Overview of Control System Design

Transcription

1 Overview of Control System Design Chapter 10 General Requirements 1. Safety. It is imperative that industrial plants operate safely so as to promote the well-being of people and equipment within the plant and in the nearby communities. Thus, plant safety is always the most important control objective and is the subject of Section Environmental Regulations. Industrial plants must comply with environmental regulations concerning the discharge of gases, liquids, and solids beyond the plant boundaries. 3. Product Specifications and Production Rate. In order to be profitable, a plant must make products that meet specifications concerning product quality and production rate.

2 4. Economic Plant Operation. It is an economic reality that the plant operation over long periods of time must be profitable. Thus, the control objectives must be consistent with the economic objectives. Chapter Stable Plant Operation. The control system should facilitate smooth, stable plant operation without excessive oscillation in key process variables. Thus, it is desirable to have smooth, rapid set-point changes and rapid recovery from plant disturbances such as changes in feed composition.

3 Steps in Control System Design Chapter 10 After the control objectives have been formulated, the control system can be designed. The design procedure consists of three main steps: 1. Select controlled, manipulated, and measured variables. 2. Choose the control strategy (multiloop control vs. multivariable control) and the control structure (e.g., pairing of controlled and manipulated variables). 3. Specify controller settings.

4 Chapter 10 Control Strategies Multiloop Control: Each output variable is controlled using a single input variable. Multivariable Control: Each output variable is controlled using more than one input variable.

5 10.2 THE INFLUENCE OF PROCESS DESIGN ON PROCESS CONTROL Traditionally, process design and control system design have been separate engineering activities. Chapter 10 Thus in the traditional approach, control system design is not initiated until after the plant design is well underway and major pieces of equipment may even have been ordered. This approach has serious limitations because the plant design determines the process dynamic characteristics, as well as the operability of the plant. In extreme situations, the plant may be uncontrollable even though the process design appears satisfactory from a steady-state point of view.

6 10.2 THE INFLUENCE OF PROCESS DESIGN ON PROCESS CONTROL (continued) Chapter 10 A more desirable approach is to consider process dynamics and control issues early in the plant design. This interaction between design and control has become especially important for modern processing plants, which tend to have a large degree of material and energy integration and tight performance specifications. As Hughart and Kominek (1977) have noted: "The control system engineer can make a major contribution to a project by advising the project team on how process design will influence the process dynamics and the control structure. The interaction of the process design and control system design teams is considered in Chapter 23. Next, we consider an example of heat integration.

7 Chapter 10 Figure 10.1 Two distillation column configurations.

8 Chapter 10 Figure 10.3 Batch reactor with two temperature control strategies.

9 10.3 Degrees of Freedom for Process Control Chapter 10 The important concept of degrees of freedom was introduced in Section 2.3, in connection with process modeling. The degrees of freedom N F is the number or process variables that must be specified in order to be able to determine the remaining process variables. If a dynamic model of the process is available, N F can be determined from a relation that was introduced in Chapter 2, NF = NV NE (10-1) where N V is the total number of process variables, and N E is the number of independent equations.

10 Chapter 10 Case 2. The set point is adjusted frequently by a higher level controller. The set point is now considered to be a variable. Consequently, the introduction of the control law adds one new equation and one new variable, y sp. Equations 10-1 and 10-2 indicate that N F and N FC do not change. The importance of this conclusion will be more apparent when cascade control is considered in Chapter 16. Selection of Controlled Variables Guideline 1. All variables that are not self-regulating must be controlled. Guideline 2. Choose output variables that must be kept within equipment and operating constraints (e.g., temperatures, pressures, and compositions).

11 Chapter 10 Figure 10.7 General representation of a control problem.

12 Guideline 3. Select output variables that are a direct measure of product quality (e.g., composition, refractive index) or that strongly affect it (e.g., temperature or pressure). Chapter 10 Guideline 4. Choose output variables that seriously interact with other controlled variables. Guideline 5. Choose output variables that have favorable dynamic and static characteristics.

13 Selection of Manipulated Variables Guideline 6. Select inputs that have large effects on controlled variables. Chapter 10 Guideline 7. Choose inputs that rapidly affect the controlled variables. Guideline 8. The manipulated variables should affect the controlled variables directly rather than indirectly. Guideline 9. Avoid recycling of disturbances.

