Basic Concepts

CONTROL SYSTEMS BASIC CONCEPTS

NTTF

Control Systems - Basic concepts

1

Introduction • Society relies heavily on automatic control systems for its day-to-day operation. • These control systems range from very simple to fairly complex in nature. • Automatic washing machines, refrigerators, and ovens are examples of some of the simpler systems used in the home. • Aircraft automatic pilots, welding and inspection robots used in manufacturing, and electric power generation and distribution systems represent complex control systems. . NTTF

Control Systems - Basic concepts

2

Introduction • In this chapter some of the terms commonly used to describe the operation, analysis, and design of control systems are discussed • The concept of a control system is introduced and its operation is explained.

NTTF

Control Systems - Basic concepts

3

Control Terminology • System – Subsystem

• • • • • •

Element (Component) Block Control Automatic Control System Automatic Control System

NTTF

Control Systems - Basic concepts

4

System • A system is a collection, set, or arrangement of elements (subsystems). • The concept of a system is perhaps the most important-but also the most misunderstood-concept. • A system is an entity that can be considered on its own. • It may be very large and complex, such as a planetary system, or it may be small and simple, such as the thermostatic temperature control of a steam iron or the water-level control system in a toilet. NTTF

Control Systems - Basic concepts

5

The concept of a system

NTTF

Control Systems - Basic concepts

6

The concept of a system • Figure shows various elements (objects) A, B, C,..., F. • System Z contains only one element F. • System Y ctmtains D, and E, whereas system X contains elements A, B, and C. In other words, system Y is concerned only with the interaction of elements D and E . • If the operation of a system is dependent upon elements A, B, C, D, and E, a larger system, XX, is formed. • X and Y, formerly known as systems, are now better described as subsystems. • Similarly, in case of system XXX,XX is termed a subsystem of XXX

NTTF

Control Systems - Basic concepts

7

Element • An electrical control system contains several active and passive components, some of which may work together to form an active filter. It may be sufficient to deal with a filter network containing operational amplifiers, resistors, and capacitors network as a whole. • Even though individual component properties dictate the filter characteristics, it is sufficient to treat the filter as a black box, which contains all the characteristics of an active filter network. NTTF

Control Systems - Basic concepts

8

Element • It is then fair to say that the active filter can be treated as an element of the control system, without any reference to its components.

• An element (component) is the smallest part of a system that can be treated as a whole (entity) • In a small socioeconomic system, a community may be treated as an element, even though it may incorporate several families, with every family consisting of one or more members.

NTTF

Control Systems - Basic concepts

9

Block • A block is a set of elements that can be grouped together, with overall characteristics described by an input output relationship.

NTTF

Control Systems - Basic concepts

10

Block • The block is an important concept in analysis and design of control systems • Input and output characteristics of entire group of elements within the block can be described by suitable mathematical expressions.

NTTF

Control Systems - Basic concepts

11

Block

• Block diagram representation contains only the input output characteristics and leaves no indication about the physical arrangement of the elements within the block (Figure). It is nearly impossible to determine the true nature of elements from a block representation, such as whether it is electrical, pneumatic, mechanical, thermal, etc. NTTF

Control Systems - Basic concepts

12

Automatic control system • Control means to regulate, direct, or govern. • Automatic implies self-action without any human intervention. • A control system is a group of physical components arranged to control themselves or another system. • An automatic control system is a control system that is self-regulating, without any human intervention. NTTF

Control Systems - Basic concepts

13

Automatic control system • An automatic control system does not require human intervention for its operation. It monitors the output variable through a feedback loop and manipulates the plant or process to maintain the output at or near the desired value. Once the controller set point input has been entered, an automatic control system will attempt to maintain the output variable at or near the desired value. NTTF

Control Systems - Basic concepts

14

Automatic control system • An oven is an example of an automatic control system. Once the desired temperature is set on the dial, the oven-temperature controller takes over and maintains the oven temperature to the desired value. • A manual control system is a control system regulated through human intervention.

