Dynamic safety of active trailer steering systems

Abstract

The dynamic safety of an active steering system for an articulated heavy goods vehicle is investigated. The vehicle is a tractor semi-trailer with two independently steerable axles on the trailer. Several different vehicle dynamics modelling approaches are used to investigate the aspects of the safety of the steering system. These include ‘back of envelope’ calculations, a single degree-of-freedom yaw model, a simplified yaw-plane model using Matlab SimMechanics, with realistic controller frequency response assumptions, and a complex multi-body model of the whole vehicle using TruckSim. Specific safety issues of concern associated with the primary active steering function are: (a) the necessary actuation bandwidth for stable response at high speeds, and (b) the performance implications of disturbance rejection requirements, e.g. side winds and split friction braking. It is found that vehicle tracking improves with increased bandwidth up to 8.3 Hz, but beyond this, performance is limited by other factors. Also, the steering system is able to reject off-tracking disturbances from side winds and split-friction braking, although the latter has a small effect. Additional ‘failsafe’ issues of concern are: (a) whether an independent centring system is necessary on each steerable axle or whether failure of an axle can be safely managed by steering the remaining axles in opposition, (b) the force levels needed in the automatic safety centring system, and (c) the maximum slew rate for centring the axles in an emergency. It is found that individual centring systems for each axle are necessary because axle ‘opposition’ is not a safe strategy for a trailer with two steered axles. The steering actuator is required to generate 32 kN during all modes of operation in order to maintain safety during the specified manoeuvre. A maximum steering slew rate of 11°/s is found to limit additional lateral acceleration to less than 0.2 g.