What is a Programmable Logic Controller?

Programmable Logic Controllers (PLCs) are miniature industrial computers. The hardware and software in a PLC are meant to perform control functions. Specifically, a PLC helps in the automation of industrial electromechanical processes. This includes controlling machinery on assembly lines in a factory, rides in an amusement park, or instruments in a food processing industrial establishment.

Most PLCs are designed to facilitate multiple arrangements of analog and digital inputs and outputs. They typically operate with extended temperature range, resistance to impact or vibration, and immunity to electrical noise and disturbances. The basic sections of a PLC usually consist of two sections—the first, the central processing unit (CPU), and the second, an Input/Output (I/0) interface system.

The CPU uses its processor and memory systems to control all system activity. Within the CPU is the micro-controller, memory chips, and other integrated circuits for controlling logic, monitoring, and communications. The CPU may operate in different modes—programmable or run. The programming mode allows the CPU to accept changes to the logic received from another computer. In the run mode, the CPU will execute the program to operate the process.

In the run mode, the CPU will accept input data from connected field devices such as switches, sensors, and more. After processing the data, it will execute or perform the control program stored in its memory system. As the PLC is a dedicated controller, the single program in its memory is processed and executed repeatedly. The scan time, the time taken for one cycle through the program, is typically of the order of one-thousandth of a second. The memory within the system stores the program, while at the same time holding the status of the I/O and provides a means to store values.

Typically, industrial users can fit a wide range of I/O modules to a PLC to accommodate various sensors and output devices. For instance, there are discrete input modules for detecting the presence of objects or events using photoelectric or proximity sensors, limit switches, and pushbuttons. Similarly, with discrete output modules it is possible to control loads such as motors, lights, solenoid valves, mainly to turn them On or Off.

The PLC can be fitted with analog input modules to accept signals generated by process instrumentation such as temperature, pressure, flow, and level transmitters. The modules interpret the signal from their sensors, and present a value within the range determined by the electrical specification of the device.

In the same way, the PLC can use analog outputs to command loads requiring a varying control signal, such as analog flow valves, variable frequency drives, or panel meters. PLCs can also use specialized modules such as serial or Ethernet communications, and high-speed I/O or motion control.

The greatest benefits of a PLC are its ability to change and replicate or repeat the operation of a process while simultaneously collecting and communicating critical information. In the industry, all aspects of a PLC—cost, power consumption, and communication capabilities—are subject to consideration when selecting the right one for the job. Industry automation owes a lot to the PLC or Programmable Logic Controller.