  Combining 3D vision with tactile sensing could unlock a greater level of
dexterity for robots and improve several manipulation tasks. However, obtaining
a close-up 3D view of the location where manipulation contacts occur can be
challenging, particularly in confined spaces, cluttered environments, or
without installing more sensors on the end effector. In this context, this
paper presents StereoTac, a novel vision-based sensor that combines tactile
sensing with 3D vision. The proposed sensor relies on stereoscopic vision to
capture a 3D representation of the environment before contact and uses
photometric stereo to reconstruct the tactile imprint generated by an object
during contact. To this end, two cameras were integrated in a single sensor,
whose interface is made of a transparent elastomer coated with a thin layer of
paint with a level of transparency that can be adjusted by varying the sensor's
internal lighting conditions. We describe the sensor's fabrication and evaluate
its performance for both tactile perception and 3D vision. Our results show
that the proposed sensor can reconstruct a 3D view of a scene just before
grasping and perceive the tactile imprint after grasping, allowing for
monitoring of the contact during manipulation.
