1. PROCESS CONTROL SYSTEM (KNC3213) FEEDFORWARD AND RATIO CONTROL MOHAMED AFIZAL BIN MOHAMED AMIN FACULTY OF ENGINEERING, UNIMAS MOHAMED AFIZAL BIN MOHAMED AMIN FACULTY OF ENGINEERING, UNIMAS SEMESTER 1 ( 2013-2014)1

2. CONTENT 1.INTRODUCTION 2.FEEDFORWARD CONTROL 3.RATIO CONTROL 4.FEEDFORWARD CONTROLLER DESIGN BASED ON STEADY STATE MODEL 5.FEEDFORWARD CONTROLLER DESIGN BASED ON DYNAMIC MODELS SEMESTER 1 ( 2013-2014)2

3. INTRODUCTION Main Advantages of feedback control 1.Corrective action occurs as soon as the controlled variable deviates from the set point, regardless of the source 2.Feedback control requires minimal knowledge about the process to be controlled; in particular a mathematical model of the process is no required, although it can be very useful for control system design 3.The ubiquitous PID controller is both versatile and robust. If process conditions change, re-tuning the controller usually produces satisfactory control SEMESTER 1 ( 2013-2014)3

4. INTRODUCTION Disadvantages of feedback (FB) control 1.No corrective action is taken until after a deviation in the controlled variable occurs. Thus, perfect control, where the control variables does not deviate from the set point during disturbance or set-point changes, is theoretically impossible. 2.It does not provide predictive control action to compensate for the effects of known or measurable disturbances 3.It may not be satisfactory for processes with large time constants and/or long time delays. If large and frequent disturbances occur, the process mat operate continuously in a transient state and never attain the desired steady state. 4.In some situations, the controlled varaible cannot be measured on-line and consequently feedback control is not feasible SEMESTER 1 ( 2013-2014)4

5. Feedforward Control The basic concept of Feedforward control is to measure important disturbance variables and take corrective action before they upset the process In contrast, a feedback controller does not take corrective action until after the disturbance has upset the process and generated a non-zero error signal. Figure 1 shows the simplified block diagrams for feedback and feedforward. SEMESTER 1 ( 2013-2014)5 Figure 1: Simplified block diagrams for feedforward and feedback control

6. Feedforward Control Control Objective Maintain Y at its set point, Y sp, despite disturbances. Feedback Control Measure Y, compare it to Y sp, adjust U so as to maintain Y at Y sp. Widely used (e.g., PID controllers) Feedback is a fundamental concept Feedforward Control Measure D, adjust U so as to maintain Y at Y sp. Note that the controlled variable Y is not measured. SEMESTER 1 ( 2013-2014)6

7. Feedforward Control Advantages of Feedforward (FF) Control 1.Takes corrective action before the process is upset (cf. FB control.) 2.Theoretically capable of "perfect control" 3.Does not affect system stability Disadvantages of Feedforward (FF) Control 1.Disturbance must be measured on-line (capital, operating costs) 2.Requires more knowledge of the process to be controlled (depends on the accuracy of the process model). We need to know how the controlled variable responds to changes in both the disturbance and manipulated variables 3.Ideal controllers that result in "perfect control”: may be physically unrealizable. SEMESTER 1 ( 2013-2014)7

8. Feedforward Control For example, a boiler drum with a feedback control system, Figure 2(a) The level of the boiling liquid is measured and used to adjust the feedwater flowrate This control system tends to be quite sensitive to rapid changes in the disturbance variable (Steam flowrate) – when liquid capacity of the boiler drum become lower. Rapid disturbance changes are produced by steam demands. Large controller gains cannot be used because level measurements exhibit rapid fluctuations for boiling liquids and lead to amplify the measurement noise and produce unacceptable variations in the feedwater flow rate. SEMESTER 1 ( 2013-2014)8 Figure 2(a): Feedback control of the liquid level in a boiler

9. Feedforward Control The feedforward control scheme can provide better control of the liquid level by measured the steam flowrate and adjusts the feedwater flowrate in order to balance the steam demand. In this control scheme, the controlled variable (liquid level) is not measured. As an alternative, steam pressure could be measured instead of stem flowrate. Figure 2(b) shows a feedforward control system for boiler drum SEMESTER 1 ( 2013-2014)9 Figure 2(b): Feedforward control of the liquid level in a boiler

10. Feedforward Control In practical applications, feedforward control is normally used in combination with feedback control. Feedforward control is used to reduce the effects of measurable disturbances, while feedback trim compensates for inaccuracies in the process model, measurement errors and unmeasured disturbances. For this configuration the outputs are added together and the combined signal is sent to the control valve SEMESTER 1 ( 2013-2014)10 Figure 2(c): Feedforward- feedback control of the liquid level in a boiler

11. Ratio Control SEMESTER 1 ( 2013-2014)11

12. Ratio Control Typical applications of ratio control:  Specifiying the relative amounts of components in blending operations  Maintaining a stoichiometric ratio of reactants to a reactor  Keeping specified reflux ratio for a distillation column  Holding the fuel-air ratio to a furnace at the optimum value SEMESTER 1 ( 2013-2014)12

13. Ratio Control SEMESTER 1 ( 2013-2014)13 Figure 3(a): Ratio Control Method I

14. Ratio Control SEMESTER 1 ( 2013-2014)14 Figure 3(b): Ratio Control Method II

15. Feedforward Controller Design Based on Steady State Model SEMESTER 1 ( 2013-2014) 15 Figure 4: Feedforward control of exit composition in the blending system

16. Feedforward Controller Design Based on Steady State Model SEMESTER 1 ( 2013-2014) 16 Eqn. 14.11

17. Feedforward Controller Design Based on Steady State Model SEMESTER 1 ( 2013-2014)17 Eqn. 14.12

18. Feedforward Controller Design Based on Steady State Model SEMESTER 1 ( 2013-2014)18 Eqn. 14.15

19. Feedforward Controller Design Based on Steady State Model SEMESTER 1 ( 2013-2014)19 Eqn. 14.15

20. Feedforward Controller Design Based on Dynamic Models SEMESTER 1 ( 2013-2014)20

21. Feedforward Controller Design Based on Dynamic Models SEMESTER 1 ( 2013-2014)21 Eqn. 14.21

22. END OF SLIDE SEMESTER 1 ( 2013-2014)22