# Reglerteknik: Exercises

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1 Reglerteknik: Exercises Exercises, Hints, Answers Liten reglerteknisk ordlista Introduktion till Control System Toolbox ver. 5 This version: January 3, 25 AUTOMATIC CONTROL REGLERTEKNIK LINKÖPINGS UNIVERSITET

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3 Exercises This version: January 3, 25

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5 Mathematics. Determine the one-sided Laplace transform of the following signals. {, t < a) u(t) =, where A is a constant. A, t {, t < b) u(t) =, where A is a constant. At, t c) u(t) = e 2t for t. d) u(t) = cos 5t for t. Express the following in U(s), the Laplace transform of u(t). e) u(t) f) u(t), when u(t) = for t. g) ü(t) h) ü(t), when u(t) = u(t) = for t. i) u(t T ).2 Consider the differential equation ẏ(t) + 2y(t) = u(t) a) If u(t) is constant then ẏ(t) when time goes to infinity. What value will y(t) approach as t if u(t) = 5? b) Determine the transfer function relating U(s) and Y (s) for the differential equation above..3 Determine the general solution of the differential equation d 3 y dt 3 + y 4d2 dt 2 + 5dy + 2y = 3 sin(2t) dt.4 Below, differential equations that describe dynamic systems are given together with system inputs and initial conditions. Use the Laplace transform to determine the system outputs. a) b) d 2 y dt 2 + 3dy dt + 2y = σ(t) σ(t) = dy () = y() = dt ẏ(t) + y(t) = u(t) {, t <, t u(t) = + sin t y() =.5 Write the following complex numbers in polar form, that is, determine their absolute value and argument. a) + i b) +i 5i(+ 3i) Write the following complex numbers on rectangular form: c) 2e i π 3 d) 5e iπ.6 A system has amplification. What is the amplification expressed in decibel (db 2 )? What is the amplification corresponding to 2 db 2, 3 db 2, db 2, db 2, and db 2 respectively?.7 Verify that the following rule for inversion of 2 2 matrices holds. ( ) ( ) a a A = 2 A a22 a = 2 a 2 a 22 a a 22 a 2 a 2 a 2 a.8 Determine eigenvalues and eigenvectors of the matrix 2 A = 3 6 2

6 .9 Determine a transformation matrix T, such that T AT is a diagonal matrix, where A = Characterize the range space and null space of the matrix A by specifying bases for them. Find the rank of the matrix. 2 2 A = What are the time functions corresponding to the Laplace transforms below? What values will the time functions approach as time goes to infinity? a) b) c) F (s) = s 2 + s F (s) = s 2 F (s) = (s + ) 2.2 The water level, y, in a tank is modelled by the differential equation ẏ(t) + y(t) = z(t) where z denotes the inflow. The inflow is a function of a valve position, which in turn is controlled by the electric control signal u. The relation between control signal and flow is given by the differential equation z(t) + ż(t) + z(t) = u(t) What differential equation relates the water level y to the control signal u? 2

7 2 Dynamic Systems R a L a i r + Σ e K u Amp Motor y u v θ J, f Figure 2.2a Im Load Figure 2.a 2. A common component in a control system is a DC-motor. A schematic picture of the motor is shown in Figure 2.a. The motor is characterized by a number of physical relationships as will now be explained. The rotating axis is described by J θ = f θ + M, where θ is the angle of rotation, M is the torque, J is the moment of inertia of the load and f is the frictional coefficient. The interplay between rotor and stator is given by M = k a i and v = k v θ where i is the anchor current, k a a proportional constant characteristic for the motor, v is voltage induced by the rotating axis and k v is a proportional constant. The input voltage u is the control signal and θ is the output. a) Use the equations above and Kirchhoff s voltage law to write a differential equation that relates u and θ. The inductance L a can be neglected. b) Determine the transfer function of the system from u to θ. c) Study the behavior of the system by calculating θ when u is a step. K = 3. K =.5 K = 2.5 K =. K = 2.5 K = 3. K =.5 K =. K =.5 K =. Re K = 2.5 K = 3. Figure 2.2b 2.2 A servo system for positioning of a tool in a tooling machine is depicted in Figure 2.2a. In Figure 2.2b, the poles of the closed loop system are plotted for different values of the gain K. Find (without calculations), for each of the step responses in Figure 2.2c, the corresponding value of K used when generating the step response. 3

8 Step Step 2 and that is the input used in the experiment for which the resulting thickness profile d (t) is shown in Figure 2.3b. In production the thickness cannot be measured directly behind the rollers for practical reasons, and instead the thickness d 2 (t) is measured L length units after the rollers. Find the transfer function from the position of the screws to the thickness d 2. The sheet moves with speed V. Step 3 Step 4 Figure 2.2c. All comparable axes have equal scaling. Screw u d t [s] Figure 2.3b Roller d d 2 L Plate 2.4 The step response of the following system G(s) = s 2 + s + is shown as the dashed line in each part of Figure 2.4a. Figure 2.3a 2.3 Consider the simple model of the roller depicted in Figure 2.3a. To obtain a simple model we describe the relationship between the position of the screw and the thickness of the sheet d directly after the rollers as a first order transfer function G(s) = β + st To determine the constants β and T we register the effect of a sudden change in the position of the screw. The units used in the model are chosen such that a unit step will make an appropriately sized input for identification purposes, a) The step response of the system G(s) = s 2 + as + is shown as the solid line in the left of Figure 2.4a. Determine if a > or a <. b) The step response of the system G(s) = b 2 s 2 + bs + b 2 is shown as the solid line in the right of Figure 2.4a. Find b. 4

9 a) b) Pole-zero map Im Step response A Re 5 5 t [s] t [s] Figure 2.4a. Dashed: original system. Solid left: part a). Solid right: part b). B Im Re 2.5 Pair the step responses and pole-zero diagrams in Figure 2.5a. Im 2.6 Consider the systems G A (s) = s 2 + 2s + G C (s) = s 2 + 5s + 4 G E (s) = s 2 + 2s + 4 G B (s) = s 2 +.4s + G D (s) = s 2 + s + a) Use Matlab to plot the step responses of the systems. Find T r (rise time), T s (settling time) and M (overshoot) for the five step responses. b) Compute the poles of the systems G A (s), G B (s), G C (s), G D (s), and G E (s) respectively. c) How is the location of the poles related to the properties of the step responses? C D E Re Im Re Im Re 2.7 Consider a system with the transfer function Im G(s) = αs + s 2 + 2s + Compute and plot the step response of the system for some different values of α in the range < α <. How are the properties of the step response affected by the location of the zero of the system? F Real part Re Time 5 Figure 2.5a. All comparable diagrams have equal scaling. In the pole-zero maps, imaginary and real parts have equal scaling, marks poles, and marks zeros.

