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EEEN20030 Control Systems I William Heath Revised January 2012

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EEEN20030 Control Systems I William Heath Revised January 2012 Contents Sections marked * are not examinable 1 Introduction 6 1.1 Feedbacksystems........................... 6 1.2 Historyoffeedbackcontrollers . 6 1.3 Example:automotivecontrol . 8 1.4 Scopeofthecourse.......................... 9 1.5 Limitationsofthecourse....................... 9 1.6 Furtherreading............................ 10 2 Feedback 12 2.1 Negativefeedbackamplifier . 12 2.2 Standardcontrolloop ........................ 12 2.3 Standardcontrolloopwithoutdynamics . 13 2.4 Stability and dynamics . 14 2.5 Casestudy:edibleoilrefining. 15 2.6 Keypoints .............................. 18 3 Differential equations 19 3.1 Lineardifferentialequations . 19 3.2 Example: electrical circuit . 20 3.3 Example:mechanicalsystem . 20 3.4 Example:flowsystem ........................ 21 3.5 Discussion............................... 22 3.6 Keypoint ............................... 23 4 Laplace transforms and transfer functions 24 4.1 Laplacetransformdefinition. 24 4.2 Laplacetransformexamples . 24 4.2.1 Unitstep ........................... 24 4.2.2 Exponentialfunction. 25 4.2.3 Sinusoid............................ 25 4.2.4 Dirac delta function . 26 4.3 Laplacetransformproperties . 26 4.3.1 Linearity ........................... 26 4.3.2 Differentiation . 26 4.3.3 Integration .......................... 27 4.3.4 Delay ............................. 27 4.3.5 Finalvaluetheorem ..................... 28 4.4 Transferfunctions .......................... 28 4.4.1 Definitions .......................... 28 4.4.2 Simulation . 29 4.4.3 Block diagrams and closed-loop response . 30 4.4.4 Sensitivity and complementary sensitivity . 32 4.5 Bigpicture .............................. 34 5 Step responses 35 5.1 Steadystategain........................... 35 5.2 Firstordersystems.......................... 36 5.2.1 First order system with delay . 39 5.3 Secondordersystems. .. .. .. .. .. .. .. .. .. .. 39 5.3.1 Thepolesareseparateandreal . 40 5.3.2 The poles are identical and real . 41 5.3.3 Thepolesarecomplex . .. .. .. .. .. .. .. .. 41 5.3.4 Time domain step response specifications . 44 5.3.5 Secondordertransferfunctions with azero . 46 5.4 Keypoints .............................. 47 6 Frequency response analysis 48 6.1 Firstordersystem .......................... 49 6.2 Bodeplot ............................... 50 6.3 Closed-loopresponse ......................... 52 6.4 Firstorderelements ......................... 54 6.5 Secondordersystems. .. .. .. .. .. .. .. .. .. .. 56 6.5.1 Separaterealpoles .. .. .. .. .. .. .. .. .. 56 6.5.2 Identicalrealpoles .. .. .. .. .. .. .. .. .. 57 6.5.3 Complexconjugatepoles . 58 6.6 *Derivationofthemainresult. 60 6.7 Keypoints .............................. 61 7 Review of analysis tools: first order response to a sine wave 62 7.1 Time domain analysis . 62 7.2 Laplace domain analysis . 63 7.3 Frequencydomainanalysis . 64 8 PID Control 65 8.1 Proportionalcontrol . .. .. .. .. .. .. .. .. .. .. 65 8.1.1 Stablefirstorderplant. 65 8.1.2 Unstablefirstorderplant . 66 8.1.3 Stablesecondorderplant(withnozero) . 67 8.1.4 Higher order plants and plants with delay . 68 8.2 IntegralactionandPIcontrol . 68 8.3 DerivativeactionandPDcontrol . 71 8.3.1 PD and PDγ control..................... 71 8.3.2 Frequencyresponse. 72 8.3.3 Secondorderplant ...................... 72 8.4 PID control and Ziegler-Nichols tuning . 74 8.5 PIDimplementation ......................... 76 8.6 Set-pointtracking .......................... 77 8.7 Anti-windup.............................. 77 8.8 Internalmodelprinciple . 78 8.9 Keypoints .............................. 81 9 Root locus analysis 82 9.1 Therootlocusplot.......................... 82 9.2 Rootlocusexamples ......................... 84 9.3 Gridlines ............................... 92 9.4 Keypoints .............................. 92 10 The Nyquist criterion 93 10.1Introduction.............................. 93 10.2TheNyquistplot ........................... 93 10.3 The 1 point ............................ 98 10.4 TheNyquistcriterion− ........................ 98 10.5 Stability margins . 103 10.6*Gridlines ..............................105 10.7*Nicholscharts ............................ 107 10.8 *Justification of the Nyquist criterion . 108 10.9Keypoints .............................. 111 11 Design of compensators 112 11.1 Design considerations and constraints . 112 11.2 Phaseleaddesign........................... 113 11.3Phaselagdesign ........................... 118 11.4 Lead-lagcompensators. 120 11.5Notchfilters.............................. 121 11.6Keypoints .............................. 124 12 Case studies 125 CaseStudy1:Op-amps .. .. .. .. .. .. .. .. .. .. .. 125 CaseStudy2:Excitationsystems . 