Introduction to PLC
What does ‘PLC’ mean?
A PLC (Programmable Logic Controllers) is an industrial computer used to monitor inputs, and depending upon their state make decisions based on its program or logic, to control (turn on/off) its outputs to automate a machine or a process.
NEMA defines a PROGRAMMABLE LOGIC CONTROLLER as:“A digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions by implementing specific functions such as logic sequencing, timing, counting, and arithmetic to control, through digital or analog input/output modules, various types of machines or processes”.
Traditional PLC Applications
*In automated system, PLC controller is usually the central part of a process control system.
*To run more complex processes it is possible to connect more PLC controllers to a central computer.
Disadvantages of PLC control– Too much work required in connecting wires.
– Difficulty with changes or replacements.
– Difficulty in finding errors; requiring skillful work force.
– When a problem occurs, hold-up time is indefinite, usually long.
Advantages of PLC control* Rugged and designed to withstand vibrations, temperature, humidity, and noise.
* Have interfacing for inputs and outputs already inside the controller.
* Easily programmed and have an easily understood programming language.
Major Types of Industrial Control Systems
Industrial control system or ICS comprise of different types of control systems that are currently in operation in various industries. These control systems include PLC, SCADA and DCS and various others:
PLC
They are based on the Boolean logic operations whereas some models use timers and some have continuous control. These devices are computer based and are used to control various process and equipments within a facility. PLCs control the components in the DCS and SCADA systems but they are primary components in smaller control configurations.
DCS
Distributed Control Systems consists of decentralized elements and all the processes are controlled by these elements. Human interaction is minimized so the labor costs and injuries can be reduced.
Embedded Control
In this control system, small components are attached to the industrial computer system with the help of a network and control is exercised.
SCADA
Supervisory Control And Data Acquisition refers to a centralized system and this system is composed of various subsystems like Remote Telemetry Units, Human Machine Interface, Programmable Logic Controller or PLC and Communications.
Communications abilities began to appear in approximately 1973. They could also be used in the 70’s to send and receive varying voltages to allow them to enter the analog world.
The 80’s saw an attempt to:
standardize communications with manufacturing automation protocol (MAP), reduce the size of the PLC, and making them software programmable through symbolic programming on personal computers instead of dedicated programming terminals or handheld programmers.
The 90’s have seen a gradual reduction in the introduction of new protocols, and the modernization of the physical layers of some of the more popular protocols that survived the 1980’s.
The latest standard “IEC 1131-3” has tried to merge plc programming languages under one international standard. We now have PLCs that are programmable in function block diagrams, instruction lists, C and structured text all at the same time.
A PLC (Programmable Logic Controllers) is an industrial computer used to monitor inputs, and depending upon their state make decisions based on its program or logic, to control (turn on/off) its outputs to automate a machine or a process.
NEMA defines a PROGRAMMABLE LOGIC CONTROLLER as:“A digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions by implementing specific functions such as logic sequencing, timing, counting, and arithmetic to control, through digital or analog input/output modules, various types of machines or processes”.
Traditional PLC Applications
*In automated system, PLC controller is usually the central part of a process control system.
*To run more complex processes it is possible to connect more PLC controllers to a central computer.
Disadvantages of PLC control– Too much work required in connecting wires.
– Difficulty with changes or replacements.
– Difficulty in finding errors; requiring skillful work force.
– When a problem occurs, hold-up time is indefinite, usually long.
Advantages of PLC control* Rugged and designed to withstand vibrations, temperature, humidity, and noise.
* Have interfacing for inputs and outputs already inside the controller.
* Easily programmed and have an easily understood programming language.
Major Types of Industrial Control Systems
Industrial control system or ICS comprise of different types of control systems that are currently in operation in various industries. These control systems include PLC, SCADA and DCS and various others:
PLC
They are based on the Boolean logic operations whereas some models use timers and some have continuous control. These devices are computer based and are used to control various process and equipments within a facility. PLCs control the components in the DCS and SCADA systems but they are primary components in smaller control configurations.
DCS
Distributed Control Systems consists of decentralized elements and all the processes are controlled by these elements. Human interaction is minimized so the labor costs and injuries can be reduced.
Embedded Control
In this control system, small components are attached to the industrial computer system with the help of a network and control is exercised.
SCADA
Supervisory Control And Data Acquisition refers to a centralized system and this system is composed of various subsystems like Remote Telemetry Units, Human Machine Interface, Programmable Logic Controller or PLC and Communications.
PLC History
PLC development began in 1968 in response to a request from an US car manufacturer (GE). The first PLCs were installed in industry in 1969.Communications abilities began to appear in approximately 1973. They could also be used in the 70’s to send and receive varying voltages to allow them to enter the analog world.
The 80’s saw an attempt to:
standardize communications with manufacturing automation protocol (MAP), reduce the size of the PLC, and making them software programmable through symbolic programming on personal computers instead of dedicated programming terminals or handheld programmers.
The 90’s have seen a gradual reduction in the introduction of new protocols, and the modernization of the physical layers of some of the more popular protocols that survived the 1980’s.
The latest standard “IEC 1131-3” has tried to merge plc programming languages under one international standard. We now have PLCs that are programmable in function block diagrams, instruction lists, C and structured text all at the same time.
