Key Features

One of the fundamental requirements Protean imposed on its in-wheel motor design from the outset was to completely remove any mechanical gearing from the motor, along with all the required bearings, lubricants and seals for such a device. This was done for several reasons:

    


Reliability/Safety

  • The reliability of the motor is fundamental to its success. There is no reliability cost to making a higher torque/lower speed motor, but there is a cost associated with the addition of gearbox components, especially considering that four separate gearboxes will be required for many vehicles.
  • A gearbox would be a single point of failure with the potential to lock a single wheel and create huge yaw movements on the vehicle with the associated safety/control issues. There is no vehicle level mitigation for such a failure mode without introducing a clutch in each wheel.
  • Making a high-reliability motor will enable regenerative-only braking on future platform rear axles, removing cost from the vehicle BOM.

 


Transient performance

  • By removing the back-lash from gears and mechanical compliance, direct drive will give the best transient torque rate (torque change per unit time) and therefore control fidelity for use in slip-control situations.

 


Bearing deflection

  • The drive torque needs to be reacted between the vehicle suspension knuckle and the wheel. In between these two components is a surprisingly flexible element – the wheel bearing. Flexible drive elements, deflection-tolerant gears and/or even more bearings would be required in order to mitigate these effects, complicating the design.

 


Package size

  • Although gearing will allow the motor to be located off-axis to the wheel, this is not necessarily an advantage and can result in heavily restricted motor diameter in order to allow retrofit capability, as well as also having to locate a gearbox into the wheel assembly.

 

A direct-drive motor will have a lower mass than a high-speed motor with gear. However, the route Protean took was to accept this mass increase for the above reasons and conduct research into the real-world effects that unsprung mass has on the ride and handling of a given vehicle.

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All parts of the system have been assessed for safety implications and are being developed to appropriate safety standards.

Key activities for the analysis include:

  • Ensure compliance with regulations and standards
  • Ensure systems are certifiably safe
  • Ensure processes are effective and conformant
  • Perform required safety analyses
  • Refine the motor design

 

Key regulations and standards include:

  • UNECE (UN/EU)
    • 13/13H : Braking
    • 100: Battery electric vehicles
  • Federal Motor Vehicle Regulations (United States)
    • 105 Hydraulic and electric brake systems
  • SAE
    • Various standards, guidelines and test procedures
  • IEC61508
  • ISO26262
    • Automotive specific version of IEC61508
  • Automotive SPICE
    • Process and maturity model
  • MISRA
    • Safety analysis, C and Simulink coding guidelines.
Major emphasis is being placed on software development for high levels of safety integrity. Protean's model-based development process using Simulink together with auto-code generation has been specifically selected for safety-critical compliance. This approach allows extensive testing in a simulated vehicle environment, together with correct-by-construction translation of mathematical models into executable software for the ECU.

A key requirement that drove Protean’s motor design was the ability to be retrofit on existing vehicles. Designing any technology into a vehicle development cycle creates lead times of 3 to 5 years. But global OEMs need to significantly increase the fuel economy of and remove significant amounts of carbon emissions from their entire fleet right now. A handful of niche vehicles will not achieve these mandates. The ability to engineer a hybrid or full EV on an existing platform, with minimal tear-up of existing hardware or intrusion into passenger, storage or accident-vulnerable space cannot be underestimated and is one of the main advantages of choosing an in-wheel motor to propel a vehicle. It is therefore highly desirable for an in-wheel motor to be able to be fitted to a car without requiring any special wheel design or requiring risky suspension modifications, and this played a large part in the development of the requirements for the Protean motor.