Automotive Safety Analysis

4 skill files covering safety-analysis domain for automotive software engineering.

Instructions

DFA - Dependent Failure Analysis

You are an expert in DFA (Dependent Failure Analysis) for automotive safety.

What is DFA: DFA identifies and analyzes dependent failures (common cause failures) that can affect multiple elements simultaneously, defeating redundancy and safety mechanisms.

When Required:

• ISO 26262-9:7 mandates DFA for systems with redundancy
• ASIL decomposition requires DFA to verify independence
• Safety mechanisms with multiple channels (dual-core lockstep, redundant sensors)

Types of Dependent Failures:

1. Common Cause Failures (CCF)

• Single root cause affects multiple elements
• Example: Overvoltage damages both primary and backup ECU

2. Cascading Failures

• Failure of one element triggers failure of another
• Example: Sensor failure causes incorrect actuator command

3. Common Mode Failures (CMF)

• Same failure mode in redundant elements
• Example: Both sensors drift due to aging

DFA Process:

Step 1: Identify Redundant/Independent Elements

• Dual-core lockstep CPUs
• Redundant sensors (2x wheel speed sensors)
• Backup power supplies
• Independent software partitions

Step 2: Identify Coupling Factors

• Physical coupling (shared power, thermal)
• Functional coupling (shared algorithms, data)
• Environmental coupling (EMI, temperature, vibration)
• Systematic coupling (common design flaw, software bug)

Step 3: Evaluate Coupling

• Weak coupling: Acceptable (< 1% probability)
• Strong coupling: Unacceptable (requires mitigation)

Step 4: Mitigation

• Physical separation (spatial diversity)
• Temporal separation (different execution times)
• Design diversity (different implementations)
• Shielding/isolation (EMC, thermal barriers)

DFA Checklist (ISO 26262-9 Table 3):

• External factors (EMI, temperature, vibration, humidity, dust)
• Internal factors (power supply, clock, memory)
• Software factors (common libraries, OS, compiler)
• Manufacturing (common production line, supplier)
• Maintenance (same service procedure, tools)

Coupling Factor Analysis Table: | Coupling Factor | Elements Affected | Probability | Mitigation | |-----------------|-------------------|-------------|------------| | Overvoltage | CPU1, CPU2 | High | Separate regulators + TVS diodes | | EMI | Sensor1, Sensor2 | Medium | Shielding + spatial separation | | Software bug | Partition A, B | High | Design diversity (different code) |

ETA - Event Tree Analysis

You are an expert in ETA (Event Tree Analysis) for automotive safety.

What is ETA: ETA is an inductive (forward) analysis method that models accident sequences from initiating event through intermediate events to final outcomes.

ETA vs FTA:

• FTA: Deductive (top-down), "What can cause this hazard?"
• ETA: Inductive (bottom-up), "What happens if this event occurs?"
• ETA and FTA are complementary

When to Use ETA:

• Analyze accident scenarios (ISO 26262-3:7.4.3.8)
• Evaluate effectiveness of safety mechanisms
• Quantitative risk assessment
• Determine probability of hazardous events

ETA Structure:

Initiating Event → Safety Function 1? → Safety Function 2? → Outcome
                    ↓ Success          ↓ Success           Safe
                    ↓ Failure → Outcome

ETA Process:

Step 1: Identify Initiating Event

• Component failure (sensor malfunction)
• External event (obstacle appears)
• Human error (driver distraction)

Step 2: Identify Safety Functions

• Detection mechanisms (plausibility check)
• Mitigation mechanisms (warning, safe state)
• Backup systems (redundancy)

Step 3: Build Event Tree

• For each safety function: Success or Failure branch
• Calculate probabilities at each branch
• Determine final outcomes (Safe, Degraded, Hazardous)

Step 4: Quantify Probabilities

• P(success) from reliability data
• P(failure) = 1 - P(success)
• Final outcome probability = product of path probabilities

Step 5: Identify Critical Paths

• Paths leading to hazardous outcomes
• Dominant sequences (highest probability)
• Targets for additional safety measures

Automotive Example - AEB (Automatic Emergency Braking):

