Nina Petelina. Submitted to the Department of Mechanical Engineering in Partial Fulfillment of the Requirements for the Degree of

Transcription

1 Hydrodynamics of Magnet-Coil Actuated Robotic Fish by Nina Petelina Submitted to the Department of Mechanical Engineering in Partial Fulfillment of the Requirements for the Degree of Bachelor of Science in Mechanical Engineering at the Massachusetts Institute of Technology June 2017 MASSACHUSETTS INSTITUTE OF TECHNOLOGY JUL, 2 5Z?1-1 LIBRARIES ARCHIVES k 2017 Massachusetts Institute of Technology. All rights reserved. Signature of Author: Sig nature redacted Department of Mechanical Engineering May 17, 2017 Certified by: Accepted by: Signature redacted - Alexandra H. Techet Associate Professor of Mechanical and Ocean Engineering Signature redacted Thesis Supervisor Rohit Karnik Associate Professor of Mechanical Engineering Undergraduate Officer

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3 Hydrodynamics of Magnet-Coil Actuated Robotic Fish by Nina Petelina Submitted to the Department of Mechanical Engineering on May 12, 2017 in Partial Fulfillment of the Requirements for the Degree of Bachelor of Science in Mechanical Engineering ABSTRACT The hydrodynamics of two robotic fish were analyzed: a low cost toy robotic fish for developing live fish experimental techniques, HEXBUGT-"! Aquabot, and a low cost robotic fish for swarm robotics experiments, Scuba Fish. Both of these robotic fish use a magnet-coil actuation method in order to create caudal fin motion. A velocity imaging technique,, Particle Image Velocimetry (PIV), was used in order to characterize the wake structure created by the tail beat. Both robotic fish were towed through water as a high speed camera recorded the movement of seeding particles around the caudal fin, which are illuminated with a laser. From 2D PIV and 3D Synthetic Aperture PIV experiments for the Aquabots it has been identified that the discrete tail beat from the bang bang control creates vortex pairs at each start or stop motion of the caudal fin. Moreover, the wake structure from the shark Aquabot tail beat creates a wake structure similar to live dogfish sharks. Since the design of the Scuba Fish allowed more control over the motion of the tail, an additional ramp pwm caudal fin control was designed and tested in order to analyze the wake from a continuous tail beat. The results show that the vortex shedding pattern created by the pwm ramp design is different from the bang bang cases; the method creates a negative vortex ring with a small vortex pair at the end of the motion. This suggests that further designs of a continuous control have to be investigated in order to achieve a more real fish-like swimming behavior. Thesis Supervisor: Alexandra H. Techet Title: Associate Professor of Mechanical and Ocean Engineering

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5 Acknowledgments. I would like to acknowledge the amazing people working in the Experimental Hydrodynamics Laboratory and thank them for hiring me as a sophomore, who didn't even take I would like to thank Leah Mendelson for helping and teaching me so much throughout my UROPs, APS presentation, and my thesis. I would also like to thank Professor Alexandra Techet for being my thesis advisor and the graduate students in the lab, Abhishek Bajpayee, Andrea Lehn, Aliza Abraham, and Barry Scharfman. I would also like to thank Florian Berlinger and Jeff Dusek from the Self-organizing Systems Group in Harvard University for giving me the chance to work with their Scuba Fish. Additionally, I would like to thank the professors and lab instructors in Mechanical Engineering department at MIT for the knowledge I got in their classes that allowed me to build and work on this experiment. I want to also thank my peers for helping me with ideas of how to make things work and the MIT MakerWorkshop for giving me access to the tools in order to build parts for my experiments. And lastly, I want to thank the HEXBUGTMs, the Scuba Fish, and the giant danios.

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7 Table of Contents Abstract 3 Acknowledgements 5 Table of Contents 7 List of Figures 8 List of Tables 9 1. Introduction Interest in simple swimmers Thesis goals Thesis structure Background Evaluating kinematics Particle Image Velocimetry (PIV) HEXBUGTM Aquabots What are the HEXBUGTI Aquabots How do they work? What alterations do HEXBUGT! Aquabot require Experimental Setup D PIV instrumentation and methods D PIV results D SA PIV on the shark HEXBUGT' instrumentation D SA PIV results Conclusions and future work Scuba Fish Experiments Introduction to the Scuba Fish Experimental design for the Scuba Fish PIV methods and instrumentation PIV results Conclusions and future work Overall conclusions Bibliography 44

