INDUSTRIAL AUTOMATION

ABSTRACT: This is one of a series of white papers on systems modelling, analysis and control, prepared by Control Systems Principles.co.uk to give insights into important principles and processes in control. In control systems there are a number of generic systems and methods which are encountered in all areas of industry and technology. These white papers aim to explain these important systems and methods in straightforward terms. The white papers describe what makes a particular type of system/method important, how it works and then demonstrates how to control it. The control demonstrations are performed sing models of real systems designed by our founder - Peter Wellstead, and developed for manufacture by TQ Education and Training Ltd in their CE range of equipment. Where possible, results from the real system are shown. This white paper is about the 'work horse' of industrial automation - the programmable logic controller. What is Programmable Logic Control? Programmable Logic Control, or PLC as it is universally called, is the 'work horse' of industrial automation. My boss also calls it the 'Cinderella of Control', because it is not well treated in colleges and universities, but when control engineers use PLCs in industry they wonder how they lived without them, because they are so important and so wide spread. We think that there are a lot of three term controllers in the world, but there are many times more PLCs! Programmable Logic Control is important because all production processes go through a fixed repetitive sequence of operations that involve logical steps and decisions. A PLC is used to control, time and regulate the sequence. Examples of production processes that are controlled using PLCs are - metal machining sequences, product assembly lines and batch chemical processes. Logical control has been used to control sequences of actions in automatic manufacturing systems for many years. Originally, a logical control system was 'hard wired' using electronic relays, timers and logical units. These systems were inflexible. Once a 'hard wired' logic system had been constructed, then if the machining schedule was altered for another type of product, the logic control system had to be manually rewired for the new application. This was inflexible and time consuming, it restricted the production scheduling of a factory and made changing of products difficult. This situation began to change in the 1970's when mini-computers, became available. It is easy to perform switching, time delays and logic in computer programmes. A computer programme is also easy to change when the control problem changes and allows many more logical functions than are possible with hardwired logic control. When reliable microprocessors became available in the 1980's, with cheap memory and flexible input-output features, then the modern generation of PLCs began to emerge. Today the microcomputer based PLC is a robust, reliable instrument with many functions and features. Even cheap PLC's are able to control a medium scale automatic machining station or chemical batch reactor. Large PLC systems are capable of running an entire factory automation system. PLC's have the basic structure, shown in 1.

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a logic sequence has been programmed and downloaded to the Programme Memory unit, then the programme can be started, paused, single stepped and stopped from the PLC unit itself. The normal structure of a PLC sequence is a repetitive chain of elementary actions that are determined by the status of the input devices from the industrial process. The following paragraphs describe a typical PLC sequence with reference to a chemical engineering automation process. All industrial production processes follow a fixed sequence of actions that are determined by the identified steps in the production process and the known reactions that must be made during the process. Industrial production processes can be the manufacture of mechanical or electrical products in a machining and assembly line, processing of raw materials, the formation of chemical or pharmaceutical products in a chemical process, plus many more. In this section I will describe a simple liquid processing sequence that is typical of many liquid processes sequences that are found in the process industries, [1]. To keep the illustration simple, a two-tank batch process from the chemical industry (as shown in 2) is used. The process consists of two tanks, one mounted above the other. There is a pump that can be switched on or off, to pump liquid to the upper tank (Tank 1). Tank 1 has a solenoid operated valve in the base. This valve (Valve 3) is normally closed but it can be switched open so as to release fluid from Tank 1 into the lower tank (Tank 2). Tank 2 has a solenoid-operated valve (Valve 4) in its base so that Tank 2 can be emptied into a reservoir. The pump and valve inputs to the process are binary

