Curvature sensing with a spherical tactile sensor using the color-interference of a marker array

The only way to perceive a small object held between our fingers is to trust our sense of touch. Touch provides cues about the state of the contact even if its view is occluded by the finger. The interaction between the soft fingers and the surface reveals crucial information, such as the local shape of the object, that plays a central role in fine manipulation. In this work, we present a new spherical sensor that endows robots with a fine distributed sense of touch. This sensor is an evolution of our distributed tactile sensor that measures the dense 3-dimensional displacement field of an elastic membrane, using the subtractive color-mixing principle. We leverage a planar manufacturing process that enables the design and manufacturing of the functional features on a flat surface. The flat functional panels are then folded to create a spherical shape able to sense a wide variety of objects.The resulting 40mm-diameter spherical sensor has 77 measurement points, each of which gives an estimation of the local 3d displacement, normal and tangential to the surface. Each marker is built around 2 sets of colored patches placed at different depths. The relative motion and resulting hue of each marker, easily captured by an embedded RGB camera, provides a measurement of their 3d motion. To benchmark the sensor, we compared the measurements obtained while pressing the sensor on a curved surface with Hertz contact theory, a hallmark of contact mechanics. While the mechanics did strictly follow Hertz contact theory, using the shear and normal sensing, ChromaTouch can estimate the curvature of an object after a millimeter-size indentation of the sensor.