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Wim Michiels
Department of Computer Science
K.U.Leuven
Leuven, Belgium
Silviu-Iulian Niculescu
Laboratoire des Systèmes et Signaux
Supélec
Gif-sur-Yvette, France
SOCN Course Leuven
November – December 2011
Stability and control of time-
delay systems
Overview of the course
Lecture 1 motivating examples, basis concepts, eigenvalue problems associated with linear time-delay systems,
spectral properties
Lecture 2 computation of characteristic roots, robustness and performance specifications, computation of H-2
and H-infinity norms
Lecture 3 computation of stability regions in general parameter spaces, stability regions in delay parameter
spaces
Lecture 4 control design I: fundamental limitations induced by delays, design of fixed-order controllers using
eigenvalue optimization
Lecture 5 delay differential equations of neutral type / delay differential algebraic equations: qualitative properties
of solutions, delay sensitivity, design of fixed-order controllers
Lecture 6 control design II: prediction based controllers, using delays as controller parameters
Aim
• the student understands fundamental properties of systems subjected to
time-delays (what becomes different when including a delay?);
• he/she has an overview of methods, techniques, and software for the analysis and
control design -mainly frequency domain based and grounded in numerical
linear algebra and optimization);
• he/she has a view on applications in various domains and on the applicability of
the methods.
Course material
- book “Stability and stabilization of time-delay systems”, SIAM, 2007
- copy of slides (distributed at the beginning of each lecture)
Home-works
- optional (3 instead of 2 ECTS credits if evaluation positive)
- 3 homeworks (distributed after Lectures 2, 3, and 4)
- solutions must be sent to the lecturers by e-mail, one week after the distribution
Contacting lecturers
- Wim Michiels ([email protected])
- Silviu-Iulian Niculescu ([email protected])
Course website
slides, homeworks, supplementary material,…
) http://people.cs.kuleuven.be/wim.michiels/courses/socn/
Overview Lecture I
Examples and applications
Basic properties of time-delay systems
Representation by functional differential equations
The initial value problem
Properties of solutions
Reformulation in a standard, first order equation
Spectrum of linear time-delay systems
Two eigenvalue problems related to linear time-delay systems
Qualitative properties of the spectrum
Continuity propertties of the spectrum
Examples and
applications
• networks
- biology (e.g. interactions between neurons)
- car following models
- time-based spacing of airplanes
- distributed and cooperative control, sensor networks
- congestion control in communication networks
- biochemical networks
• mechanical engineering
- haptic interfaces and motion synchronization
- machine tool vibrations (cutting and milling machines)
• parallel computing (load balancing)
• biological systems
- population dynamics
- cell/virus dynamics
• laser physics (lasers with optical feedback)
Motivating examples
Fluid flow model for a congested router in TCP/AQM
controlled network
pTC
tQtR
QCtR
tWtN
QCtR
tWtN
tQ
tRtptRtR
tRtWtW
tRtW
)()(
0,0,)(
)()(max
0)(
)()(
)(
))(())((
))(()(
2
1
)(
1)(
Hollot et al., IEEE TAC 2002
Model of collision-avoidance type:
W: window-size
Q: queue length
N: number of TCP sessions
R: round-trip-time
C: link capacity
p: probability of packet mark
Tp: propagation delay
AQM is a feedback control problem:
Sender Receiver Bottleneck
router
link c
rtt R
queue Q
acknowledgement
packet marking
• networks
- biology (e.g. interactions between neurons)
- car following models
- time-based spacing of airplanes
- distributed and cooperative control, sensor networks
- congestion control in communication networks
• mechanical engineering
- haptic interfaces
- machine tool vibrations (cutting and milling machines)
• parallel computing (load balancing)
• population dynamics
• cell dynamics, virus dynamics
• laser physics (lasers with optical feedback)
Motivating examples
Successive passages of teeth
delay
Rotation of each tooth
periodic coefficients
Delay inversely proportional to
speed
Goal: increasing efficiency while avoiding undesired oscillations
(chatter)
Model:
Rotating milling machines
• networks
- biology (e.g. interactions between neurons)
- car following models
- time-based spacing of airplanes
- distributed and cooperative control, sensor networks
- congestion control in communication networks
- biochemical networks
• mechanical engineering
- haptic interfaces and motion synchronization
- machine tool vibrations (cutting and milling machines)
• parallel computing (load balancing)
• biological systems
- population dynamics
- cell/virus dynamics
• laser physics (lasers with optical feedback)
Motivating examples
Delays appear as intrinsic components of the system, or in approximatations
of (mostly PDE) models describing propragation and wave phenomena
Heating system
Linear system of dimension 6,
5 delays
temperature to be controlled setpoint
Lab. Tomas Vyhlidal, CTU Prague
Model
Control law (PI+ state feedback)
• networks
- biology (e.g. interactions between neurons)
- car following models
- time-based spacing of airplanes
- distributed and cooperative control, sensor networks
- congestion control in communication networks
- biochemical networks
• mechanical engineering
- haptic interfaces and motion synchronization
- machine tool vibrations (cutting and milling machines)
• parallel computing (load balancing)
• biological systems
- population dynamics
- cell/virus dynamics
• laser physics (lasers with optical feedback)
Motivating examples
Haptic Interfaces and Motion Synchronization
Haptic Interfaces and Motion Synchronization
Smith predictor: transforming interconnections
“delay-out-of-the-loop”
Haptic Interfaces and Motion Synchronization
Haptic Interfaces and Motion Synchronization
• networks
- biology (e.g. interactions between neurons)
- car following models
- time-based spacing of airplanes
- distributed and cooperative control, sensor networks
- congestion control in communication networks
- biochemical networks
• mechanical engineering
- haptic interfaces and motion synchronization
- machine tool vibrations (cutting and milling machines)
• parallel computing (load balancing)
• biological systems
- population dynamics
- cell/virus dynamics
• laser physics (lasers with optical feedback)
Motivating examples
Evolution of anti-cancer T cells (T) Evolution of cancer cells (C)
Immune Dynamics in Leukemia Models
Interactions between T/C cells
Immune Dynamics in Leukemia Models
Post-transplantation dynamics of the immune response to chronic
myelogenous leukemia:
where:
T: anti-cancer cell population,
C: cancer cell population (functions of time t).
