By their very nature, SCADA systems can be very complex. Our experience has shown that there is nothing worse than a continuing change of software engineers modifying them, each with their own programming methods.That's where our strength lies.We involve no one else in programming your systems other than ourselves. From design through to commissioning, gradually honing it to a well structured programming philosophy.Apart from specific modifications that you may require, typical areas of improvement includeFaster communicationsFaster screen updatesData integrityAbsolute reliabilityUser friendly front ends with superior graphics.Our aim is to provide a friendly and professional service, working closely with you and your specific project. This could be an existing system requiring modification and support, or it could be a new project where we would work from your specification and be involved in the design. If you only have a rough idea what you want then get us involved and we will fill in the gaps.
Tel: 0845 838 7048Wolf Automation. Old Meldrum House, Ball Lane, CovenHeath, Staffordshire. WV10 7EY© 2011 All Rights Reserved.PLC ProgrammerRecent Wolf Automation NewsOur engineer has just returned froma successful installation of an OEE data collection system in Dubai.The touch screen PC based system collects information via sensors on afolding machine in a major printing comapny.
Data such as Good Copy, BadCopy, Setting / Good Production is collated by the system and transformedinto statistical data which is used to monitor the efficiency of the machineand operators. The system also integrates into the businesses management software allowingthe ordering of raw materials and the scheduling of jobs to be automated.Wolf Automation - Any Application - Any Where!Ourengineer has just completed the commissioning of the software for thePLC Control system wecreated, for the automatic fuel changeover system on two oil tanker vesselsin Amsterdam. New European legislation means that all ships have toburn Low Sulpher Deisel Oil rather than the standard Heavy Fuel Oil while inport. New tanks pipework and valves have to be added as well asmodifications to the boiler burners. In this case automation was added sothat the changeover can be made safely from the engine control room at thepush of a button. A Mitsubishi FX3u was implemented along with FX2n-2ADmodules to enable the reading of flow meters in the fuel lines.
The controlis operated and monitored from a Mitsubishi E1100 HMI. Feedback from valvesand motor starters as well as fuel flow can be easily monitored on thescreen. An alarm list has also been generated to integrate with the existingvessel management system. Wolf Automation - Any Application - Any Where!Wolf were called in tosetup a 'special effect' in the largest show venue in Athens Greece. The effect involved dropping two large show curtains and 'flying'them through the venue, over the heads of the audience for Nikos Vertis 2009show.
The unfinished control panel was already on site when we arrived, unfortunately there wasno software and the Mitsubishi inverters (FR-A520-5.5) required configuration. We programmed thePLC Control System comprising a Koyo DL06 PLC to orchestrate the effect sequence andcontrol the speed of the5.5kw winches, Chabuki (kabuki) release devices were used to drop the curtainsbefore the winches accelerated to full speed in half a second. The curtainsfly over the heads of the audience at a speed of 20ft/second. Obviouslythe timing of the drop and wind up is importan to avoid dropping the curtainsontothe audience. Using our industrial automation experience we were able to hit the groundrunning and even though there was only a couple of days available, the effect was ready foropening night.
Once again our engineer displayed the versatility of Wolf Automation - Any Application - Any Where!Wolf Automation replaced the controls and drive system on an existingtube cutting and forming machine. The machine takes long lengths of stainless steel tube and cuts andshapes them into various different styles of heater modules.
You may click something and it comes right up and 2 minutes later you click something else and it takes forever and then a minute later it's fast again. Fps counter mass effect.
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Wolf engineers realised that the problems with the machine were mostlygenerated by the control system and so the mechanical system was refurbishedsaving many thousands of pounds on the job. The existing stepper motors werereplaced with DC servo motors and gearheads which improved the reliabilityand repeatability of the system.
A motion control system and HMI were addedbringing the machine into the 21st century and easing operation. Due to the success of the the refurbishment, the same is now pending on 2 moremachines for the same customer, a multinational manufacturer of specialheaters and controls. Wolf Automation specialise in retro fitting controlsystems to existing machines improving quality, production, reliability andsaving thousands of pounds against buying a new machine.