8 List of Figures Figure 2-1: The schematic for the experimental setup for 2D PIV 15 Figure 2-2: The schematic for the experimental setup for 3D SA PIV experiments 15 Figure 3-1: The shark and angelfish HEXBUGTm Aquabots 16 Figure 3-2: The schematic for the Aquabot caudal fin magnet-coil mechanism schematic 17 Figure 3-3: All components of the HEXBUGTM Aquabots 18 Figure 3-4: Surface distortion for a free swimming Aquabot 19 Figure 3-5: The CAD and picture of the robotic fish mount 21 Figure 3-6: Sample Arduino for controlling the motor-winch mechanism 22 Figure 3-7: Circuit schematic for the motor-winch mechanism 23 Figure 3-8: Picture of the motor-winch mechanism 23 Figure 3-9: The overall setup for the 2D PIV experiment with the Aquabots 24 Figure 3-10: The vortex patterns for the Aquabot towed at the original speed 25 Figure 3-11: The vortex patterns for the Aquabot towed at the adjusted speed 26 Figure 3-12: The overall setup for the 3D SA PIV experiment with the Aquabots 27 Figure 3-13: The resultant 3D wake structure for shark Aquabot 28 Figure 4-1: The Scuba Fish robot assembly that was used for the PIV experiments 31 Figure 4-2: The mechanical parts of the Scuba Fish used in the experiments 32 Figure 4-3: The H-bridge circuit schematic for the caudal fin motion and frequency control 33 Figure 4-4: The photograph of the two circuits used for the Scuba Fish experiment 33 Figure 4-5: Sample Arduino for the Scuba Fish caudal fin motion control 34 Figure 4-6: An example plot from the waveform calibration for pwm ramp control 35 Figure 4-7: The motor-winch mechanism for towing the Scuba Fish 36 Figure 4-8: The experimental setup for the 2D PIV with the Scuba Fish 37 Figure 4-9: The time series velocity field for the bang bang control of the Scuba Fish 39 Figure 4-10: The time series velocity field for the pwm ramp control of the Scuba Fish bottom plane of the caudal fin 40 Figure 4-11: The time series velocity field for the pwm ramp control of the Scuba Fish middle plane of the caudal fin 41 8

9 TABLE 3-1: TABLE 4-1: List of Tables Frequencies of the HEXBUGrT 1 Aquabots Main dimensions of the Scuba Fish robot used in the experiments

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11 1 Introduction 1.1. Interest in simple swimmers Fish have interested researchers and served as a source of inspiration for underwater vehicle design for a long time due to their unbeatable maneuverability and efficiency of swimming. Insight on fish swimming hydrodynamics can improve the design of the robotic fish in order to achieve a more lifelike fish behavior. Experiments with live fish can show what strategies fish use in order to achieve different swimming performances [6]. The main non-invasive technique used for flow quantification is Particle Image Velocimetry (PIV), which evaluates the velocity fields behind the fish created by the fin motion. The PIV experiments require optimization of many different parameters, such as proper lighting, particle seeding, data collection methods, and algorithm and masking optimization. For example, an improvement to the 3D Synthetic Aperture PIV (3D SA PIV) methods was developed using data from giant danio fish swimming [3]. However, since the fish are hard to control and it is hard to predict the good laser alignment for data collection, the development of the live-fish study techniques using live fish can be time consuming. Therefore, in order to optimize the initial methods for live fish experiments, a simple and predictable robotic fish model that displays a swimming performance similar to live fish has to be used. Moreover, a simple robot with a fish-like swimming performance can be used for experiments, where many cheap robots are required. This thesis will evaluate the swimming performances of two robotic fish, HEX BUGT' Aquabot, a toy robotic fish that can be purchased in a store for live fish experimental method development, and Scuba Fish, a simple robotic fish for studies of swarm robotics. Both of these fish use a similar magnet-coil actuation method for the caudal fin that uses magnetic field created in coils that moves the magnet attached to the tail, and therefore, creates a tail beat motion. I I

12 1.2. Thesis goals Even though both robotic fish use the same caudal fin actuation mechanism, the goals for the experiments are not exactly the same, and the study of the Scuba Fish is more of an extension from the study of the Aquabots. In the case of the HEXBUGT' Aquabots, the main goal is to analyze the swimming performance of the toy robotic fish and determine whether this low-cost and simple robot has a good enough swimming performance in order to be used as a validation method for developing experimental techniques for live fish studies. Another motivation is to determine whether there are simple modifications to the design of Aquabots in order to improve the swimming performance and make it more fish-like. The goal of the Scuba Fish study is to develop a better understanding of the hydrodynamics of the locomotion of the designed robotic fish. From the collected results, some directions for improvement can be determined and then incorporated into the design, before the Scuba Fish will be used for swarm systems experiments Thesis structure This thesis is divided into three main parts, focusing on the background, experimental methods, and results for the HEXBUGT' Aquabots and Scuba Fish separately. Chapter 2 gives an overview of the techniques used for the experiments with both the HEXBUGTI Aquabots and the Scuba Fish. At first, an overview of the kinematics measurements and analysis is presented. Secondly, the methods and principles of 2D PIV are discussed. Lastly, the methods for 3D SA PIV are discussed. Chapter 3 describes the experiments with the HEXBUGTh Aquabots. This section presents the motivation, experimental setup, results from multiple different cases, and the directions for future work, in order to enhance the swimming performance of HEXBUG'm Aquabots. I 2'

