Ball sensor rattle: Experimental and numerical sensitivity study for seatbelt applications

A ball sensor, part of virtually any seatbelt retractor, is investigated experimentally and by numerical simulation to predict its acoustical behavior under vertical excitation. Impact forces among mating surfaces are computed with flexible multibody system analysis, employing Craig-Bampton modes to approximate the vibrations of acoustically relevant substructures. Acoustic radiation is determined by the boundary element method. Two distinct rattle plateaus are identified on a test rig via acceleration-amplitude ramp-ups with harmonic base frequency. These plateaus are caused by (i) ball lift-off bouncing against the sensor cradle and (ii) impact of the lever on the clutch disk. The dominant peaks in the sound pressure spectrum (SPL) are associated with system-eigenmodes and are therefore independent from the excitation as long as this excitation is strong enough. On sensitivity analysis, variation of the stiffness of sensor housing and lever, sensor gap, excitation frequency and sensor mass does not cause significant SPL reduction. Conversely, changing contact stiffness between sensor ball and cradle reduces SPL within certain frequency bands. This observation warrants further research to improve robustness of retractors to rattle. The methodology presented predicts rattle induced noise reliably, providing new impetus to take optimization of retractor design to a higher level.