Scratching the itch: Feedback loops

At the heart of every controls system is a feedback loop. An event occurs, it is measured, there is a response and then the disturbances are measured again; continue until “controlled.” Without thinking, on a daily basis we engage with feedback loops; someone says something to you, you respond. If your answer clarifies things you move closer to a “closed loop,” if not, there is an error detected and you and your conversational partner resolve the difference.

Within Simulink models the closing of a loop is realized using a state variable; often a unit delay that provides data from the last pass to the controller.

Being a controls engineer requires two things, to understand physically what a controls loop is and programmatically how it is implemented. Today I will give examples of the 3 everyday ways in which we engage in feedback loops…

Cruise control

As you drive along the road, you see the cars in front of you. As you get closer you either slow down or switch lanes to pass. In this example your speed is the controlled variable in response to the distance between you and the other cars.

AC system

In the heat of the summer you may open up your home at night to let the cool evening air in; both freshening up your dwelling and bringing the temperature down. As the sun rises you close up the window and draw the blinds to keep the heat from making your home too hot. Opening and closing blinds and shades; this is your response to the heating input.

Scratching and Itch

There is nothing quite like that feeling when you hit the spot that itches, you scratch for a few moments and the issue goes away. However, this is a good example of a potential problem with un-dampened feedback; if you don’t stop you end up doing more harm than good.

Now it is your chance. Drop me a line in the feedback section to help me “tune” this blog.

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