High Dynamic Position Control for a Typical Hydraulic Quadruped Robot Leg Based on Virtual Decomposition Control

The dynamics of hydraulic robots are complicated due to the closed-chain joints formed by cylinder articulation. This article is focused on presenting a model-based control framework for rapid locomotion, integrating closed-chain dynamics without a substantial increase in computational costs. The virtual decomposition control (VDC) approach has been adapted and innovatively extended to a leg system for the first time, featuring a floating base and variable contact constraints. In this article, a position control framework is proposed, consisting of three VDC-based controllers designed specifically for the stance phase and the swing phase, respectively. During the stance phase, a constrained estimation model is developed to recursively compute the previously incalculable dynamic equations. Furthermore, the control laws are designed to ensure that the virtual power flows caused by contact constraints do not affect the stability. In the swing phase, a noninertial frame is established to transform the underactuated system into a fully actuated fixed-base system. Despite being position controlled, our framework enables the leg system to generate compliance by setting a separate low-gain VDC-based controller during the landing stage. Experiments reveal that the proposed framework exhibits better position trajectory tracking performance and jumping ability in highly dynamic motion compared with the state-of-the-art position controller. Additionally, the impedance characteristics of the leg system can be actively adjusted to adapt to uneven terrain.