Case Study of an ASIL D Torque Vectored Electric Power Train
Aligned to ISO26262
Presented by David Blackburn
Introduction
Short Introduction to Zytek
•Specialist powertrain and vehicle engineering company.
•Originally developing engine management systems for the automotive industry, Zytek’s focus has
progressed to include Electric and Hybrid vehicles.
Intent of example
•To illustrate an initial approach to applying ISO26262 requirements to a Safety Critical Electric Power Train
application.
•Focus on the processes associated with;
•Hazard Identification.
•Safety Goal Definition.
•Functional Safety Concept Specification.
•Technical Safety Concept Specification.
•System Architecture.
•Design Implications.
•Strategy for evaluation of Hardware Metrics. Revision 1
Vehicle Application
Rear wheeled drive lightweight sports car.
•High Level Architecture
•x2 Independent Electric Machines and Power Electronics (PE’s).
•Single communications bus between the PE’s and the rest of the vehicle.
•Single stage gearing
•No mechanical differential.
•Directly coupled to the drive wheel with no mechanical disconnect  / master clutch.
•Performance
•Power: 300 kW (Approx. 400 hp).
•Torque: 850 Nm @ Machine Shaft (combined): 5100 Nm @ Wheels.
•Performance: 0 – 100 km/h approx. 4 seconds.
•Torque Vectoring available at all speeds.
Revision 1
Safety Targets
Vehicle Level Hazard
•Unbalanced lateral torque across the vehicle, leading to the loss of control.
Hazard Analysis
•Considering the maximum differential wheel torque error possible (5100 Nm) under different  road surface conditions yields different Fault Tolerant Time Intervals (FTTI).
•Dry surface  (high coefficient of friction)
•5100 Nm Torque Error.
•FTTI = 100 ms.
•Wet surface (medium coefficient of friction)
•5100 Nm Torque Error.
•FTTI = 60 ms.
•Icy surface (low coefficient of friction)
•5100 Nm Torque Error.
•FTTI = 40 ms.
•The Probability of Exposure to the different road surface conditions then links these Fault Tolerant Time Intervals to ASIL targets.
Revision 1
ASIL Determination
Common Factors
•Severity - Classified as S3 – Life-threatening injuries, survival uncertain / fatal.
•Controllability - Classified as C3 – Difficult to control / uncontrollable.
Influence of Exposure to Different Environmental Factors
•Dry surface  (high coefficient of friction)
•Classified as E4 – High Probability
•Hazard classified as ASIL D
•Wet surface (medium coefficient of friction)
•Classified as E3 – Medium Probability
•Hazard classified as ASIL C
•Icy surface (low coefficient of friction)
•Classified as E2 – Low Probability
•Hazard classified as ASIL B
Revision 1
reaction in the shaft

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