Flexible Three‐Axial Force Sensor for Soft and Highly Sensitive Artificial Touch

摘要

A soft tactile sensor able to detect both normal and tangential forces is fabricated with a simple method using conductive textile. Owing to the multi-layered architecture, the capacitive-based tactile sensor is highly sensitive (less than 10 mg and 8 μm, for minimal detectable weight and displacement, respectively) within a wide normal force range (potentially up to 27 N (400 kPa)) and natural touch-like tangential force ranges (from about 0.5 N to 1.8 N). Being flexible, soft, and low cost, this sensor represents an original approach in the emulation of natural touch. Furthermore, in addition to being flexible and mechanically robust over a wide pressure range, the sensor is also able to detect in-plane forces. We investigated the performance of the device under tangential stimulation while applying a static normal force Fz of, either, 0.5 N or 1 N; thus mimicking the tactile indentation needed to induce a tangential force through horizontal displacement (Figure 3d). During the experiments, observations of the cross section under an optical microscope suggest that no shear stress is induced in the dielectric fluorosilicone film and that the top electrode is free to slide over the bottom set of electrodes (see video S6, Supporting Information). In particular, the latter aspect is due to the low friction properties of the fluorosilicone. Independently of the static normal applied force, a 0.4 N tangential force is needed before a significant capacitance variation can be observed (see Figure 3d). This minimum force could reflect the shear strain induced in the thick PDMS layer during the transmission of the tangential displacement. Indeed, such shear force could be needed to initiate the sliding of the top electrode over the bottom electrodes. Then, starting from about 0.4 N, a second regime is observed in which varies linearly with the applied tangential force. As a higher static normal force must be applied initially to the device, a higher friction force is needed before sliding occurs, and we can observe that, for the same capacitance variation (same overlapping area of the electrodes), doubling the initial load applied to the sensor contributes to a tangential force divided by a factor two. Minimal detected tangential displacements of 8 μm and 14 μm, under 1 N and 0.5 N initial static normal forces, respectively, were evaluated. It should be noted that this result is about one order of magnitude lower than minimal displacements reported in the literature (i.e., 60 μm).13 Linear behavior is observed within the 0.4–1.2 N and 0.8–1.6 N ranges, for 0.5 N and 1 N static normal forces, respectively. The slope represents the sensitivity of the sensor in the tangential mode. The obtained performances were 0.32 ± 0.02 N−1 and 0.34 ± 0.02 N−1 for the 0.5 N and 1 N initial contact force curves, respectively. Hence, we may conclude that, due to the design of the sensor, in this regime the tangential sensitivities are almost independent of the normal component of the applied force, and they are equal to about 0.3 N−1. Furthermore, when increasing the tangential force, a deviation of the response from the linear behavior is observed, suggesting that the overlapping area of the electrodes is not dictated by a sliding mechanism of the two sets of electrodes, rather it is limited by the deformable properties of the whole device, in particular its stretchability. To conclude, the design and the materials used to develop our sensor generate high performing features for the next generation of three-axis soft tactile sensors. Although our design is simple, we have achieved an improved level of tactile information to mimic natural touch. In parallel, the materials used for the multi-layer structure converge in obtaining a soft, flexible and highly sensitive device that offers a low-cost technological approach. New perspectives are thus open for the design of soft and smart interfaces, and for all kinds of applications where natural-like tactile