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Development of an Amphibious Mother Spherical Robot Used as the Carrier for Underwater Microrobots

Shuxiang Guo1,3, Shilian Mao2, Liwei Shi1, Maoxun Li2 1Faculty of Engineering, Kagawa University, 2217-20, Hayashichou, Kagawa, Japan

2Graduate school of Engineering, Kagawa University, 2217-20, Hayashichou, Kagawa, Japan 3Harbin Engineering University

[email protected], [email protected], [email protected] Abstract – Nowadays, smart materials actuated microrobots are widely used when dealing with complicated missions in limited spaces. But problems still exist in this kind of solutions, such as low locomotion speed and short operating time. To solve these problems, we propose a mother-son multi-robots cooperation system, named GSL system, which included several microrobots as son robots, and a novel designed amphibious spherical robot as the mother robot. The mother robot, called GSLMom, was designed to be able to carry microrobots and provide power supply for them. This paper will talk about the structure and mechanism of the GSLMom robot. The GSLMom robot was designed as an amphibious spherical one. The robot was equipped with a 4 unit locomotion system, and each unit consists of a water-jet propeller and two servo motors. Each servo motor could rotate 90°in horizontal and 120°in vertical direction respectively. When moving in water, servo motors controlled the directions of water jet propellers and the 4 propellers work to actuate the robot. In the ground situation, propellers were used as legs, and servo motors actuated these legs to realize walking mechanism. After discussed structures, experiments were conducted to evaluate performance of the actuators. Index Terms – Spherical underwater robot. Amphibious robot. Water-jet propeller. Quadruped walking. Mother robot.

I. INTRODUCTION

Nowadays, one important research orientation of robotics is microrobots that can be implemented in very limited spaces such as narrow pipelines or complicated underwater spaces full with reefs. Many researchers focus on downsizing their robots by optimizing structures and employing small motors. But traditional motor actuated robots have their problems because the limits in size and power consumption of motors. For this reason, smart actuators are more and more being used in the development of microrobots. And because the smart actuators are usually very simple and light, and some of them also have low power consumption, a verity of this kind of robot have been reported to have small sizes, new locomotion methods and consume little energy while working. For example, ICPF actuator based microrobots [1]-[4], SMA actuator based microrobots [5]-[8]. Although a lot of achievements have been got on microrobot kinematics and control strategy, it still seems far from being used in practical tasks. The first reason is that the velocity of these robots is still very low, and because of this their application field is limited. It is difficult for the microrobots to get the position where tasks are conducted with

a velocity of several millimetres per second. The second reason is the power supply problem. As there must be a tradeoff between the size of microrobots and volume of batteries, the enduring time is limited. Solutions like EMA actuating [9] and wireless powering [10] are also reported, but a huge outside device is needed in these situations. The last reason is that it is difficult to develop intelligent microrobots, and this is also because of the size of microrobot. It is hard to realize intelligent control or multi-robot cooperation on microrobots. To solve these problems, we proposed a mother-son robot system in this paper, and named it GSL system. The inspiration of this system is aircraft carrier system. In the system, a mother robot called GSLMom is used as the base station, and several microrobots can be transported and controlled by it. When a task is being conducted, firstly GSLMom robot carrying microrobots move to an appropriate place near the destination, and stabilize itself. After that, microrobots walking or swimming out of the GSLMom robot and deal with tasks. In this system, because the destination and the mother robot are quiet near, it is possible to conduct cable control between mother robot and microrobots. By using this mechanism, power supply and control units can be equipped on the GSLMom robot, thus the microrobots can be designed more compact and concentrate on the kinematic design. Details of this newly proposed GSL system will be talked in chapter II, and from chapter III, we will talk about the amphibious spherical GSLMom robot in detail as follows: total design and mechanism of actuating system will be introduced in chapter III, follows analysis and experiments of water-jet actuating in chapter IV. After that, chapter V will be conclusions.

II. GSL SYSTEM

As has been talked in the introduction part, this research aims at a robotic system to deal with the situations in which tasks are too complicated for normal microrobots, and at the same time environments are too narrow for normal-sized robots to enter. In order to solve this contradiction, we proposed a 2-level robot system, the GSL system to make use of the advantages of both the two kind robot. In this system, microrobots are used to conduct tasks practically, and the GSLMom robot, which is in the upper level, acts as the controller and transporter. As shown in Fig.1, at the first of a task, GSLMom robot with microrobots inside of it move to a proper place near the

Proceedings of the 2012 ICME International Conference on Complex Medical Engineering July 1 - 4, Kobe, Japan

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destination where tasks will be conducted. The robot can move in water-jet mode or quadruped walking mode depending on the environment. After got near the destination, or encountered a narrow way which is hard to get through, GSLMom robot will take a stable gesture and act as a base station for microrobots. After that, microrobots, which were kept in the space inside the lower hemisphere of the GSLMom robot before this step, will move out to conduct tasks by themselves. Because of the sizes of microrobots are small, they can be sent into very limited spaces, for example, narrow pipelines shown in Fig.1. Control units and batteries of microrobots are located in the GSLMom robot, and cables are used between it and microrobots.

