Modeling and Fabrication of Scalable Tactile Sensor Arrays for Flexible Robot Skins

Artificial robot skins capable of multi-modal tactile feedback are anticipated to revolutionize how robots perceive their immediate surroundings and how they collaborate with humans on tasks that are more complicated and dynamic than currently possible. Efforts to develop the tactile skins must tackle significant hurdles related to flexibility, robustness, and scalability of sensors. In this paper, a flexible tactile sensor array is developed to address these hurdles via a novel sensor geometry and thermal compensation technique. This tactile skin relies on a polymeric piezoresistive array of strain gauges with a novel star-shaped geometry. Due to their radial symmetry, the proposed strain gauges exhibit a biaxial response to local strain-stress loading, avoiding "blind spots" in the vicinity of individual tactels. The sensors are fabricated using gold electrodes and piezoresistive polymers (PEDOT:PSS) micro-patterned on a flexible polyimide substrate enclosed in a compliant silicone matrix. The sensor is first modeled using multiphysics finite elements software to guide the design choices for a variety of geometric parameters under various load profiles and locations with respect to the gauge's center. A novel homogeneous thermal compensation technique is proposed, which packages the sensors on the opposite sides of a flexible carrier. This thermal compensation technique achieves a 10-fold attenuation in temperature drifts as well as a double sensitivity in terms of strain measurement. After numerical modeling, a 4 x 4 tactile array with the optimal geometry obtained through simulation was fabricated and interfaced to a custom-made electronic data acquisition system. Samples were tested using an automated dynamic force experimental test bed. The tactile skin array was experimentally characterized in terms of force sensitivity, spatial response, dynamic bandwidth, and temperature drift. The array exhibits a sensitivity of 1125 nV/N in the range of 0-0.5 N, a sensitivity of 412 nV/N in the range of 0.5-23 N, a hysteresis of 11.13%, a dynamic bandwidth of 1.49 Hz, and a temperature sensitivity of 6 nV/degrees C, averaged among individual sensors.