NUSense: Robust Soft Optical Tactile Sensor

Abstract

While most tactile sensors rely on measuring pressure, insights from continuum mechanics suggest that measuring shear strain provides critical information for tactile sensing. In this work, we introduce an optical tactile sensing principle based on shear strain detection. A silicone rubber layer, dyed with color inks, is used to quantify the shear magnitude of the sensing layer. This principle was validated using the NUSense camera-based tactile sensor. The wide-angle camera captures the elongation of the soft pad under mechanical load, a phenomenon attributed to the Poisson effect. The physical and optical properties of the inked pad are essential and should ideally remain stable over time. We tested the robustness of the sensor by subjecting the outermost layer to multiple load cycles using a robot arm. Additionally, we discussed potential applications of this sensor in force sensing and contact localization.

Figures (6)

• Figure 1: NUSense tactile sensor. a) Sensing principle: surface displacement caused by a small axi-symmetric punch (load); the soft layer is made of silicone rubber that is dyed with color inks. The resulting pattern is projected onto camera's pixels. b) A robot arm with the sensor and the indenter for the point load. c) Surface construction using B-spline patches from camera images during resting state and under mechanical load. The region of shear deformation creates an annulus. Shear is estimated as the distance between the two surfaces represented by B-spline control nets $C_0$ and $C_1$.
• Figure 2: Bio-mimetic sensing principle: a) Human glabrous skin. The epidermis has ridges projecting into the dermis in the human glabrous skin. b) The structure and mechanics of the artificial sensing skin.
• Figure 3: Left: Assembly of NUSense tactile sensor with a camera and elastomer. Right: The silicone molding-based fabrication of NUSense. (a) Manufaturing process with all molding layers, (b) front and (c) back of the fabricated elastomer.
• Figure 4: Tactile image processing: A. Raw Tactile image from fish eye lens camera, B. Undistorted image, C. Quadrilaterals as a polygons around yellow markers, D. Midpoints extracted as control points for the B-Spline surface with magnified view, E. Sampled points from target B-Spline surface, F. Sampled points from reference B-Spline surface.
• Figure 5: Left: Experimental Setup. Indenters with three different areas of contact. Normal force response in the ranges 1 N to 3 N and 3 N to 8 N was obtained by (a) round and (b) flat tips, respectively. (c) Long tip was used for edge detection. (d) Torn off elastomer sensor.

Theorems & Definitions (1)

• Definition 1: Shear Strain