Insect-inspired Biomimetic Underwater Microrobots for a Father-son Robot System

Maoxun Li 1, Shuxiang Guo 2,3, Kazuhiro Yamashita 1 1 Graduate school of Engineering, Kagawa University, 2217-20, Hayashichou, Kagawa, Japan

2 Faculty of Engineering, Kagawa University, 2217-20, Hayashichou, Kagawa, Japan 3 Beijing Institute of Technology, China

s14d505@stmail.eng.kagawa-u.ac.jp, guo@eng.kagawa-u.ac.jp

Abstract – In previous researches, a number of underwater microrobots powered by smart actuators have realized basic motions such as walking, swimming, floating and grasping motions. However, most of them are actuated by an open-loop control mode and wired control mode, which limit their mobility, because of the compact structure. In order to overcome these limitations, a father-son robot system was proposed. In this system, microrobots are carried by a high-speed father robot, which provides the power and sends the control signals to microrobots. In this paper, two kinds of ICPF (ionic conducting polymer film) actuator-based insect-inspired microrobots for mounting on the father robot are developed. The basic operations of the robots are confirmed. In confined spaces, a 9 ICPF actuator-based microrobot carrying a proximity sensor is developed to realize the operation of object recovery. Then we evaluate the performance of photodiode and carry two light sensors on the front of an 8 ICPF actuator-based microrobot to realize finding the target position and recovery of the microrobot. Using the light sensors, multi-robots can realize the tracking motion and also can get back to the father robot. Index Terms – Underwater microrobot. ICPF(ionic conducting polymer film) actuator. Microrobot recovery. Tracking motion .

I. INTRODUCTION

Owing to the increasing needs for underwater tasks, underwater robots have been applied more and more widely. Many researches of underwater robots are fabricated using traditional actuators, multiple motors, joints, links and so on, which can achieve high moving speed and long operating times. However, these kinds of robots cannot get through confined spaces and do tasks in restricted spaces due to the large size, and have higher power consumptions.

Researches of underwater biomimetic microrobots actuated by smart actuators, on the other hand, has been focused on the applications, such as cleaning the micro-pipeline in a radiation environment, submarine sampling and data collecting and object recovery in limited and dangerous spaces and so on [1] [2]. Smart materials can be used as actuators directly and they can easily perform flexible and complex movements without the need of additional parts, which is in contrast to traditional actuators. Therefore, compared to the motor-based robots, smart actuator-based robots have light weight and small size and also make low noise. Based on these advantages, applications of smart materials, including ionic conducting polymer film (ICPF), piezoelectric elements, pneumatic actuator, and shape memory

alloy (SMA), become more and more wide in the field of microrobots [3] [4].

A fish robot can realize a body and caudal actuator-based swimming mode by an SMA actuator embedded into its polymer [5], which has a maximum speed of 112 mm/s. Another fish-like robot mimics the general swimming motion of the real fish using ICPF actuator passive tail fin, which gets a maximum speed of 22 mm/s [6]. A snake-inspired robot, composed of three parts, was developed to mimic the behavior of a snake [7]. Each part of the robot is connected by an ICPF actuator. And the robot can move at a maximum speed of 8 mm/s. Another robot, jellyfish robot, driven by a jet propulsion system, was developed [8]. By regulating the buoyancy with a balloon, the robot can achieve a maximum speed of 2.3 mm/s.

ICPF actuators have been widely researched in the way of actuating microrobots, with the advantages of compact structure, soft characteristic, low voltage driving, low noise driving, driving in water or wet environments, and having the similar density to the water. For their quick response properties, ICPF actuators are used as oscillating fins in swimming microrobots, and legs in walking underwater microrobots.

These biomimetic microrobots actuated by smart actuators exhibit good performance in some respects. However, developing a microrobot to implement compact structure, flexibility, and multi-functions at the same time still seems hard because of the conflicts among these three properties. Their compact structure limits the function of the robot, as well as the mobile velocity and operating times. By the reason of the properties of the smart actuators, microrobots usually have low velocity. And it is hard for wireless microrobots to carry a large power supply, which limits their operating times. Wired microrobots can get enough power though the cable, but the range of movement is also limited by the cable.

For solving these problems, we described a father-son robot configuration, in which the motor-based father robot carries a number of smart actuator-based microrobots. The father robot has a high mobile velocity and can provide power and send control signals to microrobots, as shown in Fig. 1.

