A Programmable Logic Controller is a user-friendly electronic computer that carries out control functions of many types and levels of complexity. It can be calculated, monitored and operated by a person unskilled in operating computers. The programmable logic controllers mostly draw the lines and devices of ladder diagrams. The resulting drawing in the computer replaces much of the external wiring required for control of a process. The programmable logic controllers can operate any system that has output devices that go ON and OFF. It can also run any system with analog outputs. The Programmable Logic Controllers can be performed on the Input side by ON / OFF devices or by analog input devices.
In 1978, the National Electrical Manufacturers Association (NEMA) released a standard for PLC’s. This specification, NEMA Standard ICS3-1978, was the result of four years of work by a committee made up of representatives from PC manufacturers, Part ICS3-304 of the standard defines a PLC as:
“A digitally operating electronic apparatus, which uses a programmable memory for the internal storage of instructions for implementing specific
functions such as logic, sequences, timing, counting and arithmetic to control, through digital or analog Input / Output modules, various types of machines of a process.”
Evaluation to the present PLC
The first PLC system evolved from the conventional computers in the late 1960s and early 1970s. The first PLC’s were mostly installed in automotive plants. Traditionally, the auto plants had to be shut down for up to a month at a change over time. The first PLC’s we used along with other new automation techniques to shorten the changeover time.
Through late 1970, the improvements were made in PLC programs to make them somewhat more user-friendly. In 1978, the introduction of the microprocessor chips increased computer power for all kind of automation and lowered the computing cost. Robotics, automation devices, and equipment of all types, including the PLC, consequently underwent many improvements. PLC programs became more understandable to people. PLC’s became more affordable, as well.
In the 1980s with more computer power available per Dollar, the use of PLC rises exponentially. Some electronics and computer companies and some different corporate electronic divisions found that the PLC became their greatest valued product. The market for PLC’s grew from a volume of $80 million in 1978 to 1 billion dollars per year by 1990 and is still growing.
Advantages of PLC
The following are some of the major benefits of using a Programmable Logic Controller.
Flexibility
In the past, each another electronically controlled production machine required its controller; fifteen devices might require fifteen drivers. Now it is possible to use just model of a PLC to run anyone of the fifteen machines. Furthermore, you would probably need fewer than fifteen controllers, because one PLC can easily run many machines. Each of the fifteen devices under PLC control would have its distinct program.
Implementing changes and correcting errors
With a wired relay type panel, any program alterations require time for rewiring of panels and devices. While a PLC program can be modified from a Keyboard sequence in a matter of minutes. No extra rewiring is required for a PLC controlled system. Also, if a programming error has occurred in a PLC control ladder diagram, change can be typed in quickly.
Large quantities of contacts
The PLC has a lot of contacts for each coil available in its programming. Suppose that a panel wire relay has four connections and all are in use when a design change, requiring three more contacts, is made. It would mean that time must be taken to procure and install a new relay or relay contact block. Using a PLC, however, would only require that three more contacts be typed in. These three connections would be automatically available in the PLC. Indeed, hundreds of contacts can be used from one relay if sufficient computer memory is available.
Lower Cost
Increased technology makes it possible to compact more functions into smaller and less expensive packages. In the 1990s, you can purchase a PLC
with numerous relays, timers, counters, a sequencer and other features for a few hundred dollars.
Pilot Running
A PLC programme circuit can be pre-run and evaluated in the office or lab. The record can be typed in, tested, observed and modified if needed, saving valuable factory time. In contrast, conventional relay system has been best tested on the plant floor, which can be very time-to consume.
Visual Observation
A PLC circuit’s operation can be seen during operation directly on a CRT screen. The operation or miss-operation of a circuit can be observed as it happens. Logic paths light up on the screen as they are energized. Troubleshooting can be done quicker by visual observation.
Speed of Operation
The relay can take an un-expectable amount of time to actuate. The operational speed for the PLC programming is breakneck. The rate of PLC logic operation is determined by scan time, which is a matter of milli seconds.
Ladder or Boolean Programming Method
The PLC programming can be accomplished in the Ladder mode by an electrician or technician, alternately, a PLC programmer who works in digital or Boolean Control systems can also easily perform PLC programming.
