# Feedback Principles

Feedback principles are the broad and fundamental concepts that underlay all systems (both human made and in nature) which rely on feedback for their proper operation.  While the specific application, models, parameters, and conditions of any one system/environment can dramatically alter the behavior of the feedback dependent system, the principles themselves are immutable.  Furthermore, while learning the mathematics needed to do most control design and control systems engineering is complicated, the feedback principles themselves are fairly intuitive and straightforward.

A feedback loop consists of something that is being controlled, usually a physical object or subsystem within a larger object or system.  Some aspect (or aspects) of the system are the signals being regulated and the first and most fundamental operation of feedback is the measurement of that signal.  (We cannot control what we cannot measure.)  The next piece of a feedback system is that this measurement is compared to some sort of desired or reference signal and -- at the simplest level -- a difference is computed.  That difference is used to drive an adjustment mechanism. The adjustment mechanism then creates a new signal which is used to act upon the system so as to adjust the behavior of the measured signal.

When we measure something and then use that measurement in order to adjust the behavior of that something, this is feedback.  It is a fundamental mechanism of nature and each of our bodies has hundreds if not thousands of feedback loops big and small.  Moreover people have been building devices that implement automatic feedback loops for millennia without realizing that they were connected by some  fundamental principles of operation.  It was the flyball governor of the late 1800s CE, that regulated the pressure of steam engines, thereby accelerating the industrial revolution.  These devices generated a lot of interest from famous scientists of the day (including James Clerk Maxwell -- he of the equations).  The early 1900s brought telephony -- communicating over wires -- that required both amplifiers and repeaters.  Those amplifiers behaved much better when they were run in a feedback loop.  World War II brought both both automated weapons (the V1 and V2 bombs built by the Nazis) and radar guided anti-aircraft guns (the M4 Gun Director built by the Americans).  Slowly, engineers working in these different areas started realizing that they were speaking different dialects of the same language and the field of automatic control -- where humans teach machines to use feedback to improve their operation -- was born.

So, feedback relies on:

• Measurement of the thing we wish to adjust
• A comparison with some reference level of that thing
• A decision mechanism for

And these adjustments have to be made in a "Goldilocks" way:  not too much and not too little.  And they have to be made not too late. (No point speeding up your pace through the airport if the plane has already taken off.)

These are the fundamental principles of every feedback system, be it small or large, made by humans or occurring in nature.

This topic includes the following resources and journeys:

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## Introducing Feedback Control to Middle and High School STEM Students, Part 2...

60 min
Intermediate
Article / Blog
Demonstration

This paper was presented at the 2019 IFAC Advances on Control Education Conference (IFAC-ACE), Philadelphia, PA, USA, July 7-9, 2019, and is in the conference proceedings. This paper aims at...

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## Introducing Feedback Control to Middle and High School STEM Students, Part 1...

60 min
Intermediate
Article / Blog
Demonstration

This paper was presented at the 2019 IFAC Advances on Control Education Conference (IFAC-ACE), Philadelphia, PA, USA, July 7-9, 2019, and is in the conference proceedings. This paper aims at...

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## What's a Control System and Why Should I Care? A whirlwind tour through the ...

60 min
Beginner
Article / Blog
Theory

This paper aims to provide some introduction, a cheat sheet, and some context for college level STEM students about to take that first controls class. In some cases, it provides context...

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## Understanding Control Systems: Open-Loop Control Systems

6 min
Beginner
Video
Theory

This video explores open-loop control systems by walking through some introductory examples.

Learn how open-loop systems are found in everyday appliances like toasters or showers, and...

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## Understanding Closed-Loop Control

9 min
Beginner
Video
Demonstration

This lecture discusses the differences between open loop and closed loop control in a very easy and intuitive way. Daily examples, like a dishwasher, plant irrigation, and car speed systems...

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## Understanding Control Systems: The Disturbance Rejection Problem

6 min
Beginner
Video
Demonstration

This video provides a demonstration using a car to show how you can simulate open- and closed-loop systems in Simulink®.

First, you will learn how to model and tune open-loop systems. The...

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## Understanding Control Systems: Components of a Feedback Control System

5 min
Beginner
Video
Demonstration

This video introduces the components of a feedback control system and how they interact with each other.

Learn basic terminology by walking through examples that include driving a car...

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## Resonance in Nature and Bioinspired Squid Robots

18 min
Beginner
Video
Demonstration

Bioinspired robots combine the best of both biology and engineering. Using tools learned from nature, we can build squid-like and other squishy robots to improve current capabilities in...

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## Understanding Control Systems: Feedback Control Systems

6 min
Beginner
Video
Theory

This video provides introductory examples to learn about the basics of feedback control (closed-loop control) systems.

Learn how feedback control is used to automate processes and...

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## Virtual Lab for a Two-tanks system

Beginner
Virtual Lab
Application

This is a virtual lab for a two-tank system that can be used for modelling and control learing/teaching purposes. Open-loop tests and closed-loop simulatons based on PI control or PI plus...

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## TCLab: An Inexpensive Experimental Platform for Students to Learn Feedback

5 min
Beginner
Video
Demonstration

The temperature control lab (TCLab) reinforces process feedback control  with real data. The TCLab hardware consists of an Arduino® shield that fits onto a standard Arduino Leonardo...

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## What Is a Con­trol Sys­tem and Why Should I Care? (Part 2)

34 min
Beginner
Video
Demonstration

This second video introduces some of the methods that engineers use to build control systems.  It shows how we use science to help us derive models of systems from both measurements and...

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## What Is a Control System and Why Should I Care?

25 min
Beginner
Video
Demonstration

This is a 25 minute abbreviated version of the Part 1 & Part 2 talk.  It goes through the basic ideas while skipping some of the details and examples of the longer talks.   The talk abstract...

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## What Is a Control System and Why Should I Care? (Part 1)

38 min
Beginner
Video
Demonstration

This video is the first of a two part series.  It in introduces the basic concepts of feedback with lots of visual examples.  The idea is to show that not only are these principles...

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## Feedback in the Squishy Stuff: Introducing Chemical and Biological Process ...

29 min
Beginner
Video
Demonstration

Feedback doesn’t just happen in robots and self-driving cars, but in the regulation of our very bodies and in the ways in which we transform raw ingredients into materials we can use, be...

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## What Is a Control System and Why Should I Care? (Part 2)

34 min
Beginner
Video
Theory

This talk gives a glimpse of some of the methods and math that allow us to understand feedback systems. Continuing on from Part 1, it gives a description of how we use scientific principles...

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## What Is a Control System and Why Should I Care? (Part 1)

38 min
Beginner
Video
Application

This talk introduces the basic concepts of feedback with lots of visual examples.

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