1 Study Material CONTROL SYSTEM ENGINEERING (As per SCTE&VT,Odisha new syllabus) 4th Semester Electronics & Telecom Engineering By Sri Asit Kumar Acharya, Lecturer ETC, Govt. Polytechnic Dhenkanal & Sri Gadadhar Maharana, Sr. Lecturer ETC S.K.D.A.V, Govt. Polytechnic, Rourkela
2 CONTENTS Chapter 1 FUNDAMENTAL OF CONTROL SYSTEM Chapter 2 TRANSFER FUNCTION Chapter 3 CONTROL SYSTEM AND MATHEMATICAL MODELING OF PHYSICAL SYSTEM Chapter 4 Block Diagram & Signal Flow Graphs Chapter 5 Time Domain Analysis Chapter 6 FEEDBACK CHARACTERISTICS Chapter 7 STABILITY CONCEPT & ROOT LOCUS METHOD Chapter 8 FREQUENCY RESPONSE ANALYSIS Chapter 9 STATE VARIABLE SYSTEM
3 CHAPTER 1 :- FUNDAMENTAL OF CONTROL SYSTEM Control system engineering is the branch of engineering which deals with the principles of control theory to design a system which gives desired behaviour in a controlled manner. Hence, this is interdisciplinary. Control system engineers analyze, design, and optimize complex systems which consist of highly integrated coordination of mechanical, electrical, chemical, metallurgical, electronic or pneumatic elements. Thus control engineering deals with diverse range of dynamic systems which include human and technological interfacing. Control system engineering focuses on analysis and design of systems to improve the speed of response, accuracy and stability of system. The two methods of control system include classical methods and modern methods. The mathematical model of system is set up as first step followed by analysis, designing and testing. Necessary conditions for the stability are checked and finally optimization follows. In classical method, mathematical modelling is usually done in time domain, frequency domain or complex s domain. Step response of a system is mathematically modelled in time domain differential analysis to find its settling time, % overshoot etc. Laplace transforms are most commonly used in frequency domain to find the open loop gain, phase margin, band width etc of system. Concept of transfer function, sampling of data, poles and zeros, system delays all comes under the classical control engineering stream. Modern control engineering deals with Multiple Input Multiple Output (MIMO) systems, State space approach, Eigen values and vectors etc. Instead of transforming complex ordinary differential equations, modern approach converts higher order equations to first order differential equations and solved by vector method. Automatic control systems are most commonly used as it does not involve manual control. The controlled variable is measured and compared with a specified value to obtain the desired result. As a result of automated systems for control purposes, the cost of energy or power as well as the cost of process will be reduced increasing its quality and productivity. Before I introduce you the theory of control system it is very essential to know the various types of control systems. Now there are various types of systems, we are going to discuss only those types of systems that will help us to understand the theory of control system and detail description of these types of system are given below:
4 Linear Control Systems In order to understand the linear control system, we should know the principle of superposition. The principle of superposition theorem includes two the important properties and they are explained below: Homogeneity: A system is said to be homogeneous, if we multiply input with some constant A then output will also be multiplied by the same value of constant (i.e. A). Additivity: Suppose we have a system S and we are giving the input to this system as a 1 for the first time and we are getting output as b 1 corresponding to input a 1. On second time we are giving input a 2 and correspond to this we are getting output as b 2. Now suppose this time we giving input as summation of the previous inputs ( i.e. a 1 + a 2 ) and corresponding to this input suppose we are getting output as (b 1 + b 2 ) then we can say that system S is following the property of additivity. Now we are able to define the linear control systems as those types of control systems which follow the principle of homogeneity and additivity. Examples of Linear Control System Consider a purely resistive network with a constant dc source. This circuit follows the principle of homogeneity and additivity. All the undesired effects are neglected and assuming ideal behaviour of each element in the network, we say that we will get linear voltage and current characteristic. This is the example of linear control system. Non-linear Systems We can simply define non linear control system as all those system which do not follow the principle of homogeneity. In practical life all the systems are non-linear system. Examples of Non-linear System A well known example of non-linear system is magnetization curve or no load curve of a dc machine. We will discuss briefly no load curve of dc machines here: No load curve gives us the relationship between the air gap flux and the field winding mmf. It is very clear from the curve given below that in the beginning there is a linear relationship between winding mmf and the air gap flux but after this, saturation has come which shows the non linear behavior of the curve
