PLC introduced
The graduation project topic is the application of PLC in the servo servo system, the PLC application and understanding of the graduate design, following the basic knowledge of PLC to make a brief introduction.
The PLC can be programmed controller is the new industrial control devices based on computer technology. The PLC in the PLC draft standards issued by the International Electrotechnical Commission in 1987 made the following definition: "the PLC is a specially designed for applications in industrial environments digital computing operation of electronic devices which can be programmed memory, with to implementation of the logical, sequential operation, timing, counting and arithmetic operations, such as operating instructions in its internal storage and can be digital or analog inputs and outputs to control various types of machinery or production processes. PLC and its related peripherals should be easy and industrial control systems to form a whole, easy to extend the principle of functional design. "
A.PLC features
(a)High reliability, strong anti-jamming capability
High reliability is the key to performance of the electrical control equipment.PLC is due to the introduction of modern LSI technology, strict production process, the internal circuit to take the advanced anti-jamming technology, with high reliability. For example, the Mitsubishi F series PLC MTBF up to 300,000 hours. Redundant CPU PLC, the average time between failures is longer. From a machine outside the circuit of the PLC, the PLC constitutes a control system, and compared to the same scale relay contactor system, electrical wiring and switch contacts have been reduced to hundreds or even thousands, the fault will be greatly reduced. In addition, PLC with a self-detection of hardware failure, failure to timely alert. In the application software, application can also be incorporated into the peripheral devices and fault diagnostic procedures, also received the fault self-diagnostic protection circuit outside the PLC system and equipment. In this way, the entire system with high reliability is also not surprising.
Complete, fully functional, and applicability
(b)Complete, fully functional, the applicability
PLC development today, has formed a large, medium and small scale series products. Can be used for industrial control applications of all sizes. In addition to the logical processing functions, the mode
rn PLC mostly data computing power can be used for a variety of digital control. In recent years the emergence of a large number of functional units of the PLC, PLC penetrated into the position control, temperature control, CNC and other industrial control. With PLC communication capability enhancement and development of human-computer interface technology, the use of PLC composed of a variety
of control systems have become very easy.
(c)Easy to use, well received by engineers and technicians welcome
PLC as a general-purpose industrial control computer, industrial control equipment for industrial and mining enterprises. It easy to interface, programming language is easy to be accepted for engineering and technical personnel. The ladder language graphic symbol and expression and relay circuit is very close, only a small number of switch PLC logic control instructions can easily relay circuit. Not familiar with the electronic circuit, people do not understand computer theory and assembly language using a computer to engage in industrial control to open the door.
(d)System design, construction workload is small, easy to maintain, easy to transform
PLC storage logic instead of wiring logic, greatly reduce the external wiring of control equipment, control system design and construction of the cycle is much shorter, while maintaining also become easily. More important is that the same device after the change the change the production process become possible. It is suitable for the occasion of many varieties, small batch production.
(e)Small size, light weight, low energy consumption
Ultra-Small PLC, for example, the newly produced species at the bottom of size less than 100mm, weighs less than 150g, only a few watts of power. Due to small size it is easy to load the internal machinery is the ideal control device for mechanical and electrical integration.
B.The composition of the PLC
PLC is divided into fixed and modular (modular) two points from the structure. Fixed PLC CPU board, I / O board, display panel, memory blocks, power, and these elements are combined into a non-removable as a whole. Modular PLC CPU module, I / O modules, memory, power supply modules, backplane or rack, these modules can be combined according to certain rules configuration.
(a)The composition of the CPU
CPU is the core of the PLC, since the role of the nerve center, at least one CPU in each PLC, the PLC system program, the functions entrusted to the reception and storage of user programs and data collected from field input devices sent a scanned stateor data, and deposit shall be required to register, at the same time, the diagnostic power and the working status of the PLC internal circuitry and programming process in syntax errors. Into operation from the user program memory, one by one read command, the corresponding control signal analysis and then according to the mandate of instruction, to command the control circuit.
CPU is the main contact between the computing device, controller, register, and to implement them, control and status bus, a CPU unit also includes a peripheral chips, bus interface, and related circuit. Memory is mainly used for storing programs and data is an integral unit in PLC.
User opinion, unnecessarily detailed analysis of the internal circuitry of the CPU, but on the part of the working mechanism should have sufficient understanding. The CPU controller controls the CPU to read instructions, which explain the instructions and execute instructions. However, the pace of work is controlled by the oscillation signal. The arithmetic unit is used for digital or logic operation under the command of the controller. Register to participate in operations, and store intermediate results of operations, it is under the command of the controller.
CPU speed and memory capacity are important parameters in the PLC, they determine the pace of work of the PLC, IO number and software capacity, thus limiting the size of the control.
(b)I / O modules
The interface of the PLC and the electrical circuit is completed by the input and output portion (I / O). I / O modules with integrated PLC's I / O circuit, the input registers reflect the state of the input signal, output point to reflect the state of the output latch. The input module electrical signals into a digital signal into a PLC system, the output module is the opposite. I / O module is divided into the digital input (DI), digital output (DO), analog input (AI), analog output (AO).
The I / O are classified as follows:
Switch: the voltage level points, 220VAC, 110VAC, 24VDC, points by way of isolation and separation of relays and transistor isolation.
