Towards Generation and Transition of Diverse Gaits for Quadrupedal Robots Based on Trajectory Optimization and Whole-Body Impedance Control

Trajectory optimization (TO) combined with whole-body control (WBC) have been a widely accepted approach for dynamic gait control of quadruped robots. However, there are still open issues in this framework, one is the lack of a unified description of intrinsic inter-limb coordination for wide range of gaits and their transitions in TO, another is motion compliance against disturbances arisen from transitions while maintaining accurate tracking performance. In this letter, we introduce the reduced antero-posterior sequence (APS) gait parametrization approach into the model predictive control (MPC) based TO. The APS gait parametrization, which is enforced as equality constraints in optimization model, enables the representation of diverse gaits, symmetrical and asymmetrical, with five parameters and offers an intuitive way to efficiently implement gait transitions with linear interpolation of gait parameters. We also construct a whole-body impedance controller that integrates the operational impedance controller into WBC, allowing to compliantly track the optimized torso state trajectories and contact forces. The effectiveness of the proposed approach is verified in simulations and experiments. The results indicate that the test robot is able to robustly locomote with various gaits, including diagonal walk, trot, bound, flying trot and traverse gallop at a variety of speeds, and smoothly implement gait transitions among these gaits. Compared to existing gait transition approach, ours can obtain better reference tracking performance and higher energy efficiency during gait transitions.