SISTEMA program for the design of safe machine control systems

Status:

completed 11/2007

Aims:

ISO 13849-1:2007, "Safety of machinery - Safety-related parts of control systems - Part 1: General principles for design", is a generic safety standard which sets out requirements for safety-related controls for machinery. The 1997 version defines five typical control system architectures, termed Categories, independent of the technology employed (mechanical, pneumatic, hydraulic, electrical). A thorough revision of EN ISO 13849-1 became necessary in order for greater consideration to be given to new technologies (electronics, software). The emphasis of the revision lay upon determining the probability of failure. Despite simple approaches, the analysis methods and the handling of data are complex in practice. A Windows-based program Safety Integrity Software Tool for the Evaluation of Machine applications (SISTEMA)was planned as an aid to apply the standard. One purpose was to enable the probability of failure of control systems, whether planned or already implemented, to be analyzed quickly and easily. Besides greater acceptance of the new methods, structured user guidance was to support the avoidance of errors in application. The tool was intended to support machine manufacturers, control system manufacturers, users and test bodies in the design, integration and assessment of safety-related aspects.

Activities/Methods:

The requirements for the program were defined following systematic examination of the standard in its final form, and with the incorporation of users' experience with a software prototype. The various methods set out in ISO 13849-1 were modelled in the software such that users need only enter their data in manageable input dialogs with tree structure, and the result is calculated automatically. Besides robust computing functions, user-friendly functionality was also implemented, such as a results prognosis, highlighting of missing or incomplete data, a database for standard components and control systems which have already been analyzed, a summary/report, and preparation for different language versions.

An important requirement was the separation of the user interface from the database for projects, safety functions and components. Selection of a suitable database necessitated a comprehensive survey. The draft was produced in UML (Unified Modelling Language) with the aid of tools. Delphi 7 was employed for object-oriented implementation. The program was tested against actual control systems by testers at the BGIA Institute for Occupational Safety and Health. It was then approved and prepared for distribution, including on the Internet.

Results:

With the publication of Version 1.0 of SISTEMA, a comprehensive and user-friendly tool is now available for the application of EN ISO 13849. At the same time, all normative aspects are considered for the quantitative assessment of the Performance Level (PL) in accordance with the simplified methods proposed by the standard. All important properties of a project are documented in detail. Library functions simplify the exchange of data between manufacturers of components and of control systems. Existing subsystems and projects can be re-used and exchanged with ease. The user interface, which is internally separate from the database, comprises a workspace, a navigation window in tree format, a toolbar, a help window with integral wizard functionality (introductory information for new users), a context help window and a PL preview. A report function and the provision of language support complete the program. Simultaneous development of the software and production of a BGIA Report on the application of EN ISO 13849 resulted in many adaptations and interpretations, required in practical application, being incorporated into the design of SISTEMA. These include an interpolation method for intermediate DC and MTTF_d) values.

In conjunction in particular with the BGIA Report which appeared at the same time, SISTEMA represents a major aid for the application of EN ISO 13849, as a result of which the standard's acceptance is substantially promoted. SISTEMA primarily supports manufacturers in the design and safety assessment of a safety control system. It can however also be employed for consultancy purposes.

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