5 or characteristics of the non linear control system. In recent years, control systems plays main role in the development and advancement of modern technology and civilization. Practically every aspects of our day-to-day life is affected less or more by some control system. A bathroom toilet tank, a refrigerator, an air conditioner, a geezer, an automatic iron, an automobile all are control system. These systems are also used in industrial process for more output. We find control system in quality control of products, weapons system, transportation systems, power system, space technology, robotics and many Requirement Of Good Control System Accuracy: Accuracy is the measurement tolerance of the instrument and defines the limits of the errors made when the instrument is used in normal operating conditions. Accuracy can be improved by using feedback elements. To increase accuracy of any control system error detector should be present in control system. Sensitivity: The parameters of control system are always changing with change in surrounding conditions, internal disturbance or any other parameters. This change can be expressed in terms of sensitivity. Any control system should be insensitive to such parameters but sensitive to input signals only. Noise: An undesired input signal is known as noise. A good control system should be able to reduce the noise effect for better performance. Stability: It is an important characteristic of control system. For the bounded input signal, the output must be bounded and if input is zero then output must be zero then such a control system is said to be stable system. Bandwidth: An operating frequency range decides the bandwidth of control system. Bandwidth should be large as possible for frequency response of good control system. Speed: It is the time taken by control system to achieve its stable output. A good control system possesses high speed. The transient period for such system is very small.
6 Oscillation: A small numbers of oscillation or constant oscillation of output tend to system to be stable. Types Of Control Systems There are two main types of control system. They are as follow 1. Open loop control system 2. Closed loop control system Open Loop Control System A control system in which the control action is totally independent of output of the system then it is called open loop control system. Open loop system is also called as Manual control system. Fig 1 shows the block diagram of open loop control system in which process output is totally independent of controller action. Practical Examples Of Open Loop Control System 1. Electric Hand Drier Hot air (output) comes out as long as you keep your hand under the machine, irrespective of how much your hand is dried. 2. Automatic Washing Machine This machine runs according to the pre-set time irrespective of washing is completed or not. 3. Bread Toaster This machine runs as per adjusted time irrespective of toasting is completed or not. 4. Automatic Tea/Coffee Maker These machines also function for pre adjusted time only. 5. Timer Based Clothes Drier This machine dries wet clothes for pre adjusted time, it does not matter how much the clothes are dried. 6. Light Switch lamps glow whenever light switch is on irrespective of light is required or not. 7. Volume on Stereo System Volume is adjusted manually irrespective of output volume level. 8. Advantages Of Open Loop Control System 1. Simple in construction and design. 2. Economical. Easy to maintain.
7 Closed Loop Control System Control system in which the output has an effect on the input quantity in such a manner that the input quantity will adjust itself based on the output generated is called closed loop control system. Open loop control system can be converted in to closed loop control system by providing a feedback. This feedback automatically makes the suitable changes in the output due to external disturbance. In this way closed loop control system is called automatic control system. Figure below shows the block diagram of closed loop control system in which feedback is taken from output and fed in to input. Practical Examples Of Closed Loop Control System 1. Automatic Electric Iron Heating elements are controlled by output temperature of the iron. 2. Servo Voltage Stabilizer Voltage controller operates depending upon outputvoltage of the system. 3. Water Level Controller Input water is controlled by water level of the reservoir. 4. Missile Launched & Auto Tracked by Radar The direction of missile is controlled by comparing the target and position of the missile. 5. An Air Conditioner An air conditioner functions depending upon the temperature of the room. 6. Cooling System in Car It operates depending upon the temperature which it controls. Advantages OF Closed Loop Control System 1. Closed loop control systems are more accurate even in the presence of non-linearity. 2. Highly accurate as any error arising is corrected due to presence of feedback signal. 3. Bandwidth range is large. 4. Facilitates automation. 5. The sensitivity of system may be made small to make system more stable. 6. This system is less affected by noise.
