Prioritized multi-task motion coordination of physically constrained quadruped manipulators

A research paper by scientists at Shandong University presented a novel coordinated motion distribution and tracking algorithm for quadruped manipulators.

The research paper, published on Mar. 19, 2025 in the journal Cyborg and Bionic Systems.

Quadruped manipulators can use legs to mimic legged animals for crossing unstructured environments. They can also use a bionic arm to execute manipulation tasks. The increasing demands for such robots have pushed research progress. However, there remain challenging works in their usage of a high degree of freedom. To solve this problem, this paper proposes a novel motion coordination framework based on multi-task prioritization and null-space projection. “Our framework can adaptively generate optimal motion for different parts of the robot considering 3 prioritized tasks. The tasks include end-effector trajectory tracking, motion redistribution to meet physical constraints, and manipulability enhancement.” said the author Aizhen Xie, a researcher at Shandong University, “The motion is executed by a whole-body controller incorporating dynamics, inverse kinematics, multiobjective priorities, and force constraints.”

The overall process of this framework is as follows: Authors adopt the simplification method by Murphy et al. to establish the system’s kinematics and dynamics model. The quadruped base is modeled as the extended joints of the manipulator. Authors also take advantage of the whole-body controller (WBC) by Bellicoso et al. and propose a motion execution method based on prioritized task projection. The projection idea is adopted from the “saturation in the null space” algorithm presented by Flacco et al. and extended to quadruped manipulators. It is used in both motion generation and execution to effectively address the redundant system’s limitations. Limitations on different working modes are established. A tracking camera with an integrated environment perception algorithm is used in the system. Motion coordination is to finish 3 prioritized tasks. The first and most important task is trajectory tracking of the end effector. The motion redistribution of the robot is then programmed in the null space to enhance tracking accuracy and meet physical constraints. A compensation velocity is designed as the third task to realize singularity avoidance. The generation results are executed using the proposed WBC.

The advantages and effectiveness of the proposed algorithm have been verified in simulation experiments and robot platform experiments. The robot can finish robust and accurate operational space end-effector tracking with errors less than 3 cm. “In the near future, we will extend our framework to plan the manipulation trajectory online. Vision-based recognition will be added to automatically finish grasping of objects. Furthermore, we want to extend this work for confined space manipulation and highly dynamic object catching through reinforcement learning.” said Aizhen Xie.

Authors of the paper include Aizhen Xie, Xuewen Rong, Guoteng Zhang, Yibin Li, Yong Fan, Zhi Li, and Teng Chen.

This work was supported in part by the National Key Research and Development Program of China (Grant No. 2020AAA0108900), the National Natural Science Foundation of China (Nos. 62203268 and 62373223), and the Innovation Capability Improvement Project for Small and Medium-Sized Scientific and Technological Enterprises in Shandong Province (No. 2022TSGC2079).

The paper, “Prioritized Multi-task Motion Coordination of Physically Constrained Quadruped Manipulators” was published in the journal Cyborg and Bionic Systems on Mar. 19, 2025, at DOI: 10.34133/cbsystems.0203.

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