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Programmable logic controllers (PLCs) are members of the computer family capable of storing instructions to control functions such as sequencing, timing, and counting, which control a machine or a process. The PLC is composed of two basic sections, the Central Processing Unit (CPU) and the Input/Output (I/O) interface system. The PLC measures input signals coming from a machine and through the internal program provides output or control back to the machine.
Ladder logic is the programming language used to represent electrical sequences of operation. In hardwired circuits the electrical wiring is connected from one device to another according to logic of operation. In a PLC the devices are connected to the input interface, the outputs are connected to the output interface and the actual wiring of the components is done electronically inside the PLC using ladder logic. This is known as soft wired.
PLC is a device that is capable of being programmed to perform a controlling function. Before the advent of PLC, the problem of industrial control was usually solved by relays or hardwired solid-state logic blocks. These are very flexible in design and easy for maintenance personal to understand. However, they involved a vast amount of interconnection. For the wiring cost to be minimized, relays and logic blocks had to be kept together. This led to development of control panel concept for larger and more complex logic control system.
The PLC was first conceived by group of engineers from hydramatic division of GM in 1968.This was designed to provide flexibility in control based on programming and executing logic instruction. Adopting the ladder diagram programming language, simplifying maintenance and reducing the cost of spare parts inventories realized major advantages.
2. PLC HISTORY
In the late 1960's PLCs were first introduced. The primary reason for designing such a device was eliminating the large cost involved in replacing the complicated relay based machine control systems. Bedford Associates (Bedford, MA) proposed something called a Modular Digital Controller (MODICON) to a major US car manufacturer. Other companies at the time proposed computer based schemes, one of which was based upon the PDP-8. The MODICON 084 brought the world's first PLC into commercial production.
When production requirements changed so did the control system. This becomes very expensive when the change is frequent. Since relays are mechanical devices they also have a limited lifetime which required strict adhesion to maintenance schedules. Troubleshooting was also quite tedious when so many relays are involved. Now picture a machine control panel that included many, possibly hundreds or thousands, of individual relays. The size could be mind boggling. How about the complicated initial wiring of so many individual devices! These relays would be individually wired together in a manner that would yield the desired outcome. Were there problems You bet!
These "new controllers" also had to be easily programmed by maintenance and plant engineers. The lifetime had to be long and programming changes easily performed. They also had to survive the harsh industrial environment. That's a lot to ask! The answers were to use a programming technique most people were already familiar with and replace mechanical parts with solid-state ones.
In the mid70â„¢s the dominant PLC technologies were sequencer state-machines and the bit-slice based CPU. The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs. Conventional microprocessors lacked the power to quickly solve PLC logic in all but the smallest PLCs. As conventional microprocessors evolved, larger and larger PLCs were being based upon them. However, even today some are still based upon the 2903.(ref A-B's PLC-3) Modicon has yet to build a faster PLC than their 984A/B/X which was based upon the 2901.
Communications abilities began to appear in approximately 1973. The first such system was Modicon's Modbus. The PLC could now talk to other PLCs and they could be far away from the actual machine they were controlling. They could also now be used to send and receive varying voltages to allow them to enter the analog world. Unfortunately, the lack of standardization coupled with continually changing technology has made PLC communications a nightmare of incompatible protocols and physical networks. Still, it was a great decade for the PLC!
The 80â„¢s saw an attempt to standardize communications with General Motor's manufacturing automation protocol(MAP). It was also a time for reducing the size of the PLC and making them software programmable through symbolic programming on personal computers instead of dedicated programming terminals or handheld programmers. Today the world's smallest PLC is about the size of a single control relay!
The 90â„¢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! PC's are also being used to replace PLCs in some applications. The original company who commissioned the MODICON 084 has actually switched to a PC based control system.
3. PLC HARDWARE
A programmable logic controller consists of the following components:
Â¢ Central Processing Unit (CPU).
Â¢ Input modules.
Â¢ Output modules and
Â¢ Power supply.
