Lecture Notes: EEEC4340318 Instrumentation and Control Systems - Introduction to Control Systems

Course: Diploma in Electrical and Electronic Engineering

1. EEEC4340318 INSTRUMENTATION AND CONTROL SYSTEMS Introduction To Control Systems FACULTY OF ENGINEERING AND COMPUTER TECHNOLOGY DIPLOMA IN ELECTRICAL AND ELECTRONIC ENGINEERING Ravandran Muttiah BEng (Hons) MSc MIET

2. 1 Introduction To Control Systems Control engineering is based on the fundamentals of feedback theory and linear system analysis, and it generates the concepts of network theory and communication theory. Accordingly, control engineering is not limited to any engineering discipline but is applicable to aeronautical, chemical, mechanical, environmental, civil, and electrical engineering. A control system is an interconnection of components forming a system configuration that will provide a desired system response. The basis for analysis of a system is the foundation provided by linear system, which assumes a course effect relationship for the components of a system. A component or process to be controlled can be represented by a block as shown in figure 1.

3. 2 Control System Input Output Figure 1: C ontrol system

4. 3 Based on some parameters, we can classify the control systems into the following ways. Continuous Time and Discrete-Time Control Systems Control Systems can be classified as continuous time control systems and discrete time control systems based on the type of the signal used. In continuous time control systems, all the signals are continuous in time. But, in discrete time control systems, there exists one or more discrete time signals. Single Input Single Output (SISO) and Multiple Input Multiple Output (MIMO) Control Systems Control S ystems can be classified as SISO control systems and MIMO control systems based on the number of inputs and outputs present. SISO control systems have one input and one output. Whereas, MIMO control systems have more than one input and more than one output. Classification Of Control Systems

5. 4 Open Loop and Closed Loop Control Systems Control Systems can be classified as open loop control systems and closed loop control systems based on the feedback path. In open loop control systems, output is not fed-back to the input. So, the control action is independent of the desired output. Actuating Device Input Output Actuating Signal Figure 2: O pen loop control system In open loop control systems, an input is applied to a controller and it produces an actuating signal or controlling signal. This signal is given as an input to a plant or process which is to be controlled. So, the plant produces an output, which is controlled. Process

6. 5 In closed loop control systems, output is fed-back to the input. So, the control action is dependent on the desired output. Controll er Input Output Plant Actuating Signal Figure 3 : N egative feedback closed loop control system The error detector produces an error signal, which is the difference between the input and the feedback signal. This feedback signal is obtained from the block feedback elements by considering the output of the overall system as an input to this block. Instead of the direct input, the error signal is applied as an input to a controller. Error Signal Feedback Signal Feedback Elements

7. 6 The controller produces an actuating signal which controls the plant. In this combination, the output of the control system is adjusted automatically till we get the desired response. Hence, the closed loop control systems are also called the automatic control systems. Traffic light control system having sensor at the input is an example of a closed loop control system. Open Loop Control Systems Closed Loop Control Systems Control action is independent of the desired output Control action is dependent of the desired output Feedback path is not present Feedback path is present These are also called as non-feedback control systems These are also called feedback control systems Easy to design Difficult to design These are economical These are costlier Inaccurate Accurate Table 1: The difference between the open loop and the closed loop control systems

8. 7 Figure 4 shows a temperature control of an electric furnace. The temperature in the electric furnace is measured by a thermometer, which is an analogue device. The analogue temperature is converted to a digital temperature by an Analogue to Digital (A/D) converter. The digital temperature is fed to a controller through an interface. This digital temperature is compared with the programmed input temperature, and if there is any error, the controller sends out a signal to the heater, through an interface, amplifier and relay, to bring the furnace temperature to a desired value. Temperature Control Systems

9. 8 Analogue to Digital Converter Interface Relay Amplifier Interface Input Thermometer Figure 4: Temperature control system

10. 9 Control System Design Establish control g oals Identify the variables to control Write the specifications for the variables Establish the system configuration and identify the actuators Obtain a model of the process, the actuator, and the sensor Describe a controller and select key parameters to be adjusted Optimise the parameters and analyse the performance Figure 5 : Control system design process

11. 10 Figure 5 shows the control system design process. Variables to control are the quantities or conditions that are measured and controlled. Process is a natural, progressively continuing operation marked by a series of gradual changes that succeed one another in a relatively fixed way and lead toward certain result or end. A system is a combination of components that act together and perform a certain objective. Control Systems Design Process

