Transfer Functions

ECM2105 retain applied science Dr Mustafa M Aziz (2010) ________________________________________________________________________________ TRANSFER FUNCTIONS AND BLOCK DIAGRAMS 1. asylum 2. impart Function of linear Time-Invariant (LTI) Systems 3. Block Diagrams 4. Multiple Inputs 5. counterchange Functions with MATLAB 6. Time Response Analysis with MATLAB 1. institution An important step in the analysis and design of control organizations is the mathematical modelling of the controlled process. There be a number of mathematical representations to describe a controlled processDifferential equalitys You have learned this before. Transfer dish up It is defined as the ratio of the Laplace understand of the payoff variable to the Laplace transform of the stimulation variable, with tot all(prenominal)y cipher initial conditions. Block diagram It is subroutined to represent all types of corpses. It jackpot be employ, together with transference functions, to describe the ca routine and effect relationships throughout the system. State-space-representation You will study this in an advanced book Systems Design course. 1. 1. Linear Time-Variant and Linear Time-Invariant SystemsDefinition 1 A meter-variable derived function comp be is a differential coefficient equation with one or more of its coefficients are functions of time, t. For example, the differential equation d 2 y( t ) t2 + y( t ) = u ( t ) dt 2 (where u and y are dependent variables) is time-variable since the term t2d2y/dt2 depends explicitly on t through the coefficient t2. An example of a time-varying system is a space vehicle system which the mass of spacecraft changes during flight callable to fuel consumption. Definition 2 A time-invariant differential equation is a differential equation in which none of its coefficients depend on the independent time variable, t.For example, the differential equation d 2 y( t ) dy( t ) m +b + y( t ) = u ( t ) 2 dt dt where the coefficients m and b are constants, is time-invariant since the equation depends solely implicitly on t through the dependent variables y and u and their derivatives. 1 ECM2105 escort engineer Dr Mustafa M Aziz (2010) ________________________________________________________________________________ Dynamic systems that are expound by linear, constant-coefficient, differential equations are called linear time-invariant (LTI) systems. 2. Transfer Function of Linear Time-Invariant (LTI) SystemsThe transfer function of a linear, time-invariant system is defined as the ratio of the Laplace (driving function) U(s) = transform of the production signal ( repartee function), Y(s) = y(t), to the Laplace transform of the introduce u(t), under the assumption that all initial conditions are zero. u(t) System differential equation y(t) taking the Laplace transform with zero initial conditions, U(s) Transfer function System transfer function G (s) = Y(s) Y(s) U(s) A dynamic system can be described by t he pursuit time-invariant differential equation an d n y( t ) d n ? 1 y( t ) dy( t ) + a n ? 1 + L + a1 + a 0 y( t ) n ? 1 dt dt dt d m u(t) d m ? 1 u ( t ) du ( t ) = bm + b m ? 1 + L + b1 + b 0 u(t) m m ? 1 dt dt dt Taking the Laplace transform and considering zero initial conditions we have (a n ) ( ) s n + a n ? 1s n ? 1 + L + a 1s + a 0 Y(s) = b m s m + b m ? 1s m ? 1 + L + b1s + b 0 U(s) The transfer function amidst u(t) and y(t) is tending(p) by Y(s) b m s m + b m ? 1s m ? 1 + L + b1s + b 0 M (s) = = G (s) = U(s) N(s) a n s n + a n ? 1s n ? 1 + L + a 1s + a 0 where G(s) = M(s)/N(s) is the transfer function of the system the root of N(s) are called poles of the system and the roots of M(s) are called zeros of the system.By setting the denominator function to zero, we obtain what is referred to as the characteristic equation ansn + an-1sn-1 + + a1s + a0 = 0 We shall see later that the stability of linear, SISO systems is completely governed by the roots of the characteristi c equation. 2 ECM2105 declare Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ A transfer function has the hobby properties The transfer function is defined only for a linear time-invariant system. It is not defined for nonlinear systems. The transfer function between a mates of remark and output variables is the ratio of the Laplace transform of the output to the Laplace transform of the input. All initial conditions of the system are set to zero. The transfer function is independent of the input of the system. To derive the transfer function of a system, we use the following procedures 1. Develop the differential equation for the system by using the physical laws, e. g. Newtons laws and Kirchhoffs laws. 2. Take the Laplace transform of the differential equation under the zero initial conditions. 