This paper is organized as follows. In section II, we described the father-son robot configuration. Then we introduced the structures of the two kinds of microrobots and the motion mechanisms in section III. After that, the prototype microrobots were given in section IV. And through the underwater experiments, the basic operations were confirmed.

By carrying the proximity sensor and light sensors, the functions of object recovery and automatic tracking were realized. Finally, we drew the conclusions in section V.

Fig. 1 Father-son robot configuration [9]

II. FATHER-SON ROBOT CONFIGURATION

A. An Amphibious Spherical Father Robot

The father-son robot configuration is composed of a father robot and several microrobots, which combines the advantages of a motor-based mobile robot and smart actuator-based microrobot [9] [10]. The amphibious father robot has two openable covers, four actuating units and a stand for carrying microrobots. For adapting to complex environments, the robot has a water-jet propulsion mode and a quadruped walking mode. The control center of the father-son robot system is based on an AVR ATMEGA micro-controller. The PWM channels are used to control the servo motors of the father robot. Furthermore, I/O ports are used to control the water-jet propellers of the father robot and ICPF actuators of the microrobots, and also two light sensors carried on the microrobot. Use data transmission ports, through analog-to-digital conversion, to control the infrared proximity sensors and gyroscope sensor carried on the microrobot and father robot respectively.

For carrying and deploying the microrobots, we developed a fixture mechanism. A mechanism with no actuating parts was proposed to simplify the control system, as shown in Fig. 2. A block that can move up and down freely on a plate is used to fix position of the microrobots. The block falls down to fix the position of the microrobots by gravity during the walking and swimming motions of the father robot. When deploying the microrobots, the four actuating units rotate upwards to allow the robots to be supported by the plate, and push the block up so that the microrobots can leave the father robot.

B. Microrobots

Two kinds of insect-like biomimetic microrobots are developed for mounting on the father robot, as shown in Fig. 3. In order to implement the underwater tasks of object recovery, microrobots should be capable of finding both the target position and the father robot, and carrying target object automatically. The eight-legged microrobot and the hexapod microrobot are developed to realize the abilities to find the target and father robot positions, and recover the target object respectively.

(a)

(b)

Fig. 2 Fixture mechanism of the microrobots: (a) locked state and (b) free state [9]

Fig. 3 Two kinds of microrobots: (a) eight-legged microrobot and (b) hexapod

microrobot

III. DESIGN AND MOTION MECHANISMS OF MICROROBOTS

A. ICPF Actuator

ICPF is an innovative material made of an ionic polymer membrane, chemically plated with gold electrodes on both sides. When an external electric stimulus is applied to this material, a bending deformation occurs due to the change in the chemical structure. Based on this phenomenon, this material can be used as an actuator. When voltage is applied on both electrodes, ICPF will bend toward the positive electrode.

B. Structures of the Underwater Microrobots

We developed two insect-like underwater microrobots, including the eight-legged microrobot and the hexapod microrobot. The eight-legged microrobot consists of a main body and eight ICPF actuators, four of them are used as supporters and others are used as drivers. The body of microrobot is 24 mm long, 24 mm wide and 3.5 mm height. Eight actuators are all 17 mm long, 3 mm wide and 0.2 mm thick and have one degree of freedom. And two light sensors are mounted on the front of the microrobot; all of them are facing forward.

The hexapod microrobot is composed of a main body, a passive tail fin and nine ICPF actuators, which are used as legs, hands and tail respectively. Two middle actuators are used as supporters. The robot is 67mm long, 56mm wide and 16mm height. Nine actuators are all 17mm long, 3mm wide and 0.2mm thick and have one degree of freedom.

C. Mechanism of the Walking/Rotating Motion

The two kinds of microrobots have the same mechanism of walking and rotating motions. The robots can implement stick insect-inspired walking/rotating motions by using supporters and drivers. The two robots can finish one step cycle of moving forward and rotating motions through four steps, as shown in Figs. 4 and 5 respectively. The drivers provide the propulsion for the robots and the supporters are used to reduce the resistance from the ground. Set the oscillating frequencies of drivers and supporters to the same value, and set the phase of supporters to a value which lags behind the phase of the drivers by 90°. According to the above, we can realize a series of walking and rotating motions, including walking forward/backward, rotating in clockwise/ counterclockwise.

Assuming that the tip displacement of ICPF actuator is equals to /2, we can get the average speed by (1). (1)

where is the forward distance of the robot, is the average speed and is the control frequency. Through the previous research results, we know that the tip displacement is in inverse proportion to the frequency. Thus, with the frequency increasing, the average speed will increase first and then decrease to zero.