8 Disadvantages Of Closed Loop Control System 7. They are costlier. 8. They are complicated to design. 9. Required more maintenance. 10. Feedback leads to oscillatory response. 11. Overall gain is reduced due to presence of feedback. 12. Stability is the major problem and more care is needed to design a stable closed loop system. Comparison of Closed Loop And Open Loop Control System OPEN LOOP CONTROL SYSTEM CLOSED LOOP CONTROL SYSTEM The feedback element is absent. The feedback element is always present. An error detector is not present. An error detector is always present It is stable one. It may become unstable. Easy to construct. Complicated construction. It is an economical. Having small bandwidth. It is inaccurate. Examples: Hand drier, tea Maker It is costly. Having large bandwidth It is accurate. Examples:Servo voltage stabilizer, Feedback Loop Of Control System A feedback is a common and powerful tool when designing a control system. Feedback loop is the tool which take the system output into consideration and enables the system to adjust its performance to meet a desired result of system. In any control system, output is affected due to change in environmental condition or any kind of disturbance. So one signal is taken from output and is fed back to the input. This signal is compared with reference input and then error signal is generated. This error signal is applied to controller and output is corrected. Such a system is called feedback system. Figure below shows the block diagram of feedback system.
9 When feedback signal is positive then system called positive feedback system. For positive feedback system, the error signal is the addition of reference input signal and feedback signal. When feedback signal is negative then system is called negative feedback system. For negative feedback system, the error signal is given by difference of reference input signal and feedback signal. Effect Of Feedback Refer figure beside, which represents feedback system where R = Input signal E = Error signal G = forward path gain H = Feedback C = Output signal B=Feedback signal 1. Error between system input and system output is reduced. 2. System gain is reduced by a factor 1/(1±GH). 3 Improvement in sensitivity. 4. Stability may be affected. 5. Improve the speed of response. Standard Input Test Signals : These are also known as test input signals. The input signal is very complex in nature, it is complex because it may be a combination of various other signals. Thus it is very difficult to analyze characteristic performance of any system by applying these signals. So we use test signals or standard input signals which are very easy to deal with. We can easily analyze the characteristic performance of any system more easily as compared to non standard input signals. Now there are various types of standard input signals and they are written below: Unit Impulse Signal : In the time domain it is represented by (t). The Laplace transformation of unit impulse function is 1 and the corresponding waveform associated with the unit impulse function is shown below.
10 Unit Step Signal : In the time domain it is represented by u (t). The Laplace transformation of unit step function is 1/s and the corresponding waveform associated with the unit step function is shown below. Unit Ramp signal : In the time domain it is represented by r (t). The Laplace transformation of unit ramp function is 1/s 2 and the corresponding waveform associated with the unit ramp function is shown below. Unit Ramp Signal Parabolic Type Signal : In the time domain it is represented by t 2 / 2. The Laplace transformation of parabolic type of the function is 1 / s 3 and the corresponding
11 waveform associated with the parabolic type of the function is shown below.
12 CHAPTER 2 TRANSFER FUNCTION TRANSFER FUNCTION For any control system there exists a reference input termed as excitation or cause which operates through a transfer operation termed as transfer function and produces an effect resulting in controlled output or response. Thus the cause and effect relationship between the output and input is related to each other through a transfer function.. It is not necessary that the output will be of same category as that of the input. For example in case of an electrical motor, the input is an electrical quantity and output is a mechanical one. In control system all mathematical functions are converted to their corresponding Laplace transforms. So the transfer function is expressed as a ratio of Laplace transform of output function to Laplace transform of input function. The transfer function can be expressed as While doing Laplace transform, while determining transfer function we assume all initial conditions to be zero.
