Introduction to Feedforward Control Techniques in Process Control

1. Feedforward Control Prof. Ing. Michele MICCIO Dip. Ingegneria Industriale (Università di Salerno) ProdalScarl (Fisciano)  adapted from Romagnoli & Palazoglu’sChapter 16: Model-Based Control  see also Stephanopoulos,1984 Chapter 21  §21.1-4 rev. 3.4 of April 30, 2019

2. Introduction to Model-Based Control In this course we consider the following control design techniques that explicitly use the process model: • Delay Compensation (Smith Predictor) • Inverse Response Compensation • Feedforward control • Model Predictive Control (MPC) Romagnoli & Palazoglu, “Introduction to Process Control “

3. Detailed Process Understanding Intelligent Use of Modern Control Systems Improved Profitability $ Introduction to Model-Based Control Definition of Model-Based Control Combination of detailed process understanding (advanced mathematical modeling) with the intelligent use of modern control systems (hardware, software and technology). Romagnoli & Palazoglu, “Introduction to Process Control “

4. Disturbance Manipulated variable Controlled variable Set-point Process Controller Back to Feedback Control • Feedback control can never achieve perfect control of a chemical process. It reacts to the changes in the controlled variable after a deviation is detected in the output. Sensor Romagnoli & Palazoglu, “Introduction to Process Control “

5. Feedforward Control • A feedforward controller measures the disturbance directly and takes control action to compensate for its eventual impact on the output variable. • Feedforward controllers have the theoretical potential for perfect control. Romagnoli & Palazoglu, “Introduction to Process Control “

6. Controller Process Feedforward Control Consider the following feedforwardflow of information about disturbance … Disturbance Set point Controller output Final control element Feedforward Controller Controlled variable Manipulated variable  the feedforward controllerpredicts the effect of disturbances Romagnoli & Palazoglu, “Introduction to Process Control “

7. d(s) gd(s) m(s) y(s) gp(s) Feedforward Control Design We want to achieve the following control objective: y(t) = ysp(t) Therefore, in the Laplace domain: Process We shall require perfect control : 1 Romagnoli & Palazoglu, “Introduction to Process Control “

8. disturbance measurement y y sp sp d + − g g g 1 1 1 g g g md md md ff ff ff g g gd 2 2 2 g g g ff ff ff + y y + g g g g g g f f f p p p final control element Feedforward Control Design We introduce a suitable structure for the feedforward controller. Then, we further determine m(s) from the block algebra: 2 3 process vs. disturbance actual feedforward controller o m process Romagnoli & Palazoglu, “Introduction to Process Control “

9. Feedforward Control • The feedforward control elements are notconventional controllers (P, PI or PID) • The feedforward controller: • needs the gff1 block in order to make the set point comparable to the measured disturbance • depends on the knowledge of process and disturbance models • can be developed for more than one disturbanceand for multiple controlled variables Romagnoli & Palazoglu, “Introduction to Process Control “

10. Feedforward vs Feedback Feedforward - Advantages • Acts before disturbances affect the process • Cannot cause instability • Good for slow process dynamics Feedforward - Disadvantages • Must identify and measure ALL disturbances • Fails for unmeasured disturbances • Needs to have a reliable process dynamic model • Fails for changes within the process • No indication of control quality Romagnoli & Palazoglu, “Introduction to Process Control “

11. Feedforward vs Feedback • Feedback - Advantages • No disturbance measurements needed • Limited or even no process model needed • Can cope with changes within process •  Feedback - Disadvantages • Will always be some error • Poor for slow process dynamics, interaction, etc. • Instability is possible Romagnoli & Palazoglu, “Introduction to Process Control “

12. Feedforward-Feedback Control Use a combination of Feedforward and Feedback control • We expect that a combined feedforward-feedback control system will retain, • The superior performance of a feedforward controller, and • The insensitivity of the feedback controller to uncertainties in model and inaccuracies in model parameters. Romagnoli & Palazoglu, “Introduction to Process Control “

13. Example 1:Process with dead time Consider the following process TFs: gmd and gf purely algebraic • Design a Feedforward Controller • Compare with PI feedback design Romagnoli & Palazoglu, “Introduction to Process Control “

14. Example 1: process with dead time(disturbance rejection) FF Controller Feedback with a PI controller  No modeling error modeling error with +20% error in gdgain (Kd = 1.2) NB: Output signal = controlled variable different scales on axes ! Romagnoli & Palazoglu, “Introduction to Process Control “

15. Tout,sp Steam Feedforward Controller Tout Example 2: Heat Exchanger Manipulated variable Controlled variable TT FT Disturbances Design a FF controller to compensate for variations in the feed flow rate and temperature. Romagnoli & Palazoglu, “Introduction to Process Control “

16. Example 3 Distillation Column Design a FF controller to compensate for variations in feed composition and flow rate. Manipulated variable Disturbances Controlled variable CT FT Cout,sp FF Controller Romagnoli & Palazoglu, “Introduction to Process Control “