What is a Programmable Logic Controller?

What is a PLC Programmable Controller?

A Programmable Logic Controller (PLC) is a digital logic controller used for automatic control, which can load electronic control instructions into memory for storage and execution at any time. PLC can receive input and send and output various types of electronic signals, which can be used to control or supervise almost all devices, automatic mechanical equipment, or power monitoring. PLC has the characteristics of versatility, convenient use, wide adaptability, high reliability, strong anti-interference ability, and simple program editing.

Knowledge and programming of basic and advanced instructions for using programmable logic controllers (PLC). Familiarity with related application skills, analogy and temperature control, NC servo positioning, industrial communication, etc. Flexible use of its powerful functions enables machines and equipment to operate automatically and intelligently. And use to control or supervise almost all kinds of mechanical and electrical systems.

The initial programmable logic controller only has the function of circuit logic control. With continuous development, these computer modules with simple functions have already included logic control, timing control, analog control, multi-machine communication, etc. Function. Before the emergence of programmable logic controllers, it is generally necessary to use hundreds of relays and counters to form an automation system with the same function. But now, the programmed simple programmable logic controller modules have replaced these large installations. The system program of the programmable logic controller is initialized before leaving the factory, and the user can edit the corresponding user program according to his own needs to meet different automatic production requirements.

Now the programmable logic controller used in the industry is quite close to the composition of a lightweight computer, and even a programmable automation controller (PAC). Integrates a personal computer (using an embedded operating system) and a PLC has appeared, which can pass digital or Analog I/O modules to control machinery, manufacturing processes, and other electronic systems that control modules. Programmable logic controllers are used in the current industrial control field. In the field of industrial control, the application of PLC control technology has become an indispensable member of the industry.

What is the Definition of PLC?

PLC has the characteristics of versatility, convenient use, wide adaptability, high reliability, strong anti-interference ability, and simple programming. A Programmable Logic Controller is an electronic system that operates digitally and is designed for use in industrial environments. It uses a type of programmable memory for internal storage of programs, execution of user-oriented instructions such as logic operations, sequence control, timing, counting, and arithmetic operations, and controls various types of devices through digital or analog input/output machinery or production process. The programmable logic controller and its related external devices are designed according to the principle that it is easy to integrate with the industrial control system and expand its functions.

• Easy to program, the program can be modified on-site.
• It is easy to maintain and adopts a modular structure.
• The reliability is higher than the relay control device.
• The volume is smaller than the relay control device.
• Data can be directly sent to the computer.
• Cost competitive with relay controls.
• The input can be AC 115V.
• The output is AC 115V, above 2A, which can directly drive solenoid valves, contactors, etc.
• When expanding, the original system only needs to be slightly changed.
• The capacity of the user program memory can be expanded.

The Inner Workings of a PLC:

Although many names such as relays, timers, and counters are often used in the ladder diagram programs used by PLCs, the PLC does not physically have this hardware but uses memory and program programming to do logic control editing, and through the output, the element is connected to an external mechanical device for physical control. The hardware space required by the controller can be reduced. The way PLC executes the ladder diagram program is to read the program code line by line into the CPU by scanning, and finally execute the control operation.

1. Step 1: Enter Status Check
   The PLC first checks the state of each point switch or sensor connected to the input element and writes the state to the corresponding position Xn in the memory.
2. Step 2: Program Execution
   Load the ladder diagram program line by line into the CPU for calculation. If the state of the contacts needs to be input during the program execution, the CPU will query and fetch it directly from the memory. The operation result of the output coil is stored in the corresponding position in the memory, and will not be reflected in the output terminal Yn for the time being.
3. Step 3: Output status update
   Update the output status in step 2 to the PLC output contact, and return to step 1.

These three steps are called the scan cycle of the PLC, and the time required to complete them is the response time of the PLC. If the time of the PLC input signal is less than this response time, there is a possibility of misreading. After each program execution and before the next program execution, the output and input status will be updated once, so this mode of operation is called output and input end program end regeneration.

PLC Hardware Structure:

PLC is divided into box type and module type. But their composition is the same. For the box-type PLC, there is a CPU board, I/O board, display panel, memory block, power supply, etc. Of course, it is divided into several models according to the CPU performance, and according to the number of I/O points. There are several specifications. For modular PLCs, there are CPU modules, I/O modules, memory, power modules, backplanes, or racks. Regardless of the structure type of PLC, it belongs to the bus-type open structure, and its I/O capabilities can be expanded and combined according to user needs.

