lyapunov and invariance methods in control design · logo lyapunov and invariance methods in...

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Lyapunov and invariance methods in control design

Franco Blanchini 1

1Dipartimento di Matematica e InformaticaUniversita degli Studi di Udine

IFAC Joint conference, Grenoble, February 4, 2013

F. Blanchini Lyapunov and invariance methods in control design

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General

The purposes of the talk are


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General


Overview of set theoretic concepts;


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General



Invariance and Lyapunov techniques;


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General



Invariance and Lyapunov techniques;

Ideas and applications.


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Dynamic Programming 1

A general problem

x(t+1) = f (x(t),u(t),w(t))


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A general problem

x(t+1) = f (x(t),u(t),w(t))

Constraints:

- On the disturbance w(t) ∈ W ;- On the control input u(t) ∈ U ;- On the state x(t) ∈ X ;


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A general problem

x(t+1) = f (x(t),u(t),w(t))

Constraints:

- On the disturbance w(t) ∈ W ;- On the control input u(t) ∈ U ;- On the state x(t) ∈ X ;

Problem–State in a tube: Find a control u =Φ(x) andXini ⊆ X such that and for all x(0) ∈ Xini and w(t) ∈ W

x(t) ∈ X , t ≥ 0,

u(t) ∈ U , t ≥ 0.


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v

ρ

ζ

θ

φ

η

Figure: Tracking a fly


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r



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horizontal

vert

ical

r



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Example

Horizontal motion.x1: relative horizontal position of the flyx2: relative horizontal speed of the fly

x(t+1) =

[

1 10 1

]

x(t)+

[

01

]

u(t)+

[

1 00 1

][

w1(t)w2(t)

]

Constraint sets

X = x : |x1| ≤ 4 |x2| ≤ 4

U = u : |u| ≤ 4

W = w : |w1| ≤ 1 |w2| ≤ 1


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k=0

X(0)

Figure: The sequence of sets


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k=0 k=−1

X X(1)(0)



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k=0 k=−1

k=−2

X X

X(2)

(1)(0)



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k=0 k=−1

k=−2 k=−3

X X

X(2)

(1)(0)

(3)

X



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k=0 k=−1

k=−2 k=−3

X X

X(2)

(1)(0)

(3)

X =X(2)=X

8( )



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One–step controllability set to S

C (S ) = x : ∃u ∈ U : x+ = f (x ,u,w) ∈ S , ∀w ∈ W


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C (S ) = x : ∃u ∈ U : x+ = f (x ,u,w) ∈ S , ∀w ∈ W

Procedure


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C (S ) = x : ∃u ∈ U : x+ = f (x ,u,w) ∈ S , ∀w ∈ W

Procedure

1 Define X0 = X ;


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C (S ) = x : ∃u ∈ U : x+ = f (x ,u,w) ∈ S , ∀w ∈ W

Procedure

1 Define X0 = X ;

2 Define recursively backward in time

X(k−1) = C (X (k))

⋂

X


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C (S ) = x : ∃u ∈ U : x+ = f (x ,u,w) ∈ S , ∀w ∈ W

Procedure

1 Define X0 = X ;

2 Define recursively backward in time

X(k−1) = C (X (k))

⋂

X

3 DefineXmax =

⋂

k≤0

X(k)


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Theorem

Assume that U , X and W are compact. The problem of keepingthe state in a tube has a solution iff

Xmax 6= /0

Xmax : set of all initial states for which the problem has a solution.


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Theorem


Xmax 6= /0


Control:

u =Φ(x) ∈ Ω(x) = u ∈ U : f (x ,u,w) ∈ Xmax , ∀w ∈ W


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Theorem


Xmax 6= /0


Control:


x ∈ Xmax ⇔ Ω(x) 6= /0.


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Theorem


Xmax 6= /0


Control:


x ∈ Xmax ⇔ Ω(x) 6= /0.

Bertsekas and Rhodes – Glower and Scheppe (1972)


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Related problems

x(t+1) = f (x(t),u(t),w(t))


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Related problems

x(t+1) = f (x(t),u(t),w(t))

Target set. Given T ⊆ X assure that x(t) ∈ T for t ≥ T ;


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Related problems

x(t+1) = f (x(t),u(t),w(t))


Full information control u =Φ(x ,w).


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Related problems

x(t+1) = f (x(t),u(t),w(t))



Guaranteed convergence ‖x(t)‖→ 0 if w(t) = 0.