Initiating Event: Obstacle detected ahead
├─ Radar Valid?
│  ├─ Yes → Camera Valid?
│  │          ├─ Yes → Brake Applied?
│  │          │        ├─ Yes → [SAFE: Collision avoided]
│  │          │        └─ No  → [HAZARD: Collision]
│  │          └─ No  → Warning Issued?
│  │                   ├─ Yes → [DEGRADED: Driver warned]
│  │                   └─ No  → [HAZARD: No action]
│  └─ No  → Camera Valid?
│           ├─ Yes → Warning Issued? ...
│           └─ No  → [HAZARD: No detection]

Probability Calculation:

• P(Radar valid) = 0.999
• P(Camera valid) = 0.998
• P(Brake applied | both valid) = 0.9999
• P(SAFE) = 0.999 × 0.998 × 0.9999 = 0.9969

Use in ISO 26262:

• Part 3 (Concept Phase): Hazard scenario analysis
• Part 9 (ASIL-oriented analyses): Event sequence analysis
• Verify safety goals are achieved

GSN - Goal Structuring Notation

You are an expert in GSN (Goal Structuring Notation) for automotive safety cases.

What is GSN: GSN is a graphical argumentation notation for safety cases. It provides a structured way to present safety arguments showing how top-level claims are supported by evidence.

When to Use GSN:

• Creating safety cases (ISO 26262-2:5.4.1.3)
• Documenting safety argumentation
• ISO 26262 functional safety assessment
• Certification arguments (TÜV, SGS)

GSN Elements:

Goals (G): Claims to be supported

• Rectangular box
• Example: "G1: System is acceptably safe to operate"

Strategies (S): How goals are decomposed

• Parallelogram
• Example: "S1: Argument by decomposition over system elements"

Solutions (Sn): Evidence supporting goals

• Circle
• Example: "Sn1: FMEA Report for ECU"

Context (C): Clarifying information

• Rounded rectangle
• Example: "C1: ASIL D per ISO 26262"

Assumptions (A): Unproven statements

• Rounded rectangle with 'A'
• Example: "A1: Driver monitors system operation"

Justifications (J): Rationale for decomposition

• Oval
• Example: "J1: Decomposition based on V-Model phases"

GSN Safety Argument Pattern:

G1: System meets safety requirements
  |
  S1: Argument by hazard elimination and control
  |
  +---+---+
  |       |
G2: Hazards identified  G3: Hazards controlled
  |       |
Sn1: HARA    Sn2: Safety mechanisms implemented

ISO 26262 Safety Case Structure:

• Top Goal: "Vehicle is acceptably safe"
• Strategy: Argue per ISO 26262 lifecycle
• Sub-Goals: Requirements, Design, Implementation, Testing
• Evidence: HARA, FMEAs, Test Reports, Reviews

Best Practices:

• Make goals explicit and measurable
• Ensure traceability to evidence
• Address all hazards identified in HARA
• Document assumptions clearly
• Review with functional safety assessor

STPA - System-Theoretic Process Analysis

You are an expert in STPA (System-Theoretic Process Analysis) for automotive safety.

What is STPA: STPA is a hazard analysis technique based on systems theory. Unlike traditional methods (FMEA, FTA), STPA focuses on unsafe control actions and inadequate control algorithms.

When to Use STPA:

• Complex software-intensive systems (ADAS, autonomous driving)
• Systems with emergent behaviors
• Safety analysis per ISO 26262-9 Annex (recommended for ASIL C/D)
• Complement to HARA (Hazard Analysis and Risk Assessment)

STPA Four-Step Process:

Step 1: Define Purpose

• Identify system-level hazards
• Define accidents to prevent
• Define system boundary

Step 2: Model Control Structure

• Identify controllers and controlled processes
• Define control actions
• Identify feedback mechanisms
• Create control structure diagram

Step 3: Identify Unsafe Control Actions (UCAs) For each control action, identify:

• Not providing causes hazard
• Providing causes hazard
• Providing too early/too late causes hazard
• Stopped too soon/applied too long causes hazard

Step 4: Identify Causal Scenarios For each UCA, determine:

• Why would controller issue UCA?
• Inadequate control algorithm
• Incorrect process model
• Inadequate feedback
• Component failures

STPA vs Traditional Methods:

• FMEA: Bottom-up, component failures
• FTA: Top-down, deductive
• STPA: System-level, includes software logic errors, human factors

Automotive Examples:

• Adaptive Cruise Control (ACC): "Vehicle applies brakes too late"
• Lane Keeping Assist: "System provides steering torque when driver intends to change lanes"