Towards a Fault Tolerant AHS Design Part I: Extended Architecture

54 Pages·2003·1.84 MB·English

CALIFORNIAPATHPROGRAM
INSTITUTEOFTRANSPORTATION STUDIES
UNIVERSITY OFCALIFORNIA,BERKELEY
Towards a Fault Tolerant AHS Design
Part I: Extended Architecture
John Lygeros
Datta N. Godbole
Mireille Broucke
California PATH Research Report
UCB-ITS-PRR-96-14
This work was performed as part ofthe California PATH Program ofthe
University ofCalifornia, in cooperation with the State ofCaliforniaBusiness,
Transportation, and Housing Agency, Department ofTransportation; and the
United States Department ofTransportation, Federal Highway Administration.
The contents ofthis report reflect the views ofthe authors who are responsible
for the facts and the accuracy ofthe data presented herein. The contents do not
necessarily reflect the official views or policies ofthe State ofCalifornia. This
report does not constitute a standard, specification, orregulation.
June 1996
ISSN 1055-1425

Towards a Fault Tolerant ARS Design*
Part I: Extended Architecture
John Lygeros, Datta N. Godbole, Mireille Broucke
Department of Electrical Engineering and Computer Sciences
University of California, Berkeley
Berkeley, CA 94720
lygeros, godbole, [email protected]
Abstract
We propose a hierarchical control architecture for dealing with faults and adverse environ
mental conditions on an Automated Highway System (AHS). Our design builds on a previously
designed control architecture that works under normal conditions of operation. The faults that
are considered in our design are classified according to the capabilities remaining on the vehicle or
roadside after the fault has occurred. Information about these capabilities is used by supervisors
in each of the layers to select appropriate control strategies. We outline the extended control
strategies that are needed by these supervisors in each layer ofthe hierarchy and, in certain cases,
give examples oftheir detailed operation. A companion paper develops and verifies protocols for
extended coordination strategies and maneuvers.
Keywords
Automated Highway System Design, Fault Tolerant Control, Safety
•Research supported by the California PATH program, InstituteofTransportation Studies, University ofCalifornia,
Berkeley, under MOU-135

