Development of the Insect-inspired Biomimetic Underwater Microrobot for a Father-son Robot System

Wenbo Sui1, Shuxiang Guo2,3,4*, Liang Zheng1, Ruochen An1 and Awa Tendeng1 1Graduate School of Engineering, Kagawa University, Takamatsu, Kagawa 761-0396, Japan

2Department of Intelligent Mechanical Systems Engineering, Faculty of Engineering and Design, Kagawa University, 2217-20 Hayashi-cho, Takamatsu, Kagawa, Japan.

3Key Laboratory of Convergence Medical Engineering System and Healthcare Technology, the Ministry of Industry and Information Technology, School of Life Science, Beijing Institute of Technology, Beijing, China.

4Tianjin Key Laboratory for Control Theory and Application in Complicated Systems and Intelligent Robot Laboratory, Tianjin University of Technology, Tianjin, China

E-mail: tjswb@outlook.com, *corresponding author: guo@eng.kagawa-u.ac.jp

Abstract – In this paper, the lifting platform structure of the father robot and the development of the insect-inspired biomimetic underwater microrobot are presented. The objective is to decrease the load of microrobot and realize more functions. In previous researchers, the platform of the father robot is fixed, which is not conducive to adopt and release the son robot. The structure of the lifting platform can effectively solve the problem in theory. The insect-inspired microrobot used wood which has high hydrophilism. It causes the body of microrobot increased weight and short-term works. The fillable structure with waterproof character which is made by a 3D printer. The new structure also can be changed weight for achieving different functions underwater efficiently. Then, two kinds of communication between the light source and microrobot like chasing rotation and fixed distance chasing are described in order to prove the feasibility of the software. In all these cases, experimental results show the performance of the microrobot. Index Terms - Lifting Platform, Insect-inspired Microrobot, IPMC Actuator, Fillable Structure.

I. INTRODUCTION

Nowadays, various underwater intervention tasks are performed by underwater robots. The underwater robot can be divided into two categories: ROV (Remote Operated Vehicle) and AUV (Autonomous Underwater Vehicle) on the basis of the control method. On the grounds of the shape, the sorts of the underwater robot are streamlined shape and special shape. For achieving more kinds of underwater tasks, using the underwater robot equipped with microrobots is selected as the project. Generally, the microrobots are diffusely used in various fields of underwater monitoring such as pollution determination, visual mapping, and detection of underwater unstructured environments. Thus, it is necessary to establish a father-son underwater robot system [1]-[3]. Communication is a fundamental capacity of the father-son robot system, which consists of the underwater robot and microrobot [4]-[8]. Among the various underwater robots, the spherical robot has the characteristics of stable structure, and is almost not entangled by the surrounding marine plants, and has flexible mobility. Considered about the IPMC (Ion-exchange Polymer Metal Composite) has compact structure and characteristics of low-powered, low-noise, soft, and quick response properties in the

water, the actuator is appropriate to be the actuator of the microrobot [9]-[13].

A kind of spherical underwater robot which is drove by 4 vectored water-jet thrusters is developed [14]-[21]. Because of the water-jet thruster and sphericity, the robot owns low noise and high flexibility. The robot has requirements of underwater detection, including detecting underwater organisms and collecting samples. Due to the spherical structure of the robot has the characteristics of compact and strong stability, and it is expediently to pass through the dense water plant areas. In regard to the design of the son robot, the restrictions such as the size of the storage space on the father robot and the limited power supply of the father robot should be considered. Hence, microminiaturization, multiple degrees of freedom (DOF), lightweight, low power, simple structure, and driven by smart actuators are keys of design.

Several underwater microrobots have been developed and the design methods are summarized accurately for precise underwater locomotions [22]-[26]. Among the many kinds of microrobot, the insect-inspired microrobot has the superior capability of locomotions. In previous research, the material of the microrobot body is wood without waterproof. With the passage of time, the main body will be heavier due to the absorption of water, and not conducive to preserving. In addition, by the communication between the spherical underwater robot and microrobot, the microrobot's mobility is not enough.

An insect-inspired microrobot are used as the son robot to perform the underwater manipulation for the complicated underwater tasks [27]-[30]. The design concept of the father-son robot system shows in Fig.1. We design a new structure of microrobot to ensure the feasibility of adjusting the weight.

The rest of this paper is organized as follows. In the section Ⅱ, we introduced the design of the father robot and son robot respectively. Then the structure of the robot platform and the insect-inspired underwater microrobot is introduced in the section Ⅲ. After that, the underwater experiments were shown in the section Ⅳ. The four kinds of manipulations were verified. By setting up the optical sensors, the functions of automatic tracking and fixed distance tracking were implemented. Finally, the section Ⅴ is the conclusions.

