Cascade Control Systems ( 串级控制系统 )

Cascade Control Systems()Lei Xie

Institute of Industrial Control, Zhejiang University, Hangzhou, P. R. China

2013/04/03

Summary for Single-loop PID ControllersDetermination of Process CharacteristicsType Selection of Control ValvesAction Selection of PID ControllersSelection of PID Controller TypesTuning of PID Controller ParametersFlow Control & Tight / Average Level ControlPrevention of Reset Windup

ContentsConcept of Cascade ControlCharacteristics of Cascade Control Design Principle of Cascade Control Implementation and Tuning of Controllers Simulation Examples

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

TT23

TC23

Pgas (t)

Process Fluid

Fuel Gas

T(t)

u(t)

Tm(t)

%, TO

%, CO

Tsp

Ti (t)

+

Tsp

+

+

_

Tm (t)

+

+

T(t)

MV

D2 (t)

TT 23

ControlValve

Furnace

TC23

u(t)

D1 (t)

Analysis of Simple Control SystemProblem: it will take very long time from disturbances entrance to feedback controller starting responseInlet 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

TT23

TC23

Pgas (t)

Process Fluid

Fuel Gas

T(t)

u(t)

Tm(t)

%, TO

%, CO

Tsp

Ti (t)

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 ?

TT23

TC23

Pgas (t)

Process Fluid

Fuel Gas

T(t)

u(t)

Tm(t)

%, TO

%, CO

Tsp

Ti (t)

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 ?

Fgas

Fsp

T

Tsp

u(t)

TC23

TT23

Process Fluid

Fuel Gas

Ti (t)

FC13

FT13

Fm

Tm

Cascade Control Diagram for Outlet Temp. of Process Fluidwhere TC 23 is called primary/master controller (), and FC 13 is called secondary/slave controller (); D1 denotes disturbances entering the outer loop, D2 denotes disturbances entering the inner loop.

FT 13

FC13

Control Valve

Fm

Fgas

TT 23

D2

D1

Fsp

Tm

T

Tsp

Fuel Supply Subsystem

TC23

Furnace

Terms in Cascade Control Note: Primary/Secondary Disturbances are called /,

y1,sp

y2,sp

y2

ym2

y1

ym1

Primary CV()S & T

Control Valve

Primary / Master Controller()

Secondary / Slave Controller()

Secondary Plant()

Primary Plant()

Primary Disturbances

Secondary Disturbances

Secondary CVS & T

General Cascade Control DiagramNote: 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.

Secondary / Slave / Inner Loop

Gm2

y1,sp

y2,sp

y2

ym2

y1

ym1

Gm1

Gc1

Gc2

Gv

Gp2

D2

D1

Gp1

Primary / Master / Outer Loop

Equivalence of Inner Loop and Extended Control Valve

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

Design Principles of Cascade Control SystemsThe 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 plantTypical cascaded loops:temp. to flow, concentration to flow, pressure to flow, level to flow, temp. to pressure, temp. to temp.

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

Fgas

Fsp

T

Tsp

u(t)

TC23

TT23

Process Fluid

Fuel Gas

Ti (t)

FC13

FT13

Fm

Tm

TT23

TT25

T1sp

TC 25

T2m

T2

TC 23

Fuel Gas

Process Fluid

T1m

T2sp

T1

Fgas

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

Tuning of Cascade ControllersStep 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 StudyCascade Control SchemeSingle-Loop Control Scheme

TC

Furnace

Tsp

Tm

T

Fgas

Process Fluid

Ti (t)

Pgas

Fuel Gas

u

FC

TC

Furnace

Fuel Gas

Tsp

Tm

Fsp

Fm

T

Fgas

Process Fluid

u

Ti (t)

Pgas

Process ModelingDescription of linear and nonlinear dynamic model.

Primary Plant

u(t)

Pgas

MPa

T/hr

Fgas

_

Ti

_

T,

+

+

+

+

+

T 0

%

Secondary Plant

TC

Furnace

Tsp

Tm

T

Fgas

Process Fluid

Ti (t)

Pgas

Fuel Gas

u

Single-loop Control System

Secondary Plant

Valve

Fuel Supply

Furnace

T

Tsp

TCPID1

Tm

Fgas

u(t)

Pgas

Ti

Primary Plant

Temp. Sensor & Transmitter

+

T 0

+

_

+

Tm


T

Dynamic Model ParametersInitial ConditionKgas = 0.4u0 = 60 %Pgas0= 0.25 MPaFgas0 = 12 T/hrT0 = 300 T10= 120 Instrument SpanFlow of fuel gas: 0 ~ 40 T/hr, Outlet temp.: 200 ~ 400 Dynamic ParametersTf1 = 2 minK1 = 5T1 = 10 min1 = 5 minK2 = 1T2 = 1 minTm1 = 2 minTm2 = 0.2 min

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

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

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

Valve

Fuel Supply

Furnace

Flow Sensor & Transmitter

T

Fgas

Tsp

TCPID21

Tm

Pgas

Ti

Secondary Plant

Primary Plant


Fm

u(t)

Fsp

FCPID22

+

T 0

+

_

+

Tm


T

Fm

Fgas

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

PID Tuning for the Cascade Control SystemStep 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 ControlPlease see the following simulation example /CascadePID/CasPidwithResetWindup.mdl

Gm2

y1,sp

y2,sp

y2

ym2

y1

ym1

Gm1

Gc1

Gc2

Gv

Gp2

D2

D1

Gp1



e1

KC

y2c

y2,sp

Prevention of Reset Windup for Cascade ControlPlease see the example /CascadePID /CasPidwithoutResetWindup.mdl

Gm2

y1,sp

y2,sp

y2

ym2

y1

ym1

Gm1

Gc1

Gc2

Gv

Gp2

D2

D1

Gp1



KC

y2c

e1

y2,sp

ym2

Three-Level Cascade SystemsDiscuss the design principle and PID tuning

T1sp

TT25

TC25

TC23

PC31

Fuel Gas

Process Fluid

T1m

T2sp

T1

Pgas

PT31

Fgas

T2m

T2

TT23

Cascade Control Examplesof Heat ExchangerScheme #1Scheme #2Scheme #3

Process Fluid

Steam

Ti

Condensate

T

RV

TT27

TC27

FT17

FC17

Fm

Tm

Tsp

Fsp

PV

T

RV

Tsp

Psp

PV

Process Fluid

Condensate

Steam

Ti

TT27

TC27

PT37

PC37

Pm

Tm

T

RV

Tsp

Psp

PV

Process Fluid

Condensate

Steam

Ti

TT27

TC27

PT37

PC37

Pm

Tm

SummaryIntroduction of Cascade Control SystemsCharacteristics of Cascade Control SystemsDesign Principle of Cascade ControlImplementation and Tuning of ControllersSimulation Result Comparison of Cascade Control and Simple Control

Exercise 4.1For 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. 1Please describe the complete control diagram, and note the inputs and outputs for each block;2Select the action of control valve (failed-open or failed-close) and explain the selection reason; 3Determine the action of controller PC, TC (direct or inverse action).

Process Fluid

Fuel Gas

Ti (t)

PC31

PT31

Pm

Tm

Pgas

PS P

T

P

TS P

u(t)

TC23

TT23

Discussion problemsWhy do people prefer to apply the cascade control strategy ?When can we use the cascade control?Whats the differences among cascade control, simple control and feedforward /feedback control ?Next Topic: Feedforward Control

Documents

Cascade Control Systems ( 串级控制系统 )