Assuming the rotation angle in one step cycle of the robot is an angle of θ , as shown in Fig. 6 (a), we arrive at the following relationships:

(2) · 2 (3)

10 17.5 (4) · . ·

(5) where is the arc length which the moving side of the driver turns, is rotation radius with the centre of point O, is the displacement between initial position and final position of the moving side of the driver, which is approximately equal to , ω is the rotational speed and is the control frequency. And the rotational speed is described by (5).

D. Mechanism of the Grasping Motion

The hexapod microrobot with two ICPF hands can realize the grasping motion by bending the two front ICPF actuators inwards. The bending force generated by the two ICPF hands is determined by the driving voltage and the tip displacement.

At a given voltage, as the deformation increases, the bending force decreases. Hence, the performance of grasping motion is determined by the size of the target object.

E. Mechanism of the Floating and Swimming Motions

At a control frequency of driving voltage lower than 0.5 Hz, the water around the surface of the ICPF actuators can be electrolyzed to generate bubbles. The buoyancy of the robot increases with the increasing volume displacement generated by the bubbles. For the hexapod microrobot, through swing the tail fin and pushing water, it can realize the swimming motion. The swimming direction can be controlled by changing the swing frequency and amplitude.

Fig. 4 One step cycle of moving forward motion (The marks ● indicate which

actuator contacts the ground)

Fig. 5 One step cycle of rotating motion (The marks ● indicate which actuator contacts the ground)

Fig. 6 (a) The rotation angle in one step cycle. (b) The calculation of the value

of . (Only drivers are drawn)

IV. PROTOTYPE MICROROBOTS AND EXPERIMENTS

A. Prototype Microrobots

The two prototype insect-inspired underwater microrobots are constructed, as shown in Fig. 7. The microrobots are connected to the father robot by a kind of enamel covered copper wires with a diameter of 0.03mm. The resistance of the wires could be ignored by the reason of its high flexibility. The eight-legged microrobot is 24 mm long, 24 mm wide and 3.5 mm height. The weight of it is 2.8 g. The hexapod

microrobot is 67 mm long, 56 mm wide and 16 mm height. The weight of it is 5.3 g.

B. Walking/ Rotating Experiments in a Water Tank

Walking and rotating experiments for the two kinds of microrobots are conducted in a water tank to evaluate the performance of the robot at a given control voltage of 8V. During the experiments, we changed the applied signals and calculated the walking speed and rotational speed of the microrobot in each signal by recording the time separately. All experiments were repeated 5 times at a set of control signals to achieve an average speed.

The experimental results of walking and rotating motions for the two kinds of robots are shown in Figs. 8 and 9. From the results of the eight-legged microrobot, at a control frequency of 3 Hz a maximum walking speed of 18.5 mm/s and a maximum rotational speed of 0.5 rad/s were achieved. From the results of the hexapod microrobot, a maximum walking speed of 2.83 mm/s at a control frequency of 1.5 Hz and a maximum rotational speed of 0.39 rad/s at a control frequency of 1.25 Hz were achieved.

Fig. 7 Prototype microrobots: (a) eight-legged microrobot and (b) hexapod

microrobot

Fig. 8 Experimental results for the eight-legged microrobot

Fig. 9 Experimental results for the hexapod microrobot

Fig. 10 The swimming speed of the hexapod microrobot

C. Swimming Experiments of the Hexapod Microrobot

We conducted the swimming experiments of the hexapod microrobot on the calm water. The robot can generate thrust by bending the caudal fin. We did the experiments and calculated the swimming speed at a control frequency ranged from 0.25 Hz to 3.25 Hz. The experiment was also repeated 5 times to get the average speed at a voltage of 8 V. Figure 10 shows the experimental results of the swimming motion.

From the results, with the increasing frequency, the average swimming speed increased to the peak and then decreased. A maximum swimming speed of 8.25 mm/s was achieved at a frequency of 1.75 Hz.

D. Grasping Experiments of the Hexapod Microrobot

For realizing the application of object recovery, a proximity sensor was mounted on the bottom of the robot behind the two hands, as shown in Fig. 11. The dimemsion of the sensor is 12 5 mm and the weight of it is 0.8 g. The measurable distance range of the sensor is from 0 cm to 6 cm and the output voltage of it is from 150 mV to the power voltage. The measurable angle distance of the sensor is from -

30 º to 30 º. During the experiments, the robot will be controlled to move along the path which has been configured. When the robot gets closed to the object target, the micro-controller will receive data from the proximity sensor and judge the distance to the target in real time, and then change the motion of the hinds to grasp the object.

