Simple and reliable method to estimate the fingertip static coefficient of friction in precision grip

The static coefficient of friction (μstatic) plays an important role in dexterous object manipulation. Minimal normal force (i.e. grip force) needed to avoid dropping an object is determined by the tangential force at the fingertip-object contact and the frictional properties of the skin-object contact. Although frequently assumed to be constant for all levels of normal force (NF, the force normal to the contact), μstatic actually varies nonlinearly with NF and increases at low NF levels. No method is currently available to measure the relationship between μstatic and NF easily. Therefore, we propose a new method allowing the simple and reliable measurement of the fingertip μstatic at different NF levels, as well as an algorithm for determining μstatic from measured forces and torques. Our method is based on active, back-and-forth movements of a subject’s finger on the surface of a fixed six-axis force and torque sensor. μstatic is computed as the ratio of the tangential to the normal force at slip onset. A negative power law captures the relationship between μstatic and NF. Our method allows the continuous estimation of μstatic as a function of NF during dexterous manipulation, based on the relationship between μstatic and NF measured before manipulation.