A programmable logic controller or PLC is a small computer used for automation of real-world processes, such as control of machinery on factory assembly lines.
PLCs often replace bulky cabinets full of electro-mechanical relay logic. Where older automated systems would use hundreds or thousands of relays, 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 the field connected "sensor" inputs (pushbuttons, limit-switches, photo-eyes, analog transmitters, etc.) and controls the attached output "actuators" (motor starters, solenoids, pilot lights/displays, speed drives, valves, etc.) according to a user-created, logic program. The program is generally created using "relay ladder logic" development software in a PC and then downloaded and 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, DCS and complex networking have now been added to the PLC's list of functions.
A simple program could maintain the level of water 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.
Analog vs Digital Inputs and Outputs
Some signals behave as switches, yielding simply an On or Off signal. Others behave like volume controls, yielding a range of values between On and Off.
PLCs typically have a limited number of connections for signals such as digital inputs, digital outputs, analog inputs and analog outputs. Typically expansions are available if the base model of a PLC does not have sufficient I/O.
Ladder Logic
Early PLCs were designed to be used by electricians who train on the job. They were usually programmed in a language called "ladder logic", which strongly resembles a schematic of relay logic. Modern PLCs can be programmed in ladder logic or in more traditional programming languages.
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 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.
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 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. Some PLCs may only communicate over a network to some other system, such as a computer running a web browser.
Links
Many companies manufacture PLCs including Zworld, GE Fanuc, Microchip (makers of PIC microcontrollers).