PLC Training
PLC
PLC also known as the Programmable Logical Controller is the Brain of any Process or Plant.
The PLC is a Microprocessor based of programmable device
It’s a Programmable interface Between Field input Devices and Field output Devices again which is Programmable and reprogrammable as per the user or as per the Application. In here Field Input Devices can be Different Parameter based sensor like Temperature, Pressure, Flow, Level, Velocity, Voltage, Current, Frequency, RPM, Weigh, Height etc.
The most basic function of a programmable controller is to emulate the functions of electromechanical relays. Discrete inputs are given a unique address, and a PLC instruction can test if the input state is on or off. Just as a series of relay contacts perform a logical AND function, not allowing current to pass unless all the contacts are closed, so a series of “examine if on” instructions will energize its output storage bit if all the input bits are on. Similarly, a parallel set of instructions will perform a logical OR. In an electromechanical relay wiring diagram, a group of contacts controlling one coil is called a “rung” of a “ladder diagram “, and this concept is also used to describe PLC logic. Some models of PLC limit the number of series and parallel instructions in one “rung” of logic. The output of each rung sets or clears a storage bit, which may be associated with a physical output address or which may be an “internal coil” with no physical connection. Such internal coils can be used, for example, as a common element in multiple separate rungs. Unlike physical relays, there is usually no limit to the number of times an input, output or internal coil can be referenced in a PLC program.
Some PLCs enforce a strict left-to-right, top-to-bottom execution order for evaluating the rung logic. This is different from electro-mechanical relay contacts, which, in a sufficiently complex circuit, may either pass current left-to-right or right-to-left, depending on the configuration of surrounding contacts. The elimination of these “sneak paths” is either a bug or a feature, depending on the programming style.
More advanced instructions of the PLC may be implemented as functional blocks, which carry out some operation when enabled by a logical input and which produce outputs to signal, for example, completion or errors, while manipulating variables internally that may not correspond to discrete logic.
It’s a Programmable interface Between Field input Devices and Field output Devices again which is Programmable and reprogrammable as per the user or as per the Application. In here Field Input Devices can be Different Parameter based sensor like Temperature, Pressure, Flow, Level, Velocity, Voltage, Current, Frequency, RPM, Weigh, Height etc. or its can be switch or Button and for Field Output Devices its can be Motor’s, Pump, Compressor, Light, Valve.
Types of PLC
There are mainly two types of PLC in terms of physical hardware:
- Compact PLC or Fixed PLC
- Modular PLC
- Compact Type PLC: This type of PLC generally has a fixed number of Input /Output modules.. This is generally used for small Application.
- Modular Type PLC: This types of PLC generally various modules. Inputs and Outputs modules can be easily expanded just by adding the modules. It’s also known as expandable type of PLC.
- PLC Basic Architecture Consist of the Following.
- CPU.
- Input Module.
- Output Module.
- Power Supply Module.
- Communication Module.
- Chassis.
a. CPU
Central Processing Unit, we can say that this is the brain of this system. The program in the form of logical instructions is stored in the CPU.
The Memory in the CPU is used to store programs and data from the different equipment connected to PLC
The CPU Module is consist of the Micro Controller, Memory Register and Bus System. The Bus System Consist of 3 type of System.
- Control Bus.
– The Control Bus is a part of Bus System the generally carry Control information from System.
- Data Bus.
- -The Data Bus carries the information from the input Module to CPU and from CPU to Output Module
- Address Bus.
-The Address Bus is used by CPU and send Address of memory location from Input to Output.
b. Input Module.
-PLC Input Module is intimidate between Field input and CPU. It’s where Field input device are connected in PLC. They are generally Classified into 2 type Signal.
- Digital Input Module.
-The Digital input module is Generally used for Digital Input Devices and also known as Discrete Input Module. The Device like Proximity Sensor, Switch, Pushbutton.
- Analog Input Module.
-The Analog input module is Generally used for Analog Input Devices and also known as Analog to Digital Module. The Device like to work analog input signals operate in the range of 4-20 mA, 0-20 mA, 1-5 V, etc.
c. Output Module.
-PLC Output Module is intimidate between CPU and Field Output Device. It’s where Field Output device are connected in PLC. They are generally Classified into 2 type Signal.
- Digital Output Module.
-The Digital Output module is Generally used for Digital Out Devices and also known as Discrete Output Module. The Device like LED, Relay, Valve, Motor.
- Analog Output Module.
-The Analog Output module is Generally used for Analog Output Devices and also known as Digital to Analog Module. The Device like to work analog Output signals operate in the range of 4-20 mA, 0-20 mA, 1-5 V, etc.
d. Power Supply Module
The Power Supply Module is one which Carry energy to each part of PLC to carry out its function. The Power Supply Ranges from 230VAC, 110VAC, 24VDC, etc.
e. Communication Module
The Communication Module is the module where Communication cable are connected with PLC and HMI or SCADA.
