Strategy on Safety Function Implementation: Case Study Involving Risk Assessment and Functional Safety Analysis for a Power Assist System

Power assist systems are expected to bring many benefits in various fields, and some of them have already been introduced in the welfare and manufacturing industries. As power assist systems demand manual contact with a human operator, it is important to prevent hazards that originate from system faults. The objectives of this paper are to introduce a strategy on safety function implementation by means of a case study for a power assist system and to propose an approach for safety function design. This paper describes details of the strategy for Skill-Assist — the power assist system adopted as the experimental platform. First, the safety integrity level (SIL) required for Skill-Assist was determined, following which top-down and bottom-up risk assessments were conducted. A safety-related system (SRS) with a fail-safe fault detection device and dual-channel voting architecture was then constructed based on the risk assessment result. A functional safety analysis was performed for the SRS and we found that the SRS satisfied the required SIL. An experiment on the functionality of the SRS was performed and we verified the safety function for Skill-Assist set by the proposed strategy.