjun, et al. principle, application and maintenance of programmable control system [M]. Beijing: Tsinghua University publishing house, 1992.
[2] Wang. Can be compiled into a controller tutorial [M]. Beijing: Machinery Industry Press, 2000.
Author introduction: Wang Qiong (1980-), male, from Shengzhou, Zhejiang Province, is a thermal control engineer of Shangyu Hangxie Thermal Power Co., Ltd., and his research interests include management and maintenance of power plant automation control system, daily maintenance of hardware and software programming.