Investigations On Mechanism Of Single Steering Wheel Shimmy And Optimization Design

The steering wheel shimmy is a serious problem of wheeled device dynamics system. The mechanism of shimmy and stability analysis of different dynamics models were developed by simplified single steering wheel system. Meanwhile the mathematic steering wheel models were valid by shimmy test bench. The linear and nonlinear stability characters of steering wheel model were analyzed by stability theory of ordinary differential equation and Hopf bifurcation theory of nonlinear dynamics. The methods that can avoid shimmy were summed up from engineering viewpoints. The shimmy problem of steering wheel in a lever propulsion wheelchair has been solved based on the methods and conclusions of analysis of stability and optimization design. The main researches are as follows:The single Degree of Freedom (DoF) model of the typical steering wheel with vertical axis and inclined axis was established and their stability analyses were conducted respectively. The linear dynamics model of the typical steering wheel system was established to study the stability effect with different parameters. In particular, an analytical solution has been found for a system where the wheel has a mechanical trail and both the yaw and lateral damping of the hinge point are taken into account. The stability analysis of one and two DoFs typical steering wheel model was conducted using root locus theory and Routh Criterion. Furthermore we are applying control theory to explain the mechanism of shimmy in frequency domain.Lever propulsion wheelchair steering wheel dynamics model of3DoFs was established by analytic mechanics. This model is derived from Lagrange equation and its numerical solutions are solved by Ordinary Differential Equation (ODE) method. And the derivatives models are introduced by simplifying the3DoFs caster wheel model. Its different stability characters were given by changing the geometry, mass and damping parameters of the steering wheel system.In the experimental stage, a new steering wheel shimmy test bench was established. The whole structure and the working process of the test bench were introduced. Vibration measurements successfully verified the theoretical results obtained from mathematical model as well as the dynamics behavior of towed wheels within different cases.Steering wheel model bifurcation character of the self-excited shimmy was analyzed by Hopf bifurcation theory of nonlinear dynamics and by stability theory of ordinary differential equation. The computation results indicate that the self-excited shimmy is a stable limit cycle occurring after Hopf bifurcation and it can occur on the wheelchair with a certain set of parameters. The shimmy angle and the range of critical velocities of the wheelchair have effects on shimmy and the intensity of shimmy is also able to represent good or bad stability characters.To avoid and eliminate shimmy, some new practical suggestions are offered due to the effects of system parameters on shimmy, for instance, by increasing length of the trail, applying a damped bearing and adopting reasonable parameters of geometry and mass.To avoid the shimmy in the design stage, the genetic algorithm was applied to optimization design of the steering wheel system. According to the results obtained from the experiment to analyze the stability characters. Shimmy can be avoided after replacing a damping bearing with a set of reasonable parameters. Using the same method, the optimization of a real wheel chair was conducted. Base on the stability analysis and optimization design conclusion to solve the shimmy problem of steering wheel in a lever propulsion wheelchair.