10 2.8 Consider a system described by the model Y (s) = G(s)U(s) as shown in Figure 2.8a. Given G(s), how is the step response computed? How can the step response be determined using experiments? u G Figure 2.8a y 2. Figure 2.a shows the step responses of four different systems. Combine each step response with a transfer function from the alternatives below. Transfer function Poles Zeros G() G (s) = s 2 +2s+ ± i G 2 (s) = s+2 2 /2 G 3 (s) = s2 +2s+2 (s+)(s 2 +s+), 5 ± 8.7i ± i 2 2 G 4 (s) = (s 2 +s+)(s+2) 2, 5 ± 8.7i 6 G 5 (s) = (s 2 +s+)(s+3) 3, 5 ± 8.7i 2 4 G 6 (s) = (s 2 s+)(s+2) 2, 5 ± 8.7i Figure 2.9a shows the step response of a system Y (s) = G(s)U(s). The input step has amplitude. Use the figure and determine a) Steady state value. b) Overshoot M in % of the final value. c) Rise time T r. d) Settling time T s. Step A Step B y t [s] Step C Step D Figure 2.a. All comparable axes have equal scaling. Figure 2.9a 6

11 Acid process flow NaOH solution q, c A, q 2, c A,2 V, c A Flow with desired ph Figure 2.a q, c A 2. In the continuously stirred tank, see Figure 2.a, an acid process flow is neutralized by adding a concentrated NaOH solution. The acid process flow has a tendency to vary its ph with time, which gives undesired variation of the ph in the outflow. In an effort to reduce these variations one has decided to use control. a) Classify the different signals as input, output, and disturbance signal. b) Draw a block diagram of the system with a control strategy. Figure 2.2a 2.2 Two flows with different concentrations of a chemical component A are mixed in a continuously stirred tank, see Figure 2.2a. The volume can be assumed constant, V = m 3. a) During a stationary period the values of the concentrations and flows are, c A, =. kmol/m3, q =. m 3 /min, c A,2 = 4. kmol/m3 and q 2 =.5 m 3 /min. What are c A (t) and q(t) during this period? b) Write down the dynamical balance equation for component A. State all your assumptions. Is the dynamical model linear? c) Assume that c A, changes value from. kmol/m 3 to.2 kmol/m 3 at t =. Show that the expression for c A (t) can be written as ( ) c A (t) = k + k e t/τ Determine the constants k, k and τ. 7

12 V i y i L i x i - The equilibrium is described by αx i y i = + (α )x i a) Write the two differential equation that describes M i (t) and x i (t). b) Linearize the model under the assumption that the change of the mass on the plate is zero. 2.4 Figure 2.4a shows the step response from a system with the transfer function M i, x i V i+ y i+ L i x i ω 2 G(s) = s 2 + 2ζω s + ω 2 for the following sets of values. Combine values and step responses. i) ω = 4 ζ =.5 ii) ω = 4 ζ =.3 iii) ω = 2 ζ =.5 iv) ω = 2 ζ =.3 Figure 2.3a 2.3 The distillation plate, see Figure 2.3a, has the following variables: L i V i M i - Liquid flow from plate i (kmol/min) - Steam flow from plate i (kmol/min) - Amount of liquid on plate i (kmol) x i - Mole fraction of the most volatile component in the liquid on plate i. y i - Mole fraction of the most volatile component in the steam from plate i. Assumptions: - Perfect mixing of liquid on the plate. - Binary separation - y i is in equilibrium with x i. 8

13 .5 Step Response A.5 Step Response B Amplitude.5 Amplitude Time (sec) Time (sec).5 Step Response C.5 Step Response D Amplitude.5 Amplitude Time (sec) Time (sec) Figure 2.4a 9

14 3 Feedback Systems Valve x b) The transfer function of the valve is assumed to be Inflow G v (s) = k v + T s To find k v and T a unit step change in u has been applied. The step response, that is, the resulting liquid flow, is shown in Figure 3.b. Determine the constants k v and T. u y Tank m 2 y c) Draw a block diagram of the system, in which the valve and the tank are represented by one block each. v 2.5 x Figure 3.a Pump 3. Consider a part of a process industry, consisting of a valve and a tank. The system is described by Figure 3.a and the following information: t [s] Figure 3.b The valve is operated electrically, and the input voltage is denoted u. The resulting liquid flow is denoted x. The level in the tank is denoted y, and the flow out from the tank, v, is determined by a pump located further down the process. The cross section area of the tank is m 2. The pipes are always filled with incompressible liquid, so there are no transport delays in the system. a) Use mass balance, that is, the fact that the change in tank volume per time unit is proportional to the difference between inflow and outflow, to determine a transfer function G t (s) for the tank. 3.2 Consider the system studied in Problem 3.. The aim is now to control the tank level automatically, and therefore PID feedback control is introduced according to Figure 3.2a. This means that the valve input is determined according to the expression u(t) = K P e(t) + K I t where e(t) denotes the control error, that is, e(t) = r(t) y(t) e(τ) dτ + K D de(t) dt and r(t) denotes the reference level, that is, desired tank level. (3.)