• Power module:
  The power supply in some PLCs is combined with the CPU module, and some are separated. Its purpose is to provide working power for the integrated circuits of the PLC modules. Some also provide 24V working power for the input circuit. If the power supply is an AC power supply, it is usually 220VAC or 110VAC, and if it is a DC power supply, it is usually 24V.
• Central processing unit:
  The CPU in the PLC is the core of the PLC. It receives and stores user programs and data according to the functions given by the PLC system program, collects the status or data sent by the on-site input device by scanning, and stores them in the planned temporary. At the same time, it can diagnose the working status of the power supply and the internal circuit of the PLC, the syntax errors in the programming process, etc. After entering the operation, read the instructions one by one from the user program memory, and then generate corresponding control signals according to the tasks specified by the instructions after analysis, to direct the relevant control circuits. Like a personal computer, it is mainly composed of arithmetic units, The controller, registers, and the data, control, and status buses that realize the connection between them, as well as peripheral chips, bus interfaces, and related circuits. Determine the scale of the control, work speed, memory capacity, etc.
• Memory:
  Memory is used to store programs and data and is an indispensable component of PLC. The program instructions and data that have been written and edited are stored inside the PLC. Usually, it can be expanded by using a dedicated memory card such as RAM or EEPROM, but the expansion capacity varies with each brand and model.
• I/O unit:
  The external functions of the PLC are mainly connected with the outside world through various input/output modules. The specifications and quantities of the modules are determined according to the number of I/O points. The I/O modules can be more or less, but the maximum number is limited by the CPU. The ability to manage basic configurations that are limited by the maximum number of backplane or rack slots. The I/O module integrates the I/O circuit of the PLC, its input register reflects the state of the input signal, and the output points reflect the state of the output latch. The input unit is used to connect and capture the signal action of the input element and send the data to the memory through the internal bus, and the CPU processes the driver instruction part. PLC input module The architecture of the PLC system and the selection of input module products depend on the level of the input signal to be monitored. Variable signals from different types of monitored sensors and process controls can cover input signal ranges from ±10mV to ±10V. The output unit is an interface used to drive external loads. The main principle is that the CPU processes the program instructions written in the PLC to judge the drive output unit and then controls the external loads, such as indicator lights, electromagnetic contactors, relays, air/oil pressure valves, etc. PLC output modules are used in industrial environments to control brakes, valves, motors, etc. PLC system analog output ranges include ±5V, ±10V, 0V to 5V, 0V to 10V, 4 to 20mA, or 0 to 20mA, etc.
• Communication:
  Most PLCs have the function of expandable communication network modules. Simple PLCs are connected by BUS cable or RS-232, while higher-level PLCs are connected by USB or Ethernet. It enables the exchange of information between PLCs, between PLCs and personal computers, and other intelligent devices to form a unified whole and realize decentralized and centralized control. Now almost all new PLC products have the function of a communication network. It has an RS-232 interface like a computer. Through twisted pairs, coaxial cable, or optical cable, information can be exchanged within a few kilometers or even tens of kilometers. The communication network between PLCs is dedicated to each manufacturer. For communication between PLCs and computers, some manufacturers use industrial standard busbars and approach standard communication protocols. This will make PLCs of different models. PLCs It is convenient to communicate and network with the computer. PLC communication protocol specifications can be divided into RS-232, RS-422, RS-432, RS-485, IEEE 1394, and IEEE-488 (GPIB). The most commonly used communication protocols in the world are MODBUS-ASCII mode and MODBUS-RTU mode.

The External Device of PLC:

• Programming device: There are simple programmers and intelligent graphic programmers, which are used for programming, making some settings for the system, and monitoring the working status of the PLC and the system controlled by the PLC. The programmer is an indispensable component for PLC development and application, monitoring operation, inspection, and maintenance, but it does not directly participate in on-site control operations.
• Monitoring device: Data monitor and graphics monitor. Monitor the data directly or monitor the data through the screen.
• Storage device: There are memory cards, storage tapes, floppy disks, or read-only memory, which are used to permanently store user data so that user programs are not lost, such as EPROM, EEPROM writers, etc.
• Input and output devices: Used to receive signals or output signals, generally there are barcode readers, potentiometers for inputting analog quantities, printers, etc.

The Internal Components of the PLC:

In the programming process of PLC, it will use the internal memory to plan many components that are often used in sequential control programs. These components include input relays, output relays, auxiliary relays, counters, timers, data registers, and other main components.

• Input contact and output contacts: Used for status transmission between PLC and external components. External components can be connected, as well as push button switches, selector switches, photoelectric switches, digital switches, etc. If excessive current is used, the internal contact components will be damaged.
• Auxiliary relay: Used to replace the relay in traditional sequence control. Traditional relays include contacts and coils. PLC uses internal memory to memorize the status of auxiliary relays.
• Counter: Used in the program to count the number of repetitions.
• Timer: Used to count the duration of an action.
• Data register: Used to store the value of characters or character data.