TEL421 — Control Engineering: Lecture Note

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This is the complete lecture note for TEL421 Control Engineering. You can read it online below, open it in the full viewer, or download the PDF for offline study. It pairs with the course's video lectures, so you can read and watch together.

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Course code: TEL 421 Course title: Control Engineering Course credit load: 2 Units Series 1: Feedback Control Concepts and System Classification • Part 1.1: The Concept of Feedback Control o Sub Part 1.1.1: Open-Loop and Closed-Loop Systems Revisited o Sub Part 1.1.2: The Feedback Concept o Sub Part 1.1.3: Advantages of Feedback Control • Part 1.2: Classification of Control Systems o Sub Part 1.2.1: Linear and Nonlinear Systems o Sub Part 1.2.2: Time-Invariant and Time-Varying Systems o Sub Part 1.2.3: Continuous-Time and Discrete-Time Systems • Part 1.3: Control System Structures o Sub Part 1.3.1: Single-Loop and Multi-Loop Structures o Sub Part 1.3.2: Cascade Control Structures o Sub Part 1.3.3: Feedforward and Combined Structures • Part 1.4: Process Control Fundamentals o Sub Part 1.4.1: The Process Control Loop o Sub Part 1.4.2: Process Variables and Control Objectives o Sub Part 1.4.3: Communicating Control System Concepts Introduction This is the opening Series of TEL 421: Control Engineering. Feedback is the single organising idea behind almost every practical control system in industrial and process engineering, from a simple household thermostat to a large chemical plant's temperature regulation loop, and before

any specific controller design, transfer function, or stability criterion examined later in this course can be properly understood, a learner needs a firm grasp of what feedback actually is, why it is used so widely, how control systems are classified, and how they are structured. The aim of this Series is to equip you with the feedback concept and its advantages, the classification of control systems, common control system structures, and the fundamentals of process control, forming the conceptual foundation on which the components, transient analysis, controller design, and stability analysis examined in the Series that follow are all built. We shall commence this Series by examining the concept of feedback control. Next, we will consider the classification of control systems. Following that, we will examine control system structures. Finally, we will conclude by examining process control fundamentals. Series Learning Outcomes Upon completing this Series, you should be able to: • SLO 1.1: explain the feedback concept and distinguish open-loop from closed-loop control, • SLO 1.2: explain the advantages of feedback control, • SLO 1.3: classify control systems as linear/nonlinear and time-invariant/time-varying, • SLO 1.4: classify control systems as continuous-time or discrete-time, • SLO 1.5: describe single-loop and multi-loop control structures, • SLO 1.6: describe cascade and feedforward control structures, • SLO 1.7: describe the process control loop and process variables, • SLO 1.8: communicate control system concepts effectively. 1.1 The Concept of Feedback Control 1.1.1 Open-Loop and Closed-Loop Systems Revisited An open-loop system applies a control action based purely on a predetermined input or programmed sequence, with no measurement of the actual output and therefore no way of correcting for any deviation from the intended result, making it simple and inexpensive but inherently vulnerable to disturbances, load changes, and inaccuracies in the underlying process model, since nothing in an open-loop arrangement can ever detect, let alone correct, an error once it has occurred. A closed-loop system, by contrast, continuously measures the actual

