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

Sammanfattning

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 designedfor paved road driving and experiments testing it in off-road scenarios have resultedin longer braking distances compared to braking with ABS disabled. Research inoff-road conditions is mainly focused on traction control and lateral motion ratherthan braking.A physics based tire model has been implemented and analyzed along with an automaticbraking controller tailored for off-road conditions. The model describes theacquired forces from a tire when it sinks into soft terrain and includes parameterswith values specific for different terrain types such as clay and sand. According tothis 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 valueis −100%. This differs from paved road driving which is why ABS aims to regulateslip between −30% and −10%. As a slip of −100% significantly sacrifices theability to steer the vehicle in the lateral direction, the objective of the project is tobrake 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 slipvalue. Test cases were simulated where the vehicle was decelerated from 30km/h invarious conditions such as varying terrain type and load. Based on the weights in theH∞ configuration, decoupled PI-controllers were tuned to compare the centralizedH∞ controller against individual slip controllers acting on each tire axis. It wasfound that the centralized controller exhibits superior slip tracking performance inmost test cases compared to the decoupled controllers. The H∞ controller caneffectively track the slip value it was linearized around, namely −24% for fronttires and −20% for rear tires. It was able to track this slip reference on differentterrain types, including terrain shifts during the braking action, while also beingheavily laden with 20 · 103 kg. In addition, slip tracking was also successful whenthe slip reference was set to −90% for all tires which reduced the braking distanceto approximately 9m in comparison to roughly 12m, depending on terrain type andextra load.The implemented tire model gives insight about why ABS can be ineffective on softterrain as it regulates slip around −20% where longitudinal braking force is suboptimal.With slip references of around −24% for front tires and −20% for reartires, the H∞ does not bring obvious advantages over the ABS. However, the factthat the developed controller showed effective slip tracking performance in variousscenarios, including the more aggressive slip reference of −90%, enables the possibilityto expand the H∞ controller to control more dimensions. Further research isencouraged to include lateral dynamics into the control configuration such that atradeoff between trying to maximize longitudinal braking force while keeping lateraltraction is balanced by weights in the H∞ control configuration.