Programmable logic controller
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Image:Automate télémécanique tsx17.jpg Image:Automate siemens codeur analyseur de trame.JPG 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 is usually created by a skilled technician at an industrial site, rather than a professional computer programmer. The program is stored in battery-backed memory.
The main difference from other computers are the special input/output arrangements. These connect the PLC to a process's sensors and actuators. PLCs read limit switches, dual-level devices, 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 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.
The earliest PLCs expressed all decision making logic in simple ladder logic inspired from the electrical connection diagrams. The electricicians 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.
Recently, inspired from Grafcet, the PLC have integrated the Sequencial Function Charts : a new graphical language which allows now to directly program the sequencial nature of processes.
Today, the line between a programmable computer and a PLC is thinning. With the IEC-1138 standard, it is now possible to program these devices using structured programming languages (such as C), and logic elementary operations.
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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 digital signal. Digital signals are judged using either voltage or current, where a specific range is denominated as On and another as Off. A PLC might use 24VDC I/O, with values above 22VDC representing On and values below 2VDC representing Off. Initially, PLCs had only digital I/O. Current inputs are less sensitive to electrical noise (i.e. from welders or electric motor starts) than voltage inputs.
Analog signals are like volume controls, with a range of values between zero and full-scale. These are typically interpreted as integer values 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. On a 0-24VDC scale, 25VDC may be interpreted by the PLC as any value.
Example: Digital vs Analog
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 one of the PLC's analog outputs. This system fills the tank faster when there's 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 PLC's 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.
The average amount of inputs installed in the world 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.
Programming
PLCs programs are generally written in a special application on a personal computer then downloaded over a custom cable to the PLC. The program is typically stored in the PLC either in battery-backed-up RAM or some other non-volatile memory (flash).
Early PLCs were designed to be used by electricians who would learn PLC programming on the job. These PLC's were programmed in "ladder logic", which strongly resembles a schematic 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 5 programming languages for programmable control systems: FBD (Function Block Diagram), LD (Ladder Diagram), ST (Structured Text, Pascal type language), IL (Instruction List) and SFC (Sequential Function Chart). These techniques emphasize logical organization of operations.
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 control. Proportional control dictates that large deviations should be corrected by large amounts and small deviations should be corrected by small amounts. A PID loop could be used to control the pH level of water in a swimming pool.
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 SCADA system or web browser.
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 and trucks 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 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 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 from 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.
