Wireless Communication Techniques and Methodology for Functional Safety in Industry

• DIRECTORS: Iñaki Val Beitia & Pablo Angueira Buceta
• UNIVERSITY: UPV-EHU

ABSTRACT

This doctoral thesis examines the impact of wireless propagation in industrial environments on safety communications systems. The thesis addresses a methodology for analyzing the satisfaction of the strict requirements imposed by the regulations that standardize safety applications. Confronting the challenging conditions of the wireless channel in industrial environments poses a significant challenge. Engineers and designers of safety systems must be aware of the degradation and new issues imposed by the wireless medium’s nature. The aim is to ensure that wireless communications do not cause potentially dangerous situations for system production, the people using it, or the surrounding environment.

Existing regulations for standardizing safety applications are deeply rooted in various industrial sectors. There are vast regulations and regulatory documentation on this matter, each defining and particularizing different application scenarios. Additionally, an essential aspect of such regulations is that they are constantly evolving. Most standards are based on an initial definition grounded in wired systems, which also constitute the majority of current systems. These wired systems exhibit much higher performance than wireless systems, although they lack the latter’s advantages, such as flexibility, maintenance, scalability, etc. On the other hand, the characterization of wireless systems for safety is an unexplored field. Proper characterization, following a precise methodology, could provide engineers with design guidelines for systems that meet Safety Integrity Level (SIL) limits, the primary criterion for classifying the safety performance of an application.

This doctoral thesis reviews the principal safety regulations, particularly interested in those that play a role in communication characterization. Safety characterization methodologies in these communications are identified, highlighting the evaluation of the Safety Communication Layer (SCL). This additional layer within the protocol stack consists of a set of countermeasures to limit the occurrence of hazardous events.

The work identifies message integrity as the main problem for safety validation of a Safety Instrumented Function (SIF). The main tools for preserving integrity and methods for characterizing their performance are reviewed and studied. The impact of the wireless medium on the ability of Cyclic Redundancy Check (CRC) codes to detect corrupted frames is examined. This degradation must be taken into account when defining a safety layer. In order to improve the performance of CRC-based systems, the thesis explores the possibility of using redundant schemes with a dual

objective. On the one hand, provide the system with sufficient integrity capability to meet some SIL limits. Furthermore, on the other hand, not neglect other aspects of functional safety, such as reliability and availability.

The thesis also covers the characterization of the remaining countermeasures in a generic SCL. The impact of the wireless medium on all of them is studied. This evaluation is carried out in two steps. The first, more straightforward step analyzes the performance of the SCL on a narrowband system. The second, more complex step involves using an OFDM modulation scheme, considering disturbances such as temporal dispersion leading to selective fading. The benefit of coding as a tool to address specific cases that may result in hazardous events in the SIF is also considered. This methodology would allow characterizing the safety performance of a system with these characteristics.

The thesis contributes with analytical models and empirical studies using different laboratory prototypes. Among these empirical studies is one conducted at the National Institute of Standards and Technology (NIST), where an SCL is integrated into an industrial collaborative robot prototype. These tests have identified some of the integration issues of SCLs in industrial environments, such as the relationship between availability and safety, or latency problems associated with wireless systems in applications with strict safety requirements.