A Model-Based Hierarchical Controller for Legged Systems Subject to External Disturbances

Abstract

Legged robots have many potential applications in real-world scenarios where the tasks are too dangerous for humans, and compliance is needed to protect the system against external disturbances and impacts. In this paper, we propose a model-based controller for hierarchical tasks of legged systems subject to external disturbance. The control framework is based on projected inverse dynamics controller, such that the control law is decomposed into two orthogonal subspaces, i.e., the constrained and the unconstrained subspaces. The unconstrained component controls multiple desired tasks with impedance responses. The constrained space controller maintains the contact subject to unknown external disturbances, without the use of any force/torque sensing at the contact points. By explicitly modelling the external force, our controller is robust to external disturbances and errors arising from incorrect dynamic model information. The main contributions of this paper include (1) incorporating an impedance controller to control external disturbances and allow impedance shaping to adjust the behaviour of the motion under external disturbances, (2) optimising contact forces within the constrained subspace that also takes into account the external disturbances without using force/torque sensors at the contact locations. The techniques are evaluated on the ANYmal quadruped platform under a variety of scenarios.

Analytic Model for Quadruped Locomotion Task-Space Planning

“…The identification of efficient and effective planning strategies in unstructured environments is critical to the development of mobile robotics platform that can enter daily living environments to improve human life quality and take over dangerous tasks [1], [3] environments (e.g., deep mines, offshore oil rigs and disaster sites), offering both greater flexibility than wheeled robots and better stability characteristics compared to bipeds [1]. However, the high redundancy of the system kinematics implies a higher complexity in the planning stage, making the system less adaptable to sudden changes in the environment which trigger a replanning of the entire action.…”

Online Optimal Impedance Planning for Legged Robots

“…2(a,b)) and high impedance ( The aim of this paper is thus to plan the impedance of the robot online, without relying on direct force sensor measurements. Applying a task-space impedance control technique to a generic robotic system leads to a resulting behavior described by the model [15], [3]…”

“…Applying impedance control to legged robots is a common practice in literature [2], [3], [4], [5], [6]. However, there is currently no established basis upon which to choose the impedance gains for a specific system or scenario.…”

“…In this work we show that variable gains lead to better performance and robustness to unexpected interactions. We will rely on the impedance controller proposed in [3], developing a module that will plan the gains of this controller online. In particular, we will focus on the impedance modulation of the torso of the robot.…”

Model Predictive Control for Motion Planning of Quadrupedal Locomotion

“…The advantage of wheels comes together with the invention of pavement and roads, and while facing difficult terrains, a biological setup such as legs has greater advantages in dealing with rugged surfaces, slopes, rocks, stairs, ledges, sand, and snow. Quadrupedal robots are one type of legged machines that are built for multi-terrain purposes [1], [2], [3], [4]. Quadrupedal robots exhibit various gait patterns, depending on the locomotion speed, terrain conditions as well as animal species [5], [6].…”