12. 11 DC Amplifier Battery Actuator (DC Motor) Control Device (Amplifier) Process (Turntable) Turntable DC Motor Actual Speed Desired Speed Figure 6: Open loop control of speed of a turntable and a block diagram model

13. 12 DC Amplifier Battery Turntable DC Motor Figure 7 : Closed loop control of the speed of a turntable Tachometer

14. 13 Control Device Figure 8 : Closed loop control system for disc drive Actuator and Read Arm Sensor Actual Head Position Desired Head Position Error Design Example: Disk Drive Read System

15. 14 A hard disk uses round, flat disks called platters , coated on both sides with a special media material designed to store information in the form of magnetic patterns. The platters are mounted by cutting a hole in the center and stacking them onto a spindle . The platters rotate at high speed, driven by a special spindle motor connected to the spindle. Special electromagnetic read/write devices called heads are mounted onto sliders and used to either record information onto the disk or read information from it. The sliders are mounted onto arms , all of which are mechanically connected into a single assembly and positioned over the surface of the disk by a device called an actuator . A logic board controls the activity of the other components and communicates with the rest of the computer.

16. 15 Figure 9 : A hard disk

17. 16 Figure 10: Components of a hard disk

18. 17 Feedback Control Of A n Antiaircraft Gun Control System Gun Dynamics Figure 11: Feedback control of an antiaircraft system Demanded Azimuth (Elevation) Gun Azimuth (Elevation)

19. 18 Exercise 1 A precise optical signal source can control the output power level to within 1 %. A laser is controlled by an input current to yield the output power. A microprocessor controls the input current to the laser. The microprocessor compares the desired power level with a measured signal proportional to the laser power output obtained from a sensor. Draw the block diagram representing the closed loop control system. Micro-processor Laser (Process) Desired Power Output Output Power Error Current Figure 12: Microprocessor controls input current to laser + - Sensor (Measurement) Measured Power

20. 19 Exercise 2 Automated highways may be prevalent in the next decade. Consider two automated highway lanes merging into a single lane, and describe a control system that ensures that the vehicle merge with a prescribed gap between two vehicles Computer ( Controller) Active Vehicle Desired Gap Actual Gap Error Brakes Figure 13: A closed loop control system of an automated highway + - Steering Sensor (Radar) Measured Gap

21. 20 Problem 1 Many luxury automobiles have thermostatically controlled air-conditioning system for the comfort of the passengers. Sketch a block diagram of an air-conditioning system where the driver sets the desired interior temperature on a dashboard panel. Thermostat ( Controller) Automobile Cabin Desired Temperature Cabin Temperature Error Figure 14: Thermostatically controlled air-conditioning system + - Sensor (Measurement) Measured Gap

22. 21 Problem 2 The role of air traffic control systems is increasing as airplane traffic increases at busy airports. Engineers are developing air traffic control systems and collision avoidance systems using the Global Positioning System (GPS) navigation satellites. GPS allows each aircraft to know its position in the airspace landing corridor very precisely. Sketch a block diagram depicting how an air traffic controller might utilise GPS for aircraft collision avoidance. Autopilot ( Controller) Allerons , Elevators, Rubber, and Engine Power Desired Flight Path From Traffic Controller Flight Path Figure 15: Air traffic control systems and collision avoidance using GPS + - Global Positioning System Measured Flight Path Aircraft (Process)

23. 22 Problem 3 The potential of employing two or more helicopters for transporting payloads that are two heavy for a single helicopter is a well-addressed issue in the civil and military rotorcraft design arenas. A case of a multi-lift arrangement wherein two helicopters jointly transport payloads has been named twin lift as shown in figure 16. Develop the block diagram describing the pilots action, the position of each helicopter, and the position of the load. Figure 16: Two helicopters jointly transport payloads Load 1 2

24. 23 Radar (Measurement) Pilot Helicopter Altimeter (Measurement) Desired Separation Distance Desired Altitute Separation Distance Altitute Measured Altitute Measured Separation Distance + + - - Figure 17: Control system for two helicopters jointly transport payloads

25. 24 Design Problem Many cars are fitted with cruise control that, at the press of a button, automatically maintains a set speed. In this way, driver can cruise at a speed limit or economic speed without continually checking the speed meter. Design a feedback control in block diagram for a cruise control system. Electric Motor 1/k Valve Auto/Engine k Shaft Speed M eter Controller Desired Speed of Auto Set by Driver Desired Shaft Speed Measured Shaft Speed Drive Shaft Speed + - Figure 18: Feedback control of automatic cruise control system

26. 25 References (1) Richard C. Dorf and Robert H. Bishop, Modern Control Systems, Prentice Hall, 2001.