Brake-Actuated Steering of Heavy Goods Vehicles

Abstract

Large-scale adoption of higher capacity vehicles in road freight transport can significantly improve transport productivity and reduce fuel consumption and greenhouse gas emissions. To date, their adoption in the UK is limited by their reduced low-speed manoeuvrability. Active rear-steering technologies have been developed to mitigate this, but they rely on heavy and costly electro-hydraulic actuation systems. This dissertation describes the development of a ‘brake-actuated steering’ (BAS) concept to actively steer articulated heavy good vehicles (HGV) using low-cost and lightweight steering technology. The system actuation is performed using individually controlled fast-acting pneumatic brake-modulator valves previously developed by the Cambridge Vehicle Dynamics Consortium. Existing trailer steering technologies and modern braking systems for motion control are presented in Chapter 1. This is followed by an introduction to the dual-action flow concept for the brake-actuator valves in Chapter 2. It is shown that the use of switchable restrictions in the pneumatic brake system can provide both fine pressure control and high flow-rate emergency braking control. Simulations of the dual-flow braking system and of an improved pressure control strategy are developed and experimentally validated using a laboratory test-rig. In Chapter 3, nonlinear simulations of a tractor-semitrailer are developed and linearized for a trailer pendulum model. The linear model is used for the selection of key axle parameters and in the controller design. The BAS controller implements a path-following steering control (PFC) strategy to determine the steering demands for the trailer axles. An inner-loop steering controller is devised to control the steer angles. Simulations of various steering-control strategies are presented. It is shown that the BAS controller has the potential for similar path-following accuracy to electro-hydraulic PFC steering systems, with acceptable levels of longitudinal forces. Both systems provide significantly better directional performance than an un-steered vehicle, and reduce the energy consumed in the manoeuvre. Chapter 4 focusses on the design and construction of a novel BAS axle prototype for installation on an experimental test vehicle. Prototype axle performance and safety requirements are defined. The system is intended to steer at low speeds and be locked at high speeds and during emergency conditions. The experimental vehicle setup including instrumentation, controllers, and pneumatics is also described. In Chapter 5, the feasibility of the BAS concept is demonstrated in full-scale tests on the tractor-semitrailer combination described in Chapter 4. The performance of the BAS controller is evaluated for different pneumatic cases by varying the restriction valve diameters. Performance criteria are defined in terms of path-following accuracy, air usage, and energy consumed by the tractor unit. The BAS system significantly improves low-speed cornering relative to a conventional vehicle with fixed axles. The swept path width is reduced by about 25%, tail swing is decreased by 35-55%, and trailer cut-in is reduced by about 70%. Comparison of different air-flow modes reveals a trade-off between the air usage and the path-following accuracy, with a 2.5mm diameter restriction valve offering the best performance. Chapter 6 draws the conclusions and makes recommendations for future work.