Reliability
Solid-state devices a more reliable, in general, than mechanical or electrical relays and timers, the PLC is built up of solid state electronic components with very high-reliability rates.
Simplicity of Ordering Control System Components
A PLC is a device with one delivery date when the PLC arrives; all the counters relay another component also arrive. In designing a relay panel, on the other hand, you may have twenty different relays and timers from twelve different suppliers. Obtaining the parts on time involve various delivery dates and availability. With a PLC you have one product and one lead-time for delivery. In a relay system, forgetting to buy one component would mean delaying the startup of the control system until that part arrives with the PLC one more relay is always available providing you an ordered PLC with enough extra computing power.
Documentation
An immediate printout of the real PLC circuit is available in minutes if required. There is no want to look for the blue print of the course in remote files. The PLC print out actual circuit in operation at a given moment often the file prints for relay panels are properly kept up to date. PLC print out is the course at present; no wire tracing is needed for verification.
Security
A PLC program change can’t be made unless PLC is properly unlocked and programmed. Relay panels tend to undergo undocumented changes. People on the late shift don’t always record panel alternations made when the office area is locked up for the night.
Ease of Changes by Reprogramming
Since the PLC can be reprogrammed quickly, so a mixed production processing can be accomplished. For example, if a part B comes down the assembly line while part A is still being processed, a schedule for part B’s processing can be reprogrammed into the production machinery in a matter of seconds.
Disadvantages of the PLC
Following are some of the disadvantages of or perhaps precautions for using PLC’s.
Newer Technology
It 's hard to change some personnel’s thinking from ladders and relays to the PLC computer concepts.
Fixed Program Application
Some applications are single function application. It can’t play to use a PLC that includes multiple programming capabilities if they are not needed.
Environmental Consideration
Individual process environments such as high heat and vibrations interfere with electronic devices in PLC’s, which limits their use.
Fail Safe Operations
In relay systems, the stop button electrically cuts, the circuit; if the power fails the operation ends. Furthermore, the relay system doesn’t automatically restart when power is restored. This, of course, can be programmed into the PLC however in some PLC plans you may have to implement an input voltage to produce a device to stop. These systems are not “Fail Safe.” These disadvantages can be overcome by adding safety relays to a PLC system.
Fixed Circuit Operations
If the circuit in operation never altered, a fixed control system might be less costly than a PLC. The PLC is most useful when periodic changes in operations are made.
In 1978, the National Electrical Manufacturers Association (NEMA) released a standard for PLC’s. This specification, NEMA Standard ICS3-1978, was the result of four years of work by a committee made up of representatives from PC manufacturers, Part ICS3-304 of the standard defines a PLC as:
“A digitally operating electronic apparatus, which uses a programmable memory for the internal storage of instructions for implementing specific
functions such as logic, sequences, timing, counting and arithmetic to control, through digital or analog Input / Output modules, various types of machines of a process.”
Evaluation to the present PLC
The first PLC system evolved from the conventional computers in the late 1960s and early 1970s. The first PLC’s were mostly installed in automotive plants. Traditionally, the auto plants had to be shut down for up to a month at a change over time. The first PLC’s we used along with other new automation techniques to shorten the changeover time.
Through late 1970, the improvements were made in PLC programs to make them somewhat more user-friendly. In 1978, the introduction of the microprocessor chips increased computer power for all kind of automation and lowered the computing cost. Robotics, automation devices, and equipment of all types, including the PLC, consequently underwent many improvements. PLC programs became more understandable to people. PLC’s became more affordable, as well.
In the 1980s with more computer power available per Dollar, the use of PLC rises exponentially. Some electronics and computer companies and some different corporate electronic divisions found that the PLC became their greatest valued product. The market for PLC’s grew from a volume of $80 million in 1978 to 1 billion dollars per year by 1990 and is still growing.
Advantages of PLC
The following are some of the major benefits of using a Programmable Logic Controller.
Flexibility
In the past, each another electronically controlled production machine required its controller; fifteen devices might require fifteen drivers. Now it is possible to use just model of a PLC to run anyone of the fifteen machines. Furthermore, you would probably need fewer than fifteen controllers, because one PLC can easily run many machines. Each of the fifteen devices under PLC control would have its distinct program.