the PLC has four main units: 1. The Programme Memory -the instructions for the logical control sequence are stored here 2. The Data Memory - the status of switches, interlocks, past values of data and other working data is stored here 3. The output devices - these are hardware/software drivers for the industrial process actuators, such as solenoid switches, motors and valves 4. The input devices - these are hardware/software drivers for the industrial process sensors such as switch status sensors, proximity detectors, interlock settings and so on. In addition to this the PLC has a programming unit. This is either a special module, a PC connected to the PLC by the serial link, or both. The programming unit is used to build, test and edit the logical sequence that the PLC will execute. At a basic level PLCs are programmed in a simple form of assembly code. Each manufacturer has their own standards and definitions for these codes. There are other programming languages, including the IEC 61131-3 standard, Sequential Function Chart, Function Block Diagrams.However, a long established standard programming language for programmable logic, called 'Ladder Logic', is universally understood by PLC programmers. In recent years PLC's have become more sophisticated. As a result it is sometimes difficult to put all their features into the ladder logic framework, and so the alternative programming languages are becoming more widely used. At this time however, ladder logic is the standard way to describe a PLC programme. Engineering Motivation - A typical batch process

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input signals that actuate the process by switching the pump on or off, or opening or closing the valves. The sensors are also binary; they tell the PLC whether a switch is open or closed. The exception is a flow meter, which gives a pulsed signal, which can be processed to find pump flow rates. The pulsed input for the flow meter is an example of how the scope of PLCs is expanding beyond pure logical signals. All the inputs and outputs for the process in 1 are listed in the table below: The variables for the PLC system A simple batch sequence for this system might be as follows: 1. Open valve V2, close V3, and set pump P1 on (this starts filling Tank 1) 2. Wait until L4 is set and set pump P1 off (this stops filling Tank 1 when it is full) 3. Wait for 10 seconds (this could be to allow fluid mixing, reactions or settling to take place) 4. Open valve V3, close V4 (this starts emptying the contents of Tank 1 into Tank 2). 5. Wait until L3 is set and close V3 (this waits until all of Tank 1 is emptied into Tank 2) 6. Go to step 1 to repeat the sequence indefinitely More complex fluid processing sequences can be set up with the process system in 1, but I think that the key elements of a batch sequencing system are shown here. In the next section I will explain the elements of a PLC ladder logic programme. Basic Ladder Logic Components Contacts Contacts are the input devices in a ladder diagram. They are opened and closed by either an externally applied signal (usually represented by X), internal timer( labelled T), counter (labelled C), or internal logical flag (M and S). As shown in the table contacts are available in two forms normally open and normally closed. Coils Coils are the output devices in a ladder diagram. They are used to operate external devices, and internal timers, counters and flags. Some manufacturers include coils that allow special operations to be performed which extend the capability of the PLC beyond that which can be obtained with the simple coils and contacts. The following instructions are a selection of the extra features that can be operated using the special coil feature from one manufacturer (Mitsubusi). Master Control (MC) and Master Reset Elements of Ladder Logic The basic components in a ladder logic programme are the contact and the coil. The contact is the name given to a general input device - in can set by an external switch, an internally set logic or timer function. The coil is the name given to a general output device - and is used to drive motors, solenoids and other process actuators. These two basic devices are shown in 3. section I will show some actually examples of programmes written for the PLC process