14 Selection of Measured Variables Guideline 10. Reliable, accurate measurements are essential for good control. Chapter 10 Guideline 11. Select measurement points that have an adequate degree of sensitivity. Guideline 12. Select measurement points that minimize time delays and time constants

15 10.5 Process Safety and Process Control Process safety has been a primary concern of the process industries for decades. Chapter 10 But in recent years, safety issues have received increased attention for several reasons that include increased public awareness of potential risks, stricter legal requirements, and the increased complexity of modern industrial plants. Overview of Process Safety Process safety is considered at various stages in the lifetime of a process: 1. An initial safety analysis is performed during the preliminary process design.

16 2. A very thorough safety review is conducted during the final stage of the process design using techniques such as hazard and operability (HAZOP) studies, failure mode and effect analysis, and fault tree analysis. Chapter After plant operation begins, HAZOP studies are conducted on a periodic basis in order to identify and eliminate potential hazards. 4. Many companies require that any proposed plant change or change in operating conditions require formal approval via a Management of Change process that considers the potential impact of the change on the safety, environment, and health of the workers and the nearby communities. Proposed changes may require governmental approval, as occurs for the U.S. pharmaceutical industry, for example.

17 5. After a serious accident or plant incident, a thorough review is conducted to determine its cause and to assess responsibility. Multiple Protection Layers Chapter 10 In modern chemical plants, process safety relies on the principle of multiple protection layers (AIChE, 1993b; ISA, 1996). A typical configuration is shown in Figure Each layer of protection consists of a grouping of equipment and/or human actions. The protection layers are shown in the order of activation that occurs as a plant incident develops. In the inner layer, the process design itself provides the first level of protection.

18 Chapter 10 Figure Typical layers of protection in a modern chemical plant (CCPS 1993).

19 The next two layers consist of the basic process control system (BPCS) augmented with two levels of alarms and operator supervision or intervention. Chapter 10 An alarm indicates that a measurement has exceeded its specified limits and may require operator action. The fourth layer consists of a safety interlock system (SIS) that is also referred to as a safety instrumented system or as an emergency shutdown (ESD) system. The SIS automatically takes corrective action when the process and BPCS layers are unable to handle an emergency. For example, the SIS could automatically turn off the reactant pumps after a high temperature alarm occurs for a chemical reactor.

20 Relief devices such as rupture discs and relief valves provide physical protection by venting a gas or vapor if overpressurization occurs. Chapter 10 As a last resort, dikes are located around process units and storage tanks to contain liquid spills. Emergency response plans are used to address emergency situations and to inform the community.

21 Chapter 10 Types of Alarms Type 1 Alarm: Equipment status alarm. Indicates equipment status, for example, whether a pump is on or off, or whether a motor is running or stopped. Type 2 Alarm: Abnormal measurement alarm. Indicates that a measurement is outside of specified limits. Type 3 Alarm: An alarm switch without its own sensor. These alarms are directly activated by the process, rather than by a sensor signal. Type 3 alarms are used for situations where it is not necessary to know the actual value of the process variable, only whether it is above (or below) a specified limit. Type 4 Alarm: An alarm switch with its own sensor. A type 4 alarm system has its own sensor that serves as a backup in case the regular sensor fails. Type 5 Alarm: Automatic Shutdown or Startup System. These important and widely used systems are described in the next section on Safety Interlock Systems.

22 Chapter 10 Fig A general block diagram for an alarm system.

23 Chapter 10 Fig Two flow alarm configurations.

24 Chapter 10 Fig Two interlock configurations.

25 Safety Interlock System (SIS) The SIS in Figure serves as an emergency back-up system for the BPCS. Chapter 10 The SIS automatically starts when a critical process variable exceeds specified alarm limits that define the allowable operating region. Its initiation results in a drastic action such as starting or stopping a pump or shutting down a process unit. Consequently, it is used only as a last resort to prevent injury to people or equipment.

26 It is very important that the SIS function independently of the BPCS; otherwise, emergency protection will be unavailable during periods when the BPCS is not operating (e.g., due to a malfunction or power failure). Chapter 10 Thus, the SIS should be physically separated from the BPCS (AIChE, 1993b) and have its own sensors and actuators.