Implementing changes and correcting errors
With a wired relay type panel, any program alterations require time for rewiring of panels and devices. While a PLC program can be modified from a Keyboard sequence in a matter of minutes. No extra rewiring is required for a PLC controlled system. Also, if a programming error has occurred in a PLC control ladder diagram, change can be typed in quickly.
Large quantities of contacts
The PLC has a lot of contacts for each coil available in its programming. Suppose that a panel wire relay has four connections and all are in use when a design change, requiring three more contacts, is made. It would mean that time must be taken to procure and install a new relay or relay contact block. Using a PLC, however, would only require that three more contacts be typed in. These three connections would be automatically available in the PLC. Indeed, hundreds of contacts can be used from one relay if sufficient computer memory is available.
Lower Cost
Increased technology makes it possible to compact more functions into smaller and less expensive packages. In the 1990s, you can purchase a PLC
with numerous relays, timers, counters, a sequencer and other features for a few hundred dollars.
Pilot Running
A PLC programme circuit can be pre-run and evaluated in the office or lab. The record can be typed in, tested, observed and modified if needed, saving valuable factory time. In contrast, conventional relay system has been best tested on the plant floor, which can be very time-to consume.
Visual Observation
A PLC circuit’s operation can be seen during operation directly on a CRT screen. The operation or miss-operation of a circuit can be observed as it happens. Logic paths light up on the screen as they are energized. Troubleshooting can be done quicker by visual observation.
Speed of Operation
The relay can take an un-expectable amount of time to actuate. The operational speed for the PLC programming is breakneck. The rate of PLC logic operation is determined by scan time, which is a matter of milli seconds.
Ladder or Boolean Programming Method
The PLC programming can be accomplished in the Ladder mode by an electrician or technician, alternately, a PLC programmer who works in digital or Boolean Control systems can also easily perform PLC programming.
Reliability
Solid-state devices a more reliable, in general, than mechanical or electrical relays and timers, the PLC is built up of solid state electronic components with very high-reliability rates.
Simplicity of Ordering Control System Components
A PLC is a device with one delivery date when the PLC arrives; all the counters relay another component also arrive. In designing a relay panel, on the other hand, you may have twenty different relays and timers from twelve different suppliers. Obtaining the parts on time involve various delivery dates and availability. With a PLC you have one product and one lead-time for delivery. In a relay system, forgetting to buy one component would mean delaying the startup of the control system until that part arrives with the PLC one more relay is always available providing you an ordered PLC with enough extra computing power.
Documentation
An immediate printout of the real PLC circuit is available in minutes if required. There is no want to look for the blue print of the course in remote files. The PLC print out actual circuit in operation at a given moment often the file prints for relay panels are properly kept up to date. PLC print out is the course at present; no wire tracing is needed for verification.
Security
A PLC program change can’t be made unless PLC is properly unlocked and programmed. Relay panels tend to undergo undocumented changes. People on the late shift don’t always record panel alternations made when the office area is locked up for the night.
Ease of Changes by Reprogramming
Since the PLC can be reprogrammed quickly, so a mixed production processing can be accomplished. For example, if a part B comes down the assembly line while part A is still being processed, a schedule for part B’s processing can be reprogrammed into the production machinery in a matter of seconds.
Disadvantages of the PLC
Following are some of the disadvantages of or perhaps precautions for using PLC’s.
Newer Technology
It 's hard to change some personnel’s thinking from ladders and relays to the PLC computer concepts.
Fixed Program Application
Some applications are single function application. It can’t play to use a PLC that includes multiple programming capabilities if they are not needed.
Environmental Consideration
Individual process environments such as high heat and vibrations interfere with electronic devices in PLC’s, which limits their use.
Fail Safe Operations
In relay systems, the stop button electrically cuts, the circuit; if the power fails the operation ends. Furthermore, the relay system doesn’t automatically restart when power is restored. This, of course, can be programmed into the PLC however in some PLC plans you may have to implement an input voltage to produce a device to stop. These systems are not “Fail Safe.” These disadvantages can be overcome by adding safety relays to a PLC system.
Fixed Circuit Operations
If the circuit in operation never altered, a fixed control system might be less costly than a PLC. The PLC is most useful when periodic changes in operations are made.