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(MCR) A master control block is a set of ladder programmes that are executed when an input device is activated. The end of a master control block is marked by the master reset instruction. It is the ladder logic equivalent of a subroutine or procedure in a conventional programming language. Set (SET) and Reset (RST) Devices These instructions are used to permanently set or reset an output device when a designated input is set. The output device holds the given value even if the input state is changed. Reset (RST) Timer or Counter This instruction resets a designated counter or timer. Any output device set by the timer will also be reset. Pulse Rising Edge (PLS) and Pulse Falling Edge (PFE) This instruction sets a designated output device for one cycle when either a rising (PLS) or falling edge (PFE) is detected from an input device. Using the basic elements described here it is possible to build very complex logical sequences. In the next PLC Systems. There are many, many companies offering PLC systems. The choice is large and confusing, and so it is best restrict the selection by first defining the features that are required in a particular application, (e.g. number of inputs, number of outputs, etc). This will reduce the number of options and there are several web sites that will help refine the choice beyond this. To motivate this, consider the application of controlling the PLC process described in Section 2. The PLC process has 6 outputs (level, flow rate and detectors) and 5 inputs (valve solenoids and motor). This gives the minimum I/O requirement of the PLC system needed to drive it. In addition for an application the system should be able to achieve a certain scan time, allow sufficient instructions for the most complex programme planned, and have a large enough programme and data memory. There will also be system requirements for input and output drive option, programming tools and communications protocols. In a training and prototyping situation, we have found the CE123 PLC Trainer shown in 4 to be excellent. It incorporates a 14 I/O channel industrial PLC system. The important features of the CE132 system is the packaging with buffered input and output connectors, manual input output switches and status indicators. The CE123 PLC System As I have remarked in Section 1, PLCs have a programming module that allows the user to enter the instructions that make up the logic sequence to be followed. There are a number of languages possible, but the most widely understood is the ladder logic scheme. Ladder logic instructions are entered into a ladder diagram that describes the entire logical sequence of a particular process. As an example, the 5 shows a screen shot of a simple ladder diagram to be executed in the CE123 PLC system for the control of the PLC process. The aim of this logic controller circuit is to fill Tank 1 and hold the level in the tank at the level of the 'Tank 1 HIGH level switch' (see Table 1). The programme is started by setting the input X6 to '1' (this

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2 when the pump is high (or the reservoir level is too low) the Tank 2 valve is opened. (This is todrain water back to the reservoir and is not part of the control cycle.) Rung 3 when the Tank 1 high level indicator is low and the input X6 is high then the Tank 1 feed is valve is opened. would be done with one of the manual switches on the CE123. The programme operates a level control cycle whereby Valves V2 and V3 are opened and closed, according to the state of the L4 sensor in Tank 1. The objective is to maintain the tank level in Tank 1 around the high mark. Going through the ladder diagram rung by rung: Rung 1 starts the pump when X6 is high and if the reservoir level is high, it also opens the by-pass valve if the feed to Tank 1 is closed. Rung 4 when the Tank 1 high level indicator is high the drain valve to Tank 1 is opened
response to input conditions within a bounded time, otherwise unintended operation will result.

The PLC was invented in response to the needs of the American automotive manufacturing industry. Programmable logic controllers were initially adopted by the automotive industry where software revision replaced the re-wiring of hard-wired control panels when production models changed. Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was accomplished using hundreds or thousands of relays, cam timers, and drum sequencers and dedicated closedloop controllers. The process for updating such facilities for the yearly model changeover was very time consuming and expensive, as electricians needed to individually rewire each and every relay. Digital computers, being general-purpose programmable devices, were soon applied to control of industrial processes. Early computers required specialist programmers, and stringent operating environmental control for temperature, cleanliness, and power quality. Using a general-purpose computer for process control required protecting the computer from the plant floor conditions. An industrial control computer would have several attributes: it would tolerate the shop-floor environment, it would support discrete (bit-form) input and output in an easily extensible manner, it would not require years of training to use, and it would permit its operation to be monitored. The response time of any computer system must be fast enough to be useful for control; the required speed varying according to the nature of the process. [1] In 1968 GM Hydramatic (the automatic transmission division of General Motors) issued a request for proposal for an

A programmable logic controller (PLC) or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory. A PLC is an example of a hard real time system since output results must be produced in