27 A Final Thought Chapter 10 As Rinard (1990) has poignantly noted, The regulatory control system affects the size of your paycheck; the safety control system affects whether or not you will be around to collect it.

28 Dynamic Behavior and Stability of Closed-Loop Control Systems In this chapter we consider the dynamic behavior of processes that are operated using feedback control. This combination of the process, the feedback controller, and the instrumentation is referred to as a feedback control loop or a closed-loop system.

29 Block Diagram Representation Figure 11.7 Block diagram for the entire blending process composition control system.

30 Closed-Loop Transfer Functions The block diagrams considered so far have been specifically developed for the stirred-tank blending system. The more general block diagram in Fig contains the standard notation: Y = controlled variable U = manipulated variable D = disturbance variable (also referred to as load variable) P = controller output E = error signal Y m = measured value of Y Y sp = set point Y % sp = internal set point (used by the controller)

31 Figure 11.8 Standard block diagram of a feedback control system.

32 Y u = change in Y due to U Y d = change in Y due to D G c = controller transfer function G v = transfer function for final control element (including K IP, if required) G p = process transfer function G d = disturbance transfer function G m = transfer function for measuring element and transmitter K m = steady-state gain for G m

33 Set-Point Changes Next we derive the closed-loop transfer function for set-point changes. The closed-loop system behavior for set-point changes is also referred to as the servomechanism (servo) problem in the control literature. Y = Y + Y d u (11-14) Yd = GdD= 0 (because D= 0) (11-15) Y = G U (11-16) u p Combining gives Y = G U p (11-17)

34 Figure 11.8 also indicates the following input/output relations for the individual blocks: U = Gv P P= Gc E (11-18) (11-19) E = Y% Y (11-20) Y% Y sp m = = sp K Y m m sp G Y Combining the above equations gives m (11-21) (11-22) Y= GGP= GGGE p v p v c = GGG Y% Y ( ) p v c sp m ( ) = GGG KY GY p v c m sp m (11-23) (11-24) (11-25)

35 Rearranging gives the desired closed-loop transfer function, Y Y Disturbance Changes KmGcGvGp = 1 + GGG G sp c v p m (11-26) Now consider the case of disturbance changes, which is also referred to as the regulator problem since the process is to be regulated at a constant set point. From Fig. 11.8, Y = Yd + Yu = GdD+ GpU (11-27) Substituting (11-18) through (11-22) gives Y = G D+ G U = G D+ G G G K Y G Y d p d p v c m sp m ( ) (11-28)

36 Because Y sp = 0 we can arrange (11-28) to give the closed-loop transfer function for disturbance changes: Y D Gd = 1 + GGG G c v p m (11-29) A comparison of Eqs and indicates that both closed-loop transfer functions have the same denominator, 1 + G c G v G p G m. The denominator is often written as 1 + G OL where G OL is the open-loop transfer function, G G G G G At different points in the above derivations, we assumed that D = 0 or Y sp = 0, that is, that one of the two inputs was constant. But suppose that D 0 and Y sp 0, as would be the case if a disturbance occurs during a set-point change. To analyze this situation, we rearrange Eq and substitute the definition of G OL to obtain OL c v p m.

37 G KmGcG d vgp Y = D+ Y 1+ G 1+ G OL OL sp (11-30) Thus, the response to simultaneous disturbance variable and setpoint changes is merely the sum of the individual responses, as can be seen by comparing Eqs , 11-29, and This result is a consequence of the Superposition Principle for linear systems.

38 General Expression for Feedback Control Systems Closed-loop transfer functions for more complicated block diagrams can be written in the general form: where: Z is the output variable or any internal variable within the control loop Z i is an input variable (e.g., Y sp or D) Π f Z Z i Π f = 1 +Π e (11-31) = product of the transfer functions in the forward path from Z i to Z = product of every transfer function in the feedback loop Π e

39 Example 11.1 Find the closed-loop transfer function Y/Y sp for the complex control system in Figure Notice that this block diagram has two feedback loops and two disturbance variables. This configuration arises when the cascade control scheme of Chapter 16 is employed. Figure Complex control system.

