Lateral and longitudinal dynamics of an off road vehicle with hydropneumatic suspension system

Suspension system is one of the most important vehicle subsystem that affects the ride and handling performance of on and off-road or a military vehicle. Modelling and simulation of the suspension systems which define the ride comfort and road holding characteristics of a vehicle, has a key role in research and product development studies in automotive industry. This study mainly focuses on modelling a HPS system for a 10 tonness class armoured vehicle and simulating that model with different scenarios about lateral and longitudinal dynamics of that vehicle. Before modelling the HPS system, a base vehicle with conventional suspension system which consists of helical springs with constant stiffnes and dampers with different constant damping coefficients for jounce and rebound motion is generated. After that, a base HPS system which can be replaced with the conventional one that shows similar springing and damping characteristics with respect to that, is modeled. This base HPS system is further developed for advanced suspension functions such as cross plumbing of the suspension accumulators for improved roll and pitch stiffness and a system with leveling capability. HPS system models are created using MATLAB-Simulink toolbox and "Simscape" sub-toolboxes.Thus, a simple and an understandably visualized block diagram of the suspension system is utilized for further developments. Since the comfort dynamics of the vehicles is directly related to the seat dynamics, a seat model with constant springing and damping coefficients is created using "Simscape" toolbox blocks. This model is integrated to the vehicle model by making necessary adjustments in both IPG TruckMaker graphical user interface and the Simulink model of the suspension model. All testing scenarios are carried out by IPG TruckMaker-Simulink co-simulation. In order to use the user defined suspension model in co-simulations, technical support is received from M.Sc. Zhou Huang, from CarMaker Service Team. By his guidance, some initial parameters for suspension system are adjusted and the base model have been made functional for test simulations. In preliminary simulations, predefined parameters such as sprung mass natural frequency and damping ratio of both conventional and HPS systems are tested for validation and confirmed for being comparable for further scenarios. This confirmation is made by running the vehicles with those systems on a road that includes a step type disturbance. FFT's of the vehicle bodies' vertical accelerations have shown that the natural frequency of the both systems are overlapped with the design parameters. To compare and approximate the damping characteristics, damping ratios of the vehicles are calculated by graphical methods using the vertical displacements of the vehicle bodies. For vertical dynamics simulations, two NATO standart test procedures are applied to to the vehicles with conventional and HPS systems. The first one is called the absorbed power technique, which regards the power absorbed by a human subjected to ride vibration as ride quality. The two vehicles are run over two predefined road profiles which are designated by APG-11 and APG-IV with different speeds in order to define the cruising speed at which the absorbed power on the driver is reached to 6 W. Toxxii calculate the absorbed power, FFT of the seat acceleration signals are calculated to get the RMS accelerations of the seat for each spectral band and used in the absorbed power calculation formulae that is given by "Aberdeen TOP 1-1-014 Ride Dynamics" procedure. Constants used in that formulae is also given by the procedure. Results of the absorbed power tests have shown that; for both road profiles, vehicle with the HPS system provides higher cruising speed without violating 6 Watts of maximum absorbed power criterion. The second simulation type for vertical dynamics is the half round obstacle technique, that is done by running the vehicle on a half round cylinder geometry at different speeds and control if the vertical seat acceleration reaches 2.5 g. Interesting results are obtained from these simulations that are further examined in the simulation results section of this study. Vehicles with both suspension systems performed well at higher speeds such as 20, 30, 50 kmh in terms of maximum seat vertical accelerations however at lower speeds, vehicles with both suspension systems violated the 2.5 g of maximum seat acceleration criterion for half round obstacle with 8 inch radius. For 4 inch radius half round obstacle tests, vehicles with both suspension systems have passed the tests succesfully for all speeds, again they have shown better results at higher speeds. For lateral dynamics simulations, ISO Lane Change or double lane change procedure adapted for off-road vehicles is performed for 4 vehicles with different suspension systems which are the conventional system, another conventional system with stabilizers on the front and rear axle derived from the former, base HPS system and the HPS system with cross-plumbing derived from the former. Trajectory limits for the maneuver are defined with the pylons in the IPG TruckMaker Road module and the simulation log files provided the data of the pylon hits which includes the identification code, the location where and the time when the pylon hit occurs. 4 vehicles with different suspension models have been run at speeds increased 5 kmh gradually starting from 35 kmh. The suspension model which couldn't have past the maneuver at the current speed, haven't been tested at upper speeds. The vehicles with conventional system and the conventional system with the front and rear stabilizers could make the maneuver at 50 kmh while the vehicle with the HPS system could only made the maneuver at 40 kmh. The worst performance has been shown by the vehicle with HPS system with cross plumbing which could only make the maneuver at 35 kmh speed. For suspension leveling simulations, increasing and decreasing the piston positions 10 cm and returning the piston position to the design position after a loading case are investigated. To get the final results of the leveling function for more sensitive, steady and faster action, parameters of the leveling device elements such as the control algorithm, the valve actuator dynamics and the spool valve configuration is tried to be optimized. In the conclusion and the recommendation section of the thesis, a general comparison of the created suspension models is summarized and further studies that can be derived from this work are suggested to the researchers studying o