PLC Hardware
Hardware Components of a PLC SystemProcessor unit (CPU), Memory, Input/Output, Power supply unit, Programming device, and other devices.
Central Processing Unit (CPU)CPU – Microprocessor based, may allow arithmetic operations, logic operators, block memory moves, computer interface, local area network, functions, etc.
CPU makes a great number of check-ups of the PLC controller itself so eventual errors would be discovered early.
System BussesThe internal paths along which the digital signals flow within the PLC are called
busses.
The system has four busses:
– The CPU uses the data bus for sending data between the different elements,
– The address bus to send the addresses of locations for accessing stored data,
– The control bus for signals relating to internal control actions,
– The system bus is used for communications between the I/O ports and the I/O unit.
Memory
System (ROM) to give permanent storage for the operating system and the fixed data used by the CPU.
RAM for data. This is where information is stored on the status of input and output devices and the values of timers and counters and other internal devices. EPROM for ROM’s that can be programmed and then the program made permanent.
I/O Sections
Inputs monitor field devices, such as switches and sensors.
Outputs control other devices, such as motors, pumps, solenoid valves, and lights.
Power Supply
Most PLC controllers work either at 24 VDC or 220 VAC. Some PLC controllers have electrical supply as a separate module, while small and medium series already contain the supply module.
Programming Device
The programming device is used to enter the required program into the memory of the processor.
The program is developed in the programming device and then transferred to the memory unit of the PLC.
CPU makes a great number of check-ups of the PLC controller itself so eventual errors would be discovered early.
System BussesThe internal paths along which the digital signals flow within the PLC are called
busses.
The system has four busses:
– The CPU uses the data bus for sending data between the different elements,
– The address bus to send the addresses of locations for accessing stored data,
– The control bus for signals relating to internal control actions,
– The system bus is used for communications between the I/O ports and the I/O unit.
Memory
System (ROM) to give permanent storage for the operating system and the fixed data used by the CPU.
RAM for data. This is where information is stored on the status of input and output devices and the values of timers and counters and other internal devices. EPROM for ROM’s that can be programmed and then the program made permanent.
I/O Sections
Inputs monitor field devices, such as switches and sensors.
Outputs control other devices, such as motors, pumps, solenoid valves, and lights.
Power Supply
Most PLC controllers work either at 24 VDC or 220 VAC. Some PLC controllers have electrical supply as a separate module, while small and medium series already contain the supply module.
Programming Device
The programming device is used to enter the required program into the memory of the processor.
The program is developed in the programming device and then transferred to the memory unit of the PLC.
PLC Operation
Input RelaysThese are connected to the outside world. They physically exist and receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors.
Internal Utility RelaysThese do not receive signals from the outside world nor do they physically exist. They are simulated relays and are what enables a PLC to eliminate external relays.
There are also some special relays that are dedicated to performing only
one task.
CountersThese do not physically exist. They are simulated counters and they can be programmed to count pulses.
Typically these counters can count up, down or both up and down. Since they are simulated they are limited in their counting speed.
Some manufacturers also include highspeed counters that are hardware based.
TimersThese also do not physically exist. They come in many varieties and increments.
The most common type is an on-delay type.
Others include off-delay and both retentive and non-retentive types. Increments vary from 1ms through 1s.
Output Relays
These are connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc.
They can be transistors, relays, or triacs depending upon the model chosen.
Data StorageTypically there are registers assigned to simply store data. Usually used as temporary storage for math or data manipulation.
They can also typically be used to store data when power is removed from the
PLC.
Internal Utility RelaysThese do not receive signals from the outside world nor do they physically exist. They are simulated relays and are what enables a PLC to eliminate external relays.
There are also some special relays that are dedicated to performing only
one task.
CountersThese do not physically exist. They are simulated counters and they can be programmed to count pulses.
Typically these counters can count up, down or both up and down. Since they are simulated they are limited in their counting speed.
Some manufacturers also include highspeed counters that are hardware based.
TimersThese also do not physically exist. They come in many varieties and increments.
The most common type is an on-delay type.
Others include off-delay and both retentive and non-retentive types. Increments vary from 1ms through 1s.
Output Relays
These are connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc.
They can be transistors, relays, or triacs depending upon the model chosen.
Data StorageTypically there are registers assigned to simply store data. Usually used as temporary storage for math or data manipulation.
They can also typically be used to store data when power is removed from the
PLC.
Leading Brands Of PLC
AMERICAN
1. Allen Bradley
2. Gould Modicon
3. Texas Instruments
4. General Electric
5. Westinghouse
6. Cutter Hammer
7. Square D
EUROPEAN
1. Siemens
2. Klockner & Mouller
3. Festo
4. Telemechanique
JAPANESE
1. Toshiba
2. Omron
3. Fanuc
4. Mitsubishi
Areas of Application
Manufacturing / Machining
Food / Beverage
Metals
Power
Mining
Petrochemical / Chemical
Advantages of PLC
-Less wiring.
-Easier and faster to make changes.
-Trouble shooting aids make programming easier and reduce downtime.
-Reliable components make these likely to operate for years before failure.
___________________________________________________________________________
No comments:
Post a Comment