Enhancing the Flexibility of a Quadruped Robot with a 2-DOF Active Spine Using Nonlinear Model Predictive Control

Abstract

For quadrupeds, a flexible spine allows them to traverse space and make quick turns. From the perspective of mechanical design in quadruped robots, an active spine with 2 degrees of freedom (2-DOF) can achieve dynamic posture adjustment similar to biological organisms which allows for pitch and yaw control. In this work, we present a novel approach to enhance the flexibility of a quadruped robot, Yatsen Lion II, by incorporating a 2-DOF active spine, which is mechanically designed as a linkage-driven parallelogram mechanism. To optimize its motion, we utilize nonlinear model predictive control (NMPC), which combines centroidal dynamics with full kinematics. By incorporating the two extra DOFs of the spinal joint into the generalized coordinates and velocities, we represent the robot as a hybrid dynamic system, capturing the intricate interplay between the legs and spine. Centroidal dynamics act as a crucial bridge between joint movements and the robot’s overall momentum, enabling the controller to synchronize the quadruped’s movements with dynamic spinal adjustments and adaptive gait patterns. We validate our approach through both simulation and real-world experiments. We compare spinal quadruped robot to their rigid-spine counterparts across key locomotion metrics, including in-place turning, straight-line speed, and turning radius. The results indicate that the spined quadrupedal robot outperforms its rigid counterpart by up to 26%, highlighting its flexibility.