13 Chapter 4 focuses on the experiments with the Scuba Fish. It describes the experimental setup and design for the towing mechanism and the robotic fish, the original and adjusted way to activate the tail beat, results from 2D PIV experiments, and lastly, conclusions and proposed directions for future adjustments. Finally, Chapter 5 discusses the overall conclusions and suggestions for future work with low-cost robotic fish with a coil-magnet actuation design, such as HEXBUGT" Aquabots and Scuba fish. 2 Background The swimming performance of the robotic fish is evaluated from the point of kinematics and dynamics. There are multiple imaging techniques used to describe the swimming performance of HEXBUGTI Aquabots and the Scuba Fish. A brief overview of them is presented below Evaluating kinematics First of all, the kinematics of the robotic fish are determined from the recorded videos of a fish in water. The recorded videos are tracked using a MATLAB package Digitalizing Tools (DLT) developed by the Hendrik Lab [1]. The body of the fish is tracked in order to evaluate the overall velocity (U) of the fish, and then the tip of the tail is tracked in order to measure the amplitude (A) and the frequency (f) of the tail-beat. These parameters are used in order to determine the Strouhal number for the overall movement of the live or robotic fish: =(I) Later the Strouhal number values is compared to live fish Strouhal number, which is normally in the range from 0.2 < ffi< 0.4. Overall, the Strouhal number is a dimensionless number that shows how much momentum is created in comparison to how fast and far the fish moves. 1 3

14 2. 2. Particle Image Velocimetry (PIV) In order to assess the hydrodynamics of the robotic fish movement Particle Image Velocimetry (PIV) is used. This is a non-invasive technique for measuring the velocity field in fluids and getting an image of the wake structure created by a swimming fish. In PIV experiments tracer particles (in these experiments, 50 um polyamide particles) are added to water in the tank and a laser is used to illuminate the particles while the fish swims. The type of the laser depends on the type of the experimental methods and analysis algorithm. For the 2D experiments a sheet NIR laser (Lasiris Magnum II for the experiments in this thesis) with the wavelength of 8 10nm is used, and for the 3D experiments a volume NIR laser (Oxford Lasers Firefly 1000W) is used. In order to eliminate any reflections from the body of robot, all parts that are submerged in water are coated with black matte paint as shown in Figure 3-1. Afterwards, particle movement is recorded by high speed camera arrays. For a 2D crosssection image of the wake, only one high speed camera is required, whereas for a 3D image more cameras have to be used, depending on what algorithm will reconstruct the recorded wake structure. For the shark HEXBUGTm experiment, described in Section 3.6, a technique called 3D Synthetic Aperture PIV was used. The details of how the 3D SA PIV technique is implemented can be found in Belden et al (2010) [4]. For this method 8 high speed cameras record the swimming robotic or live fish. The 3D image of the wake structure is generated by a refocusing algorithm that simulates one camera with a narrow depth of field, and then transforms images over a range of depths. After the data is collected, the cameras a 30-second calibration with a sliding grid is used. The overall experimental setup schematic for 2D PIV is shown in Figure'-- I, and the 3D SA PV is shown in Figure 2-2.

15 Laser Sheet -i I j.- Laser.... C PARTICLES HIGH SPEED CAMERA Figure 2-1. The schematic for the experimental setup for 2D PIV. The main components are the High Speed Camera, NIR sheet laser, and particles. HIGH SPEED CAMERA ARRAY PARTICLES OJJ O NIR -. VOLUME LASER Figure 2-2. The schematic for the experimental setup for 3D SA PIV experiments. In this case, 8 high speed cameras are used and instead of a sheet laser a volume NIR laser is required. The collected images for the 2D PIV are processed via La Vision software DaVis 8.1. The software compares the consecutive images and tracks the movement of groups of particles. Then using this data indicates how velocity fields change from frame to frame. Different masking and post-processing parameters are calibrated in order to generate a clear image of the wake structure. These processing parameters depend on the lighting in the collected data. The processed data shows the velocity field created by fish motion, and can be used to calculate vorticity, overall momentum, impulse, and forces created by a live or robotic fish. 15

16 3 HEXBUGTM Aquabots 3.1 What are the HEXBUGTM Aquabots HEXBUGTM Aquabots are an affordable, about $10 per fish, and simple toy robotic fish that can be easily purchased in a store. These fish come in a variety of species; a shark robot with an asymmetrical caudal fin shape and an angelfish robot with a symmetrical caudal fin were analyzed in the initial study. Both fish are shown in Figure 3-1. Unlike in experiments with live fish, the simple mechanism of the HEXBUGTMs is much easier to control and modify for experiments. Therefore, the swimming performance of the toy robotic fish analyzed in order to identify whether Aquabots could be used to develop experimental techniques and analysis methods to study the hydrodynamics of live fish. Dorsal fin & Caudal fin Shark Pectoral fin Angelfish Figure 3-1: The shark and angelfish HEXBUGTM Aquabots. Both fish robots are painted with black matte paint in order to eliminate any reflections from the laser during the PLV experiments. 3.2 How do they work? HEXBUGTM Aquabots have only one actuated part that creates locomotion - the caudal fin. The toy fish start their tailbeat when the pressure sensors next to pectoral fins come in to 16