output, compares it against a desired reference value, and adjusts the control action based on the resulting error signal, a mechanism that allows the system to actively compensate for disturbances, load changes, and modelling inaccuracies in a way no open-loop arrangement, however carefully designed, can ever achieve. Almost every practical control engineering application examined throughout this course, from the process control loop introduced later in this Series to the servo-mechanisms examined in Series 3, is implemented as a closed-loop system precisely because of this self-correcting capability, and the specific mechanism by which a closed-loop system continuously measures and corrects for error, examined in detail in the very next sub-Part of this Series, is what engineers formally call feedback, the single concept from which this entire Series, and much of this course, takes its name. Worked example: A simple timer-controlled sprinkler that waters a lawn for fifteen minutes regardless of actual soil moisture is open-loop, while a soil-moisture-sensing irrigation system that measures actual moisture and adjusts watering duration accordingly is closed-loop, correcting for whatever rainfall or evaporation has already occurred. Application: Correctly identifying whether a given industrial control problem calls for an open-loop or closed-loop solution is the very first analytical judgement a control engineer makes, directly shaping every design decision examined throughout the rest of this course. Fill in the blank: A closed-loop system continuously measures actual output and compares it against a ________ value. 1.1.2 The Feedback Concept Feedback is the process by which a portion of a system's output is measured, returned to the input side of the system, and used to compute an error signal, the difference between the desired reference value and the actual measured output, that then drives the controller's subsequent control action, a continuous measure-compare-correct cycle that repeats for as long as the control system remains in operation. This is more precisely called negative feedback, since the error signal is computed by subtracting the measured output from the reference, driving the system's control action in the direction that reduces the error toward zero, in contrast to positive feedback, which instead reinforces and amplifies any deviation rather than correcting it, an arrangement that is generally destabilising and is deliberately avoided in the vast majority of practical control engineering applications examined throughout this course, negative feedback being overwhelmingly the norm. Worked example: In a room-temperature control system, the thermostat measures actual room temperature, subtracts it from the desired setpoint to compute an error signal, and increases heating output when the error is positive (room too cold) and reduces it as the error approaches zero, a textbook illustration of the negative feedback cycle.

Application: Understanding the measure-compare-correct feedback cycle is the essential conceptual foundation for every transfer function, block diagram, and stability analysis technique examined throughout the remainder of this course. Fill in the blank: Negative feedback drives a system's control action in the direction that reduces the ________ toward zero. 1.1.3 Advantages of Feedback Control Feedback control offers several decisive advantages over open-loop control: disturbance rejection, the ability to automatically compensate for unpredictable external influences, such as a sudden load change or an ambient temperature shift, that an open-loop system, examined earlier in this Part, has no means of detecting or correcting for at all; and reduced sensitivity to model inaccuracy, since a feedback system corrects based on the actual measured output rather than relying entirely on how accurately the plant's behaviour was originally modelled or predicted at the design stage. Additional advantages include improved accuracy, since persistent errors are continuously detected and driven toward zero rather than being allowed to accumulate unchecked, and the practical ability to control genuinely unstable processes that would be entirely unusable in open-loop form, at the cost of additional complexity, sensor cost, and the introduction of potential stability concerns, examined in detail in Series 5, that simply do not arise in an open-loop arrangement lacking any feedback path at all. Worked example: A feedback-controlled industrial oven automatically compensates for a door being briefly opened, letting heat escape, by increasing heater output the moment temperature is measured to have dropped, a correction an open-loop, timer-based oven control would have no way of making. Application: Weighing feedback's advantages, disturbance rejection, model-error tolerance, and improved accuracy, against its added complexity and potential stability cost is a judgement made explicitly at the very start of nearly every real control system design project. Fill in the blank: Feedback control reduces sensitivity to ________ inaccuracy since it corrects based on actual measured output. Short description: A block diagram showing a closed-loop feedback control system, with a reference input, a summing junction computing the error signal, a controller block, a plant block, and a feedback path returning the measured output to the summing junction.

Figure 1.1: A standard closed-loop feedback control block diagram. Pilot questions 1.1 1. Distinguish open-loop from closed-loop control. (Linked to SLO 1.1) 2. Define feedback. (Linked to SLO 1.1) 3. Distinguish negative from positive feedback. (Linked to SLO 1.1) 4. Name two advantages of feedback control. (Linked to SLO 1.2) 5. Explain why feedback reduces sensitivity to model inaccuracy. (Linked to SLO 1.2) 1.2 Classification of Control Systems 1.2.1 Linear and Nonlinear Systems A linear system is one whose output responds proportionally to its input, obeying the principles of superposition (the response to a sum of inputs equals the sum of the individual responses) and homogeneity (scaling an input scales the output by the same factor), properties that make a linear system's behaviour highly predictable and, critically, allow it to be analysed using the powerful transfer function and signal flow graph techniques examined in detail in Series 4 of this course.

A nonlinear system violates superposition or homogeneity, exhibiting behaviour such as saturation (output cannot exceed a physical maximum regardless of how large the input becomes), dead zone (no output response below a certain input threshold), or hysteresis (output depends on the input's recent history, not merely its present value), and while many

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