A, the definition of programmable controller

Programmable logic controller (PLC) is a new type of industrial control equipment based on computer technology. In 1987, the draft PLC standard issued by the International Electric Commission defined PLC as follows: "PLC is an electronic device specially designed for digital operation in industrial environment. It uses programmable memory to store instructions for performing logical operations, sequential operations, timing, counting and arithmetic operations, and can control various types of machines or production processes through digital or analog input and output. PLC and its related peripherals should be designed according to the principle of easy integration with industrial control system and easy function expansion. "

Second, the characteristics of PLC

1 has high reliability and strong anti-interference ability.

High reliability is the key performance of electrical control equipment. Because PLC adopts modern large-scale integrated circuit technology and strict production technology, its internal circuit adopts advanced anti-interference technology and has high reliability. For example, the average time between failures of F series PLC produced by Mitsubishi Company is as high as 300,000 hours. The average trouble-free working time of PLC with redundant CPU is longer. As for the external circuit of PLC, compared with the relay contactor system of the same scale, the electrical wiring and switch contacts are reduced to hundreds or even thousands, and the faults are greatly reduced. In addition, PLC has the function of hardware fault self-checking, which can send out alarm information in time when there is a fault. In the application software, users can also write the fault self-diagnosis program of peripheral equipment, so that the circuits and equipment in the system except PLC can also obtain fault self-diagnosis protection. It is not surprising that the whole system has high reliability.

2 complete sets, perfect function and strong applicability.

Today, PLC has developed into a series of large, medium and small-scale products. It can be used in industrial control occasions of various scales. In addition to the logic processing function, most modern PLCs also have perfect data operation ability, which can be used in various digital control fields. In recent years, a large number of functional units of PLC have emerged, which makes PLC penetrate into various industrial controls such as position control, temperature control and numerical control. Coupled with the enhancement of PLC communication ability and the development of man-machine interface technology, it is very easy to form various control systems with PLC.

Easy to learn and easy to use, it is welcomed by engineers and technicians.

As a general industrial control computer, PLC is the industrial control equipment of industrial and mining enterprises. Its interface is simple, and the programming language is easily accepted by engineers and technicians. The graphic symbols and expressions of ladder diagram language are quite close to the relay circuit diagram, and the functions of relay circuit can be realized conveniently with only a few PLC switching logic control instructions. It opens the door for people who are not familiar with electronic circuits, computer principles and assembly languages to use computers to engage in industrial control.

4. The workload of system design and construction is small, and it is easy to maintain and transform.

PLC uses storage logic instead of wiring logic, which greatly reduces the external wiring of control equipment, shortens the design and construction cycle of control system, and makes maintenance easier. More importantly, it is possible for the same equipment to change the production process by changing the program. This is very suitable for multi-variety and small batch production.

Small size, light weight and low energy consumption

Take the subminiature PLC as an example. The bottom size of the newly produced product is less than 100mm, the weight is less than 150g, and the power consumption is only a few watts. Because of its small size, it is easy to be installed in machinery, and it is an ideal control device to realize electromechanical integration.

3. In the automatic line of modular machine tools, three kinds of sliding tables are generally set according to different machining accuracy requirements.

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 usage can be roughly summarized into the following categories.

Logical control of 1 switch value

This is the most basic and extensive application field of PLC. It replaces the traditional relay circuit to realize logic control and sequence control. It can be used for single machine control, 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 analog control

In the process of industrial production, there are many continuously changing quantities, such as temperature, pressure, flow, liquid level and speed, which are all analog quantities. In order to make the programmable controller deal with analog quantity, it is necessary to realize A/D conversion and D/A conversion between analog quantity and digital quantity. PLC manufacturers all produce matching A/D and D/A conversion modules, so that programmable controllers can be used for analog control.

3 Motion control

PLC can be used to control circular motion or linear motion. From the configuration of control mechanism, I/O module was used to directly connect the position sensor and actuator in the early days, but now it is generally used with special motion control module. Such as a single-axis or multi-axis position control module that can drive a stepping motor or a servo motor. Almost all the products of major PLC manufacturers in the world have motion control functions, which are widely used in various machinery, machine tools, robots, elevators and other occasions.