Wolf Automation - Any Application - Any Where!Wolf were asked to commission a copper tube straightening machine in Revda SverdlovskayaOblast, Russia. When we arrived on site tosetup the Bronx machine, we found that it hadn't even been wired. We were able to source cable locally and connect themachine to the manufacturers drawings before commissioning the S7-300 basedPLC control system that utilised a WinCC front end, the positioning systemutilised profibus mounted absolute encoders. We stayed on site during a whole week of productiontrials. Wolf Automation - Any Application - Any Where!Wolf Automation have completed on a brand new control system for a Cold Roll Formingline in the Midlands of Great Britain.
The PLC control system utilises a Mitsubishi FX PLC, Indramat (Bosch Rexroth)servos, Control Techniques Mentor II 350Kw DC drive and CT commander SKinverters. The system controls and automatesall parts of the line, Decoiler, Rollfeed, Pierce Punching Press, 2 Accumulator pits,Rolling Mill, Flying Cutoff with High speed hydraulics, Runoff conveyer andProduct Ejector system to deliver the products to the operator. Wolf designed, built, programmed, installed and commissioned the line, ontime and on budget. We deliver High-spec,High-quality, value for money solutions. Wolf Automation - AnyApplication - Any Where!Wolfwere tasked with producing a servo control system to control two actuatorswhich divert steel section from a conveyor to a packing station. Wolf used the new Bosch Rexroth Indradrive with integrated PLC along witha VCP08 HMI. This simple point to point application gave us the chanceto acquaint ourselves with some of the latest technology form Bosch Rexroth.Once again Wolf designed, built, programmed, installed and commissionedthe system.
Wolf Automation - Any Application - Any Where!Wolf Automation visited Lloyds TSB in Bristol recently to fault findon their backup generators. The system has two generators aswell as a UPSsystem and the whole thing is monitored by 3 24v Mitsubishi F2-60MR PLC's.Wewere able to interrogate the software with the old MEDOC software F2-20 GF1and SC-03 interfaces to quickly find the cause of the problem on the system.Due to the importance of this system to the entire banking system we had tocarry out all works between 6pm and midnight on a Saturday evening. Wolf Automation - Any Application - Any Where!Wolfcompleted the installation of a high speed hydraulic punching unit fora blue chip manufacturer of strut products. The unit controls the impressionof the companies name into the product synchronising with the crank pressthat pierces the product.
The hydraulic system supplied by H&L Hydraulics (VoithTurbo) is able to apply 20 tonnes of stamping pressure and cycles at a rateof over 5hz. Wolf designed the control system to drive and monitor thehydraulic control card and hydraulic power pack, we also integrated thesystem to the existing press and Indramat roll feed system. Plans are inplace to duplicate the system later this year.
Wolf Automation -Any Application - Any Where! PLCControl SystemsIndustrial control systemsIndustrial control system (ICS) is a general term that encompassesseveral types of control systems, including supervisory control and dataacquisition (SCADA) systems, distributed control systems (DCS), and othersmaller control system configurations such as skid-mounted programmable logiccontrollers (PLC) often found in the industrial sectors and criticalinfrastructures. ICSs are typically used in industries such as electrical, water, oil and gas,data. Based on information received from remote stations, automated oroperator-driven supervisory commands can be pushed to remote station controldevices, which are often referred to as field devices. Field devices controllocal operations such as opening and closing valves and breakers, collectingdata from sensor systems, and monitoring the local environment for alarmconditions.A historical perspectiveIndustrial control system technology has evolved over the past three to fourdecades. DCS systems generally refer to the particular functional distributedcontrol system design that exist in industrial process plants (e.g., oil andgas, refining, chemical, pharmaceutical, some food and beverage, water andwastewater, pulp and paper, utility power, mining, metals).
The DCS concept cameabout from a need to gather data and control the systems on a large campus inreal time on high-bandwidth, low-latency data networks. It is common for loopcontrols to extend all the way to the top level controllers in a DCS, aseverything works in real time. These systems evolved from a need to extendpneumatic control systems beyond just a small cell area of a refinery. The PLC (programmable logic controller) evolved out of a need to replaceracks of relays in ladder form. The latter were not particularly reliable, weredifficult to rewire, and were difficult to diagnose. PLC control tends to beused in very regular, high-speed binary controls, such as controlling ahigh-speed printing press. Originally, PLC equipment did not have remote I/Oracks, and many couldn't even perform more than rudimentary analog controls.
