Advanced wheel hub drive passes extensive driving tests

November 25, 2014 // By Christoph Hammerschmidt
Besides high design freedom and low-wear characteristics, wheel-integrated electric drives offer a high potential to improve safety and driving dynamics of electric vehicles. However, they pose high challenges to drive design and manufacturing - in particular with respect to the safety assessment and to the specific measures for series deployment. A research consortium led by Fraunhofer IFAM now succeeded in proving the safety of such drives even in the case of malfunctions.

The consortium tested two wheel hub drives, mounted at the rear axle of a Fraunhofer-developed demo vehicle under various difficult road surface conditions. The wheel hub drive generated a torque of up to 900 Nm per wheel. Tests included dynamic turning manoeuvres on wet surface, directional stability on slick, icy roads as well as during heavy braking - and all of them in combination with various system malfunctions which have been controlled selectively by the scientists. Subject to the test were innovative concepts of drive monitoring and error handling of fault-tolerant drive technologies under the aspect of operational safety.

In a first phase, the project partners collected and analysed data on possible effects of errors. By means of measurements on test rigs and numerical simulations, they determined the brake torque during short circuits in the coils as well as during failures of sensors and other components. Based on this knowledge they could develop specific hardware and software concepts for failure management. In this context, they developed a novel error recognition method that enables localising any errors reliably.

Failures of the current sensor and the angle sensors can be detected safely through a comparison of values obtained though measurements and through model-based calculation. In the case of an error, the model data are used. Handling of complications like short circuits in the windings or converters is based on a fault-tolerant design of the entire drive system. Towards this end, the electric motor consists of multiple redundant subsystems, enabling designers to isolate any errors and to compensate brake torque resulting from a failure, guaranteeing safely stopping the vehicle and even continue operation in emergency mode.

The test proved that in particularly during rides at the physical limits under difficult road and weather conditions the implementation of the project results led to significant safety improvements. At any operational situation the formation of improper brake torques could be avoided. Even during fast turns and