Delay Description
four distinct delays: and ;
relevant values approximately
Lossless Propagation: from hyperbolic PDEs to DDAEs
nonlinear characteristic of the tunnel diode.
Non linear circuit with LC line and tunnel diode
Lossless Propagation: from hyperbolic PDEs to DDAEs
Cauchy problem defined on the domain:
“propagation triangle”.
Propagation infinite angle Propagation triangle
Lossless Propagation: from hyperbolic PDEs to DDAEs
denote:
• networks
- biology (e.g. interactions between neurons)
- car following models
- time-based spacing of airplanes
- distributed and cooperative control, sensor networks
- congestion control in communication networks
- biochemical networks
• mechanical engineering
- haptic interfaces and motion synchronization
- machine tool vibrations (cutting and milling machines)
• parallel computing (load balancing)
• biological systems
- population dynamics
- cell/virus dynamics
• laser physics (lasers with optical feedback)
Motivating examples
Biochemical Network
(Monotone) Cyclic System with delay
subject to “negative feedback”
given by:
or
leading to:
where:
Basic properties of
time-delay systems
Representation by a functional differential equation
: Banach space of continuous function over [-¿, 0], equipped
with the maximum norm,
t t-¿
Functional Differential Equation of retarded type
functional
Linear FDE of retarded type
F: bounded variation in [-¿, 0]
F(0)=0
) unifying theory available
Linear FDE
F: bounded variation in [-¿, 0]
F(0)=0
examples:
pointwise
(discrete) delay
distributed
(continuous) delay
F
µ
-¿
-A0-A1
-A0
0
+A1
+A0
The initial value problem
Delay differential equation
linear
initial data required = function segment
x
-¿ 0 t
Ordinary differential equation
linear
x
0 t
The initial value problem
recall:
Theorem holding under mild conditions of f:
t
x
-¿ 0 t-¿ t
→ infinite-dimensional system
Properties of solutions
“Method of steps (single delay)”
Divide the solution in “couplets”:
General property: solutions become smoother as time evolves !
“Small solutions”
Backward continuation of solutions is in general not possible !
Asymptotic growth rate of solutions and stability
t
x
-¿ 0 t-¿ t
-¿ 0 µ
t=0
t=t1
t1
x
-¿ 0 t1-¿ t
Reformulation in a standard, first order form
Abstract ordinary differential equation over the function space X
-¿ 0 µ
t=0
t=t1
t1
x
-¿ 0 t1-¿ t
a time-delay system is a distributed parameter system with a special
structure: “spatial” and “temporal” variable are coupled
Related PDE formulation
Abstract ordinary differential equation
Functional differential equation
Delay systems are distributed parameter systems /infinite-dimensional
scalar examples
oscillatory solutions
chaotic attractor
Analysis: complex behavior
Controller synthesis:
any control design problem involving the determinination of a finite number of
controller parameters is a reduced order controller design problem
! inherent limitations
! control design almost exclusively ends up in
an optimization problem
! two main type of representations lead two
two mainstream control design techniques
The spectrum of linear time-
delay systems
Two eigenvalue problems associated with linear time-
delay systems
Finite-dimensional
nonlinear eigenvalue problem
Infinite-dimensional
linear eigenvalue problem
Functional differential equation Abstract ODE
substitution of exponential solution
Extension to general linear system
Outlook to Lecture 2:
Computing characteristic roots via a two-step approach
1. discretize linear-infinite-dimensional operator;
compute eigenvalues of resulting matrix
2. correct the individual characteristic root approximations
using the nonlinear equation (2)
(2)
Qualitative properties of the spectrum
Real axis
Continuity properties of the spectrum
“Shower example”
The following situations may occur.
• no smoothing of solutions
• infinitely many characteristic roots in a right half plane
• exponential and asymptotic stability not equivalent for linear systems
• multiple delays: stability may be sensitive to infinitesimal delay
perturbations
The presented results do NOT carry over time-delay systems of
neutral type (Lecture 5)
or, more generally,
Conclusions of Lecture 1
• Examples
• Properties of time-delay systems of retarded type
(solutions, representation, spectral properties)
Note: generalization of Lyapunov’s second method
Delay differential equation of retarded type Ordinary differential equation
sufficient condition for exponential stability of null solution
converse theorems exist, quadratic function(al) works in linear case
But: V is a functional ! gap between sufficient and
necessary conditions when restricting
to subclass characterized by a finite
number of parameters
system candidate Lyapunov functional
exact stability regions:
a¿
k¿
bounded by red curves
Example:
Note: time-stepping (numerical integration of solutions)
Main difference with time-steppers for ordinary differential equations
• memory / dependence of past data, often interpolation needed
• taking into account discontinuities in derivatives of solution
Overview of software available at
http://twr.cs.kuleuven.be/research/software/delay/software.shtml