PLC
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. Where older automated systems would use hundreds or thousands of relays and cam timers, a single PLC can be programmed as a replacement.
The PLC is a microprocessor based device with either modular or integral input/output circuitry that monitors the status of field connected "sensor" inputs and controls the attached output "actuators" (motor starters, solenoids, pilot lights/displays, speed drives, valves, etc.) according to a user-created, logic program stored in the microprocessor's battery-backed RAM memory. The functionality of the PLC has evolved over the years to include capabilities beyond typical relay control; sophisticated motion control, process control, Distributed Control System and complex networking have now been added to the PLC's list of functions.
Examples
A simple program could maintain the level of water in a tank between two float switches by opening and closing an electric valve. A slightly more complex arrangement could involve a scale under the tank (as an input) and a flow controller (as an output) allowing water to flow at a controlled rate. A typical industrial application might control several tanks in a process such as sewage treatment. Each tank might be watched for a variety of conditions such as being too full or too empty or having the wrong pH.
Analog versus digital inputs and outputs
Digital signals behave as switches, yielding simply an On or Off signal (logical 1 or 0, respectively). These are interpreted as boolean values by the PLC. Pushbuttons, limit switches, and photo-eyes are examples of devices providing a digital signal. Analog signals behave as volume controls, yielding a range of values between On and Off. These are typically interpreted as floating-point values by the PLC, with various ranges of accuracy depending on the device and the number of bits available to store the data. Pressure transducers, scales and gas leak detectors can provide analog signals.
Digital signals generally use either voltage or current, where a specific range is denominated as On (logical 1) and another as Off (logical 0). A typical PLC might use 24VDC I/O (with 24V representing On and 0V representing Off). Analog signals generally use voltage or current as well, but do not have discrete ranges for On or Off. They define a range of valid values, typically the range in which the I/O device operates reliably. Other methods of signal I/O include serial communications (typically RS-232 or RS-485), and proprietary networks like Allen-Bradley's Data Highway, Opto 22's OptoMux or open and standardised networks like Profibus.
PLCs have a limited number of connections built in for signals such as digital inputs, digital outputs, analog inputs and analog outputs. Typically, expansions are available if the base model does not have sufficient I/O.
The average amount of inputs installed in the world is three times that of outputs for both analog and digital. The need for this rises from the PLC's need to have redundant methods to monitor a instrument to appropriately control another.
Examples
In the simple example above, the PLC is accepting two digital inputs from float switches. The PLC is controlling a single digital output to actuate the inlet valve into the tank. If both float switches are off (down) the PLC will open the valve to let more water in. It will continue to fill until both floats lift making their switches turn on.
The slightly more complex example (scale and flow controller) uses analog inputs and outputs. The scale is connected to one of the PLC's analog inputs and the flow controller is connected to one of the PLC's analog outputs. In this system, we can fill at different rates based on how much water is left. If the water level drops rapidly, the flow controller can be opened wide. If water is only dripping out of the tank, the flow controller can be set to allow only a small amount of water back into the tank.
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 stored in the PLC either in battery-backed-up RAM or some other non-volatile memory.
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.
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 variety of methods are employed.
A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. Most of 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. In 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.
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