Fig.1. Conceptual design for father-son robot system.

Fig.2. Conceptual design of the underwater microrobot.

II. THE FATHER-SON ROBOT SYSTEM

A. Spherical Father Robot The configuration of the father-son robot system is composed of a father robot and several microrobots. The system has the overall superiorities compare with only a motor-based underwater robot or an intelligent actuator-based underwater microrobot. The spherical underwater father robot has a fixture mechanism that is used to carry the son robot when the robot is in idle mode. When the son robot needs to perform tasks, the fixture mechanism is used to be a platform for the son robot to enter the working environment. In order to adapt to complex environments, the father robot has hybrid propulsion devices including vectored water-jet and propeller thrusters. The ATMega2560 microcontroller is used as the control center of the father-son robot system. The servo motors of the father robot are controlled by the PWM channels. In addition, I/O interfaces are used as a channel to transmit the information to the propellers of the father robot and IPMC actuators of the son robot, as well as two optical sensors on the microrobot to achieve the control function. B. Son Robot A kind of insect-inspired biomimetic microrobot that is carried on the father robot is developed, as shown in Fig.2. On behalf of achieving various underwater interventions, more locomotions, long-time work underwater and orientation ability should be developed.

III. DESIGN OF THE UNDERWATER MANIPULATION SYSTEM

A. Design of the Robot Platform Structure The platform is designed to release the son robot when it needs to perform the task and receive and carry it when the son robot finishes the task. According to formula (1), within a right triangle, the length of A (connecting rod) is fixed, and when the angle 𝛼 between A and B increases change from 0 ° to 90 °, 𝑐𝑜𝑠𝛼 will decrease, and the length of B will decrease, so as to shorten the distance, as shown in Fig.3. Considering the movement mode of the son robot, the design of the platform is a cylindrical lifting platform. The upper platform and the casing of the father robot are fixed together, and the connecting rod is rotated into the tank by the four waterproof motors to achieve the task of lifting the lower platform, as shown in Fig.4.When the connecting rod rotates to 90 degrees, the lower platform descends, the hatch opens, and the son robot can enter or leave the platform. When the connecting rod rotates to 42 degrees, the lower platform rises and the hatch closes. The open and closed state of the tank is shown in Fig.5. The bottom radius of the cylindrical tank is about 35 mm and the height is about 80 mm.

𝐵 𝐴𝑐𝑜𝑠𝛼 (1)

B. Structure of the Underwater Microrobot The insect-inspired underwater microrobot is a kind of the hexapod microrobot. The structure of the microrobot includes the main body, the optical sensor, and the IPMC actuator. The length of the main body is 40 mm, the width is 30mm and the height is 4 mm. There are six IPMC actuators which are 17 mm long, 3 mm wide, 0.1 mm thick with one degree of freedom.

Fig.3. Principle of lifting platform.

Fig.4. Design of the robot platform structure.

Fig.5. Movement of the lifting platform.

(a) Phase1 (b) Phase2 (c) Phase3 (d) Phase4

Fig.6. One step cycle of moving forward locomotion (The marks ● indicate which actuator is supporting the ground. The graph without ● indicate which

actuator is leaving the ground).

(a) Phase1 (b) Phase2 (c) Phase3 (d) Phase4 Fig.7. One step cycle of rotating locomotion (The marks ● indicate which

actuator is supporting the ground. The graph without ● indicate which actuator is leaving the ground).

Four actuators are set up at the front and the tail of the main body as drivers and the other two are located in the middle of the main body as supports. And two optical sensors are installed on the head of the microrobot, all of the receiving surfaces of sensors are facing forward. In order to be able to control the weight of the microrobot, we designed the main part of the robot to be a fillable structure. The frame of the microrobot is printed by a 3D printer. The material is Polylactic Acid (PLA). The density of PLA is 1.24g/cm³. It is similar to the underwater environment and the microrobot can perform tasks conveniently. Since the two optical sensors at the front of the microrobot will increase the weight of the robot, it is necessary to set an object for adding buoyant on the depression to basically maintain the balance of the entire robot. C. Mechanism of the Walking/Rotating Motion

The microrobot can realize each step cycle of walking forward and rotating motion like insect crawling. Locomotions are achieved by assigning different commands to drivers and supporters at different times. The principles of walking forward and rotating motion are shown in Fig.6 and Fig.7 respectively.

