1. 1 Introduction to control system (ECEg 3153) Chapter 1 Introduction to Control Systems Electrical and Computer Engineering

2. Introduction Control system engineering :  is the branch of engineering which deals with the principles of control theory, to design a system which gives yields the desired behavior in a controlled manner.  Tasks of Control engineers Determining the process variable Determining the measurement site Assessing the disturbance Selecting the manipulator Selecting/installing a suitable controller Installation of the controller Determining the process variable Determining the measurement site Assessing the disturbance Selecting the manipulator Selecting/installing a suitable controller Installation of the controller 2

3. Introduction (…) System: A box which has an input and an output with collection of interacting components around which a boundary is set. An interconnection of elements and devices for a desired purpose Examples Power Station Input Fuel Output Electricity Input Electric Power Output Mechanical Rotation Electric Motor Classification of system study Model Development System Synthesis System Analysis  Includes parameter identification  Predicting the system behaviour:  Can be done either by mathematical analysis or numerical simulation  To force the system to behave as we would like it to  This course concerns 3

4. Introduction (…) Lumped parameter vs Distributed parameter System Classification Continuous time vs Discrete time Deterministic vs Stochastic Time invariant vs Time varying Linear vs Nonlinear Analogue vs Digital 4

5. Control System When a number of elements are combined together to form a system to produce desired output then the system is referred to as control system The main feature of a control system is that there should be a clear mathematical relationship between input and output of the system Example: Speed Control of DC motor by varying the Supply voltage Types of Control Systems 1. Open-loop Control Systems 2. Closed-loop Control Systems (Automatic) 5

6. Open-Loop Control Systems System which doesn’t automatically correct for variations in its output No information is feedback to the system to adjust itself and maintain a constant output An input is chosen on the basis of experience of such a system to give the value of the required output. Examples: 1. An electric heater with a selection switch 2. Systems which operate by preset timing mechanisms like Traffic Lights, Washing Machine 6

7. Features of an open loop Control System  Positive feature  Relatively simplehencelowcostwithgenerally good reliability  Inherently Stable  Negative feature  Often inaccurate since there is no error correction  More sensitive to changes in component characteristics  More sensitive to disturbances 7

8. Closed-Loop Control Systems A feedback signal to the input from the output will be sent and used to modify the input so that the output is maintained constant regardless of any changes in conditions. Actual System InputOutput Feedback e Comparison element E xample: The open-loop heating system could be made a closed-loop if some one with a thermometer monitors the temperature in the room and switches the selection switch on and off. 8

9. Features of Closed-loop Control Systems  Positive features  Ability to match the actual to the required value since there is continuous error correction  Less sensitivity to disturbances  Increased speed of response and hence increased band width Electric Heater Required Temp. Signal Output Temp. Measurement + e - Input Temperature 9

10. Features of Closed-loop Control Systems  Negative features  Instabilitybecauseoftimedelayswhentransferring corrective action  More complex than open-loop and so more costly with a ofgreater greater chance of break downasaresult number of components 10

11. Basic Components AnyControlSystemconsistsofanumberofbasicsub- systems or elements Thoseelementscouldappearseparateorintegratedasa single entity The basic elements are: 1. Comparison Element 2. Control Element 3. Correction Element 4. Process Element (plant) 5. Measurement Element (Transducers) 11

12. Reference Value Measurement Element + e - Input Process Input Signal Output Controlled Variable Correction Element Control Element Process Output Controlled Variable Correction Element Control Element feedback Open-Loop Controller Closed-Loop Controller Comparison element 12

13. Terminology : 1. Comparison Element: compares the reference value of the variable being controlled with the measured value of the actual output and produces an error signal. 2. Control Element: decides what action to be taken after it gets an error signal. 3. Correction Element (actuator): used to produce a change in the process in order to avoid the error. 4. Process Element (Plant): is the system of which a variable is being controlled 5. Measurement Element: Produces a Signal related to the actual output and provides a feedback signal to the comparison element. 13