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electronic replacement for hard-wired relay systems. The winning proposal came from Bedford Associates of Bedford, Massachusetts. The first PLC, designated the 084 because it was Bedford Associates' eighty-fourth project, was the result. [2] Bedford Associates started a new company dedicated to developing, manufacturing, selling, and servicing this new product: Modicon, which stood for MOdular DIgital CONtroller. One of the people who worked on that project was Dick Morley, who is considered to be the "father" of the PLC.[3] The Modicon brand was sold in 1977 to Gould Electronics, and later acquired by German Company AEG and then by French Schneider Electric, the current owner. One of the very first 084 models built is now on display at Modicon's headquarters in North Andover, Massachusetts. It was presented to Modicon by GM, when the unit was retired after nearly twenty years of uninterrupted service. Modicon used the 84 moniker at the end of its product range until the 984 made its appearance. The automotive industry is still one of the largest users of PLCs. Logic, a very high-level programming language designed to program PLCs based on state transition diagrams. Many early PLCs did not have accompanying programming terminals that were capable of graphical representation of the logic, and so the logic was instead represented as a series of logic expressions in some version of Boolean format, similar to Boolean algebra. As programming terminals evolved, it became more common for ladder logic to be used, for the aforementioned reasons and because it was a familiar format used for electromechanical control panels. Newer formats such as State Logic and Function Block (which is similar to the way logic is depicted when using digital integrated logic circuits) exist, but they are still not as popular as ladder logic. A primary reason for this is that PLCs solve the logic in a predictable and repeating sequence, and ladder logic allows the programmer (the person writing the logic) to see any issues with the timing of the logic sequence more easily than would be possible in other formats.

Programming
Early PLCs, up to the mid-1980s, were programmed using proprietary programming panels or special-purpose programming terminals, which often had dedicated function keys representing the various logical elements of PLC programs. [2] Programs were stored on cassette tape cartridges. Facilities for printing and documentation were very minimal due to lack of memory capacity. The very oldest PLCs used non-volatile magnetic core memory. More recently, PLCs are programmed using application software on personal computers. The computer is connected to the PLC

Development
Early PLCs were designed to replace relay logic systems. These PLCs were programmed in "ladder logic", which strongly resembles a schematic diagram of relay logic. This program notation was chosen to reduce training demands for the existing technicians. Other early PLCs used a form of instruction list programming, based on a stack-based logic solver. Modern PLCs can be programmed in a variety of ways, from ladder logic to more traditional programming languages such as BASIC and C. Another method is State

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through Ethernet, RS-232, RS-485 or RS422 cabling. The programming software allows entry and editing of the ladder-style logic. Generally the software provides functions for debugging and troubleshooting the PLC software, for example, by highlighting portions of the logic to show current status during operation or via simulation. The software will upload and download the PLC program, for backup and restoration purposes. In some models of programmable controller, the program is transferred from a personal computer to the PLC though a programming board which writes the program into a removable chip such as an EEPROM or EPROM. or input status with the extreme consistency in timing as is expected from PLCs. Still, such desktop logic applications find use in less critical situations, such as laboratory automation and use in small facilities where the application is less demanding and critical, because they are generally much less expensive than PLCs. In more recent years, small products called PLRs (programmable logic relays), and also by similar names, have become more common and accepted. These are very much like PLCs, and are used in light industry where only a few points of I/O (i.e. a few signals coming in from the real world and a few going out) are involved, and low cost is desired. These small devices are typically made in a common physical size and shape by several manufacturers, and branded by the makers of larger PLCs to fill out their low end product range. Popular names include PICO Controller, NANO PLC, and other names implying very small controllers. Most of these have between 8 and 12 digital inputs, 4 and 8 digital outputs, and up to 2 analog inputs. Size is usually about 4" wide, 3" high, and 3" deep. Most such devices include a tiny postage stamp sized LCD screen for viewing simplified ladder logic (only a very small portion of the program being visible at a given time) and status of I/O points, and typically these screens are accompanied by a 4-way rocker push-button plus four more separate push-buttons, similar to the key buttons on a VCR remote control, and used to navigate and edit the logic. Most have a small plug for connecting via RS-232 or RS-485 to a personal computer so that programmers can use simple Windows applications for programming instead of being forced to use the tiny LCD and push-button set for this purpose. Unlike regular PLCs that are usually modular and greatly expandable, the PLRs are usually not modular or

Functionality
The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems and networking. The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. PLC-like programming combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications. Regarding the practicality of these desktop computer based logic controllers, it is important to note that they have not been generally accepted in heavy industry because the desktop computers run on less stable operating systems than do PLCs, and because the desktop computer hardware is typically not designed to the same levels of tolerance to temperature, humidity, vibration, and longevity as the processors used in PLCs. In addition to the hardware limitations of desktop based logic, operating systems such as Windows do not lend themselves to deterministic logic execution, with the result that the logic may not always respond to changes in logic state

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expandable, but their price can be two orders of magnitude less than a PLC and they still offer robust design and deterministic execution of the logic. hydraulic cylinders, magnetic relays, solenoids, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.