A PLC hardware block diagram is shown in Figure. The programming terminal in the diagram is not a part of the PLC, but it is essential to have a terminal for programming or monitoring a PLC. In the diagram, the arrows between blocks indicate the information and power flowing directions.
Fig: PLC Hardware Block Diagram
Like other computerized devices, there is a Central Processing Unit (CPU) in a PLC. The CPU, which is the brain of a PLC, does the following operations:
Â¢ Updating inputs and outputs. This function allows a PLC to read the status of its input terminals and energize or deenergize its output terminals.
Â¢ Performing logic and arithmetic operations. A CPU conducts all the mathematic and logic operations involved in a PLC.
Â¢ Communicating with memory. The PLCâ„¢s programs and data are stored in memory. When a PLC is operating, its CPU may read or change the contents of memory locations.
Â¢ Scanning application programs. An application program, which is called a ladder logic program, is a set of instructions written by a PLC programmer. The scanning function allows the PLC to execute the application program as specified by the programmer.
Â¢ Communicating with a programming terminal. The CPU transfers program and data between itself and the programming terminal.
A PLCâ„¢s CPU is controlled by operating system software. The operating system software is a group of supervisory programs that are loaded and stored permanently in the PLCâ„¢s memory by the PLC manufacturer.
Memory is the component that stores information, programs, and data in a PLC. The process of putting new information into a memory location is called writing. The process of retrieving information from a memory location is called reading.
The common types of memory used in PLCs are Read Only Memory (ROM) and Random Access Memory (RAM). A ROM location can be read, but not written. ROM is used to store programs and data that should not be altered. For example, the PLCâ„¢s operating programs are stored in ROM.
A RAM location can be read or written. This means the information stored in a RAM location can be retrieved and/or altered. Ladder logic programs are stored in RAM. When a new ladder logic program is loaded into a PLCâ„¢s memory, the old program that was stored in the same locations is over-written and essentially erased.
The memory capacities of PLCs vary. Memory capacities are often expressed in terms of kilo-bytes (K). One byte is a group of 8 bits. One bit is a memory location that may store one binary number that has the value of either 1 or 0. (Binary numbers are addressed in Module 2). 1K memory means that there are 1024 bytes of RAM. 16K memory means there are 16 x 1024 =16384 bytes of RAM.
Input modules and output modules
A PLC is a control device. It takes information from inputs and makes decisions to energize or de-energize outputs. The decisions are made based on the statuses of inputs and outputs and the ladder logic program that is being executed.
The input devices used with a PLC include pushbuttons, limit switches, relay contacts, photo sensors, proximity switches, temperature sensors, and the like. These input devices can be AC (alternating current) or DC (direct current). The input voltages can be high or low. The input signals can be digital or analog. Differing inputs require different input modules. An input module provides an interface between input devices and a PLCâ„¢s CPU, which uses only a low DC voltage. The input moduleâ„¢s function is to convert the input signals to DC voltages that are acceptable to the CPU. Standard discrete input modules include 24 V AC, 48 V AC, 120 V AC, 220 V AC, 24 V DC, 48 V DC, 120 V DC, 220 V DC, and transistor-transistor logic (TTL) level.
The devices controlled by a PLC include relays, alarms, solenoids, fans, lights, and motor starters. These devices may require different levels of AC or DC voltages. Since the signals processed in a PLC are low DC voltages, it is the function of the output module to convert PLC control signals to the voltages required by the controlled circuits or devices. Standard discrete output modules include 24 V AC, 48 V AC, 120 V AC, 220 V AC, 24 V DC, 48 V DC, 120 V DC, 220 V DC, and TTL level.
PLCs are powered by standard commercial AC power lines. However, many PLC components, such as the CPU and memory, utilize 5 volts or another level of DC power. The PLC power supply converts AC power into DC power to support those components of the PLC.
