SkinAxis: 3D force-sensing soft robotic skins balancing touch sensitivity and impact protection for safe pHRC

Physical human-robot collaboration (pHRC) requires robotic skins that simultaneously ensure impact safety, as established by ISO/TS 15066, and preserve touch sensitivity for delicate human interactions. Existing soft skins enhance compliance and multimodal sensing, but their design is rarely guided by a quantitative evaluation of the trade-off between touch sensitivity and impact mitigation, and systematic impact testing against ISO/TS 15066 limits is often absent. The SkinAxis modular soft robotic skin addresses this gap by embedding a compact 3D force sensor in the soft material, directly enabling touch localization, and using a material-based design that jointly optimizes quasi-static touch sensitivity and impact safety. This purpose is achieved by combining the mechanical properties of elastomeric materials, selected through dedicated mechanical tests, to identify a stiff elastomer that efficiently transfers forces to the sensor and a highly dissipative elastomer that absorbs impact energy. To highlight the trade-off achieved by the selected materials, two main designs are benchmarked: a single-material baseline and a hybrid patch with a compliant surface, a rigid load-transmission plate, and a stiff sensor support sleeve. The designs are compared in terms of quasi-static force sensitivity $\left(k_{\epsilon }\right)$ , touch localization performance, and maximum pressure, as well as peak-force and transmitted-energy reduction efficiencies ( $P_{\text{max}}$ , ${\eta }_{\text{F}}^{\left(-\right)}$ , ${\eta }_{\text{E}}^{\left(-\right)}$ ), measured respectively in quasi-static compression, force-indentation, and impact experiments on a human head surrogate (PILZ PRMS setup). The results demonstrate that the hybrid solution achieves higher quasi-static force sensitivity $\left(k_{\epsilon }=0.089\right)$ , a more reliable model for touch localization; it reaches 44.66% reduction in peak force and 18.95% reduction of transmitted energy under impact at cobot's operational velocity of 1 $\frac{m}{s}$ , with a maximum transmitted pressure of 1.11 MPa in accordance with ISO/TS 15066 limits. In parallel, finite element simulations predict the forces transmitted to the embedded force sensor in quasi-static compression and impact conditions, supporting the experimental findings.