Unifying Discrete and Continuous Simulation with Discrete Events:

DEVS as the Next Modeling Standard

Bernard P. Zeigler

Arizona Center for Integrative Modeling and SimulationUniversity of Arizona

Tucson

www.acims.arizona.edu

Outline

• Why is a formal framework for M&S needed?– for independent V&V– for simulation component reuse– for standardization at the right level

• Systems-based framework for M&S– Discrete Event System Specification (DEVS)– Representing Continuous Systems in DEVS– Speed up advantages

Simulation-based V&V as an afterthought

development of the system

simulation tests of the system

Government

Contractor

Government’s Desire for Independent Simulation-based V&V of new Systems

simulation environment/ test plans/events for V&V

Another Contractor

specification of the system

development of the system

Government

Contractor

simulation tests of the system

How is simulation software different from other software?

• It represents the behavior of dynamic systems whose states are functionally dependent on time

• Properly controlling the flow of time is critical• Simulation software may combine:

– continuous (time-driven) and discrete (event-driven) processes

– actual operating hardware and software representations

– wall clock and {faster/slower} than real time advance

independent specification of the desired deliverable – how the dynamic system behaves over time

methods needed to compare the behaviors of the system representation and the independent specification

A Formal Framework for Modeling and Simulation should provide

DEVS

Experimental

Frames

• DEVS = Discrete Event System Specification

• Provides formal M&S framework: specification,simulation

• Derived from Mathematical dynamical system theory

• Supports hierarchical, modular composition

• Object oriented implementation

• Supports discrete and continuous paradigms

• Exploits efficient parallel and distributed simulation techniques

DEVS Background

DEVS Hierarchical Modular Model Framework

Atomic: lowest level model, contains structural dynamics -- model level modularity

Atomic

AtomicAtomic

Atomic

+ coupling

Atomic

Atomic

Atomic

Coupled: composed of one or more atomic and/or coupled models

hierarchical construction

DEVS Examples

• Lockheed Martin’s Managed M&S

• Univ. New Mexico’s Virtual Lab

Managed Modeling in Lockheed’s “System of Systems” M&S Environment

• DEVS (Discrete Event Modeling Formalism)

– Separates Model and Simulators

– Defines Couple Models and Atomic Models

– Modularized via Ports and Defined Events

• SES (System Entity Structure)

– Provides a well defined structure for model reuse

– Maintains: kind-of, part-of, multiplicity relationships

– Supports constraints on model compatibility

• Architecture based on SES/DEVS supports component model reuse

during last decade

Component Model Reuse MatrixProject

Model

Critical Mobile Target

Global Positioning System III

Arsenal Ship

Coast Guard

Deep Water

Space Operations Vehicle

Common Aero

Vehicle

Joint Composit

e Tracking Network

Integrated System

Center

Space Based Laser

Space Based

Discrimination

Missile Defense

(Theater / National)

Radar Model x x x x x x x

IR Sensor Model x x x x x x xMissile Model x x x x xLaser Model x x x xComm.

Model x x x x x xCommand Control

Modelx x x

Earth & Terrain Model

x x x x xWeather Model x x

Waypoint &

Heading Nav

Model

x x x x x x x

Orbital Propagate Model

x x x x x x xBallistic Trajectory Model

x x x x x

University of New Mexico Virtual Lab for Autonomous Agents

V-Lab-a virtual laboratory for autonomous agents-SLA-based learning controllers El-Osery, A.I.; Burge, J.; Jamshidi, M.; Saba, A.; Fathi, M.; Akbarzadeh-T, M.-R.; Systems, Man and Cybernetics, Part B, IEEE Transactions on , Volume: 32 Issue: 6 , Dec. 2002 Page(s): 791 -803

Physics Terrain Dynamic

SimEnv

Control Agents SimMan

Computer Network

Middleware (HLA,CORBA,JMS)

DEVS Simulator

IDEVS SimEnv

V-Lab developed on top of DEVSJAVA includes a simulation environment for robotic agents with physics, terrain and dynamics. It extends DEVS to provide a layer for specifying intelligent automation and soft computing algorithms (IDEVS).

"Priorities for M&S Standards”Monday, 31 March 03

Standardization should be aimed at the modeling/specification level…

"Priorities for M&S Standards”

Monday, 31 March 03

Partial

nD

(n-1)D

X>0

X<0

D

ta(nD) = |D/x|

nD

D

ta(q) = ((n+1)D-q)/x

e

X>0

X<0

q

ta(q) = |q-nD/x|

(n+1)D

Mapping Differential Equation Models into DEVS Integrator Models

DEVSinstantaneous

function

DEVS Integrator

d s1/dt s1f1x

d s2/dt s2f2

d sn/dt snfn

sx

sx

sx

...

d s1 /dt s1f1x

d s2 /dt s2f2

d sn /dt snfn

sx

sx

sx

...

DEVSSDEVS

DEVS

F

F

F

)(tf

t

quantum

Number of crossings = Activity/quantum

Activity – a characteristic of continuous models

dttfd

q)(

Activity = |f(t1) – f(t0)|

DEVS Efficiency Advantage where Activity is Heterogeneous in Time and Space

Time Period

T

time stepsize

# time steps

=T/

tt

activityA

quantumq

# crossings=A/q

Potential Speed Up=

#time stops /# crossings

X

numberof

cells

1 D Gas Dynamics Example

Heterogeneous activity in

time and space

More: http://www.ece.arizona.edu/~salila/movies

Activity as unifying continuous and discrete paradigms

Heterogeneous activity in

time and space

Quantization allows DEVS to naturally focus computing resources on high activity regions

DEVS represents all decision making and continuous dynamic components in the

scene

Summary• The theory of modeling and simulation has provides a framework to represent

classes of continuous and discrete systems and to develop object-oriented simulation frameworks to interoperate such models.

• There is an urgent need to come up with new principles to unify discrete and continuous modeling paradigms, to encode them into modeling standards and to develop simulation standards for effectively and efficiently executing these models.

• This talk presented a basis for discrete event abstractions by formalizing the concept of activity which relates to the characterization and heterogeneous distribution of events in space and time.

• This formulation offers a new way to unify the computational representation of both continuous and discrete phenomena and simulate them with the greater efficiency and flexibility afforded by object-oriented discrete event environments. .

• On this basis, the DEVS (Discrete Event System Specification) formalism is the right vehicle for standardization at the modeling and simulation levels, augmenting existing interoperability standards such as HLA.