SCADA's history is rooted in distribution applications, such as power,natural gas, and water pipelines, where there is a need to gather remote datathrough potentially unreliable or intermittent low-bandwidth/high-latency links.SCADA systems use open-loop control with sites that are widely separatedgeographically. A SCADA system uses RTUs (remote terminal units, also referredto as remote telemetry units) to send supervisory data back to a control center.Most RTU systems always did have some limited capacity to handle local controlswhile the master station is not available. However, over the years RTU systemshave grown more and more capable of handling local controls. The boundaries between these system definitions are blurring as time goes on.The technical limits that drove the designs of these various systems are nolonger as much of an issue.
Many PLC platforms can now perform quite well as asmall DCS, using remote I/O and analog control loops, and are able tocommunicate supervisory data. It is not uncommon to have telecommunicationsinfrastructure that is so responsive and reliable that some SCADA systemsactually manage closed loop control over long distances. With the increasingspeed of today's processors, many DCS products have a full line of PLC-likesubsystems that weren't offered when they were initially developed. This has led to the concept of a PAC (programmable automation controller orprocess automation controller). It is an amalgamation of these three concepts.Time and the market will determine whether this can simplify some of theterminology and confusion that surrounds these concepts today.DCSsDCSs are used to control industrial processes such as electric powergeneration, oil and gas refineries, water and wastewater treatment, andchemical, food, and automotive production.
DCSs are integrated as a controlarchitecture containing a supervisory level of control overseeing multiple,integrated sub-systems that are responsible for controlling the details of alocalized process. Product and process control are usually achieved by deploying feed back orfeed forward control loops whereby key product and/or process conditions areautomatically maintained around a desired set point. To accomplish the desiredproduct and/or process tolerance around a specified set point, specificprogrammable controllers are used ONLY.PLCsPLCs provide boolean logic operations, timers, and (in some models)continuous control. The proportional, integral, and/or differential gains of thePLC continuous control feature may be tuned to provide the desired tolerance aswell as the rate of self-correction during process upsets.
DCSs are usedextensively in process-based industries. PLCs are computer-based solid-statedevices that control industrial equipment and processes. While PLCs can controlsystem components used throughout SCADA and DCS systems, they are often theprimary components in smaller control system configurations used to provideregulatory control of discrete processes such as automobile assembly lines andpower plant soot blower controls. PLCs are used extensively in almost allindustrial processes.DevelopmentEarly PLCs were designed to replace relay logic systems. These PLCs wereprogrammed in 'ladder logic', which strongly resembles a schematic diagram ofrelay logic.
This program notation was chosen to reduce training demands for theexisting technicians. Other early PLCs used a form of instruction listprogramming, based on a stack-based logic solver. Modern PLCs can be programmed in a variety of ways, from ladder logic to moretraditional programming languages such as BASIC and C.
Another method is StateLogic, a very high-level programming language designed to program PLCs based onstate transition diagrams. ProgrammingEarly PLCs, up to the mid-1980s, were programmed using proprietaryprogramming panels or special-purpose programming terminals, which often haddedicated function keys representing the various logical elements of PLCprograms. Programs were stored on cassette tape cartridges. Facilities forprinting and documentation were very minimal due to lack of memory capacity. Thevery oldest PLCs used non-volatile magnetic core memory.FunctionalityThe functionality of the PLC has evolved over the years to include sequentialrelay control, motion control, process control, distributed control systems andnetworking. The data handling, storage, processing power and communicationcapabilities of some modern PLCs are approximately equivalent to desktopcomputers.
PLC-like programming combined with remote I/O hardware, allow ageneral-purpose desktop computer to overlap some PLCs in certain applications. The main difference from other computers is that PLCs are armored for severeconditions (such as dust, moisture, heat, cold) and have the facility forextensive input/output (I/O) arrangements. These connect the PLC to sensors andactuators. PLCs read limit switches, analog process variables (such astemperature and pressure), and the positions of complex positioning systems.Some use machine vision.
On the actuator side, PLCs operate electric motors,pneumatic or hydraulic cylinders, magnetic relays, solenoids, or analog outputs.The input/output arrangements may be built into a simple PLC, or the PLC mayhave external I/O modules attached to a computer network that plugs into thePLC. SystemscaleA small PLC will have a fixed number of connections built in for inputs andoutputs. Typically, expansions are available if the base model has insufficientI/O. Modular PLCs have a chassis (also called a rack) into which are placedmodules with different functions.
