Cascade Control Systems (串级控制系统) Lei Xie Institute of Industrial Control, Zhejiang University, Hangzhou, P. R. China 2012/3/21

Summary for Single-loop PID Controllers Determination of Process Characteristics Type Selection of Control Valves Action Selection of PID Controllers Selection of PID Controller Types Tuning of PID Controller Parameters Flow Control & Tight / Average Level Control Prevention of Reset Windup

Contents Concept of Cascade Control（串级控制概念） Characteristics of Cascade Control（串级控制系统的特性分析） Design Principle of Cascade Control （串级控制的设计原理） Implementation and Tuning of Controllers （串级控制器的实现与参数整定） Simulation Examples（仿真举例）

Process Example: A Simple Control System MV: flow of fuel gas, CV: outlet temp. of process fluid from furnace Final control element: fail-closed valve

Analysis of Simple Control System Inlet pressure of fuel gas↑→ flow of fuel gas even if u(t) keeps constant↑→(by combustion) temp. in furnace chamber ↑→ outlet temp. of process fluid T ↑→(by feedback path) CO↓ Problem: it will take very long time from disturbances entrance to feedback controller starting response。

Idea behind Cascade Control Upsets in the fuel supply system first affect Fgas (the flow of fuel gas), then affect the outlet temp. of process fluid from furnace. it is logical to start manipulating the fuel valve as soon as a variation in Fgas is sensed, before T starts to change. This corrective action uses an intermediate variable, Fgas, to reduce the effect of some dynamics in the process. How to reduce the effect of Pgas ?

Process Example: A Cascade Control Scheme (1) This scheme consists of two sensors, two transmitters, two controllers, and one valve. (2) This scheme results in two control loops, one loop controlling T and the other loop controlling Fgas. Note: The flow of fuel gas is used only as an intermediate variable to improve control performance. Please plot block diagram of the system ?

Cascade Control Diagram for Outlet Temp. of Process Fluid where TC 23 is called “primary/master controller (主控制器)”, and FC 13 is called “secondary/slave controller (副控制器)”; D1 denotes disturbances entering the outer loop, D2 denotes disturbances entering the inner loop.

Terms in Cascade Control Note: Primary/Secondary Disturbances are called “一次/二次干扰”,

General Cascade Control Diagram Note: D1 denotes the effect of primary disturbances on primary CV, D2 denotes the effect of secondary disturbances. “Primary Loop” presents the outer loop where the inner loop is closed and set in remote set point or cascade mode.

Equivalence of Inner Loop and Extended Control Valve

Characteristics of Cascade Control Systems Secondary loop (or Inner loop) usually responds fast and can overcome the effect of secondary disturbances on primary CV efficiently Secondary loop could reduce the nonlinearity of control valves and secondary processes. Why ?

Design Principles of Cascade Control Systems The secondary variable must respond faster to changes in some disturbances than the primary variable does — the faster, the better Secondary loop or inner loop must include some obvious disturbances to primary variable— the more the better If possible, secondary loop should include some nonlinear plant Typical cascaded loops: temp. to flow, concentration to flow, pressure to flow, level to flow, temp. to pressure, temp. to temp.

Example for Secondary Variable Selection Scheme #1 Scheme #2 About the response speed of secondary loop and secondary disturbances included in secondary loop

Type Selection of Cascade Controllers Secondary/slave/inner controllers usually select P or PI strategy Main reason: response speed to secondary disturbances (strong P + weak I action) Primary/master/outer controllers frequently select PI or PID strategy Because of slow response of primary variables such as temperatures, D action is common used in cascade control systems

Tuning of Cascade Controllers Step 1: Set the primary controller to manual mode, tune the PID parameters of slave controller using single-loop PID tuning techniques. Step 2: Set the secondary controller to remote automatic mode and the primary controller to manual mode, make a step change in the primary controller output to obtain the open-loop characteristic for the extended controlled plant. Step 3: Tune the PID parameters of primary controller using single-loop offline tuning techniques.

Simulation Study Cascade Control Scheme Single-Loop Control Scheme

Process Modeling Description of linear and nonlinear dynamic model.

Single-loop Control System

Dynamic Model Parameters Initial Condition Kgas = 0.4，u0 = 60 %，Pgas0= 0.25 MPa，Fgas0 = 12 T/hr，T0 = 300 ℃，T10= 120 ℃ Instrument Span Flow of fuel gas: 0 ~ 40 T/hr, Outlet temp.: 200 ~ 400 ℃ Dynamic Parameters Tf1 = 2 min，K1 = 5，T1 = 10 min，τ1 = 5 min，K2 = 1，T2 = 1 min，Tm1 = 2 min，Tm2 = 0.2 min

Single-loop PID Simulation (Please see …/CascadePID/SinglePidwithLimit.mdl)

PID Tuning Steps for Single-loop Control System Step 1: Apply Step Response Test for the Extended Controlled Process Step 2: Obtain Process Characteristic Parameters K, T ,τ Step 3: Tune the Initial PID Parameters Step 4: Optimize PID Parameters Based on the Control Performances Simulation study for the above single-loop control system

Cascade Control System for Furnace Outlet Temperature Problem: discuss the extended controlled plant for the inner and outer loop

Cascade Control Simulation for the Furnace Outlet Temp. (Please see …/CascadePID/CascadePidwithLimit.mdl)

PID Tuning for the Cascade Control System Step 1: Set the inner controller to manual mode, tune the PID parameters of inner controller. Step 2: Set the inner controller to remote automatic mode and the outer controller to manual mode, make a step change in the outer controller output to obtain the characteristic parameters for the extended controlled plant. Step 3: Tune the PID parameters of outer controller using single-loop tuning techniques. Simulation study for the cascade system

Simulation Result Comparison of Cascade and Simple Control

Reset Windup Problem in Cascade Control Please see the following simulation example …/CascadePID/CasPidwithResetWindup.mdl

Prevention of Reset Windup for Cascade Control Please see the example …/CascadePID /CasPidwithoutResetWindup.mdl

Three-Level Cascade Systems Discuss the design principle and PID tuning

Cascade Control Examples of Heat Exchanger Scheme #2 Scheme #1 Scheme #3

Summary Introduction of Cascade Control Systems Characteristics of Cascade Control Systems Design Principle of Cascade Control Implementation and Tuning of Controllers Simulation Result Comparison of Cascade Control and Simple Control

Exercise 4.1 For the control system shown in the right figure, the controlled variable is T, and P represents the pressure of fuel gas after control valve. u represents the position of control valve. Psp, Tsp denote the setpoints of controllers TC and PC, respectively. （1）Please describe the complete control diagram, and note the inputs and outputs for each block; （2）Select the action of control valve (failed-open or failed-close) and explain the selection reason; （3）Determine the action of controller PC, TC (direct or inverse action).

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 ? Next Topic: Feedforward Control