Two supporters bend downwards to hoist the microrobot body, and the drivers are far from the ground. Whereafter, the four drivers bend forward. The supporters bend upwards so as to leave the ground, and the drivers support the body subsequently. In order to make the robot move forward, the four drivers bend backward to generate the friction. The oscillating

frequencies of drivers and supporters are the same. The phase of the supporters is set to be 90° behind the phase of the drivers. In view of the principle, we can realize walking forward / backward, rotating in clockwise / counterclockwise.

D. Sensors To realize the cooperation of the father-son robot system for practical applications, the communication system is very important. In order to receive the signals from the spherical underwater robot to operate the underwater mission, the communication system needs to be designed. Optical communication is appropriate for the underwater communication system for short-haul communication because of low power, and no stringent requirement on directionality and high-data rate. In order to reduce the weight of the overall micro-robot as much as possible, when the optical sensor is installed, the excess part will be removed, leaving only the internal circuit and the sensing element. The optical sensor is shown in Fig.8. Two optical sensors fixed in the front of the son robot are used in this research for achieving chasing light. E. Control circuit In this study, the control module of the father-son robot system is an AVR processor which is AVR ATMEGA2560 microcontroller. The control system of the father-son robot system is shown in Fig.9. The six IPMC actuators are controlled by twelve I/O ports of the microcontroller. One IPMC actuator is controlled by two I/O ports. The input of positive voltage and negative voltage to the IPMC smart actuator alternates between the two I/O ports, thereby AC power is generated to start the IPMC smart actuator. The microcontroller receives the data transmitted by the optical sensors through the data transmission ports and then reads the data by analog-to-digital conversion to realize the data transmission control of the two optical sensors. The closed-loop control implemented.

Fig.8. The optical sensor.

Fig.9. Control circuit of the father-son underwater robot system.

Fig.10. Prototype of the son robot.

(a) at=0s (b) t=28s (c) t=40s

Fig.11. Tracking experiment of going forward.

(a) t=0s (b) t=24s (c) t=72s

Fig.12. Tracking experiment of drawing back.

(a) t=0s (b) t=64s (c) t=88s

Fig.13. Tracking experiment of clockwise turning.

(a) t=0 (b) t=61s (c) t=113s

Fig.14. Tracking experiment of counter-clockwise turning.

IV. MICROROBOTS AND EXPERIMENTS

A. Prototype Microrobots The insect-inspired hexapod microrobot is 40 mm long, 30 mm wide and 4 mm height. The weight of it is 4.90 g. The prototype of the son robot as shown in Fig.10. The enamel-covered copper wires have high flexibility and low resistance that can be ignored.

B. Tracking Motion of the Microrobot The confirmatory experiment for verifying the performance of the microrobot with a fillable structure is carried out. The experiment setup is for testing the communication between the son robot and the light. We put the microrobot in a water basin. And the light is set in the front of the microrobot. Two optical sensors were fixed on the head of the microrobot symmetrically. We set a reference distance of 10cm between the light source and the optical sensor so that the microrobot moves backward when the distance between the light source and the optical sensor is less than the reference distance and keeps away from the light source. On the contrary, the forward movement keeps getting closer to the light source. Then, to contrast the output voltage of the two optical sensors, the microrobot will change the locomotion from moving straight locomotion and rotating locomotion. As shown in Fig.11, when the distance between the light source and the optical sensors is longer than the standard distance, the microrobot will go forward, and the chasing locomotion and the change of the scale can be observed. On the contrary, when the distance between the light and the light sensor is shorter than the standard distance, the microrobot will realize the drawing backward locomotion, as shown in Figure 12.As shown in Fig. 13, by observing the change of angle, it can be found that when the light received by the right optical sensor is stronger than the left optical sensor, the microrobot will turn clockwise. Instead, as shown in Fig. 14, the microrobot will turn counterclockwise.

V. CONCLUSIONS

In this paper, a kind of lifting platform of father robot to adopt and release microrobot is proposed. The IPMC actuator-based insect-inspired microrobot with the fillable structure was developed. The kind of microrobot and the spherical underwater robot can combine an underwater father-son robot system. The system can be used in more underwater performance. The operations of the micro3robot are verified by the underwater experiments, including chasing rotation and fixed distance chasing. From the results of experiments, by the guidance on the light source, the going forward speed of 0.025 mm/s at a frequency of 1 Hz, the drawing back speed of 0.035mm/s at a frequency of 1 Hz, the rotational speed of clockwise turning is 0.002 rad/s and the rotational speed of counterclockwise turning is 0.007 rad/s at a frequency of 1Hz were achieved.

ACKNOWLEDGMENT

This research is partly supported by National High Tech. Research and Development Program of China (No.2015AA043202), and SPS KAKENHI Grant Number 15K2120.

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