In our previous research, the performance evaluation of the sensor has been finished [11]. By changing the distance between the sensor and the obstacle from 60 mm to 0 mm, we got the output voltage. As shown in the results, the output voltage change rapidly when the distance is less than 25 mm.

Thus, when the robot does the application of object recovery, the hands will open at a distance of 20 mm to the object and then close to catch the object at a distance of 10 mm.

Fig. 11 Proximity sensor mounted on the robot

E. Tracking Motion of the Eight-legged Microrobot

For practical applications, it is important to realize the autonomic father-son robot cooperation system. For the microrobots, they should find the target object automatically and then carry it. For the robot system, it is necessary for the father robot to recover the microrobots with the object after the work finished.

Before installing the light sensors on the robot, the performance of the photodiode was evaluated. An infrared LED and a photodiode constitute a optical coupler. When the centers of the infrared LED and the photodiode are in a straight line, we set the angle of this position to be 0 º. By changing the direction and displacement of the infrared sensor, we measured the output voltage of the photodiode in the angular range from 0 º to 60 º and at a certain displacement of 0.5 cm, as shown in Fig. 12. From the results, the output voltage will increase as the distance between the infrared LED and the light sensor get close. At a given displacement, the output voltage are approximately equal at an angular range from 0 º to 20 º. Furthermore, at a given angle, the output voltage is in inverse proportion to the displacement. At an angular range over 30 º, the output voltage is in inverse proportion to the input angle.

The results of the detectable maximum displacement of the sensor in different angles are shown in Fig. 13. The amplitude of the angle is in inverse to the displacement. Within this range of angle and displacement, the light sensor can detect the light source well.

Thus, two light sensors were mounted on the front of the robot symmetrically, facing to the moving direction of the robot, as shown in Fig. 7 (a). By comparing the value of the output voltage of the two light sensors, the robot will change

its motion among walking straight, turning right and turning left. We have conducted the tracking experiments of the eight-legged microrobot in a water tank, as shown in Fig. 14. During the experiments, we carried a red LED with the infrared LED for making the experiments to be observed easily. The microrobot will face to the LEDs by rotating motion, and then move to the LEDs straightly.

In order to realize the function of the microrobot recovery, we put several infrared LEDs on the same place on the father robot, facing to different directions. Using the same priciple, the microrobot can find the father robot and get back to it with the light sensors.

Fig. 12 Calibration results for the light sensor

Fig. 13 Detectable range of the light sensor

Fig. 14 Tracking experiments

V. CONCLUSIONS

In this paper, two kinds of ICPF actuator-based insect-inspired microrobots for mounting on the father robot were developed. They are an eight-legged microrobot and a hexapod microrobot. Through the underwater experiments, we confirmed the basic operations of the two robots, including walking, rotating, swimming and grasping motions. From the experimental results of the eight-legged microrobot, at a control frequency of 3 Hz a maximum walking speed of 18.5 mm/s and a maximum rotational speed of 0.5 rad/s were achieved. From the experimental results of the hexapod microrobot, a maximum walking speed of 2.83 mm/s at a control frequency of 1.5 Hz, a maximum rotational speed of 0.39 rad/s at a control frequency of 1.25 Hz and a maximum swimming speed of 8.25 mm/s at a frequency of 1.75 Hz were achieved.

Using a proximity sensor, the hexapod microrobot can realize the operation of object recovery. We also evaluated the performance of photodiode and carried two light sensors on the front of the eight-legged microrobot to realize the tracking motion and microrobot recovery.

ACKNOWLEDGMENT

This research is partly supported by National Natural Science Foundation of China (61375094), Key Research Program of the Natural Science Foundation of Tianjin (13JCZDJC26200), National High Tech. Research and Development Program of China (No.2015AA043202), and SPS KAKENHI Grant Number 15K2120.

REFERENCES [1] B. Gao, S. Guo, “Development of an Infrared Sensor-based Wireless

Intelligent Fish-like Underwater Microrobot”, Proceedings of 2010 IEEE International Conference on Information and Automation, pp. 1314-1318, Harbin, China, June 20-23, 2010c.

[2] Q. Pan, S. Guo, T. Okada, “A Novel Hybrid Wireless Microrobot”, International Journal of Mechatronics and Automation, Vol.1, No.1, pp. 60-69, 2011.

[3] Gill, J.J., Ho, K., Carman, G.P., “Three-dimensional thin-film shape memory alloy microactuator with two-way effect”, J. Microelectromech. Syst. 11, pp. 68-77, 2002.