17 sh- contact with water. The activation of the switch allows the current to flow through coils that are located next to the caudal fin, resulting magnetic fields within the coils. The change of the current flow direction switches the direction of the magnetic field, and hence moves the magnet that is attached to the caudal tail, which creates the tail motion. The working mechanism schematic is shown in Figure 3-2. The frequency of the caudal fin motion is controlled via a small circuit board. All components of the Aquabots are shown in Figure 3-3. per Coils Magnet \ I / Caudal Fin Fish Body Figure 3-2: The schematic for the Aquabot caudal fin magnet-coil mechanism schematic. The current runs through the copper coils and creates a magnetic field that moves the magnet from side to side, and therefore, moves the caudal fin. In most HEXBUGTm Aquabots these frequencies are randomized in order to create a more natural swimming appearance. However, in the later versions of the RC controlled version of the Aquabots the swimming frequency can be now controlled via a small remote control. 17

18 Fish Body Magnet Holder Coils Coil Holder Magnet Fully assembled HEXBUGTM Aquabot Controller Dorsal and Caudal Fins Figure 3-3: All components of the HEXBUGTM Aquabots. 3.3 What alterations do HEXBUGTM Aquabot require? From the initial testing of the toy robotic fish it was identified that multiple swimming parameters, such as buoyancy, speed, and direction of swimming, have to be controlled and changed in order to achieve a more natural live fish behavior. Firstly, the overall body weight of the HEXBUGTM Aquabots is rather low, which results in positive buoyancy and the toy robot swims close to the surface of water and the caudal fin is not fully submerged in water. This creates ripples on the surface of the water as can be seen in Figure 3-4, and the wake structure from the caudal tail motion cannot be properly analyzed using PIV. In order to have a fully-submerged caudal fin the robotic fish had to be submerged in water via a mounting device and towed through water. In addition, this method allows to align the 18

19 height of laser at different levels of the HEXBUGTM caudal fin, and therefore, acquire the wake structure cross-section image at different heights. Surface Distortion Figure 3-4: This photograph shows the surface distortion as the result of the positive buoyancy of the HEXBUGTm Aquabot. The tip of the caudal fm is about the water surface and creates ripples. Secondly, it has been identified that the toy robotic fish has a preferred direction of tail beat that results in non-straight swimming pattern. For PIV experiments, the laser and the camera focus has to be focused on the particles around the caudal fin of the Aquabot. Therefore, a towing mechanism would also provide a locomotion in a straight line and therefore, allow a more controlled and efficient experiments. Lastly, from the initial calibration experiments it was identified that each toy robotic fish have three different frequency modes and these values are presented in Table 3-1. The approximate speeds are proximately 5.5 cm/sec for the angelfish and 7.5 cm/sec for the shark robot, however, these values will change as the batteries run low. In order to measure these values, a high speed camera recorded the swimming toy robotic fish while it was freely moving in water. Using the DLT package, the body and tail motions were tracked in each video, and the 19

20 using the tracking data, the plots of the tail motion were determined. These plots allowed to fit in a sinusoidal function, that determined the amplitude, frequency of the tail beat motion. Shark Angelfish Low Frequency Mode (Hz) Medium Frequency Mode (Hz) High Frequency Mode (Hz) Table 3.1: The table shows the values for different frequency modes for both the shark and the angelfish Aquabots. These results show that the Strouhal number for different swimming modes are in a range of 0.65 to 0.9 for the shark robot and 0.35 to 0.55 for the angelfish, which are higher than the Strouhal number range for live fish swimming. Therefore, in order to achieve a vortex shedding pattern of the Aquabot tail motion with a Strouhal number in the same range as live fish, the fish has to be towed at a higher speed. Moreover, since the medium and high frequencies are higher than the real fish tailbeat frequency, the towing speed was calibrated only for the low frequency, and only the low-frequency data was recorded and analyzed in the 2D PIV and 3D SA PIV experiments. 3.4 Experimental Setup. The overall towing mechanism has three main parts: the attachment of the HEXBUGTM to the moving mechanism, the sliding block that moves the fish through water, and the motor that controls the movement of the sliding block The fish mount There are multiple design requirements for the Aquabot attachment that have to be satisfied for the experiments. First of all, the shape of the Aquabot attachment to the moving mechanism had to have a streamline shape in order to not disrupt the vortex shedding. Secondly, 20

