Modeling Of Displacement-dependent Damper And Study On Its Effects On Vehicle Performance

Damper as the major damping component of the vehicle suspension system, and as a complicated nonlinear system, its performance has direct influence on the ride performance and handling stability. The basic damper characteristics depend on the piston and rod diameters, and on the characteristics of the four valves. Additional factors include the reservoir pressure and the fluid properties: density, viscosity, temperature, gas absorption, emulsification and compressibility, and so on. An important measure of damper performance design is realized through adjusting the inner valves. The traditional damper development method is to determine the design parameters according to experience, and then to adjust through experiments, with a major weakness of a division between vehicle dynamic performance and the study of the inner structure as well as the outer dynamic characteristics. In order to reduce the trial production and real test for damper, it is an inevitable currency to apply CAD/CAE technology to the damper development, with a key problem of accurate prejudge of the damper characteristics on the design stage.The acting force of displacement-dependent damper is not only related to velocity but also influenced by displacement. This dissertation studies two forms of displacement-dependent damper: one is the by-pass-displacement-dependent damper, with added by-pass grooves on the inner wall of the working cylinder of a normal damper according to the running distance, so to realize a change in damp force characteristics along with the change of running distance; the other one is the hydraulic-stopper- displacement-dependent damper, besides the normal shock absorption function, which can replace the rubber bumpstoper and reboundstoper of the normal suspension, and with better buffer characteristics than rubber stopper, it can improve ride performance on bad roads.In order to study the displacement-dependent damper and its influence on vehicle performance, a dynamic model is set up according to the structure and working principles of displacement-dependent damper, so to investigate the dynamic performance respectively from the experimental and simulative aspect. The damper working status of the benchmark vehicle is analyzed under ordinary working conditions, and displacement-dependent damper parameters are reasonably determined; the outer characteristics of the displacement-dependent damper are assembled onto the benchmark vehicle dynamics model to do simulation analysis of its improvement on the performance of the benchmark vehicle.First, the structure and working principles of the normal twintube hydraulic damper are analyzed, by abstracting it into an equivalent hydraulic transmission system and applying ADAMS/Hydraulics to set up the dynamic model. On this basis, the equivalent models of by-pass grooves and hydraulic stoppers are added, and the dynamic model of the displacement-dependent damper is built. The normal damper, the by-pasas and the hydraulic-stopper-displacement-dependent damper are tested respectively. Model and testing data are compared, and the two are in sound accordance.Second, simulation analysis is done on the aberration of the outer characteristics, the force-velocity hysteresis, and the valve characteristics parameters'effects on the outer characteristics, etc. which are the focuses in the damper performance design. Dynamic simulation is done on the displacement-dependent damper, and the by-pass grooves parameter influence is analyzed.Next, a mass produced mini-van is used as the benchmark vehicle, and the ADAMS/Car is applied to set up vehicle dynamic model, with random road profile generation as the focus of the illustration. The ADAMS/Car Ride tool for generating road profiles with roughness uses a mathematical model developed by Sayers which is based on the observed characteristics of many measured profiles of roads of various types. Tire model for ride use is chosen and subsystem models such as the suspension are built, which help to point out that there is a discrepancy of a constant coefficient between the random GB7031 road power-spectral density model and the Sayers model, and outer characteristics of the displacement-dependent damper are assembled by using the GSE Damper. Tests on the ride performance and handling stability of the target vehicle are done according to the international GB standard, and the emulation and the test results are in good accordance.Then, the working status of the original damper of the benchmark vehicle on typical working condition is analyzed, and the length of the by-pass grooves as well as other design parameters are determined; the hydraulic stop position is determined according to posiotn of the rebound buffer. by-pass-displacement-dependent damper offers comparatively soft damping in condition of normal ride, and so provides satisfactory comfort; in condition of emergency, the running distance of the damper is large, and the piston is out of the groove, and the damping of the vehicle is hard, so to make sure the safety. Hydraulic-stopping damper has better buffer characteristics than rubber stopper. The energy produced when the rubber stopper is impacted is saved and given out as potential energy, and the liberation energy of the system does not reduce, while the impact energy of the hydraulic-stopping damper is transformed into irreversible heat energy which emits in the air and greatly reduces the liberation energy, and so the ride performance of the vehicle on bad roads is well improved.Finally, for the completeness of the dissertation, the influence of the bush on top of the damper on the vehicle performance is analyzed. Flexibility in series with the damper, often called a"relaxation spring", has only a small effect on the mean RMS acceleration of the body (comfort level), but a significant effect on the wheel transmissibility. In practice, the best use of damper and rubbers is with nonlinear rates, with significant flexibility occurring over only very small deflections; the rubbers stiffen up on larger deflections. This allows the benefits of lower transmissibility at high-frequency small-amplitude inputs, while maintaining control over the wheel at larger amplitudes。Main Innovations of the Dissertation:(1) To simplify the displacement-dependent damper to equivalent hydraulic transmission system, and apply ADAMS/Hydraulics to set up the dynamic model. This method is also applicable in the modeling of normal hydraulic damper, which can clearly describe the damping mechanism of the damper. Each element and its corresponding parameter in the hydraulic loop has definite physical meaning, and so the parameters of the damper can be easily changed in the dynamic performance analysis, structure design, element selection of the hydraulic damper, and in the vehicle dynamic performance study.(2) To assemble the force-velocity characteristics of the displacement-dependent damper to the ADAMS/Car vehicle model by using GSE Damper. This method is applicable not only to the damper of the new structure in the vehicle application study, but also to the semi-active suspension study.(3) To analyze the improvement effect of the displacement-dependent damper on the vehicle dynamic performance by ADAMS/Car. The structure of the displacement-dependent damper is simple, and can significantly improve the vehicle performance in ultimate working condition. (4) To analyze the influence of the bush on top of the damper on the vehicle performance from frequency domain and time domain. The bush on top of the damper maintains a significant control over wheel transmissibility at high-frequency small-amplitude inputs, which should draw enough attention in design.