Ali Karimpour Associate Professor Ferdowsi University of Mashhad

Transcription

1 CONTROL IN INSTRUMENTATION Ali Karimpour Associate Professor Ferdowsi University of Mashhad Reference: - Advanced PID Control by Karl j. Astrom and Tore Hagglund, ISA, 2006 مدلسازی و کنترل صنعتی تالیف فرشاد مریخ بیات The Michigan Chemical Process Dynamics and Controls Open Text Book

2 Topics to be covered Introduction Lecture 9 Control Paradigms Cascade Control Mid-Range and Split-Range Control Selective and Override Control Ratio Control 2

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. Slave/master control Inner/Outer control Secondary/Primary control Primary loop Secondary loop It is also possible to have a cascade control with more nested loop. Cascade control Feedback control Advantages: One PI can change with two P controller with better performance. 3

4 Cascade Control Example: Improved load disturbance rejection s P s P 2 3 Using a conventional PI controller as: C p 0.37 s Using a cascade control as: C s 5 G( s) C ( s C s s ) s C p 0.55 s 4

5 Cascade Control Example: Improved load disturbance rejection Yellow line: Answer to conventional control Blue line: Answer to cascade control 5

6 Cascade Control Example: Small improvement in set-point response Yellow line: Answer to conventional control Blue line: Answer to cascade control control_in_inst_8_example.mdl control_in_inst_8_example2.mdl 6

7 Cascade Control How a cascade controller help? 7

8 Cascade Control Choice of secondary measured variables Basic rules for secondary variable selection 8

9 Cascade Control Choice of secondary measured variables Basic rules for secondary variable selection 9

10 Cascade Control Example of cascade systems Position control of a motor. Heat exchanger 0

11 Cascade Control Design procedure through an example(example 3) Consider following system: P? (0.5s )( s ) P? 0.s - In which condition cascade control suggested? P 2 P (0.5s )( s ) 0.s 2- Have a look on feedback controller. C p K p ( ) s C p PP 2 K p 20 s( s 2)( s 0) Stability margin for K p 2 K p by some method K p 6 by Ziegler

12 Cascade Control Design procedure through an example(example 3) Consider following system: P 2 (0.5s )( s ) P 0.s 3- First tune the secondary loop. P 0.s C s 5 2

13 Cascade Control Design procedure through an example(example 3) Consider following system: P 2 (0.5s )( s ) P 0.s 4- Tune the primary loop. CsP C P s P s 6 (0.5s )( s ) C p K p ( ) s Loop tf K p 00 s( s 2)( s 60) Stability margin for K p K p K p 37 by Ziegler by some method 74 3

14 Cascade Control Design procedure through an example(example 3) Consider following system: Feedback control Cascade control control_in_inst_8_example3.mdl 4

15 Mid-Range and Split-Range Control Cascade Control Number of measurement signals > Number of controlled signals Dual situation for Cascade Control is mid-range and split-range control Split-range control Number of control signals > Number of measurement signals Mid-range control Use two control signal simultaneously 5

16 Mid-Range Control Small with high resolution Large with low resolution Take care of v to be in middle usp u ysp Take care of control problem is in the middle of its operation y 6

17 Mid-Range Control Block diagram of mid-range control C is controlled by driving the process output y away from the set point. If this is done slowly, the deviation from the set point can be kept small, Otherwise use following: If the forward compensator is: Controller C 2 will perform the mid-ranging control without any disturbance of the process output y. 7

18 Split-Range Control Split-range control Number of control signals > Number of measurement signals Reactor system with split-range control Output signal and Valve coordination 8

19 Split-Range Control Reactor system with split-range control Output signal and Valve coordination 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 20

21 Override Control(Selective Control) [The Michigan Chemical Process Dynamics and Controls Open Text Book] Important application of override control are: - Equipment Protection 2- Auctioneering 2

22 Override Control(Selective Control) [The Michigan Chemical Process Dynamics and Controls Open Text Book] Important application of override control are: 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. 22

23 Override Control(Selective Control) [The Michigan Chemical Process Dynamics and Controls Open Text Book] Important application of override control are: 4- Artificial Measurements 23

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. 24

25 Ratio Control Example 4: Pulp Bleaching Control Pulp flow P 5 s P 2 2 s C s Hydrosulphite flow C s y Let y / y2 0 so 0. control_in_inst_8_example4.mdl Drawback: Delay in the hydrosulphite flow????????????? 0y 2 25

26 Ratio Control r Use a blend station instead of ratio station ) r ( t) ( ) y ( ) 2( t t control_in_inst_8_example4.mdl Pulp flow Hydrosulphite flow y y y 2 0y 2 26

27 Exercises - 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 = 0 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? 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 controls on this reactor to meet safety regulations and to ensure continuous operation. Which controls do you add? And where? Suppose sensors have downscale failure. 27