A PLC requires a programming terminal and programming software for operation. The programming terminal can be a dedicated terminal or a generic computer purchased anywhere. The programming terminal is used for programming the PLC and monitoring the PLCâ„¢s operation. It may also download a ladder logic program (the sending of a program from the programming terminal to the PLC) or upload a ladder logic program (the sending of a program from the PLC to the programming terminal). The terminal uses programming software for programming and talking to a PLC.
4. WORKING OF PLC
Bringing input signal status to the internal memory of CPU
Â¢ The field signals are connected to the I/P module. At the output of I/P module the field status converted into the voltage level required by the CPU is always available.
Â¢ At the beginning of each cycle the CPU brings in all the field I/P signals from I/P module & stores into its internal memory called as PII, meaning process image input.
Â¢ The programmable controller operates cyclically meaning when complete program has been scanned; it starts again at the beginning of the program.
A PLC works by continually scanning a program. We can think of this scan cycle as consisting of 3 important steps. There are typically more than 3 but we can focus on the important parts and not worry about the others. Typically the others are checking the system and updating the current internal counter and timer values.
Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or off. In other words, is the sensor connected to the first input on How about the second input How about the third... It records this data into its memory to be used during the next step.
Step 2-Execute Program-Next the PLC executes your program one instruction at a time. Maybe your program said that if the first input was on then it should turn on the first output. Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the state of the first input. It will store the execution results for use later during the next step.
Step 3-Update Output Status-Finally the PLC updates the status of the outputs. It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true.
Process Control and Automation
The process of recognizing the state of the process at all times, analyze the information according to the set rules and guidelines and accordingly actuate the control elements is referred to as process control.
In control of process all these actions can be taken manually with human involvement or in a semiautomatic or fully automatic manner.
Automation is basically the delegation of human control functions to technical equipment aimed towards achieving:
Â¢ Higher productivity.
Â¢ Superior quality of end product.
Â¢ Efficient usage of energy and raw materials.
Â¢ Improved safety in working conditions etc.
Methods adopted for Process Control and Automation
Â¢ Manual control
Â¢ Hard wired logic control
Â¢ Electronics control
Â¢ PLC control
Â¢ Manual Control
Â¢ This was considered to be the first step towards automation.
Â¢ Here the contractor & relays together with timers & counters were used.
Â¢ With the advent of electronics, the logic gates started replacing the relays & auxiliary contractors in the control circuits & timers.
Â¢ With changes, the benefits are:
Â¢ 1) Reduced space requirements
2) Energy saving
3) Less maintenance and hence greater reliability etc.
Â¢ With electronics, the implementation of changes in the control logic as well as reducing the project lead-time was not possible.
Programmable Logic Controller
Â¢ With microprocessor and associated peripherals chips, the process of control and automation went a radical change.
Â¢ Instead of achieving the desired control or automation through physical wiring of control devices, in PLC it is through a program or software. Thus these controllers are referred to as programmable logic controllers.
Â¢ The programmable controllers have experienced an unprecedented growth as universal element. It can be effectively used in applications ranging from simple control like replacing small number relays to complex automation problem.
5. PROGRAMMING THE PLC
Ladder logic is the main programming method used for PLCs. The ladder logic has been developed to mimic relay logic. The decision to use the relay logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and trades people was greatly reduced.
Modern control systems still include relays, but these are rarely used for logic. A relay is a simple device that uses a magnetic field to control a switch, as pictured in Fig. When a voltage is applied to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch. The contact that closes when the coil is energized is called normally open. The normally closed contacts touch when the input coil is not energized. Relays are normally drawn in schematic form using a circle to represent the input coil. The output contacts are shown with two parallel lines. Normally open contacts are shown as two lines, and will be open (non-conducting) when the input is not energized. Normally closed contacts are shown with two lines with a diagonal line through them. When the input coil is not energized the normally closed contacts will be closed (conducting).
