What is a programmable logic controller (PLC)?
What Is a Programmable Logic Controller?
Programmable Logic Controllers (PLCs) are industrial computers, with various inputs and outputs, used to control and monitor industrial equipment based on custom programming.
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.
How Does a PLC Work?
A PLC’s operation is broken down into three stages: inputs, program execution, and outputs. PLCs capture data from the plant floor by monitoring inputs from any connected machines or devices. These inputs are checked against the program logic, which changes the outputs to any connected output devices. It is possible to have the same machine connected to both inputs and outputs on the same PLC, such as a valve position sensor connected to the inputs with the control of that valve position connected to the outputs. A program could read the current position of that valve, check to see if it needs to move, then move the valve position with the output.
PLCs often make a distinction between Digital (or Discrete) and Analog I/O. Digital I/O acts like a standard light switch where the state is either on or off, with no states between. Analog I/O acts like a dimmer switch, where the state can be anywhere between on and off.
It is easy to think of there being two sources of input data for PLCs: Device input data, automatically generated by a machine or sensor, or User input data, generated by a human operator using an HMI or SCADA system.
The Device input data comes from sensors and machines that send information to the PLC. This can include:
- On/Off states for things like mechanical switches and buttons
- Analog readings for things like speed, pressure, and temperature
- Opened/Closed states for things like pumps and valves
Human-facilitated inputs can include button pushes, switches, sensors from devices like keyboards, touch screens, remotes, or card readers.
PLC outputs are very similar to inputs, but can also include audible or visual indicators for the user, such as turning on a warning light, or sounding an alarm beacon. Other outputs can include:
- Opening or closing a valve
- Adjusting the speed on a motor
- Turning a heater On or Off
PLC programs operate in cycles. First, the PLC detects the state of all input devices that are connected to it. The PLC executes the user-created program, using the state of the inputs to determine the state that the outputs should be changed to. The PLC then changes the output signals to each corresponding device. After completing all these steps, the PLC then does a housekeeping step, which includes an internal diagnostic safety check to ensure that everything is within normal operating conditions. The PLC restarts the cycle each time the process is completed, starting again by checking inputs.