130 13 Worked examples 136 Problem1..................................136 Problem2..................................138 Problem3..................................139 Problem4..................................140 Problem5..................................141 Problem6..................................143 Problem7..................................144 Problem8..................................145 14 Worked example solutions 146 Problem1..................................146 Problem2..................................150 Problem3..................................152 Problem4..................................154 Problem5..................................156 Problem6..................................158 Problem7..................................161 Problem8..................................163 Tutorial 1 164 Question1.1................................. 164 Question1.2................................. 165 Tutorial 2 166 Question2.1................................. 166 Question2.2................................. 166 Question2.3................................. 167 Tutorial 3 168 Question3.1................................. 168 Question3.2................................. 169 Question3.3................................. 169 Tutorial 4 170 Question4.1................................. 170 Question4.2................................. 170 Question4.3................................. 171 1 INTRODUCTION 6 1 Introduction 1.1 Feedback systems Feedback mechanisms are ubiquitous in both nature and man-made systems. We are concerned with the use of negative feedback to control or regulate various system attributes to desired levels. There are many examples in biology. In the eye the iris opens and closes to control the amount of light that falls on the retina. In bright light it closes while in dim light it opens. Similarly the form of the lens is modified using feedback to ensure correct focus of an object as its distance varies (so-called accommodation). The ability of biological systems to maintain attributes at a desired even level is known as “homeostasis.” Almost all mammals are “warm- blooded”—that is to say they regulate their blood temperature. There are also many examples at the level of cell biology. For example, most enzyme production is regulated by negative feedback. Too much of a product acts as an inhibitor on an earlier stage of production. Negative feedback operates on a macro scale in ecological systems. For exam- ple feedback mechanisms maintain the proportion of oxygen in the atmosphere near 21%, and the proportion of carbon dioxide near 0.03%. This phenomenon gives rise to the “Gaia hypothesis” that the earth is alive. Mankind is currently testing both this hypothesis and the feedback mechanisms to their limit. Some control mechanisms in nature are highly sophisticated. Consider a tennis player. He/she has highly developed actuators (the tennis racquet, limbs and muscles) and highly developed sensors (vision and feel). But to be a good player, he/she also needs considerable skill and finesse to co-ordinate the sen- sors and actuators into a winning combination. Nevertheless feedback is crucial. When catching a high ball, sportsmen/sportswomen do not calculate the tra- jectory of the ball. Rather they use feedback heuristics such as the following: “fix your gaze on the ball, start running, and adjust your running speed so that the angle of the gaze remains constant.”∗ 1.2 History of feedback controllers In this course we are concerned with the design of feedback controllers for engi- neering systems. Some common examples are extremely old. For example the ballcock in a lavatory prevents the cistern from overflowing. Note that as this involves a switching mechanism, it beyond the scope of this course. Watt’s governor (or the centrifugal governor) was one of the catalysts for the industrial revolution (Fig 1). When used as a governor for a steam engine, its function can be described heuristically as follows: as the flyballs spin faster, they move out and open a valve which releases steam; this in turn slows the speed at which the flyballs spin; similarly as the speed drops the flyballs move in, and close the valve. There is an interesting example of such a governor at Quarry Bank Mill in Styal, where the governor is used to regulate the amount ∗G. Gigerenzer: Gut Feelings, Allen Lane, 2007. 1 INTRODUCTION 7 of water falling onto a mill wheel. Until very recently the same principle was used for the control of Diesel engines. flyball pivot valve Figure 1: Watt’s governor. The invention of the feedback amplifier in 1927 ushered in long distance telecommunications. The feedback amplifier has stable linear characteristics despite uncertainties and nonlinearities in the high gain amplifier (originally a vacuum tube amplifier) around which it is built. Traditionally control has been divided into
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