Optimised wheel hub drive makes vehicles more agile

Optimised wheel hub drive makes vehicles more agile

Technology News |
By Christoph Hammerschmidt

In theory at least, the disadvantages of the wheel hub drive are offset by a number of important advantages. The motors can thus be produced more cost-effectively than motors normally used in electric vehicles, which are either installed centrally in the bodywork or close to the axle. Even more important is the possibility of increasing the steering effect of the wheel hub drive by means of electronic torque vectoring of the individual motors, thus increasing the agility of the vehicle.

A third advantage is that with this type of drive, body designs can be realized with better space utilization than with conventional designs; experts call this a “skateboard configuration”: Since the wheel hub concept does not require a central motor or transmission, this drive topology is low-maintenance and robust. Typical wear parts such as drive shafts are eliminated. The task of the SR4Wheel R&D project was to prove these theoretical advantages on a design close to series production. 

At the heart of the Cologne researchers’ concept are four rim motors which, together with associated power electronics, have been integrated into the space between the brake system and the rim of the standard wheels of a production vehicle. Each motor consists of a static part with 20 coils and a rotating part with 24 teeth. When a coil is energized, it attracts the nearest tooth of the rotor etc, thus generating the rotational movement of the wheel. The underlying physical principle is reluctance force, which minimizes the magnetic resistance between coil and rotor.

The 2 tonne carrier vehicle accelerates to 100 kmph (about 60 mph) in 8 seconds and reaches a top speed of 160 kmph. The prototype consumes only insignificantly more energy than a normal electric motor, but is said to be much cheaper to manufacture. Permanent magnets are often used in conventional electric motors and rare earths are used as a raw material in their production. These magnets are produced under difficult ecological conditions and have experienced a significant price increase in recent years. The rim motors, on the other hand, are made entirely of sheet iron, aluminium and copper. “The production is therefore extremely price-stable and, in large quantities, around 30 to 40 percent cheaper than comparable conventional electric motors,” says research associate Martin Voßwinkel.

Within the project, the Technical University of Cologne also developed an electronic control system that transforms the conventional experimental vehicle into an all-wheel drive vehicle. Each wheel can be controlled individually and is regulated every two milliseconds. This increases the agility of the vehicle. Based on the steering angle, the electronics calculates how the driver wants to take a bend and can intercept a vehicle breaking out. “The outer wheels are then automatically driven stronger and the inner wheels weaker,” explains Voßwinkel. In the practical test, it was possible to drive up to 30 percent faster in curves with absolutely stable driving behaviour.

A further challenge is the inherent noise development in magnetic-free electric drives – another reason why the wheel hub motor has not yet been able to establish itself on the market. The presented prototype is also currently still quite noisy, especially when starting up. However, the research team also hopes to find a solution for this problem. “So far, we have concentrated on the design of the engine in the project and have only been working on the acoustic problem for eight weeks. During this time the noise level has already been reduced by 50 percent by optimizing the driving modes. The goal now is to bring the machine to an acoustic level that is suitable for the mass market,” says Voßwinkel. TH Köln and Alte SW GmbH are now looking for further industrial partners to bring their drive concept to series maturity.


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