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Related problems

x(t+1) = f (x(t),u(t),w(t))




Largest feasibility domain or domain of attraction inside X

x(t+1) = f (x(t),w(t))


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Related problems

x(t+1) = f (x(t),u(t),w(t))




Largest feasibility domain or domain of attraction inside X

x(t+1) = f (x(t),w(t))

Constrained control u ∈ U x ∈ X

x(t+1) = f (x(t),u(t))


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Claim

First message: 40 years ago we had the basic theory to deal withthe “state in a tube problem”.


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Positive invariance 1


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Definition

The set P is (robustly) positively invariant for the system

x(t+1) = f (x(t),w(t))

if x(τ ) ∈ P implies x(t) ∈ P for t ≥ τ .


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Definition

The set P is (robustly) positively invariant for the system

x(t+1) = f (x(t),w(t))

if x(τ ) ∈ P implies x(t) ∈ P for t ≥ τ .

Definition

The set P is (robustly) controlled invariant for the system

x(t+1) = f (x(t),u(t),w(t))

if there exists a control law u =Φ(x) ∈ U such that P ispositively invariant for

x(t+1) = f (x(t),w(t),Φ(x(t)))


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Theorem

The “state in a tube” problem has a solution iff there exists acontrolled–invariant set

P ⊆ X


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Theorem


P ⊆ X

The set Xmax is the maximal–controlled invariant set.


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Theorem


P ⊆ X

The set Xmax is the maximal–controlled invariant set.

In practice we might be satisfied with some P smaller butsimpler


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Problem

Input-output gain.

w(k) y(k)x(k+1) = A x(k) + Bw(k)

y(k) = C x(k)

Problem: compute

J = supw(·) 6=0

‖y‖∞

‖w‖∞

where‖w‖∞

.= sup

k≥0|w(k)|


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The question is not well–posed since x(0) is not specified.


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If x(0) = 0, thenJ = ∑

k≥0

|CAkB |.


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If x(0) = 0, thenJ = ∑

k≥0

|CAkB |.

Set theoretic condition

Proposition

J ≤ µ iff there exists a positively invariant set P such that

P ⊂ x : |Cx | ≤ µ


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Set–theoretic interpretation:

2x

1x


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2x

1x

|Cx| <µ


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2x

1x

|Cx| <µP


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x

x

P

µ

1

2

g(x) <

x =f(x,w) +

Figure: Advantages: Set–theoretic works for nonlinearF. Blanchini Lyapunov and invariance methods in control design

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The condition |y | ≤ µ is satisfied for all x(0) ∈ P.


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The set theoretic conditions works for uncertain and nonlinearsystems.


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The set theoretic conditions works for uncertain and nonlinearsystems.

Synthesis case: potential advantages of nonlinear controllers


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Invariant sets and Lyapunov functions 1

x(t) = f (x(t)), 0 = f (0)

orx(t+1) = f (x(t))


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x(t) = f (x(t)), 0 = f (0)

orx(t+1) = f (x(t))

Definition

A Lyapunov function is a positive–definite function which isdecreasing (non–increasing) along the system trajectories:

V (x(t2))< V (x(t1)), if t1 ≤ t2

Typically: V (x) = ∇ V (x)f (x)< 0 or ∆V (x)< 0.


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Controlled systems:

x(t) = f (x(t),u(t)), 0 = f (0,0)

or

x(k+1) = f (x(k),u(k)),


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Controlled systems:

x(t) = f (x(t),u(t)), 0 = f (0,0)

or

x(k+1) = f (x(k),u(k)),

Definition

A Control Lyapunov function is a positive–definite function whichbecomes a Lyapunov function if a proper feedback u = κ (x) isapplied:

x(t) = f (x(t),κ (x(t))),


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Fact

The sub-level set

N [V ,κ ] .= x : V (x)≤ κ

are positively invariant.


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Fact

The sub-level set

N [V ,κ ] .= x : V (x)≤ κ


Any LF generates invariant sets.


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Fact

The sub-level set

N [V ,κ ] .= x : V (x)≤ κ


Any LF generates invariant sets.

Given an invariant set, there is no obvious way to associate aLF with it.


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Linear systems: Given

x(t) = A(w(t))x(t)

and a convex and compact set P including 0 in its interior (C-set)

S

x

x2

1

Minkowski functional.


