PLC Programming
Freelance PLC Programmer

I am a PLC programming engineer with over 20 years of experience on all types of PLC.  I am based in the Midlands of the UK and so ideally situated to help where ever you are in the UK or the rest of the world.

I have worked in many industries including steel, rubber & tyres, food & beverage, power generation, chemical process, automatically guided vehicles, steel & copper tube manufacture, packaging, material handling, industrial cleaning, special purpose machinery, conveyors, automated warehouse.

Any Application, Any industry, Anywhere.

Automation & Controls or PLC programming, with an extensive range of test equipment and software tools, I can help with

  • Machine breakdowns
  • Upgrades & changes
  • SCADA / HMI Control Systems
  • Industrial Control Systems
  • Control Systems Installation & Commissioning both at home & Overseas
  • Sourcing or replacing obsolete or legacy equipment.

  • Emergency Callout available!

    Call Alan for Free & Friendly Advice














     
     

    Recent Wolf Automation News

    OEE Installation in Dubai

    Our engineer has just returned from a successful installation of an OEE data collection system in Dubai.  The touch screen PC based system collects information via sensors on  a folding machine in a major printing comapny. Data such as Good Copy, Bad Copy, Setting / Good Production is collated by the system and transformed into statistical data which is used to monitor the efficiency of the machine and operators. The system also integrates into the businesses management software allowing the ordering of raw materials and the scheduling of jobs to be automated. Wolf Automation - Any Application - Any Where !

     

    Automation for OIL Tankers Fuel System

    Our engineer has just completed the commissioning of the software for the PLC Control system we created, for the automatic fuel changeover system on two oil tanker vessels in Amsterdam. New European legislation means that all ships have to burn Low Sulpher Deisel Oil rather than the standard Heavy Fuel Oil while in port. New tanks pipework and valves have to be added as well as modifications to the boiler burners. In this case automation was added so that the changeover can be made safely from the engine control room at the push of a button. A Mitsubishi FX3u was implemented along with FX2n-2AD modules to enable the reading of flow meters in the fuel lines. The control is operated and monitored from a Mitsubishi E1100 HMI. Feedback from valves and motor starters as well as fuel flow can be easily monitored on the screen. An alarm list has also been generated to integrate with the existing vessel management system. Wolf Automation - Any Application - Any Where !

     

    Athens Theatre Special Effect

    Wolf were called in to setup 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 2009 show. The unfinished control panel was already on site when we arrived, unfortunately there was no software and the Mitsubishi inverters (FR-A520--5.5) required configuration. We programmed the PLC Control System comprising a Koyo DL06 PLC to orchestrate the effect sequence and control the speed of the 5.5kw winches, Chabuki (kabuki)  release devices were used to drop the curtains before the winches accelerated to full speed in half a second. The curtains fly over the heads of the audience at a speed of 20ft/second. Obviously the timing of the drop and wind up is importan to avoid dropping the curtains onto the audience. Using our industrial automation experience we were able to hit the ground running and even though there was only a couple of days available, the effect was ready for opening night. Once again our engineer displayed the versatility of Wolf Automation - Any Application - Any Where !

     

    Tube Former Success

    Wolf Automation replaced the controls and drive system on an existing tube cutting and forming machine. The machine takes long lengths of stainless steel tube and cuts and shapes them into various different styles of heater modules. Wolf engineers realised that the problems with the machine were mostly generated by the control system and so the mechanical system was refurbished saving many thousands of pounds on the job. The existing stepper motors were replaced with DC servo motors and gearheads which improved the reliability and repeatability of the system. A motion control system and HMI were added bringing the machine into the 21st century and easing operation. Due to the success of the the refurbishment, the same is now pending on 2 more machines for the same customer, a multinational manufacturer of special heaters and controls. Wolf Automation specialise in retro fitting control systems to existing machines improving quality, production, reliability and saving thousands of pounds against buying a new machine. Wolf Automation - Any Application - Any Where !