NTTF

Control Systems - Basic concepts

15

Some more examples • Opening or closing of a window in a room, thus regulating air temperature or air quality (manual control) • Activation of a light switch to change illumination in a room (manual control) • Porch-light activation through an optical sensor at dusk or dawn • Thermostatic temperature control of home (automatic control) NTTF

Control Systems - Basic concepts

16

Some more examples • River water-flow control system (may be a very complex system involving several hydroelectric stations and reservoirs) (combination of manual and automatic control) • Boiler temperature control in a nuclear reactor (complex automatic control) • Autopilot control system in a commercial airliner (complex automatic control) NTTF

Control Systems - Basic concepts

17

Open-loop control system • An open-loop control system is a control system in which the control (regulating) action is independent of the output.

Closed-loop control system A closed-loop control system is a control system in which the control (regulating) action is influenced by the output. NTTF

Control Systems - Basic concepts

18

Feedback • An important feature of a closed-Ioop control system is that a control system output signal is fed back to the input stage,providing information to the controller about the current output state and facilitating an appropriate control action. It is not possible to have a fully functional automatic or even manual control system without a feedback signal.

NTTF

Control Systems - Basic concepts

19

Example • Consider a car being driven on a road. – In this case, the driver acts as the controller; – The car, its components, and the road become part of a closed-loop control system. – Visual feedback provides information to the driver about the location of the car with respect to the surroundings. – Through the help of this feedback, the driver can slow down or speed up and manipulate the steering in order to maintain a desired heading. – If his (visual) feedback signal is removed (e.g., by a newspaper covering the windshield), the control system becomes an open-loop control system, rendering safe driving nearly impossible. – Admittedly, if there is no traffic or curves on the road, it is still possible to maintain the car heading without a disaster. – But the system will fail as soon as there is a change in the operating environment. NTTF

Control Systems - Basic concepts

20

A regulator system • A regulator system is a control system where the reference (input) is normally fixed. – A home-heating system is an example of a regulator system. – The reference temperature is normally set to a constant value, and the furnace is regulated to maintain the output temperature at a steady value. – Variation in outside temperatures and wind velocity place varying heating loads and tend to vary the house temperature. – A temperature controller senses the variation in temperature and corrects it by changing the amount of heat added through the furnace. NTTF

Control Systems - Basic concepts

21

A regulator system • Regulator systems also experience variation in reference input. – In a home-heating control system, the night time temperature setting may be different than the daytime temperature setting. – However, these changes are less frequent, and so the control system has to cater mainly to load variation due to external disturbances.

NTTF

Control Systems - Basic concepts

22

Servomechanism • A servomechanism (follower system) is a control system where the reference (input) varies continuously and the system operates so that the output follows the input.

NTTF

Control Systems - Basic concepts

23

Servomechanism • Servomechanism is (sometimes) used to describe control systems where the output variable is a mechanical position, velocity, or acceleration of an object. It originated from the Greek term seruus, meaning servant. • A solar power panel with continuous tracking of the sun (for optimum power generation) falls in this category. • Other examples include robotics-movement, plotterposition, and hard-disk-drive control systems. NTTF

Control Systems - Basic concepts

24

Single-input, Single-output (SISO) system • A single-input, single-output (SISO) system is a system where only one parameter is entered as input and only one parameter represents the output.

NTTF

Control Systems - Basic concepts

25

Single-input, Single-output (SISO) system

• A home-heating (or cooling) system involves a SISO system. • A single parameter, the desired room temperature, is entered as the reference variable and results in only one controlled (output) variable: actual room temperature. • It is highly desirable to incorporate control of humidity, but that is left to a separate humidifier control system. NTTF

Control Systems - Basic concepts

26

Multiple-input, multiple-output (MIMO) system • A multiple-input, multiple-output (MIMO) system is a system where several parameters may be entered as input and output is represented by multiple variables.