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Controlled invariance of ellipsoids

E = x : xTPx ≤ 1, P positive definite


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x(t) = Ax(t)+Bu(t)


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x(t) = Ax(t)+Bu(t)

Controller u = Kx the control-invariance condition is

(A+BK )TP+P(A+BK )< 0.

Set R = KP−1 = KQ to get

QAT +AQ+BR+RTBT < 0


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x(t) = Ax(t)+Bu(t)




QAT +AQ+BR+RTBT < 0

For discrete-time we get similar conditions.


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x(t) = Ax(t)+Bu(t)




QAT +AQ+BR+RTBT < 0

For discrete-time we get similar conditions.

Claim

(Almost) nothing is forgotten in the LMI world: disturbance,constraints, performances, delays, uncertainties, LPV, disturbancerejection ... Boyd, El Ghaoui, Feron, Balakrishnan


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Competitors?


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Competitors?

Question

Are there other convenient classes of Lyapunov functions?


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Competitors: polyhedral Lyapunov functions

V (x) = maxi

Fix∑ pi : pi PERLOSPAZIO


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Competitors: polyhedral Lyapunov functions

V (x) = min∑ pi : pi ≥ 0, x = Xp


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Controlled invariance of polyhedra 1

Consider the set

P = x = Xp, p ≥ 0, ∑pi = 1

and x(t) = Ax(t)+Bu(t). Controlled invariance requires

Pij ≥ 0, i 6= j

AX +BU = XP

1TP ≤ −β 1T , β ≥ 0

where 1T = [ 1 1 . . . 1 ].

Non–linear unless x is fixed.

But we can use backward–recursive procedures.


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Consider the set

P = x = Xp, p ≥ 0, ∑pi = 1


Pij ≥ 0, i 6= j

AX +BU = XP

1TP ≤ −β 1T , β ≥ 0

where 1T = [ 1 1 . . . 1 ].

Non–linear unless X is fixed....

... but we can use backward–recursive procedures.


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Consider the set

P = x = Xp, p ≥ 0, ∑pi = 1


Pij ≥ 0, i 6= j

AX +BU = XP

1TP ≤ −β 1T , β ≥ 0

where 1T = [ 1 1 . . . 1 ].

Non–linear unless X is fixed....

...but we can use backward–recursive procedures.


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In general a controlled–invariant polyhedron does not admit linearcontrollers, but a piecewise–linear control.

Gutman and Cwikel (1986)... yes ... but how many sectors are needed?


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u = K x

h

Gutman and Cwikel (1986)... yes ... but how many sectors are needed?


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u = K x

h

Gutman and Cwikel (1986)... yes ... but how many?


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Controlled invariance of polyhedra

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1

234

567

89

10

Controlled–invariant with constraints|h1− h1| ≤ 0.1m |h2− h2| ≤ 0.1m, |u− u| ≤ 0.05m/s.


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Ellipsoidal approximation.


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Limits and advantages of quadratic sets/functions

Quadratic functions are conservative because they provide


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a very rough approximation of the maximal admissible set;


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infinitely conservative robustness margin, for instance


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[

x1x2

] [

0 −11 0

] [

x1x2

]

+

[

δ1

]

u, |δ(t)| ≤∆

is stabilizable for all ∆, but quadratically stabilizable iff|δ(t)| ≤∆< 1.


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[

x1x2

] [

0 −11 0

] [

x1x2

]

+

[

δ1

]

u, |δ(t)| ≤∆

is stabilizable for all ∆, but quadratically stabilizable iff|δ(t)| ≤∆< 1.

However they involve efficient algorithms!!


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Limits and advantages of polyhedral sets/functions

Polyhedral functions are computationally demanding because


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No bounds for the complexity of the function andcompensator.


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Non–conservative: stability (stabilizability) of

x(t+1) = A(w(t))x(t)+B(w(t))u(t), w(t) ∈ W

is equivalent to polyhedral stability (stabilizability) Braytonand Tong 1984, Molchanov and Piatnitsky 1986, (Blanchini1995).


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Non–conservative: stability (stabilizability) of

x(t+1) = A(w(t))x(t)+B(w(t))u(t), w(t) ∈ W

is equivalent to polyhedral stability (stabilizability) Braytonand Tong 1984, Molchanov and Piatnitsky 1986, (Blanchini1995).

ε–approximation of the maximal controlled-invariant setMorris and Brown 1975, Gutman and Cwickel 1986, Kheertiand Gilbert 1987, (Blanchini Miani 1996).


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Other functions


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Other functions

Piecewise quadratic.