[edit] PLC topics

Scan time
A PLC program is generally executed repeatedly as long as the controlled system is running. The status of physical input points is copied to an area of memory accessible to the processor, sometimes called the "I/O Image Table". The program is then run from its first instruction rung down to the last rung. It takes some time for the processor of the PLC to evaluate all the ladders and update the I/O image table with the status of outputs. This scan time may be a few milliseconds for a small program or on a fast processor, but older PLCs running very large programs could take much longer (say, up to 100 ms) to execute the program. If the scan time was too long, the response of the PLC to process conditions would be too slow to be useful. As PLCs became more advanced, methods were developed to change the sequence of ladder execution, and subroutines were implemented. This simplified programming and could also be used to save scan time for high-speed processes; for example, parts of the program used only for setting up the machine could be segregated from those parts required to operate at higher speed. Special-purpose I/O modules, such as timer modules or counter modules, could be used where the scan time of the processor was too long to reliably pick up, for example, counting pulses from a shaft encoder. The relatively slow PLC could still interpret the counted values to control a machine, but the accumulation of pulses was done by a

Control panel with PLC (grey elements in the center). The unit consists of separate elements, from left to right; power supply, controller, relay units for in- and output

The main difference from other computers is that PLCs are armored for severe conditions (such as dust, moisture, heat, cold) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, analog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some use machine vision. On the actuator side, PLCs operate electric motors, pneumatic or

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dedicated module that was unaffected by the speed of the program execution. 485 or Ethernet. Modbus, BACnet or DF1 is usually included as one of the communications protocols. Other options include various fieldbuses such as DeviceNet or Profibus. Other communications protocols that may be used are listed in the List of automation protocols. Most modern PLCs can communicate over a network to some other system, such as a computer running a SCADA (Supervisory Control And Data Acquisition) system or web browser. 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 coordinate over the communication link. These communication links are also often used for HMI devices such as keypads or PC-type workstations.

System scale
A small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model has insufficient I/O. Modular PLCs have a chassis (also called a rack) into which are placed modules with different functions. The processor and selection of I/O modules is customised for the particular application. Several racks can be administered by a single processor, and may have thousands of inputs and outputs. A special high speed serial I/O link is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants.

User interface
See also: User interface and List of humancomputer interaction topics

Programming
PLC programs are typically written in a special application on a personal computer, then downloaded by a direct-connection cable or over a network to the PLC. The program is stored in the PLC either in battery-backed-up RAM or some other nonvolatile flash memory. Often, a single PLC can be programmed to replace thousands of relays.[4] Under the IEC 61131-3 standard, PLCs can be programmed using standards-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers. Initially most PLCs utilized Ladder Logic Diagram Programming, a model which emulated electromechanical control panel devices (such as the contact and coils of relays)

PLCs may need to interact with people for the purpose of configuration, alarm reporting or everyday control. A humanmachine interface (HMI) is employed for this purpose. HMIs are also referred to as man-machine interfaces (MMIs) and graphical user interface (GUIs). A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. More complex systems use programming and monitoring software installed on a computer, with the PLC connected via a communication interface.

