A Functional Safety Approach to Wireless Sensor Networks for Heterogeneous Applications

The extensive use of sensors represents an opportunity for industrial applications to become more precise, more comfortable for the operator and more safe. The possibility to collect data, for immediate use or offline analysis, is opening new business opportunities that were not possible until just some years ago. For this reason, it is becoming always more recurrent to have industrial applications with sensors involved; here, the use of a Wireless Sensor Network (WSN) is an attractive solution that would greatly reduce costs. In those applications that can be considered safety-relevant the use of a WSN is currently discouraged or prohibited because the wireless communication channel is not considered safe. However, recent technology and conceptual advances such as the black channel principle, and the recent evolution of functional safety standards, suggest that this can change in the future; in order to make this possible, a thorough analysis on the implication of requiring a certain safety level on the development of a system is required. Referring to the most used safety standards, the IEC 61508, IEC 62061, ISO 13849 and the ISO 25119 standards, it is possible to define an hardware and software co-design to make wireless sensor modules suited to be used in safety-critical applications. The work described in this thesis cover the technical aspects of the design and development of a safe WSN node, from hardware aspects to software and protocol issues. Compared to existing real-time protocols for WSN, this thesis propose an enhancement which decreases the worst-case communication latency, allowing for faster control loops or a greater number of sensors to be employed. Software issues are then analysed, with reference to the requirements of functional safety standards, showing the inadequacy of common WSN operating systems and proposing the use of hard-real time operating systems as a starting code base. Finally, the hardware architectures proposed in this thesis are derived applying international functional safety standards, mainly ISO 61508 and ISO 25119. Different architectural variants are then possible, depending on the characteristics of the chosen components. One of the main results of this thesis is a prototype implementation that has been designed and is being realised at CNR-IMAMOTER, and will be used to develop a proof-of-concept device which can then be used to realise a test-bed.