Rigorous work flow and data model for risk assessments of machine control systems

Modern machine control systems involve very often programmable electronics. In Europe, risk assessment of machinery, including the control systems, is obligatory by the Machinery Directive 2006/42/EC. A harmonised standard for risk assessment exists, namely EN ISO 14121-1:2007 (Safety of machinery - Risk assessment - Part 1: Principles). It defines a process model for assessing risks of machinery, but it does not define the exact work flow as to in which phases of the control system development process the risk assessments should be made. Furthermore, the structure of the input data to the risk assessment tasks as well as the structure of the results of the risk assessments need to be defined further than done in the standard to facilitate systematic risk assessment. The paper suggests using the three phase risk assessment practice defined and used by VTT in industrial risk assessment commissions. A model for risk assessment work flow and a data model for risk assessment artefacts are described in the paper. Such a rigorous model standardises the cooperation between the systems engineers and safety engineers. Also the deployment of third parties to carry out risk assessment tasks becomes well coordinated. The paper discusses the challenges, advantages and prospects of the proposed work flow and data model.