Programmable logic controller
The early machines were made up of cams, gears, levers, and some other basic mechanical equipment. With the increasing complexity of the machine, also need a more complex control system to control the connection function modules of this system includes the control switch and relay, these modules according to the needs of some logic functions and some specific types of machine operation group are connected into a whole, which is not a need to change the hardware but with the improvement of production technology can be easily ported to new products, with the majority of equipment upgrades can easily control machines, change circuit switching and switching logic is very cumbersome and time consuming, for the production of a new circuit to change the control scheme and reconnect is inevitable, modify it very the difficulties and design up to spend a lot of time, some small problems in the process of installing the design will become the lead line Systems need to be redesigned for key factors must be re modify the control circuit, the American auto industry lead the development and promotion of the new technology, became a leading figure in the industry as a whole, this new technology in the late 1960s and early 1970s obtained vigorous development, from the birth programming controller, also known as PLC, automobile manufacturing industry facing manufacturing process of all-round improvement, such as improvements in some aspects must be linked to other aspects, PLC provides a simple method by re programming instead of the line of the whole system as a whole change.
In the program development during the development of the PLC system is not an easy thing, for the design of PLC programming language is a very troublesome thing, this work requires an experienced programmer and trained with regularity to complete,
early device functions only by frequent change of relay to achieve PLC, PLC now gradually, the rapid development in recent years has become a complex and highly versatile control system components, each module has the ability to perform complex mathematical functions including integral and differential operation, these functions are through a fast microprocessor, old PLC discrete input and output (i.e. switch input signal) today, the system can receive and generate analog voltage and current, wide level and pulse signal, the PLC design process is very difficult, Unlike in the existing computer technology based on the design of the new computer can easily withstand the external environment such as vibration, shock, temperature rise, and the interference of electromagnetic noise.
With more and more manufacturers involved in the company's production and development, PLC function expand rapidly, programming language also great rich, this is the development of every new thing must go through the process, also various manufacturers tend to develop their own ladder logic language (for a PLC programming language). This complex PLC language can not applicable to all types of equipment. However, with the other computer languages like, once this ladder language has been widely used, then the language being used in most of the equipment on the possibility is big, most of the system designers will find production of a particular manufacturer of products for the writing system.
It should be noted that the programmable logic controller is often referred to as \"PLC\" or \"programmable controller\". Although the term \"programmable logic controller\" has been widely accepted, it is not like the abbreviation \"personal computerhe abbreviation \"personal computer\" usually refers to the personal computer used. In
this article we will see that PLC does not mean personal computer.
Can be programmable controller is the personal computer users as free many configuration options, as personal computers selected which is better some completely rely on their own experience, as a controller of a lot of setup options to good control it made him work needs to rely on their own experience.。Modular PLC including the system components are:
1 processor
Processor (sometimes referred to as CPU) is a program of the central processor can execute, which is full of the module with a number of specific features, these features some processing more complex mathematical operations and some of the PID control loop and programmable command processor is composed of a micro processor, a system memory, serial communication port and printer, PLC LAN links and external programming equipment system through the I / O port supply.
2 Architecture
Generally, the metal frame and the printed circuit board are connected with the PLC input and output module and the processor, and the number of the mounting frames required for the system is realized according to the specified module. The assembly frame provides data and power through the motherboard and the module connection processor. Without power supply structure to preserve the modular power supply, the processor of the system is independently installed and the cable rack are connected
together, frame assembly can is mounted directly to the panel, or can be installed in the standard width of 19 and multiple mounting frame between is linked to ensure that adapt to the large number of I / O module in the whole system.
3 input and output modules
The input and output (I / O) modules are connected to a particular application program based on the specified input and output signals, which can be divided into discrete categories, analog high-speed counters or register types.
Discrete I / O modules are generally able to 8-bit or 16 bit data processing, in some cases, switch input and output module is specified as an input or output, but manufacturers generally provide a free allocation of inputs and outputs can be cured within the same module.
Analog input and output module is feasible and applicable to the expected resolution and voltage or current range, these have become the general discrete input or output module, some manufacturers between the same module provides analog input and output, to simulate the PLC can also be accepted directly thermocouple temperature measurement and monitoring.
Pulse input to PLC can be accepted using high speed countermodule. This module can be used to measure the frequency of the input signal from the tachometer or frequency generator. These modules can also count the incoming pulses if required. In general, both the frequency and count of the same module, if both are required in the
application.
The register input and output modules transmit information from 16 or 8 bits, and from a programmable controller. These words are usually numbers (BCD code or binary) which are the input data from the encoder switch or encoder system to be generated by the PLC output display device
4, power supply
The specified power depends on the manufacturer's programmable controller used in the application. As mentioned above, in some cases, the power supply to provide the required power of the system as a part of the processor module. If the power supply is a separate module, it must be able to provide a greater capacity than the sum of all the currents needed for the other modules. For the internal processor module of power supply system, may have some modules, in the system, much power, rather than from the processor, because voltage or current requirements, only through the addition of a power supply. This is generally true if the analog or external communication module, as these requirements of the DC power supply, in the case of the analog module, must be well regulated.
