Experimental verification of understeer compensation by four wheel braking

Paper in proceeding, 2014

This study is designed to validate a new approach to understeer mitigation chassis control, based on a particle motion reference: parabolic path reference (PPR). Considering the scenario of excess entry speed into a curve, related to run-off-road crashes, the aim is that automatic braking minimizes lateral deviation from the target path by using an optimal combination of deceleration, cornering forces and yaw moments. Previous simulation studies showed that four-wheel braking can achieve this much better than a conventional form of yaw moment control (DYC). The aim of this work is to verify this on a test track with an experimental vehicle, and to compare performance with DYC and an uncontrolled vehicle. Experiments were performed with a front-wheel-drive passenger vehicle equipped with an additional four identical brake callipers controlled via an electro-hydraulic brake (EHB) system, enabling individual brake control. Minimizing the maximum deviation from the intended curve radius is the control objective. Feedback to the controller consists of the available steering wheel angle, wheel speeds, yaw rate and lateral acceleration sensors in the vehicle. Additional to these variables, also the vehicle position was logged using a GPS system. It was found that PPR is superior to DYC in reducing the maximum deviation from the intended path, confirming the trends previously found in simulations. Furthermore, the PPR concept is found to be inherently more stable than DYC since more brake force is applied to the outer wheels than the inner wheels throughout the manoeuvre. The experiments involve a first implementation of a PPR control which is not a fully closed-loop control intervention and tuned to a step steer (transition from straight to fixed-radius curve. This is the first study to explicitly and systematically evaluate this new approach to understeer mitigation. The approach is fundamentally different from common DYC and suggests the potential for a new generation of controllers based on trajectory control via chassis actuators.