table of contents
What is PLC?
About PLC type
Where is the PLC used?
What language (program) is required for PLC?
What is PLC?
businessman's hand draws gear to success concept-1
PLC is an abbreviation for Programmable Logic Controller, which is a device for controlling sequences of manufacturing equipment by a dedicated computer.
PLC is used in various factory equipment from semiconductor manufacturing to metal processing machines, including FA (factory automation) systems in manufacturing lines such as automobiles and electrical equipment, and is also used to control familiar mechanical equipment such as elevators. .. PLC, which has evolved from a control circuit that uses electromagnetic relays, has made it possible to change and improve sequence control programmatically.
The PLC gives us a high degree of control over the operation of manufacturing equipment, which makes it possible to carry out everything from planning to execution and control in supply chain management ( SCM ).
To understand the mechanism of PLC and mechanical equipment, it is important to correctly understand sequence control and relay sequence control. Here, we will introduce sequence control and relay sequence control in detail.
Supply Chain Management (About SCM) is explained in the following article.
Related article: What is SCM (Supply Chain Management)? Introducing the background and system components that are becoming more necessary
What is sequence control?
Sequence control is one of the automatic control methods using electric signals. Sequences have meanings such as "order" and "array", and sequence control operates machines in a fixed order.
For example, a washing machine operates in the order of water supply, washing, rinsing, and dehydration. A series of operations of this washing machine is sequence control, which is defined in the JIS standard as "control that sequentially advances each stage of control according to a predetermined order or procedure".
What is relay sequence control?
Relay sequence control is a sequence control that uses an electromagnetic relay as a switch. Also called "contact sequence", the switch uses the contact that opens and closes the coil of the electromagnetic relay to control the sequence. Relay sequence control has the characteristics of being resistant to large load capacity and electrical noise, and is applied to the control of motors. By using an electromagnetic relay that controls an electric signal by the movement of mechanical contacts, it is possible to remotely control a high-risk current by converting a low-risk low current into a signal. However, there are also disadvantages such as higher power consumption compared to other methods, slow operation because it opens and closes to mechanical contacts, and wear of contacts.Advantages of PLC
Prior to the widespread use of PLCs, sequence control circuits were constructed using a huge number of electromagnetic relays, which caused problems such as complexity and space pressure. If you wanted to change the operation controlled by using an electromagnetic relay, you had to change or improve the wiring that connects the electromagnetic relays. PLCs have the advantages of eliminating the need for wiring between the machines to be controlled and enabling space-saving control of complex equipment. Since the PLC has the same basic structure as a general-purpose computer, the sequence control can be easily changed by rewriting the program. By using PLC, it is not necessary to stop for a long time when changing the operation of the controlled machine, and the efficiency of the product manufacturing process can be realized. Furthermore, since PLC is controlled by software instead of hardware control using electromagnetic relays, mass production of control devices is easy. In addition, PLC has high durability because it is controlled by a program instead of mechanical contacts that are consumed like electromagnetic relays.About PLC type
Young technician working on broken computer in his office The internal structure of PLC is divided into functions and roles, and mainly consists of the following functions. Input module: Input information from switches and sensors Calculation unit: Calculates the input signal Memory part: Stores programs and data Output module: Outputs a signal to the device Power supply module: Supply necessary power to each part Depending on the structure, PLC can be roughly divided into "small block type" that packages the above functions and "building block type" that has an independent configuration for each function. From here, we will introduce the mechanism and advantages / disadvantages of each PLC.Small block type
The small block type (package type, compact type) is a PLC that integrates each function. structure It has a structure in which the input module, arithmetic unit, memory unit, output module, and power supply module of the basic functions are integrated. Since the small block type has an integrated input / output unit, it can be used by connecting a power supply. Control operations can be executed by connecting a computer programming support tool to the PLC, writing a program, and connecting each device to the input / output terminals.merit and demerit
