Study On Modeling And Simulation Of Dynamic Process For Vehicle Self-excited Shimmy

It is well known that shimmy is the continued swing of front wheels around thekingpin and occurs when certain vehicles run in certain conditions. Shimmy is influenced bymany factors, and its mechanism is very complex. In the early stages of design anddevelopment it is difficult to predict and control shimmy. Shimmy is one of difficultproblems about chassis design.Shimmy is divided by domestic and foreign experts into compulsive shimmy andself-excited type. For compulsive shimmy, the main research is about the resonancephenomenon caused by external periodic excitation forces. For the self-excited type shimmy,the limit cycle oscillation phenomenon is caused by the nonlinear components in the system,such as tire cornering nonlinear features, tire relaxation effects, steering stem friction,clearance, etc. By shimmy study research found that self-excited type shimmy simplifiedshimmy mathematical model for eigenvalue analysis to identify the natural vibration modesof the system or the stability criterion for stable analysis and the research on self-excitedtype to shimmy dynamic process simulation is relatively small. The existing vehicledynamics models lack the impact of self-excited type shimmy key link, the model cannotsimulate the dynamic process of self-excited type shimmy.Based on the above-mentioned status quo, on the basis of the key aspects of research onself-excited type shimmy, try to apply dynamic simulation method to explore the self-excitedtype shimmy simulation, focusing on the key issues of self-excited type to shimmy dynamicprocess modeling, the balance of the dynamics simulation system, self-excited shimmy dynamic process simulation and the key factors.First, the key issues of self-excited type shimmy dynamic process modeling. Some ofthe key aspects of the vehicle systems have important implications for self-excited typeshimmy, in order to make the model have the ability to simulation of self-excited typeshimmy, needing accurate modeling of these key links in the model.(1) The coordinationability of left and right steering wheels has a great impact on shimmy. This paper hasestablished a complete dynamic model of the steering system. Left and right wheels, axles,steering tie rod, rack form a complete dynamical systems. Ackerman agencies achieve theforce input to establish about the mechanical contact between the wheels, adaptivecoordination of left and right wheels.(2) Static friction in steering system has the ability toinhibit shimmy. Dry friction static and dynamic friction state switching can cause changes inthe system eigenvalues and the natural frequency of system. Drawing D.Karnopp the frictionmodel to establish static and dynamic friction separation of steering kingpin dry frictionmodel that accurately describes the static friction characteristics of the steering system andachieve the simulation of the dead zone, so that the steering system model to have a certainability to resist outside interference.(3) The elastic characteristics of the carcass has animportant impact on shimmy. This paper established a wheel model respecting carcassflexibility. Simplifying the wheel rim and carcass, the introduction of the carcass springdamping and carcass dynamics, an accurate description of the elastic characteristics of thetire, achieve the force at the wheel center input to the steering system dynamics andaccurately calculate the tire ground imprinted at the state of motion.(4) In order to modelsimulation in non-level road, non-level road detection algorithm, this paper establishesdetection model for non-level road, real-time detection of the elevation and the normalvector of the point of contact of tire and non-level road, make the model applicable to thesimulation of non-level road.Second, research the equilibrium method for dynamic simulation system. The initialstate is a necessary prerequisite to stimulate self-excited type shimmy. In order to determinethe correct vehicle model the initial state, the balance of the dynamics simulation system were studied and proposed a balanced approach in full working condition.(1) Traditionalstatic balance method applies only to the wheel ground contact, while the special conditionsthat the wheels are off the ground cannot be solved. On base of the Powell method establishoptimization algorithm about wheels off the ground conditions, body vertical displacementdegrees of freedom are as the optimization objective to achieve the balance of the wheelfrom the ground conditions.(2) Owing to the Jacobian matrix analytic expression must becalculated in the traditional non-linear iterative algorithm-Newton Raphson iterativemethod. This paper established a quasi-Newton non-derivative iteration method, the use ofalternative means to solve the system Jacobian matrix, so that the iterative algorithm is notdifficult to obtain the limit of the analytical expressions of the Jacobian matrix.(3) Balancedapproach divided the unknown variables into the boundary conditions and initial state, theouter loop optimization method for solving the boundary conditions, the Inner loop with anonlinear iterative algorithm for the initial state.Third, the study of dynamic process simulation for self-excited shimmy. Establishvehicle dynamics model, experimental verification of the correctness of the modelsteady-state performance. Through the simulation of typical operating conditions, verify themodel with the simulation of self-excited type shimmy dynamic process capacity, analyzethe importance of some key aspects of modeling self-excited type shimmy through dynamicprocess simulation, conduct simulation and analysis about the key factors of self-excitedtype shimmy, including: the size of outside incentives, carcass elasticity, inertia of thesteering system, caster angle, vehicle speed, the following conclusions are summed up:(1)Outside incentives must be large enough, the self-excited type shimmy can be excited.(2)The smaller of the elasticity of the carcass is, the more prone to self-excited type shimmy.(3)The greater the inertia of the steering system is, the more prone to self-excited type shimmy.(4) The greater the Kingpin inclination is, the more prone to self-excited type shimmy.(5)Self-excited type shimmy has certain speed range, shimmy will not occur when less than thisspeed range. The obtained simulation conclusions and engineering experience is consistent,further validate the model more accurate simulation of self-excited type shimmy dynamic process.