4 process control

Process control refers to the closed-loop control of temperature, pressure, flow and other analog quantities. As an industrial control computer, PLC can compile various control algorithms and complete closed-loop control. PID regulation is a widely used regulation method in general closed-loop control system. Large and medium-sized PLC has PID module, and many small PLCs also have this function module at present. General PID processing is to run a special PID subroutine. Process control is widely used in metallurgy, chemical industry, heat treatment, boiler control and other occasions.

5 data processing

Modern PLC has the functions of mathematical operation (including matrix operation, function operation and logic operation), data transmission, data conversion, sorting, table lookup, bit operation and so on, which can complete data collection, analysis and processing. These data can be compared with the reference values stored in the memory to complete some control operations, and can also be transmitted to other intelligent devices by using communication functions, or printed and made into tables. Data processing is generally used in large-scale control systems, such as unmanned flexible manufacturing systems; It can also be used in process control systems, such as some large-scale control systems in papermaking, metallurgy, food and other industries.

6 communication and network

PLC communication includes communication between PLCs and communication between PLCs and other intelligent devices. With the development of computer control and the rapid development of factory automation network, all PLC manufacturers attach great importance to the communication function of PLC and have launched their own network systems. The newly produced PLC has communication interface, which makes communication very convenient.

7 PLC-controlled CNC sliding table structure

The CNC sliding table in the automatic line of modular machine tools generally adopts the open-loop servo mechanism driven by stepping motor. The numerical control sliding table controlled by PLC is composed of programmable controller, annular pulse distributor, stepping motor driver, stepping motor and servo transmission mechanism.

The backlash of gears Z 1 and Z2 in the servo drive mechanism should be eliminated to avoid the reverse dead zone or reduce the machining accuracy. The lead screw drive pair shall determine whether to select the ball screw pair according to the machining accuracy requirements of the unit. The use of ball screw pair has the advantages of high transmission efficiency, good system stiffness, high transmission accuracy and long service life, but it is expensive and cannot be self-locked.

8 software structure of control system

The software structure is designed according to the control requirements, and it is mainly divided into five modules: stepping motor control module, positioning control module, data dialing input and data transmission module, digital output display module, and component fault automatic detection and alarm module.

Due to the huge structure of the whole software, the pulse controller generates a control pulse of 0. 1 sec to shift the shift register and provide six-beat timing pulses. The three output relays Y430, Y43 1 and Y432 control the stepping motor in a single, double and six-beat power-on way through a three-phase six-beat ring distributor. In order to realize positioning control, different counters are used to control the coarse positioning stroke and the fine positioning stroke respectively, and the set value of the counter depends on the stroke. For example, if a tool or workbench moves from point A to point C, it is known that AC = 200 mm, AC is divided into AB and BC, AB = 196 mm, BC = 4 mm, AB is the rough positioning stroke, and it moves quickly with the pulse equivalent of 0. 1mm/ step. The 6-bit counter (C660/C6665438) is used for precise positioning with the pulse equivalent of 0.0 1mm/ step. At the same time of coarse positioning and fine positioning, the PLC automatically turns on the output point Y433 of the electromagnetic clutch to realize the replacement of the transmission mechanism.

9 grounding method of PLC control system

(1) Due to various reasons (such as corrosion, insulation damage, etc. ), the metal shell of electrical equipment such as PLC cabinet, operation console and power distribution cabinet and the normally uncharged metal part of control equipment may have dangerous voltage, and should be protected and grounded. Unless there are special requirements, grounding protection is not required for equipment with power supply below 36V.

(2) 2) The reference potential in the PLC control system is the reference potential of each loop. The connecting line of reference potential is called system ground, which is usually the zero-volt conductor of DC power supply in control loop. The grounding modes of the system include floating ground, direct grounding and capacitive grounding.

(3) To prevent electrostatic induction and magnetic field induction, the shielded grounding terminal should be grounded. Among them, signal loop grounding and shielding grounding are also called working grounding.