15 Valve x.2 and K P = respectively. How does the choice of K P affect the properties of the closed loop system? Inflow u PIDcontroller r y Tank m 2 v y b) Assume that the reference level is constant, r(t) = 5, and that the outflow is constant, v(t) = 2. Determine the steady state tank level when the proportional feedback F (s) = K P is used. Is it possible for the output signal to reach the desired level? What happens with the steady state level and other properties of the closed loop system if K P is chosen very large? c) Assume that a PI controller is used, that is, u(t) = K P e(t) + K I e(τ) dτ What can be said about the steady state tank level in this case? What is the possible benefit of introducing the integrating part in the feedback? d) Finally, assume that a PD controller is used, that is, Figure 3.2a Pump a) Verify, using Laplace transform, that the relationship in (3.) can be expressed U(s) = F (s)(r(s) Y (s)) where F (s) = K P + K I s + K Ds b) Let the PID controller be represented by a block and draw a block diagram of the entire feedback control system. c) Use the block diagram from above together with the transfer functions G v (s), G t (s), and F (s), and derive the transfer functions of the closed loop system, that is, the transfer functions from the reference level and the outflow to the tank level. 3.3 Consider the level control system studied in Problems 3. and 3.2. a) Assume that the tank level is controlled using proportional control, that is, F (s) = K P. Compute the poles of the closed loop system for K P = u(t) = K P e(t) + K D de(t) dt Assume also that K P = and calculate a value of K D so the damping ratio of the closed loop poles will be greater than / 2. This corresponds to placing the poles in the grey area in Figure 3.3a. What is the benefit of including the derivative part in the feedback? Figure 3.3a 3.4 Figure 3.4a shows the block diagram for a hydraulic servo system in an automatic lathe (Swedish: svarv). The output signal y(t) represents the position of Im Re

16 y ref + Σ Controller e F (s) f c + + Σ Figure 3.4a Hydraulic cylinder ms 2 + ds the lathe tool (Swedish: svarvstål), y ref (t) is the desired position of the lathe tool, m is the mass of the tool slide (Swedish: verktygsslid) and the hydraulic piston (Swedish: kolv), d is the viscous damping of the tool slide, F (s) is the transfer function for the controller, f c is the cutting power. a) How large, in steady state, is the error e(t) between the actual value of the lathe tool and its desired reference value, when there is a step disturbance in the cutting power f c (t)? The controller is assumed to be an amplifier with a constant gain F (s) = K. b) How is this error changed if the amplifier is replaced by a PI controller with transfer function F (s) = K + K 2 /s? 3.5 Consider the system Y (s) = G(s)U(s) =.2 (s 2 + s + )(s +.2) U(s). a) Suppose G(s) is controlled by a proportional controller with gain K P, that is, U(s) = K P (R(s) Y (s)). Use Matlab to compute the closed loop system, and to plot the step response of the closed loop system. Choose some values for K P in the range. to. How are the properties of the step response affected by K P? What happens with the steady state error when K P increases? Is it possible to obtain a well damped closed loop system and small steady state error using proportional control? b) Let us now introduce integration in the regulator and use y Put K P = and try some values of K I in the range < K I < 2. How are the step response and the steady state error affected by the introduction of the integrating part and the value of K I? c) Finally we will introduce the differentiating part in the regulator and use U(s) = (K P + K I s + K Ds )(R(s) Y (s)). st + Since true differentiation is difficult to implement, the derivative part is approximated by K Ds +st. (This will low-pass filter the error signal before differentiation.) Put K P =, K I = and T =. and try some values of K D in the range < K D < 3. How does the D-part affect the step response of the closed loop system? + Σ G o (s) Figure 3.6a 3.6 Draw a root locus with respect to K for the system in Figure 3.6a, with G o (s) given below. For which values of K are the systems stable? What conclusions on the principal shape of the step response can be drawn from the root locus? a) A Ferris wheel (Swedish: Pariserhjul): b) A Mars rover: c) A magnetic floater: G o (s) = G o (s) = G o (s) = K(s + 2) s(s + )(s + 3) K s(s 2 + 2s + 2) K(s + ) s(s )(s + 6) U(s) = (K P + K I )(R(s) Y (s)). s 2

17 θ ref + Σ + Σ K α Figure 3.7a k + sτ 3.7 Consider the servo system in Figure 3.7a with a DC-motor. Suppose that the angular velocity can be measured with a tachometer and let the control law be as in the block diagram. Let τ =.5 and k = 2. a) Draw the root locus with respect to K for the system without the tachometer feedback (that is, α = ). b) Draw the root locus with respect to K for α =. c) Draw the root locus with respect to K for α = /3. d) Let K = and draw the root locus with respect to α. Discuss, using the results from a), b), c), and d), what is gained by using the tachometer. θ s θ If we consider small deviations from a reference value θ, we get the transfer function from δ to ω for a specific aircraft as G 2 (s) = s + (s + 4)(s 3) This model is valid when the aircraft is flown with a large θ. The elevator (Swedish: höjdroder) is driven by a hydraulic servo amplifier with the transfer function G (s) = s + from elevator command δ ref to δ. a) What happens with ω if one gives a constant elevator command δ ref? Motivate! b) The angular velocity ω is measured and a control law is used so that the input δ ref to the servo amplifier is K(ω ref ω). Draw root locus with respect to K. For which values of K is the system stable? c) Is there any value of K such that the closed loop system is stable and all poles are real? θ θ ref + Σ + Σ (s + )(s + ) K θ s θ δ Figure 3.8a 3.8 Consider an aircraft where the pitch angle θ is controlled by the elevator deflection (Swedish: höjdroderutslag) δ, see Figure 3.8a. Let ω be the angular velocity, ω = θ. K 2 Figure 3.9a 3.9 The block diagram in Figure 3.9a shows a cascade controlled DC-motor where K > and K 2 >. a) Draw root locus with respect to K 2 for the characteristic equation of the closed loop system. For which K 2 > are the closed loop system asymptotically stable? 3