13 The transfer function of a control system is defined as the ration of the Laplace transform of the output variable to Laplace transform of the input variable assuming all initial conditions to be zero. Procedure for determining the transfer function of a control system are as follows : 1. First deduce the equations for the system 2. Now take Laplace transform of the system equations, assuming initial conditions as zero. 3. Specify system output and input 4. Lastly take the ratio of the Laplace transform of the output and the Laplace transform of the input which is the required transfer function Methods of obtaining a Transfer function: There are major two ways of obtaining a transfer function for the control system.the ways are Block diagram method : It is not convenient to derive a complete transfer function for a complex control system. Therefore the transfer function of each element of a control system is represented by a block diagram. Block diagram reduction techniques are applied to obtain the desired transfer function. Signal Flow graphs: Signal Flow Graph is a modified form of a block diagram which gives a pictorial representation of a control system. Signal flow graph further shortens the representation of a control system. The transfer function of a system is completely specified in terms of its poles and zeroes and the gain factor. Let us know about the poles and zeroes of a transfer function in brief. Where, K = system gain, z 1, z 2, z m = zero s of the transfer function p 1, p 2, p n = pole s of the transfer function Putting the denominator of equation (i) equal to zero we get the poles value of the transfer function. For this the T.F is infinity. Putting the numerator of equation (ii) equal to zero we get the value of zero of the transfer function. For this T.F is equal to zero. There are two types of transfer functions :- i) Open loop transfer function( O.L.T.F) : Transfer function of the system without feedback path or loop.
14 ii) Closed loop transfer function (C.L.T.F) : Transfer function of the system with feedback path or loop.
17 Chapter3 CONTROL SYSTEM COMPONENTS & MATHAMATICAL MODELING OF PHYSICAL SYSTEM In a control system, the devices which are used to convert the process variables in one form to another form is known as Transducer. Transducer can also be defined as a device which transforms the energy from one form to another. For example a thermocouple converts the heat energy to electrical voltage. In control system the following devices are used as a transducer 1. Potentiometer 2. DC Servomotor 3. AC servomotor 4. Synchros 5. Stepper Motor 6. Magnetic Amplifier 7. Tachogenerator 8. Gyroscope 9. Differential Transformer Potentiometer - A Potentiometer is a simple device which is used for mechanical displacement either linear or angular. Thus a Potentiometer is electro mechanical transducer which converts the mechanical energy to electrical energy. The input to the device is in the form of linear mechanical displacement or rotational mechanical displacement. When the voltage is applied across the fixed terminal. The output voltage is proportional to the displacement. Let Ei=Input voltage Eo=Output voltage
18 and Time Domain Analysis and Design of
19 Diagram Reduction Analysis and Design of Control Systems: Block Diagram Reduction
23 CHAPTER- 4 Block Diagram & Signal Flow Graphs(SFG)
24 BLOCK-DIAGRAM REDUCTION 10.1 INTRODUCTION Block Diagram: Pictorial representation of functions performed by each component of a system and that of flow of signals. R (s) G ( s ) C (s) C ( s ) = G ( s ) R ( s) Figure Single block diagram representation. Components for Linear Time Invariant System(LTIS): Figure Components for Linear Time Invariant Systems (LTIS).
