  We present a modified TacTip biomimetic optical tactile sensor design which
demonstrates the ability to induce and detect incipient slip, as confirmed by
recording the movement of markers on the sensor's external surface. Incipient
slip is defined as slippage of part, but not all, of the contact surface
between the sensor and object. The addition of ridges - which mimic the
friction ridges in the human fingertip - in a concentric ring pattern allowed
for localised shear deformation to occur on the sensor surface for a
significant duration prior to the onset of gross slip. By detecting incipient
slip we were able to predict when several differently shaped objects were at
risk of falling and prevent them from doing so. Detecting incipient slip is
useful because a corrective action can be taken before slippage occurs across
the entire contact area thus minimising the risk of objects been dropped.