18 b) How is the stability requirement on K 2 affected by the size of the velocity feedback K? 3. We want to control the temperature of an unstable chemical reactor. The transfer function is (s + )(s )(s + 5) a) Use a proportional controller and draw a root locus with respect to the amplification K. Calculate which K in the compensator that stabilizes the system. b) Use a PD controller. The control law is given by u = K(e + T D de dt ) where e is the error. Let T D =.5 and draw a root locus with respect to K. For which values of K does the controller stabilize the system? b) Use k = 6 and assume that the noise is a high frequency sinusoid. The amplitude of y f when y m (t) = sin(t) is used as a measurement of how effective the noise reduction is. What is the smallest value you can obtain (after transients) by choosing a suitable a. We also want y to tend to the steady state value when r(t) is a unit step. 3.2 Figure 3.2a shows the root locus for the characteristic equation of a P- controlled process G with respect to the gain K. In Figure 3.2b four step responses for the closed loop system with different values of K are shown. Match the plots in Figure 3.2b with the K-values below. Justify your answer. K = 4 K = K = 8 K = 5 Im r + Σ u k s(s + 2) y 2 y f a s + a y m + Σ + Measurement noise Re Figure 3.a 3. Consider the system in Figure 3.a. In a realistic situation what you really measure is not y(t) but a signal y m (t) which is the sum of y(t) and measurement noise. To avoid that the control is based on noisy measurements one uses y f (t) instead of y m (t). The signal y f (t) is y m (t) filtered through the low pass filter a s + a a) First we assume that the measurement noise is negligible. Choose k = 6. Draw a root locus for the closed loop system with respect to the time constant of the low pass filter /a. Find for which a > the system is stable. Figure 3.2a. Starting points are marked and end points. -2 4

19 Step A Step B h, and changes in the flows, q in, and q out,, is given by d dt (Ah ) = q in, q out, where A is the area of the lake. In order to try to keep the level of the lake constant the flows through the lake are controlled at the inflow such that q dam, = K(h ref, h ) Step C Step D Figure 3.2b. All comparable axes have equal scaling. 3.3 Consider a system with the transfer function G(s) = sn + b s n 2 + b n s n + a s n + + a n that has all zeros strictly in the left half plane. Show that such a system always can be stabilized by u(t) = Ky(t) if K is selected large enough. Dam Lake q dam q in q out Figure 3.4a Dam where h ref, is the reference value. The reservoir that controls the outflow is controlled so that q out is constant, that is, q out, =. Since it takes time before a change in q dam gives a result in q in we have q in, = q dam, (t T ) where T =.5 hours. How large can the quotient K/A be at the most before the system becomes unstable? Σ K G(s) + Figure 3.5a 3.5 A system G(s) is controlled using feedback with a proportional controller according to Figure 3.5a. a) For K =, the open loop system KG(s) has the Nyquist diagram according to (i), (ii), (iii), or (iv) in Figure 3.5b. Is the closed loop system stable in each case? G(s) has no poles in the right half plane. b) If K >, for which values of K are the different closed loop systems stable? 3.6 a) Draw the Nyquist curve for an integrator G(s) = /s. b) Draw the Nyquist curve for the double integrator G(s) = /s We want to control the level in a lake by controlling the flow using flood gates, see Figure 3.4a. The relationship between changes in the level of the lake, 5

20 Im Im Im Re - Re.5 2 Re ω = ω = ω = (i) (ii) Im Im ω = 2 3 Figure 3.7a -2 - Re Re y ref + Σ e K G(s) y Figure 3.7b (iii) Figure 3.5b (iv) 3.7 The system G(s) is asymptotically stable and has the Nyquist curve in Figure 3.7a. It is controlled using feedback according to Figure 3.7b. a) For what values of K (K > ) is the closed loop system asymptotically stable? b) Determine the steady state error, e, as a function of K if y ref is a unit step. c) Assume that G is controlled using an I controller according to Figure 3.7c. For what values of K is the closed loop system stable? y ref e + Σ 3.8 Consider the DC-motor It is controlled by K s Figure 3.7c G(s) τÿ(t) + ẏ(t) = u(t) u(t) = K(r(t T ) y(t T )) Here τ and T are positive constants. K is slowly increased until the system oscillates with the angular frequency ω =. K is then set to 33% of this value. After a while the system starts to oscillate again, now with the angular frequency ω =.5. This is due to the fact that the time delay T has changed y 6

21 to T. Can the parameters τ, T, and T be decided from these data? If so determine τ, T and T. 3.2 The Nyquist curve for the system G(s) = B(s) A(s) can be seen in Figure 3.2a. Determine which one of the root loci in Figure 3.2b that matches y ref + Σ e F (s) G(s) y for this system. A(s) + KB(s) = Im Figure 3.9a 3.9 A system G(s) is to be controlled using the regulator F (s) = K s Re according to the Figure 3.9a. The controller has positive gain K that, however, is not completely known. For what values of K is the closed loop system asymptotically stable? The Nyquist curve for G(s) is given in Figure 3.9b. Im ω = 3 ω = ω = Re ω = ω = / The system s(s+) Figure 3.2a with input u and output y has the controller U(s) = F (s)(r(s) Y (s)) F (s) = b s 2 + b s + b 2 s How should the coefficients of F be chosen to achieve pure P, pure I and pure D control respectively? 3.22 The equations for the P, PI, and PID controllers to be used in this problem are given in Problem 3.5. a) Let the system Figure 3.9b Y (s) = G(s)U(s) =.2 (s 2 + s + )(s +.2) U(s) be controlled by a proportional controller with gain K P. Use Matlab to plot the root locus with respect to K P of the characteristic equation of the closed loop system. For which values of K P > is the closed loop system asymptotically stable? 7