5, programming unit
The programming unit allows the engineer or technical personnel to enter and edit the program execution. In its simplest form, it can be a handheld device that shows the steps or functions of the input and display devices (light emitting diodes or liquid crystal
displays) to the keyboard input program, as shown in fig.. More advanced systems use a single personal computer, which allows programmers to write, view, edit, and download the program into a programmable controller. This is achieved by the manufacturer with proprietary software available from the programmable controller. The software also allows the programmer or engineer to monitor the program as it is running the program. With this monitoring system, such as internal coils, registers, timers and other items that are not visible externally can be monitored to determine the correct operation. In addition, the internal register data can be changed, if required to fine tune the program operation. This may be advantageous when debugging a program. Communication with the system's programmable controller is a cable connected to a particular programming port on the controller. Connected to personal computers can be installed through a serial port or from a special card.
Programmable controller is a kind of special computer. Because it is a computer, it has all the basic components of a computer, a central processing unit, memory, input interface and output interface. The central processing unit (CPU) is an important part of programmable logic controller (PLC) control. It can explain the same procedures and commands directly from memory and behavior to those commands. This unit in the current programmable logic controller (PLC) is a microprocessor based system. Processor module of central processing unit (CPU) in system module.
Memory systems are usually of two types, read-only memory and random access memory. ROM's memory contains program information that allows the central processor to interpret and act on the ladder logic program stored in the RAM store. RAM storage is usually removed in order not to lose the power of the ladder programming
system. The battery can be a standard type of dry battery charge. Now the programmable controller (PLC) unit with electrically programmable read - only memory (EEPROM) does not require a battery. Processor module for storing in a modular system.
Any different types of input units can be determined by the expected input signal. Enter a discrete analog signal. Today's controllers provide two discrete signal input AC and DC voltage TTL 250 VDC, from 5 to 250 VAC. Analog input unit can accept input level, such as + 10 V DC, + 5 V DC current loop value. Each CPU as a single 1 or 0 in the analog input unit in analog / digital conversion circuit and current input voltage to the central processor, discrete input unit inputs in binary format to a maximum number of words for transmission. The number of bits is determined by the unit of the system, representing the input voltage or current. Usually there is a definition of the number of size and number of symbols. Input from the input unit to the central processing unit, as it is obtained by a binary or BCD code.
Operating the same output unit and the input unit, except that the unit is provided with one or a purchase (to provide a voltage) discrete voltage or the purchase of analog voltage or current. Under the guidance of the output signal, the central processing unit is proposed. Discontinuous output current can be TTL and higher DC voltage or AC voltage output. The application of a larger current is used to make the intermediate relay contacts available at the time of closing. These larger currents are generally limited to 2 to 3 amps. Analog circuit output digital analog conversion to generate a variable voltage or current output.
The first time the programmable logic controller (PLC) was used, it started to update
the function of I/O. This means that all the discrete input states are recorded from the input unit to transfer all the discrete state output to the output unit. The initial data generally have a specific address and the associated input and output data, both called the input and output registers. These debug registers are input and output modules that require them to be updated with discrete data. Since this is the input / output update, it is called the I/O update. Update the discrete input and output information is accomplished using the image input and output registers to hold the PLC storage. Each discrete input associated with the associated input state. That is, each discrete output point has a small output image with which it is combined. When the I/O is updated, each input point is the first set of addresses that will be associated with that particular point in time. If the input is closed, a 0 will be sent to the address. PLC memory in today's configuration is usually 16 words. This means that a word can store the state of the memory of the discrete input. Thus, it is possible that the number of memory set aside is the image input and output register. I/O updates the status of the input image is set by the discrete state, from the input and output of the image status register transfer to the output unit. Usually only transfer information occurs when the I/O is updated. It may be forced to occur in other programmable controller (PLC) times with a very clear I/O update. This command will force the PLC to update I/O at any other time, although this is a special case.
At the beginning of the study, PLC programming can be a very important module. Get an understanding of the various types of PLC, as well as the programmable controller (PLC) how to execute a program. Open framework and modular PLC are suitable for the application of environmental conditions on the basis of specific types, the number of input and output expansion and the access control program. In addition,
programming requires a priority to receive input information, the implementation of the PLC program, and output information. Based on this information, we begin to study the PLC program design.
When the program is written, ladder diagram control machine. The reason is that in a basic standard logic PLC program is very similar to the electrical diagram, this is not a coincidence. Engineers are very sensitive to the development of PLC programming language, most engineers, technicians and electricians electrical mechanical staff every day will be familiar with this method of control logic, on behalf of which will allow you to update, only familiar with PLC control chart can adapt to continuously update the programming language. PLC programming language is one of the most easy to learn programming language
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