Bioinspired additive manufacturing material optimization for increased stiffness and improved strain sensing in robotic limbs

Robots traversing uneven terrain may require precise calculation of limb kinematics (e.g. foot position from joint angles and segment dimensions) and sensing of force for adaptive walking control. To facilitate these calculations, robots often have very rigid limb segments with a stiff load cell between the limb and end effector (e.g. foot). Insects also must compute kinematics and measure force when walking, but sense forces quite differently, using strain sensors embedded in their exoskeletons distributed across their legs to infer force. Due to this configuration, more rigid leg segments would simplify the computation of kinematics (e.g. foot position), but would reduce the strain that could be measured to infer force, creating a trade-off between structural stiffness and force sensitivity. Insects appear to balance leg segment rigidity and force measurement sensitivity through heterogeneous cuticle that creates stress (and thus strain) concentrations near strain sensing organs but keeps the segment rigid overall. To engineer robot limbs that balance rigidity and force sensitivity, we leveraged these biological principles and additive manufacturing to explore the effect of localized Kevlar® fiber reinforcement on the stiffness and strain sensing performance of 3D printed robotic limbs. We fabricated limbs with no, partial, and full Kevlar® fiber reinforcement and evaluated their performance through beam bending, robot stepping, and fatigue tests. The limbs were assessed for endpoint stiffness, strain sensitivity, and structural integrity under repeated loading. Results reveal that partial fiber reinforcement offers the most effective compromise, increasing endpoint stiffness to facilitate kinematics calculations while amplifying strain signals for high signal-to-noise ratios. Furthermore, partially reinforced limbs demonstrated superior fatigue resistance, retaining sensing capability after prolonged cyclic loading. These findings suggest that localized, heterogeneous reinforcement that mimics the structural and sensory trade-offs found in insect exoskeletons can enhance both the mechanical and sensing performance of robotic limbs.