Modelling and validation of off-road vehicle ride dynamics

Increasing concerns on human driver comfort/health and emerging demands on suspension systems for off-road vehicles call for an effective and efficient off-road vehicle ride dynamics model. This study devotes both analytical and experimental efforts in developing a comprehensive off-road vehicle ride dynamics model. A three-dimensional tire model is formulated to characterize tire–terrain interactions along all the three translational axes. The random roughness properties of the two parallel tracks of terrain profiles are further synthesized considering equivalent undeformable terrain and a coherence function between the two tracks. The terrain roughness model, derived from the field-measured responses of a conventional forestry skidder, was considered for the synthesis. The simulation results of the suspended and unsuspended vehicle models are derived in terms of acceleration PSD, and weighted and unweighted rms acceleration along the different axes at the driver seat location. Comparisons of the model responses with the measured data revealed that the proposed model can yield reasonably good predictions of the ride responses along the translational as well as rotational axes for both the conventional and suspended vehicles. The developed off-road vehicle ride dynamics model could serve as an effective and efficient tool for predicting vehicle ride vibrations, to seek designs of primary and secondary suspensions, and to evaluate the roles of various operating conditions

Integration of torque blending and slip control using nonlinear model predictive control

Antilock Braking System (ABS) is an important active safety feature in preventing accidents during emergency braking. Electrified vehicles which include both hydraulic and regenerative braking systems provide the opportunity to implement brake torque blending during slip control operation. This study evaluates the design and implementation of a new torque allocation algorithm using a Nonlinear Model Predictive Control (NMPC) strategy that can run in real-time, with results showing that wheel-locking can be prevented while also permitting for energy recuperation

Theoretical analyses of roll- and pitch-coupled hydro-pneumatic strut suspensions

Vehicle suspension design and dynamics analysis play a key role in enhancement of automotive system performance. Despite extensive developments in actively-controlled suspensions, their commercial applications have been limited due to the associated high cost and weight. Alternative designs in either passive or semi-active suspensions are highly desirable to achieve competitive vehicle performance with relatively lower cost and greater reliability. This dissertation research proposes two hydro-pneumatic suspension strut designs, including a twin-gas-chamber strut, and systematically investigates various concepts in roll- and pitch-coupled suspensions employing hydraulic, pneumatic and hybrid fluidic interconnections between the wheel struts. The proposed strut designs, including single- and twin-gas-chamber struts, offer larger working area and thus lower operating pressure, and integrate damping valves. Nonlinear mathematical models of the strut forces due to various interconnected and unconnected suspension configurations are formulated incorporating fluid compressibility, floating piston dynamics, and variable symmetric and asymmetric damping valves, which clearly show the feedback damping effects of the interconnections between different wheel struts. The properties and dynamic responses of the proposed concepts in roll- and pitch-coupled suspension struts are evaluated in conjunction with in-plane and three-dimensional nonlinear vehicle models. The validity of the vehicle models is demonstrated by comparing their responses with the available measured data. The analyses of the proposed coupled suspensions are performed to derive their bounce-mode, anti-roll, anti-pitch and warp-mode properties, and vehicle dynamic responses to external excitations. These include road roughness, steering and braking, and crosswinds. The results suggest that the fluidically-coupled passive suspension could yield considerable benefits in enhancing vehicle ride and handing performance. Furthermore these offer superior design flexibility. The suspension struts offer a large number of coupling possibilities in the three-dimensions, some of which however would not be feasible, particularly for commercial vehicles where suspension loads may vary considerably. A generalized analytical model for a range of interconnected suspensions is thus developed, and a performance criterion is formulated to assess the feasibility of a particular interconnection in a highly efficient manner. The handling and directional responses of a three-dimensional vehicle model employing X-coupled hydro-pneumatic suspension are evaluated under split-o straight-line braking and braking-in-a-turn maneuvers. The results clearly show that the X-coupled suspension offers enhanced anti-roll and anti-pitch properties while retaining the soft vertical ride and warp properties. Fundamental pitch and vertical dynamics of a road vehicle are also considered to derive a set of essential design rules for suspension design and tuning for realizing desirable pitch performanc

High-precision hydraulic pressure control based on linear pressure-drop modulation in valve critical equilibrium state

High precision and fast response are of great significance for hydraulic pressure control in automotive braking systems. In this paper, a novel sliding mode control based high-precision hydraulic pressure feedback modulation is proposed. Dynamical models of the hydraulic brake system including valve dynamics are established. An open loop load pressure control based on the linear relationship between the pressure-drop and coil current in valve critical open equilibrium state is proposed, and also experimentally validated on a hardware-in-the-loop test rig. The control characteristics under different input pressures and varied coil currents are investigated. Moreover, the sensitivity of the proposed modulation on valve's key structure parameters and environmental temperatures are explored with some unexpected drawbacks. In order to achieve better robustness and precision, a sliding mode control based closed loop scheme is developed for the linear pressure-drop modulation. Comparative tests between this method and the existing methods are carried out. The results validate the effectiveness and superior performance of the proposed closed loop modulation method

Effect of handling characteristics on minimum time cornering with torque vectoring

In this paper, the effect of both passive and actively-modified vehicle handling characteristics on minimum time manoeuvring for vehicles with 4-wheel torque vectoring (TV) capability is studied. First, a baseline optimal TV strategy is sought, independent of any causal control law. An optimal control problem (OCP) is initially formulated considering 4 independent wheel torque inputs, together with the steering angle rate, as the control variables. Using this formulation, the performance benefit using TV against an electric drive train with a fixed torque distribution, is demonstrated. The sensitivity of TV-controlled manoeuvre time to the passive understeer gradient of the vehicle is then studied. A second formulation of the OCP is introduced where a closed-loop TV controller is incorporated into the system dynamics of the OCP. This formulation allows the effect of actively modifying a vehicle's handling characteristic via TV on its minimum time cornering performance of the vehicle to be assessed. In particular, the effect of the target understeer gradient as the key tuning parameter of the literature-standard steady-state linear single-track model yaw rate reference is analysed

A Situation-Aware Collision Avoidance Strategy for Car-Following

In this paper, we discuss how to develop an appropriate collision avoidance strategy for car-following. This strategy aims to keep a good balance between traffic safety and efficiency while also taking into consideration the unavoidable uncertainty of position/speed perception/measurement of vehicles and other drivers. Both theoretical analysis and numerical testing results are provided to show the effectiveness of the proposed strategy