     

    Wolf Automation Commissioning in Russia

    Wolf were asked to commission a copper tube straightening machine in Revda Sverdlovskaya Oblast, Russia. When we arrived on site to setup the Bronx machine, we found that it hadn't even been wired. We were able to source cable locally and connect the machine to the manufacturers drawings before commissioning the S7-300 based PLC control system that utilised a WinCC front end, the positioning system utilised profibus mounted absolute encoders. We stayed on site during a whole week of production trials. Wolf Automation - Any Application - Any Where !

     

    Rolling Mill Control System

    Wolf Automation have completed on a brand new control system for a Cold Roll Forming line 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 SK inverters. The system controls and automates all parts of the line, Decoiler, Rollfeed, Pierce Punching Press, 2 Accumulator pits, Rolling Mill, Flying Cutoff with High speed hydraulics, Runoff conveyer and Product Ejector system to deliver the products to the operator. Wolf designed, built, programmed, installed and commissioned the line, on time and on budget. We deliver High-spec, High-quality, value for money solutions.  Wolf Automation - Any Application - Any Where !

     

    Bosch Rexroth Servo Ejector System

    Wolf were tasked with producing a servo control system to control two actuators which divert steel section from a conveyor to a packing station. Wolf used the new Bosch Rexroth Indradrive with integrated PLC along with a VCP08  HMI. This simple point to point application gave us the chance to acquaint ourselves with some of the latest technology form Bosch Rexroth. Once again Wolf designed, built, programmed, installed and commissioned the system. Wolf Automation - Any Application - Any Where !

     

    Lloyds Bank Generator Backup System

    Wolf Automation visited Lloyds TSB in Bristol recently to fault find on their backup generators. The system has two generators aswell as a UPS system and the whole thing is monitored by 3 24v Mitsubishi F2-60MR PLC's. We were able to interrogate the software with the old MEDOC software F2-20 GF1 and 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 to carry out all works between 6pm and midnight on a Saturday evening. Wolf Automation - Any Application - Any Where !

     

    High Speed Hydraulic Control System

    Wolf completed the installation of a  high speed hydraulic punching unit for a blue chip manufacturer of strut products. The unit controls the impression of the companies name into the product synchronising with the crank press that pierces the product. The hydraulic system supplied by H&L Hydraulics (Voith Turbo) is able to apply 20 tonnes of stamping pressure and cycles at a rate of over 5hz. Wolf designed the control system to drive and monitor the hydraulic control card and hydraulic power pack, we also integrated the system to the existing press and Indramat roll feed system. Plans are in place to duplicate the system later this year.  Wolf Automation - Any Application - Any Where !
















































    PLC's Supported Include




    Mitsubishi
  • FX1S PLC
  • FX1N PLC
  • FX2N PLC
  • FX3U PLC
  • FX3UC PLC
  • System Q PLC
  • AnS PLC
  • AlphaPLC


  • Direct Logic / KOYO
  • DL05 PLC
  • DL06 PLC
  • DL105 PLC
  • DL205 PLC
  • DL305 PLC
  • DL405 PLC
  • CLICK PLC


  • Omron
  • CJ1M
  • CJ1G
  • CJ1H
  • CS1
  • CJ1
  • CQM1
  • CPM1
  • CPM2A
  • CPM2C
  • ZEN
  • C200


  • Schneider Telemecanique
  • Zelio
  • Twido
  • TSX






























  • Software Tools Available Include


    Allen Bradley
  • AB RSLogix5
  • AB RSLogix500
  • AB RSLogix5000


  • KOYO Direct Logic
  • Direct Soft 5
  • Omron
  • Omron Syswin
  • Omron CX One
  • Omron CX Programmer
  • Siemens
  • Seimens S7
  • Mitsubishi Medoc
  • Mitsubishi GX Developer


















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    PLC Control Systems

     

    Industrial control systems

    Industrial control system (ICS) is a general term that encompasses several types of control systems, including supervisory control and data acquisition (SCADA) systems, distributed control systems (DCS), and other smaller control system configurations such as skid-mounted programmable logic controllers (PLC) often found in the industrial sectors and critical infrastructures. ICSs are typically used in industries such as electrical, water, oil and gas, data. Based on information received from remote stations, automated or operator-driven supervisory commands can be pushed to remote station control devices, which are often referred to as field devices. Field devices control local operations such as opening and closing valves and breakers, collecting data from sensor systems, and monitoring the local environment for alarm conditions.

