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System Structure Tree

Build hierarchical product breakdowns for structured FMEA analysis (AIAG/VDA requirement).

What is a Structure Tree?​

A Structure Tree breaks down your product into hierarchical levels:

System (Level 1)
├── Subsystem A (Level 2)
│   ├── Component A1 (Level 3)
│   │   ├── Part A1a (Level 4)
│   │   └── Part A1b (Level 4)
│   └── Component A2 (Level 3)
└── Subsystem B (Level 2)
    └── Component B1 (Level 3)

Why Use Structure Trees?​

AIAG/VDA FMEA Handbook requires structured analysis for:

  • ✅ Systematic top-down analysis (System → Component)
  • ✅ Complete coverage (no missed elements)
  • ✅ Function definition per element
  • ✅ Requirements traceability
  • ✅ Failure Chain linkage (Effect → Mode → Cause)

Creating a Structure Tree​

Step 1: Start with System Level​

Click "Structure Tree" tab in your FMEA analysis:

  1. Click "Add Element"

  2. Fill in:

    Element Name: Electric Vehicle Powertrain
    Element Type: System
    Level: 1

  3. Add Functions (what does it do?):

    • Convert electrical energy to motion
    • Regenerate energy during braking
    • Manage thermal load

  4. Link to Requirement (optional):

    • Select requirement from dropdown
    • Creates traceability link

Step 2: Add Subsystems​

Click the System element and "Add Child":

Element Name: Battery Pack
Element Type: Subsystem
Level: 2
Parent: Electric Vehicle Powertrain

Functions:
- Store electrical energy (85 kWh)
- Provide 400V DC to inverter
- Support fast charging (150kW)

Linked Requirement: REQ-PWR-001 "System shall provide 300km range"

Step 3: Add Components​

Continue breaking down:

Element Name: Battery Management System (BMS)
Element Type: Component
Level: 3
Parent: Battery Pack

Functions:
- Monitor cell voltages (96 cells)
- Balance charge across cells
- Protect from over-current
- Report State of Charge to vehicle

Linked Requirement: REQ-PWR-005 "BMS shall prevent overcharging"

Step 4: Add Parts (if needed)​

For detailed analysis:

Element Name: Temperature Sensor Array
Element Type: Part
Level: 4
Parent: Battery Management System

Functions:
- Measure temperature at 24 locations
- Report to BMS every 100ms
- Trigger thermal protection

Defining Functions​

Good function definitions are clear and measurable:

✅ Good: "Convert 400V DC to 3-phase AC (0-14,000 RPM)" ✅ Good: "Monitor battery temperature (±2°C accuracy)" ✅ Good: "Provide emergency shutdown within 50ms"

❌ Avoid: "Work properly" ❌ Avoid: "Do its job" ❌ Avoid: "Function as designed"

Linking to Failure Modes​

Once your Structure Tree is built, link failure modes to specific elements:

  1. Create Failure Mode
  2. Select Structure Element: "Battery Management System"
  3. Analysis now knows which component this failure affects

Example:​

Structure Element: Temperature Sensor Array
Failure Mode: Sensor provides incorrect reading
Effects: BMS does not detect overheating
→ Links to parent BMS
→ Links to parent Battery Pack
→ Traces to System-level failure

Failure Chains​

Connect failures across hierarchy levels:

Example Chain:​

Level 4 (Part): Temperature sensor fails
     ↓ (causes)
Level 3 (Component): BMS does not detect overheat
     ↓ (causes)
Level 2 (Subsystem): Battery pack thermal runaway
     ↓ (causes)
Level 1 (System): Vehicle fire

In NirmIQ:

  1. Create failure mode at Part level
  2. Set "Linked Mode" to BMS failure
  3. Set BMS failure's "Linked Mode" to Battery failure
  4. Creates traceable chain

Best Practices​

Start High-Level​

Build System and Subsystem levels first, then drill down. Don't try to define every part upfront.

Function Per Level​

Each level should have clear functions:

  • System: Overall product functions
  • Subsystem: Major capabilities
  • Component: Specific tasks
  • Part: Detailed operations

Not every element needs a requirement link - focus on critical ones.

Use Consistent Naming​

✅ Good: "Brake System → Hydraulic Unit → Master Cylinder → Piston Seal"
❌ Avoid: "Brakes → Hydraulic → MC → seal"

Example: Medical Device​

System: Insulin Pump
├── Subsystem: Drug Delivery
│   ├── Component: Syringe Driver
│   │   ├── Part: Stepper Motor
│   │   └── Part: Lead Screw
│   └── Component: Infusion Set
│       ├── Part: Cannula
│       └── Part: Tubing
├── Subsystem: Control System
│   ├── Component: Microcontroller
│   ├── Component: User Interface
│   └── Component: Alarm System
└── Subsystem: Power Supply
    ├── Component: Battery
    └── Component: Charging Circuit

Each element has:

  • Clear functions (e.g., "Deliver precise insulin dose ±5%")
  • Linked requirement (e.g., REQ-SAFE-003)
  • Associated failure modes

Viewing the Tree​

Tree View: Hierarchical display with expand/collapse Table View: Flat list with level indicators Search: Find elements by name or function

Common Questions​

Q: How many levels should I use? A: Typically 2-4 levels. Use more only if needed for detail.

Q: Can I reuse Structure Trees? A: Each FMEA has its own tree, but you can copy common structures between analyses.

Q: What if my product doesn't fit a hierarchy? A: Use functional breakdown instead of physical. Group by capability rather than location.

Q: Can I link one element to multiple requirements? A: Currently one requirement per element. Use the most critical one, or create multiple elements.

Next Steps​