Analog: Signal type, current-(4-20mA, 0-20mA), voltage type (0-10V,
0-5V,-10-10V), etc., precision points, 12bit, 14bit, 16bit.
In addition to these general-purpose IO, there are special IO module, such as thermal resistance, thermocouple, pulse module.
I / O points to determine the module specification and quantity, I / O modules can be more or less, but the maximum number of the ability of the CPU can manage the basic configuration, that is, by the largest number of floor or rack slot limit.
(c)Power supply module
PLC power supply IC for the PLC module supplies power to operate. At the same time, some of the input circuit to provide 24V power supply. Power input type: AC power (220VAC or 110VAC), DC power supply (usually
24VDC).
C. Working principle
Put into operation when the programmable logic controller, the course of their work is generally divided into three stages, that is, the input sample, the user program execution and output refresh three stages. Completion of the three stages is called a scan cycle. During the entire operation, the
CPU of the programmable logic controller scan speed, repeat the above three stages. (a)Input sampling phase
The input sampling stage, the programmable logic controller in order to
scan read into the state of all input and data, and deposit them with the corresponding unit in the I / O image area. Transferred to the user program execution and output refresh period after the end of the input samp le. In both phases, even if the input state and data changes, the state and data of the corresponding unit in the I / O image area will not change. Therefore, if the input pulse signal, the signal of the pulse width must be greater than one scan cycle, in order to ensure that in any case, the input can be read into the.
(b)User program execution stage
In the user program execution stage, programmable logic controllers are always followed by a top-down order to scan the user program (ladder). Scan each one ladder, and always scan by the contact of the ladder left of the control of line, and first left and right first and then the order of logical operations on the control circuit by the contact form , then the logical result of the operation, refresh the logic of the coil system RAM storage area corresponding state; or refresh the state of the output
coil corresponding bits in the I / O image area; or determine whether to perform the ladder the provisions of the special function instruction.
That is, during user program execution, only the input status and data points in the I / O image area does not change, the output point and soft devices in the I / O image area or system RAM storage areastatus and data are likely to change, but came in above the ladder program execution results will be ranked below those who used these coils or data ladder work; the contrary, the row below the ladder, thethe status of logic coils to be refreshed or the data can only be to the next scan cycle can be ranked on top of the program work.
If you use the I / O instruction immediately you can directly access the I / O points in the process of program execution. That the use of I / O instruction, the input process image register values are not updated, the program directly from the I / O module value, the output process image register is immediately updated, this immediately enter some difference.
(c)Output refresh periodtransform英文
When the scan the user program after the end of the programmable logic controller to enter the output refresh period. During this time, the CPU in accordance with the I / O image area correspondi
ng to the state and data refresh all of the output latch circuit driven by the output circuit corresponding peripherals. At this time, is the real output of the programmable logic controller.
D.PLC applications
At present, the PLC at home and abroad has been widely used in iron and steel, petroleum, chemicals, electricity, building materials, machinery manufacturing, automobile, textile, transportation, environmental protection and cultural entertainment industry, usage can be broadly divided into the following categories.
(a)Switch logic control
This is the PLC basic, wide range of applications, which replaces the
traditional relay circuit, logic control, sequential control, both for the control of a single device, can also be used for multi-group control and automated assembly line.Such as injection molding machines, printing presses, stapler machine, combined machine tools, grinding machines, packaging production lines, electroplating lines and so on.
(b)Analog Control
In the industrial production process, there are many continuous variables, such as temperature, pressure, flow, level and speed are analog. To make the programmable controller to handle analog, analog (Analog) and Digital (Digital) A / D conversion and D / A conversion. PLC manufacturers are supporting the production of A / D and D / A converter module, the programmable controller for analog control.
(c)Motion Control
PLC can be used in a circular motion or linear motion control.From the configuration of the control mechanism, early used directly in the switch I / O modules to connect the position sensors and actuators, is now generally use a dedicated motion control module. Drive a single or multi-axis position of the stepper motor or servo motor control module. The world on all major PLC manufacturers products almost every motion control functions, widely used in a variety of machinery, machine tools, robots, elevators and other occasions. (d)Process control
Process control refers to the closed-loop control of temperature, pressure, flow analog. As an industrial control computer, the PLC can prepare a variety of control algorithm, the complete closed-loop control. PID regulation is a general closed-loop control system using more regulation. And medi
um-sized PLC have PID modules, and many small PLC also has this feature module. PID processing is generally run a dedicated PID subroutine. Process control in metallurgy, chemical industry, heat treatment, boiler control and so on a very wide range of applications.
(e)Data processing
Modern PLC math (including matrix operations, functions, operation, logic operation), data transfer, data conversion, sorting, look-up table, bit operation functions, you can complete the data collection, analysis and processing.These data can be compared with the reference value stored in memory to complete the control operation can also use the communication functions to other intelligent devices, or print them tabulation. The data processing is generally used for large-scale control systems, flexible manufacturing systems such as unmanned control; can be used for process control systems, such as papermaking, metallurgy, food industry, some large-scale control system.
(f)Communications and networking
PLC communication with the communication between PLC and communication between the PLC and other intelligent devices.With the development of computer control, factory automation, network development,

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