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Other functions


Polynomials.


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Other functions


Polynomials.

Truncated quadratic.


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Other functions


Polynomials.

Truncated quadratic.

Composite quadratic.


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Applications

Topics which successfully involve set–invariance techniques include


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Applications


Constrained control with disturbances


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Applications



Reference management


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Applications




Receding horizon control


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Applications





Switching among compensators


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Applications






Obstacle avoidance


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Applications






Obstacle avoidance

. . .


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Control with constraints and disturbances

Boyle–Turbine model: Usoro, Schweppe, Gould, Wormley (1982)


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Example

x1x2x3

=

−0.0075 −0.0075 00.1086 −0.149 0

0 0.1415 −0.1887

x1x2x3

+

0−0.05380.1187

w +

0.003700

u

|x1| ≤ 0.1, |x2| ≤ 0.01, |x3| ≤ 0.1, |u| ≤ 0.25

Maximize the disturbance size α : |w | ≤ α


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Quadratic L. F. performance: αQUAD = 1.27


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Quadratic L. F. control: u =−37.85x1−4.639x2+0.475x3


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Polyhedral L. F. performance αPOLY = 1.45


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Polyhedral L. F. control: Number of sectors (and gains)ns = 60.


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Polyhedral L. F. control: Number of sectors (and gains)ns = 60.

Trade–off: if you use the linear one, you loose about 12%


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Life is a trade–off

Claim


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Claim

There is almost always a trade–off between the optimalsolution which is complex or unrealistic and simpler butsub–optimal solutions.


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Claim


Optimal solutions support the sub–optimal ones, because ...


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Claim


Optimal solutions support the sub–optimal ones, because ...

... once you know the optimal you are aware of how much youare loosing when you use the sub–optimal one (Boyd–Barratt).


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Reference management 1

Tracking under constraints: why adopting controlled invariantsets?

X


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X

Too late!


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X

P

Too late!


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ReferenceRegulator PLANT

r e y+

−manager

m

Figure: Open loop Reference Management Device


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ReferenceRegulator PLANT

r e y+

−manager

m

Figure: Closed loop Reference Management Device


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Kapasouris, Athans and Stain (1988); Tan and Gilbert (1991);Gilbert, Kolmanowski and Tan (1995); Bemporad, Casavola andMosca (1996)...

Example

1z − a

kz − 1

r

Manager

Referencey

u

m

−+

Let a= 0.8 and k = 0.08, assume x(0) = 0 let r(t)≡ 0.8.Constraint: |y | ≤ 1.


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−1.5 −1 −0.5 0 0.5 1 1.5−1

−0.8

−0.6

−0.4

−0.2

0

0.2

0.4

0.6

0.8

1

Figure: Evolution without RMD (dashed) and with closed–loop RMD(plain)


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0 10 20 30 40 50 60 70 800

0.2

0.4

0.6

0.8

1

1.2

1.4

Figure: Time evolution without RMD (dashed), with open–loop RMD(dash–dotted) and with closed loop RMD (plain)


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Advantage of nonlinear controllers

Claim


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Advantage of nonlinear controllers

Claim

Control performance can considerably benefit fromnon-linearities.


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Receding horizon control 1

Consider the following discrete-time system

x(k+1) = f (x(k),u(k))

RHC algorithm (on-line).


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x(k+1) = f (x(k),u(k))


1 (1) measure (or estimate) x(k);


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x(k+1) = f (x(k),u(k))



2 Compute u(k), u(k+1), ... , u(k+T −1), u(i) ∈ U ,x(i) ∈ X ,

JT =k+T−1

∑i=k

g(x(i),u(i))


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x(k+1) = f (x(k),u(k))




JT =k+T−1

∑i=k

g(x(i),u(i))

3 apply u(k);


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x(k+1) = f (x(k),u(k))




JT =k+T−1

∑i=k

g(x(i),u(i))

3 apply u(k);

4 set k = k+1 and go to (1)


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The following problems arise


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recursive feasibility?


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stability?


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stability?

(sub) optimality?


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A possible solution: use invariant sets!

minJ =k+T−1

∑i=k

g(x(i),u(i))

s.t.

x(i +1) = f (x(i),u(i))

u(i) ∈ U , x(i) ∈ X , i = k ,k+1, . . . ,k+T −1,

x(k+T ) ∈ P

P controlled invariant.