[4] Brunetto, P., Fortuna, L., Graziani, S., Strazzeri, S., “A model of ionic polymer–metal composite actuators in underwater operations”, Journal of Smart Material and Structures, 17, pp. 025-029, 2008.

[5] Kim, H. J., “Smart Soft-morphing Structure: Design, Manufacturing, and Application,” Ph.D. Thesis, School of Mechanical and Aerospace Engineering, Seoul National University, 2011.

[6] Chen, Z., Shatara, S., and Tan, X., “Modeling of biomimetic robotic fish propelled by an ionic polymer–metal composite caudal fin,” IEEE/ASME Transactions on Mechatronics, Vol. 15, No. 3, pp. 448-459, 2010.

[7] Kamamichi, N., Yamakita, M., Asaka, K., and Luo, Z. W., “A snake-like swimming robot using IPMC actuator/sensor,” Proc. 2006 IEEE International Conference on Robotics and Automation (ICRA 2006), pp. 1812-1817, 2006.

[8] Yeom, S. W. and Oh, I. K., “A biomimetic jellyfish robot based on ionic polymer metal composite actuators,” Smart Materials and Structures, Vol. 18, Paper No. 085002, 2009.

[9] M. Li, S. Guo, H. Hirata, H. Ishihara, "Design and Performance Evaluation of an Amphibious Spherical Robot", Robotics and Autonomous Systems, doi:10.1016/j.robot.2014.11.007, Vol. 64, pp. 21–34, 2015.

[10] S. Guo, M. Li, C. Yue, “Performance Evaluation on Land of an Amphibious Spherical Mother Robot in Different Terrains”, Proceedings

of 2013 IEEE International Conference on Mechatronics and Automation, pp. 1173-1178, 2013.

[11] L. Shi, S. Guo, K. Asaka, S. Mao, “Development and Experiments of a Novel Multifunctional Underwater Microrobot”, Proceedings of the 2010 IEEE International Conference on Nano/Molecular Medicine and Engineering, pp. 1-2, Hong Kong, China, December 5-9, 2010.

[12] L. Shi, S. Guo, M. Li, S. Mao, N. Xiao, B. Gao, Z. Song, K. Asaka,“A Novel Soft Biomimetic Microrobot with Two Motion Attitudes”, Sensors, Vol. 12, No. 12,pp. 16732-16758, 2012.

[13] S. Guo, M. Li, L. Shi and S. Mao, “A Smart Actuator-based Underwater Microrobot with Two Motion Attitudes”, Proceedings of 2012 IEEE International Conference on Mechatronics and Automation, pp.1675-1680, 2012.

[14] S. Guo, L. Shi, K. Asaka, and L. Li, “Experiments and Characteristics Analysis of a Bio-inspired Underwater Microrobot”, Proceeding of the 2009 IEEE International Conference on Mechatronics and Automation, pp.3330-3335, Changchun, China, August 9-12, 2009.

[15] Z. Wang, G. Hang, J Li, Y. Wang, K. Xiao, “A microrobot fish with embedded SMA wire actuated flexible biomimetic fin”, Sensors and Actuators, A 144, pp.354–360, 2008.

[16] B. Behkam, M. Sitti, “Design methodology for biomimetic propulsion of miniature swimming robots,” Journal of Dynamic Systems, Measurement, and Control, Vol.128, No. 1, pp. 36-43, 2006.

[17] Cilingir, H., Menceloglu, Y., Papila, M., “The effect of IPMC parameters in electromechanical coefficient based on equivalent beam theory”, Journal of Electroactive Polymer Actuators and Devices (EAPAD), Proc. of SPIE. Vol. 6927, 69270L, 2008.

[18] S. Kim, I. Lee and Y. Kim, “Performance enhancement of IPMC actuator by plasma surface treatment,” Journal of Smart Material and Structures, Vol. 16, pp.N6-N11, 2007.

[19] Carrozza, M. C., Arena, A., Accoto, D., Menciassi, A., Dario, P.,“A SMA-actuated miniature pressure regulator for a miniature robot for colonoscopy”, Sensors and Actuators A: Physical, Vol. 105, No. 2, pp.119-131, 2003.

[20] S. Liu, M. Lin, Q. Zhang, “Extensional Monomeric Polymer Conductor Composite Actuators with Ionic Liquids”, Electroactive Polymer Actuators and Devices (EAPAD), Proc. of SPIE, Vol. 6927, pp. 69270H, 2008.