What is a control system?

Some people can get a bit confused by the “control system” term. A quick search on the internet yields a lot about proportional-integral-derivative (PID) controllers and other similar devices. It makes sense in a way, as they are the most widely used in control theory.

However, PID is a method used to control a system, not the system itself. I just wanted to clarify that before we dig into the definition and types of control systems.

Let’s start with the definition! A control system is a number of mechanical and/or electronic devices which regulate, manage, and command other devices or systems using control loops.

This definition makes sense if you know what a control loop is. But don’t worry. We’re here to clarify everything for you, so let’s define a control loop.

A control loop consists of all the physical components and control functions needed to adjust or maintain the value of a measured process variable (PV) to your desired value, or setpoint (SP). To do this, your loop needs at least three basic elements – an input, a controller, and an output. Make sense?

Now let’s review the two types of control loop, open and closed. We’ve put out some rather technical definitions so far – not very Visaya-like, is it? So now we’ll use some analogies to make it easier to understand open-loop and closed-loop control systems.

Open-loop system

As the simplest type, an open-loop system doesn’t measure variables nor send output values to the controller.

Take your toaster, for instance. When you put a slice of bread in the toaster, you set a number, your input, which represents how long you want your slice to heat. The toast, your output, could come out barely browned to very crunchy, depending on the number you chose. Note here that no sensor measures the color of your bread or its crunchiness, so you don’t know the condition of your slice until the toaster finishes. If you set your number too high, your toast will burn.

image of control system open loop
Image courtesy of polytechnichub.com

Another common open-loop system a bit more complex than a toaster is your washing machine. When you set a washing mode and turn the machine on, it calculates the time to run, based on the weight of the clothes inside. It also controls a number of devices: the motor to rotate the drum, pumps or valves to add water or detergent, and so on. However, nothing measures whether your clothes got cleaned or just wet.

Closed-loop system

Now that we know how an open-loop control system works, we’ll move on to the closed-loop system. The main difference between the two systems is the feedback.

In the closed-loop system, you have a sensor to constantly monitor your output. This sensor feeds data back to the controller. With this data, the controller compares the output with your setpoint. If this comparison doesn’t match, the controller analyzes this error, using one of the control methods. Then it sends a signal to an actuator to change your process value and bring it closer to your setpoint.

image of control system close loop
Image courtesy of polytechnichub.com

If you drive, then you’ve used this system. Imagine you want to go from point A to point B as fast as the law allows. There is a 60 mile-per-hour speed limit on your route, so you want to maintain the speed of the car at 60.

Here, your eyes (the sensor) monitor the car’s velocity on the speedometer (the output). If you haven’t reached your setpoint, your brain (the controller) will tell you to press the gas pedal (the final control element) further down with your foot (the actuator). When you reach 60, you’ll set your foot in a position to maintain that speed.

It all works out great if you encounter no disturbances, such as a hill. If you go up a hill, your speed will decrease unless you change the position of your foot.

As soon as your speed decreases, your brain analyzes this error and adjusts your foot. However, if you crest the hill without readjusting your foot, you may wind up speeding, which is also an error. You then monitor your speed and adjust yet again. This process repeats itself every time you have a disturbance in your system.

Process control system

Even though we defined a control system in the beginning, we talked about the loops and not the system. In the industry, you’ll see mostly closed-loop control systems, since most processes need to stay at the desired values.

A closed-loop control system is the whole package of equipment and logic behind the process, including the software and hardware needed to control all your process values:

  • Field instruments to monitor your PVs and send data to your controller.
  • A controller to connect all the inputs and outputs of your system.
  • Programming and coding to analyze and store the data, then send signals to final control elements.
  • A human-machine interface (HMI) to display data from the field in a way people can understand.
  • Actuators, devices which act to change your PV, such as pumps, valves, and heat systems.
  • Field communication, which includes all types of wiring and protocols (the standard “languages” in which the controller and field devices communicate).

Conclusion

We have control systems in devices as simple as a toaster or in industrial processes as complex as those in a petroleum refinery. The complexity of the system will depend on the tasks you need done.

For simple daily tasks, sometimes an open-loop control system will suffice. For more complex monitoring and automation, you should definitely go with closed-loop systems.

Regardless of the complexity of the system, a control system must perform tasks without the need for human intervention.

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