21 mount design required no holes or other intrusive attachments that could damage the electronics inside body of the fish. The last two requirements are to allow yaw motion and fit both the angelfish and the shark HEXBUGThs. From prototyping a simple vest-like mechanism that uses the pectoral fins as the attachment method was chosen. The design of the attachment incorporates three aluminum parts that were made out of aluminum sheet via a waterjet. The parts are held together using 3 #4 fasteners, and are attached to the sliding component of the towing mechanism with the one of the holes at the sides of the attachment mount. This design also allows a simple and fast change of the toy robotic fish during the experiments. Both the CAD model and the picture of the fish mount are presented in Figure 3-5. Figure 3-5: The left image is the CAD model of the fish mount with the angelfish Aquabot. The right image shows the final manufactured attachment with the shark Aquabot. The vest-mount is made out of aluminum and coated with matte black paint in order to eliminate reflection during the PIV experiments Sliding mechanism and the tank. The Aquabot mount is attached via an acrylic piece made using a laser cutter. This piece is attached to an 8020 aluminum extrusion, which allows to control the depth at which the fish is submerged in water. The assembly is attached to a sliding 8020 block that slides on an

22 extrusion. These materials are simple to assemble and adjust, and therefore, they can fit both the angel fish and the shark Motor, winch, and Arduino control. In order to move the Aquabot through water with a controllable speed a winch and motor mechanism is used. This method is simple, adjustable, does not require a mounting surface on the tank, and instead can be installed on a table or a bench next to the tank. A piece of string connects the sliding block to the winch, and the winch is rotated with a Tamiya Planetary Gearbox motor. The motor and winch block are attached to an acrylic plate that can be mounted on an optical table via standard - 20 fasteners. The winch is connected to the motor shaft via a ball-bearing joint, which allows an almost frictionless rotation of the winch. The speed of the shaft rotation of the motor is controlled with an Arduino Uno microcontroller. Since the robotic fish has to be towed at a constant speed, Arduino supplies a pwm signal with a constant input, which was calibrated for the mechanism for each desired towing speed. Since the Arduino does not provide enough power to the motor by itself, the motor is connected to the output pin of the Arduino via a simple circuit. The circuit schematic is presented in Figure 3-7, and requires a couple of cheap components such as transistor and diode. In order to supply power to the circuit a TENMA DC Regulated Power supply was used, that supplied a constant voltage of 5V. // Define which pin to be used to communicate with Base pin of TIP120 transistor int TIP120pin = 11; //for this project, I pick Arduino's P.M pin 11 vcid setup() pinmcde(tip120pin, OUTPUI); // Set pin for output to control TIP120 Base pin void loop() I analcgiirite(tip120pin, 140);//calibrate the values for the setup Figure 3-6: Sample Arduino code for controlling the motor-winch mechanism 22

23 -j MOTOR GROUND DIOD E N-channel MOSFET Resistor ARDUINO PIN 11 Figure 3-7: The circuit schematic that allows to control the motor with an Arduino Uno microcontroller. Winch Motor*, Motor Mount.r Circuit Arduino Uno Figure 3-8: The motor-winch mechanism setup for the experiments D PIV instrumentation and methods PIV instrumentation. After the parameters for the towing speeds were calibrated, the swimming behavior of the HEXBUGTM Aquabots was recorded with a Vision Research Phantom Miro 310 high speed 23

24 ,Am camera, which is controlled on a computer. The camera was set underneath the tank, in order to record the bottom up view that shows the cross-section of the vortex pattern. For PIV processing a sheet Lasiris Magnum diode continuous wave NIR laser with a wave length of 81 Onm with a 10 degrees fan light sheet. The NIR laser illuminated particles, which are 50 micron polyamide Dantec Dynamics particles. In addition, the body of the Aquabots were painting with a matte black spray paint, in order to eliminate any reflections from the body and the electronics inside the fish, as can be observed in Figure 3-1. In order to achieve proper light collection, the lens was set to the maximum f-stop value, 1.8, this means that the lens gives a narrow depth of field and lets in as much light as possible, and the overhead lights in the room were turned off. Images were recorded at a frame rate of 600 fps with an exposure time of 50 gsec. The overall setup is presented in Figure 3-9 below. NIR sheet laser.-- - Aquabot High peed C mera "Motor and Winch. Figure 3-9: The overall setup of the 2D PIV experiment. One high speed camera is used to capture the motion of particles illuminated by an NIR sheet laser. The Aquabot is towed with a motor and winch mechanism. 24

25 D PIV methods For both of the fish species, the shark and the angelfish, the swimming behavior was recorded for two different cases: natural HEXBUGTM Aquabot speed and the calibrated speed for a natural fish swimming Strouhal number. For each of these cases with five sets for five different laser heights were recorded. This allowed to acquire 2D PIV images for different locations of the caudal fin, and hence for different layers of the vortex patterns created by the motion of the caudal fin D PIV Results Natural HEXBUGTM Aquabot speed The initial set of experiments was recorded for the natural speed of the HEXBUGTM Aquabots. As can be seen on Figure 3-10, for both species, the shark and the angel fish, each caudal fin motion produces a vortex pair. Moreover, since due to low swimming speed of the HEXBUGTMs, the toy robotic fish doesn't move far enough from the vortex pair ater it was created by the tail beat. This causes the wake structure from the caudal tail motion to be disrupted by next tail motion. a) b) 30 Figure 3-10: Graphs for the angel fish and shark Aquabots that were towed at the original speed. Graph a) shows a disrupted vortex pair for the angelfish Aquabot. Graph b) shows the disrupted vortex pair for the shark Aquabot. 25

