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Design and Characterization of a Fully Autonomous Under actuated Soft Batoid like Robot T V Truong1 V K Viswanathan1 V S Joseph2 and P Valdivia y Alvarado1 2 3 Abstract Batoids use their large pectoral fi ns to achieve unique maneuverability and propulsive performance In this work the design fabrication and characterization a soft batoid like robot is presented The robot is designed to mimic un dulatory rajiform locomotion The design is under actuated simple and robust and well suited for propulsive performance experiments thanks to its full autonomy long battery life and wireless recharging capabilities The robot has a 180mm body length a maximum fl apping amplitude of 60deg and reaches a peak speed of 0 93 body length per second In order to charac terize its swimming kinematics and propulsive forces a special setup was built using an instrumented holder and high speed video Experiments were conducted to characterize propulsive force generation with varying fi n fl apping frequencies from 1hz to 2 4hz amplitudes and wavelengths The results show that while both fl apping frequency and amplitude infl uence propulsive forces for the locomotion modes tested fl apping frequency has a stronger effect on both thrust and side forces Furthermore larger side forces than thrust forces are produced for the same swimming parameters I INTRODUCTION Batoids cartilaginous fi shes including rays skates and their relatives are a type of fi sh that has garnered special attention due to their superior maneuverability 1 They are characterized by dorsoventrally fl attened bodies and enlarged pectoral fi ns 2 and by swimming modes in which propulsive forces are generated by either oscillation or undulation of their enlarged pectoral fi ns 3 These span wise and chord wise fi n deformations enable impressive control over three dimensional body motions 3 4 Batoids using undulatory locomotion display high maneuverability during turning backward swimming and impressive position control at slow speeds 4 As a result many studies have investigated the fl uid mechanics bio mechanics materials and mechanisms related to this type of locomotion in order to gain insights on the principles that enable such per formance 3 5 6 An effective approach has been to utilize bio inspired mechanisms such as robots to test various hypotheses and clarify the basic principles enabling batoid performance 5 7 Various groups have attempted to develop underwater robots capable of replicating batoid movement features Wang et al presented the design of a wireless manta ray robot actuated by shape memory alloys SMAs embedded This project was supported by SUTDs Digital Manufacturing and Design Centre DManD 1SUTD International Design Centre IDC 2SUTD Digital Manufacturing and Design Centre DManD 3Engineering Product Development Pillar Singapore University of Tech nology and Design SUTD Corresponding author pablov sutd edu sg within a latex membrane fi n Their design could achieve a maximum swimming speed of 0 23 body lengths per second BL s 8 Gao et al developed a cownose ray robot with two actuators and fi ns made of compliant silicone rubber which could reach a maximum swimming speed of 1 0 BL s 9 Chen et al used ionic polymer metal composites as artifi cial muscles for a robotic manta ray 10 The robot was able to swim at 0 067 BL s with portable power consumption under 2 5 W Ma et al developed a cownose ray inspired robotic fi sh which was propelled by oscillating and chord wise twisting pectoral fi ns 11 The chord sections of their pectoral fi ns were designed with standard airfoil sections and its fl exible ribs were made from silicone rubber with outstanding elasticity and anti tear properties The maximum reported forward swimming speed was of 0 94 BL s Autonomous robotic manta rays have also been developed using assemblies of crank sliders subsystems individually actuated by servomotors 12 13 The fi rst RoMan I 12 and second RoMan II 13 prototypes were driven by a total of 20 and 6 servomotors respectively in order to actuate two fi ns The steady forward swimming speed of RoMan II reached 0 8 BL s However a high number of actuators can cause an increase in control complexity an increase of power consumption and a low mean time to failure value 14 An advantage strategy adopted by Valdivia Y Alvarado whose robotic stingrays are made out of a soft material and only require one actuator per fi n 15 16 17 18 Chew et al presented a prototype of the Robot Manta Ray in which the oscillatory motion of each fi n is controlled by only one servo motor 19 The fi n is designed with a leading edge structure spare that supports a fl exible fi n fi lm material made of PVC fi lm with uniform thickness and the fl apping fi n frequency could archive to 1 1Hz and the highest speed was recorded to be 1 783 BL s To measure thrust generated by a single fi n the replaced servomechanism was clamped above the water while the fi n was fl apping near the water surface However this fore measurement setups have the limitations such as the effect of water surface and the movement of separated actuator might not consistent with the robot manta ray and the thrust performance of a Robo manta ray has not been measured Despite the considerable progress in implementations of fi n ray like mechanisms for robots there is a lack of studies which characterize the three dimensional propulsive force and kinematics of batoid like robots In this work we de signed and fabricated an autonomous soft batoid like robot with a small wingspan that can mimic undulatory raijform locomotion The design is small robust fully autonomous 2019 IEEE RSJ International Conference on Intelligent Robots and Systems IROS Macau China November 4 8 2019 978 1 7281 4003 2 19 31 00 2019 IEEE5826 a b c d Fig 1 Robot design a Isometric view of under actuated soft Batoid like robot The majority of the robot s body i e its large fi ns are made of a soft polymer and only one rigid fl apper actuates