The processor and selection of I/O modules iscustomised for the particular application. Several racks can be administered bya single processor, and may have thousands of inputs and outputs. A special highspeed serial I/O link is used so that racks can be distributed away from theprocessor, reducing the wiring costs for large plants. UserinterfacePLCs may need to interact with people for the purpose of configuration, alarmreporting or everyday control. AHuman-Machine Interface (HMI) is employed for this purpose. HMIs are alsoreferred to as MMIs (Man Machine Interface) and GUI (Graphical User Interface). A simple system may use buttons and lights to interact with the user.
Textdisplays are available as well as graphical touch screens. More complex systemsuse a programming and monitoring software installed on a computer, with the PLCconnected via a communication interface. PLC programs are typically written in a special application on a personalcomputer, then downloaded by a direct-connection cable or over a network to thePLC.
The program is stored in the PLC either in battery-backed-upRAMor some other non-volatileflashmemory. Often, a single PLC can be programmed to replace thousands ofrelays. Under theIEC61131-3 standard, PLCs can be programmed using standards-based programminglanguages. A graphical programming notation calledSequential Function Charts is available on certain programmable controllers.Initially most PLC's utilized Ladder Logic Diagram Programming, a model whichemulated electromechanical control panel devices (such as the contact and coilsof relays) which PLC's replaced.
This model remains common today. IEC 61131-3 currently defines five programming languages for programmablecontrol systems: FBD (Functionblock diagram), LD (Ladderdiagram), ST (Structuredtext, similar to thePascal programming language), IL (Instructionlist, similar toassembly language) and SFC (Sequentialfunction chart). These techniques emphasize logical organization ofoperations. While the fundamental concepts of PLC programming are common to allmanufacturers, differences in I/O addressing, memory organization andinstruction sets mean that PLC programs are never perfectly interchangeablebetween different makers. Even within the same product line of a singlemanufacturer, different models may not be directly compatible.PLCcompared with other control systemsPLCs are well-adapted to a range of automationtasks. These are typically industrial processes in manufacturing where the costof developing and maintaining the automation system is high relative to thetotal cost of the automation, and where changes to the system would be expectedduring its operational life.
PLCs contain input and output devices compatiblewith industrial pilot devices and controls; little electrical design isrequired, and the design problem centers on expressing the desired sequence ofoperations. PLC applications are typically highly customized systems so the costof a packaged PLC is low compared to the cost of a specific custom-builtcontroller 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 thenon-recurring engineering charges are spread over thousands or millions ofunits. For high volume or very simple fixed automation tasks, different techniquesare used. For example, a consumerdishwasherwould be controlled by an electromechanicalcam timercosting only a few dollars in production quantities.
Amicrocontroller-based design would be appropriate where hundreds orthousands of units will be produced and so the development cost (design of powersupplies, input/output hardware and necessary testing and certification) can bespread over many sales, and where the end-user would not need to alter thecontrol. Automotive applications are an example; millions of units are builteach year, and very few end-users alter the programming of these controllers.However, some specialty vehicles such as transit busses economically use PLCsinstead of custom-designed controls, because the volumes are low and thedevelopment cost would be uneconomic. Very complex process control, such as used in the chemical industry, mayrequire algorithms and performance beyond the capability of evenhigh-performance PLCs. Very high-speed or precision controls may also requirecustomized solutions; for example, aircraft flight controls. Programmable controllers are widely used in motion control, positioningcontrol and torque control.
Some manufacturers produce motion control units tobe integrated with PLC so thatG-code(involving aCNCmachine) can be used to instruct machine movements. PLCs may include logic for single-variable feedback analog control loop, a'proportional, integral, derivative' or 'PIDcontroller.' A PID loop could be used to control the temperature of amanufacturing process, for example. Historically PLCs were usually configuredwith only a few analog control loops; where processes required hundreds orthousands of loops, adistributed control system (DCS) would instead be used.
As PLCs have becomemore powerful, the boundary between DCS and PLC applications has become lessdistinct. PLCs have similar functionality asRemote Terminal Units. An RTU, however, usually does not support controlalgorithms or control loops.
