1. Feedforward Control (前馈控制)
Lei Xie
Institute of Industrial Control,
Zhejiang University, Hangzhou, P. R. China
2013

2. Contents Feedforward Concept Design of Linear Feedforward Controllers
Design of Nonlinear Feedforward Controllers
Design Examples of Feedforward Control
Feedforward-Feedback Control
Simulation Results
Summary

3. Problem Discussion
Consider the heat exchanger shown in the figure, it is required to control the outlet temperature of process fluid, T, by manipulating the steam flow, RV. Please design the relevant control schemes on the diagram for the following cases:
Case #1: both of the flow, RF, and steam inlet pressure, PV, are stable;
Case #2: RF is stable, but PV varies frequently;
Case #3: PV is stable, but RF varies frequently;
Case #4: both of PV and RF varies frequently.

4. Feedback or Feedforward Control Examples
Feedback Control Scheme
Feedforward Control Scheme

5. Feedforward Concept
D1(t), …, Dn(t) represent some measurable disturbances to the controlled variable
Idea: to compensate for some measurable disturbances before they affect the controlled variable.

6. Feedforward / Feedback Control

7. Design of Linear Feedforward Controllers
Design Objective:

8. Design of Linear Feedforward Controllers (cont.)
Design formula for the feedforward controller:
Design Objective:

9. Design of Linear Feedforward Controllers (cont.)
Design formula for the feedforward controller:
( Why ? )

10. Types of Feedforward Controllers
Steady-State or Dynamic Feedforward Controller (静态前馈控制与动态前馈控制)
Linear or nonlinear Feedforward Controller (线性前馈控制与非线性前馈控制)
Discuss the common-used feedforward controller

11. Simulation Results of Linear Feedforward Controllers
(Please see…/FFControl /ExHeaterLinearFFC.mdl)

12. Nonlinear Steady-state Feedforward Control
Steady-state Equation
Discuss the realization of nonlinear feedforward controller and the effect of relevant measurements

13. Nonlinear Steady-State Feedforward Control Simulation
(Please see …/FFControl/ExHeaterStaticFFC.mdl)

14. Comparison of Feedforward and Feedback Control
Feedforward Control
Feedback Control
Disturbances are measurable
CV is measurable
Control MV based on disturbances
Control MV based on control ERROR
Open-loop, No Stability Problem
Closed-loop, Stability is the most important
Only some disturbances are detected
All disturbances are detected
Accurate model needed for both of Control and Disturbance Paths
No accurate model needed
Not adaptable to nonlinear or time-varied systems
Adaptable to nonlinear or time-varied systems

15. FFC+FBC Scheme #1 for Heat Exchanger
Steady-state output and smooth switch problem ?

16. FFC+FBC Scheme #2 for Heat Exchanger

17. Summary Cases which may introduce feedforward control
(1) Primary Controlled Variable is not measurable
(2) Primary CV is measurable, but some disturbances are too strong for feedback control systems
Conditions to apply feedforward control
(1) Main disturbances are measurable
(2) Response speed of disturbance path is slower than that of control path
(3) Characteristics of disturbance and control path are almost invariable

18. Thanks a lot !