Feedforward Control ( 前馈控制 ) Dai Lian-Kui Shen Guo-jiang Institute of Industrial Control, Zhejiang University

Discussion problems Why do people prefer to apply the cascade control strategy ? When can we use the cascade control ? What’s the differences among cascade control, simple control and feedforward /feedback control ?

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

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

Feedback or Feedforward Control Examples Feedback Control SchemeFeedforward Control Scheme

Feedforward Concept D 1 (t), …, D n (t) represent some measurable disturbances to the controlled variable Idea: to compensate for some measurable disturbances before they affect the controlled variable.

Feedforward / Feedback Control D(t) represent some main measurable disturbances

Design of Linear Feedforward Controllers Design Objective:

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

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

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

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

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

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

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

Comparison of Feedforward and Feedback Control Feedforward ControlFeedback Control Disturbances are measurableCV 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

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

FFC+FBC Scheme #2 for Heat Exchanger

FFC+FBC Scheme #3 for Heat Exchanger Steady-state Equation is If main disturbances are from P V, R F ; how can you improve control performances ?

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

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