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Adaptive Cruise Control Ilana Davidi Margaret Stringfellow Herring Paul Wheeler Agenda Hazard Analysis Safety Constraints Partial STPA Completeness Criteria Requirements Changes High-Level Design Intent Specifications Design Limitations Hazard Analysis

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adaptive cruise control

Adaptive Cruise Control

Ilana Davidi

Margaret Stringfellow Herring

Paul Wheeler

agenda
Agenda
  • Hazard Analysis
  • Safety Constraints
  • Partial STPA
  • Completeness Criteria
  • Requirements Changes
  • High-Level Design
  • Intent Specifications
  • Design Limitations
hazard analysis
Hazard Analysis
  • Constructive way to learn ACC system
  • Tens of hazards stemmed from three
    • Rear end car in front
    • Hit by car in back
    • Lose vehicle control
safety constraints
Safety Constraints
  • Matched hazards with safety constraints
  • Natural outfalling of hazards to safety
  • Constraints kept simple but precise
  • Leads to high-level design requirements
completeness criteria example
Completeness Criteria Example
  • The system and software must start in a safe state.
    • Software initial state is “ACC off.”
    • No direct transitions to hazardous states from “ACC off”
    • Cannot transition out of “ACC off” state unless
      • The ignition is turned to the “on” position.
      • The driver has subsequently pushed the “ACC on” button.
      • The ACC system passes a self-diagnostic test for system faults
      • Brakes are not engaged
      • Speed is greater than 45 mph
    • ACC then transitions to “ACC standby” state.
      • There are further conditions to transition from “ACC standby” to “ACC Active”
requirement changes examples
Requirement Changes Examples
  • Minimum speed will be 45 mph instead of 25 mph
  • Alarm will sound during shutdown
    • Not only in response to driver disengagement from the steering wheel
  • Set speed is not retained in memory after coast button is pushed
    • Current speed is used as set speed
partial stpa
Partial STPA
  • Used control loops to discover states
high level design
High-Level Design

Distance SP

Speed SP

Mode

Brake SW 1

Actuate

Acceleration

Actuate

Brakes

Brake Control

Module

Engine

Power

Driver

Accelerator

Brake

Pedal

Acceleration

Actuate

Increasing

Speed

Of Car

Closing Speed

Distance

Decreasing

Speed

Of Car

Acceleration

Distance to

Car In Back

Brake

Lights

Warning

Decreasing

Speed

Of Car

Distance to

Car In Front

Radar

ACC State

Target Speed

Brake switch 1&2

Cruise switch Req

Target Speed

Engine Control

Module

CAN

ACC

Break Switch 2

ACC State

Brake Request

Target Speed

Input to Display

ACC Input to Display

Instrument

Cluster

Vehicle

Speed

Brake Actuator Command

ACC State

intent specification 1 assumptions
Intent Specification 1: Assumptions
  • A licensed driver is operating a car with no malfunctions or problems.
  • The road is smooth and unobstructed.
  • The road is continuous and does not suddenly terminate.
  • The ACC system will interface and communicate with five parts of the car
    • Braking system
    • Engine
    • Accelerator
  • Ignition
  • Steering Wheel
design limitations
Design Limitations
  • Human behavior
    • Sudden lane changes
    • Human as cruise control monitor
  • Auto-off on steering wheel
    • Clamp
  • No system redundancy for radar
lessons learned
Lessons Learned
  • Documenting assumptions & decision reasoning
    • Generates single mental model across different people
    • Prevents loss of information over time
  • Safe systems can be achieved.
    • Rigorous approach to requirements generation
    • SpecTRM links hazards, constraints, and assumptions in one document
      • Provides visibility and traceability
  • Paul can consult on SpecTRM software
    • For a price.