Programmable Logic Controllers (PLCs)
Programmable Logic Controllers (PLCs) are industrial computers, with various inputs and outputs, used to control and monitor industrial equipment based on custom programming. A PLC is designed to take inputs, from either automated data capture points or through human input (e.g. via switches or buttons), and make decisions out it's output based upon it's programming. PLCs are typically located close to the devices that they are required to control.
PLCs are used in both SCADA and DCS systems as the control components of an overall hierarchical system to locally manage processes through feedback control. In the case of SCADA systems, they may provide similar functionality to RTUs. When used in DCS systems, PLCs are implemented as local controllers within a supervisory control scheme.
How does a PLC Work?
The PLC Input modules gather data from field input devices and transmit it to the PLC processor (CPU) as voltage or current signals. The received signals are the PLC inputs.
The CPU then processes the received information as pre-programmed data, which it uses together with the execution results of its programmed logic to control the field output devices via the Output modules. Thus, PLC outputs are the appropriate control commands that the PLC CPU sends out to control/operate physical devices. The controller uses ROM to store its operating system and system-defined variables and RAM to execute its functions and store input/output instructions.
The CPU has different operating modes that perform different tasks. For example, in programming mode, the PLC accepts the downloaded logic from a PC. On the other hand, when the PLC is in the run mode, it executes the program and runs the program.
The block diagram below illustrates the principle of operation of a PLC system:
Examples of PLC Inputs and Outputs
Inputs may come from IoT devices, robots, safety sensors, human-machine interfaces, or almost any other type of data entry point, including:
START/STOP Push Buttons
Proximity Sensors
Limit Switches
The output of the PLC can control various switches, motor starters, relays, and other devices connected to its outputs. This allows the PLCs to take control of mechanical processes such as the operation of a machine. Engineers can also link multiple system parts by programming PLCs to send their output signal to another PLC in a chain.
PLCs come in many different sizes and form factors. Some are small enough to fit in your pocket, while others are large enough to require their own heavy-duty racks to mount. Some PLCs are more modular, with only basic I/O (Inputs and Outputs), but can be customized with additional back planes and functional modules (such as analog I/O, communications modules, or display modules) to fit different types of industrial applications.
PLCs are designed to withstand harsh conditions, with protections against environmental factors dangers seen in industrial environments such as dust, vibration, condensation and high heats.
Programming Languages
PLCs are widely used in a variety of industries because they are fast, easy to operate, and considered easy to program. There are five standard PLC programming languages. The most commonly used language is Ladder Logic, but it is also possible to use Function Block Diagrams, Sequential Function Charts, Structured Text, or Instruction Lists to achieve the same functionality.
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