Fig. 1 GSL mother-son robot system

Compared to individual microrobots, there exit several advantages of this structure.

1. The moving range of the whole system is expended due to the relatively high moving speed and long enduring time of the mother robot.

2. Cables are used between mother robot and microrobots, so that microrobots can get a relatively stable power supply.

3. Because microrobots are all controlled by the mother robot, and can get communications with each other through mother robot, it is easier to conduct task in which multi-robot cooperation is needed.

4. Because power supply and control units are equipped in the mother robot, microrobots can realize a more compact structure with only actuating units and sensors.

III. STRUCTURE DESIGN OF THE GSLMOM ROBOT

A. General design

The designed GSLMom robot is shown in Fig.2 and Fig.3. It was designed to be a spherical one to maximum the inner space, and an amphibious one to expend the moving range of the whole system. The robot is shaped by a hemisphere upper hull of 250mm in diameter and 2 quarter sphere openable hulls of 266mm in diameter. Each of the openable hulls can rotate around an axles fixed on the upper hemisphere for about 90 degrees. The upper hemisphere is water proofed and control units and batteries were installed in

it. However the space in the lower hemisphere was connected to the outside through gaps and holes on the quarter sphere hulls. Actuating system and space for transporting microrobots were installed here. There are 2 long holes on each quarter hemisphere hull so that water-jet system can work normally when the hulls are closed.

Fig. 2 GSLMom robot in closed state

Fig. 3 GSLMom robot in opening state

When actuated by water-jet in the water, the two openable hulls are closed to keep the robot a spherical shape like what Fig2 shows. The advantages of this shape are: 1, closed hulls can protect microrobots and actuation system inside them, 2, a spherical shaped robot is relatively easy to control in water because of symmetry, 3, a spherical shaped robot do little disturbance to the underwater environment, 4, a spherical robot is hard to be found by sonar so it can be used in military applications. But when the robot works in walking mode or in the situation microrobots are sent out, the 2 openable hulls have to be opened, so that the actuation system and the space for

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transport microrobots are exposed to the outside directly. This situation is shown in Fig.3.

B. Opening mechanism

Fig. 4 shows the opening mechanism of the GSLMom robot. When being closed, the upper hemisphere and the lower 2 quarter spheres are concentric. And the inner diameters of quarter spheres are 10mm larger than the outer diameter of the upper hemisphere. Axles are fixed in the upper hemisphere, and there is a 20mm distance between the two axles so that the two quarter spheres can be controlled independently. From Fig.5 we can see that with this structure, the quarter spheres can be opened freely without any collision with the upper hemisphere.

Fig. 4 Opening mechanism of the GSLMom robot

C. Actuating system

Fig.5 shows the whole image of the 4 unit actuating system installed under the central plate of GSLMom robot. Actuating units were suspended under the central plate by axles and were controlled independently. Each unit was composed of one carriage, one water-jet motor and two servo motors. As shown in Fig.6, each of them has 2 degree of freedom, and can generate one more actuating force in water through water-jet mechanism. Here we used the JR DS3836 servo motors, which has a compact size of 21.5*21.5*11mm, and each of them can rotate 120 degrees in max and provide a maximum torch of 2kg*cm. The water-jet motor is sized 21*31*46mm without nozzles. Water proof was conducted on servo motors and water-jet motors before experiment. With this structure, both vectored water-jet and quadruped walking can be realized in one actuating system. so it is called a hybrid actuating system, and act as the key part of the GSLMom robot.

IV. WATER-JET ACTUATING

A. Mechanism The robot can realize a three degree of freedom motion in underwater environment by using the vectored water-jet mechanism. The 3 degrees of freedom are horizontally moving forward and turning, and vertically moving up and down. In the situation of vertical motion, the water-jet direction should be adjusted to generate the desired actuating force. The mechanism is shown in Fig.7. The water-jet motor should be deployed horizontally in horizontal motion mode, and detailed

mechanisms are shown in Fig.8. By turning on and off appropriate water-jet motors, robot can realize moving forward and turning motion.