As hardware rapidly becomes more powerful andcheaper,RTUs, PLCs andDCSs are increasingly beginning to overlap in responsibilities, and manyvendors sell RTUs with PLC-like features and vice versa. The industry hasstandardized on the IEC 61131-3 functional block language for creating programsto run on RTUs and PLCs, although nearly all vendors also offer proprietaryalternatives and associated development environments.Digital and analogsignalsDigital or discrete signals behave as binary switches, yielding simply an Onor Off signal (1 or 0, True or False, respectively). Push buttons, limitswitches, andphotoelectric sensors are examples of devices providing a discrete signal.Discrete signals are sent using eithervoltage orcurrent, where a specific range is designated as On and another asOff. For example, a PLC might use 24 V DC I/O, with values above 22 V DCrepresenting On, values below 2VDC representing Off, andintermediate values undefined. Initially, PLCs had only discrete I/O. Analog signals are like volume controls, with a range of values between zeroand full-scale.
These are typically interpreted as integer values (counts) bythe PLC, with various ranges of accuracy depending on the device and the numberof bits available to store the data. As PLCs typically use 16-bit signed binaryprocessors, the integer values are limited between -32,768 and +32,767.Pressure, temperature, flow, and weight are often represented by analog signals.Analog signals can use voltage orcurrent with a magnitude proportional to the value of the process signal.For example, an analog4-20 mA or 0 - 10 V input would beconverted into an integer value of 0 - 32767.Current inputs are less sensitive to electrical noise (i.e. From welders orelectric motor starts) than voltage inputs. ExampleAs an example, say a facility needs to store water in a tank. The water isdrawn from the tank by another system, as needed, and our example system mustmanage the water level in the tank. Using only digital signals, the PLC has two digital inputs fromfloatswitches (Low Level and High Level).
When the water level is above theswitch it closes a contact and passes a signal to an input. The PLC uses adigital output to open and close the inletvalve into thetank. When the water level drops enough so that the Low Level float switch is off(down), the PLC will open the valve to let more water in. Once the water levelrises enough so that the High Level switch is on (up), the PLC will shut theinlet to stop the water from overflowing. This rung is an example of seal inlogic. The output is sealed in until some condition breaks the circuit. An analog system might use a waterpressure sensor or aload cell,and an adjustable (throttling) dripping out of the tank, the valve adjusts toslowly drip water back into the tank.
In this system, to avoid 'flutter' adjustments that can wear out the valve,many PLCs incorporate 'hysteresis'which essentially creates a 'deadband'of activity. A technician adjusts this deadband so the valve moves only for asignificant change in rate. This will in turn minimize the motion of the valve,and reduce its wear. A real system might combine both approaches, using float switches and simplevalves to prevent spills, and a rate sensor and rate valve to optimize refillrates and preventwaterhammer. Backup and maintenance methods can make a real system verycomplicated.About PLC ProgrammerI began working with PLC's ata major Japanese manufacturer of office equipment in based in Shropshire.There were many PLC's installed in various production lines, assemblyequipment and robots. I installed Omron PLC's into a fleet of AutomaticallyGuided Vehicles that I designed.
The AGV's carried photocopiers around theplant, automatically transferring between one production line and the next.The PLC installed on board took care of managing the route to take and whatto do when it got there. The AGV PLC communicated to the production linePLC's in order to instigate and manage transfer from the vehicle to theconveyor, the PLC also commanded the motion controllers which tookcare of the drive and differential steering. Another PLC was staticallybased and kept track of each AGV in the fleet, effectively managing thewhole system. This was quite a first PLC project, eventually saving thecompany over £300,000 against a similar system bought from their usualsupplier. After 10 years and studying ONC and HNC I moved on to a newposition.
PLC based Special purpose machinery for the rubber and plasticindustries. Most equipment went into tyre (tire) plants all over theworld. I designed PLC control systems, wrote the PLC and motioncontrol software, installed it and commissioned in house and on site allover the world.
This was an interesting position with the great opportunityto travel the world while still being involved with PLC control systems.Some small machines such as Tube splicers were installed with various brandsof brick PLC as specified by the customer, the larger machines such as Tirebuilders generally had modular PLC's such as Allen Bradley SLC505. The fullyautomated bias cutter machines with PLC I/O counts of over 400 had modularPLC's with distributed I/O and SCADA systems.
