1. Multivariable Control Systems
Ali Karimpour
Assistant Professor
Ferdowsi University of Mashhad
<<<1.1>>>
###Control System Design###
{{{Control, Design}}}

2. Multivariable Control System Design:
Chapter 9
Multivariable Control System Design:
Quantitative Feedback Theory
Topics to be covered include:
Introduction
Uncertainty Model and Plant Templates
QFT Design Procedure
Example
Extension to MIMO systems 2

3. Multivariable Control System Design:
Quantitative Feedback Theory
Topics to be covered include:
Introduction
Uncertainty Model and Plant Templates
QFT Design Procedure
Example
Extension to MIMO systems 3

4. Introduction
Many design methods require design objectives to be stated in terms which are familiar
from classical SISO design methods.
Some of these specification, may be derived from an accurate knowledge of noise and
disturbance statistics, or of possible perturbations to the nominal plant model, but they
are more often obtained in a less quantitative manner.
Typically they are initially obtained from previous experience with similar plants, and
then refined in a specify-design-analyze cycle until derived acceptable C.L. behavior.
This approach has been forcefully criticized as inadequate by Horowitz (1982).
The basic reason for using feedback is:
Unpredictable noises or disturbances.
To combat uncertainty
Unpredictable variations in the behavior of the plant.
If we have a quantitative description of the amount of uncertainty which may be present,
and a precise specification of the range of behaviors which may be tolerated in the face
of such uncertainty, then we should aim to develop a design technique ………

5. Multivariable Control System Design:
Quantitative Feedback Theory
Topics to be covered include:
Introduction
Uncertainty Model and Plant Templates
QFT Design Procedure
Example
Extension to MIMO systems 5

6. Uncertainty Model and Plant Templates
The various origins of model uncertainty:
I) Parametric uncertainty
Parametric uncertainty implies specific knowledge of variations in parameters of the transfer function.
II) Non-parametric uncertainty
The main source of non-parametric
uncertainty is error in the model. 6 6

7. Uncertainty Model and Plant Templates
I) Parametric uncertainty 7 7

8. Uncertainty Model and Plant Templates
I) Parametric uncertainty
Sometimes edges are not ok! 8 8

9. Uncertainty Model and Plant Templates
I) Parametric uncertainty
Sometimes edges are not ok! 9 9

10. Uncertainty Model and Plant Templates
How a QFT controller works?
Frequency (rad/sec)
Amplitude (db)
Frequency (rad/sec)
Amplitude (db) 10

11. Uncertainty Model and Plant Templates
How a QFT controller works?
System with and without Controller
Frequency (rad/sec)
Amplitude (db) 11

12. Uncertainty Model and Plant Templates
How a QFT controller works?
Step Response of System with and without Controller (K(s)=1) 12

13. Multivariable Control System Design:
Quantitative Feedback Theory
Topics to be covered include:
Introduction
Uncertainty Model and Plant Templates
QFT Design Procedure
Example
Extension to MIMO systems 13

14. QFT Design Procedure Choice of Frequency Array Choice of Nominal Plant
An appropriate frequency band for a computing templates and bounds has to be selected.
Choice of Nominal Plant
In order to compute bounds, it is necessary to choose a plant from the uncertainty set as the nominal plant. It is common practice to select a nominal plant which we think is most convenient for design.
QFT Bounds Computations
Given the plant templates, QFT converts closed-loop magnitude specifications into magnitude and phase constraints on a nominal open-loop function.
QFT Loop-shaping
The final step in a QFT design involves the design (loop shaping) of a nominal loop function that meets its bounds. The controller design then proceeds using the Nichols chart and classical loop-shaping ideas. 14 14

15. QFT Design Procedure
The QFT approach assumes that the plant uncertainty is represented by a set of templates
on the complex plane at some frequency ωk.
It also assumes that the design specification is in the form of bounds on the magnitudes
of the frequency-response transfer functions
The QFT technique leads to a design which satisfies these specifications for all
permissible plant variations.
Horowitz and Sidi (1972) have obtained sufficient conditions on frequency-domain
bounds which imply the satisfaction of time-domain bounds, the frequency-domain
bounds obtained in this way do not appear to be unduly conservative.