40 Figure Block diagram for reduced system.

41 Figure Final block diagrams for Example 11.1.

42 Solution Using the general rule in (11-31), we first reduce the inner loop to a single block as shown in Fig To solve the servo problem, set D 1 = D 2 = 0. Because Fig contains a single feedback loop, use (11-31) to obtain Fig a. The final block diagram is shown in Fig b with Y/Y sp = K m1 G 5. Substitution for G 4 and G 5 gives the desired closed-loop transfer function: Y Km1Gc1Gc2GG 1 2G3 = Y 1 + G GG + G G G G G G sp c2 1 m2 c1 2 3 m1 c2 1 Closed-Loop Responses of Simple Control Systems In this section we consider the dynamic behavior of several elementary control problems for disturbance variable and setpoint changes.

43 The transient responses can be determined in a straightforward manner if the closed-loop transfer functions are available. Consider the liquid-level control system shown in Fig The liquid level is measured and the level transmitter (LT) output is sent to a feedback controller (LC) that controls liquid level by adjusting volumetric flow rate q 2. A second inlet flow rate q 1 is the disturbance variable. Assume: 1. The liquid density ρ and the cross-sectional area of the tank A are constant. 2. The flow-head relation is linear, q 3 = h/r. 3. The level transmitter, I/P transducer, and control valve have negligible dynamics. 4. An electronic controller with input and output in % is used (full scale = 100%).

44 Figure Liquid-level control system.

45 Derivation of the process and disturbance transfer functions directly follows Example 4.4. Consider the unsteady-state mass balance for the tank contents: dh ρa ρq1 ρq2 ρ q3 (11-32) dt = + Substituting the flow-head relation, q 3 = h/r, and introducing deviation variables gives dh h A = q1 + q2 (11-33) dt R Thus, we obtain the transfer functions 2 ( s) ( ) H K p = Gp ( s) = Q s τs+ 1 (11-34)

46 1 ( s) ( ) H K p = Gd ( s) = Q s τs+ 1 (11-35) where K p = R and τ = RA. Note that G p (s) and G d (s) are identical because q 1 and q 2 are both inlet flow rates and thus have the same effect on h. Proportional Control and Set-Point Changes If a proportional controller is used, then G c (s) = K c. From Fig and the material in the previous section, it follows that the closed-loop transfer function for set-point changes is given by ( s) ( ) ( s+ ) ( ) H KKK c v pkm/ τ 1 = H s 1 + KKK K / τs+ 1 sp c v p m (11-36)

47 Figure Block diagram for level control system.

48 This relation can be rearranged in the standard form for a firstorder transfer function, where: ( ) ( ) H s K1 = H s τ s+ 1 sp 1 (11-37) KOL K1 = (11-38) 1 + K OL τ τ 1 = (11-39) + K 1 OL and the open-loop gain K OL is given by K = K K K K OL c v p m (11-40)

49 From Eq it follows that the closed-loop response to a unit step change of magnitude M in set point is given by ( ) ( / τ 1 ) h t = K1M 1 e t (11-41) This response is shown in Fig Note that a steady-state error or offset exists because the new steady-state value is K 1 M rather than the desired value of M. The offset is defined as ( ) h ( ) offset h (11-42) sp For a step change of magnitude M in set point, hsp = M. From (11-41), it is clear that h ( ) = K1M. Substituting these values and (11-38) into (11-42) gives M offset (11-43) = M K1M = 1 + K OL ( )

50 Figure Step response for proportional control (setpoint change).

51 Proportional Control and Disturbance Changes From Fig and Eq the closed-loop transfer function for disturbance changes with proportional control is 1 ( s) ( ) ( s+ ) ( ) H Kp / τ 1 = Q s 1 + K / τs+ 1 OL (11-53) Rearranging gives ( ) ( ) H s K = 2 Q s τ s where τ 1 is defined in (11-39) and K 2 is given by (11-54) K p K2 = (11-55) 1 + K OL

52 A comparison of (11-54) and (11-37) indicates that both closedloop transfer functions are first-order and have the same time constant. However, the steady-state gains, K 1 and K 2, are different. From Eq it follows that the closed-loop response to a step change in disturbance of magnitude M is given by ( ) ( / τ 1 ) h t = K2M 1 e t (11-56) The offset can be determined from Eq Now h sp = since we are considering disturbance changes and h ( ) = K2M for a step change of magnitude M. Thus, ( ) 0 KpM offset = 0 h ( ) = K2M = (11-57) 1 + K OL