Fig: Simple Relay Layouts and Schematics
Relays are used to let one power source close a switch for another (often high current) power source, while keeping them isolated. An example of a relay in a simple control application is shown in Figure. In this system the first relay on the left is used as normally closed, and will allow current to flow until a voltage is applied to the input A. The second relay is normally open and will not allow current to flow until a voltage is applied to the input B. If current is flowing through the first two relays then current will flow through the coil in the third relay, and close the switch for output C. This circuit would normally be drawn in the ladder logic form. This can be read logically as C will be on if A is off and B is on.
Fig: A Simple Relay Controller
The example in Figure does not show the entire control system, but only the logic. When we consider a PLC there are inputs, outputs, and the logic. Figure 4 shows a more complete representation of the PLC. Here there are two inputs from push buttons. We can imagine the inputs as activating 24V DC relay coils in the PLC. This in turn drives an output relay that switches 115V AC that will turn on a light. Note, in actual PLCs inputs are never relays, but outputs are often relays. The ladder logic in the PLC is actually a computer program that the user can enter and change. Notice that both of the input push buttons are normally open, but the ladder logic inside the PLC has one normally open contact, and one normally closed contact. Do not think that the ladder logic in the PLC needs to match the inputs or outputs. Many beginners will get caught trying to make the ladder logic match the input types.
Fig: A PLC Illustrated With Relays
Many relays also have multiple outputs (throws) and this allows an output relay to also be an input simultaneously. The circuit shown in Figure 5 is an example of this; it is called a seal in circuit. In this circuit the current can flow through either branch of the circuit, through the contacts labeled A or B. The input B will only be on when the output B is on. If B is off, and A is energized, then B will turn on. If B turns on then the input B will turn on and keep output B on even if input A goes off. After B is turned on the output B will not turn off.
Fig: A Seal-in Circuit
The first PLCs were programmed with a technique that was based on relay logic wiring schematics. This eliminated the need to teach the electricians, technicians and engineers how to program a computer - but, this method has stuck and it is the most common technique for programming PLCs today. An example of ladder logic can be seen in Fig. To interpret this diagram imagines that the power is on the vertical line on the left hand side, we call this the hot rail. On the right hand side is the neutral rail. In the figure there are two rungs, and on each rung there are combinations of inputs (two vertical lines) and outputs (circles). If the inputs are opened or closed in the right combination the power can flow from the hot rail, through the inputs, to power the outputs, and finally to the neutral rail. An input can come from a sensor, switch, or any other type of sensor. An output will be some device outside the PLC that is switched on or off, such as lights or motors. In the top rung the contacts are normally open and normally closed. Which means if input A is on and input B is off, then power will flow through the output and activate it Any other combination of input values will result in the output X being off.
Fig: A Simple Ladder Logic Diagram
There are other methods for programming PLCs. One of the earliest techniques involved mnemonic instructions. These instructions can be derived directly from the ladder logic diagrams and entered into the PLC through a simple programming terminal. An example of mnemonics is shown in Figure. In this example the instructions are read one line at a time from top to bottom. The first line 00000 has the instruction LDN (input load and not) for input A. This will examine the input to the PLC and if it is off it will remember a 1 (or true), if it is on it will remember a 0 (or false). The next line uses an LD (input load) statement to look at the input. If the input is off it remembers a 0, if the input is on it remembers a 1 (note: this is the reverse of the LDN). The AND statement recalls the last two numbers remembered and if they are both true the result is a 1; otherwise the result is a 0. This result now replaces the two numbers that were recalled, and there is only one number remembered. The process is repeated for lines 00003 and 00004, but when these are done there are now three numbers remembered. The oldest number is from the AND, the newer numbers are from the two LD instructions. The AND in line 00005 combines the results from the last LD instructions and now there are two numbers remembered. The OR instruction takes the two numbers now remaining and if either one is a 1 the result is a 1; otherwise the result is a 0. This result replaces the two numbers, and there is now a single number there. The last instruction is the ST (store output) that will look at the last value stored and if it is 1, the output will be turned on; if it is 0 the output will be turned off.
