CONTROL IN INSTRUMENTATIONkarimpor.profcms.um.ac.ir/imagesm/354/stories/con_ins/control_inst… · Reactor system with split-range control Output signal and Valve coordination. lecture

Ali KarimpourAssociate Professor

Ferdowsi University of Mashhad

CONTROL IN CONTROL IN INSTRUMENTATIONINSTRUMENTATION

Reference:1- Advanced PID Control by Karl j. Astrom and Tore Hagglund, ISA, 20062- و کنترل صنعتی تالیف فرشاد مریخ بیات مدلسازی3- The Michigan Chemical Process Dynamics and Controls Open Text Book

lecture 9

Ali Karimpour Nov 2013

2

Lecture 9

Control ParadigmsControl Paradigms

Topics to be covered Introduction

Cascade Control

Mid-Range and Split-Range Control

Selective and Override Control

Ratio Control

lecture 9


3

Cascade Control

Cascade Control: Cascade control uses two or more controllers one driving the other.

Number of measurement signals > Number of controlled variables

Cascade control is particularly useful when there are significant dynamics, e.g, long dead time or long time constants between the control variable and the process variable.

It is also possible to have a cascade control with more nested loop.

Secondary loop

Primary loop

Cascade control Feedback controlAdvantages: One PI can change with two P controller with better performance.

Slave/master control

Inner/Outer control

Secondary/Primary control

lecture 9


4

Cascade Control

Example1:Improved load disturbance rejection

11

1 s

P 32 11

s

P

sCp

168.037.0

31)1()(

sCsCsGs

s

Using a conventional PI controller as:

Using a cascade control as:

5sC

sCp

29.055.0

lecture 9


5

Cascade ControlExample1:

Improved load disturbance rejection

Yellow line: Answer to conventional control

Blue line: Answer to cascade control

lecture 9


6

Cascade ControlExample1:

Small improvement in set-point response

Yellow line: Answer to conventional control

Blue line: Answer to cascade control

control_in_inst_8_example1.mdlcontrol_in_inst_8_example2.mdl

lecture 9


7

Cascade Control

How a cascade controller help?

lecture 9


8

Cascade Control

Choice of secondary measured variables

Basic rules for secondary variable selection

lecture 9


9

Cascade Control

Choice of secondary measured variables

Basic rules for secondary variable selection

lecture 9


10

Cascade Control

Example of cascade systems

Position controlof a motor.

Heat exchanger

lecture 9


11

Cascade Control

Design procedure through an example(Example 3)

Consider following system:

)1)(15.0(1

?

ssP

11.01

?

sP

1- In which condition cascade control suggested?

)1)(15.0(1

2

ssP

11.01

1

sP

2- Have a look on feedback controller.

)11(s

KC pp )10)(2(20

21

sssKPPC pp

12formarginStability pK by Ziegler 6pKmethod someby 1pK

lecture 9


12

Cascade Control



)1)(15.0(1

2

ssP

11.01

1

sP

3- First tune the secondary loop.

5sC

11.01

1

sP

lecture 9


13

Cascade Control



)1)(15.0(1

2

ssP

)1)(15.0(1

61.05

1 21

1

sss

PPCPCs

s

4- Tune the primary loop.

)11(s

KC pp )60)(2(

100

sss

KtfLoop p

74formarginStability pK

by Ziegler 37pK

method someby 1pK

11.01

1

sP

lecture 9


14

Cascade Control



Feedback control Cascade control

control_in_inst_8_example3.mdl

lecture 9


15

Mid-Range and Split-Range Control

Cascade Control Number of measurement signals > Number of controlled signals

Split-range control

Dual situation for Cascade Control is mid-range and split-range control

Number of control signals > Number of measurement signals

Mid-range control Use two control signal simultaneously

lecture 9


is in the middle of its operation1u

y

16

Mid-Range Control

Small with high resolution

Large with low resolution

Take care ofcontrol problem

Take care of v1to be in middle spu spy

lecture 9


17

Mid-Range Control

Block diagram of mid-range control

C1 is controlled by driving the processoutput y away from the set point.

If this is done slowly, the deviationfrom the set point can be kept small,Otherwise use following:

If the forward compensator is:

Controller C2 will perform the mid-ranging control without any disturbance of the process output y.

lecture 9


18

Split-Range Control

Split-range control Number of control signals > Number of measurement signals

Reactor system with split-range control

Output signal andValve coordination

lecture 9


19

Split-Range Control

Reactor system with split-range control

Output signal andValve coordination

lecture 9


20

Override Control(Selective Control)[The Michigan Chemical Process Dynamics and Controls Open Text Book]

Override Control

Number of measurement signals > Number of controlled variables

This controllers are used in cases where a choice must be made between inputs.

This controller uses some switches (electronic and pneumatic selectors ):

• High Selective Switch (HSS) • Low Selective Switch (LSS)

• Median Value Selector

lecture 9


21

Important application of override control are:

1- Equipment Protection 2- Auctioneering


lecture 9


22


3- Instrumentation Redundancy

In the case of failure in a sensor others will help us to have a continuous operation.

Different type of failure:

• Downscale failure

• Upscale failure

Suitable for downscale failure.


lecture 9


23


4- Artificial Measurements


lecture 9


24

Ratio Control

The objective of ratio control is to control the ratio of two variables, at a certain value.

Examples of ratio control are:

• Fuel-to-air supply ratio.

• Blending chemicals.

lecture 9


25

Ratio Control

Example 4: Pulp Bleaching Control

Pulp flow

Hydrosulphite flow

151

1 s

P 121

2

sP

sC 05.02.01

sC 073.0078.02

1.010/ 21 soyyLet

1y

210y

Drawback: Delay in the hydrosulphite flow

?????????????


lecture 9


26

Ratio Control

Use a blend stationinstead of

ratio station

Pulp flow

Hydrosulphiteflow )()1()()( 112 tytrtr

1y

210y

1y

210y

0 75.0


lecture 9


27

1- There is a reaction that normally goes from A B under extremely high pressures. However, the pressure in the reactor must not goes below P = 10 atm, and also the temperature in the reactor must not goes below 300 C. What type of a selectors would be used to protect this equipment?

Exercises

2- We have a reactor that carries out the reaction A + B C. However, if there is too much A fed,a highly exothermic side reaction,A + C D occurs and will quickly melt the entire reactor. We need two sets of redundant controlson this reactor to meet safety regulationsand to ensure continuous operation.Which controls do you add? And where?Suppose sensors have downscale failure.

Documents

CONTROL IN INSTRUMENTATIONkarimpor.profcms.um.ac.ir/imagesm/354/stories/con_ins/control_inst… · Reactor system with split-range control Output signal and Valve coordination. lecture