Fig. 5 Actuating system

Fig. 6 Single actuating unit

Fig. 7 Water-jet mechanism

Fig. 8 Mechanism of horizontal moving (top view)

B. Single water-jet motor kinematics

Fig.9 shows the water-jet motor used in our robot. When the motor is working, water was actuated by rotating blades and jetted out from the nozzle. Fig.10 gives the flow model of the water-jet motor. The shaft is perpendicular to the pipe, and there are four blades fixed on the shaft. We assume that the

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axis flow velocity of the pipe Va equals to its centre flow velocity Vc. Ω is the rotation velocity of motor shaft, Vi is the incoming velocity of flow, Vo is the flow velocity jetted out of the pipe and D is the diameter of the pipe.

Fig. 9 Water-jet motor

Fig. 10 Flow model inside the nozzle of water-jet motor

According to the equation of continuity, we get Eq.1. And because the pipe is a cylinder tube with even diameter, so Ac=Ao, and also the flow is considered as incompressible flow with uniform density ρc=ρo. Then we can get Eq.2. For simplification, we consider the axis velocity Vc as a linear combine of incoming flow velocity and the linear velocity of shaft, as Eq.3 shows. Where, Vi = Vfcosθ, Vf is the ambient flow velocity. Then the thrust force Ft can be directly expressed as Eq.4. Where m=ρAVc, then we can substitute it in Eq.4 to obtain:

ρcVcAc= ρoVoAo (1)

Vc= Vo (2)

Vc= k1Vi+k2DΩ (3)

Ft = mVc (4)

Ft = ρAVc

2 (5)

Therefore, we can see the output force of water-jet motor depends on incoming angles, flow velocity and the shaft velocity. In next chapter, we will introduce the experiment of dynamics characteristics of the water-jet motor.

C. Actuating force experiment of single water-jet motor

In order to figure out the relationship between input voltage and actuating force of a single water-jet motor, and to

evaluate the actuating forces under different situations, we did some experiments. As shown in Fig.11, we made use of leverage principle and used an electromagnetic scale to measure the actuating force of the water-jet motor. Assume that the two arm of the lever is a and b respectively, and the reading of the electromagnetic scale is m, then we can compute out the actuating force F of the water-jet motor:

F=m*g*a/b (6)

We considered three situations, which are illustrated in Fig.12:

Situation 1: water is jetted out from the nozzle, which is the common situation of water-jet actuation mode. We considered this situation because in our design, nozzle is necessary because water should be guided to be jetted out from the holes on the robot, see Fig. 7.

Situation 2: water is jetted out from the opposite direction of nozzle.

Situation 3: water is jetted out from the opposite direction of nozzle. And there is a plate in the direction of water-jet. This is to simulate the situation when robot is moving down in water. And the lengths and angle of the plate is decided as the same with the situation of the underwater moving down motion of the robot.

From the experiment results shown in Fig.13, we can know that:

1. With the most common situation, the situation 1, a maximum actuating force of 0.127N can be got.

2. The actuating force of situation 1 is smaller than that of situation 2, which means that the nozzle is harmful to water-jet.

3. The difference between situation 2 and situation 3 is not obvious, from which we can know that the plate in this distance does not influent the actuating force much, thus the underwater moving down mechanism is realizable.

Fig. 11 Actuating force measurement experiment for one propeller

Fig. 12 Three situations

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Fig. 13 Water-jet actuating force

VI. CONCLUSION

In this paper, we proposed a mother-son multi-robot system to deal with problems such as conducting complicated missions in limited spaces. Compared with individual microrobots, there were several advantages such as high locomotion ability, long enduring time and intelligence in this system. In this mother-son robot system, the mother robot was designed with a compact spherical structure, and the ability of amphibious locomotion. So a hybrid actuating system was designed for the robot, which could realize both water-jet actuation and walking actuation. In underwater environment, a vectored water-jet mechanism could help the robot to realize a 3 degree of freedom motion. While on the ground, four legs with 2 degrees of freedom each could realize a quadruped walking mechanism. In order to realize the hybrid actuating system in a hemisphere with a diameter of only 250mm, water-jet motors were also used as legs in walking motion, which is an originate design. At last, we also proposed water-jet actuation strategy and evaluated the actuating performance of the water-jet motor. The results obtained in this paper could help us design the control strategies and optimize the structure of the robot in the future.

ACKNOWLEDGMENT

This research was supported by the Kagawa University Characteristic Prior Research Fund 2011.

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