PLCs use built-in ports, such as USB, Ethernet, RS-232, RS-485, or RS-422 to communicate with external devices and systems software, SCADA, HMI. Communication is carried over various industrial network protocols, like Modbus, or EtherNet/IP. Many of these protocols are vendor specific.
PLCs used in larger I/O systems may have peer-to-peer (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over the communication link. These communication links are also often used for HMI devices such as keypads or PC-type workstations.
f. PLC Chassis
The PLC Chassis or Rack is the hardware assembly where the Input and Output, CPU, Power Supply, Communication and Additional function Module.
PLC Programming
User needs to download the program to control the processes. This program is stored in the CPU.
The program is prepared using a language which is called programming language. This is the list of PLC programming language:
- Ladder Diagram(LD)
- Instruction List(IL)
- Structured Text(ST)
- Function Block Diagram(FBD)
- Sequential Function Charts(SFC)
Out of all these languages, Ladder Diagram (LD) is widely used in the Automation industry because of its easy understanding.
The most basic function of a programmable controller is to emulate the functions of electromechanical relays. Discrete inputs are given a unique address, and a PLC instruction can test if the input state is on or off. Just as a series of relay contacts perform a logical AND function, not allowing current to pass unless all the contacts are closed, so a series of “examine if on” instructions will energize its output storage bit if all the input bits are on. Similarly, a parallel set of instructions will perform a logical OR. In an electromechanical relay wiring diagram, a group of contacts controlling one coil is called a “rung” of a “ladder diagram “, and this concept is also used to describe PLC logic. Some models of PLC limit the number of series and parallel instructions in one “rung” of logic. The output of each rung sets or clears a storage bit, which may be associated with a physical output address or which may be an “internal coil” with no physical connection. Such internal coils can be used, for example, as a common element in multiple separate rungs. Unlike physical relays, there is usually no limit to the number of times an input, output or internal coil can be referenced in a PLC program.
Some PLCs enforce a strict left-to-right, top-to-bottom execution order for evaluating the rung logic. This is different from electro-mechanical relay contacts, which, in a sufficiently complex circuit, may either pass current left-to-right or right-to-left, depending on the configuration of surrounding contacts. The elimination of these “sneak paths” is either a bug or a feature, depending on the programming style.
More advanced instructions of the PLC may be implemented as functional blocks, which carry out some operation when enabled by a logical input and which produce outputs to signal, for example, completion or errors, while manipulating variables internally that may not correspond to discrete logic.
Uploading and Downloading of Program.
- Uploading: It’s the process by which PLC program is shared from PLC CPU to PC.
- Downloading: It’s the Process by which PLC program from PC to PLC CPU.
Communication Protocol
Communication Protocol is used to exchange data between PLC and connected devices. These are the most popular communication protocols used in the PLC system,
What are a scan cycle and a scan time?
Each PLC has a scan time and a scan cycle. This considers how fast PLC work.
The Scan cycle is the cycle in which the PLC sense the inputs, executes the PLC programs, and then updates the outputs. This will take some amount of time usually in milliseconds.
The amount of time it takes for the PLC to finish one complete cycle is called the scan time of PLC.
Popular PLC Brands
There are multiple brands available for PLC systems are as below:
- Siemens
- Allen Bradley
- ABB
- GE
- Delta
- Mitsubishi
- Omron
- Toshiba
- Koyo
- Rexroth
- Yokogawa
- IDEC
- Honeywell
- Panasonic
- Schneider
A Programmable Logic Controller (PLC), is more or less a small computer with a built-in operating system (OS). This OS is highly specialized to handle incoming events in real time, i.e. at the time of their occurrence.
A PLC is primarily used to control machinery. Programs written for PLCs consist in simple terms on instructions to turn on and off outputs based on input conditions and the internal program.
The PLC has input lines where sensors are connected to notify upon events (e.g. temperature above/below a certain level, liquid level reached, etc.), and output lines to signal any reaction to the incoming events (e.g. start an engine, open/close a valve, etc.).
The system is user programmable. It uses a languages are:
1. Ladder Diagram, Sequential Function chart.
Functional block diagram, Structured Text, Instruction List. PLC used in Different Industries and Process as per below:
PLC in manufacturing, PLC in production, PLC in power generation, PLC in fabrication, PLC in waste water collection and treatment, PLC in oil and gas pipelines, PLC in electrical power transmission and distribution, PLC in civil defense siren systems, and PLC in large communication systems. PLC for Monitor and control heating, ventilation, air conditioning systems (HVAC), access, and energy consumption.