I installed and serviced PLCbased tire machinery in the UK, USA, Canada, India, China, Indonesia and gotto meet some great people. When I started my own PLC control company Icontinued to work all over the world but in many different industries, Ihave installed PLC control systems in Breweries, Power stations, Potatoprocessing plants, steel plants, rubber plants, chemical processing plantsand many more, all over the world. As with any technology PLC's progress andPLC's installed 10 or 15 years ago may not be operating your machineryto the optimum, with increased flexibility in good PLC systems such asintegrated motion control, increases in quality and efficiency can beachieved. Replacing an outdated control system, PLC with an uptodate PLC control system can yield significant benefits.About PLC'sThe main difference from other computers is that PLCs are armored for severe conditions (dust,moisture, heat, cold, etc) and have the facility for extensive input/output(I/O) arrangements.
These connect the PLC to sensors and actuators. PLCsread limit switches, analog process variables (such as temperature andpressure), and the positions of complex positioning systems. Some even usemachine vision.
On the actuator side, PLCs operate electric motors,pneumatic or hydraulic cylinders, magnetic relays or solenoids, or analogoutputs. The input/output arrangements may be built into a simple PLC, orthe PLC may have external I/O modules attached to a computer network thatplugs into the PLC. System scaleA small PLC will have afixed number of connections built in for inputs and outputs. Typically,expansions are available if the base model does not have enough I/O.Modular PLCs have achassis (also called a rack) into which are placed modules with differentfunctions.
Plc Programmer For Complex Machines And Systems Vietenplus Bielefeld In Germany
The processor and selection of I/O modules is customised for theparticular application. Several racks can be administered by a singleprocessor, and may have thousands of inputs and outputs. A special highspeed serial I/O link is used so that racks can be distributed away from theprocessor, reducing the wiring costs for large plants.PLCs may need tointeract with people for the purpose of configuration, alarm reporting oreveryday controlA Human-MachineInterface (HMI) is employed for this purpose.
HMIs are also referred to asMMIs (Man Machine Interface) and GUI (Graphical User Interface).A simple system may usebuttons and lights to interact with the user. Text displays are available aswell as graphical touch screens. More complex systems use a programming andmonitoring software installed on a computer, with the PLC connected via acommunication interface.CommunicationsPLCs have built incommunications ports usually 9-Pin RS232, and optionally for RS485 andEthernet. Modbus or DF1 is usually included as one of the communicationsprotocols. Others' options include various fieldbuses such as DeviceNet orProfibus. Other communications protocols that may be used are listed in theList of automation protocols.Most modern PLCs cancommunicate over a network to some other system, such as a computer runninga SCADA (Supervisory Control And Data Acquisition) system or web browser.PLCs used in larger I/Osystems may have peer-to-peer (P2P) communication between processors.
Thisallows separate parts of a complex process to have individual control whileallowing the subsystems to co-ordinate over the communication link. Thesecommunication links are also often used for HMI (Human-Machine Interface)devices such as keypads or PC-type workstations.
Some of today's PLCs cancommunicate over a wide range of media including RS-485, Coaxial, and evenEthernet for I/O control at network speeds up to 100 Mbit/s.PLC compared with other control systemsPLCs are well-adaptedto a range of automation tasks. These are typically industrial processes inmanufacturing where the cost of developing and maintaining the automationsystem is high relative to the total cost of the automation, and wherechanges to the system would be expected during its operational life. PLCscontain input and output devices compatible with industrial pilot devicesand controls; little electrical design is required, and the design problemcenters on expressing the desired sequence of operations in ladder logic (orfunction chart) notation. PLC applications are typically highly customizedsystems so the cost of a packaged PLC is low compared to the cost of aspecific custom-built controller design. On the other hand, in the case ofmass-produced goods, customized control systems are economic due to thelower cost of the components, which can be optimally chosen instead of a'generic' solution, and where the non-recurring engineering charges arespread over thousands or millions of units.For high volume or verysimple fixed automation tasks, different techniques are used. For example, aconsumer dishwasher would be controlled by an electromechanical cam timercosting only a few dollars in production quantities.A microcontroller-baseddesign would be appropriate where hundreds or thousands of units will beproduced and so the development cost (design of power supplies andinput/output hardware) can be spread over many sales, and where the end-userwould not need to alter the control. Automotive applications are an example;millions of units are built each year, and very few end-users alter theprogramming of these controllers.
