Control Engineering

Introduction What we will discuss in this introduction: – What is control engineering? – What are the main types of control systems? – How to learn control engineering?

What is Control Engineering Control engineering or automatic control engineering is theory and engineering aiming to create the best performance of control systems. A control system consists of electrical, mechanical, optical components and computers, with which we can handle the issues such as how to make robot joints positioned accurately, how to guide a missile flying towards a target, how to get a power system operated stably under disturbances, how to open and close train doors, how to start and stop a motor, etc..

What are the main types of control systems? There are two types of control systems, continuous time control systems and on-off control systems. The continuous time control systems are featured by control signals changing continuously with time; while control signals of the on-off systems change between an ON state to an OFF state. These two types of systems are widely used in engineering and daily life nowadays, some examples are given in the next slide

Main types of control systems

Continuous time Control System Open loop and Closed Loop System

Example: Automatic Control of a Steam Turbine Generator

Stability problems of system turbine generator system Disturbances like load variation or short-circuits sometime occur, which result in instability in generator rotor angle, voltage and frequency. For example a short circuit leading to a line isolation by circuit breakers will cause variations in the power flows, network power voltages and generator’s rotor speeds. The voltage variations will actuate voltage regulators in the system. Generator speed variations will actuate the prime mover governors. The Voltage and frequency variations will affect the system loads. An unstable condition will lead to cascade outages and a shut down of a major portion of the power system. We call the above phenomena as three stability problems of power systems, namely rotor angle stability, voltage stability and frequency stability. 1.Rotor angle stability 2.Voltage stability 3.Frequency stability

How to study continuous time control systems Normally control engineering utilizes mathematical models to represent a physical control system described with parameters like mass, a frictional coefficient, elasticity and electrical quantities, and then to investigate the system performance. Control engineering is broken down into two categories: analysis and synthesis. –Analysis refers to understanding all the aspects of an available control system, such as description, analysis, evaluation and performance improvement of the system. –Synthesis means to design a control system with control principles.

A general procedure to perform the control system analysis Step 1: Separating a control system (sub-system) from the entire engineering system. It is normally called a physical model of a control system. Step 2: Creating a schematic model to represent the control system. The inherent parameters are used for description of the system. These parameters may include mechanical ones like mass, inertia, transmission ratio, damping (or friction), elasticity, and electric ones like resistance, capacitance, inductance, etc. Step 3: Establishing a set of mathematical models to represent the schematic model. The mathematical models have to be differential equations that are established by the physical laws in the first place, and then we can create other types of mathematical models by some techniques in order to solve problems easier than by the differential equations. These models include transfer functions, signal flow graphs, Bode-plots, etc. By establishing mathematical models of a system, people can investigate the effect of the system configuration and parameters on the characteristics and performance of the system.

A general procedure to perform the control system analysis (Contd) Step 4: Characteristics and performance: system characteristics and performance are observed in the system response, i.e., the system output, to the input, normally a unit step signal. The system performance are described as follows, –Performance 1. the system response should be stable – but not oscillatory infinitely, or even divergent. –Performance 2. the transient time of the system response should not be too long, in other words, the system should response to the input fast. –Performance 3. The steady state error of the system response should be within a permitted limit. Step 5: Analysis of effect of the system configuration and parameters on characteristics and performance by the mathematical models. Step 6: Improvement of characteristics and performance of the system with a controller like a PID controller. PID is the abbreviation for Proportional, Integral and Directive.