Generally, the control principle of the above grounding mode is that the protective ground and the working ground cannot be mixed, because there will be a potential difference of several millivolts or even several volts between the two points of the protective ground of each power supply, which is very serious interference to the low-level signal circuit. Shielded grounding: When the signal circuit is grounded at a single point, the shielding layer of the low-frequency cable should also be grounded at a single point. If there is more than one grounding point of the shielding layer of the cable, it will produce noise current and form a noise interference source.

The grounding resistance used in this system needs to be within the specified range. For the control system composed of PLC, it should generally be less than 4Ω, and it should have sufficient mechanical strength, and it needs anti-corrosion treatment in advance. The control system composed of PLC can be equipped with a separate grounding system or "equipotential bonding" design according to field conditions.

10 PLC control ladder diagram:

Ladder diagram is a connected diagram which connects the ladder diagram symbols of PLC instructions together through wires to express the PLC instructions used and their sequence, which is very similar to the electrical schematic diagram. There are two kinds of connections: one is the bus, and the other is the internal horizontal and vertical lines. The horizontal and vertical lines inside connect the ladder diagram symbol instructions into an instruction group, which usually starts with the load (LD) instruction, followed by several input instructions (including LD instruction), and is used to establish logical conditions when necessary. Finally, it is the output instruction to realize output control, or the instruction of data control, flow control, communication processing, monitoring and so on, so as to carry out the corresponding work. The bus is used to connect command groups. The following figure is the simplest trapezoidal legend of Mitsubishi FX2N series products:

It has two groups, the first group is used to realize start and stop control. The second group has only one end instruction to end the program.

1 1 The corresponding relationship between ladder diagram and mnemonic;

There is a strict correspondence between mnemonic instructions and ladder diagram instructions, and the connection of ladder diagram can reflect the order of instructions. Generally speaking, the order is: input first, then output (including other processing); First up, then down; Turn left first, then right. With the ladder diagram, it can be translated into a mnemonic program. The above mnemonic program is:

Address instruction variable

0000 LD X000

000 1 or X0 10

0002 instead of X00 1

0003 in 0000

End of 0004

On the contrary, according to the mnemonic, a ladder diagram corresponding to it can also be drawn.

The relationship between 12 ladder diagram and electrical schematic diagram;

If only logic control is considered, ladder diagram and electrical schematic diagram can also establish a certain corresponding relationship. For example, the output (out) instruction of the ladder diagram corresponds to the coil of the relay, while the input instructions (such as LD, AND, OR) correspond to the contacts, and the interlocking instructions (il, ILC) can be regarded as the master switch, and so on. In this way, the original relay control logic can be transformed into a ladder diagram, and then further transformed into a statement table program.

With this correspondence, it is easy to express relay logic with PLC program. This is also the inheritance of PLC technology to traditional relay control technology.

Fourth, the PLC control method of numerical control slide table

There are three main control factors of CNC slide table:

1 stroke control

The stroke control of hydraulic sliding table and mechanical sliding table is generally realized by position or pressure sensor (stroke switch/dead stop iron); The stroke of NC slide table is realized by digital control. According to the structure of numerical control sliding table, the stroke of sliding table is proportional to the total rotation angle of stepping motor, so it is only necessary to control the total rotation angle of stepping motor. According to the working principle and characteristics of stepping motor, the total rotation angle of stepping motor is proportional to the number of input control pulses. Therefore, the number of pulses output by PLC can be determined according to the displacement of servo mechanism:

n= DL/d ( 1)

Where dl- displacement of servo mechanism (mm)

D —— pulse equivalent of servo mechanism (mm/pulse)

2 feed speed control

The feed speed of servo mechanism depends on the rotating speed of stepping motor, which in turn depends on the input pulse frequency; Therefore, according to the feed speed required for this process, the pulse frequency of its PLC output can be determined:

F=Vf/60d (Hz) (2)

Where Vf—— is the feed speed of the servo mechanism (mm/min)

3 feed direction control

Feed direction control is the steering control of stepping motor. By changing the energization sequence of each winding of the stepping motor, the steering of the stepping motor can be changed; For example, when the power-on sequence of the three-phase stepping motor is A-AB-B-BC-C-CA-A…, the stepping motor rotates forward; When the winding is energized in the order of A-AC-C-CB-B-BA-A ..., the stepping motor rotates in the opposite direction. Therefore, it can be realized by changing the output sequence of hardware circular distributor through the direction control signal output by PLC, or by changing the output pulse sequence through programming to change the energization sequence of stepping motor windings.