each fi n by regulating local curvature b Robot performing autonomous free swimming maneuvers c Exploded view of robot mechanism showing all its internal components d Block diagram of overall circuit architecture The current design uses a wireless charger module to recharge the on board LiPo battery which powers both the micro controller unit and two servo motors The inductive charging unit can operate through the robot soft body structure The microcontroller supports BLE connection which allows data to be transmitted and received without the need of opening up the robot structure and well suited for these types of performance studies An apparatus to measure three component forces and kinematics of the batoid like robots was also designed and experiments were conducted to characterize propulsive force generation with varying fi n fl apping frequencies from 1hz to 2 4hz amplitudes and wavelengths This paper is organized in the following order In Section II the design and fabrication of soft batoid like robots is presented Section III describes the experimental setup and testing procedures The kinematic characterization force measurement and swimming speed are summarized in Sec tion IV Finally the conclusions of this study and suggested future work are presented in Section V II DESIGN AND FABRICATION A Mechanism Design The design approach of this robot follows our previous studies in which a soft polymer based body mimics the morphology and kinematics of rajiform batoids A platinum cure silicone mix is used to create a continuum between the robot fl exible fi ns and its body by encapsulating a central shell were actuation power and the microcontroller unit are housed all delicate components are fully encapsulated by the silicone mix A fl exible fi n is embedded inside each fi n Periodic curvature control of each fi n forces structural waves to propagate downstream the soft fi ns generating propulsive forces The fi n structure mimics the lepidotrichia of ray fi nned fi shes The passive waveform reduces the com plexity of the actuation and is less susceptible to mechanical failure 17 The robot has a body length of 180 mm and weighs 310 g Fig 1 shows details of the robot mechanism design and its main electronic components The robot structure is composed of two major parts a soft skin and a core shell Fig 1 a The core shell houses all the electronic components The shell fl apper and several internal components were manufactured using a Fortus 3D printer FDM and ABS thermoplastic fi lament The shell with all the actuation mechanisms Fig 1 c are later placed inside a mold to cast the batoid like body shape The robot is designed to be neutrally buoyant and its center of mass CM is closely aligned with its center of buoyancy CB The two fl appers on either side of the robot are actuated to 5827 steer the robot forward sideways and to perform pitch yaw and roll turns B Electronics Initial experiments were conducted with a single fl apper actuator embedded in a sample skin and rigidly mounted on a platform Actuation signals were generated from an external computer An INA219 sensor was used to measure the total power consumed by the actuator and control electronics at different fl apping frequencies f and amplitudes a The maximum power consumed at peak frequency and amplitude was 15 2 W A corresponding power management module was designed with a 20 safety factor The small nature of the current robot prevents the imple mentation of a split power unit for actuation and control As a result a single cell Li Po battery 3 7V 2 2Ah Adafruit USA powers the entire system The cylindrical battery cell with a higher power to weight ratio is placed in the belly of the robot to help balance the CM location The higher operating voltage of the actuators and micro controller is achieved using a step up voltage converter A 6V regulator U3V50F6 Pololu USA is employed for this purpose and used in conjunction with a relay and reed switch combination to switch between operation and charging mode As shown in Fig 1 c the control circuit and power management modules are housed in the inner shell with support structures designed to hold the servo motors battery micro controller wireless charger and DC DC converter To ensure full autonomy a wireless receiver module 5V 1A from DFRobot China is added in the head of the robot as close as possible to the silicone body surface 3mm to achieve maximum power transfer effi ciency approximate 90 percent at 2mm The corresponding wireless transmitter module is mounted on a 3D printed mold that has the same 3D surface as the stingray and acts as a charging station The wireless receiver module is coupled to the Micro USB Li Po charger and provides a stable charging current of 500mA to the battery Fig 1 d The charger is con nected to a high current capacity 7 5A 3V dual latching relay TX2L3TH Panasonic Japan that switches the battery terminals between charger charging mode and the DC DC converter operation mode Since all electronic components are housed inside the shell a magnetically activated reed switch 08642 Sparkfun USA was used to activate the relay without compromising water tightness The reed switch is placed at the posterior end of the robot so that there is minimal magnetic interference from the servo motors An external neodymium magnet drives the robot to the operation ON mode and upon placing the robot on the charging station it switches to charging mode Once the battery is full the charging circuit shuts off The charging circuit is capable of providing 5V 1A MAX 1 2A The circuit is based on resonant magnetic coupling that reduces the electricity consumption during power transmission with up to 90 percent effi ciency Robot control is implemented using a Bluno Nano mod ule DFRobot China based on ATmega328 which is a low power 8 bit RISC micro controller with an oscillator frequency of up to 20MHz The on board BLE chip TI CC2540 enables wireless programming and data trans mission through low power Bluetooth connectivity