Nonlinear Model Predictive Control of Active Air Suspension for Roll Stability in Tractor-Semitrailer Vehicles

Abstract

Tractor–semitrailer combinations have a high center of gravity and strong coupling between the two vehicle units. Steering and combined steering-braking maneuvers can therefore produce large body roll and lateral load transfer. Active air suspension can reduce this risk by changing the vertical support force at each side of the tractor. Its control authority is limited by pressure, valve flow, and the pneumatic system’s response speed. This thesis builds on an earlier active air-suspension and NMPC framework. The vehicle configuration considered is a 4x2 two-axle tractor with a three-axle semitrailer. Active air suspension is installed only on the tractor, while the semitrailer keeps its passive suspension. The controller model is reformulated so that air-pressure and mass-flow dynamics are not included among the prediction states. Their physical limits are instead represented by constraints related to force, force rate, pressure, and mass flow. Pitch states and pitch weights are added. This reformulation reduces the order of the prediction model and improves computational efficiency while preserving the key actuator constraints. The model also includes forces and moments transmitted through the fifth wheel, together with a bounded preview correction for the trailer-induced roll moment. The high-fidelity Volvo Transport Model (VTM) is used as the closed-loop simulation plant. A nonlinear two-track model provides the control-oriented vehicle prediction. The upper-layer NMPC calculates increments in tractor suspension force. A lower layer converts these force requests to pressure references and on–off valve commands. The strategy is evaluated under step steering, ramp steering, lane change, and braking-in-a-turn maneuvers. Compared with the passive baseline, the NMPC controller reduces the semitrailer Load Transfer Ratio (LTR) and improves roll response in the main steering scenarios. These results indicate that, within the simulation environment and actuator constraints, the NMPC-based active air suspension strategy improves the roll stability of tractor-semitrailer combinations and provides a useful reference for future active suspension control in heavy-duty vehicles.