1. ERT 210/4 Process Control Hairul Nazirah bt Abdul Halim Email: hairulnazirah@unimap.edu.myhairulnazirah@unimap.edu.my Office: 04-9798840 CHAPTER 8 Feedback Controllers

2. Process Control in our daily life… Controlling the water temperature of a shower Let’s analyze this process: Control objective: to control the shower temp. process: shower sensor: person’s skin Controlled variable (CV): shower temp. Set point: desired shower temp. Manipulated variable (MV): flow of cold water Final control element: valve on cold water line Controller: person in the shower

3. ExxonMobil Chemical Plant

4. Benefits of improved process control: 1. improve product quality 2. faster 3. greater production rates 4. less expensive process validation procedure

5. Overview – Temperature Control heat exchanger - uses steam to heat cold water. How to control the temperature?? Manual Control Automatic Control

6. MANUAL CONTROL Operator Compare Temp. with desired value open/close valve To admit more/less steam

7. AUTOMATIC CONTROL SENSOR - TRANSMITTER (measure the temperature) (transmits it to electronic signal) FEEDBACK CONTROLLER (compares the measured temp. to the set point value & make corrective action) I/P TRANSDUCER (Convert electronic signal to pneumatic signal) CONTROL VALVE (Receive signal & adjust the manipulated variables (flow))

8. COURSE OUTCOME: CH 8, 9 & 10 CO2: Ability to identify measure and investigate Feedback controllers, Control system instrumentation and the control system design.

9. CHAPTER 8: FEEDBACK CONTROLLERS DEFINE and DISCUSS the standard feedback control algorithm (control laws), IDENTIFY and COMPARE Proportional-integral-derivative (PID) control and on-off control types of feedback control Subtopic: 1.Definition 2.Basic Control Modes 3.PID Controllers 4.On-Off Controllers

10. Chapter 8 Figure 8.1 Schematic diagram for a stirred-tank blending system. Feedback Controllers

11. Basic components in a feedback control loop are: 1.Process being control (blending system) 2.Sensor-transmitter combination (AT) 3.Feedback controller (AC) 4.Current-to-pressure transducer (I/P) 5.Final control element (control valve) 6.Transmission line between the various instrument Chapter 8

12. Figure 8.2Flow control loop Chapter 8

16. Three basic control modes: 1)Proportional Control 2)Integral Control 3)Derivative Control Next, we consider : Proportional-Integral-Derivative (PID) Control BASIC CONTROL MODES Chapter 8

17. PROPORTIONAL CONTROL Objective: To reduce the error signal to zero.

18. Chapter 8 For proportional control, the controller output is proportional to the error signal, where:

19. Chapter 8

20. Some controllers have a proportional band setting instead of a controller gain. The proportional band PB (in %) is defined as PB correspond to K c and vice versa

21. Chapter 8 The transfer function for proportional-only control:

22. Disadvantage of proportional-only control: Offset : Steady-state error occurs after a set-point change or a sustained disturbance. Chapter 8

23. INTEGRAL CONTROL Controller output depends on the integral of the error signal over time. = Integral Time

24. Chapter 8 Integral control action is widely used: the elimination of offset. Integral control action is normally used in conjunction with proportional control as the proportional-integral (PI) controller: Proportional-Integral (PI) Control

25. Chapter 8 Transfer function for the PI controller:

26. Response to unit step change in e: Chapter 8 Figure 8.6. Response of PI controller to unit step change in e(t).

27. Chapter 8 RESET WINDUP The integral term becomes quite large due to a sustained error The controller is saturated undesirable because the controller is already doing all it can to reduce the error. Commercial controllers provide antireset windup. Antireset windup reduce the windup by temporarily halting the integral control action

28. Chapter 8 Reset windup

29. Chapter 8 DERIVATIVE CONTROL anticipate the future behavior of the error signal by considering its rate of change. ideal derivative action, = derivative time, has units of time.

30. Chapter 8 “ideal” PD controller has the transfer function: Derivative control action tends to improve dynamic response of the controlled variable

31. Chapter 8 the transfer function for “real” PD controller, the value of constant : 0.05 < α <0.2, (0.1 being a common choice)

32. Chapter 8 Proportional-Integral-Derivative (PID) Control Combination of P, I & D Control modes 3 most common forms of PID Control: 1.Parallel 2.Series 3.Expended

33. Chapter 8 Figure 8.8Block diagram of the parallel form of PID control (without a derivative filter) Parallel Form of PID Control

34. Chapter 8 Parallel Form of PID Control Transfer function for “Ideal” PID control is: Transfer function for “Real” PID control, with derivative filter is:

35. Chapter 8 Series Form of PID Control Transfer function for “Ideal” PID control is: Figure 8.9Block diagram of the series form of PID control (without a derivative filter)

36. Series Form of PID Control Commercial versions of the series-form controller have a derivative filter @ “Real PID control”: Chapter 8

37. Expanded Form of PID Control Use in MATLAB Control parameters are 3 gains: K c, K I and K D

38. PID- Most complicated to tune (K c,  I,  D ). - Better performance than PI - No offset - Derivative action may be affected by noise PI- More complicated to tune (K c,  I ). - Better performance than P - No offset - Most popular FB controller P- Simplest controller to tune (K c ). - Offset with sustained disturbance or setpoint change. Controller Comparison Chapter 8

39. Automatic and Manual Control Modes Automatic Mode Controller output, p(t), depends on e(t), controller constants, and type of controller used. ( PI vs. PID etc.) Chapter 8

40. Manual Mode Controller output, p(t), is adjusted manually. Manual Mode is very useful when unusual conditions exist: i) plant start-up ii)plant shut-down iii)emergencies

41. On-Off Controllers Simple Cheap Used in residential heating and domestic refrigerators Limited use in process control due to continuous cycling of controlled variable Less widely used than PID controllers – not versatile @ effective. Chapter 8

42. Figure An example of the operation of an on-off temperature controller.

43. Typical Response of Feedback Control Systems Chapter 8 Figure 8.12. Typical process responses with feedback control. No control: the process slowly reaches a new steady state P – speed up the process response & reduces the offset PI – eliminate offset & the response more oscillatory PID – reduces degree of oscillation and the response time

44. Figure 8.13. Proportional control: effect of controller gain. Chapter 8 -Increasing Kc tends to make the process response less sluggish (more faster) -Too large of Kc, results in undesirable degree of oscillation or even become unstable -Intermediate value of Kc usually results in the best control.

45. Chapter 8 -Increasing τ I tends to make the process response more sluggish (slower) -Too large of τ I, the controlled variable will return to the set point very slowly after a disturbance change @ set-point change occurs. Figure 8.14. PI control: (a) effect of reset time (b) effect of controller gain.

46. Chapter 8 Figure 8.15. PID control: effect of derivative time. -Increasing τ D tends to improve the process response by reducing the maximum deviation, response time and degree of oscillation. -Too large of τ D : measurement noise is amplified and process response more oscillatory. -The intermediate value of τ D is desirable.

47. Selection of Controller Should consider the combined dynamic behavior of: - final control element - process - sensor P-only control: for process with non-sluggish dynamic behavior & offset is not important. (pressure @ level) PI control: when offset elimination is important (Flow, Temp., Composition, DO) PID: for sluggish process (Biomass Concentration, Temp., Composition)