22 Root locus A Root locus C Im Im Re Re Root locus B Root locus D Figure 3.2b. Starting points (K = ) are marked, and end points (K ) are marked. All diagrams have equal scaling. In Problem 3.5, we found that the step response was slow but well damped for small values of K P, while it became faster but more oscillatory when K P was increased. For large values of K P the system became unstable. We also found that the steady state error was reduced when K P was increased. Can these results be interpreted using the plot of the root locus? b) Now assume that the system is controlled by a PI controller where K P =. Plot the root locus of the characteristic equation, with respect to K I, and determine for which values of K I > the closed loop system is asymptotically stable. Problem 3.5 showed that an integrator eliminates the steady state error. Im Im Re Re A small value of K I gives a large settling time, while a too large value gives an oscillatory, and perhaps unstable closed loop system. Give an interpretation of these results using the root locus. c) Finally, let the system be controlled by a PID controller where K P =, K I = and T =.. Plot a root locus of the closed loop characteristic equation, with respect to K D >, and relate the behavior of the root locus to the simulation result in Problem 3.5, that is, that the derivative part increases the damping of the closed loop system, but a too large K D will give an oscillation with a higher frequency, and finally an unstable closed loop system Consider the system Y (s) = G(s)U(s) =.2 (s 2 + s + )(s +.2) U(s). a) Use Matlab to plot the Nyquist curve of the open loop system when G(s) is controlled by a proportional regulator. Try some different values of K P and find for which K P the closed loop system is asymptotically stable. Compare your results with those from Problem 3.22a. b) Assume now that the system is controlled by a PI controller where K P =. Investigate how K I affects the Nyquist curve and determine for which values of K I the closed loop system is asymptotically stable. Do you get the same results as in Problem 3.22b? c) Finally test a PID controller with K P =, K I = and T =. (cf Problem 3.5). How is the Nyquist curve affected by the value of K D? 3.24 a) Assume that the system Y (s) = G(s)U(s) =.4 (s 2 + s + )(s +.2) U(s) is controlled by a proportional controller where K P =. Use Matlab to make a Bode plot of the open loop system and determine ω c (gain crossover frequency), ω p (phase crossover frequency), ϕ m (phase margin) and A m (gain margin) respectively. Compute the closed loop system and plot the step response. b) Now let K P = 2.5. How does the change of K P affect ω c, ω p, ϕ m, and A m? Simulate the step response of the closed loop system and plot the result. How have the properties of the step response changed? 8

23 c) How much can K P be increased before the closed loop system becomes unstable? How does this value relate to the value of A m that was obtained for K P =? Compute and plot the step response of the closed loop system for this value of K P. How does the closed loop system behave in this case? 3.25 A system is controlled by a PID controller, U(s) = (K P + K I s + K Ds)E(s) In Figure 3.25a four step responses from a unit step for the parameter triples A B i) K P = K I = K D = ii) K P = K I = K D = iii) K P = K I = K D = iv) K P = K I = K D = are shown. Match each one of the parameter triples to one of the step responses. Justify your answer! 3.26 Assume that a DC-motor of the type described in Problem 2. is controlled by a proportional controller, that is, u(t) = K P (θ ref θ). a) Write down a block diagram for the control system. Compute the closed loop transfer function and determine how the poles of the closed loop system depend on the control gain K P. Discuss what this means for the behavior of the system for different values of K P. b) Determine the transfer function from the reference signal to the error. Let the reference signal be a step and a ramp respectively and determine what the control error will be in steady state in these two cases. c) Let the controller be a PI controller. What will the steady state error be in this case if the reference signal is a ramp? 3.27 Determine the transfer function for the loop gain and the closed loop system for the control system given by the block diagram in Figure 3.27a Figure 3.28a shows a block diagram of a control system. Determine the transfer function C Figure 3.25a. Four step responses. All comparable axes have equal scaling. a) of the loop gain, r u + Σ + Figure 3.25b G o (s) Figure 3.27a b) of the closed loop system from R(s) to Y (s), c) from the disturbance N(s) to the output Y (s), d) from the reference signal R(s) to the error signal E(s) Consider again the control system in Figure 3.28a, with n = and G(s) = (s + )(s + 3) a) Assume F (s) = K. Determine the steady state control error when r(t) is a step. D y 9

24 r + Σ e F u G + Σ y + n Figure 3.28a b) Determine a regulator F (s) such that the steady state error is zero when r(t) is a step. c) Assume F (s) =. Determine the poles and zeros of the closed loop system. A B 3.3 The system Y (s) = is controlled using PID feedback (s/.6 + )(s + ) U(s) U(s) = (K P + K I s + K Ds)(R(s) Y (s)) Figure 3.3a shows the step responses for the following four combinations of coefficient values. Combine the step responses and coefficients. () K P = 4 K I = K D = (2) K P = 4 K I = 3 K D = (3) K P = 4 K I = K D = (4) K P = 4 K I = K D = Continuously stirred tanks have an extensive use in chemical processes. They are often supplied with some sort of heat exchange system. If the tank is used for a chemical or biochemical reaction it is often important to keep a certain temperature to obtain desired productivity. A continuously stirred tank with a common type of heat exchange system is shown in Figure 3.3a. The tank is surrounded with a heating/cooling layer in which a liquid flows through in order to heat or chill the liquid in the tank. C Figure 3.3a. Four step responses. All comparable axes have equal scaling. a) Determine the important signals of the system. Suggest a control strategy based on feedback and draw the block diagram. b) To be able to construct and evaluate different controllers it is necessary to have a process model. Determine a dynamical model for this system. Assume that the volumes in the tank and in the heat exchange system are constant. c) Linearize the model. (Stationary values: T t = 5. C, T c = 75. C.) d) The parameters of the model are as follows ρ t = ρ c =. kg/m 3 c p t,in = cp c = 4.2 kj/kg C U = 672 kj/ Cmin F t =. m 3 /min F c =.2 m 3 /min V t =. m 3 V c =. m 3 T t,in =. C D T c,in = 95. C Determine the transfer function from flow in the heat exchange system to the temperature in the tank. e) Let the system be controlled by a P controller. Draw the root locus for the system. 2

