Introduction To Open And Closed Loop Control Systems

Everybody in the world will at some point have used a control system, perhaps without realising. Here Geraint Thomas expands on the difference between open and closed loop systems.

We all use control systems in one form or another. It could be the switch on the electric heater warming up your home, or it could be when you start your car up in the morning to leave for work. Let’s look in more detail at what a control system is.

A typical control system comprises input and output stages, where an input signal is given and a process happens; an action and a subsequent reaction. Looking again at the electric heater, this is a good example of a simple open loop control system. The input decision here involves switching the switch on or off.  When the switch is on electricity is transmitted to heat the element. This is the ‘process’ and the ‘output’ is the room being heated up.

So from this simple example we have an input, a process, and an output. These are the fundamentals of control systems.

Looking at the electric heater it is a very simple open loop system. The term ‘open loop’ means there is no feedback controlling the temperature of the heater. It will continue to get hotter with no adjustment, up to its maximum level, allowing you to keep the temperature at a set point. The open loop system is a simple design, the accuracy dependent on the calibration of the devices within the process. The result is that the temperature cannot be regulated and will continue to rise.

How can we make this control system better at regulating the temperature?

By introducing a closed loop system, allowing the overall system to be more accurate. Now, when the room is hot enough it will recognise this, switch off the heat source and then become a more stable system overall.

To make this a closed loop system there are a few changes that need to be made. Chief among them is the need to introduce a feedback loop, which will ‘report back’ to the process section. In the case of the heater, this will likely be a thermometer. This will give us a reading of the temperature of the room so the system knows if it has reached the right level.

When adding a feedback loop to the system, a change to the input will be given. Instead of having a switch that operates the system to turn only on or off, now a desired output set point will be needed. This, coupled with a summing block, will allow the error to be calculated (the difference in the two temperature heater and room) then find the difference and change the process to adjust for the correct reading.

Within the automotive industry, manufacturing or any operation that has an input, a process and an output, the closed loop system can be developed to provide more precise and accurate control systems. The way that this is achieved is through the use of a controller, such as a PLC. The controller acts directly on the error signal. With the insertion of the controller we change the summing block into a multiplying block. Thus, by manipulating the error signal we can change the system’s dynamic behaviour.

These types of system are known as proportional, integral and differential (PID). To look into these three types of control we can again use driving to work as an example.

When driving your car you will typically use proportional control to help reach the speed you wish to travel at. This happens when you notice the speed you are going at is not the correct speed needed, so you push the accelerator pedal down or, alternatively, ease off if your speed is too high.

Derivative control is when you know that you will need to change speed for instance, this is best described when you see a steep hill in front of you and push down on the accelerator pedal to maintain speed up the hill. When we have large changes in speed the proportional control makes these corrections. When we need to make a sudden change in speed derivative control does this for you.

Integral control, on the other hand, is constantly adding up the error from the speed you are at currently and providing feedback proportional to the speed you want to achieve.

When used in conjunction with one another, we achieve cruise control. This uses proportional control to keep car set to the correct speed, the derivative control to make sure the car reacts and the integral control to constantly monitor the difference

PID loops are used in all types of control system, from the new electric car produced by Tesla, to refrigerator systems in supermarkets where temperatures need to stay constant.

So, when you drive home tonight, you will yourself be a form of closed loop control system. And, when you put your heater on during the cold winter nights you’ll understand a bit more about the differences between open and closed loop systems.