Balancing Braking Performance and Lateral Traction on Soft Ground in Off-Road Heavy Vehicles

Abstract

Off-road heavy vehicles operate in various terrains and industries such as agriculture, mining and on construction sites. The widely used braking system ABS is designed for paved road driving and experiments testing it in off-road scenarios have resulted in longer braking distances compared to braking with ABS disabled. Research in off-road conditions is mainly focused on traction control and lateral motion rather than braking. A physics based tire model has been implemented and analyzed along with an automatic braking controller tailored for off-road conditions. The model describes the acquired forces from a tire when it sinks into soft terrain and includes parameters with values specific for different terrain types such as clay and sand. According to this model, the braking force is monotonically increasing as braking slip increases, thus finding the maximum braking force when the tire is locked and the slip value is −100%. This differs from paved road driving which is why ABS aims to regulate slip between −30% and −10%. As a slip of −100% significantly sacrifices the ability to steer the vehicle in the lateral direction, the objective of the project is to brake the vehicle with the greatest possible force while not loosing more than 40% of lateral traction relative to its peak value. An H∞ controller has been designed to brake the vehicle by tracking a desired slip value. Test cases were simulated where the vehicle was decelerated from 30km/h in various conditions such as varying terrain type and load. Based on the weights in the H∞ configuration, decoupled PI-controllers were tuned to compare the centralized H∞ controller against individual slip controllers acting on each tire axis. It was found that the centralized controller exhibits superior slip tracking performance in most test cases compared to the decoupled controllers. The H∞ controller can effectively track the slip value it was linearized around, namely −24% for front tires and −20% for rear tires. It was able to track this slip reference on different terrain types, including terrain shifts during the braking action, while also being heavily laden with 20 · 103 kg. In addition, slip tracking was also successful when the slip reference was set to −90% for all tires which reduced the braking distance to approximately 9m in comparison to roughly 12m, depending on terrain type and extra load. The implemented tire model gives insight about why ABS can be ineffective on soft terrain as it regulates slip around −20% where longitudinal braking force is suboptimal. With slip references of around −24% for front tires and −20% for rear tires, the H∞ does not bring obvious advantages over the ABS. However, the fact that the developed controller showed effective slip tracking performance in various scenarios, including the more aggressive slip reference of −90%, enables the possibility to expand the H∞ controller to control more dimensions. Further research is encouraged to include lateral dynamics into the control configuration such that a tradeoff between trying to maximize longitudinal braking force while keeping lateral traction is balanced by weights in the H∞ control configuration.