Design and Human Evaluation of Tactile Withdrawal Reflexes for a Skin-Covered Robot Arm

2026-07-24

AI Summary

This study addresses the design of human-like, safe, and user-acceptable nociceptive response mechanisms for robotic arms endowed with whole-body tactile sensing. The authors implement a complete reflexive control pipeline that maps pressure inputs from compliant artificial skin to biomimetic withdrawal motions, and systematically compare three strategies for the first time: uniform joint motion, biologically inspired position-dependent joint-space withdrawal, and Cartesian-space withdrawal along the contact surface normal. A nonlinear continuous model translates skin pressure into pain gain, and user evaluations are conducted using the Godspeed questionnaire alongside a custom survey. Results indicate that the uniform withdrawal strategy is significantly superior in perceived human-likeness, animacy, preference, and naturalness; Cartesian withdrawal best aligns with the physical contact context; and the biologically inspired approach demonstrates no clear advantage.

Abstract

Nociception is a protective biological mechanism that links harmful stimulation to a reaction. This paper investigates artificial nociception for a robotic arm with whole-body tactile sensing. We present a complete pipeline that maps pressure changes from sensitive skin on a robot manipulator to bio-inspired withdrawal motions. The system first converts skin pressure into a scalar pain gain using a nonlinear continuous model. We compare three reflexes: (i) uniform reflex moves four robot joints by a fixed amount, whereby the withdrawal is approximated by a movement of the arm "toward the base", independent of where the robot was touched; (ii) biologically motivated location-dependent joint-space withdrawal derived from human withdrawal reflex characteristics; (iii) Cartesian space withdrawal along the surface normal of the contacted skin pad. All behaviors are integrated in a reflex controller that interrupts the task, executes the withdrawal, and returns to a pre-contact pose. A user study with 15 participants compared the strategies using Godspeed questionnaire subscales, custom perceived-naturalness and safety items, forced-choice comparisons, and qualitative feedback. Interestingly, participants rated more highly the uniform reflex behavior over one or both competitors on the anthropomorphism, animacy, and likeability Godspeed subscales and on the Naturalness and Realism custom scale. When asked to compare the conditions, the uniform reflex was scored best in "felt safest", "most human-like", and "most natural". This suggests that predictability of the robot behavior is key for user acceptance. The Cartesian reflex was judged the most appropriate reaction to touch. The bio-inspired reflex did not lead any evaluated measure. This may be partly attributed to the embodiment gap between the robot arm and human arm and participants having different expectations from a robot manipulator.

Problem

• tactile withdrawal reflex
• artificial nociception
• human-robot interaction
• bio-inspired robotics
• user evaluation

Innovation

• artificial nociception
• tactile withdrawal reflex
• bio-inspired robotics
• human-robot interaction
• whole-body tactile sensing