Fig: An Example of a Mnemonic Program and Equivalent Ladder Logic
The ladder logic program in Figure is equivalent to the mnemonic program. Even if you have programmed a PLC with ladder logic, it will be converted to mnemonic form before being used by the PLC. In the past mnemonic programming was the most common, but now it is uncommon for users to even see mnemonic programs.
Sequential Function Charts (SFCs) have been developed to accommodate the programming of more advanced systems. These are similar to flowcharts, but much more powerful. The example seen in Figure is doing two different things. To read the chart, start at the top where is says start. Below this there is the double horizontal line that says follow both paths. As a result the PLC will start to follow the branch on the left and right hand sides separately and simultaneously. On the left there are two functions the first one is the power up function. This function will run until it decides it is done, and the power down function will come after. On the right hand side is the flash function; this will run until it is done. These functions look unexplained, but each function, such as power up will be a small ladder logic program. This method is much different from flowcharts because it does not have to follow a single path through the flowchart.
Fig: An Example of a Sequential Function Chart
Structured Text programming has been developed as a more modern programming language. It is quite similar to languages such as BASIC. A simple example is shown in Figure 9. This example uses a PLC memory location i. This memory location is for an integer, as will be explained later in the book. The first line of the program sets the value to 0. The next line begins a loop, and will be where the loop returns to. The next line recalls the value in location i, adds 1 to it and returns it to the same location. The next line checks to see if the loop should quit. If i is greater than or equal to 10, then the loop will quit, otherwise the computer will go back up to the REPEAT statement continue from there. Each time the program goes through this loop i will increase by 1 until the value reaches 10.
i: = 0;
i: = i + 1;
UNTIL i >= 10
Fig: An Example of a Structured Text Program
When a process is controlled by a PLC it uses inputs from sensors to make decisions and update outputs to drive actuators, as shown in Figure. The process is a real process that will change over time. Actuators will drive the system to new states (or modes of operation). This means that the controller is limited by the sensors available, if an input is not available, the controller will have no way to detect a condition.
Fig: The Separation of Controller and Process
The control loop is a continuous cycle of the PLC reading inputs, solving the ladder logic, and then changing the outputs. Like any computer this does not happen instantly. Figure shows the basic operation cycle of a PLC. When power is turned on initially the PLC does a quick sanity check to ensure that the hardware is working properly. If there is a problem the PLC will halt and indicate there is an error. For example, if the PLC power is dropping and about to go off this will result in one type of fault. If the PLC passes the sanity checks it will then scan (read) all the inputs. After the inputs values are stored in memory the ladder logic will be scanned (solved) using the stored values - not the current values. This is done to prevent logic problems when inputs change during the ladder logic scan. When the ladder logic scan is complete the outputs will be scanned (the output values will be changed). After this the system goes back to do a sanity check, and the loop continues indefinitely. Unlike normal computers, the entire program will be run every scan. Typical times for each of the stages are in the order of milliseconds.
Fig: The Scan Cycle of a PLC
Ladder Logic Inputs
PLC inputs are easily represented in ladder logic. In Figure there are three types of inputs shown. The first two are normally open and normally closed inputs, discussed previously. The IIT (Immediate Input) function allows inputs to be read after the input scan, while the ladder logic is being scanned. This allows ladder logic to examine input values more often than once every cycle.
Fig: Ladder Logic Inputs
Ladder Logic Outputs
In ladder logic there are multiple types of outputs, but these are not consistently available on all PLCs. Some of the outputs will be externally connected to devices outside the PLC, but it is also possible to use internal memory locations in the PLC. Six types of outputs are shown in Figure 13. The first is a normal output, when energized the output will turn on, and energize an output. The circle with a diagonal line through is a normally on output. When energized the output will turn off. This type of output is not available on all PLC types. When initially energized the OSR (One Shot Relay) instruction will turn on for one scan, but then be off for all scans after, until it is turned off. The L (latch) and U (unlatch) instructions can be used to lock outputs on. When an L output is energized the output will turn on indefinitely, even when the output coil is deenergized. The output can only be turned off using a U output. The last instruction is the IOT (Immediate Output) that will allow outputs to be updated without having to wait for the ladder logic scan to be completed.
