What Is a PLC? Basics of PLCs Featuring the CLICK PLC from AutomationDirect

The PLC is the key element behind today's Industrial automation. But, what exactly is a PLC? In this video, I’ll walk you through the basics of what a PLC is, some of its practical uses, and even go into some of the PLCs inner workings. Let's start by learning what the commonly used acronym PLC stands for. PLC is short for Programmable Logic Controller. In general, a PLC is an industrialized computer used to automate production equipment and or processes. The PLCs hardware and software allow it to monitor the status of incoming events and control the outcomes. It also has the flexibility to reprogram its decision making behavior through software as often as needed. Did you know you probably encounter PLC based controls every day without even realizing it? Aside from their primary use in automating factories and processes, PLCs are also used in simple applications such as car washes, elevators, and even amusement parks. More complex PLC applications include water and waste water treatment plants, manufacturing assembly lines, machinery, bottling lines, just to name a few. Let's take a quick look at the PLCs history. The PLC is mostly recognized as being introduced by Bedford Associates in nineteen sixty eight. Originally, called a modular digital controller or Modicon. It became the product of choice in meeting specifications developed by General Motors Hydra-matic Division Plan for replacing traditional relay-based machine control. Its use of ladder logic programming was one of the main features that attracted it to factory personnel. Because it could be programmed the PLC allowed for quick changes and a reduction in wiring and troubleshooting time. This was a huge advantage over the older relay-based control system. Making the same changes on relay-based control systems meant equipment would have to be shut down for long periods of time to perform extensive rewiring. The PLC also has the advantage of taking up less panel space and consuming less energy. What do the individual words in “PLC” mean? It may make more sense to take the words in reverse order. “Controller” is the key word in Programmable Logic Controller. The PLC monitors various conditions and based on these conditions determines an outcome. In other words, it has the ability to control devices wired to its output terminals based on the status of devices wired to its input terminals. “Logic” is how we treat the various conditions we monitor which determines our final outcome. The decisions are based on logical rules that we teach the PLC by way of its programming software. A simple example might be that we detect someone has opened a door and it's nighttime so light is turned on so they can see where they are stepping. Programmable software for the PLC is used to create these logical rules. Because you can program the PLC to behave however you want it can be adapted to virtually any application. The ability to program and reprogram the PLC makes it very versatile and for this reason if future conditions change the PLC can be reprogrammed to meet these needs. We can envision how a PLC functions by examining the interaction of four internal areas. The primary area the CPU or central processing unit. This is the brain. Or in other words, where the decision-making takes place. The second area is the memory. This is where the CPU stores the users control program, I/O status and data. The program is stored non-volatile memory. So it is not lost in the event of a power failure. The third area is communications. One or more communication ports are available to allow the users program to be loaded into its memory from a personal computer. And to also communicate with external devices including other PLCs in order to exchange data. The final area is the PLCs I/O. The input and output signals to real world devices. Let’s cover the I/O in more detail. After decisions are made by the users program real world control devices is carried out by the CPUs outputs. Here we see a schematic diagram example showing how inputs and outputs are wired to devices such as pushbuttons, limit switches, pilot lights, motor starters and solenoid valves. 0:05:12.039,0:05:17.960 The PLCs I/O connections are often available with different input and output types, such as DC and AC voltage inputs and also DC, AC and relay contact type outputs. So how are PLCs programmed? In most cases application software running on a personal computer is used to create the PLCs decision-making logic. Although, other programming methods exist, the most widely-used is still ladder logic which is a carryover from relay-based control systems. Ladder logic consists of contacts, coils and other functions arranged on rungs. Not unlike the rungs on a ladder. In the simple example shown here, a timer is activated by either an external device wired to input X001 or an internal control relay labeled C1. Notice the logic “OR” in the example. After a pre set amount of time the timer signals completion by closing its T1 contact and activated output Y001 which controls a real word device wired to the PLCs output terminals. An END coil is shown in the last rung to tell the program to go back to the start and check the rungs in order again. To better understand the internal workings of the PLC it helps to know how the ladder logic is examined during execution. This is referred to as the program scan. When the PLC is in its RUN mode the CPU executes the users program with the first step being to read all inputs. This is the same as taking a snapshot of the current status of all the inputs including all the registers. Next, routine housekeeping is performed such as checking communications from peripheral devices and updating registered values. Now the ladder logic is ready to be scanned. The scan is performed left to right for each instruction on the ladder logic rung starting with the first rung and then each succeeding rung until the end coil is encountered. The results for the programs logic conditions are determined, computed, and the outputs are updated. Diagnostics are performed and if all is well the scan is repeated. An entire scan can take place in less than one to two milliseconds. The scan time is dependent on the size of the ladder logic program. Scan continues until the PLC is taken out of the RUN mode. Now that you have learned a bit more on what a PLC is, how it works, and what they can do for you, why not take a look at some of the other topics within the getting started series for our CLICK PLC? We'll show in quick step-by-step sessions how to set up programs and test simple PLC programs to help get you off the ground. Until next time, thank you for watching this tutorial from Learn.AutomationDirect.com Have a great day.