• Ladder Diagram (The abbreviation is LAD)

This is the most popular language today, especially in the US. It is based on the graphical representation of Relay Ladder Logic. Most current PLCs, whether compliant or not compliant with IEC61131-3, will support this ladder language.

Elements of a circuit diagram, such as contacts that are normally closed or normally open, and coils are connected together to form networks. To create logical diagrams for complex implementations, we can insert branches to create parallel logic circuits. The parallel branches are opened in the down direction or are directly connected to the signal bus. We end the branches in an upward direction.

• Instruction List ( Siemens is called STL )

This is like a version of ladder language but in text form. It has the same structure as assembly language.

This language is less used in practice than LAD

• Function Block Diagram (The abbreviation is FBD)

Describe the connection of functions, function blocks, and programs as a set of connected graphic blocks. It looks like the electronic circuit diagram we often use.

Like the LAD language, the FBD language is also a graphical programming language. The display of the logic circuit is based on the graphic logic symbols used in Boolean algebra.

Math functions and other complex functions can be expressed directly in combination with logical boxes. To create logic for complex operations, we insert parallel branches between the boxes.

#Both LAD and FBD languages ​​use signal EN and ENO for some “box” commands.These parameters relate to the signal flow and determine when the command is executed during that scan.

+ EN (Enable In) is a Boolean input for boxes in LAD and FBD languages. The signal current (EN = 1) must be present at this input for the command box to be executed. If the EN input of an LAD box is connected directly to the left signal bar, the box will always be executed.

+ ENO (Enable Out) is a Boolean output for boxes in LAD and FBD languages. If the box has a signal stream at the EN input and the box performs its functions without error, then the ENO output will allow the signal stream (ENO = 1) to pass to the next element. If an error is detected during the execution of the command box, the signal stream will then be interrupted (ENO = 0) at the command box that generated the error.

• Structured Text ( Siemens is called SCL )

This is a very powerful high-level language for PLC, derived from the Pascal / C language. This is a language that is being increasingly used by engineers because it saves time

It can be used to define complex function blocks, which can be used in other languages. As a high-level language, SCL is very intuitive and easy to understand.

• Sequential Function Chart ( Siemens is called SFC )

This is a powerful graphical programming style to describe the sequential states of a control program. However, it is a bit difficult to learn and time consuming to program, so it is the least used language of the 5 languages ​​according to IEC61131-3.

It is mainly used in programming control problems in sequential cycles

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In the past, controllers were only made to serve a control purpose and cannot be changed ( called the hard connection controller ), this has created huge limitations and disadvantages in programming control so PLC was born. Through the PLC controller, users can completely change the control algorithm through PLC programming ( written in programming languages ).

Some famous PLC manufacturers in the world : Siemens (Germany), ABB (Switzerland), RockWell (U.S), GE-Fanuc (U.S), Omron (Japan), Keyence (Japan), Mitsubishi (Japan), Delta (Taiwan)…

• Advantages of PLC over hard-connected control systems

# Features of the hard connection control system (using relay, timer, counter, contactor …) :

+ Lots of wires.

+ Replacement is very complicated.

+ Need skilled repair workers.

+ It is difficult to update the diagram, making it difficult for maintenance and replacement.

+ There is no high aesthetic

+ Takes the area of ​​the control cabinet, large power consumption.

+ Time to repair or change operating procedures takes a lot of time.

# Characteristics of PLC control system :

+ Reduce 80% number of extension cord.

+ PLC power consumption is very low.

+ Has a self-diagnosis function so that the repair work is quick and easy.

+ Control functions can be easily changed by programming devices (software, programming device) without hardware changes if there is no requirement to add or remove devices.

+ Communicate with other smart devices such as computers, networking, expansion modules.

+ Large memory capacity to accommodate complex programs.

+ The number of relays and timers is much less than the classical control system.

+ The number of contacts in the program used is not limited.

+ The time to complete a control cycle is very fast (several ms).

+ Easy to learn programming languages> easy programming and changing programs.

+ Compact > easy to maintenance, repair.

+ High reliability, small size, completely reliable in industrial environments.

+ Adaptation in harsh environments: Temperature, humidity, voltage fluctuations …

• Structure of PLC

Image of PLC structure

# Central Processing Unit (CPU) : It is a microprocessor that controls all PLC activities such as program execution, Input/Output processing and communication with external devices.

# Memory (RAM / ROM) : Used to store programs and data. Most PLCs have to use batteries to feed memory, but newer PLCs do not need batteries to store programs and data.

# Input Signal : The intelligence of a control system depends mainly on the ability of the PLC to read different data from the sensors as well as by manual input devices

+ Manual input devices : Push buttons, keypads and switches, …

+ Sensor : Cruise switch, photoelectric sensor, fiber optic sensor, magnetic sensor, pressure sensor, …

The input signal to the PLC can be a digital or analog signal, these signals are communicated with the PLC through different input modules DI (Digital Input) or AI (Analog Input), …

# Output Signal : A control system will have no practical meaning if it is not possible to communicate with external equipment, common external devices such as motors, valves, relays, indicator lights, electric bells, etc. just like the equipment. In, external devices are connected to the output ports of the output module.

These output modules can be DO(Digital Output) or AO(analog output) , Relay output or Transistor / Triac output .

# Other Modules : There are many modules with special functions that help the PLC to communicate with peripheral devices

Enthernet Module, CC-Link Module, RS232/485 Module, Position Module, Modbus Module , EtherCAT Module, …

• Working principle of PLC

+ The signals from external devices (sensors, contacts, etc.) are first fed to the CPU via the input module. After receiving the input signal, the CPU will process and send the control signals through the output module to external controlled devices according to a preprogrammed program.

+ A cycle consists of reading input signals, executing programs, internal communication, checking errors, sending updates to the output signal called a scan cycle or a scan cycle (Scan Cycle).
Usually, a scan takes a very short time (from 1ms to 100ms). The time taken for this scan depends on the speed of the PLC’s command processing, the short length of the program, the communication speed between the PLC and peripheral devices.

>>> The first PLC was created in 1968 by General Motors, so far PLC has become an integral part of industrial automation. Currently with the Industrial Revolution 4.0, the PLC has more modern features (IoT, SCADA) and is becoming cheaper, so the PLC will become increasingly popular in the future.

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