Technical Paper Characterization of automotive shock absorbers using random excitation

In a previous paper [see reference (41, it was showp that the restoring force surfa_ce.(RFS) procedure provides a direct and clear method for characterizing the dynamic properties of.’automotive shock absorber7 ordamper$+ The procedure was based on repetitive harmonic testing of the absorbers at fixed frequency but with varying.amplitud~~The &rent paper describes how the surfaces can be obtainedfrom tests using random excitation. The merits and demerits are discussed relative to the harmonic test procedure. It is shown that thqrandom excitatiobpproach offers a useful alternative but produces force surfaces which are corrupted by small stochastic componerits; an explanatio‘iz of the distortion is given in terms of the mathematical model proposed in the previous paper. The implica-tions for identijication of shock absorbers are discussed. Key words: shock absorbers, non-linear damper, restoring force surfaces, random excitation NOTATION Overdots denote differentiation with respect to time. chamber cross-sectional areas polynomial model parameters linear damping coefficients restoring force surface damping force function Coulomb friction force hysteresis function linear stiffness coefficients mass or effective mass absorber chamber pressures coefficient of linear pressure loss coefficient of linear pressure loss in compres-sion coefficient of linear pressure loss in rebound coefficient of quadratic pressure loss coefficient of quadratic pressure loss in com-pression coefficient of quadratic pressure loss in rebound time chamber volume volumetric flowrate mean volume excitation force displacement ‘hysteresis ’ variable = p1- p 2 hyperbolic tangent model parameters fluid compressibility fluid density phase circular frequency 1