25 F t,in, T t,in F c,in, T c,in 3.34 While working in space an astronaut has to be able to move. Necessary force is obtained by letting out gas from thrusters. For such a positioning control system the control law F c, T c Tank V t, T t Cooler F t, T t Figure 3.3a. Process consisting of a tank and a cooler. Input flows have a in subscript, while outputs have no such subscript since they are also the same as the quantities found inside the tank or cooler Consider a continuously stirred tank with a cooling system. In the tank a component A reacts to form component B in an exothermic reaction. This reaction is unstable, but possible to stabilize with feedback. A model for the purpose of control has been established u = K (r y) K K 2 dy dt is used, where u = thruster force, r = set point for the astronaut s position and y = the actual value of the astronaut s position. Draw a block diagram of the system, and use physics (Newton s law) to make a model of the astronaut. Also determine K and K 2 such that If the set point r(t) = t there has to be a time T so that r(t) y(t) < for t T. The damping ratio of the closed loop system will be.7. The mass of the astronaut is assumed to be kg, equipment included. Y (s) = s 2 + 2s 3 U(s) where y(t) is the temperature in the tank and u(t) is the cooling flow. a) Show that the system is unstable. b) Prove that the system can be stabilized by a P controller Bacterial growth is described by the equation ẏ = µy where y is the amount of bacteria and µ is a positive constant. Assume that we have a control signal u available that affects the speed of growth so that ẏ = µy + u One can then use a P controller u = K(r y) where r is a reference signal. For which K-values will the system approach an equilibrium? 2

26 4 Frequency Description 4. A mercury thermometer can be described with high accuracy as a first order linear time invariant dynamic system. The input is the real temperature and the output is the thermometer reading. In order to decide the transfer function in a thermometer it is placed in liquid where the temperature is varied as a sinusoid. The obtained result is shown in Figure 4.a. Find the transfer function of the thermometer. Ψ δ Period =.34 min min Figure 4.2a Thermometer reading Ψ ref + Σ e F (s) u G r (s) δ G s (s) Figure 4.2b Ψ Transient Bath temperature Stationary state engine. Figure 4.2b shows a block diagram of the auto pilot. The auto pilot has the transfer function Figure 4.a 4.2 We want to keep a ship on a given course, Ψ, with an automatic control system using the rudder angle δ. See Figure 4.2a. If ω denotes the angular velocity of the ship, ω = Ψ (4.) the following differential equation is valid for small values of ω and δ, while G r is given by F (s) = K + s a + s, a =.2, b =.5 b G r (s) = and G s (s) is defined by (4.) and (4.2). + st 2, T 2 = a) Make a Bode plot for the transfer function F G r G s, for K =.5. T ω = ω + K δ (4.2) where T = and K =.. The desired course, Ψ ref, and the measured course, Ψ, are fed in to the auto pilot, which gives the signal u to the rudder 22

27 b) At the testing of the auto pilot we do the following experiment. The gain of the auto pilot K is increased until the control system oscillates with constant amplitude. At what value of K does this occur? What is the period time of the oscillation? c) Ψ ref is allowed to vary as a sinusoid Ψ ref (t) = A sin αt Step response Bode plot where A = 5 and α =.2. When the movements of the ship have stabilized we have Ψ(t) = B sin(βt + ϕ) A What values do B, β, and ϕ have if K =.5? Im B Re C Figure 4.3a 4.3 a) In Figure 4.3a the Nyquist curve for a system is shown. Draw the Bode plot for the same system. The scale on the ω-axis is not important, as long as the amplitude and phase curve are in agreement. b) Draw a diagram for the poles and zeros for the system. The relative placement is important, not the scale. D Time Frequency 4.4 Figure 4.4a shows the step responses (when the input is a unit step) and Bode gain plots of four different systems, in no particular order. Identify the pair of plots that belongs to each system. That is, for each step response, find the corresponding Bode gain plot (amplitude curve). Motivate your answer by pointing out a set of unique features for each system. Figure 4.4a. All comparable diagrams have equal scaling. 23

28 4.5 a) Consider the transfer functions G A (s), G B (s), G C (s), G D (s), and G E (s) in Problem 2.6. G A (s) = s 2 + 2s +, G B(s) = s 2 +.4s + G C (s) = s 2 + 5s +, G D(s) = s 2 + s + 4 G E (s) = s 2 + 2s + 4 Study the amplitude curves of the Bode plots for the systems and find the static gain and bandwidth of the systems. In cases when it is relevant find also the resonance frequency and resonance peak. b) Describe qualitatively (without formulas) the relationships between T r (rise time) and ω B (bandwidth) and between M (overshoot) and M p (resonance peak) respectively? a) Determine the gain ( G(iω) ) and phase (arg G(iω)) for the system for each value of ω. b) Determine the gain values in db 2 (2 log ( G(iω) )). c) Sketch the Bode plot using the values determined above. 4.9 Combine the transfer functions below with the Bode plots in Figure 4.9a. G (s) = s +, G 6(s + ) 2(s) = (s + 2)(s + 3) G 3 (s) = s, G 4(s) = s(s + ) 5 G 5 (s) = s 2 (poles: ± i2) + 2s A system has the transfer function G(s) = e 2s s(s + ) What is the output (after transients) when the input is u(t) = 2 sin(2t /2) 4.7 For the systems below the input is chosen as u(t) = sin(2t). Determine the output signal y(t) after transients have faded away, provided that it exists. a) Y (s) = s+ U(s) b) Y (s) = s U(s) c) Y (s) = (s+)(2s+) U(s) d) Y (s) = e.5s s+ U(s) 4.8 A system is described by Y (s) = G(s)U(s). Figure 4.8a shows u(t) = sin(ωt) and the corresponding output y(t) (after all transients have faded away) for the frequencies ω =, 5,, and 2 rad/s (from top to bottom). 24

