Improving emergency braking performance of heavy vehicles

Doktorsavhandling, 2013

Previous studies by the Cambridge Vehicle Dynamics Consortium (CVDC) have suggested that, by improving the control bandwidth of conventional Heavy Goods Vehicle (HGV) brake actuators and using a ‘slip control’ braking strategy, HGV stopping distances could be reduced by up to 30% over existing systems. The work covered in this thesis looks to: validate the CVDC’s previous braking performance predictions through Hardware-in-the-Loop (HiL) simulation, investigate how in-plane tyre dynamics may influence such a system’s performance, develop and build a next generation high-bandwidth braking system for vehicle implementation and carry out comparative back-to-back vehicle tests between the new system and commercially available HGV ABS. An introduction to HGV brake hardware and control is presented in Chapter 1. This is followed by straight-line braking simulations of a HGV using a HiL test rig in Chapter 2. In these tests, a conventional ABS is compared to a prototype braking system consisting of highspeed pneumatic valves (designed by a previous CVDC researcher) and a sliding mode slip controller. Results indicate that the novel system could reduce stopping distances and air consumption by 21% and 17% respectively. Chapter 3 considers the influence of longitudinal tyre dynamics on the straight-line braking performance of a HGV slip control system. A novel, HGV specific, dynamic tyre model is presented. Two vibration modes involving the tyre tread band and wheel hub (an in-phase mode at 20Hz, and an anti-phase mode at 55Hz) are identified that can influence the performance of a slip control system. It is shown that a gain scheduled sliding mode slip controller can be designed to be robust to oscillations in wheel speed associated with these modes. Overspinning of the wheel hub during brake release (a phenomenon seen in conventional ABS vehicle test data) is also correctly predicted by the novel tyre model. Chapter 4 focuses on the design of a next generation high-speed ABS modulator valve suitable for installation on a test vehicle. Prototype valve blocks are built and shown to meet the requirements of a typical UK semi-trailer. Frequency response tests with the new valves indicate that the system can achieve a pressure control bandwidth of 10Hz, significantly higher than the previous generation high-speed valve system (6Hz) and conventional ABS hardware (1.5Hz). Chapter 5 discusses the installation of high-speed ABS modulator valves on a tri-axle semi-trailer. Straight-line braking tests are then carried out in Chapter 6 – comparing the new valve system with conventional ABS. On average, stopping distance and air use are reduced by 15±3% and 50±10% respectively. An increase in Mean Fully Developed Deceleration (MFDD) of 16±4% is also achieved by the new system.