Enhancing Sensitivity Across Scales with Highly Sensitive Hall Effect-Based Auxetic Tactile Sensors

In recent years, Within such landscape of human–robot interaction (HRI) and human–machine interaction (HMI), tactile sensors have emerged as indispensable components, especially in fields requiring the exchange of tactile information among interacting entities. Diverse research initiatives have been initiated with the aim at enhancing sensitivity and resolution by delving into sensing mechanisms, coupled with the exploration of materials to increase the effectiveness of these mechanisms. Among these, Hall sensing-based (or magneto) tactile sensors have recently garnered significant attention due to the impressive spatial resolution achievable with a single Hall sensor: traditional resistive or capacitive tactile sensors, in contrast, require a substantial number of electrode arrays to achieve high spatial resolution, resulting in structural complexity with numerous input/output (I/O) elements. Moreover, these sensors face limitations in sensing range, as substantial external forces can interrupt their conductivity paths. Importantly, issues related to the reproducibility of stable sensing performance arise due to poor adhesion at the interface between electrode arrays and the underlying active soft layer composed of elastomers and receptors.<br/><br/>In wearable applications, there is a growing trend to directly integrate auxetic structures into tactile sensors, largely because of their crucial function in enhancing sensitivity and flexibility at the interface with human or robotic skin. Diverse auxetic structures have found application across various tactile sensing mechanisms. Nevertheless, challenges persist in addressing issues related to the interface of electrode arrays and the limited force range for interrupting the conductivity path. Ongoing efforts are dedicated to overcoming these challenges and further enhancing the performance of auxetic-based tactile sensors to ensure improved usability and effectiveness in diverse applications.<br/><br/>We present a Hall effect-based auxetic tactile sensor (HEATS) that integrates a magnetic auxetic structure with a negative Poisson's ratio into a homemade Hall sensor array for the first time, as we have recently published in ref.[1]. HEATS introduces the capability of real-time calibration of the Earth's magnetic field by integrating a reference Hall sensor near a measuring Hall sensor. This innovative approach enables the collection of signals solely from the structural deformation of the active soft layer, addressing a current weakness in Hall effect-based tactile sensors. Utilizing finite element analysis (FEA), we conducted a comprehensive analysis of the mechanical properties of various auxetic structures and identified the optimal pattern—a rotating square plate—that facilitated achieving higher sensitivity. Additionally, our observations indicate that as the Poisson's ratio of the auxetic structure becomes more negative, the sensor's sensitivity increases significantly. The experimental results showcase remarkable improvements in sensitivity for HEATS across a broad sensing range, with a 20-fold and 10-fold enhancement at tensile rates of 0.9% and 30%, respectively, compared to the non-auxetic sensor. Finally, our experiments demonstrated the successful detection of changes in muscle movement and motion associated with physical activities using HEATS. By strategically placing HEATS on various body parts, we established the feasibility of accurately capturing these changes in both humans and robots. Moreover, building upon the preceding findings, we conducted additional experiments to assess HEATS on the knee joint of a robot, where we verified that the values exhibited a linear change corresponding to the rotation angle of the robot's joint. Our technology represents a significant breakthrough in tactile sensor technology, offering the potential for enhanced sensitivity and versatility for diverse applications.<br/><br/>[1] Y. Yun, et al. Advanced Intelligent Systems, 2400337(2024).