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A possible solution: use invariant sets!

minJ =k+T−1

∑i=k

g(x(i),u(i))+h(x(T ))

s.t.

x(i +1) = Ax(i)+Bu(i)

u(i) ∈ U , x(i) ∈ X , i = k ,k+1, . . . ,k+T −1,

x(k+T ) ∈ P

P controlled invariant.


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Constrained LQR

minJ∞ =∞

∑t=0

xT (k)Qx(k)+uT (k)Ru(k)

Final setE = x : xTPx ≤ µ

where

P solution of the Riccati equation;

E ⊂ X

−Koptx ∈ U (with Kopt = BTP the optimal unconstrained).

h(x(T )) = xTPx


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Constrained LQR

minJ∞ =∞

∑t=0

xT (k)Qx(k)+uT (k)Ru(k)

Final setE = x : xTPx ≤ µ

where

P solution of the Riccati equation;

E ⊂ X

−Koptx ∈ U (with Kopt = BTP the optimal unconstrained).

h(x(T )) = xTPx

Fact

For T large enough to assure h(x(T ))≤ µ the scheme isinfinite–time optimal! (Sznaier 1987).


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x(0)


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X

P


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Final state weight g(x(T));

Cost–to–go function;

Explicit LQ solution (piecewise affine control) Bemporad,Morari, Dua, Pistikopulos (2002).

Multiparametric programming Mayne, Morari, Bemporad,Borrelli, Rakovic, Filippi, Johansen, Jones, Pistikopulos .....


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Disturbances

Blanchini (1991), Mayne, Rakovic and Seron (2005), Mayne,Rakovic, Findeisen and Allgower (2006)


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Switching among compensators 1

Plant

K

K

K

1

2

r

xu


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Plant

K

K

K

1

2

r

xu

Gain–increasing as the state approaches the origin


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Plant

K

K

K

1

2

r

xu

Gain–increasing as the state approaches the origin

Question: how can we switch in a safe way?


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A possible solution: associate an invariant set with each gain.Construct these invariants sets in such a way they are nested.Then the scheme converges.

SS

S

1

2

r


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But: You need state feedback or a reliable observer!

k

k

k

1

2

r

OBSERVER

PLANT


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Example

y(t)+µ(t)y(t)+ y(t) = 0

the friction µ is the control constrained as 0.1≤ µ ≤ 10.

Damping control strategies


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Example

y(t)+µ(t)y(t)+ y(t) = 0



Constant: e.g. minimum/maximum/critical damping;


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Example

y(t)+µ(t)y(t)+ y(t) = 0




Heuristic switching: maximum damping when |y | ≤ ε, elseno–damping;


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Example

y(t)+µ(t)y(t)+ y(t) = 0




Heuristic switching: maximum damping when |y | ≤ ε, elseno–damping;

Invariant–set–based switching.


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y

y

Figure: Heuristic strategy (blue) and the invariant–set–based strategy(red)


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−2 −1.5 −1 −0.5 0 0.5 1−2

−1.5

−1

−0.5

0

0.5

1

1.5

2

Figure: State–space evolution of the heuristic strategy (dashed) and theinvariant–set–based strategy (plain)


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0 1 2 3 4 5 6 7 8 9 10−2

−1.5

−1

−0.5

0

0.5

1

1.5

2

undamped

overdamped

critically damped

heuristic

switched

Figure: Performances of the considered strategies


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Obstacle avoidance 1


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Tracking a reference in environments with obstacles;


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Avoid collisions due to human errors;


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Covering by a family of connected controlled–invariant setswith ”crossing points”.


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Covering by a family of connected controlled–invariant setswith ”crossing points”.

Hierarchical controlLower level: in each set reaching the crossing point to thenext one (or the reference).Upper level: decision about the sequence of sets to becrossed;


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q2

q1

Figure: A manipulator in a constrained environment


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Figure: The admissible region in the configuration set and itsapproximate covering


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Figure: The upper level control is a graph


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B

A


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Conclusions

By means of set–invariance methods one can achieve several goals,especially if applied to special problems of practical importanceincluding


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Conclusions


Overcoming the limits of linear compensators by properlyintroducing nonlinearities in the loop;


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Conclusions



Supporting receding–horizon techniques;


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Conclusions




Improving performances by combining compensators andswitching among them;


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Conclusions





Solving constrained problems efficiently such as tracking andcollision avoidance;


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Conclusions






Having fun with mathematics....


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Conclusions






Having fun with mathematics....

Too many others to be all included here!


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The end


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The end

Merci!


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