The advantage of the small block type is that it is generally cheaper than the building block type described later. Since it is possible to reduce the trouble of selecting and purchasing each unit one by one, the small block type PLC can reduce the installation cost. And if you don't need an analog signal or if you have about 10 inputs and outputs, you don't have to consider the specifications, and it's easy to incorporate into your design, which is a big advantage. On the other hand, the building block type is superior in terms of scalability and performance. Therefore, the small block type is not suitable for large-scale control.Building block type
The building block type is a PLC in which each unit that composes the PLC is independent, and the units are used in combination according to the specifications. structure Unlike the small block type that packages the basic functions, the building block can add individual units of the required functions to customize the functions required for the production process. In addition to basic units such as input / output units, power supply units, and arithmetic units, there are also special units such as Ethernet communication and servo motor control. The building block formula is often used for complex and large-scale control and is used in various manufacturing sites.merit and demerit
The advantage of the building block type is its high expandability and applicability. For example, even if there is a shortage of input / output terminals, you can add new input / output units to add input terminals. Furthermore, it can be expanded to a large-scale control system by additionally connecting an expansion base. On the other hand, the disadvantages are that it is more expensive than the small block type and that knowledge is required to select each unit. However, since it has higher functionality and accuracy than the small block type, the building block type is more suitable for situations where complicated control and applicability are required.Where is the PLC used?
Developed as an alternative to electromagnetic relays, PLCs are used to control a variety of automated machines. It's mainly stored inside the control panel and the machine, so you won't have a chance to see it. However, PLCs support the control of machines in all fields and are used not only in factory equipment but also in household appliances. From home appliances such as air conditioners and dishwashers to equipment such as escalator and automatic doors, to FA systems and power plants at manufacturing sites, PLCs are used in a variety of situations where machines need to be controlled efficiently. .. In particular, PLC has dramatically improved the FA system production line and automatic control technology for various machine tools. The method of controlling the machine is not limited to PLC, but there is also a method of using a microcomputer board (embedded board), but many manufacturing sites use PLC-based control methods. The reason is that PLC is a device specialized in controlling machines efficiently. Performance such as durability, robustness, processing power, reliability, and responsiveness, as well as ease of programming and maintenance, are important for all manufacturing site controls. PLCs perform excellently as equipment used in the manufacturing floor, and have extremely high performance in terms of reliability, robustness, and ease of maintenance. Although the microcomputer board is inexpensive and can be controlled with reliability and space saving, PLC is superior in terms of maintenance. In addition, the microcomputer board basically does not allow the user to change the control circuit, etc., but the PLC allows the user to easily change the control circuit by programming. Since PLC has the performance to support complicated and flexible control even by itself, it is widely used as a control device that can be variously customized according to customer's request.What language (program) is required for PLC?
Computer keyboard with glowing codes, programming concept-1 As mentioned above, PLC is characterized by being able to customize the operation of the machine by changing the program. The general method is to create a program using programming support software provided by the PLC manufacturer and write it to the PLC using the software. There was a time when each manufacturer's own language was used as the programming language for PLC, and even if the basic method was the same, the specifications differed for each manufacturer, so the problem was that the programming language could not be diverted. Currently, PLCs of each manufacturer are increasingly adopting five programming languages defined in "IEC61131-3" issued by the IEC (International Electrotechnical Commission). The following is an overview of the five programming languages defined in "IEC 61131-3".LD (ladder diagram)
The LD language is a programming language that describes logic circuits, and is the most used language among domestic manufacturers. Unlike other programming languages, it is based on a virtual representation of relay sequence control. Therefore, it is excellent in that it is easy for engineers who have knowledge of relay sequence schematics to understand and learn.However, although easy to understand, it does require some knowledge and experience. Manufacturers such as Mitsubishi, Hitachi, Keyence, and Omron offer LD language programming tools for their products because the manufacturers and equipment are not fully compatible. In addition, even if the product is made by the same manufacturer, the description may differ depending on the model number, and when porting the program, the ability to decipher the existing program is required.