At the ground charging station a Bluno Link dongle 1220 DFRobot China acts as a master node that can collect robot data from the Nano module and upload executable code over riding the operation mode with a stable connection up to 15m Each lepidotrichia inspired fl apper is actuated by a micro high torque servo motor HD 1810MG Pololu USA The micro controller computes the required fl apper angles using a simple sinusoidal function based on the input frequency and amplitude independently for each actuator The correspond ing PWM signals are then fed to the servos The servos are powered via the step up regulator and a super capacitor to fi lter out current spikes C Soft Fin Fabrication Ecofl ex 0030 SLO JO Silicone Thinner and pigments Silc Pig were purchased from Smooth on and thoroughly pre mixed before using A silicone mix was prepared mixing 1 part Ecofl ex 0030 Part A 1 part Ecofl ex 0030 part B 4wt SLO JO Platinum Silicone Cure Retarder and adding 10wt Silicone Thinner to the resultant mixture In addition 0 2wt of yellow pigment was added Once the proper ratios of the constituents are combined they are mixed at 2000 rpm in a ARE 310 Thinky mixer for 2 min followed by de foaming at 2200 rpm for 1 min The mix was immediately poured into the molds to encapsulate the core shell and fl appers The curing process took 24h and then carefully removed from the molds III EXPERIMENTS Fig 2 shows the experiment setup and the robotic stingray used in this work The robot was attached to a specially designed clamp 3D printed in ABS to perfectly match the contour of the robot body and immersed inside a water tank 100cm long 100cm wide and 70cm deep The fi n span is small 6 cm and thus the side walls have no noticeable effect on the force measurements Regardless a waiting period of 1 minute is observed between each experiment to ensure any refl ected wake has completely dissipated The robot was placed diagonally close to one corner of the tank when measuring Fx to maximize the space behind the robot again to minimize potential refl ection effects of the robot wakes bouncing back from the walls Motions of the soft fi ns were recorded with a high speed camera Photron APX at 400 fps and a pixel resolution of 1200 x 800 fi tted with a 50 mm Nikon lens A six axis force torque sensor Nano17 ATI Industrial Automation Inc Apex NC USA is used to measure the instantaneous forces on the stingray robot The force sensor 17 mm diameter 20 1 mm length is attached to the center of the mounting clamp Fig 2 a The F T software ATI industrial Automation was used to record force data from the sensor The bias level of the load cell was set to zero 5828 a b Fig 2 Experiment setup a Tests are carried in a 1 2m 1 2m 0 4m tempered glass tank The robot is mounted to a rigid fi xture and held at the middle of the tank A 6 axis load cell attached to the holding fi xture and high speed imaging are used to characterize the robot 3D kinematics and propulsive forces during tests b Sequential images of the fi n motions at the end of the down stroke at four fl apping frequencies captured at 400 fps Flapping amplitudes of 60 deg and 30 deg are used in the fi rst column and second column respectively in the F T software before the fi ns moved Using a data acquisition board the force data were recorded at a sampling rate of 120Hz The raw data were then exported and a spectral analysis was done to identify the noise due to vibration The force signal was then fi ltered offl ine with a zero phase delay low pass digital Butterworth fi lter For all force measurements the force value was averaged from 10 cycles A pause of 1 minute was taken between each experiment to ensure there was no convection fl ow inside the tank IV RESULTS A Kinematic Characterization The fl apping angle instantaneous angle of the driving servo motor of each fl apper t is given by t A 2 sin 2 ft 2 1 where A is the desired maximum angular displacement of the servo in degrees and f is the desired fl apping frequency in hertz The maximum angular displacement of the cur rent design is 60deg In order to investigate the effects of fl apping amplitude on propulsive performance experiments were conducted for four different input amplitudes 30 40 50 and 60 degrees The robot does not show any signifi cant motion and the force generation is too small when its fi ns are fl apping below 30 degrees of amplitude For the experiments the fl apping frequency was changed from 1Hz to 2 4Hz with 0 2Hz increments Fig 2 b shows snapshots of the fi n motion recorded at 400 fps at a fl apping frequency of 1 8hz and input amplitudes of 30 and 60 degrees The undulation mode can be clearly identifi ed as the fi n motions display wavelengths shorter than the fi n length The wavelength was defi ned as the distance between two peaks or troughs along the fi n In Fig 2 b the fi ns were at the end of a downstroke and it was observed that shorter wavelengths were produced as fl apping frequency increased Flapping at the lower amplitudes also produced shorter wavelengths Fig 3 shows the input amplitude the measured output of the fl apping angle and angular velocity of the tip fl apper for the case with an amplitude of 60deg The shaded region corresponds to the upstroke period The non dimensional time t T where T 1 f is the fl apping period is 0 5 for the upstroke and 0 51 for the downstroke The output amplitude was 63 50 which is higher than the input amplitude due to the bending of the fl apper caused by the initial forces of the fi ns The output ratio h BL between the displacement of the fi n tip h at the location of the fl apper and body length BL for the input amplitudes of 30 40 50 and 60 deg is 0 18 0 23 0 29 and 0 31 respectively The wave number 2 which is indicative of the swimming mode e g undulatory or oscillatory is impor tant when analyzing the behavior of the wave propagation Fig 3 Time history of the fl aping angle and angualar velocity of fi n kinematics 5829 Fig 4 Fin w

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