14. Examples of open-loop control systems An electric switch in which a man-made control system controls the flow of electricity. The apparatus or person flipping the switch is not part of this control system. Flipping the switch on or off may be considered as the input, i.e. the input can be in one of the two states, on or off. The output is the flow or non-flow of electricity. This becomes an open-loop control system because the control action is independent of the output. The operation of ordinary traffic signals which control traffic at roadway intersections is another example of an open-loop control systems wherein all control signals are pre-set by timing mechanisms. 1414

15. Examples of Closed-loop control systems A thermostatically controlled heater or furnace automatically regulating the temperature of a room or enclosure is an example of a closed-loop control system. When the thermostat detects that the output temperature is less than the desired temperature input, the furnace provides heat until the temperature of the enclosure becomes equal to the reference input.Then the furnace is automatically turned off as soon as the temperature reaches the desired value. A part of the human controlsystem is the perspiration system. When the temperature of the air exterior to the skin becomes too high, the sweet glands secrete heavily, inducing cooling of the skin by evaporation. 1515

16. Examples of Closed-loop control systems(…) The student teacher learning process 1616

17. Main control strategies 21 17 Classical controls Modern controls Robust control Optimal control Adaptive control Nonlinear control Intelligent control

18. Brain storming questions #Identify whether it is open or closed loop control system 1. A man driving an automobile. 2. A person searching for a book on the table. 3. A blind man searching for a book on the table. 4. A man walking in prescribed direction. 5. Inserting thread into a needle. 6. A traffic man regulating car traffic flow from Addis to Debre Markos with the aid of mobilewith his colleague. 7. Students’ Cafeteria ticker allowing the student to enter through without checking the capacity of sits. 21 18

19. MORE EXAMPLES ON CONTROL SYSTEMS 1919

20. James Watt's flyball governor 20

21. The first automatic feedback controller used in an industrial process is generally agreed to be James Watt's flyball governor, developed in 1769 for controlling the speed of a steam engine. The all-mechanical device, shown in Figure above, measured the speed of the output shaft and utilized the movement of the flyball to control the steam valve and therefore the amount of steam entering the engine. As depicted in the Figure, the governor shaft axis is connected via mechanical linkages and beveled gears to the output shaft of the steam engine. As the steam engine output shaft speed increases, the ball weights rise and move away from the shaft axis and through mechanical linkages the steam valve closes and the engine slows down. 21 James Watt's flyball governor

22. If the actual speed drops below the desired value due to disturbance, then the decrease in the centrifugal force of the speed governor causes the control valve to move downward, supplying more fuel, and the speed of the engine increases until the desired value is reached. On the other hand, if the speed of the engine increases above the desired value, then the increase in the centrifugal force of the governor causes the control valve to move upward. This decreases the supply of fuel, and the speed of the engine decreases until the desired value is reached. In this speed control system, the plant (controlled system) is the engine and the controlled variable is the speed of the engine. The difference between the desired speed and the actual speed is the error signal. The control signal (the amount of fuel) to be applied to the plant (engine) is the actuating signal. The external input to disturb the controlled variable is the disturbance. An unexpected change in the load is a disturbance. 22 Speed Control System- Watt’s speed governor

23. Automobile steering control system 23 The driver uses the difference between the actual and the desired direction of travel to generate a controlled adjustment of the steering wheel.

24. Manually controlled closed-loop system for regulating the level of fluid in a tank The input is a reference level of fluid that the operator is instructed to maintain. (This reference is memorized by the operator.) The power amplifier is the operator, and the sensor is visual. The operator compares the actual level with the desired level and opens or closes the valve (actuator), adjusting the fluid flow out. to maintain the desired level. 2424

25. End of Lecture 1 Next Lecture Lecture 2. Mathematical Modeling of Linear Systems 2525