53 Figure Set-point responses for Example 11.2.

54 Figure Load responses for Example 11.3.

55 PI Control and Disturbance Changes ( ) ( ) For PI control, Gc s = Kc 1+ 1/τIs. The closed-loop transfer function for disturbance changes can then be derived from Fig : H ( s) Kp / ( τs+ 1) = (11-58) Q s 1+ K 1+ 1/τ s / τs+ 1 1 ( ) OL ( ) ( ) I Clearing terms in the denominator gives H 1 ( s) K p = ( ) ( ) Q s τ s τs+ 1 + K τ s I τ I s OL I (11-59) Further rearrangement allows the denominator to be placed in the standard form for a second-order transfer function: ( ) ( ) H s Ks Q s τ s + 2ζ τs+ 1 3 = (11-60)

56 where K 3 3 = τ / KKK (11-61) I c v m 1 1+ K OL τi ζ 3 = (11-62) 2 K OL τ τ = ττ / K (11-63) I OL For a unit step change in disturbance, Q1 s = 1/ s, and (11-59) becomes K3 H ( s) = (11-64) 2 2 τ s + 2ζ τs For 0 < ζ3 < 1, the response is a damped oscillation that can be described by ( ) K3 ζ 3t / τ3 2 h () t = e sin 1 ζ 3t/ τ 3 (11-65) τ 1 ζ

57 PI Control of an Integrating Process Consider the liquid-level control system shown in Fig This system differs from the previous example in two ways: 1. the exit line contains a pump and 2. the manipulated variable is the exit flow rate rather than an inlet flow rate. In Section 5.3 we saw that a tank with a pump in the exit stream can act as an integrator with respect to flow rate changes because 3 ( s) ( ) H 1 = Gp ( s) = Q s As (11-66) 1 ( s) ( ) H 1 = Gd ( s) = Q s As (11-67)

58 Figure Liquid-level control system with pump in exit line.

59 If the level transmitter and control valve in Eq have negligible dynamics, the G m (s) = K m and G v (s) = K v. For PI control, Gc( s) = Kc( 1+ 1/τIs). Substituting these expressions into the closed-loop transfer function for disturbance changes and rearranging gives where ( ) 1 ( ) H s Gd = Q s 1+ G G G G ( ) ( ) c v p m H s Ks = Q s τ s + 2ζ τs+ 1 K (11-68) (11-69) = τ / KKK (11-70) c v m τ = τ / K (11-71) I OL ζ = 0.5 K τ (11-72) OL I And K OL = K c K v K p K m with K p = - 1/A.

60 Stability of Closed-Loop Control Systems Example 11.4 Consider the feedback control system shown in Fig with the following transfer functions: G = K G = c c v 1 2s + 1 (11-73) 1 1 G p = G d = G 5s 1 m = + s+ 1 (11-74) Show that the closed-loop system produces unstable responses if controller gain K c is too large.

61 Figure Effect of controller gains on closed-loop response to a unit step change in set point (example 11.1).

62 Stability Most industrial processes are stable without feedback control. Thus, they are said to be open-loop stable or self-regulating. An open-loop stable process will return to the original steady state after a transient disturbance (one that is not sustained) occurs. By contrast there are a few processes, such as exothermic chemical reactors, that can be open-loop unstable. Definition of Stability. An unconstrained linear system is said to be stable if the output response is bounded for all bounded inputs. Otherwise it is said to be unstable.

63 Characteristic Equation As a starting point for the stability analysis, consider the block diagram in Fig Using block diagram algebra that was developed earlier in this chapter, we obtain KmGcGvGp Gd Y = Ysp + D 1+ G 1+ G OL OL (11-80) where G OL is the open-loop transfer function, G OL = G c G v G p G m. For the moment consider set-point changes only, in which case Eq reduces to the closed-loop transfer function, Y KmGGG c v p = (11-81) Y 1+ G sp OL

64 Comparing Eqs and indicates that the poles are also the roots of the following equation, which is referred to as the characteristic equation of the closed-loop system: 1+ G OL = 0 (11-83) General Stability Criterion. The feedback control system in Fig is stable if and only if all roots of the characteristic equation are negative or have negative real parts. Otherwise, the system is unstable. Example 11.8 Consider a process, G p = 0.2/-s + 1), and thus is open-loop unstable. If G v = G m = 1, determine whether a proportional controller can stabilize the closed-loop system.