     

    A historical perspective

    Industrial control system technology has evolved over the past three to four decades. DCS systems generally refer to the particular functional distributed control system design that exist in industrial process plants (e.g., oil and gas, refining, chemical, pharmaceutical, some food and beverage, water and wastewater, pulp and paper, utility power, mining, metals). The DCS concept came about from a need to gather data and control the systems on a large campus in real time on high-bandwidth, low-latency data networks. It is common for loop controls to extend all the way to the top level controllers in a DCS, as everything works in real time. These systems evolved from a need to extend pneumatic control systems beyond just a small cell area of a refinery. The PLC (programmable logic controller) evolved out of a need to replace racks of relays in ladder form. The latter were not particularly reliable, were difficult to rewire, and were difficult to diagnose. PLC control tends to be used in very regular, high-speed binary controls, such as controlling a high-speed printing press. Originally, PLC equipment did not have remote I/O racks, 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 data through potentially unreliable or intermittent low-bandwidth/high-latency links. SCADA systems use open-loop control with sites that are widely separated geographically. A SCADA system uses RTUs (remote terminal units, also referred to 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 controls while the master station is not available. However, over the years RTU systems have 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 no longer as much of an issue. Many PLC platforms can now perform quite well as a small DCS, using remote I/O and analog control loops, and are able to communicate supervisory data. It is not uncommon to have telecommunications infrastructure that is so responsive and reliable that some SCADA systems actually manage closed loop control over long distances. With the increasing speed of today's processors, many DCS products have a full line of PLC-like subsystems that weren't offered when they were initially developed. This has led to the concept of a PAC (programmable automation controller or process automation controller). It is an amalgamation of these three concepts. Time and the market will determine whether this can simplify some of the terminology and confusion that surrounds these concepts today.

     

    DCSs

    DCSs are used to control industrial processes such as electric power generation, oil and gas refineries, water and wastewater treatment, and chemical, food, and automotive production. DCSs are integrated as a control architecture containing a supervisory level of control overseeing multiple, integrated sub-systems that are responsible for controlling the details of a localized process. Product and process control are usually achieved by deploying feed back or feed forward control loops whereby key product and/or process conditions are automatically maintained around a desired set point. To accomplish the desired product and/or process tolerance around a specified set point, specific programmable controllers are used ONLY.

     

    PLCs

    PLCs provide boolean logic operations, timers, and (in some models) continuous control. The proportional, integral, and/or differential gains of the PLC continuous control feature may be tuned to provide the desired tolerance as well as the rate of self-correction during process upsets. DCSs are used extensively in process-based industries. PLCs are computer-based solid-state devices that control industrial equipment and processes. While PLCs can control system components used throughout SCADA and DCS systems, they are often the primary components in smaller control system configurations used to provide regulatory control of discrete processes such as automobile assembly lines and power plant soot blower controls. PLCs are used extensively in almost all industrial processes.

     

    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 Logic, a very high-level programming language designed to program PLCs based on state transition diagrams.

    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. 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.

    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. 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 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.

    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

    PLCs may need to interact with people for the purpose of configuration, alarm reporting or everyday control. A Human-Machine Interface (HMI) is employed for this purpose. HMIs are also referred to as MMIs (Man Machine Interface) and GUI (Graphical User Interface). 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 a programming and monitoring software installed on a computer, with the PLC connected via a communication interface. 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 non-volatile flash memory. Often, a single PLC can be programmed to replace thousands of relays. 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 PLC's utilized Ladder Logic Diagram Programming, a model which emulated electromechanical control panel devices (such as the contact and coils of relays) which PLC's replaced. This model remains common today. IEC 61131-3 currently defines five programming languages for programmable control systems: FBD (Function block diagram), LD (Ladder diagram), ST (Structured text, similar to the Pascal programming language), IL (Instruction list, similar to assembly language) and SFC (Sequential function chart). These techniques emphasize logical organization of operations. 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.

