Vibration and sound generation between two surfaces that are smoothly sliding against each other stem from different surface properties. Although surface characterisation through acoustic responses has been largely addressed in tactile sensing, friction-induced vibrations and sounds between soft surfaces have hitherto received little attention. Scientists initiated the EU-funded project 'Acoustics of friction under light loads' (BUAFULL) to further advance studies in this field. BUAFULL related the tribological characteristics of the rubbing surfaces (mainly roughness, adhesion and geometry) to the acoustic response under light loads. Its ultimate aim was to emulate friction finger lightly rubbing a soft material. To this end, scientists developed a lightweight pendulum that acts like a finger, where a rotating beam slides on the surface area under the finger to provide the touch. A lift mechanism in the pendulum controlled the contact force. Studies showed that the pendulum lifted the surface by an amount proportional to the friction developed between them. The pendulum motion was found to be constrained, also involving discontinuous contacts. In addition, friction-induced non-linear responses such as wedge locking, jam and swerve could have an impact on the expected pendulum dynamics. Correlating the pendulum dynamic response to the friction provided important new insight into friction measurements between soft surfaces under light loads. Another significant finding was that the measurement set-up was able to separate the adhesive friction component. Dynamic modelling of a constrained pendulum described such relationships and also provided design guidelines to avoid wedge locking of the pendulum tip with the surface. Project developments are fundamental to further advancing a number of areas. These include tactile sensing, haptic systems used in robotic gripping of sensitive objects, and product characterisation ranging from fabric softness to surfactant effects on skin.
Acoustics, friction, vibration, materials, sound generation, surface properties, acoustic responses, tactile sensing, friction-induced vibrations, soft surfaces, light loads, contact force, haptic systems, robotic gripping ,fabric softness