26 -31K Due to this disrupted the wake structure of the toy fish does not have a live fish-like behavior. This can be explained by the fact, that the Strouhal number values for the cases of the original speed are higher than the range for live fish swimming. This means that the speed of the caudal fin is greater than the speed of the body. The frequency and the amplitude of the tail beat for the Aquabots cannot be adjusted, since these parameters are randomized, and therefore, an increased speed value should be tested in order to compare the hydrodynamics of the HEXBUGTM Aquabots to the hydrodynamics of live fish Adjusted speed to match the Strouhal number of a live fish. The experiments with the higher speed towing have shown that the problem of the disrupted vortex pairs was eliminated. However, for both of the fish species, vortex pairs and not a Karman street pattern was created. As the tail starts to move a vortex pair is created, and when the caudal fin stops to move another vortex pair is created. This start-stop vortex pattern is a result of the bang bang control of the tail beat. Since the magnetic field in the coils changes directions, but remains at the same amplitude, the caudal fin of the Aquabot has a discrete motion, and therefore, creates a start-stop structure. Whereas, live fish create a continuous tail beat motion. I TI=05sec. T2= 0.64Tsse3 s T3=0.100 secl Figure 3-11 a): This figure shows the resultant wake structure at the middle plane of the caudal fin for the angelfish Aquabot at an adjusted speed to match the live fish Strouhal number. Figures show the vorticity field for the angelfish Aquabot, with a St = A start stop vortex pair is created at each tail beat

27 Ad b) 0:11 TI = 0.65 sec T2= 0.75 sec T3 = 0.85 sec Figure 3-11 b): This figure shows the resultant wake structure at the middle plane of the caudal fin for the shark Aquabot at an adjusted speed to match the live fish Strouhal number. Figures show how the vorticity field at various time steps for the shark Aquabot, with a St = Similarly to angelfish a start stop vortex pair is created at each tail beat D SA PIV on the shark HEXBUGTM instrumentation In order to have a more clear understanding of what the wake structure created by the Aquabots is, a 3D image of the wake has to be analyzed. In order to acquire a 3D image of the wake structure created by HEXBUG' Aquabots, 3D Synthetic Aperture PIV technique was used, which is described in Part 2.2. For these experiments only the shark Aquabot was tested. For these experiments 8 high speed cameras were used. All of the cameras were set to a frame rate of 500 fps and an exposure time of 50psec. The setup for this experiment is shown in Figure. Camera Array-- O Aquabot Figure 3-12: The overall setup for the 3D SA PIV experiments. There are 8 high speed cameras in the camera array and the particles are illuminated with a NIR Volume Laser. 27

28 3.8 3D SA PIV Results. The resulting wake structure created by the shark caudal fm can be observed in Figure. There is a vortex pair created at the middle of the tail, which was also present in the 2D data. Moreover, there is a vortex ring created at the tip of the caudal fin. This behavior is caused by the flexibility of the tail. 30' 30' 20' Z (mm) x (mm) Z (mm) x (mm) t = 0.35 s t = 0.39 s E ga ~ -20E X (mm) X0 20' 0 Xmm) Figure 3-13: The resultant wake structure for the shark Aquabot. These figures show the 3D image of the vortex structures created and the kinematics of the tail beat at two different time steps. A vortex pair created at the middle plane of the assymetric tail is created, and moreover, there is an extra vortex ring that is a result from the flexibility of the caudal tail. The Strouhal value for this run is As compared to the results from previous research on shark tail hydrodynamics [5], it can be observed that the vortex structure created by the HEXBUGTM is similar to the one created by the dogfish shark, except for the extra vortex at the tip of the tail. The extra small vortex ring is a result of the flexibility of the asymmetric tail. This shows that the HEXBUGTM Aquabots could 28

