What Are Programmable Logic Controllers (PLC)

Programmable logic controllers (PLC) are solid state computers sturdy enough to handle industrial automation in environments where standard computer systems would fail. PLCs can be used to control a wide variety of automated tasks, from simple lighting to monitoring and directing specific processes or even entire production lines.

What Do PLCs Do?

PLCs continuously monitor information from analog or digital input devices. The PLC makes decisions based on the input received, which are delivered to monitored equipment through the PLC’s output interface.

Tasks PLCs may perform include:

  • Signal processing,
  • Data conversion,
  • Monitoring and recording temperatures and equipment productivity,
  • Starting and stopping processes,
  • Generating alarms in the event of system malfunctions,
  • Monitoring sensing devices.
  • Automating switches and pushbuttons,
  • Controlling relays.

What’s Inside a PLC?

Programmable Logic Controllers are solid state computers, meaning they have not mechanical parts. A basic PLC contains a processor, a power supply, an input/output section, semiconductors, a solid-state storage drive, and a non-volatile flash memory.

Unlike ordinary computers, PLCs are robust and can handle severe heat, cold, dust and moisture. They have their own programming languages that are easy to understand, so a little training is all that’s needed to program them.

Advantages and Disadvantages of PLCs

PLCs, as noted above, are rugged little computers. Their ability to handle analog and digital input / output signals makes them incredibly versatile, and they run on simple logic programs easily mastered by employees.

On the down side, PLCs do not handle complex data as well. Any process requiring more than ladder logic or “yes/no” decision making is better paired with computers running C++ or Visual Basic Language.

Due to the compact nature of a PLC, most don’t display data well, if at all. External monitors are usually required to work with a PLC.

How do PLCs Work?

PLCs use a simple operating cycle of input-decision-output. The system works like this:

  • PLC receives signals from external controls or sensors.
  • PLC scans of incoming signals.
  • The system’s logic program makes decisions based on incoming signals.
  • Updated output signals are updated and sent.
  • The process starts again with new incoming signals.

This simple process is the foundation for many a large-scale automation process.

What are the 5 PLC Programming Languages?

To use PLCs, your employees need to learn how to program them. While this may sound intimidating, the five PLC languages are easy to understand and put to into practice after a short four-day seminar.

Ladder Logic

The layout of a ladder logic program will be familiar to employees with even a basic understanding of electricity. Using symbols to represent the opening and closing of counters, timers, relays, and other external devices, a ladder logic program looks like a circuit map. Each rung in the ladder represents a specific action, from inputs through to output signals. Ladder logic is considered the easiest PLC language to master.

Function Block Diagram

Function block diagrams represent PLC functions with blocks. All input, outputs, and other variables connect to these blocks to create a visual map of the program. Function block diagrams are often used for interconnected control systems.

Structured Text

A higher-level language than ladder logic or function block programs, structured text relies on “if/then” and “repeat/until” sentence structures to create PLC commands.

Instruction List

An Instruction list is a very low-level PLC language, with all functions and variable defined by a single list resembling assembly line programming.

Sequential Function Chart

Intended for complex systems, a sequential function chart contains blocks of interconnected programming each running its own subroutine. This PLC language is recommended for breaking down complicated tasks into smaller, more manageable tasks for the PLC to handle.

Why Your Employees Should be Trained for PLCs

Without proper training, your employees cannot maximize a programmable logic controller’s capabilities. Attempting to program a PLC without at least a basic understanding of PLCs will cause faults capable of reducing productivity, shortening equipment life cycles, or even bringing your production line to a halt. Fortunately, employees find the logical nature of even advanced PLC programming easy to grasp and implement, making your workplace more efficient through simple but powerful automation procedures.  A more convenient option is taking TPC Training's online PLC course.  


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