Risk Assessment in Functional Safety

A crucial part of ensuring fault tolerance is properly assessing the risks involved with any safety-critical system. This involves analyzing all possible failure scenarios and their consequences to determine their severity. Risk assessment allows engineers to prioritize the mitigation of high-severity risks over lower severity ones. Some key aspects of risk assessment in fault tolerance include:

Identifying all potential failure modes of each component through methods like Failure Mode and Effects Analysis (FMEA). This helps uncover even rare or unexpected failure scenarios.

Classifying the severity of each failure's consequences based on factors like potential for injury, environmental damage, or financial loss. Automotive safety standards like ISO 26262 define severity levels from S0 (no injury) to S3 (life-threatening injuries).

Estimating the likelihood or frequency of each failure occurring based on historical failure data and predicted usage profiles. Combining severity and likelihood gives the overall risk level - high/medium/low.

Documenting the risk assessment process and its findings. This risk information later guides the selection of appropriate safety measures.

Addressing Assumptions and Functional Safety

During system design, engineers must carefully evaluate assumptions and architectures from a fault tolerance viewpoint. Overly optimistic assumptions can undermine the effectiveness of safety measures if not properly scrutinized and tested. Some areas that require attention include:

Assumptions about external conditions - Will the system always operate as expected under normal and foreseeable abnormal conditions? Robustness to uncertain environments is important.

Architectural safety - Are critical Functional Safety distributed redundantly enough to prevent common-cause failures? How does failure of one component impact others? Architecture impacts diversity and independence of safety measures.

Independence of safety functions - Do safety mechanisms have sufficient independence from each other and from the system under control? Shared resources can couple failures.

Validation of safety assumptions - Are assumptions about failure rates, environmental conditions etc. valid based on testing and historical data? Overly optimistic values undermine safety analysis.

Defining Safety Requirements

With the risks understood, engineers can define tailored safety requirements specifications for the system. These derive from standards like IEC 61508/61511 and describe:

Safety integrity levels (SIL) required for each safety function to adequately mitigate its risk. SIL 1-4 define stringency of techniques.

Fault tolerance requirements specifying the behavior needed - e.g. "brakes shall engage within 500ms of emergency stop".

Design constraints like architectural/ hardware/ software safety measures needed to achieve each SIL.

Validation requirements like type and severity of testing to verify requirements before release.

Traceability - How each safety requirement addresses an identified hazard/risk from the risk assessment.

Failure rates, fault tolerance requirements etc. based on the SIL and risk tolerability criteria.

Documenting requirements enables evaluating safety measures during implementation and compliance verification before product release.

Implementing Safety Mechanisms

With requirements defined, engineers can select and apply appropriate safety mechanisms during system/software design and manufacturing. Some techniques include:

Using dissimilar/diverse redundant sensors, actuators, processors and communications for critical functions to prevent common-cause failures. Diversity increases as SIL rises.

Adding self/continuous monitoring with voting to detect internal faults and initiate fail-safe response. Processor watchdog timers are commonly used.

Segregating safety functions from other processing on separate ECUs/circuits with separate power supplies to maintain independence.

Using fault-tolerant algorithms, error detection/correction codes that can withstand specified number of faults.

Safe states or safe life-cycles that make the system benign on detected failures or during initialization.

Limiting complexity through modularization and simplicity for high integrity portions as per "as low as reasonably practical" (ALARP) principle.

Thorough verification and validation testing including fault injection tests to validate effectiveness of safety mechanisms.

Implementing rigorous configuration management to control any design/manufacturing changes for already validated safety mechanisms. Fault tolerance requires continuous assurance throughout the system lifecycle.

With careful application of the above practices, engineers can develop complex socio-technical systems like medical devices, industrial plants or autonomous vehicles, with confidence in their fault tolerance. A thorough, well-documented risk-based approach helps select appropriate safety techniques based on each application’s requirements. Strict validation testing verifies the effectiveness of implemented safety mechanisms. Ultimately, fault tolerance engineering aims to safeguard human life by enabling trustworthy operation of safety-critical systems even in the presence of failures or faults.

Get more insights on Functional Safety

About Author:

Money Singh is a seasoned content writer with over four years of experience in the market research sector. Her expertise spans various industries, including food and beverages, biotechnology, chemical and materials, defense and aerospace, consumer goods, etc. (https://www.linkedin.com/in/money-singh-590844163)