29 be used as a verification method for live fish experiments, however, multiple adjustments have to be made. 3.9 Conclusions and Future work. Overall, multiple PIV experiments, both 2D and 3D, were conducted in order to evaluate the swimming performance of cheap and simple toy robotic fish, HEXBUGTM Aquabots. From calibration and initial testing it has been determined that the robotic fish breaks the created wake structure when swimming at the original speed. Further experiments with towed robot have shown that the caudal fin motion creates start-stop vortex pattern, which is a result of the bangbang actuation method. Lastly, the 3D SA PIV experiment with the shark Aquabot has shown that the wake structure created is similar to the wake of a live shark. This proves that the Aquabots are a promising tool as a validation technique, however multiple adjustments have to be made in order to make the swimming performance more fish-like. In order to improve the swimming performance of the HEXBUGTM Aquabots there are multiple issues that have to be addressed. First of all, a better control over the tail motion, such as frequency control and a smoother and sinusoidal behavior, is needed. This can be achieved by externally controlling the caudal fin motion via another actuator. Moreover, this would provide control over the actuation of the tail beat, which would be useful, since the toy robotic fish would turn off during the experiments. Secondly, in order to achieve fish-like Strouhal numbers and neutral buoyancy, Aquabots have to be towed with an external mechanism. Lastly, other materials with larger stiffness have to be tested for the caudal fin for the shark Aquabot, in order to eliminate the extra vortex ring at the tip of the asymmetrical tail. 29

30 4 Scuba Fish Experiments 4.1 Introduction to Scuba Fish The simplicity and the low cost of the magnet-coil actuation design makes the HEXBUGTM Aquabots great for mass manufacturing. This design solution was used by the Selforganizing Systems Group in Harvard University in order to design the Scuba Fish. The Scuba Fish is a simple and low-cost fish robot prototype for swarm robotics experiments, developed by Florian Berlinger [2]. In his masters of engineering thesis Mr. Berlinger presented a design process and the prototype specifications of the Scuba Fish. The initial prototyped robots cost was about $105. This price includes the 3-D printed hull and the electronics that control the entire motion of the fish. The design of the Scuba Fish allows to have more control over the experimental setup and also the swimming performance optimization. First of all, due to the installed Arduino Uno microcontroller the frequency of the tail motion can be easily adjusted and controlled unlike in Aquabots, for which the frequency of the tail beat is randomized. In this chapter, the Scuba Fish experiment is described in three parts. Firstly, the experimental design of the Scuba Fish will be presented, secondly, the design of the experimental setup, and lastly, the results of the experiments and advice on future work and redesign of the Scuba Fish in order to make it hydrodynamically more similar to the live fish. 4.2 Experimental design for the Scuba Fish Since only the hydrodynamics of the caudal fin was analyzed, not a full assembly of the Scuba Fish was used in the experiment. The fish assembly did not have actuated pectoral or dorsal fins, or the electronics that allow for the fish to swim autonomously in water. Due to this constraint, the robotic fish had to be towed through water, just as in the HEXBUGTM Aquabot 30

31 experiments. Therefore, the overall design of the experiment setup can be divided into 3 parts: the mechanical assembly of the Scuba Fish, the electronical parts used to control the movement of the caudal fin, and lastly, the towing mechanism. The photograph of the robot is presented in Figure 4-1. Coil Wires Fish Body F Caudal Fin Figure 4-1: The Scuba Fish robot assembly that was used for the PIV experiments Mechanical assembly Several parts of the full Scuba Fish assembly were used for the PIV experiments: the body, the tail attachments, two copper coils, rubber caudal tail, and two magnets. The main body of the fish are two 3-D printed hulls that have a special cylindrical extrusions to fit coils for the caudal, pectoral, and dorsal fins, and also an axis for caudal fin attachment. The caudal fin is a combination of a 3D printed part that rotates about the axis on the body of the fish and a rubber sheet. For the experiments a simple rectangular sheet was used. The dimensions of the fish are presented in the Table 4-1 below and all of the components can be observed in Figure 4-2 below. Body length Tail length Tail thickness 10 cm 3.5 cm 0.159cm Table 4-1: Main dimensions of the Scuba Fish robot used in the experiments. 31

32 Tail Attachment Coils Magnets Rubber Caudal Fin 3D printed hulls Figure 4-2: The mechanical parts of the Scuba Fish used in the experiment. There are two 3D printed hulls and tail attachments, coils, magnets, and rubber sheet for the caudal fin Electronic parts. Similarly to the HEXBUGTM Aquabot experiments, the only actuated part of the Scuba Fish in the PIV experiments is the magnet-coil mechanism for the movement of the caudal fin. Unlike in Aquabots, both magnets attached to the tail are perpendicular to the coils and both of them have the same polarity direction. The coil connection between each other creates magnetic fields in the same direction. The coil wires are connected to jumper wires that attach to the H- bridge controller, which is located outside of the tank, and the solder connection between the wires was sealed with heat shrink in order to prevent water leakage and electronics damage. In order to change the direction of the current flow a SN H-bridge is used in a combination with the Arduino Uno microcontroller and a TENMA DC Regulated Power Supply, which supplies 8.7 Volts and 0.03 Amps. This results in the power consumption 32

33 of Watts. The electrical circuit schematic used for the caudal fm motion control is presented in Figure 4-2 and an image of the circuit is presented in Figure 4-3. PIN 12---M PIN 6=M 0a, 'a U -5v COILS GND PIN 5=M 8.7V-mo SE 'A z 0 U U ~GND Figure 4-3: The schematic of the H-bridge circuit used for frequency. controlling the caudal fin motion and Caudal Fin Control Circuit Towing Mechanism Control Circuit V I r14, Figure 4-4: The two circuits used to control the motion of the Scuba Fish. On the left is the H- Bridge circuit for caudal fin control and the circuit for towing mechanism control. 33

