Mechanics and Morphological Compensation Strategy for Trimmed Soft Whisker Sensor

Abstract

Recent studies have been inspired by natural whiskers for a proposal of tactile sensing system to augment the sensory ability of autonomous robots. In this study, we propose a novel artificial soft whisker sensor that is not only flexible but also adapts and compensates for being trimmed or broken during operation. In this morphological compensation designed from an analytical model of the whisker, our sensing device actively adjusts its morphology to regain sensitivity close to that of its original form (before being broken). To serve this purpose, the body of the whisker comprises a silicon-rubber truncated cone with an air chamber inside as the medulla layer, which is inflated to achieve rigidity. A small strain gauge is attached to the outer wall of the chamber for recording strain variation upon contact of the whisker. The chamber wall is reinforced by two inextensible nylon fibers wound around it to ensure that morphology change occurs only in the measuring direction of the strain gauge by compressing or releasing pressurized air contained in the chamber. We investigated an analytical model for the regulation of whisker sensitivity by changing the chamber morphology. Experimental results showed good agreement with the numerical results of performance by an intact whisker in normal mode, as well as in compensation mode. Finally, adaptive functionality was tested in two separate scenarios for thorough evaluation: (1) A short whisker (65 mm) compensating for a longer one (70 mm), combined with a special case (selfcompensation), and (2) vice versa. Preliminary results showed good feasibility of the idea and efficiency of the analytical model in the compensation process, in which the compensator in the typical scenario performed with 20.385% average compensation error. Implementation of the concept in the present study fulfills the concept of morphological computation in soft robotics and paves the way toward accomplishment of an active sensing system that overcomes a critical event (broken whisker) based on optimized morphological compensation.

Tactile Resilience of Sensory Whisker by Adaptive Morphology

“…A comprehensive understanding of the correlation between sensor morphology (both geometrical and material aspects) and the robotic device outcome is crucial to establishing a proper tactic for damage compensation. In this regard, we previously introduced an analytical model that estimates the strain gauge output for a wide range of contact conditions and morphology states [1]. Based on this, an optimized whisker's layout (upon pressure Q) equivalent to the desired sensitivity is chosen to perform the tactile compensation task.…”

“…2) at each equilibrium state of the whisker body. Its expression can be fully reviewed in [1]. In the meantime, the second law is used to describe body's deflection δ by assessing the strain energy stored in each whisker body's region and applying superposition principle in beam deflection:…”

“…with δ i (i = 1, 2, 3) indicates the contribution of geometrical parameters of the region i to total deflection δ(a) (see full expressions in [1]). Also, I i (x) and E i represent the second moment of cross-sectional area and Young's modulus for region i, respectively.…”

“…Most of the attention to this research area aims to enrich information gain, it is lacking elaboration on how variable morphology can be utilized to remain sensing ability against unexpected damage to the original sensor body (e.g., being broken, eroded). Our previous work [1] tackled this issue on a whiskered tactile sensor. By actively changing the sensor's morphology thanks to air regulation of the embedded air chamber, the tactile information (mechanical strain) perceived by the broken one was compensated to get close to that of the same stimuli before being damaged [2].…”

“…Such approach, while rather complex, is not always applicable to mechanical damage of the robot body, especially parts that continuously interact with the surrounding environment. In the previous works [1] [2], we introduced an artificial whiskered sensor that exhibited resilience against physical damage by active change of its morphology around the placement of sensory elements (strain gauges), which allowed compensation of location sensing when the whisker was trimmed. In this paper, we extend the approach by using the whisker sensor for texture discrimination tasks.…”