3.Take the ratio of the output Y(s) to the input U(s). This ratio is the transfer function. showcase Consider the following RC circuit. 1) rise up the transfer function of the network, Vo(s)/Vi(s). 2) invent the reception vo(t) for a unit-step input, i. e. ?0 t 0 v i (t) = ? ?1 t ? 0 head 3 R vi(t) C vo(t) ECM2105 deem Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ workout Consider the LCR electric network shown in the figure below. determine off the transfer function G(s) = Vo(s)/Vi(s). L R i(t) vi(t) vo(t) C motion draw the time resolution of vo(t) of the above system for R = 2. 5? , C = 0. 5F, L=0. 5H and ? 0 t 0 . v i (t) = ? ?2 t ? 0 4 ECM2105 Control Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ Exercise In the mechanical system shown in the figure, m is the mass, k is the spring constant, b is the friction constant, u(t) is an outer applied force and y(t) is the resulting displacement. y(t) k m u(t) b 1) Find the differential equation of the system 2) Find the transfer function between the input U(s) and the output Y(s). 5ECM2105 Control Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ 3. Block Diagrams A shutdown diagram of a system is a pictorial representation of the functions performed by each section and of the flow of signals. The block diagram gives an overview of the system. Block diagram items Summing eyeshade Takeoff hitch Block Transfer function +_ The above figure shows the way the various items in block diagrams are represented. Ar dustups are used to represent the directions of signal flow. A summing point is where signals are algebraically added together.The hoax point is similar to the galvanic circuit takeoff point. The block is usually drawn with its transfer funciton written inside it. We will use the following terminology for block diagrams throughout this course R(s) = reference input ( assertion) Y(s) = out put (controlled variable) U(s) = input (actuating signal) E(s) = actus reus signal F(s) = feedback signal G(s) = forward path transfer function H(s) = feedback transfer fucntion R(s) Y(s) E(s) G(s) +_ F(s) H(s) Single block U(s) Y(s) Y(s) = G(s)U(s) G(s) U(s) is the input to the block, Y(s) is the output of the block and G(s) is the transfer function of the block. serial connection U(s) X(s) G1(s) Y(s) G2(s) 6 Y(s) = G1(s)G2(s)U(s) ECM2105 Control Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ correspond connection (feed forward) G1(s) + U(s) Y(s) Y(s) = G1(s) + G2(s)U(s) + G2(s) Negative feedback system (closed- circulate system) R(s) E(s) +_ The closed loop transfer function Y(s) G(s) Y(s) G(s) = R(s) 1 + G(s) Exercise Find the closed-loop transfer function for the following block diagram R(s) Y(s) E(s) G(s) +_ F(s) H(s) 7 ECM2105 Control Engineering Dr Mustafa M Aziz (2010) ________________________________ _______________________________________________ Exercise A control system has a forward path of two elements with transfer functions K and 1/(s+1) as shown. If the feedback path has a transfer function s, what is the transfer function of the closed loop system. R(s) +_ Y(s) 1 s +1 K s Moving a summing point ahead of a block R(s) Y(s) G(s) + R(s) Y(s) + G(s) F(s) 1/G(s) F(s) Y(s) = G(s)R(s) F(s) Moving a summing point beyond a block R(s) Y(s) + R(s) G(s) Y(s) G(s) + F(s) G(s) F(s) Y(s) = G(s)R(s) F(s) Moving a takeoff point ahead of a block R(s) Y(s) R(s) Y(s) G(s) G(s) Y(s)Y(s) G(s) Y(s) = G(s)R(s) 8 ECM2105 Control Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ Moving a takeoff point beyond a block R(s) Y(s) R(s) Y(s) G(s) G(s) R(s) R(s) 1/G(s) Y(s) = G(s)R(s) Moving a takeoff point ahead of a summing point R(s) Y(s) + Y(s) F(s) R(s) F(s) + Y(s) + Y(s) Y(s) = R(s) F(s) Moving a takeoff point beyond a summing point R(s) R(s) Y(s) + Y(s) + F(s) R(s) F(s) R(s) + Y(s) = R(s) F(s) Exercise Reduce the following block diagram and determine the transfer function. R(s) + _ + G1(s) G2(s) G3(s) _ Y(s) + + H1(s)G4(s) H2(s) 9 ECM2105 Control Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ Exercise Reduce the following block diagram and determine the transfer function. H1 + R(s) +_ + G H2 10 Y(s) ECM2105 Control Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ 4. Multiple Inputs Control systems often have more than one input. For example, in that location can be the input signal indicating the required value of the controlled variable and in like manner an input or inputs due to disturbances which affect the system.The procedure to obtain the relationship between the inputs and the output for such systems is 1. 2. 3. 