  Vision-based haptic sensors have emerged as a promising approach to robotic
touch due to affordable high-resolution cameras and successful computer-vision
techniques. However, their physical design and the information they provide do
not yet meet the requirements of real applications. We present a robust, soft,
low-cost, vision-based, thumb-sized 3D haptic sensor named Insight: it
continually provides a directional force-distribution map over its entire
conical sensing surface. Constructed around an internal monocular camera, the
sensor has only a single layer of elastomer over-molded on a stiff frame to
guarantee sensitivity, robustness, and soft contact. Furthermore, Insight is
the first system to combine photometric stereo and structured light using a
collimator to detect the 3D deformation of its easily replaceable flexible
outer shell. The force information is inferred by a deep neural network that
maps images to the spatial distribution of 3D contact force (normal and shear).
Insight has an overall spatial resolution of 0.4 mm, force magnitude accuracy
around 0.03 N, and force direction accuracy around 5 degrees over a range of
0.03--2 N for numerous distinct contacts with varying contact area. The
presented hardware and software design concepts can be transferred to a wide
variety of robot parts.