When power is applied (on) the output x is activated for the left output, but turned off for the output on the right.
An input transition on will cause the output x to go on for one scan. (This is also known as a one shot relay)
When the L coil is energized, x will be toggled on; it will stay on until the U coil is energized. This is like a flip-flop and stays set even when the PLC is turned off.
Some PLCs will allow immediate outputs that do not wait for the program scan to end before setting an output.
Fig: Ladder Logic Outputs
The load (LD) instruction is a normally open contact. It is sometimes also called examine if on. (XIO) (As in examine the input to see if itâ„¢s physically on) The symbol for a load instruction is shown below.
A Load (contact) symbol
This is used when an input signal is needed to be present for the symbol to turn on. When the physical input is on we can say that the instruction is true. We examine the input for an on signal. If the input is physically on then the symbol is on. An on condition is also referred to as logic 1 state.
This symbol normally can be used for internal inputs, external inputs and external output contacts. Remember that internal relays don't physically exist. They are simulated (software) relays.
The Load Bar instruction is a normally closed contact. It is sometimes also called Load Not or examine if closed. (XIC) (as in examine the input to see if itâ„¢s physically closed) the symbol for a load bar instruction is shown below.
A Load Not (normally closed contact) symbol
This is used when an input signal does not need to be present for the symbol to turn on. When the physical input is off we can say that the instruction is true. We examine the input for an off signal. If the input is physically off then the symbol is on. An off condition is also referred to as a logic 0 state.
This symbol normally can be used for internal inputs, external inputs and sometimes, external output contacts. Remember again that internal relays don't physically exist. They are simulated (software) relays. It is the exact opposite of the Load instruction.
Logic State Load Load Bar
0 False True
1 True False
The Out instruction is sometimes also called an Output Energize instruction. The output instruction is like a relay coil. Its symbol looks as shown below.
An OUT (coil) symbol
When there is a path of True instructions preceding this on the ladder rung, it will also be true. When the instruction is true it is physically on. We can think of this instruction as a normally open output. This instruction can be used for internal coils and external outputs.
The Out bar instruction is sometimes also called an Out Not instruction. Some vendors don't have this instruction. The out bar instruction is like a normally closed relay coil. Its symbol looks like that shown below.
An Out Bar (normally closed coil) symbol
When there is a path of false instructions preceding this on the ladder rung, it will be true. When the instruction is true it is physically on. We can think of this instruction as a normally closed output. This instruction can be used for internal coils and external outputs. It is the exact opposite of the Out instruction.
Logic State Out Out Bar
0 False True
1 True False
6. ADVANTAGES OF PLC
Â¢ Reduced space.
Â¢ Energy saving.
Â¢ Ease of maintenance.
Â¢ Greater life and reliability.
Â¢ Tremendous flexibility.
Â¢ Shorter project time.
Â¢ Easier storage, archiving and documentation.
Â¢ PLCs are armored for severe conditions (such as dust, moisture, heat, cold) and have the facility for extensive input/output (I/O) arrangements.
Â¢ PLCs read limit switches, analog process variables (such as temperature and pressure), and the positions of complex positioning systems.
Â¢ PLCs are used in many "real world" applications. If there is industry present, chances are good that there is a plc present. If you are involved in machining, packaging, material handling, automated assembly or countless other industries you are probably already using them. If you are not, you are wasting money and time. Almost any application that needs some type of electrical control has a need for a plc.
7. APPLICATIONS OF PLC SYSTEM
Â¢ In industry, there are many production tasks, which are of highly repetitive nature. Although repetitive & monotonous, each stage needs careful attention of operator to ensure good quality of final product.
Â¢ Many times, a close supervision of the processes cause high fatigue on operator resulting in loss of track of process control.
