Citation: Zhang, S.; Cheng, S.; Jin, Z.
A Control Method of Mobile
Manipulator Based on Null-Space
Task Planning and Hybrid Control.
Machines 2022, 10, 1222. https://
doi.org/10.3390/machines10121222
Academic Editors: Luis Payá, Oscar
Reinoso García and Helder Jesus
Araújo
Received: 21 November 2022
Accepted: 14 December 2022
Published: 15 December 2022
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Article
A Control Method of Mobile Manipulator Based on Null-Space
Task Planning and Hybrid Control
Shijun Zhang
1
, Shuhong Cheng
2,
* and Zhenlin Jin
1
1
Institute of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, China
2
Institute of Electrical Engineering, Yanshan University, Qinhuangdao 066004, China
* Correspondence: shhcheng@ysu.edu.cn
Abstract:
The mobile manipulator is a floating base structure with wide space operability. An
integrated mechanical device for mobile operation is formed through the organic combination of
the mobile platform and multi-axis manipulator. This paper presents a general kinematic modeling
method for mobile manipulators and gives the relevant derivation of the dynamic model. Secondly,
the null-space composition of the mobile manipulator is analyzed, the task space is divided, and a
variety of task-switching criteria are designed. Finally, a hybrid control model combining dynamic
feedback and synovial control based on dynamic parameter identification is designed, and stability
proof is given. The theoretical method is also verified by the experimental platform. The proposed
method can effectively improve the control accuracy of the mobile manipulator, and the hybrid
control method can effectively control the output torque to reach the ideal state.
Keywords: mobile manipulator; null-space; hybrid control
1. Introduction
A mobile manipulator is a kind of robot that can naturally interact with the working
environment, people, and other robots, independently adapt to the complex dynamic
environment and work together. Due to the functions of grasping, moving, detection and
recognition, the mobile manipulator can perfectly realize the application scenario that one
robot replaces multiple workers or multiple robots. A mobile manipulator is a very complex
multi-input and multi-output structure. It has the nonlinear dynamic characteristics of
being time-varying and with strong coupling. Coupled with the characteristics of sliding
friction uncertainty and accidental external force interference in its dynamic model, it is
a typical nonlinear uncertain system. The mobile manipulator is a complex redundant
system with a floating base. The kinematics of the platform increases the workspace of
the manipulator and the complexity of the system. Its kinematics and dynamics analysis
methods are very different from the traditional manipulator. A mobile manipulator is a
strongly coupled nonlinear system. Generally, the mobile platform has heavy mass and
slow dynamic response, while the manipulator has a light mass and fast dynamic response.
Due to the combination of a mobile platform and manipulator, the compound mobile robot
has great spatial redundancy. For the same task, it can be realized not only by moving the
mobile platform or manipulator alone but also by moving the platform and manipulator
at the same time. Therefore, there are two different research schools. The first one is to
separate the motion planning of the manipulator and the motion planning of the mobile
platform; it includes the hybrid force/position control [
1
–
3
], impedance control [
4
], and the
combination of deep learning [5] and optimal control [6–9].
At present, there are task planning methods based on the null-space for whole-body
control, and the main characteristics are as follows: different subtasks can be decomposed
according to different meanings, and priority can be divided according to the importance;
when necessary, low-priority tasks should not affect the execution of high-priority tasks.
Machines 2022, 10, 1222. https://doi.org/10.3390/machines10121222 https://www.mdpi.com/journal/machines