4. Set all inputs except one eq ual to zero situate the output signal due to this one non-zero input Repeat the above steps for each of the remaining inputs in turn The total output of the system is the algebraic sum (superposition) of the outputs due to each of the inputs. Example Find the output Y(s) of the block diagram in the figure below. D(s) R(s) +_ G1(s) + + H(s) Solution 11 Y(s) G2(s) ECM2105 Control Engineering Dr Mustafa M Aziz (2010) _______________________________________________________________________________ Exercise Determine the output Y(s) of the following system. D1(s) R(s) +_ G1(s) + + Y(s) G2(s) H1(s) + + D2(s) 12 H2(s) ECM2105 Control Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ 5. Transfer Functions with MATLAB A transfer function of a linear time-invariant (LTI) system can be entered into MATLAB using the command tf(num,den) where num and den are row vectors containing, respectively, the coefficients of the numerat or and denominator polynomials of the transfer function.For example, the transfer function G (s) = 3s + 1 s + 3s + 2 2 can be entered into MATLAB by typing the following on the command line num = 3 1 den = 1 3 2 G = tf(num,den) The output on the MATLAB command window would be Transfer function 3s+1 s2 + 3 s + 2 Once the various transfer functions have been entered, you can combine them together using arithmetic trading operations such as addition and multiplication to evaluate the transfer function of a cascaded system. The following get across lists the most common systems connections and the corresponding MATLAB commands to implement them.In the following, SYS refers to the transfer function of a system, i. e. SYS = Y(s)/R(s). System MATLAB command Series connection R(s) Y(s) G1 G2 SYS = G1*G2 or SYS = series(G1,G2) Parallel connection G1 + R(s) SYS = G1 G2 or SYS = parallel(G1,G2) Y(s) G2 Negative feedback connection R(s) Y(s) +_ G(s) SYS = feedback(G,H) H(s) 13 ECM2105 Con trol Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ R(s) Y(s) +_ G1 G2 H Example Evaluate the transfer function of the feedback system shown in the figure above using MATLAB where G1(s) = 4, G2(s) = 1/(s+2) and H(s) = 5s.Solution Type the following in the MATLAB command line G1 = tf(0 4,0 1) G2 = tf(0 1,1 2) H = tf(5 0,0 1) SYS = feedback(G1*G2,H) This produces the following output on the command window (check this result) Transfer function 4 -21 s + 2 Exercise encipher the closed-loop transfer function of the following system using MATLAB. R(s) +_ 1 s +1 14 s+2 s+3 Y(s) ECM2105 Control Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ 6. Time Response Analysis with MATLABAfter entranceway the transfer function of a LTI system, we can compute and game the time response of this system due to different input stimuli in MATLAB. In particular, we will consider the step response, the impulse response, the force response, and responses to former(a) simple inputs. 6. 1. Step response To plot the unit-step response of the LTI system SYS=tf(num,den) in MATLAB, we use the command step(SYS). We can also enter the row vectors of the numerator and denominator coefficients of the transfer function nowadays into the step function step(num,den).Example Plot the unit-step response of the following system in MATLAB Y (s) 2s + 10 =2 R (s) s + 5s + 4 Solution Step Response 2. 5 num = 0 2 10 den = 1 5 4 SYS = tf(num,den) step(SYS) Amplitude 2 or directly step(num,den) 1. 5 1 MATLAB will then produce the following plot on the screen. Confirm this plot yourself. 0. 5 0 0 1 2 3 Time (sec. ) 4 5 For a step input of magnitude other than unity, for example K, simply breed the transfer function SYS by the constant K by typing step(K*SYS). For example, to plot the response due to a step input of magnitude 5, we type step(5*SYS).Notice in the prev ious example that that time axis was scaled automatically by MATLAB. You can specify a different time range for evaluating the output response. This is done by get-go defining the required time range by typing t = 00. 110 % Time axis from 0 sec to 10 sec in steps of 0. 1 sec and then introducing this time range in the step function as follows step(SYS,t) % Plot the step response for the given time range, t This produces the following plot for the same example above. 15 6 ECM2105 Control Engineering Dr Mustafa M Aziz (2010) _______________________________________________________________________________ Step Response 2. 5 Amplitude 2 1. 5 1 0. 5 0 0 2 4 6 8 10 Time (sec. ) You can also use the step function to plot the step responses of multiple LTI systems SYS1, SYS2, and so on on a star figure in MATLAB by typing step(SYS1,SYS2, ) 6. 2. Impulse response The unit-impulse response of a control system SYS=tf(num,den) may be plotted in MATLAB using the function impulse(SYS). Exampl e Plot the unit-impulse response of the following system in MATLAB Y(s) 5 = R (s) 2s + 10 Solution Impulse Response um = 0 5 den = 2 10 SYS = tf(num,den) impulse(SYS) 2. 5 2 impulse(num,den) Amplitude or directly 1. 5 1 This will produce the following output on the screen. Is that what you would expect? 0. 5 0 0 0. 2 0. 4 0. 6 Time (sec. ) 16 0. 8 1 1. 