1 formal models for stability analysis : verifying average dwell time * sayan mitra mit,csail...
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Formal Models for Stability Analysis : Verifying Average Dwell Time*
Sayan Mitra MIT,CSAIL
Research Qualifying Exam20th December 2004
Joint work with Daniel Liberzon (UIUC) and Nancy Lynch (MIT)
* Full version of the paper has been sent for journal review.
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A common math model (HIOA) Expressive: few constraints on continuous and discrete behavior
Compositional: analyze complex systems by looking at parts
Structured: inductive verification
Compatible: application of CT results e.g. stability, synthesis
Motivation: Macro
Control Theory: Dynamical system with boolean variables
Stability
Controllability
Controller design
Computer Science: State transition systems with continuous dynamics
Safety verification model checking theorem proving
Hybrid Systems
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Motivation: Micro
Analysis of mobile algorithms (CT view) nodes: plant with continuous motion, disturbance
algorithm: controller maintaining some structure
Complexity
Stability and Robustness
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Outline
1. Background
2. Stability under slow switching
3. Formal Model
4. Invariant Approach
5. MILP Approach
6. Conclusions
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Switching and Stability
M1
M2
M1M2
M2 M1
M3
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Stability Under Slow Switchings
Theorem [Hespanha]: Assuming Lyapunov functions for the individual modes exist, global asymptotic stability is guaranteed if τa is large enough.
),( Tt# of switches on average dwell time (ADT)
t1 12 2
)()( tV t decreasing sequence
--- (1)
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Problem Statement
If all the executions of the hybrid system satisfy Equation (1), then the
system is said to have ADT τa .
Q: Given hybrid system A, does it have ADT τa ? or, what is the largest τa that is ADT for A ?
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V: set of variables, types, valuations val(V), dtypes Q: set of states, Q val(V) : start states A: set of actions D Q A Q: discrete transitions. (v,a,v) є D is written in
short as
T: set of trajectories for V, functions describing continuous
evolution
A trajectory : J val(V)
T is closed under prefix, suffix, and concatenation
Formal Definitions: Hybrid Automata
[Lynch,Segala,Vaandrager]
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Every variable is either discrete or continuous V = Vc U Vc
A set F of state models for the continuous variables Vc
A state model is a locally Lipschitz function f such that the solution to the system of differential equation d(v) = f(v) are in the dtypes of the corresp. continuous variables
A mode switching function
So, we have only continuous variables changing over trajectories:
Mode switches changing the state models
Definitions: Structured HA (SHA)
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Definitions: Executions and Invariants
Execution (fragment): sequence 0 a1 1 a2 2 …, where:
Each i is a trajectory of the automaton, and
Each (i.lstate, ai , i+1.fstate) is a discrete step
Invariant I(s) proved by base case :
induction discrete:
continuous:
Supporting TIOA software tools [Kaynar, Lynch, Mitra]
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Average Dwell Time: Invariant Approach
An SHA A has ADT if there exists N0 such that for all α
Quantification over all executions: ADT is a property of the executions of the automaton
Invariant approach: Transform the automaton A A’ so that the ADT property of A
becomes an invariant property of A’. Then use theorem proving or model checking tools to prove the
invariant(s)
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Transformation for Stability Uniform stability preserving transformation:
counter Q, for number of extra mode switches a (reset) timer t Qmin for the smallest value of Q
A A’
Theorem: A has average dwell time τa iff Q- Qmin ≤ N0 in all reachable states of A’. invariant property
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ProofIf part: we show that
t1 t2tmin
Qmin
Q(t2,t1) = Q(t2, tmin) – Q(t1,tmin)
≤ Q(t2,tmin)
= Q(t2) – Qmin(t2)
≤ N0
t1 t2tmin
Qmin
Qmin(t2) < Qmin(t1)
Q(t2,t1) = Q(t2, tmin) + Q(t1,tmin)
≤ Q(t2,tmin)
= Q(t2) – Qmin(t2)
≤ N0Only if part: Consider a state s’ = α’(t) of A’
suppose α’(t0) attains Qmin, Qmin(t) = Qmin(t0)
Q(t) – Qmin(t) ≤ N0
Q Q
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Case Study: Hysteresis Switch
Initialize
Find
no yes?
Inputs:
Under suitable conditions on (compatible with bounded .........................................................noise
and no unmodeled dynamics), can prove ADT. See CDC paper for
details [Mitra, Liberzon]
Used in switching (supervisory) control of uncertain systems
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Average Dwell Time : Optimization approach
An SHA A has ADT if there exists N0 such that for all α
An SHA A does not have ADT if for all N0 there is execution α such thatAn SHA A does not have ADT if for all N0 there is execution α such that
In general solving OPT1 is hard
• Finiteness of solution
• Completeness
# extra switches in α w.r.t. τa
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Looking at cyclic counterexample
A simple sufficient condition for violating ADT
Lemma 3: If there is a cyclic execution of A with extra switches w.r.t τa, then
A does not have ADT τa.
Q: Is this also a necessary condition ?
A: For a useful class of SHA it is. Finitely initialized SHA.
implies
is finite
Lemma 4: IF SHA A does not have ADT τa and it is finitely initialized then it
has a cyclic execution with extra switches.
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Extending to Non-initialized SHA
If there is a subset of variables Z V, such that if x.Z = y.Z then x є implies y є F(x) = F(y)
xx’ on a then there exists y’ such that yy’ on a and x’.Z = y’.Z
xx’ by traj τ then there exists y’ such that yy’ on a traj of same length and x’.Z = y’.Z
Z induces a congruence relation and partitions the state space of A into equivalence classes.
We can find a region automaton Rz(A) corresponding to A such that, any τa > 0 is an ADT for A iff it is also an ADT for Rz(A).
It is sufficient to have Rz(A) finitely initialized (and not A itself ) for the optimization approach to work.
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Case Study: Gas BurnerSHA Region automata
MILP Soultion
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Conclusions
SHA, SHIOA model, stability definitions Verification of ADT property:
Invariant approach --- general but not automatic MILP approach --- restrictive, can be fully automated
ADT preserving abstractions
Summary:
Future work:
Stability of mobile algorithms
Input-output properties (external stability)
Probabilistic HIOA [Cheung, Lynch, Segala, Vaandrager] and stability of stochastic switched systems [Chatterjee, Liberzon, FrA01.1]
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References
[Mitra, Liberzon, Lynch, “Verifying average dwell time”, 2004, http://decision.csl.uiuc.edu/~liberzon]