Bio-inspired Tactile Sensing: Analysis of the inherent characteristics of a vibrissa-like tactile sensor

Abstract

Tactile sensing is a fast-developing research area that is important, e.g., for autonomous robot systems in the context of path planning and navigation in uncertain terrains. One possibility for novel enhanced designs of tactile sensors is to analyze and adapt natural paragons. Rodents like rats have tactile sensory hairs, so-called vibrissae, on both sides of their muzzle. These hairs exhibit a sophisticated structure and geometrical shape. The slender, tapered, and inherent curved hair shaft is supported by a hair follicle that includes mechanoreceptors. Touching an object with their vibrissae, rats can recognize the shape of an object or determine properties of its surface texture by evaluating only the signals inside the hair follicle. The present work contributes to the overall goal to unfold the ability of natural vibrissa with view to applications in engineering like surface metrology or autonomous robots. The vibrissal system is described in detail, analyzed, and interpreted using the idea of a biomechatronic system and stimulus leading apparatus. The properties of a natural vibrissa contribute in a complex manner and in various ways to its functionality and determine its inherent characteristics. Therefore, properties of a natural vibrissa are systematically adapted to an artificial tactile sensor. Using this artificial vibrissa-like sensor, it is shown that three different kinds of information about the scanned object are present in the captured data: the overall object shape, a macroscopic, and a microscopic surface texture. The regarding information is encrypted in the signals at the base of the sensor and must be processed to recognize each type of information. To support this process, it is found that a larger distance to an object, around 80\% of the length of the sensor shaft, permits a good detectability of properties regarding the macroscopic surface texture. For a moderate distance, e.g., 60\% of the length of the sensor shaft, the detection of a microscopic surface texture works best. Finally, a closer distance, below 45\% of the length of the sensor shaft, is advantageous for a recognition of the overall shape. These characteristics -strongly depending on the distance- are closely related to the elasticity of the sensor shaft. For example, scanning a close object with distinct macroscopic surface texture, proper detection of the macroscopic surface texture is not possible because the bending induced curvature of the sensor shaft prevents contact between the sensor shaft and small shapes of macroscopic surface features like grooves and gaps. Therefore, the deformed sensor shaft fulfills the task of a morphological filter. The diameter of the sensor defines the limit between macroscopic and microscopic surface texture. Surface texture elements smaller than this limit belonging to the microscopic surface texture. Therefore, also the tip diameter is an inherent characteristic of the sensor. A further inherent characteristic is found while scanning a microscopic surface texture and retracting the sensor shape in the way that the concave side of the deformed elastica points in the displacement direction.\newpage This configuration and displacement cause an amplification of the recorded support reactions. Following these kinds of ideas, the inherent characteristics of the sensor system are analyzed in the present work.