Â¢ Sometimes itâ„¢s hazardous also as in the case of potentially explosive chemical processes.
Â¢ Under all such conditions we can use PLCs effectively in totally eliminating the possibilities of human error.
Â¢ Some capabilities of PLCs are as follows:
1. Logic control
2. PID control
3. Coordination & automation
4. Operator control
5. Signaling and listing etc.
Â¢ In short, wherever sequential logic control & automation is desired the PLCs are the best suited to meet the task. It includes simple interlocking functions to complicated analog signal processing to PID control action in closed loop control etc.
Â¢ Few examples of industries where PLCs are used for control & automation purpose are listed below: -
1. Tyre industry.
2. Blender reclaimer.
3. Bulk material handling system at ports.
4. Ship unloader.
5. Wagon loaders.
6. Steel plants.
7. Blast furnace charging.
8. Brick-moulding press in refectories.
9. Galvanizing plant.
10. Dairy automation.
11. Pulp factory.
12. Printing industry etc.
Â¢ Today the PLCs are used for control and automation job in a single machine and it increases up to full automation of manufacturing or testing process in a factory.
PLC is used for two tasks in robotics:
1) As the controller or unprogrammble part of robot.
2) As an overall system controller.
In flexible manufacturing system:
The logical development from linking machines in this manner is to group programmable machines into flexible manufacturing cells, each capable of machining a variety of products under fully automatic control.
In factory automation:
The Austin-rover assembling plant.
The plant produces multi style car body from individual body panels .The process consists of following activities:
1. Make up of sub assemblies from panels. Eg: doors, under frames
2. Tag sub assemblies together.
3. Pass tagged bodies to a main jig for automatic alignment & framing.
4. Conduct material transfer in which sub assemblies are selected, transported & distributed to workstation by conveyor system
5. Maintain quality control by automatic monitoring & manual inspection of each process.
The PLC tracks each component as it moves through the production area, communicating this information to each appropriate robot as necessary. Data send between PLC & robot includes handshaking signals to indicate robot busy parked, action complete etc. Data in binary coded decimal form is used to send component information & weld sequences from the PLC to root, which must acknowledge receipt of the correct data before the PLC will allow it to commence operation.
Overall planning & control: control
Plant mainframe computer main database
2 production & scheduling data
Centers minicomputer or powerful computer
3 coordination of multiple stations
Cells large PLC or computer
4 controlling plant & machinery
Stations PLC or computer
5 machinery & processes
Machinery I/P &O/P interfacing
Pyramid control hierarchy in an automated factory
8. PLC VS COMPUTER
Â¢ Designed for extreme industrial environments.
Â¢ Can operation in high temperature and humidity. Er
Â¢ High immunity to noise.
Â¢ Integrated command interpreter (proprietary).
Â¢ No secondary memory available (in the PLC).
Â¢ Optimized for Single task.Optimized
Â¢ Designed mainly for data processing and calculation.
Â¢ Optimized for speed.
Â¢ Canâ„¢t operate in extreme environments.
Â¢ Can be programmed in different languages.
Â¢ Lost of secondary memory available.
Â¢ Multitasking capability.
9. PLC DISADVANTAGES
Â¢ In contrast to microcontroller systems that have what is called an open
architecture, most PLCs manufacturers offer only closed architectures
for their products.
Â¢ PLC devices are proprietary, which means that parts and software from
one manufacturer can t easily be used in combination with parts of
another manufacturer, which limits the design and cost options.
PLC is a device that is capable of being programmed to perform a controlling function. The PLC was designed to provide flexibility in control based programming and executing logic instruction. PLC allowed for shorter installation time and faster commissioning through programming rather than wiring.
The PLC have in recent years experienced an unprecedented growth as universal element in industrial automation .It can be effectively used in applications ranging from simple control like replacing a small number of relays to complex automation problems.
Today the PLCs are used for control & automation job in a single machine & it increases up to full automation of manufacturing / testing process in a factory.