     

    PLC compared with other control systems

    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 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 end-users 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. Very complex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very high-speed or precision controls may also require customized solutions; for example, aircraft flight controls. 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. 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.

     

    Digital and analog signals

    Digital or discrete signals behave as binary switches, yielding simply an On or Off signal (1 or 0, True or False, respectively). Push buttons, limit switches, and photoelectric sensors are examples of devices providing a discrete signal. Discrete signals are sent using either voltage or current, where a specific range is designated as On and another as Off. For example, a PLC might use 24 V DC I/O, with values above 22 V DC representing On, values below 2VDC representing Off, and intermediate values undefined. Initially, PLCs had only discrete I/O. Analog signals are like volume controls, with a range of values between zero and full-scale. These are typically interpreted as integer values (counts) by the PLC, with various ranges of accuracy depending on the device and the number of bits available to store the data. As PLCs typically use 16-bit signed binary processors, 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 or current with a magnitude proportional to the value of the process signal. For example, an analog 4-20 mA or 0 - 10 V input would be converted into an integer value of 0 - 32767.

    Current inputs are less sensitive to electrical noise (i.e. from welders or electric motor starts) than voltage inputs.

    Example

    As an example, say a facility needs to store water in a tank. The water is drawn from the tank by another system, as needed, and our example system must manage the water level in the tank. Using only digital signals, the PLC has two digital inputs from float switches (Low Level and High Level). When the water level is above the switch it closes a contact and passes a signal to an input. The PLC uses a digital output to open and close the inlet valve into the tank. 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 level rises enough so that the High Level switch is on (up), the PLC will shut the inlet to stop the water from overflowing. This rung is an example of seal in logic. The output is sealed in until some condition breaks the circuit. An analog system might use 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 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 a significant 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 simple valves to prevent spills, and a rate sensor and rate valve to optimize refill rates and prevent water hammer. Backup and maintenance methods can make a real system very complicated.





    About PLC Programmer

    I began working with PLC's at a major Japanese manufacturer of office equipment in based in Shropshire. There were many PLC's installed in various production lines, assembly equipment and robots. I installed Omron PLC's into a fleet of Automatically Guided Vehicles that I designed. The AGV's carried photocopiers around the plant, automatically transferring between one production line and the next. The PLC installed on board took care of managing the route to take and what to do when it got there. The AGV PLC communicated to the production line PLC's in order to instigate and manage transfer from the vehicle to the conveyor,  the PLC also commanded the motion controllers which took care of the drive and differential steering. Another PLC was statically based and kept track of each AGV in the fleet, effectively managing the whole system. This was quite a first PLC project, eventually saving the company over £300,000 against a similar system bought from their usual supplier. After 10 years and studying ONC and HNC I moved on to a new position. PLC based Special purpose machinery for the rubber and plastic industries. Most equipment went  into tyre (tire) plants all over the world. I designed PLC control systems , wrote the PLC  and motion control software, installed it and commissioned in house and on site all over the world. This was an interesting position with the great opportunity to travel the world while still being involved with PLC control systems. Some small machines such as Tube splicers were installed with various brands of brick PLC as specified by the customer, the larger machines such as Tire builders generally had modular PLC's such as Allen Bradley SLC505. The fully automated bias cutter machines with PLC I/O counts of over 400 had modular PLC's with distributed I/O and SCADA systems. I installed and serviced PLC based tire machinery in the UK, USA, Canada, India, China, Indonesia and got to meet some great people. When I started my own PLC control company I continued to work all over the world but in many different industries, I have installed PLC control systems in Breweries, Power stations, Potato processing plants, steel plants, rubber plants, chemical processing plants and many more, all over the world. As with any technology PLC's progress and PLC's installed 10 or 15 years ago may not be operating  your machinery to the optimum, with increased flexibility in good PLC systems such as integrated motion control, increases in quality and efficiency can be achieved. Replacing an outdated control system, PLC  with an upto date PLC control system can yield significant benefits. 

