Feedback and Temperature Control

by Charles D. H. Williams

Contents

• Preface
• Introduction
• System Model
• Types of Feedback Control
  • On-Off Control
  • Proportional+Derivative Control
  • Proportional+Integral+Derivative Control
  • Proportional+Integral Control
  • Third-Order Systems
• Practical Matters
  • Varieties of PID Algorithms
  • Control Theory
  • Noise and the Frequency Domain
• Tuning a PID Temperature Controller
• Controller Circuit (and Self-Test Questions)
  • Answers to Self-Test Questions)
• Problems and Exercises
  • Hints
• Oven Controller Simulation
  • Quick Reference Guide to Parameters
  • Technical Details of the Simulation

Preface

This is an introduction to the effects that feedback can have on systems. I have chosen an oven controlled by a PID temperature controller to use as a case study but the behaviour described is characteristic of many systems that employ feedback. There is a detailed interactive simulation of the oven-controller system for you to experiment with. I'm interested in any comments - good or bad - about this document. In particular, do you find the hypertext and simulation a significant improvement on traditional textbook or lecture presentation? Also, please let me know if you spot errors or omissions, I'd like to fix them.

Introduction

It is important to have an intuitive feel for the ways that feedback can affect a system if you want to design analogue electronic circuits that work well. This document is designed to help you develop such intuition by using a model of a simple system to illustrate some of the principal points that need to be known about systems with feedback. The early sections summarise the behaviours encountered when different types of feedback are used to control the temperature of a simple model of an electric oven. Next some features of real controllers are explained and a simple manual procedure for tuning a PID controller is referred to. If you want to build your own controller there is a circuit diagram with some questions for self-assessment. Finally there is a remark about control theory, some problems, and an interactive simulation of the oven-controller system that can be used to check answers to the problems and get some hands-on experience of how such systems behave.

The original simulator was an Excel-4 workbook. It is not as accurate as the online version but, as many people have wanted their own copy of the simulator to experiment with off-line, I have decided to make it available here OVENVCTL.XLW.

This HTML document supports modules PHY3128, it should be studied in parallel with the handout which covers the same material in more mathematical detail.