1. FeedForward Prof. Ing. Michele MICCIO
Dip. Ingegneria Industriale (Università di Salerno)
Prodal Scarl (Fisciano)
 adapted from Romagnoli & Palazoglu’s Chapter 16: Model-Based Control
 see also Stephanopoulos,1984 Chapter 21  §21.1-4
rev. 3.1 of May 11, 2017

2. Romagnoli & Palazoglu, “Introduction to Process Control “
Definition of Model-Based Control
Detailed Process
Understanding
Intelligent Use of
Modern Control
Systems
Improved
Profitability $
The combination of detailed process understanding with the intelligent use of modern control systems (hardware, software and technology) to achieve improved profitability.
Romagnoli & Palazoglu, “Introduction to Process Control “

3. Romagnoli & Palazoglu, “Introduction to Process Control “
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 “

4. Romagnoli & Palazoglu, “Introduction to Process Control “
Feedforward 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.
Manipulated variable
Controlled variable
Controller
Process
Disturbance
Set-point
Sensor
Romagnoli & Palazoglu, “Introduction to Process Control “

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

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

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

9. Romagnoli & Palazoglu, “Introduction to Process Control “
Feedforward Control
The feedforward control elements are not conventional 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 disturbance and 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. Romagnoli & Palazoglu, “Introduction to Process Control “
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. Use a combination of Feedforward and Feedback control
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 and inaccuracies.
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 modelling error
+20% error in gd gain
(Kd = 1.2)
NB:
Output signal = controlled variable
Romagnoli & Palazoglu, “Introduction to Process Control “

15. Romagnoli & Palazoglu, “Introduction to Process Control “
Example 2:
Heat Exchanger
Manipulated variable
Feedforward
Controller
Tout,sp
Tout
Steam
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. Romagnoli & Palazoglu, “Introduction to Process Control “
Example 3
Distillation Column FF
Controller
Design a FF controller to compensate for variations in feed composition and flow rate.
Manipulated variable
Disturbances
Controlled variable CT FT
Romagnoli & Palazoglu, “Introduction to Process Control “