    About PLC's

    The 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. PLCs read limit switches, analog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some even use machine vision. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays or 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. 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 does not have enough 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.PLCs may need to interact with people for the purpose of configuration, alarm reporting or everyday controlA Human-Machine Interface (HMI) is employed for this purpose. HMIs are also referred to as MMIs (Man Machine Interface) and GUI (Graphical User Interface).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 a programming and monitoring software installed on a computer, with the PLC connected via a communication interface.CommunicationsPLCs have built in communications ports usually 9-Pin RS232, and optionally for RS485 and Ethernet. Modbus or DF1 is usually included as one of the communications protocols. Others' 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 co-ordinate over the communication link. These communication links are also often used for HMI (Human-Machine Interface) devices such as keypads or PC-type workstations. Some of today's PLCs can communicate over a wide range of media including RS-485, Coaxial, and even Ethernet for I/O control at network speeds up to 100 Mbit/s.PLC compared with other control systemsPLCs 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 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 in ladder logic (or function chart) notation. 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 and input/output hardware) 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 end-users 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.Very complex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very high-speed or precision controls may also require customized solutions; for example, aircraft flight controls.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.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. However, as PLCs have become more powerful, the boundary between DCS and PLC applications has become less clear-cut.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.Digital and analog signalsDigital or discrete signals behave as binary switches, yielding simply an On or Off signal (1 or 0, True or False, respectively). Push buttons, limit switches, and photoelectric sensors are examples of devices providing a discrete signal. Discrete signals are sent using either voltage or current, where a specific range is designated as On and another as Off. For example, a PLC might use 24 V DC I/O, with values above 22 V DC representing On, values below 2VDC representing Off, and intermediate values undefined. Initially, PLCs had only discrete I/O.Analog signals are like volume controls, with a range of values between zero and full-scale. These are typically interpreted as integer values (counts) by the PLC, with various ranges of accuracy depending on the device and the number of bits available to store the data. As PLCs typically use 16-bit signed binary processors, 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 or current with a magnitude proportional to the value of the process signal. For example, an analog 4-20 mA or 0 - 10 V input would be converted into an integer value of 0 - 32767.Current inputs are less sensitive to electrical noise (i.e. from welders or electric motor starts) than voltage inputs.As an example, say a facility needs to store water in a tank. The water is drawn from the tank by another system, as needed, and our example system must manage the water level in the tank.Using only digital signals, the PLC has two digital inputs from float switches (Low Level and High Level). When the water level is above the switch it closes a contact and passes a signal to an input. The PLC uses a digital output to open and close the inlet valve into the tank.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 level rises enough so that the High Level switch is on (up), the PLC will shut the inlet to stop the water from overflowing. This rung is an example of seal in logic. The output is sealed in until some condition breaks the circuit.An analog system might use 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 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 a significant 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 simple valves to prevent spills, and a rate sensor and rate valve to optimize refill rates and prevent water hammer. Backup and maintenance methods can make a real system very complicated.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 non-volatile flash memory. Often, a single PLC can be programmed to replace thousands of relays.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.Recently, the International standard IEC 61131-3 has become popular. IEC 61131-3 currently defines five programming languages for programmable control systems: FBD (Function block diagram), LD (Ladder diagram), ST (Structured text, similar to the Pascal programming language), IL (Instruction list, similar to assembly language) and SFC (Sequential function chart). These techniques emphasize logical organization of operations.hile 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.The PLC was invented in response to the needs of the American automotive manufacturing industry. Programmable 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 closed-loop controllers. The process for updating such facilities for the yearly model change-over was very time consuming and expensive, as the relay systems needed to be rewired by skilled electricians.In 1968 GM Hydramatic (the automatic transmission division of General Motors) issued a request for proposal for an 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. 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. 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.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. 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 Logic, a Very High Level Programming Language designed to program PLCs based on State Transition Diagrams.Many of the earliest PLCs expressed all decision making logic in simple ladder logic which appeared similar to electrical schematic diagrams. 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.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. 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.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.

     

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