The Safe-SADT method for aiding designers to choose and improve dependable architectures for complex automated systems

doi:10.1016/j.ress.2006.12.020

Reliability Engineering & System Safety

Volume 93, Issue 2, February 2008, Pages 179-196

https://doi.org/10.1016/j.ress.2006.12.020 Get rights and content

Abstract

Dependability evaluation is crucial to controlling the risks associated with system failure, and for this reason, it is one of the fundamental steps in automated system design. However, the dependability evaluation methods that are currently exploited are not appropriate, given the level of complexity of such industrial systems. The ineffectiveness of the existing methods led us to develop and propose the Safe-SADT (structured analysis and design technique) method. Our method allows the explicit formalization of functional interactions, the identification of the characteristic values affecting the dependability of complex systems, the quantification of the reliability, availability, maintainability, and safety (RAMS) parameters of the system's operational architecture, and the validation of that operational architecture in terms of the dependability objectives and constraints set down in the functional requirement specifications (FRS). The results presented in this paper are limited to RAM quantification.

Introduction

Technological progress has allowed the creation of autonomous, adaptive automated systems capable of making decisions in a given context [1], [2]. These systems cannot be reduced to simple concepts of process, but should be considered as sets of means and actors working together to accomplish pre-determined functions. For this reason,studying the behavior of such a priori complex systems precisely is difficult.

This article can be loosely divided into five parts. First, we summarize the concepts of complexity as related to the modeling and analysis of automated systems, and we present the issues involved in designing complex automated systems. In order to deal with these issues, we propose the Safe-SADT (structured analysis and design technique) method, which aims both to characterize and model the operational architecture, and to quantify this architecture's reliability, availability, maintainability, and safety (RAMS) parameters. We then apply and validate the proposed method on the design of an automated hydraulic system. The last section concludes the article, presenting likely future research projects based on the Safe-SADT method.

Section snippets

System complexity

In this article, we are interested in automated systems. Generally, a system is defined as a set of discrete, inter-connected or interacting elements that work together to carry out a pre-determined mission [3]. Automated systems can be highly complex, and this complexity has multiple sources, as the list below shows [4], [5]:

•
Size complexity: Large automated systems must be highly reliable in order for their behavior to be controlled. Obviously, the number of elements in a system has a direct

Methodology for designing complex automated systems

Automated system design involves constructing three distinct classes of architecture [8]:

The functional architecture of an automated system is built according to the functional specifications (see Fig. 1, activity A1) and represents the links and interactions between the system's diverse functions. The model of this architecture is composed of the elementary functions that are found when the main functions are decomposed (see Fig. 1, activity A2).

The equipment architecture reflects the choices

The issues involved in the design of complex automated systems

The design of complex automated systems must not only take into account the parts of a system (software and equipment), but must also consider the system as a whole, which means deciding how the software will be allocated onto the equipment, including allocating tasks and controlling data processing, data access, and distributed time references. Our research was inspired by the lack of languages and tools for modeling the architecture specific to automated systems (i.e. abstract architectures

Presentation of the Safe-SADT method

The main objectives of the Safe-SADT method are to evaluate the dependability of an overall system (specifically, the RAMS parameters) during the design phase and to characterize the system's operational architecture. The Safe-SADT method is based on the SADT formalism for system analysis and design [29].

The aim of SADT is to identify and model, in a diagram of information flows, the decision-making processes and management tasks associated with complex systems. SADT consists of a graphic

System description

The Safe-SADT method is applied below to an automated hydraulic system. The main goal of this system is to convey two fluids, Fl₁ and Fl₂, to the output S at the pressure defined by the operator. For the automated hydraulic system presented in Fig. 6, the designer has two alternative configurations:

(a)
two parallel flows (Fl₁ and Fl₂), each with one pump (P₁₁ and P₃₁, respectively), three valves (V₁₁, V₁₂, V₁₃ and V₄₁, V₄₂, V₄₃, respectively) and two flow meters (Fm₁₁, Fm₁₂ and Fm₃₁, Fm₂₂,

Conclusion

Integrating new technologies when designing automated systems presents certain advantages, but also introduces new dependability constraints linked to the various types of system complexity mentioned in the first part of this article. Our research was inspired by the lack of languages and tools for modeling the abstract architectures that are specific to automated systems, which are created by combining software and equipment entities. The varied complexity of such systems makes it necessary to

Acknowledgements

Our research was supported by the European project, UGTMS (Urban Guided Transport Management Systems) (5th framework research program). The authors would like to thank Lisa Ellen Spencer Services for the correction of the English version of this paper.

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The Safe-SADT method for aiding designers to choose and improve dependable architectures for complex automated systems

Abstract

Introduction

Section snippets

System complexity

Methodology for designing complex automated systems

The issues involved in the design of complex automated systems

Presentation of the Safe-SADT method

System description

Conclusion

Acknowledgements

Automatica

Integrating safety into the design process: elements and concepts relative to the working situation

Saf Sci (Special issue “Safety in design”)

Risk assessment and management for engineers and scientists

Probabilistic reliability: an engineering approach

Toward a better integration of on shelf components

Design of intelligent distributed control systems: a dependability point of view

J Reliab Eng Syst Saf