25 Terminology: Disturbance U ( s ) R (s ) ± E (s ) = R (s ) ±b (s ) G 1 (s ) m (s ) b ( s ) G 2 (s ) C (s ) H (s ) Figure Block Diagram Components. 1. Plant: A physical object to be controlled. The Plant G 2 (s ), is the controlled system, of which a particular quantity or condition is to be controlled. 2. Feedback Control System (Closed loop Control System) : A system which compares output to some reference input and keeps output as close as possible to this reference. 3. Open loop Control System: Output of the system is not feedback to the system. 4. Control Element G 1 (s ), also called the controller, are the components required to generate the appropriate control signal M ( s ) applied to the plant. 5. Feedback Element H ( s ) is the component required to establish the functional relationship between the primary feedback signal B ( s ) and the controlled output C (s ). 6. Reference Input R (s ) is an external signal applied to a feedback control system in order to command a specified action of the plant. It often represents ideal plant output behavior. 7. The Controlled Output C (s ) is that quantity or condition of the plant which is controlled. 8. Actuating Signal E (s ), also called the error or control action, is the algebraic sum consisting of the reference input R (s ) plus or minus (usually minus) the primary feedback B (s ). 9. Manipulated Variable M ( s ) (control signal) is that quantity or condition which the control elements G 1 (s ) apply to the plant G 2 (s ). 10. Disturbance U (s ) is an undesired input signal which affects the value of the controlled output C (s ). It may enter the plant by summation with M (s ), or via an intermediate point, as shown in the block diagram of the figure above. 11. Forward Path is the transmission path from the actuating signal E ( s) to the output C (s ). 2/14
26 12. Feedback Path is the transmission path from the output C (s ) to the feedback signal B (s ). 13. Summing Point: A circle with a cross is the symbol that indicates a summing point. The ( + ) or ( ) sign at each arrowhead indicates whether that signal is to be added or subtracted. 14. Branch Point: A branch point is a point from which the signal from a block goes concurrently to other blocks or summing points. Definitions G ( s ) Direct transfer function = Forward transfer function. H (s ) Feedback transfer function. G ( s ) H ( s ) Open loop transfer function. C ( s ) R ( s ) Closed loop transfer function = Control ratio C ( s ) E ( s ) Feed forward transfer function. E (s ) R (s ) G ( s ) C (s Input B (s ) H (s ) Outpu Figure Block diagram of a closed loop system with a feedback element BLOCK DIAGRAMS AND THEIR SIMPLIFICATION Cascade (Series) Connections Figure Cascade (Series) Connection. 3/14
27 Parallel Connections Figure Parallel Connection. Closed Loop Transfer Function (Feedback Connections) R (s ) E (s ) G ( s ) C (s ) B (s ) H ( s ) Figure (Repeated) Feedback connection For the system shown in Figure 10 4, the output C ( s) and input R ( s) are related as follows: where C (s ) =G (s )E (s ) E (s ) = R (s ) B (s ) = R (s ) H (s ) C (s ) Eliminating E (s ) from these equations gives C (s ) = G (s )[R (s ) H (s ) C (s )] This can be written in the form [1 + G (s )H (s )]C (s ) =G (s )R (s ) or C ( s G ( s ) R ( s 1 + G ( s ) H ( s ) = The Characteristic equation of the system is defined as an equation obtained by setting the denominator polynomial of the transfer function to zero. The Characteristic equation for the above system is 1+G ( s ) H ( s ) =0.