29 Figure 4.8a. u(t) = sin(ωt) (solid) and y(t) (dashed). Bode gain A Bode gain C Bode gain E Bode gain B Bode gain D Figure 4.9a. All diagrams have equal scaling. 4. Figure 4.a shows the Bode gain plots and step responses of four different systems, in no particular order. Identify the pair of plots that belongs to each system. That is, for each Bode gain plot (amplitude curve), find the corresponding step response. Motivate your answer by pointing out a set of unique features for each system. 25

30 y ref + Σ e F (s) u G(s) y Figure 4.a A Bode plot Step response 4. The ph in a biochemical reactor is controlled by addition of a base. The transfer function G(s) from added base to ph for the open system has been determined by experiments to be G(s) =.7 (s + )(.7s + )(.5s + ) B In a attempt to control the ph the control structure shown in Figure 4.a is employed a) Make a Bode plot for the transfer function G(s). b) Assume that a P controller is used (F (s) = K). At what value of K does the ph start to oscillate with constant amplitude? C D Frequency Time Figure 4.a. All comparable diagrams have equal scaling. 26

31 q F u G(iω) c X arg G(iω) Figure 4.2a 3 2 ω [rad/s] 4.2 Consider the biochemical reactor in Figure 4.2a. It is desirable to control the concentration of biochemical material c X (output y) by manipulating the dilute flow q F (input u). A model of this system can be described as Y (s) = 2e 5s 3s + U(s) where the time delay reflects the time it takes to measure the biochemical concentration. The bode digram of the system is shown in Figure 4.2b. Figure 4.2b responses of the closed loop systems (third row) and the poles of the closed loop systems (bottom). Assign each closed loop system, closed loop step response and closed loop pole diagram to the correct open loop system. b) Solve the same task for Figure 4.3b. a) For which values of K is a P controller going to stabilize the system? b) Construct a controller which has the crossover frequency ω c,d =. and no steady-state error. 4.3 A system is used with a controller Y (s) = G o (s)u(s) U(s) = R(s) Y (s). a) In Figure 4.3a are Bode diagrams of the open loop systerm G o for two different systems (top), the closed loop systems G c (second row), step 27

33 5 Compensation ω ref + Σ Controller Motor Mechanical resonance. 4 F (s) (s +.)(s +.5) s 2 +.4s + 4 ω See Figure 5.2a. Measurements have been made using a sinusoidal input u and the gain and phase shift have been measured at different frequencies. The results are given in the following table. Figure 5.a 5. The block diagram for speed control of a DC-motor is shown in Figure 5.a. Find a compensator F (s) such that the following specifications are fulfilled. The system should be twice as fast as for F (s) =, but with the same damping as for F (s) =. If ω ref is constant, ω ref ω /ω ref should be less than 5%. The controller should not be unnecessarily sensitive for high frequency disturbances and the open loop system s low frequency amplification should not be larger than necessary. Frequency [rad/s] Gain Phase shift a) Make a Bode plot for the system. b) What is the largest crossover frequency possible to achieve when using proportional control and wanting a phase margin of at least 5? c) Suggest a compensator that doubles the speed compared to b) and still keeps the phase margin. B u 5.3 A hydraulic system with a valve and a piston is described by the following linearized transfer function G(s) = k u /A s( s2 + 2ζ s ω 2 ω + ) where A is the area of the piston and k u the hydraulic gain. A Figure 5.2a 5.2 The outflow temperature θ in the liquid A can be controlled in a heat exchanger by controlling the flow of the liquid B by a valve with the setting denoted u. a) Make a Bode plot for the system when ω = 5 rad/s, ζ =., and k u /A = 2. b) What is the smallest value of the ramp error that can be achieved using proportional control if we want an amplitude margin of 2? What is the crossover frequency in this case? c) Suggest a compensator such that the ramp error decreases 5 times at the same time as the crossover frequency, phase margin and amplitude margin will be the same as in b). Due to physical constraints in the implementation, this regulator has to have finite amplification at all frequencies. 29

34 θ ref e + Σ F (s) u Figure 5.4a k m s( + st m )( + st e ) 5.4 Figure 5.4a shows a position servo including a DC-motor. The extra time constant T e is due to the inductance in the winding of the motor, which is usually not taken into account. The parameter values are k m =, T m =. and T e =.. We want the servo to fulfill the following specifications: Rise time. s. Overshoot %. The steady state error at step in θ ref should be zero. θ A B Bode plot Open loop system -8-8 Bode plot Closed loop system Step response Closed loop system The steady state error when θ ref is a ramp with slope /s should be less than.. Suggest a compensator such that the specifications are fulfilled. (Clue: Suppose that the relationship between rise time, overshoot, and other specifications are the same as for a second order system, that is, according to Figures 5.4b and 5.4c in Solution 5.4 (the figures can also be found in Glad&Ljung).) 5.5 In Figure 5.5a we have arranged step responses and open loop and closed loop (feedback with ) Bode plots for five different systems. Identify the three plots that belong to each of the five systems, one open loop and one closed loop Bode plot and one step response. Motivate your answer by pointing out one unique feature for each system. C D E Frequency Frequency Time Figure 5.5a. All comparable diagrams have equal scaling. 3

35 θ ref + Σ e F (s) u + st k m s( + st )( + st 2 ) θ Figure 5.6a 5.6 A DC-servo is described by the block diagram in Figure 5.6a, where T = 5 ms is a mechanical time constant, k m = is a proportional constant, T 2 = 25 ms is an electrical time constant, and T = ms is an amplifier time constant. The system is tested with F (s) = and we find that the dynamic properties are satisfactory but that the system is somewhat too slow. Find an F (s) so that the closed loop system is twice as fast as for F (s) =, without increasing the overshoot. F (s) should also give a closed loop system which fulfills the following accuracy demands: θ θ ref. rad in steady state when θ ref is constant. When θ ref is a ramp with slope rad/s we should have θ θ ref. rad in steady state. Σ K G(s) + G(iω) arg G(iω) ω [rad/s] Figure 5.7b.2 Figure 5.7a 5.7 The amplitude curves and the phase curves in Figure 5.7b have been measured for a system without poles in the right half plane. The system is controlled using feedback according to Figure 5.7a. Use the Nyquist criterion to decide for which values of K the closed loop system is stable (K > ). 5.8 A block diagram for a control system with time delay is shown in Figure 5.8a. The system G has no poles in the right half plane. a) G has a Bode plot according to the plot in Figure 5.8b. Determine for what values of the time delay T the closed loop system is stable. b) The same as in a), but for the plot in Figure 5.8c. Σ G (s) e st + Figure 5.8a 3

