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Programmable logic controller

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PLC & input/output arrangements

A Programmable Logic Controller, PLC, or Programmable Controller is a small computer used for automation of real-world processes, such as control of machinery on factory assembly lines. The PLC usually uses a microprocessor. The program can often control complex sequencing and is often written by engineers. The program is stored in battery-backed memory and/or EEPROMs. Unlike general-purpose computers, the PLC is packaged and designed for extended temperature ranges, dirty or dusty conditions, immunity to electrical noise, and is mechanically more rugged and resistant to vibration and impact. The product life cycle of a PLC may be extended by service depots specializing in repair of automation devices. PLC's are omnipresent in manufacturing facilities, precipitating the emergence of surplus industrial supplier warehouses that may stock as many as 1 million industrial parts that become available when a plant liquidates electrical or electromechanical parts as part of its asset recovery initiatives. Another asset recovery strategy is to issue the malfunctioning asset to an industrial repair service center when it is desirable to retain the asset.

Features

The main difference from other computers are the special input/output arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, temperature indicators and the positions of complex positioning systems. Some even use machine vision. On the actuator side, PLCs drive any kind of electric motor, pneumatic or hydraulic cylinders or diaphragms, magnetic relays or solenoids. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a proprietary computer network that plugs into the PLC.

PLCs were invented as less expensive replacements for older automated systems that would use hundreds or thousands of relays and cam timers. Often, a single PLC can be programmed to replace thousands of relays. Programmable controllers were initially adopted by the automotive manufacturing industry, where software revision replaced the re-wiring of hard-wired control panels.

The earliest PLCs expressed all decision making logic in simple ladder logic inspired from the electrical connection diagrams. The electricians were quite able to trace out circuit problems with schematic diagrams using ladder logic. This was chosen mainly to reduce the apprehension of the existing technicians.

The functionality of the PLC has evolved over the years to include typical relay control, sophisticated motion control, process control, distributed control systems and complex networking. Today, the line between a general purpose programmable computer and a PLC is thinning. The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to desk-top computers. PLC-like functionality, combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications.

With the IEC 61131-3 standard, it is now possible to program PLCs using structured programming languages, and logic elementary operations. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers.

PLC compared with other control systems

PLCs are well-adapted to a certain 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 everything needed to handle high power loads right out of the box; very 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 PLC is low compared to the cost of contracting a designer for a specific, one-time only design. On the other hand, in the case of mass-produced goods, customized control systems quickly pay for themselves due to the lower cost of the components, which can be optimally chosen instead of a "generic" solution.

However, it should be noted that some PLCs no longer have a very high cost. Modern PLCs with full capabilities are available for a few hundred USD.

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 controls may also require customised solutions; for example, aircraft flight controls.

Digital vs. analog signals

Digital or Discrete signals behave as switches, yielding simply an On or Off signal (1 or 0, True or False, respectively). Pushbuttons, limit switches, and photo-eyes are examples of devices providing a discrete signal. Discrete signals are judged using either voltage or current, where a specific range is denominated as On and another as Off. A PLC might use 24 V DC I/O, with values above 22 V DC representing On and values below 2VDC representing Off. 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. Pressure, temperature and weight are often analog signals. Analog signals can use voltage or current, but do not have discrete ranges for On or Off. Instead they work in a defined range of values that are reliable for a particular device. Typically an analog 4-20 mA or 0 - 10 V signal 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 the 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 (tank empty and tank full). The PLC uses a digital output to open and close the inlet valve into the tank.

If both float switches are off (down) or only the 'tank empty' switch is on, the PLC will open the valve to let more water in. If only the 'tank full' switch is on, the valve turns off. Both switches being on would signal that something is wrong with one of the switches, as the tank cannot be both full and empty at the same time. Two float switches are used to prevent a 'flutter' condition where any water usage activates the pump for a very short time causing the system to wear out faster.

An analog system might use a load cell (scale) that weighs the tank, and a rate valve. The PLC could use a PID feedback loop (see section below) to control the rate valve. The load cell is connected to one of the PLC's analog inputs and the rate valve is connected to another of the PLC's analog outputs. This system fills the tank faster when there is less water in the tank. If the water level drops rapidly, the rate valve can be opened wide. If water is only dripping out of the tank, the rate 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 have a "deadband". 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. Backup and maintenance methods can make a real system very complicated.

How PLCs package I/O capabilities: Modular, Rack, P2P

Modular PLCs have a limited number of connections built in for inputs and outputs. Typically, expansions are available if the base model does not have enough I/O.

Rack-style PLCs have processor modules with separate [optional] I/O modules, which may occupy many racks. These often have thousands of discrete and analog inputs and outputs. Often a special high speed serial I/O link is used so that racks can be remotely mounted from the processor, reducing the wiring costs for large plants.

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 devices such as keypads or PC-type workstations.

A rule-of thumb is that the average number of inputs installed is three times that of outputs for both analog and digital. The 'extra' inputs arise from the need to have redundant methods to monitor an instrument to appropriately control another, and from the need to use both manual command inputs to the system and feedback from the controlled system itself.

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. More recently, 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 very oldest PLCs used non-volatile magnetic core memory but now the program is stored in the PLC either in battery-backed-up RAM or some other non-volatile flash memory.

Early PLCs were designed to be used by electricians who would learn PLC programming on the job. 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.

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.

While the fundamental concepts of PLC programming are common to all manufacturers, differences in I/O addressing, memory organization and instruction set 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.

PID loops

PLCs may include logic for single-variable generic industrial feedback loop, a "proportional, integral, derivative" loop, or "PID controller."

A PID loop is the standard solution to many industrial process control processes that require proportional, integral("improve errors") or derivative ("faster response") control techniques. A PID loop could be used to control the temperature of a manufacturing process, for example.

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.

A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. Most modern PLCs can communicate over a network to some other system, such as a computer running a SCADA system or web browser.

Communications

PLCs usually have built in communications ports for at least RS232, and optionally for RS485 and ethernet. Modbus is the lowest common denominator communications protocol. Others are various fieldbuses such as Profibus.

History

The PLC was invented in response to the needs of the American automotive industry. Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was accomplished using relays, timers 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 Boston, 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 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.

The automotive industry is still one of the largest users of PLCs, and Modicon still numbers some of its controller models such that they end with eighty-four. PLCs are used in many different industries and machines such as packaging and semiconductor machines. Well known PLC brands are ABB Ltd., Koyo, Honeywell, Siemens, Modicon, Omron, Allen-Bradley, General Electric, Tesco Controls, Panasonic (Matsushita), and Mitsubishi.

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