28 Block Diagram Algebra for Summing Junctions C=G ( +R±X) =+GR±GX C=GR±X = G ( + R±X G) Figure Summing junctions. Block Diagram Algebra for Branch Point Figure Summing junctions. Block Diagram Reduction Rules In many practical situations, the block diagram of a Single Input Single Output (SISO), feedback control system may involve several feedback loops and summing points. In principle, the block diagram of (SISO) closed loop system, no matter how complicated it is, it can be reduced to the standard single loop form shown in Figure The basic approach to simplify a block diagram can be summarized in Table 1:
29 TABLE 10 1 Block Diagram Reduction Rules 1. Combine all cascade blocks 2. Combine all parallel blocks 3. Eliminate all minor (interior) feedback loops 4. Shift summing points to left 5. Shift takeoff points to the right 6. Repeat Steps 1 to 5 until the canonical form is obtained TABLE Some Basic Rules with Block Diagram Transformation X G 1 G 2 Y X G G 1 2 Y Y =(GG )X 1 2 X G 1 G Y X G 1 ±G 2 Y Y = (G 1 ±G 2 )X u G u G y y = G u y 1 u u 1/ G u = y G u G u G y y y y G u 1 G y u 2 1 G y u 2 G y = Gu e 2 = G ( u 1 u 2 ) u G u 1 G y y y = Gu 1 u 2 1/ G u 2 u G 2 1/ G 2 G u 2 y y = ( G 1 G 2 )u Example 1: A feedback system is transformed into a unity feedback system R ( s) G ( s) H ( s ) C ( s) R ( s ) 1 H ( s ) G (s ) H ( s ) C ( s ) C R = 1 ± G GH = H 1 1 ± GH GH =Closed loop Transfer function
30 Example 2: Reduce the following block diagrams
31 Example 3: Example 4 G 1 and G 2 are in series H 1 and H 2 and H 3 are in parallel G 1 is in series with the feedback configuration. C(s) G = G 3 G 2 1 R(s) 1+ G 3 G 2 ( H 1 - H 2 + H 3 Example 5: The main problem here is the feed forward of V 3 (s). Solution is to move this pickoff point forward.
33 Example 6:
44 Example 1 Obtain the transfer function of C/R of the system whose signal flow graph is shown in Fig.1 G 1 R G C -G 3 G 4 There are two forward paths: Gain of path 1 : p 1 =G 1 Gain of path 2 : P 2 =G 2 There are four loops with loop gains: L 1 =-G 1 G 3, L 2 =G 1 G 4, L 3 = -G 2 G 3, L 4 = G 2 G 4 There are no non-touching loops. = 1+G 1 G 3 -G 1 G 4 +G 2 G 3 -G 2 G 4 Figure 1 Signal flow graph of example 1 Forward paths 1 and 2 touch all the loops. Therefore, 1 = 1, 2= 1 C ( The transfer function T = R(s) = s ) P G 1 + G 2 P 2 = 1+ G 1 G 3 G 1 G 4 + G 2 G 3 G 2 G 4 44
45 Example 2 Obtain the transfer function of C(s)/R(s) of the system whose signal flow graph is shown in Fig.2. R(s) 1 1 G 1 G 2 G 3 1 C(s) -H 2 H 1-1 Figure 2 Signal flow graph of example 2 There is one forward path, whose gain is: P 1 =G 1 G 2 G 3 There are three loops with loop gains: L 1 =-G 1 G 2 H 1, L 2 =G 2 G 3 H 2, L 3 = - G 1 G 2 G 3 There are no nontouching loops. = 1 -G 1 G 2 H 1 +G 2 G 3 H 2 +G 1 G 2 G 3 Forward path 1 touches all the loops. Therefore, 1 = 1. C s 1 The transfer function T = ( ) R(s) = P G G G = 1 G1 G 2 H + G G H + G G G Example 3 Obtain the transfer function of C(s)/R(s) of the system whose signal flow graph is shown in Fig.3. G 6 G 7 R(s) G 1 G 2 G 3 G 4 G 5 1 C(s) X 1 -H 1 -H2 There are three forward paths. Figure 3 Signal flow graph of example 3 45
46 The gain of the forward path are: P1=G1G2G3G4G5 P2=G1G6G4G5 P3= G1G2G7 There are four loops with loop gains: L1=-G4H1, L2=-G2G7H2, L3= -G6G4G5H2, L4=-G2G3G4G5H2 There is one combination of Loops L1 and L2 which are nontouching with loop gain product L1L2=G2G7H2G4H1 = 1+G 4H1+G2G7H2+G6G4G5H2+G2G3G4G5H2+ G2G7H2G4H1 Forward path 1 and 2 touch all the four loops. Therefore 1= 1, 2= 1. Forward path 3 is not in touch with loop1. Hence, 3=1+G4H1. The transfer function T = ( ) (1+ G 4 H 1 ) C s P P P 3 3 G G1 2 G 3 G 4 G 5 + G G 4 1 G 5 G 6 + G G 2 1 G = = 7 ( R s ( ) 1+ G 4 H 1 + G 2 G 7 H 2 + G 6 G 4 G 5 H 2 + G 2 G 3 G 4 G 5 H 2 + G 2 G 4 G 7 H 1 H 2 Example 4 Find the gains for the signal flow graph shown in Fig.4 X 1 b -h X 1 a c d e X 5 f X 6 X 2 X 3 X 4 -g -i Signal flow graph There are two forward paths. The gain of the forward path P1= acdef P2= abef There are four loops with loop gains: L1=- -cdg, L2=- -eh, L3= -cdei, L4=- -bei There is one combination of Loops L1 and L2 which are non touching with loop gain product L1L2=cdgeh 46
