Next-generation In-wheel Electric Hub Drives

The series Hybrid-Electric Drive (HED) architecture brings a significant range of potential benefits to military ground platforms. Electric-drive wheeled vehicles can feature electric traction motors in a conventional driveline, but also offer potential for motor incorporation into the axle or the wheel hub. The implementation of in-wheel hub drives presents challenges both in their design and to the overall vehicle system. However, by overcoming these challenges, the vehicle designer is rewarded with greatly increased freedom in vehicle hull and suspension design and numerous other benefits arising from the elimination of much of the mechanical driveline and the provision of individual wheel control. Many hub drive development programs of the previous two decades that have produced demonstrator vehicles have been typified by a large wheel rim size in order to accommodate a traction motor of sufficient power to achieve both peak output torque requirements and peak rotational output speed through a single reduction ratio. In some cases, traction motor rating has also been driven by the need to provide full vehicle braking capability by absorbing energy in regeneration. The need for a large wheel rim limits the further development and adoption of hub drives, as it implies excessive un-sprung mass, reduced off-road tire performance, and limited off-the-shelf tire compatibility. Large motor size also implies proportionately large power electronics. QinetiQ Limited are developing next-generation hub drives as part of DARPA’s Ground-X Vehicle Technologies program, as well as under QinetiQ IRAD, with the aim of overcoming these obstacles by integration of a small, powerdense permanent magnet motor with a three-speed planetary reduction gearbox and liquid-cooled friction disk brake, packaged within the space-claim of the standard 20” wheel rim. The motor design has demonstrated capability to deliver power to exceed the power-to-weight ratio of present-day military platforms. Incorporation of a multi-ratio integrated gearbox allows the hub performance envelope to extend to both high-tractive torque at low speed for obstacle crossing/gradability, and to high vehicle speeds for on-road use. Using a cooled integrated friction brake allows for dissipation of braking energy far in excess of the rated power of the traction motor to ensure full braking capability. This system has been successfully demonstrated to TRL-5 within the space claim of a standard 20” military wheel rim. This effort takes the state of the art a significant step closer toward wide adoption of hub drives as a critical building block in the HED vehicle architecture.