Communications
PLCs have built in communications ports, usually 9-pin RS-232, but optionally EIA-

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which PLCs replaced. This model remains common today. IEC 61131-3 currently defines five programming languages for programmable control systems: function block diagram (FBD), ladder diagram (LD), structured text (ST; similar to the Pascal programming language), instruction list (IL; similar to assembly language) and sequential function chart (SFC). These techniques emphasize logical organization of operations.[4] While the fundamental concepts of PLC programming are common to all manufacturers, differences in I/O addressing, memory organization and instruction sets mean that PLC programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible. during its operational life. PLCs contain input and output devices compatible with industrial pilot devices and controls; little electrical design is required, and the design problem centers on expressing the desired sequence of operations. PLC applications are typically highly customized systems so the cost of a packaged PLC is low compared to the cost of a specific custom-built controller design. On the other hand, in the case of mass-produced goods, customized control systems are economic due to the lower cost of the components, which can be optimally chosen instead of a "generic" solution, and where the non-recurring engineering charges are spread over thousands or millions of units. For high volume or very simple fixed automation tasks, different techniques are used. For example, a consumer dishwasher would be controlled by an electromechanical cam timer costing only a few dollars in production quantities. A microcontroller-based design would be appropriate where hundreds or thousands of units will be produced and so the development cost (design of power supplies, input/output hardware and necessary testing and certification) can be spread over many sales, and where the end-user would not need to alter the control. Automotive applications are an example; millions of units are built each year, and very few endusers alter the programming of these controllers. However, some specialty vehicles such as transit busses economically use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomic.
[5]

PLC compared with other control systems

Allen-Bradley PLC installed in a control panel

PLCs are well-adapted to a range of automation tasks. These are typically industrial processes in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the automation, and where changes to the system would be expected

Very complex process control, such as used in the chemical industry, may require algorithms and performance beyond the

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capability of even high-performance PLCs. Very high-speed or precision controls may also require customized solutions; for example, aircraft flight controls. Singleboard computers using semi-customized or fully proprietary hardware may be chosen for very demanding control applications where the high development and maintenance cost can be supported. "Soft PLCs" running on desktop-type computers can interface with industrial I/O hardware while executing programs within a version of commercial operating systems adapted for process control needs. [5] Programmable controllers are widely used in motion control, positioning control and torque control. Some manufacturers produce motion control units to be integrated with PLC so that G-code (involving a CNC machine) can be used to instruct machine movements.[citation needed] PLCs may include logic for single-variable feedback analog control loop, a "proportional, integral, derivative" or "PID controller". A PID loop could be used to control the temperature of a manufacturing process, for example. Historically PLCs were usually configured with only a few analog control loops; where processes required hundreds or thousands of loops, a distributed control system (DCS) would instead be used. As PLCs have become more powerful, the boundary between DCS and PLC applications has become less distinct. PLCs have similar functionality as Remote Terminal Units. An RTU, however, usually does not support control algorithms or control loops. As hardware rapidly becomes more powerful and cheaper, RTUs, PLCs and DCSs are increasingly beginning to overlap in responsibilities, and many vendors sell RTUs with PLC-like features and vice versa. The industry has standardized on the IEC 61131-3 functional block language for creating programs to run on RTUs and PLCs, although nearly all vendors also offer proprietary alternatives and associated development environments. In recent years "Safety" PLCs have started to become popular, either as standalone models (Pilz PNOZ Multi, Sick etc.) or as functionality and safety-rated hardware added to existing controller architectures (Allen Bradley Guardlogix, Siemens Fseries etc.). These differ from conventional PLC types as being suitable for use in safety-critical applications for which PLCs have traditionally been supplemented with hard-wired safety relays. For example, a Safety PLC might be used to control access to a robot cell with trapped-key access, or perhaps to manage the shutdown response to an emergency stop on a conveyor production line. Such PLCs typically have a restricted regular instruction set augmented with safety-specific instructions designed to interface with emergency stops, light screens and so forth. The flexibility that such systems offer has resulted in rapid growth of demand for these controllers.

By :

INDUSTRIAL AUTOMATION S.Satheesh, S.Ramkumar, Pre final year (EEE), M.A.M. College of Engineering and Technology. Trichy_621 105

Contact: Email.id: ram.sargunam@gmail.com, sssatheeshshanmugasundar am@gmail.com Mobile no: 9790111186; 9790123567;

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