47 = 1+cdg+eh+cdei+bei+cdgeh Forward path 1 and 2 touch all the four loops. Therefore 1= 1, 2= 1. X 6 The transfer function T = = 1 P P2 cdef + abef = X 1+ cg + eh + cdei+ bei + cdgeh 1 CHAPTER-5 Time Domain Analysis of Control Systems When we study the analysis of the transient state and steady state response of control system it is very essential to know a few basic terms and these are described below. Standard Input Signals : These are also known as test input signals. The input signal is very complex in nature, it is complex because it may be a combination of various other signals. Thus it is very difficult to analyze characteristic performance of any system by applying these signals. So we use test signals or standard input signals which are very easy to deal with. We can easily analyze the characteristic performance of any system more easily as compared to non standard input signals. Now there are various types of standard input signals and they are written below: Unit Impulse Signal : In the time domain it is represented by (t). The Laplace transformation of unit impulse function is 1 and the corresponding waveform associated with the unit impulse function is shown below. Unit Step Signal : In the time domain it is represented by u (t). The Laplace transformation of unit step function is 1/s and the corresponding waveform associated with the unit step function is shown below. 47
48 Unit Ramp signal : In the time domain it is represented by r (t). The Laplace transformation of unit ramp function is 1/s 2 and the corresponding waveform associated with the unit ramp function is shown below. Unit Ramp Signal Parabolic Type Signal : In the time domain it is represented by t 2 / 2. The Laplace transformation of parabolic type of the function is 1 / s 3 and the corresponding waveform associated with the parabolic type of the function is shown below. 48
49 Transient Response of Control System As the name suggests transient response of control system means changing so, this occurs mainly after two conditions and these two conditions are written as follows- Condition one : Just after switching on the system that means at the time of application of an input signal to the system. Condition second : Just after any abnormal conditions. Abnormal conditions may include sudden change in the load, short circuiting etc. Steady State Response of Control System Steady state occurs after the system becomes settled and at the steady system starts working normally. Steady state response of control system is a function of input signal and it is also called as forced response. Now the transient state response of control system gives a clear description of how the system functions during transient state and steady state response of control systemgives a clear description of how the system functions during steady state. Therefore the time analysis of both states is very essential. We will separately analyze both the types of responses. Let us first analyze the transient response. In order to analyze the transient response, we have some time specifications and they are written as follows: Delay Time : This time is represented by t d. The time required by the response to reach fifty percent of the final value for the first time, this time is known as delay time. Delay time is clearly shown in the time response specification curve. Rise Time : This time is represented by t r. We define rise time in two cases: 1. In case of under damped systems where the value of ζ is less than one, in this case rise time is defined as the time required by the response to reach from zero value to hundred percent value of final value. 2. In case of over damped systems where the value of ζ is greater than one, in this case rise time is defined as the time required by the response to reach from ten percent value to ninety percent value of final value. Peak Time : This time is represented by t p. The time required by the response to reach the peak value for the first time, this time is known as peak time. Peak time is clearly shown in the time response specification curve. 49
50 Settling Time : This time is represented by t s. The time required by the response to reach and within the specified range of about (two percent to five percent) of its final value for the first time, this time is known as settling time. Settling time is clearly shown in the time response specification curve. Maximum Overshoot : It is expressed (in general) in percentage of the steady state value and it is defined as the maximum positive deviation of the response from its desired value. Here desired value is steady state value. Steady State Error : It can be defined as the difference between the actual output and the desired output as time tends to infinity. Now we are in position we to do a time response analysis of a first order system. Transient State and Steady State Response of First Order Control System Let us consider the block diagram of the first order system. From this block diagram we can find overall transfer function which is linear in nature. The transfer function of the first order system is 1/((sT+1)). We are going to analyze the steady state and transient response of control system for the following standard signal. 1. Unit impulse. 2. Unit step. 3. Unit ramp. Unit impulse response : We have Laplace transform of the unit impulse is 1. Now let us give this standard input to a first order system, we have 50
