Simulation and Experimental Assessment of the Usability of the Phase Angle Method of Examining the State of Shock Absorbers Installed in a Vehicle

Abstract

The technical condition of the shock absorbers used in automotive suspension systems is important with respect to vehicle occupants’ comfort and traffic safety. Therefore, much effort has been made for many years to find diagnostic methods that would be more effective. There is a preference for those methods where the shock absorbers do not have to be dismounted from the vehicle. Among those being in use, the ‘forced vibration methods’ have earned the widest acceptance. One of them is the solution where the angle of phase shift between the vertical displacement of the vibration plate and the tyre–plate interaction force is measured. The authors decided to assess this method’s usability by comparing simulation results with the results of experiments run on a prototype diagnostic test stand. They used two ‘quarter-car’ simulation models (linear and non-linear) and experimentally tested suspension systems of two medium-class cars. In the first stage, computations were made in the frequency domain for the linear model with two degrees of freedom, followed by simulations in the time domain, where an analogous but strongly non-linear model was used. In the latter model, the actual characteristic curves (determined during the laboratory measurements) of shock absorber damping, tyre and suspension elasticity, sliding friction in the suspension system, and tyre bouncing were taken into account. The authors have presented the computation results in the form of curves representing the phase angle as a function of the relative damping in the suspension system under test for the two medium-class cars. The suspensions of the cars had similar inertia properties but different characteristics of the spring and damping forces. The cars also differed from each other in the observed and measured level of the friction forces (twice bigger). The computation results obtained for the linear and non-linear model and the experiment results show a similar qualitative nature. In quantitative terms, however, they differ significantly from each other. The role of non-linearities is important. Nevertheless, the results show monotonicity and noticeable sensitivity to changes in the technical condition of the shock absorbers, which is an essential and desirable feature in diagnostics.