Towards Unconstrained Collision Injury Protection Data Sets: Initial Surrogate Experiments for the Human Hand

TL;DR

This work addresses the lack of injury data for unconstrained collisions in physical human-robot interaction, focusing on edged impactors. It introduces a two-pendulum surrogate setup with ex vivo pig feet to model human hands and robot effective masses across a range of masses $m_r$ and velocities $v_r$. The study yields initial injury data for injury types s-i, s-c, and t/b, showing low probabilities at $v_r < 0.5$ m/s but elevated risk with larger $m_r$ and $v_r$, and identifies force thresholds associated with different injuries. The results provide a foundational dataset for risk assessment and real-time safety control in pHRI, while acknowledging limitations such as ex vivo surrogacy and the need for broader human validation and angular-shear considerations.

Abstract

Safety for physical human-robot interaction (pHRI) is a major concern for all application domains. While current standardization for industrial robot applications provide safety constraints that address the onset of pain in blunt impacts, these impact thresholds are difficult to use on edged or pointed impactors. The most severe injuries occur in constrained contact scenarios, where crushing is possible. Nevertheless, situations potentially resulting in constrained contact only occur in certain areas of a workspace and design or organisational approaches can be used to avoid them. What remains are risks to the human physical integrity caused by unconstrained accidental contacts, which are difficult to avoid while maintaining robot motion efficiency. Nevertheless, the probability and severity of injuries occurring with edged or pointed impacting objects in unconstrained collisions is hardly researched. In this paper, we propose an experimental setup and procedure using two pendulums modeling human hands and arms and robots to understand the injury potential of unconstrained collisions of human hands with edged objects. Pig feet are used as ex vivo surrogate samples - as these closely resemble the physiological characteristics of human hands - to create an initial injury database on the severity of injuries caused by unconstrained edged or pointed impacts. For the effective mass range of typical lightweight robots, the data obtained show low probabilities of injuries such as skin cuts or bone/tendon injuries in unconstrained collisions when the velocity is reduced to < 0.5 m/s. The proposed experimental setups and procedures should be complemented by sufficient human modeling and will eventually lead to a complete understanding of the biomechanical injury potential in pHRI.

Figures (11)

• Figure 1: To structurally investigate the severity and probability of occurrence of injuries occurring in unconstrained collision situations, we model the collision scenario using a defined impact geometry, the human effective body part mass, $m_h$, robot effective mass, $m_r$, and the robot's velocity, $v_r$.
• Figure 2: Four collision scenarios: a) unconstrained and dynamic, b) unconstrained and quasi-static, c) constrained and dynamic, and d) constrained and quasi-static.
• Figure 3: Pig specimen for human hand substitution: middle foot with a) proximal and b) distal collision location, representing the human middle hand and wrist region, dew claw with c) proximal and d) distal collision location, representing human fingers.
• Figure 4: Unconstrained impact test setup.
• Figure 5: Experimental procedure for unconstrained collision injury analysis. i) aligning specimen and impactor, ii) motor and wind drag pendulum-II to the desired angle $\alpha$, iii) pendulum is held by brake while motor unwinds the strap, iv) brake is released and pendulum-II collides with pendulum-I while encoder and force sensor measurement is active, v) pendulum-I swings and is caught by the human operator, pendulum-II is stopped by the brake when swinging back.