Protective robot skin with pressure sensitivity and impact absorbability based on electrical charge-accumulative gel

Abstract

Human-robot collaboration (HRC) is effective to improve productivity in industrial fields, based on the robot’s fast and precise work and the human’s flexible skill. To facilitate the HRC system, the first priority is to ensure safety in the event of accidents, such as collisions between robots and humans. Therefore, we propose a protective and collision-sensitive robot skin to guarantee the safety in HRC. The protective robot skin is composed of polyvinyl chloride (PVC) gel, which is a functional material with piezoresistive characteristics and impact absorption capability. In particular, the PVC gel has a distinctive piezoresistive property that the relation between mechanical pressure and electrical resistance is tunable depending on an applied voltage. When a voltage is applied to the PVC gel, the electrical charges are accumulated around the anode and it shows increased piezoresistive sensitivity. In this research, we analyzed the collision detection and impact absorbability characteristics of the protective robot skin based on PVC gel. The results showed that the developed protective robot skin has an enough performance to guarantee the safety in HRC. It was verified for the PVC gel to exhibit the 4.78 times higher sensitivity by simply applying a driving voltage. Moreover, the collision sensing unit could detect the collision within 5 ms, which satisfies the safety requirement for the collaborative robot. To improve the limitation of the developed protective robot skin, we proposed the strain-insensitive pressure sensing mechanism based on the charge-accumulation of PVC gel and liquid metal electrodes. Since the mechanical stretching occurs at the joint part of protective robot skin, the strain-insensitive pressure sensing is essential for the protective robot skin for covering all the parts of the robot. Therefore, the novel structure of strain-insensitive pressure sensing is analyzed for a stable usage of the protective robot skin. To verify the applicability of the proposed mechanism, various physical characteristics are investigated including the strain-insensitivity, material reactivity and temperature stability. In addition, we suggested and verified the direction-detectable pressure sensing mechanism. The location and direction awareness of pressure sensing of the protective robot skin is essential to establish avoidance strategy of the robot manipulator. Therefore, we designed a direction-detectable pressure sensing unit and applied it to the protective robot skin. The developed protective robot skin was applicated to the robot gripper in a wearable form and it was useful to detect a collision and absorb an impact even for the deformable parts. When the robot gripper collided with the human-modeled testbed, the impact force was decreased dramatically by using the protective skin. Furethermore, when the robot gripper with the protective skin collided with various objects and human bodies, they were safely protected thanks to the impact absorption and collision detection of the skin. For future work, we will develop a new-type of robot safety skin with the pressure sensing and proximity sensing ability based on PVC gel. Moreover, the robot collision avoidance algorithm will be analyzed to establish fast avoiding strategy and minimize the human damage in collision situation. Based on these technologies, we can develop a universal protective robot skin and construct a safety system for human-robot collaboration.