Verb (abbreviation of verb) PLC at home and abroad

The first PLC recognized in the world was developed in 1969 by American Digital Equipment Corporation (DEC). Limited by the component conditions and computer development level at that time, the early PLC was mainly composed of discrete components and small and medium-sized integrated circuits, which could complete simple logic control, timing and counting functions. Microprocessors appeared in the early 1970s. People soon introduced it into programmable controller, which made PLC increase the functions of calculation, data transmission and processing, and completed the industrial control device with real computer characteristics. For the convenience of engineers who are familiar with relay and contactor systems, the programmable controller adopts ladder diagram similar to relay circuit diagram as the main programming language, and all computer storage elements involved in operation and processing are named after relay. At this time, PLC is the product of the combination of microcomputer technology and relay conventional control concept.

In the middle and late 1970s, the programmable controller entered the stage of practical development, and computer technology was fully introduced into the programmable controller, which made its function leap forward. Higher operation speed, ultra-small volume, more reliable industrial anti-interference design, analog operation, PID function and high cost performance have laid its position in modern industry. In the early 1980s, programmable controllers were widely used in advanced industrial countries. The development characteristics of programmable controllers in this period are large-scale, high-speed, high-performance and product serialization. Another feature of this stage is that more and more countries in the world produce programmable controllers, and the output is increasing. This indicates that the programmable controller has entered a mature stage.

At the end of 20th century, the development of programmable controller is more suitable for the needs of modern industry. In terms of control scale, mainframes and subminiature computers were developed during this period; In terms of control ability, a variety of special functional units have been born, which are used in various control occasions such as pressure, temperature, speed and displacement. As far as the matching ability of products is concerned, various man-machine interface units and communication units have been produced, which make it easier to match industrial control equipment using programmable controllers. At present, the application of programmable controller in machinery manufacturing, petrochemical industry, metallurgical steel, automobile, light industry and other fields has made great progress.

The introduction, application, development and production of programmable controllers in China began with the reform and opening up. Initially, programmable controllers were widely used in imported equipment. Next, the application of PLC in the production equipment and products of various enterprises is expanding. At present, China has been able to produce small and medium-sized programmable controllers. The CF series produced by Shanghai Wu Dong Electric Co., Ltd., DKK and D series produced by Hangzhou Machine Tool Electric Appliance Factory, S series produced by Dalian Modular Machine Tool Research Institute and YZ series produced by Suzhou Electronic Computer Factory have reached a certain scale and have been applied in industrial products. In addition, Sino-foreign joint ventures such as Wuxi Huaguang Company and Shanghai Island Company are also famous PLC manufacturers in China. It can be predicted that with the deepening of China's modernization process, PLC will have a broader application field in China.

Future prospect of intransitive verb PLC

2 1 century, PLC will have greater development. Technically, the new achievements of computer technology will be more applied to the design and manufacture of programmable controllers, and there will be varieties with faster operation speed, larger storage capacity and stronger intelligence; From the perspective of product scale, it will further develop in the direction of ultra-small and ultra-large; From the perspective of product compatibility, the varieties and specifications of products will be more abundant, and the perfect man-machine interface and complete communication equipment will better meet the needs of various industrial control occasions; From the market point of view, the situation that each country produces a variety of products will be broken with the intensification of international competition, and there will be a situation that a few brands monopolize the international market, and there will be an internationally common programming language; From the development of network, it is the development direction of programmable controller technology to connect programmable controller with other industrial control computers to form a large-scale control system. At present, a large number of programmable controllers have been applied to computer DCS (distributed control system). With the development of computer network, programmable controller, as an important part of automation control network and international general network, will play an increasingly important role in many fields other than industry.