36 Go(iω). arg Go(iω) ω [rad/s] Figure 5.8b G(iω).5 arg G(iω) ω [rad/s] Figure 5.8c 32

37 r e u + Σ F (s) Amplifier Motor y Figure 5.9a 5.9 A servo system based on a DC-motor has to be designed. A block diagram for the system is given in Figure 5.9a. By measuring the phase shift and the gain at different frequencies the Bode plot for the motor, see Figure 5.9b, has been determined. The amplification is a system of the first order, that is, it has the transfer function G A (s) = k A s + a. In order to find the constants k A and a, a unit step experiment has been carried out on the amplifier, giving the output shown in Figure 5.9c. a) Find the constants k A and a from Figure 5.9c. Also draw the Bode plot for the open loop system, that is, the system from u to y. b) Find a compensator F (s), such that the closed loop system fulfills the following demands: Gm(iω) arg Gm(iω) Figure 5.9b ω [rad/s] The system has to be 5 times as fast as when using F (s) =. The overshoot should not be larger than for F (s) =..4 y t [s] Figure 5.9c 33

38 u G s y a) Draw the Nyquist curve of the system. b) Assume that the system is controlled using the proportional feedback Figure 5.a U(s) = K(R(s) Y (s)) 5. A system G(s) can be split into two sub-systems G(s) = G (s) s according to Figure 5.a. The Bode plot for G (s) is given in Figure 5.b. Find a compensator for the system G(s) such that the following is fulfilled: For which K > is the closed loop system asymptotically stable? c) Assume that we choose K = 2 in the proportional controller in problem b). What will the steady state error be when r(t) = t? d) Assume that y(t) is delayed T seconds. How large is T allowed to be in order for the system to still be asymptotically stable with K = 2? The phase margin for the compensated system is 4. The closed loop system is twice as fast as what is possible to achieve using proportional control with a 4 phase margin. The steady state error when the reference signal is a ramp is % of the corresponding error with proportional control and 4 phase margin. G(iω) - -2 G(iω) arg G(iω) arg G(iω) Figure 5.a ω [rad/s] ω [rad/s] Figure 5.b 5. The Bode plot for a system is given in Figure 5.a. 34

39 5.2 a) A plot of the amplitude curve of a stable transfer function G o (s) is given in Figure 5.2a. Choose one of the following alternatives regarding the stability of the closed loop system G o : + G o. It is stable. 2. It is not stable. 3. Impossible to determine given these facts only. b) Repeat for the transfer function whose amplitude curve is given in Figure 5.2b. Justify your answers carefully. Go(iω) arg Go(iω)? Go(iω) 2 2 Figure 5.2b ω [rad/s] arg Go(iω)? 2 2 ω [rad/s] Figure 5.2a 5.3 Consider the relation where G(s) = Y (s) = G(s)U(s) 725 (s + )(s + 2.5)(s + 25) a) Assume that the system is controlled by U(s) = F (s)(r(s) Y (s)) where F (s) =. Find ω c, ω p, ϕ m, and A m for the loop gain. 35

40 b) Compute a regulator such that the open loop system fulfills the following requirements: (i) ω c = 5 (ii) ϕ m 6 and the closed loop system fulfills: (iii) e = Draw the Bode plot of the compensated open loop system and check that the requirements are satisfied. Simulate the closed loop system for a step in the reference signal and plot the step response. Check that the requirement on the steady state error is satisfied. c) Draw the amplitude curve of the Bode plot of the closed loop system with and without the compensator. Describe how the properties of the closed loop system have been changed by the compensation. d) Simulate the control error when the reference signal is a ramp and the regulator designed in b) is used. Is the stationary error zero? 5.4 When using microorganisms in production it is important to keep the oxygen concentration at a certain level to get maximum productivity. There are many ways to control the amount of dissolved oxygen, in this example we will use the speed of stirring as the controlled signal. The transfer function from the stirrer speed N to the oxygen measurement O p becomes (linearized model) G(s) = b s + T + st 2 e sτ The parameters τ = 2 s, T 2 = 2 s and b =.2 remain constant with change in stirrer speed while T can vary from.2 s to.224 s as the stirrer speed increases from 4 r/min to 2 r/min. A Bode plot for G(s) is given in Figure 5.4a. Construct a controller, for the 2 r/min case, which has a crossover frequency ω c =.2 rad/s, a phase margin ϕ m = 6 and no steadystate error. 5.5 Earlier in the history of the basic automatic control courses at LiTH, the lead and lag compensators were parameterized in a different way. This parameterization is used in this problem, which is not only meant to be a crash course in understanding exams with solutions dating back to those days, but should be G(iω) arg G(iω) G 2 G 4 G 4 G ω [rad/s] Figure 5.4a. Solid line: 4 r/min. Dash-dotted line: 2 r/min. seen as a chance of practicing the ideas of loop shaping rather than exercising recipe knowledge. The old parameterization of a lead compensator looked as follows: F lead (s) = N s + b s + b N The schematic Bode plot of such a lead compensator is shown in Figure 5.5a, and it can be shown that the maximum phase lead is obtained at the frequency b N, where the gain is N, and the phase lead is given by N / N ϕ max = arctan 2 (shown in Figure 5.5b). The old parameterization of a lag compensator looked as follows: F lag (s) = s + a s + a/m The schematic Bode plot of such a lag compensator is shown in Figure 5.5c. Just like the parameter τ I of the new parameterization, the parameter a used 36

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