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Programmable Logic Controller (PLC)

Programmable Logic Controllers (PLCs) are integral components of industrial hardware systems. Because they were designed to be used mostly by non-engineer workers, it takes just a little effort to understand how crucial PLCs are in application and the many advantages they bring to the shop floor.

Key Takeaways

  • PLC = input + output + CPU

  • Most PLCs use Ladder Logic (LD)

  • PLCs process discrete signals better than analog signals

What is a Programmable Logic Controller (PLC)?

A programmable logic controller is a solid-state drive (meaning no actively moving parts) control system with user-programmable memory. A PLC implements certain crucial communications and operations functions in a factory:

  • Input and Output control (I/O)
  • Logic and arithmetic rules
  • Timing and counting
  • Three mode PID control
  • Data and file processing

Some PLCs are small enough to be handheld devices, while others are so large they need to be housed in a separate control room. This, of course, depends on the size of the industrial operation at hand.

There are two options for programmable logic controllers: Fixed I/O and Modular

A Fixed I/O PLC can also be known as an integrated or compact PLC. This type is when your input and output generated by the machine is fully integrated into the internal microcontroller. In other words, this PLC is an impenetrable hardware unit that doesn’t allow for any internal tinkering.

A Modular PLC can be combined with other modules to make combinations of input/output processors. This is helpful nowadays because it means less downtime, easier fault detection, extended memory potential, and allows for customized industrial solutions. Some modular PLCs are often held as backups just in case one or more units needs to be switched out unexpectedly.

wall-mounted PLC

How Does a PLC Work?

A PLC takes both data and human inputs. Data inputs include information automatically sent from sensors, encoders, and detectors. Human inputs, on the other hand, are buttons, switches, keyboards, touch screens, remotes, or card readers that humans use to interact with the operational system.

The PLC takes this input data and converts it into physical or visual outputs. Examples of these are starting motors, draining valves, sending printouts, or monitoring the GPS location of remote equipment. The output data can also be in the form of a visual display using a Human-Machine Interface (HMI).

But how do PLCs read the inputs they receive?

Programmable logic controllers read inputs via signals and translate data into outputs. There are two types of signals: discrete, and analog.

Discrete signals are on or off

E.g. The light is EITHER on or off, or a machine’s cutting blade is EITHER lowered in place or not. There is no middle ground option.

Analog signals are ranges or measurements

E.g. The operational step is completed once the machine records an internal temperature WITHIN the range of 80-100 degrees, and begins the next automatic movement after a cooling off period of 30-45 seconds.

Early programmable logic controllers were programmed using Ladder Logic, also known as Ladder Diagram (LD) language. It is named so because it was designed to be read by non-engineers, and thus used electrical diagrams that looked like ladders where operations were drawn as rungs.

The idea is that all processes are outlined visually so workers can quickly understand which sequence the order of operations lies. In the case of a change or a shutdown in the logical process of commands, the right section of programming can be accessed quickly and efficiently for as little downtime as possible.

In 2015, the IEC mandated the standards for PLC programming languages. While mostly all PLCs still used Ladder Logic, they also now could easily use languages like:

  • Structured Text (ST)
  • Sequential Function Chart (SFC)
  • Function Block Diagram (FBD)
  • Instruction List (IL)

Of course, Ladder Diagram (LD) is the defining logic of the programmable logic controller. This is because the high-volume, simple operations that PLCs control work best with little complication. In fact, if a factory uses too complicated a system, breakdowns could occur frequently and it will be difficult to tell in which “ladder rungs” or operational steps the breakdowns are occurring.

Leaving complicated processes for equipment for later down the production streamlines this flow of information and decreases the energy consumption needed for initial logic processes.

Ladder Diagram of PLC-operated Tank Level Control

What Can PLCs Be Used For?

Programmable Logic Controllers are important parts of control systems architecture like SCADA (supervisory control and data acquisition), and communicate easily with HMI (human-machine interface) technology for solid worker comprehension.

PLCs also continually perform housekeeping activities like proper communication within the control system and internal diagnostic checks.

Safety PLCs take this responsibility one step further -- they are programmed with many redundancies in place so in an emergency situation, they have many fail safes to catch system errors if things truly start to degrade.

Today, in some fully optimized IIoT industrial systems, workers can communicate data to a PLC via the web, SQL databases, or even the Cloud for Smart Factory applications.

Lightbulb Pro Tip

Pro Tip

IEC 61131 is the international standard for Programmable Logic Controller procedures created by the International Electrotechnical Commission (IEC). It describes service conditions and climatic, transport, and electrical requirements. It also outlines functional requirements like the memory capacity, inputs and outputs styles, verification tests, and electromagnetic compatibility (EMC) tests that must be performed.

PLC Advantages

PLCs are built for simple inputs and outputs but are also extremely optimized for factories. They are built to be durable even around dust, dirt, and fluctuating temperatures. There are few components of programmable logic controllers so problems are easy to troubleshoot if they ever arise.

PLCs are much more durable than the old relay-based control systems that were prone to frequent failures. Nowadays, modular PLCs can be configured together or swapped in and out for better performance.

Overall, they use little power and are more efficient, easier to program, and simple to use, which makes them a crucial technology for industrial production. Most industrial operations heavily rely upon the I/O organization enabled by PLC technology.

PLC-controlled Tank Level indicator

PLC Disadvantages

A helpful way to think of PLC technology is to see it as one step behind the capability of a Distributed Control System (DCS). Programmable logic controllers alone are not advanced enough to synthesize complicated analog inputs; the system needs something with a bit more accessible memory and override options.

For example, a PLC would struggle to understand data in C++ and display complex data back to the user with enough detail to be useful.

PLCs can also be vulnerable to electromagnetic interference (EMI). In order to avoid electrical shutdown, operators must be aware of the PLC’s temperature tolerance, CPU speed, RAM space, and potential compatibility with other system hardware or electricity source.

Lightbulb Pro Tip

Pro Tip

While PLCs can be better applied with combined modules or within a DCS, they are still crucial for ANY industrial operation. Even fully integrated enterprise systems with more complicated operations still rely upon the critical I/O data compiled by PLCs. The traditional PLC may evolve, but it will never be obsolete.

Industrial PLC Examples & Trends

Okay, so you’ve described PLCs, but what’s this PAC thing I keep hearing about?

A PAC, or Programmable Automated Controller, is pretty much the same thing as a PLC except with a higher degree of automated output. The term PAC was introduced in the early 2000s to refer to the more modern PLCs then starting to come out which relied on slightly more complicated automation.

PACs today are helpful because they can be programmed using BASIC or C, instead of just Ladder Diagram language.

As the Industrial Internet of Things advances with the rise of Industry 4.0, there is more need for PLCs in edge computing at the ends of networks. This is because PLCs communicate using the cyclical poll-response method. In other words, they are constantly sending data and have to be internet-connected in order to be receptive to signals from other automatic systems in order to receive next steps.

Programmable logic relays (PLRs) are another form of PLC technology specifically designed for small operations needing few discrete output options. These PLR systems are simple, all-in-one I/O computers with HMIs for easy installation and use. They are NOT modular or expandable, but they are definitely cheaper and more efficient than modular PLC or PAC systems. This makes them best suited for smaller operating systems.

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