Driver Response to Active Front Steer and Power-Assist Failures

Active Front Steer (AFS) has the ability to apply varying road wheel angles for the same steering wheel angle, depending on the speed of the vehicle. It achieves this by changing the steering gain dynamically and can be a useful driver aid: less steering wheel input is required at low speeds than a more traditional fixedgain steering system to achieve the same radius of turn. Should an AFS system fail, it is designed to revert back to a fixed-gain system. Concerns have been expressed by designers of AFS systems that this sudden change in steering gain may prove hazardous for drivers and be particularly difficult for them to handle. The present study had three main aims: • To compare drivers’ behaviour using both fully-functioning AFS and power-assisted, fixed-gain, rack-and-pinion steering systems. • To compare driver behaviour and response to the failures of both AFS and to the power-assist of a fixed-gain, rack-and-pinion steering system. • To conclude whether failure of AFS is potentially more hazardous than failure of the power-assist of fixed ratio, rack-and-pinion steering systems. Using the fixed-base Leeds Driving Simulator, this study compared driver behaviour using both AFS and fixed-gain steering systems. Failures of each system were also investigated. Fixed-gain system failure was simulated by a loss of steering power-assist. Forty drivers, balanced for age and gender took part in the study. Results showed that fully-functioning AFS had some advantages over fully-functioning fixed-gain steering. Drivers found AFS less demanding, demonstrated by the fact that they showed significantly fewer steering reversals. They also rated AFS as easier to control in curved sections. Whilst making left-hand turns at a series of T-shaped intersections, there was a highly significant worsening of driver performance between steering functioning normally and failed steering. However, drivers found AFS failure no harder to manage than power-assist failure. Indeed, there were statistical trends suggesting that, if anything, AFS failure was easier for drivers to deal with than loss of power-assist.