NTTF

Control Systems - Basic concepts

27

Multiple-input, multiple-output (MIMO) system • A typical residential home-heating system is a SISO system where a single parameter, the room temperature, is monitored and controlled. • There is a growing consensus that humidity control should be added to the home-heating system (environmental control system). With this, both temperature and humidity are monitored and controlled, making the system an MIMO type. • A chemical process plant typically has several reference inputs and requires the control of several output variables. NTTF

Control Systems - Basic concepts

28

A linear system • A linear system is a system where input/output relationships may be represented by a linear differential equation. – Ohm's law assumes a linear relationship between the voltage drop across a resistor and the current flow through it. It is certainly valid over a wide range of currents and voltages but does not hold for very high values of current flow through a metal wire.

NTTF

Control Systems - Basic concepts

29

A time-invariant system • A time-invariant system is a system described by a differential equation with constant coefficients. – A spring mass and damper system represents a time-invariant system if the characteristics of all three components do not change with time.

NTTF

Control Systems - Basic concepts

30

A time-variant system • A time-variant system is a system described by a differential equation with variable coefficients. – A rocket burning fuel represents a time-variant system. The mass of the rocket does not remain constant during a flight but changes as fuel is burned.

NTTF

Control Systems - Basic concepts

31

Our Course • All the discussions in our course assume that all the control components and systems are linear and time-invariant. • It is further assumed that the control systems are of the single-input, single-output (SISO) type. • Furthermore, all systems considered are negative-feedback closed-loop control systems. NTTF

Control Systems - Basic concepts

32

FUNCTIONING OF A CONTROL SYSTEM • All control systems, whether simple or complex, function in a very similar manner. • The controller (brain of the control system) compares the actual value of the controlled variable to the desired value and tries to manipulate the process in such a fashion that the controlled variable approaches the desired value. NTTF

Control Systems - Basic concepts

33

Human example • Operation of a control system is no different than tasks routinely carried out by humans. • Consider the task of reaching for a glass on a table (Fig). • The objective of the task is fairly simple: the hand has to be moved so that it touches the glass. • The entire process can be summarized as follows: NTTF

Control Systems - Basic concepts

34

Human example

NTTF

Control Systems - Basic concepts

35

Human example • Reference input (objective) – Reach for the glass.

• Controller (human brain) – Look at the glass (feedback) and determine (estimate) the distance between the hand and the glass

• Manipulate (final control element) – Move the arm toward the glass.

• . Feedback (eye) – Get a new estimate of distance between the hand and the glass. NTTF

Control Systems - Basic concepts

36

Human example • Loop (repeat) – Move the arm untll feedback (eye) indicates that the hand has reached the glass.

• The visual feedback is the single most important part of the whole process. Without visual feedback, it is not possible to reach for the glass. Movement of the human hand is very imprecise. It cannot move through a precise distance on command (desire), but the use of (visual) feedback allows the hand movement to be very precisely controlled. NTTF

Control Systems - Basic concepts

37

Robot example • Have you ever considered using a fancy robot to thread a needle? With feedback, the controller can modify its action, and even imprecise machinery can be made to work satisfactorily in demanding applications. • If it can be measured, it can be controlled.

NTTF

Control Systems - Basic concepts

38

Oven-temperature control • The oven contains a closed-loop temperature control system. – A controlling relay located at the back panel of the oven is responsible for maintaining the oven at the desired temperature. – The user turns the oven on and sets the temperature dial to the desired value. – Actual oven temperature is measured by a thermocouple (temperature sensor) located at the back wall of the oven. It provides the crucial feedback of oven temperature to the controlling relay. – If the actual temperature is below the desired temperature by a (factory-) set margin, the oven-heating element is turned on. – The element is turned off when the oven temperature exceeds the desired value by the set margin. NTTF

