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in a tank between two float switchs 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.
Digital signals behave as switches, yielding simply an On or Off signal. 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. Pressure transducers, scales and gas leak detectors can provide analog signals.
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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.
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.
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In ladder logic, a [[relay]] coil, or just 'coil', can open or close any number of contacts, the switches which a relay controls. This can make ladder logic on a PLC easier then using real relays which seldom have more then four contacts. A PLC programmer lays out these virtual contacts and coils on their PC to control a process or machine in the real world.
In addition to normal relays, ladder logic allows for math functions through [[black box]] pieces that are integrated into the ladder. A simple addition function may count the number of times a button is pushed.
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 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.
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. Some PLCs may only communicate over a network to some other system, such as a computer running a web browser.
* [http://ab.com Allen-Bradley]
* [http://www.zworld.com Zworld]
* [[GE Fanuc]]
* [[Microchip]] (makers of [[PIC microcontroller]]s)
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