Compliant intervertebral mechanism for humanoid backbone: Kinematic modeling and optimization

•We propose a modular bio-mimetic intervertebral parallel mechanism.•The mechanism is composed of a flexible central part similar to a windsurf diabolo.•We define a constrained kinematic model to authorize tilting only.•We optimize the architecture and orientation of the model.•Optimization defines stress specifications of the compliant element.

This article presents a novel bio-mimetic inter-vertebral mechanism that can be incorporated into the trunk of a humanoid robot. It is composed of a central passive rod that joins two consecutive vertebrae. Two linear actuators connect both vertebrae laterally through Universal joints. Thanks to this mechanism, the top vertebra can be tilted in all directions with respect to the lower vertebra. The mechanism is modeled from a kinematic point of view to demonstrate the actual mobility, and to determine the optimal parameters regarding the placement of both lateral linear actuators taking into account maximal desired bending amplitudes. The kinematic model is also used to calculate the stress that the central rod will have to resist. The stress forces, namely shear forces, normal compression/tension forces, and twist torques are useful to fix the design specifications for the flexible rod that will be responsible for the compliance of the mechanism.