16. QFT Design Procedure Let we need |s|<2.5 so:
Bounds for S
Let we need |s|<2.5 so:
L must be outside of black curve.
Now consider the templates at one frequency with nominal plant as blue curve.
Bounds for template is red curve.
Effect of nominal plant?

17. QFT Design Procedure Let we need |s|<2.5 so:
Bounds for S
Let we need |s|<2.5 so:
L must be outside of black curve.
Changing Nominal plant
New bound
Pervious bound

18. A feedback configurations with two degree of freedom.
QFT Design Procedure
Bounds for T
A feedback configurations with two degree of freedom.
We need:
Since:
If L would not intersect any pair of M-circles whose values differed by more than
Nominal plant

19. QFT Design Procedure Bounds for T
If L would not intersect any pair of M-circles whose values differed by more than
Nominal plant

20. A feedback configurations with two degree of freedom.
QFT Design Procedure
Bounds for T
A feedback configurations with two degree of freedom.
Suppose that we know the template of all possible values of G at ω1:
But we need:
Since:
Nominal plant
If L would not intersect any pair of M-circles whose values differed by more than

21. A feedback configurations with two degree of freedom.
QFT Design Procedure
Bounds for T
A feedback configurations with two degree of freedom.
Changing Nominal plant
New bound
Leads Different Bounds
Nominal plant
Pervious bound

22. QFT Design Procedure Bounds for T
In order to meet the design specifications
the pre-filter is chosen to have the gain
Similarly one can found bounds for S

23. QFT Design Procedure QFT Algorithm:
1. Formulating of the closed-loop control performance specifications, i.e., stability margins, tracking and disturbance rejection.
2. Generating templates. For a given uncertain plant , select a series of frequency points according to the plant characteristics and the specifications.
3. Computation of QFT bounds. Find the intersection of bounds. An arbitrary member in the plant set is chosen as the nominal case.
4. Loop shaping for QFT controllers. The design of the QFT controller, K(s) is accomplished on the Nichols Chart. The QFT bounds at all frequencies must be satisfied and the closed-loop nominal system is stable;
5. Design of prefilters P(s). The Final step in QFT is to design the prefilter, P(s), such that the performance specifications are satisfied. 23 23

24. Multivariable Control System Design:
Quantitative Feedback Theory
Topics to be covered include:
Introduction
Uncertainty Model and Plant Templates
QFT Design Procedure
Example
Extension to MIMO systems 24

25. Example Suppose that the set of our plants is:
Performance specification are:
Constrain on S
a) |S|<2.5. (for robust stability)
Constrain on T
b) Zero steady state error to step input.
c) Less than 50% overshoot to step input.
d) Output must be above 90% after 1 second when step input applied. 25

26. Example a) |S|<2.5. (for robust stability) Constrain on S
Constrain on T
b) Zero steady state error to step input.
c) Less than 50% overshoot to step input.
d) Output must be above 90% after 1 second when step input applied.
0.9 1
1.5
0.5 2 3
Acceptable step response 26

27. Example 27 27

28. Example 28 28

29. Example
Templates are: 29 29

30. Example
Nominal Plant is:
a=1 and b=1 30 30

31. Example

32. Example

33. Example

34. Example
Three different controllers applied

35. Example
Three Different Controllers 35 35

36. Example
Three Different Controllers 36 36

37. Example
With Controller 37 37

38. Example
With Controller ?
Let 38 38

39. Example
With Controller 39 39

40. Example
With Controller 40 40

41. Multivariable Control System Design:
Quantitative Feedback Theory
Topics to be covered include:
Introduction
Uncertainty Model and Plant Templates
QFT Design Procedure
Example
Extension to MIMO systems 41

42. Extension to MIMO systems
This design method is extended to multivariable problems as follows.
We are going to find tuv (vth input to uth output)

43. Extension to MIMO systems

44. Refrences Maciejowski J.M. (1989). Multivariable Feedback Design:
Adison-Wesley.
Control System Design – QFT. Bo Bernhardsson, K. J. Åström
Department of Automatic Control LTH, Lund University