Control Systems - Basic concepts

39

Automobile speed control • The driver brings the automobile to the desired speed and activates the cruise control, which latches the speed data into the onboard computer memory. • The computer gets the actual vehicle speed data from the speedometer (velocity transducer) and compares it with the set speed. • It then adjusts the fuel intake into the engine manifold to bring the vehicle speed to the desired value. NTTF

Control Systems - Basic concepts

40

Electronic voltage regulator

NTTF

Control Systems - Basic concepts

41

Electronic voltage regulator • The unregulated voltage-input terminal is connected to the source voltage and the output voltage is obtained from the regulated voltage-output terminal. • In the case of a fixed-voltage regulator, the input is kept fixed (factory set) to a predefined voltage, e.g., 5 V. A buried voltage reference provides this fixedinput reference voltage. • Two resistors, Rl and R2, provide feedback information about the actual output voltage. • An onboard op-amp acts as a controller. It compares the two input voltages and controls the amount of current passing through the series pass transistor, thus controlling the output voltage. NTTF

Control Systems - Basic concepts

42

Electronic voltage regulator • An adjustable voltage regulator does not use an internal fixed reference but allows a user programmable voltage to be entered as the reference. It is generally set either using a potentiometer or as a combination of two precise resistors. A practical voltage regulator contains additional components to provide protection against excessive current and/or overheating. NTTF

Control Systems - Basic concepts

43

Aircraft instrument landing system • Instrument landing systems systems are highly complex and very sophisticated. • A computer aboard the aircraft acts as a control1er. This controller is responsible for receiving real-time data from the airport computer and operating various flight controls. • Ground-based instrumentation provides information about aircraft position, velocity, altitude, etc., to the aircraft controller. • This controller, in turn, continuously adjusts the various controls on the aircraft to gently bring it down to the airport runway. NTTF

Control Systems - Basic concepts

44

Metal-rolling mill velocity control • In metal-rolling operations, multiple rolling stands are utilized. • Material entering the first stand has the largest thickness, and the material leaving the last stand has the smallest thickness. • As metal is rolled at a stand, its thickness is reduced, and as a result the linear velocity of material leaving the stand is larger than the velocity of material entering the stand. • In order to make up for increased material velocity, the rolls at the next stand have to spin faster. NTTF

Control Systems - Basic concepts

45

Metal-rolling mill velocity control

NTTF

Control Systems - Basic concepts

46

• Each stand is driven by a dc servomotor. • The rotational velocity of the servomotor at the second stand is slightly higher than the velocity of the motor at the first stand. The actual difference in velocity depends on several factors, including the material stiffness, reduction in thickness at the first stand, and entry velocity. • A high-performance controller with fast response actuators and a precision tension load cell is required for successful implementation. • The controller is responsible for controlling the velocity of the second stand. Instead of using the linear velocity as the input, the controller works on the sheet material tension in between the stands. NTTF

Control Systems - Basic concepts

47

• If the rolls at the second stand match the velocity of incoming material (from the first stand), the tension in between the two stands will be minimal. • The operator enters the desired tension value and the controller accepts the actual tension from the load cell as the feedback signal. • It then manipulates the velocity of the secondstand servomotor, thus controlling the tension to the desired level. NTTF

Control Systems - Basic concepts

48

Metal-rolling mill velocity control

NTTF

Control Systems - Basic concepts

49

Problems 1. Provide an example of each of the following: – – – –

Manual control system Automatic control system Biological control system Natural control system.

2. Provide an example of an open-loop control system that is manually controlled. NTTF

Control Systems - Basic concepts

50

Problems 1. State whether the following systems are open-loop or closed-loop. – – – – –

Firing of a bullet Opening of a faucet Throwing a ball Turning on of a hot plate to a certain setting Operation of an oven (setting it to a specified temperature)

2. Consider the task of adjusting the water temperature in a shower. Identify the input, output, and control action. NTTF

Control Systems - Basic concepts

51