Driver and vehicle behaviour to power train failures in electric vehicles: experimental results of field and simulator studies

Peter Cocron
Isabel Neumann
Maria Kreußlein
Marta Pereira
Daniel Wanner
Lars Drugge
Maxim Bierbach

New electric power trains can be subject to different failures when compared to those arising in conventional vehicles. The objectives for active safety investigations within the EVERSAFE project (work package 2) were to address vehicle stability under these failure conditions and the driver response to relevant types of failures. Failure conditions that affect the vehicle stability are believed to be significantly different from today’s conventional internal combustion engine cars, and may potentially be a substantial safety problem if not treated in a correct manner. To study these effects, two examples of system failures and their consequences on the driver response and vehicle stability were investigated with the help of three studies.

The first two studies investigated a failure of wheel hub motors (WHMs), an emerging technology among the future generation of electric vehicles (EV). The main benefits of a WHM are its controllability, high efficiency, high power density and low weight. However, the direct connection to the wheel comes along with the potential disadvantage in case a failure occurs in the system. Therefore, those WHM failures are to be analysed in this study. A possible failure with severe consequences to the driving task could be a constant brake torque applied to one of the wheels, which leads to a sudden yaw torque and subsequently to a deviation from the desired path of driving. As a consequence, a compensatory control action by the driver is necessary. The impact of this failure of WHMs on the drivers was assessed twofold in the EVERSAFE project. On the one hand a driving simulator study investigated the impact of WHM failures under high speed conditions (110 km/h). On the other hand these failures were investigated in a test track study at lower speeds (30 km/h). Both studies examined the following three manoeuvres: a failure while straight line driving as well as two failures while driving in a curve, i.e. on the inner and the outer rear wheel respectively. Drivers were asked to evaluate the failure situation regarding perceived stress, risk, lack of control, workload and disturbance. Furthermore, objective data collected via the data logger (steering wheel angle, accelerator pedal travel, brake pressure, speed, yaw rate, longitudinal and lateral acceleration) contributed to shed light on the driver response and vehicle stability. The results of the subjective ratings of both studies revealed a low to medium level of perceived failure severity for all tested failures. It can be concluded from the outcomes of the simulator study, that WHM failures at speeds of 110km/h are rated more stressful, risky, disturbing and demanding than driving without an occurring failure. With regard to the test track study these differences occurred only for the straight inward failure. Comparing the subjective evaluations in the simulator and the field study, it can be assumed that failures are perceived as more severe at higher than at lower speeds. The simulator study showed steering as the main reaction to all failures. Regarding the test track study a steering reaction was only detected for the curve inward failure. Consequently, the outcomes of the subjective evaluations of the simulator study and driving data of both studies lead to the assumption that the curve inward failure is the most severe amongst the failures tested here. As participants in both studies used the accelerator pedal during failure activation, regardless of failure type, it could be concluded that drivers try to overrule the deceleration triggered by the failure at low as well as at high speeds. Whether this was an effect of the instruction to maintain a steady speed should be clarified by further research. Generally speaking, results of both studies showed that humans can compensate WHM failures well, at least for the specific settings of the studies. However, further research is needed to investigate if and how these conclusions are applicable, for instance, in case of a higher workload of the driver (e.g., in more complex driving situations).

The third study conducted within the active safety focus of the EVERSAFE project examined a failure of the regenerative braking (RB) system. The latter is a system designed to convert kinetic energy to chemical energy stored in the energy storage system (i.e. battery) while the vehicle decelerates. In the tested configuration this energy recapture is triggered via the accelerator pedal and therefore decelerates the car whenever the driver releases that actuator. In case the RB system fails to operate properly, hazardous situations might occur because the driver expects a deceleration of the car which is inexistent due to the failure. In order to study the effects of a RB failure on driver behaviour and perception as well as on vehicle stability, a test track study was conducted. Similar to the WHM failure testing, the failure situation was evaluated with the help of subjective ratings (stress, risk, lack of control, disturbance, workload) and data logger records (steering wheel angle, accelerator pedal travel, brake pedal pressure, speed, yaw rate, longitudinal and lateral acceleration). The RB failure was implemented during a deceleration manoeuvre when entering a curve at a speed of about 50 km/h. Whereas half of the participants were informed that a RB failure might occur, the other half was not informed. Results revealed that only about 50% of the subjects noticed the RB failure, but compensation efforts were manageable. Although the situation was rated more risky than solely driving on a road, the RB failure did not induce more stress or workload. Informing people about an upcoming failure yielded a higher frequency of mild braking manoeuvres. It is important to note that failure of RB system will incur in higher vehicle speeds than the ones originally expected by the driver under normal circumstances. Depending on the road conditions (e.g., wet/icy surface) or even the road geometry (e.g., curve driving) the unexpected high velocity might even lead to vehicle instability thus compromising the safety of the occupants as well as the general traffic flow. These aspects should be considered in future research.



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