Study On The Vibration Characteristics Of The Pneumatically Interconnected Suspensions

The demand for the vehicle ride comfort has been increasing with the rapid development of the automotive industry in recent years.It is difficult for the traditional coil spring or leaf spring suspension to achieve a breakthrough in enhancing the ride comfort as these suspensions must be stiff enough to support the load and guarantee the driving safety.As a superior vibration isolator,the air spring has been widely used in the vehicle suspension system,cab and seat suspension system of the commercial vehicle,due to its advantages of the variable spring rate and low vibration transmissibility.However,the air spring's dynamic characteristics are complicated that it exhibits nonlinear elastic performance in large displacement excitation,and its dynamic stiffness depends on the frequency and amplitude of the excitation.The passive interconnected suspensions have been used to alleviate the vehicle vibration.However,few studies have been carried out in the design of the pneumatically interconnected suspension(PIS)systems and especially in the vibration theory of the corresponding PIS system.Therefore,the aim of this thesis is to conduct the theoretical and experimental research on the modeling method of the air spring and the vibration characteristics of the PIS system.The major contents of this thesis are as follows:(1)The dynamical model of the nonlinear,frequency and amplitude dependent air spring is proposed,of which the air spring force consists of the nonlinear elastic force of the compressed air in the air bellow,the viscoelastic force and the friction of the air bellow rubber material.The nonlinear,frequency and amplitude dependent characteristics of the air spring are accounted by the elastic force,viscoelastic force and friction elements,respectively.The elastic force is derived based on thermal dynamics,the viscoelastic force with fractional derivative model,and the advanced Berg's friction by statistical theory.The air spring bench tests are carried out,and the model parameters are identified based on the experimental results of the standalone air spring.Several models for the bellow-pipe-tank system are compared with the proposed model and the measurements in harmonic excitations with different amplitudes and frequencies,and random excitations with both large and small displacement cases.The results of comparison show that the proposed model can accurately predict the air spring's dynamic characteristics in an acceptable computation time.(2)The mechanical system's dynamical equation of a roll-plane 4-degree-of-freedom(4-DOF)half-car model with PIS system is derived,and the air spring model is linearized based on thermodynamic equation.The air flow in the pipe is modeled through a linear differential equation based on Newton's second law in which the air mass inertial effects,the frictional and local pressure drops are considered.The vibration equation of the mechanical-pneumatic coupled system is obtained by integrating the pneumatic strut forces into the vehicle mechanical system.Based on this coupled system vibration equation of the vehicle with PIS system,both the vehicle free vibration modes and frequency response functions(FRFs)are compared with the unconnected air suspension.The results show that the PIS can suppress the vehicle roll vibration without affecting its bounce properties.The effects of the local loss ratio factor,pipe length,and pipe diameter on the vehicle roll vibration transmissibility properties are investigated.The design of experiments(DOE)approach is further used to obtain an optimal design of the pipe to achieve the desired roll vibration responses,i.e.the minimal vibration level and the ideal resonance frequency and amplitude,under road random excitations.The results show that the vehicle ride comfort can be conveniently improved by designing a pipe with suitable length and diameter of the PIS.(3)The quarter car models with and without considering the effect of the lever ratio are built,respectively,to study the vertical stiffness and the natural frequency of the dual-chamber air suspension.By assuming the displacement excitation is small,the mechanical-pneumatic coupled system's dynamic equations of a roll-plane 4-DOF half-car model with dual-chamber PIS system are derived,which include the dynamic equation of the mechanical system,the linear differential equations of the dual-chamber air spring and the pipe.Based on the coupled system equations of the vehicle with dual-chamber PIS system,both the vehicle free vibration modes and FRFs are analyzed.The results show that the dual-chamber PIS system can alleviate the vertical and roll vibration because it can decrease the suspension stiffness and add additional system damping effect.The effects of the dual-chamber PIS system parameters on the vehicle vertical and roll vibration transmissibility properties are further investigated.In addition,the vertical and roll stiffness of the dual-chamber PIS system are studied.It shows that the nonlinear suspension vertical and roll stiffnesses can avoid the shock,and the rollover in quick steering maneuvre.(4)The development method for the PIS system in practical engineering application is generalized.The multibody dynamics simulation platform of the full vehicle with roll-plane dual-chamber PIS system is built in MSC.ADAMS/Car software environment,and the mechanical-pneumatic coupling is realized through the ordinary differential equation(ODE)method.The corresponding test platform of the full vehicle with PIS system is set up,and the ride road tests are conducted to validate the full vehicle dynamical model and investigate the parameters' effects on the ride comfort.In addition,the anti-roll performance of the dual-chamber PIS system is evaluated by conducting the steady static circular simulation based on the validated full vehicle model.The results show that the dual-chamber PIS system can improve both the ride comfort and anti-roll performances.