Ptolemy II:系统设计与建模指南

Ptolemy

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Acknowledgements

• The Naval Research Labs (NRL), Cooperative Agreement Number N00173-12-1-G015:

Software Producibility for Systems of Systems, as part of the Ofﬁce of Secretary of De-

fense (OSD) Software-Intensive Systems Producibility Initiative.

• The Air Force Research Lab (AFRL), Contract FA8750-11-C-0023: Extensible Mod-

eling and Analysis Framework.

• The Air Force Ofﬁce of Scientiﬁc Research (AFOSR) under the Multi-University Re-

search Initiative (MURI) grant #FA9550-06-0312: High-Conﬁdence Design of Adap-

tive, Distributed Embedded Control Systems (HCDDES).

Additional funding was provided by the Center for Hybrid and Embedded Software Sys-

tems (CHESS) at UC Berkeley, which over several years has received support from the

following companies:

• Agilent

• Bosch

• Lockheed-Martin

• National Instruments

• Thales

• Toyota

Support for work on modeling of heterogeneous systems was also provided by the iCyPhy

(industrial cyber-physical systems) research consortium, sponsored by IBM and United

Technologies.

Key support for work described in Chapters 10 and 16 was provided by the Focus Cen-

ter Research Program (FCRP), a Semiconductor Research Corporation (SRC) program

funded by MARCO and DARPA, under the following projects:

• The Multiscale Systems Center (MuSyC).

• The TerraSwarm Research Center, one of six centers supported by the STARnet phase

of the FCRP.

The views and conclusions contained in this text are those of the authors and should not

be interpreted as representing the ofﬁcial policies, either expressed or implied, of any of

the sponsors. The U.S. Government is authorized to reproduce and distribute reprints for

Government purposes notwithstanding any copyright notation herein.

xvi Ptolemaeus, System Design

1. HETEROGENEOUS MODELING

Many of today’s engineered systems combine heterogeneous and often complex subsys-

tems. A modern car, for example, may combine a complex engine, electronic control units

(ECUs), traction control systems, body electronics (for controlling windows and door

locks), entertainment systems, climate control and ventilation, and a variety of safety

subsystems (such as airbags). Each subsystem may be realized with a combination of

software, electronics, and mechanical parts. Engineering such complex systems is quite

challenging, in part because even the smallest subsystems span multiple engineering dis-

ciplines.

These complex systems also challenge the design tools that engineers use to specify, de-

sign, simulate, and analyze systems. It is no longer sufﬁcient to sketch a mechanical struc-

ture and write down a few equations describing the interactions of the mechanical parts.

Neither is it sufﬁcient to rely entirely on software tools for 3D modeling of mechani-

cal parts or tools for model-based design of software systems. The complex interplay

across domains (mechanics, software, electronics, communication networks, chemistry,

ﬂuid dynamics, and human factors) reduce the usefulness of tools that address only a

single domain.

The focus of this book is on cyber-physical systems (CPS) (Lee, 2008a, 2010a; Lee and

Seshia, 2011), which combine computing and networking with physical dynamics. Cyber-

physical systems require model combinations that integrate the continuous dynamics of

physical processes (often described using differential equations) with models of software.

Diverse models are most useful in applications where timed interactions between compo-

nents are combined with conventional algorithmic computations.

They can also be used

in traditional software systems that have concurrent

interactions between algorithmic

components.

An algorithm is a ﬁnite description of a sequence of steps to be taken to solve a problem. Physical

processes are rarely structured as a sequence of steps; rather, they are structured as continuous interactions

between concurrent components.

Concurrency, from the Latin verb concurrere meaning “run together,” is often taken in computer science

to mean the arbitrary interleaving of two or more sequences of steps. However, this is a rather specialized

interpretation of a basic concept. In this book, we take concurrency to mean simultaneous operation, with no

implication of either interleaving nor sequences of steps. In particular, two continuous processes can operate

concurrently without being directly representable as sequences of steps. Consider, for example, a resistive

heating element immersed in a vat of water. Increasing the current through the heating element will cause

the temperature of the water to rise. The electrical ﬂow is one continuous process, as is the temperature of

the water, and these processes are interacting. But neither process is reasonably representable as a sequence

of steps, nor is the overall process an interleaving of such steps.

Ptolemaeus, System Design 3

Sidebar: About the Term “Cyber-Physical Systems”

The term “cyber-physical systems” emerged around 2006, when it was coined by He-

len Gill at the National Science Foundation in the United States. We are all familiar

with the term “cyberspace,” attributed William Gibson, who used the term in the novel

Neuromancer to refer to the medium of computer networks used for communication

between humans. We may be tempted to associate the term cyberspace with CPS, but

the roots of the term CPS are older and deeper. It would be more accurate to view

the terms “cyberspace” and “cyber-physical systems” as stemming from the same root,

“cybernetics,” rather than viewing one as being derived from the other.

The term “cybernetics” was coined by Norbert Wiener (Wiener, 1948), an American

mathematician who had a huge impact on the development of control systems theory.

During World War II, Wiener pioneered technology for the automatic aiming and ﬁring

of anti-aircraft guns. Although the mechanisms he used did not involve digital comput-

ers, the principles involved are similar to those used today in a huge variety of computer-

based feedback control systems. Wiener derived the term from the Greek κυβερνητ ης

(kybernetes), meaning helmsman, governor, pilot, or rudder. The metaphor is apt for

control systems.

Wiener described his vision of cybernetics as the conjunction of control and commu-

nication. His notion of control was deeply rooted in closed-loop feedback, where the

control logic is driven by measurements of physical processes, and in turn drives the

physical processes. Even though Wiener did not use digital computers, the control logic

is effectively a computation, and therefore cybernetics is the conjunction of physical

processes, computation, and communication.

Wiener could not have anticipated the powerful effects of digital computation and net-

works. The fact that the term “cyber-physical systems” may be ambiguously interpreted

as the conjunction of cyberspace with physical processes, therefore, helps to underscore

the enormous impact that CPS will have. CPS leverages a phenomenal information

technology that far outstrips even the wildest dreams of Wiener’s era.

4 Ptolemaeus, System Design

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