Underwater Motion Characteristics Evaluation of a Bio-inspired Father-son Robot Ruochen An 1 Shuxiang Guo 1,2 , Chunying Li 1 , Tendeng Awa 1 1 Graduate School of Engineering, Kagawa University Takamatsu, Kagawa 761-0396, Japan s19d501@stu.kagawa-u.ac.jp; guo.shuxiang@ kagawa-u.ac.jp 2 Key Laboratory of Convergence Medical Engineering System and Healthcare Technology, The Ministry of Industry and Information Technology, School of Life Science and Technology, Beijing Institute of Technology, Haidian District, Beijing 100081, China Abstract - Conventional underwater Sample Acquisition tasks are performed by underwater vehicles equipped with rigid multi- link arms. However, the motion of the conventional robotic arm interferes with the stability of the underwater vehicle when performing sampling tasks. In this paper, a fish-inspired small under robot is designed and developed as a son robot for sample acquisition, which is connected to the spherical underwater robot father robot by a tether. Firstly, we proposed the novel father-son robot system to realize the underwater sample acquisition task. And then, the motion control is proposed for the father-son robot system. The son robot actuated by one water-jet thruster can realize underwater basic motions. The hybrid thrusters as the thruster mechanism for the father robot. Also, the control circuit of the father-son robot system is designed. Finally, the hydrodynamic analysis and experiment results verify the validity and reliability of the father-son system. Index Terms - Father-son robot system, spherical underwater robot, hydrodynamic analysis. I. INTRODUCTION Multi-modal robot system has been extensively applied to underwater intervention missions, like underwater sample acquisition [1-3]. The father-son robot system is a form of multi-robot system, which can realize the functions of two kinds of robots. This extraordinary function is inspired by nature and can combine the advantages of different types of robots into one platform. It can realize effective movement in different underwater environments and convert between different environments underwater [4,5]. Therefore, the same robot can cover long distances and over obstacles, and effectively move in small underwater spaces. The combination of the advantages of the father-son robot system can adapt to the complex underwater environment and replace humans as much as possible. The father-son robot system is a system in which a father machine with strong dynamic balance and large carrying capacity and a son machine with small disturbance and flexible movement are used as a system for the cooperative work of the father and son. Here, the father machine chooses to have no tether, and can work independently according to the pre-settings, without the need for human remote control of the autonomous water downloading tool. Instead of using a tether wrapped with copper wires and optical fibers to connect to the working mother ship, the operator can instantly transmit commands to the remotely operated submersible of the vehicle [6,7]. Usually the underwater manipulator, as the key equipment of the underwater vehicle, consists of an external mechanical arm, which is manually operated by humans and completes the planned tasks. Underwater robotic arms are usually very large and can realize the recovery of large objects. When performing underwater recovery tasks, the robotic arm is usually connected to the remotely operated underwater vehicle or the autonomous underwater vehicle. In this process, the underwater robotic arm will produce a reaction force on the underwater vehicle, which will have a certain impact on the stability of the underwater vehicle, resulting in the underwater vehicle deviated from the target point. It is necessary to use a compensation controller to stabilize the movement of the underwater vehicle, which increases the complexity of the overall control system. In order to overcome these problems, we propose a small robot as the father-son robot system of the hand manipulator, which can achieve underwater sample acquisition tasks. The father-son robot system includes a small bionic robot and a multiple degree of freedom autonomous underwater vehicle. The Father-son Underwater Intervention Robotic System (FUIRS) that the amphibious underwater robot as the father robot and an ICPF actuators based micro-robot plays the role of the son robot was proposed in our group firstly [8]. In this system, it realized wireless communication between the father robot and the son robot. Also, the motion control of the son robot was realized. Amphibious underwater robot has developed into four generations. The amphibious underwater robot is inspired by the tortoise, which can realize the movement of the robot from land to underwater. The underwater robot has good performance in underwater and land movement. In our previous research, amphibious motion control has been realized [9-12], and communication between robots has been realized [13,14]. At same time, the function of the localization with the method of relative close-range [15] and the path tracking [16] also realized. In the subsequent research, we evaluated the performance of the amphibious robot [17-19]. There is also a spherical underwater robot in our group. The spherical underwater robot has good flexibility and good hydrodynamic performance. We have developed five generations of spherical underwater robots totally. SUR II has good motion performance. The robot is composed of two hemispheres. At the joint of the hemispheres, three vector water