51 Now taking the inverse Laplace transform of the above equation, we have It is clear that the steady state response of control system depends only on the time constant T and it is decaying in nature. Unit step response : We have Laplace transform of the unit impulse is 1/s. Now let us give this standard input to first order system, we have With the help of partial fraction, taking the inverse Laplace transform of the above equation, we have It is clear that the time response depends only on the time constant T. In this case the steady state error is zero by putting the limit t is tending to zero. Unit ramp response : We have Laplace transform of the unit impulse is 1/s 2. Now let us give this standard input to first order system, we have With the help of partial fraction, taking the inverse Laplace transform of the above equation we have On plotting the exponential function of time we have T by putting the limit t is tending to zero. Transient State and Steady State Response of Second Order Control System 51
52 Let us consider the block diagram of the second order system. From this block diagram we can find overall transfer function which is nonlinear in nature. The transfer function of the second order system is (ω 2 ) / ( s ( s + 2ζω )). We are going to analyze the transient state response of control system for the following standard signal. Unit impulse response : We have Laplace transform of the unit impulse is 1. Now let us give this standard input to second order system, we have Where ω is natural frequency in rad/sec and ζ is damping ratio. Unit step response : We have Laplace transform of the unit impulse is 1/s. Now let us give this standard input to first order system, we have With the help of partial fraction, taking the inverse Laplace transform of the above equation we have Now we will see the effect of different values of ζ on the response. We have three types of systems on the basis of different values of ζ. 52
53 1. Under damped system : A system is said to be under damped system when the value of ζ is less than one. In this case roots are complex in nature and the real parts are always negative. System is asymptotically stable. Rise time is lesser than the other system with the presence of finite overshoot. 2. Critically damped system : A system is said to be critically damped system when the value of ζ is one. In this case roots are real in nature and the real parts are always repetitive in nature. System is asymptotically stable. Rise time is less in this system and there is no presence of finite overshoot. 3. Over damped system : A system is said to be over damped system when the value of ζ is greater than one. In this case roots are real and distinct in nature and the real parts are always negative. System is asymptotically stable. Rise time is greater than the other system and there is no presence of finite overshoot. 4. Sustained Oscillations : A system is said to be sustain damped system when the value of zeta is zero. No damping occurs in this case. Now let us derive the expressions for rise time, peak time, maximum overshoot, settling time and steady state error with a unit step input for second order system. Rise time : In order to derive the expression for the rise time we have to equate the expression for c(t) = 1. From the above we have On solving above equation we have expression for rise time equal to Peak Time : On differentiating the expression of c(t) we can obtain the expression for peak time. dc(t)/ dt = 0 we have expression for peak time, Maximum overshoot : Now it is clear from the figure that the maximum overshoot will occur at peak time tp hence on putting the valye of peak time we will get maximum overshoot as Settling Time : Settling time is given by the expression 53
64 CHAPTER-6 FEEDBACK CHARACTERISTICS OF CONTROL SYSTEM 64
67 CHAPTER-7 STABILITY CONCEPT & ROOT LOCUS 67
79 Rule-6 The breakaway points on which multiple roots of the characteristics equation occur of the root locus are the solution of dk/ds =0 79
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