Abstract
Keywords: air suspension, air spring modeling, pipe modeling, interlinked air suspension, pneumatic simulation
Since a vehicle is actuated by road irregularities in many different ways the improvement of the passengers driving comfort is a major focus of automotive research and development work especially in the premium vehicle segment. Apart from the acoustic properties of a vehicle generally referred to as NVH (noise vibration harshness), the low-frequent and mechanical kind of vibrations mainly decide on the driving comfort felt by the passengers. Providing sufficient roll stiffness to counteract body roll under the influence of lateral acceleration and achieving a maximum of ride comfort are conflicting targets in terms of the body roll motion of a vehicle. Especially during straight line driving - particularly on uneven roads - high levels of roll stiffness may severely compromise driving comfort due to copy effects and increased body excitation.
A substantial improvement to resolve this conflict is the implementation of controllable interlinked air suspension systems as semi-active suspension systems. An interlinked air suspension system consists of a conventional four corner air suspension system including pneumatic interconnection lines between the two front and the two rear air spring modules. The connection lines can be switched on and off using solenoid valves so that the air flow between the left and right air spring modules can be terminated depending on the current driving condition.
The advantage of this concept lies in facilitating variable body roll stiffness and damping rates without severely increasing complexity or cost of the entire suspension system. Besides an improved ride comfort by means of reduced body roll accelerations during on-road driving conditions the system is also capable to improve the off-road capability of SUV-type vehicles in terms of an improved level of articulation between the two axles reducing the resistance to that articulation and the wheel load differences.
Therefore the system performance in terms of the potential to improve the ride comfort of a vehicle was subjectively and objectively analyzed on vehicle level and different system setups were investigated experimentally on a servo-hydraulic test bench.
The focus of the comprehensive investigations on a servo-hydraulic test bench was the analysis of the dynamic system response and the complex, highly nonlinear behavior for dynamic excitations mainly caused by the turbulent and compressible air flow between the air spring modules. A variety of design parameter variations like pipe and valve geometry variations were carried out and investigated in detail. Different excitation modes, frequencies and amplitudes were studied to cover the complete range of relevant system responses of an interlinked air suspension system.
To support the research and development work a CAE environment including thermodynamic air spring models, pipe models and different models of pneumatic resistors was established in MATLAB/Simulink to enable detailed system analyses and investigations including parameter variation studies of the system based on a CAE approach. Finally, the results of the test bench investigations were used to validate the models.