2 ECM2105 Control Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ 6. 3. Ramp response There is no ramp command in MATLAB. To obtain the unit ramp response of the transfer function G(s) multiply G(s) by 1/s, and use the resulting function in the step command.The step command will throw out multiply the transfer function by 1/s to make the input 1/s2 i. e. Laplace transform of a unit-ramp input. For example, consider the system Y(s) 1 =2 R (s) s + s + 1 With a unit-ramp input, R(s) = 1/s2, the output can be written in the form Y(s) = 1 1 1 R (s) = 2 ? s + s +1 (s + s + 1)s s 2 1 ? ?1 =? 3 2 ?s + s + s ? s which is equivalent to multiplying by 1/s and then working out the step response. To plot the unitramp response of this system, we enter the numerator and denominator coefficients of the term in square brackets into MATLAB num = 0 0 0 1 en = 1 1 1 0 and use the step command step(num,den) The unit ramp response will be plotted by MATLAB as shown below. Step Response 12 10 Amplitude 8 6 4 2 0 0 2 4 6 Time (sec. ) 17 8 10 12 ECM2105 Control Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ 6. 4. Arbitrary response To obtain the time response of the LTI system SYS=tf(num,den) to an arbitrary input (e. g. exponential function, sinusoidal function .. etc. ), we can use the lsim command (stands for linear simulation) as follows lsim(SYS,r,t) or lsim(num,den,r,t) here num and den are the row vectors of the numerator and denominator coefficients of the transfer function, r is the in put time function, and t is the time range over which r is defined. Example Use MATLAB to obtain the output time response of the transfer function Y(s) 2 = R (s) s + 3 when the input r is given by r = e-t. Solution Start by entranceway the row vectors of the numerator and denominator coefficients in MATLAB num = 0 2 den = 1 3 Then specify the required time range and define the input function, r, over this time t = 00. 16 r = exp(-t) % Time range from 0 to 6 sec in steps of 0. 1 sec Input time function put on the above information into the lsim function by typing lsim(num,den,r,t) This would produce the following plot on the screen. Linear Simulation Results 0. 4 0. 35 Amplitude 0. 3 0. 25 0. 2 0. 15 0. 1 0. 05 0 0 1 2 3 Time (sec. ) 18 4 5 6 ECM2105 Control Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ TUTORIAL PROBLEM SHEET 3 1. Find the transfer function between the input force u(t) and the output displaceme nt y(t) for the system shown below. y(t) b1 u(t) m b2 where b1 and b2 are the frictional coefficients.For b1 = 0. 5 N-s/m, b2 = 1. 5 N-s/m, m = 10 kg and u(t) is a unit-impulse function, what is the response y(t)? microprocessor chip and plot the response with MATLAB. 2. For the following circuit, vex the transfer function between the output voltage across the inductor y(t), and the input voltage u(t). R u(t) L y(t) For R = 1 ? , L = 0. 1 H, and u(t) is a unit-step function, what is the response y(t)? Check and plot the result using MATLAB. 3. Find the transfer function of the electrical circuit shown below. R L u(t) y(t) C For R = 1 ? , L = 0. 5 H, C = 0. 5 F, and a unit step input u(t) with zero initial conditions, compute y(t).Sketch the time function y(t) and plot it with MATLAB. 19 ECM2105 Control Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ 4. In the mechanical system shown in the figure below, m is th e mass, k is the spring constant, b is the friction constant, u(t) is the outer applied force and y(t) is the corresponding displacement. Find the transfer function of this system. k u(t) m For m = 1 kg, k = 1 kg/s2, b = 0. 5 kg/s, and a step input u(t) = 2 N, compute the response y(t) and plot it with MATLAB. b y(t) 5.Write down the transfer function Y(s)/R(s) of the following block diagram. R(s) Y(s) K +_ G(s) a) For G(s) = 1/(s + 10) and K = 10, determine the closed loop transfer function with MATLAB. b) For K = 1, 5, 10, and 100, plot y(t) on the same window for a unit-step input r(t) with MATLAB, respectively. Comment on the results. c) Repeat (b) with a unit-impulse input r(t). 6. Find the closed loop transfer function for the following diagram. R(s) E(s) Y(s) G(s) +_ F(s) H(s) a) For G(s) = 8/(s2 + 7s + 10) and H(s) = s+2, determine the closed loop transfer function with MATLAB. ) Plot y(t) for a unit-step input r(t) with MATLAB. 7. Determine the transfer function of the fol lowing diagram. Check your answer with MATLAB. _ R(s) +_ s s + + 1/s s 20 1/s Y(s) ECM2105 Control Engineering Dr Mustafa M Aziz (2010) ________________________________________________________________________________ 8. Determine the transfer function of the following diagram. R(s) +_ +_ 50 s +1 Y(s) s 2/s 1/s2 2 +_ a) Check you result with MATLAB. b) Plot y(t) for a unit-impulse input r(t) with MATLAB. 9. Determine the total output Y(s) for the following system. D(s)