However, some specialty vehicles such astransit busses economically use PLCs instead of custom-designed controls,because the volumes are low and the development cost would be uneconomic.Very complex processcontrol, such as used in the chemical industry, may require algorithms andperformance beyond the capability of even high-performance PLCs. Veryhigh-speed or precision controls may also require customized solutions; forexample, aircraft flight controls.Programmablecontrollers are widely used in motion control, positioning control andtorque control. Some manufacturers produce motion control units to beintegrated with PLC so that G-code (involving a CNC machine) can be used toinstruct machine movements.PLCs may include logicfor single-variable feedback analog control loop, a 'proportional, integral,derivative' or 'PID controller.' A PID loop could be used to control thetemperature of a manufacturing process, for example. Historically PLCs wereusually configured with only a few analog control loops; where processesrequired hundreds or thousands of loops, a distributed control system (DCS)would instead be used. However, as PLCs have become more powerful, theboundary between DCS and PLC applications has become less clear-cut.PLCs have similarfunctionality as Remote Terminal Units. An RTU, however, usually does notsupport control algorithms or control loops.
As hardware rapidly becomesmore powerful and cheaper, RTUs, PLCs and DCSs are increasingly beginning tooverlap in responsibilities, and many vendors sell RTUs with PLC-likefeatures and vice versa. The industry has standardized on the IEC 61131-3functional block language for creating programs to run on RTUs and PLCs,although nearly all vendors also offer proprietary alternatives andassociated development environments.Digital and analog signalsDigital or discretesignals behave as binary switches, yielding simply an On or Off signal (1 or0, True or False, respectively). Push buttons, limit switches, andphotoelectric sensors are examples of devices providing a discrete signal.Discrete signals are sent using either voltage or current, where a specificrange is designated as On and another as Off.
For example, aPLC might use 24 V DC I/O, with values above 22 V DC representing On,values below 2VDC representing Off, and intermediate valuesundefined. Initially, PLCs had only discrete I/O.Analog signals are likevolume controls, with a range of values between zero and full-scale. Theseare typically interpreted as integer values (counts) by the PLC, withvarious ranges of accuracy depending on the device and the number of bitsavailable to store the data. As PLCs typically use 16-bit signed binaryprocessors, the integer values are limited between -32,768 and +32,767.Pressure, temperature, flow, and weight are often represented by analogsignals. Analog signals can use voltage or current with a magnitudeproportional to the value of the process signal. For example, an analog 4-20mA or 0 - 10 V input would be converted into an integer value of 0 - 32767.Current inputs are lesssensitive to electrical noise (i.e.
From welders or electric motor starts)than voltage inputs.As an example, say afacility needs to store water in a tank. The water is drawn from the tank byanother system, as needed, and our example system must manage the waterlevel in the tank.Using only digitalsignals, the PLC has two digital inputs from float switches (Low Level andHigh Level). When the water level is above the switch it closes a contactand passes a signal to an input. The PLC uses a digital output to open andclose the inlet valve into the tank.When the water leveldrops enough so that the Low Level float switch is off (down), the PLC willopen the valve to let more water in. Once the water level rises enough sothat the High Level switch is on (up), the PLC will shut the inlet to stopthe water from overflowing. This rung is an example of seal in logic.
Theoutput is sealed in until some condition breaks the circuit.An analog system mightuse a water pressure sensor or a load cell, and an adjustable (throttling)dripping out of the tank, the valve adjusts to slowly drip water back intothe tank.In this system, toavoid 'flutter' adjustments that can wear out the valve, many PLCsincorporate 'hysteresis' which essentially creates a 'deadband' of activity.A technician adjusts this deadband so the valve moves only for a significantchange in rate. This will in turn minimize the motion of the valve, andreduce its wear.A real system mightcombine both approaches, using float switches and simple valves to preventspills, and a rate sensor and rate valve to optimize refill rates andprevent water hammer.
Backup and maintenance methods can make a real systemvery complicated.PLC programs aretypically written in a special application on a personal computer, thendownloaded by a direct-connection cable or over a network to the PLC. Theprogram is stored in the PLC either in battery-backed-up RAM or some othernon-volatile flash memory.