Executive Summary
The goal of the MOD 135 project is to extend the Automated Highway System (AHS) hierarchical
control architecture of PATH to work under abnormal conditions such as adverse environment and
system failures. Most ofthe research work in the AVeS area performed at PATH so far assumes that
the system is operating in the normal mode. The definition of "normal" may vary from case to case,
but, in general, it means benignenvironmental conditions and faultless operation ofall the hardware,
both on the vehicles and on the roadside. Some studies to deal with "abnormal" conditions have
been made (for example [Rao and Varaiya1994, Tomizuka et al.1994, Hitchcock1994]), but they are
mostly concerned with specific faults rather than a general framework. Our goal is to propose an
AHS design that will perform well under almost any condition. The only abnormal conditions of
operation that we do not consider are faults in the design (e.g., a deadlock in the communication
protocols) and in the implementation of the software, since we assume the design will be verified
before being implemented.
In this report, we propose a general framework for fault handling and recovery. The study
results in a fault tolerant (degraded mode) control architecture for the AHS. The new architecture
is a qualitative as well as a quantitative extension of the normal mode architecture. Qualitatively,
the degraded mode architecture maintains the hierarchical structure introduced in [Varaiya1993] but
it increases the autonomy of the system. This is achieved by adding to the design the capability to
detect faults, determine their impact on the system capabilities, decide on a new control strategy
and execute it. Moreover, the extended architecture adds new control laws and maneuvers in each
level of the hierarchy, thus quantitatively extending the normal mode architecture.
During the normal mode ofoperation, the only informationthat the layers ofthe hierarchy
need is sensor data about the current state ofthe system (including the control messages passed from
one layer to the other). The degraded mode controllers however also need information about the
current capability of the system (in addition to the sensor data) to select the best possible action.
This additional information also needs to be presented in a form compatible with the abstraction
of the controller at each layer of the hierarchy. We propose to extend the information flow by the
addition oftwo more hierarchical structures. The capability structure encodes the discrete changes in
the system capability due to faults in the vehicle and roadside hardware. The performance structure
encodes the gradual degradation in the system performance due to adverse environmental conditions
and gradual wear of the vehicle components. These additional system monitors receive their inputs
from fault detection and sensor fusion modules. We assume that the sensor structure and the fault
detection module have already been designed for the normal mode (see for example [Alag et al.1994,
Garg and Hedrick1993]).
An important aspect of the degraded mode design is the system response strategies used
to recover from the abnormal condition with safety and minimum throughput reduction. A compre
hensive list of faults, for both the vehicle and the infrastructure is presented in the Appendix. The
capability and performance monitors are designed to induce a classification of faults. The classes
reflect the capabilities remaining in the system in the presence ofa combination offaults and adverse
conditions. New coordinated control strategies are designed for each class of failures. All strategies
are implemented by a two stage process: a strategic planning stage, where a sequence of actions is
planned to deal with the given degraded conditions and an execution stage where the chosen strat
egy is executed using specialized maneuvers. In case of failures on-board the vehicle, the degraded
mode strategy is used to either stop the vehicle or take it to the nearest exit in cooperation with
the neighboring vehicles. Infrastructure based link layer strategies are designed to divert the traffic
around an incident using specialized queue management maneuvers. The strategies attempt to lo
calize the impact ofthe failure on the throughput while maintaining the safety ofautomated vehicle
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operation. New maneuvers and strategies (sequences of maneuvers) are outlined for the extended
coordination and regulation layer. A companion report [Godbole et al.1995a] presents a complete
design and verification of coordination protocols for the extended architecture.
In summary, we have proposed a framework for an AHS design that is capable ofoperating
in the presence of faults and other factors that induce performance degradation. Our framework is
hierarchical and buildsonthecontrol architectureof[Varaiya1993]. Thedesignprovidesa highdegree
ofautonomy by extending the information structure to include data about the system capability and
the control structure to make a distinction between strategic planning and execution. Once the
design of the control laws outlined in this report has been completed, the extended architecture can
be implemented, in conjunction with a fault diagnosis module, as a highly autonomous fault tolerant
control system for an AHS.
III

Contents
1 Introduction 1
1.1 Overview of Normal Mode Architecture 1
1.2 Notation . 3
2 Outline of Proposed Solution 4
2.1 Design Goals . 4
2.2 General Features . . . . . . . 4
2.2.1 Hierarchical Structure 4
2.2.2 Information Flow. 4
2.2.3 Control Structure. 5
2.3 The Design Process. 6
3 Modeling Capability 7
3.1 Capability Structure . . . . . . . . . 7
3.1.1 Physical Layer Predicates .. 7
3.1.2 Regulation Layer Predicates. 7
3.1.3 Regulation Layer Supervisor Capability Predicates 8
3.1.4 Coordination Layer Supervisor Predicates 9
3.1.5 Link Layer Supervisor Predicates 10
3.2 Performance Structure . 11
3.2.1 Robustness Analysis Framework 12
3.2.2 Robustness Enhancement 13
3.2.3 Degraded Mode Initiation 13
3.2.4 Examples . 14
4 Fault Classification 15
5 Control Design 17
5.1 Link Layer . 18
5.1.1 Overview . 18
5.1.2 Link Layer Supervisor 20
5.1.3 Link Layer Regulator 21
5.2 Coordination Layer. 21
5.2.1 Overview . 21
5.2.2 Strategies. 21
5.2.3 Maneuvers 22
5.3 Regulation Layer . 22
5.3.1 Overview . 22
5.3.2 Supervisor. 22
5.3.3 Control Laws 23
5.4 Physical Layer ... 23
5.4.1 Normal Mode Requirements. 23
5.4.2 Degraded Mode Requirements 24
6 Discussion & Further Issues 25
6.1 Design Optimality 25
6.2 Verification Issues ..... 26
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