If you’re new to the fascinating world of programmable logic controllers, you might have heard about the different control systems found within them: open loop and closed loop. So what’s the difference between them?
You already know that PLCs are crucial to the success of industrial processes, and are found across industries, factories and warehouses. They’re industrial computers that monitor input devices, making decisions based on a technician’s programming. But the systems that are designed within the PLC for it to function properly can be a mystery, especially if you’re just starting out in the field.
In this article, we’ll lay out the differences between these two systems and we’ll also talk about the definition of a control system and what it does. If you’re interested in getting started with PLCs, then this article is for you.
What Is a Control System?
Let’s get a refresher on what defines a control system. A control system manages, commands and maintains a device using control loops. They ensure the desired outcomes within a facility, such as a factory floor, are successfully achieved. Control loops use programmable software so that they can oversee the industrial or manufacturing processes they’re designed for.
To make things easier, think about control loops as a management tool that oversees a process variable. But what’s a process variable? A process variable is a measurable value, like a temperature setting for example, that requires monitoring. A control loop will measure the variable, monitor it, and control it so that it maintains what’s called a “set point.” A set point is really just another way to say “goal.”
So if you have a thermostat that needs to have a set point (desired temperature) of 22° C, the control loop works to make sure that this outcome is achieved.
The example used above is for temperature control, but control loops are found all over, in both domestic and industrial settings. With our world becoming more and more automated, the need for advanced control systems has increased. These control systems are found in toilet tanks, cars, assembly lines and, of course, PLCs.
So What’s the Difference Between Open Loop and Closed Loop?
When it comes to the industrial process, there are generally two types of control loop systems used: the open loop and the closed loop.
Open Loop
In an open loop system, the action is not dependent upon the output system. There is no feedback from the output to the input, meaning nothing is getting monitored. Imagine an open loop system as a straight line running from left to right. On the far left, you’ll see the input, which receives a signal. From there, you will see a controller, which receives an actuating signal from the input (or direction that can be put into action.) From there, on the far right, you will see the output. The output takes the signal and the action and executes.
Another way to describe this is by using a simple example of a clothes dryer. The desired time spent drying the clothes is set (input). The timer (the controller) takes this action and applies it to the output (the heating elements). The dryer will complete once the desired outcome is achieved, no more and no less. The dryer will not sense that the clothes are still wet and continue drying, nor will it sense if the clothes have been dried early.
Open loop systems are advantageous because they’re simple to construct, they’re easy to maintain, and they’re inexpensive. In terms of PLCs, open loop systems can be great for applications that need to be manually operated and with there’s a direct connection between the input and the output. However, open loop systems cannot engage in machine learning. They’re not able to correct errors or account for external disruptions. They’re not as accurate as their counterparts and they can be unreliable.
Closed Loop
A closed loop system, on the other hand, adjusts its input quality based on the output. The action it participates in depends on feedback from the output system and can adjust based on this information so that the set point is consistent.
If we view the system again from left to right, you’d see the input on the left hand side, the controller, and the output. But it doesn’t stop there. The output is connected to a feedback sensor that reaches back to the input as well as an error detector—thus creating a closed loop system.
Think about a conveyor belt in a factory. It might seem like a straightforward open loop system, but what if something became lodged in the gears or the item it was supposed to transport fell off? A closed loop system could be designed to sense disturbances and halt a machine’s processes.
Closed loop systems are also referred to as automatic or feedback control systems and they’re best used for tasks that require accuracy and customization. Both an open and closed loop system in a PLC will be programmed using ladder logic which is a programming language used within industrial control. You can learn how to use ladder logic by taking courses on PLCs and gaining hands-on experience with these industrial computers.
Open Loop vs. Closed Loop
Control systems are integral to manufacturing industries, automotive industries, robotics and automation. They’re fascinating and powerful tools that allow our world to run smoothly and provide us with basic necessities. The different types of control systems are worth understanding if you’re interested in getting involved with PLCs, as PLC Technicians have to have a clear understanding of how they work and when they should be used.
If you want to learn more, or you want to learn where you can gain hands-on experience with these systems, consider applying for a training course. George Brown College offers a PLC Technician Program that prepares you for the industry, giving you real-world experience.