Often, a single PLC can be programmed to replacethousands of relays.Under the IEC 61131-3standard, PLCs can be programmed using standards-based programminglanguages. A graphical programming notation called Sequential FunctionCharts is available on certain programmable controllers.Recently, theInternational standard IEC 61131-3 has become popular. IEC 61131-3 currentlydefines five programming languages for programmable control systems: FBD(Function block diagram), LD (Ladder diagram), ST (Structured text, similarto the Pascal programming language), IL (Instruction list, similar toassembly language) and SFC (Sequential function chart). These techniquesemphasize logical organization of operations.hile the fundamentalconcepts of PLC programming are common to all manufacturers, differences inI/O addressing, memory organization and instruction sets mean that PLCprograms are never perfectly interchangeable between different makers. Evenwithin the same product line of a single manufacturer, different models maynot be directly compatible.The PLC was invented inresponse to the needs of the American automotive manufacturing industry.Programmable controllers were initially adopted by the automotive industrywhere software revision replaced the re-wiring of hard-wired control panelswhen production models changed.Before the PLC,control, sequencing, and safety interlock logic for manufacturingautomobiles was accomplished using hundreds or thousands of relays, camtimers, and drum sequencers and dedicated closed-loop controllers.
Theprocess for updating such facilities for the yearly model change-over wasvery time consuming and expensive, as the relay systems needed to be rewiredby skilled electricians.In 1968 GM Hydramatic(the automatic transmission division of General Motors) issued a request forproposal for an electronic replacement for hard-wired relay systems.The winning proposalcame from Bedford Associates of Bedford, Massachusetts. The first PLC,designated the 084 because it was Bedford Associates' eighty-fourth project,was the result.
Bedford Associates started a new company dedicated todeveloping, manufacturing, selling, and servicing this new product: Modicon,which stood for MOdular DIgital CONtroller. One of the people who worked onthat project was Dick Morley, who is considered to be the 'father' of thePLC.
The Modicon brand was sold in 1977 to Gould Electronics, and lateracquired by German Company AEG and then by French Schneider Electric, thecurrent owner.One of the very first084 models built is now on display at Modicon's headquarters in NorthAndover, Massachusetts. It was presented to Modicon by GM, when the unit wasretired after nearly twenty years of uninterrupted service. Modicon used the84 moniker at the end of its product range until the 984 made itsappearance.The automotive industryis still one of the largest users of PLCs.Early PLCs weredesigned to replace relay logic systems.
These PLCs were programmed in'ladder logic', which strongly resembles a schematic diagram of relay logic.Modern PLCs can be programmed in a variety of ways, from ladder logic tomore traditional programming languages such as BASIC and C. Another methodis State Logic, a Very High Level Programming Language designed to programPLCs based on State Transition Diagrams.Many of the earliestPLCs expressed all decision making logic in simple ladder logic whichappeared similar to electrical schematic diagrams. This program notation waschosen to reduce training demands for the existing technicians. Other earlyPLCs used a form of instruction list programming, based on a stack-basedlogic solver.Early PLCs, up to themid-1980s, were programmed using proprietary programming panels orspecial-purpose programming terminals, which often had dedicated functionkeys representing the various logical elements of PLC programs. Programswere stored on cassette tape cartridges.
Facilities for printing anddocumentation were very minimal due to lack of memory capacity.
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By Steve Elliot, Copperline Consulting Services, Inc.1.
Contents.Introduction What is a Programmable Logic Controller (PLC)? A Programmable Logic Controller, or PLC, is more or less a small computer with a built-in operating system (OS).
This OS is highly specialized and optimized to handle incoming events in real time, i.e., at the time of their occurrence.The PLC has input lines, to which sensors are connected to notify of events (such as temperature above/below a certain level, liquid level reached, etc.), and output lines, to which actuators are connected to affect or signal reactions to the incoming events (such as start an engine, open/close a valve, and so on).The system is user programmable. It uses a language called 'Relay Ladder' or RLL (Relay Ladder Logic). The name of this language implies that the control logic of the earlier days, which was built from relays, is being simulated.Some other languages used include:. Sequential Function chart. Functional block diagram. Structured Text.
Instruction List. Continuous function chartA programmable logic controller, PLC, or programmable controller is a digital computer used for automation of typically industrial electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many machines, in many industries. PLCs are designed for multiple arrangements of digital and analog inputs and outputs, 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 response to input conditions within a limited time, otherwise unintended operation will result.Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop controllers.
Since these could number in the hundreds or even thousands, the process for updating such facilities for the yearly model change-over was very time consuming and expensive, as electricians needed to individually rewire the relays to change their operational characteristics.Digital computers, being general-purpose programmable devices, were soon applied to control 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.
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