Embedding Soft Material Channels for Tactile Sensing of Complex Surfaces—Light Propagation Modeling

In previous work, “embedding softness into structure enabling distributed tactile sensing of high-order curved surfaces” (ESSENCE) technology, a general principle for accurate and robust tactile sensing on arbitrary surfaces, has been developed. Each tactel consists of a soft material sensing channel, a light source, and an optical receiver. Force applied on the tactel is transformed through the channel to cause an axial protrusion of the soft material. By measuring the change in light intensity caused by the protrusion, the magnitude of the force can be determined. However, multiple receivers share a single light source, resulting in lighting conditions in the region of interest (ROI) to vary from place to place, which in turn affects the consistency of sensing performance. To overcome this challenge, a freeform reflective funnel of the light source that is coated with a layer of high reflective material is designed to reshape the light distribution. Theoretical analysis and experimental results show that the proposed method improves the sensing consistency. For a $3\times 3$ array-type sensor, the standard deviations of the force measuring range, linearity, and sensitivity before and after using the freeform surface are 0.29 and 0.08 N, 4.58% and 4.04%, and 0.016 and $0.007~{\text {N}}^{-{1}}$ , respectively.