Actively Steered Open On-Limb Robot with Alignment Control and Online Diameter Estimation

Wearable robotics has expanded beyond conventional exoskeletons and prosthetic devices toward compact systems that attach to or move along the human body. Within this field, on-body mobile robots have emerged as a promising platform for healthcare monitoring, haptic interaction, assistance with daily activities, rehabilitation support, and dynamic wearable interfaces. However, on-body locomotion remains challenging because local curvature changes continuously, limb diameters vary, contact is compliant, and the system must preserve user comfort while ensuring safe physical human–robot interaction. A recent study introduced an open on-limb locomotion mechanism based on spherical rollers and passive diameter adaptation. That work also revealed the need to integrate sensing, control electronics, and actuation more tightly within the wearable platform. These limitations motivate the present study, which advances a validated locomotion concept into an embedded wearable mechatronic system evaluated under more anthropomorphic conditions. To overcome these limitations, this paper presents an integrated, second-generation open on-limb robot that incorporates active steering for real-time locomotion and contact alignment. Moving beyond previous passive compliance and rigid 2D bilateral symmetry constraints, we introduce a generalized differential kinematic framework based on a complete Jacobian of the roller centers and coordinate-independent circumradius estimation. This mathematical foundation enables the system to actively leverage the geometric alignment variable as feedback for closed-loop steering corrections. Experimental results on variable-diameter surfaces demonstrate the platform’s ability to maintain longitudinal locomotion, handle non-symmetric link deflections, and perform online diameter estimation. To support reproducibility, all 3D-printable components are made openly available.