65 Figure Stability regions in the complex plane for roots of the characteristic equation.

66 Figure Contributions of characteristic equation roots to closed-loop response.

67 Solution The characteristic equation for this system is s+ 0.2K c 1 = 0 (11-92) Which has the single root, s = K c. Thus, the stability requirement is that K c < 5. This example illustrates the important fact that feedback control can be used to stabilize a process that is not stable without control. Routh Stability Criterion The Routh stability criterion is based on a characteristic equation that has the form n n 1 as + a 1s + K+ as 1 + a0 = 0 (11-93) n n

68 Row Routh array: 1 a n a n-2 a n-4 2 a n-1 a n-3 a n-5 3 b 1 b 2 b 3 4 c 1 c 2 n M+ 1 zm 1 L L L L where: b b 1 2 a a a a = a = n 1 n 2 n n 3 n 1 a a a a n 1 n 4 n n 5 M a n 1 (11-94) (11-95)

69 and: c c 1 2 = = ba a b 1 n 3 n 1 2 b 1 ba a b 1 n 5 n 1 3 M b 1 (11-96) (11-97) Routh Stability Criterion: A necessary and sufficient condition for all roots of the characteristic equation in Eq to have negative real parts is that all of the elements in the left column of the Routh array are positive.

70 Example 11.9 Determine the stability of a system that has the characteristic equation Solution s + 5s + 3s + 1 = 0 (11-98) Because the s term is missing, its coefficient is zero. Thus, the system is unstable. Recall that a necessary condition for stability is that all of the coefficients in the characteristic equation must be positive.

71 Example Find the values of controller gain K c that make the feedback control system of Eq stable. Solution From Eq , the characteristic equation is s + 17s + 8s+ 1+ K c = 0 (11-99) All coefficients are positive provided that 1 + K c > 0 or K c < -1. The Routh array is K c b 1 b 2 c 1

72 To have a stable system, each element in the left column of the Routh array must be positive. Element b 1 will be positive if K c < 7.41/0.588 = Similarly, c 1 will be positive if K c > -1. Thus, we conclude that the system will be stable if 1< K c < 12.6 (11-100) Direct Substitution Method The imaginary axis divides the complex plane into stable and unstable regions for the roots of characteristic equation, as indicated in Fig On the imaginary axis, the real part of s is zero, and thus we can write s=jω. Substituting s=jω into the characteristic equation allows us to find a stability limit such as the maximum value of K c. As the gain K c is increased, the roots of the characteristic equation cross the imaginary axis when K c = K cm.

73 Example Use the direct substitution method to determine K cm for the system with the characteristic equation given by Eq Solution Substitute s = jω and K c = K cm into Eq : jω 17ω + 8 jω + 1+ K cm = 0 or (11-105) ( 2) ( 3 K ) cm j 1+ 17ω + 8ω 10ω = 0

74 Equation is satisfied if both the real and imaginary parts are identically zero: ω = 0 (11-106a) K cm ( ) 3 2 8ω 10ω = ω 8 10ω = 0 (11-106b) Therefore, 2 ω = 0.8 ω =± (11-107) and from (11-106a), K cm = 12.6

75 Root Locus Diagrams Example Consider a feedback control system that has the open-loop transfer function, G OL ( s) = 4K ( s+ 1)( s+ 2)( s+ 3) Plot the root locus diagram for Solution c K c The characteristic equation is 1 + G OL = 0 or ( )( )( ) (11-108) s+ 1 s+ 2 s K c = 0 (11-109)

76 The root locus diagram in Fig shows how the three roots of this characteristic equation vary with K c. When K c = 0, the roots are merely the poles of the open-loop transfer function, -1, -2, and -3.

77 Figure Root locus diagram for third-order system. X denotes an open-loop pole. Dots denote locations of the closedloop poles for different values of K c. Arrows indicate change of pole locations as K c increases.

78 Figure Flowchart for performing a stability analysis.