34 In the original Scuba Fish design the caudal fin was driven in a similar way as the HEXBUGTM Aquabots, with a square wave input, when a maximum current flow is supplied to the coils in each direction. The desired frequency of the tail beat is 1.5 Hz, and the time constants were calibrated in the experimental design. However, as determined in the experiments with the Aquabots, the square wave input, or the bang-bang control, create start stop vortices and sharp tail beat kinematics. In order to achieve a more continuous motion, that has a more smooth change of direction and tail motion, a different input signal strategy was used. The amplitude of the tail deflection depends on the amplitude of the voltage supplied to the coils, because the magnetic field in the coil is proportional to the voltage supplied. Therefore, if the supplied voltage would steadily increase, the tail should steadily increase the amplitude. From this assumption, a ramp pwm input signal that increased the pwm signal gradually was tested Sum Dala Rl (smods Time (seconds) Figure 4-7: An example plot from the waveform calibration for pwm ramp control. A sinusoid, triangular, and square waves were fitted to the data. The sine wave had the best goodness of fit, and therefore, the motion of the tail is smoother than for the bang bang control case. For this test the robotic fish was submerged into water without being towed. Different pwm ramp signals were supplied for future calibration, and the tail beat was recorded via a Sony Digital Camera, with a frame rate of 30 fps. The caudal fin motion was tracked with the DLT 34

35 - -A package, and the resultant amplitude versus time plots for the caudal fm motion were fitted with different waveforms. It was determined that the sinusoidal fit has a greater R-square value than the square and triangular wave fit, and therefore, has a better goodness of fit. An example plot from the waveform calibration for pwm ramp in Figure 4-5. Knowing that the ramp input strategy allows to achieve a smoother caudal fm motion, the input ramp constants, such as the pwm duty cycle and the time delay between the pwm duty cycle increases were calibrated, so that the Strouhal number value was in the range of a live fish swimming. The example of the ramp code is presented below in Figure 4-5. //Code for variable control of tail motion #define E2 12 // Enable Pin for motor 2 #define 13 5 // Control pin 1 for motor 2 #define 14 6 // Contrcl pin 2 for motor 2 void setup () pinmode (E2, OUTPUT) ; pinmode (13, OUTPUT); pinmode(14, OUTPUT); void loop() { digitalwrite(i4, HIGH); int coilywm_4 = 0; int coilywm_ = 0; int changepwm = 50; //left side int left k = 0; for (int leftk; leftk<=5; leftk = leftk +1 ) { coilywm_3 = changepwm*eft k; analogwrite(13, coilywm_3); analogwrite (I4, 0); delay(65);//ocntrols the frequency of the tail beat //right motion int right _k = 0; for (int rightk; right k<=8; rightk = right k +1 ) { coilpwm_4 = changeywm*right k; analogwrite (14, coilpwm_4); analognwrite(i3, 0); dielay(65); Figure 4-6: A sample Arduino ramp code used for caudal fm motion control in the Scuba Fish experiments. 35

36 Towing Mechanism As in previous experiments with the HEXBUGTM Aquabots a towing mechanism with a sliding 8020 block, a motor, and a winch was used. This mechanism allowed more control over the swimming velocity of the robotic fish and also the NIR laser layer placement during the experiment. The circuit and the code of the experimental setup remained the same as described previously in Section 3.4.3, however, the some of the mechanical parts had to be adjusted and redesigned due to a larger size of the robotic fish. First of all, components of the mechanism such as the winch and motor mount, were modified after the HEXBUGTM Aquabot experiments in order to reduce friction between the moving parts. In addition, the string was replaced with fishing line, in order to fit the new winch design. Winch Tamiya Motor Motor Mount Figure 4-7: The motor-winch mechanism for towing the Scuba Fish. The fish mount attachment was also redesigned in order to fit the new shape of the robotic fish. The Scuba Fish doesn't have pectorai fish, which would aliow to use the vest design 36

37 from the HEXBUGTM Aquabot experiments. Therefore, a clamping design using the indents for the pectoral fins was used. The PWM input was experimentally calibrated in order to achieve a velocity of approximately 1 body length per second, or 0. 1m/s, which is the desired velocity of the Scuba Fish. The final design of the motor-winch mechanism is shown in Figure PIV methods and instrumentation PIV instrumentation The instrumentation for the 2D PIV experiment is the same as for the 2D PIV experiments with HEXBUG' Aquabots, previously described in Section 3.5 of the thesis and only few parameters that were adjusted. First of all, a wider angle camera lens, Phoenix 24mm, was used, due to a larger size of the robotic fish. Secondly, the f-stop was set to f/2.8, and the images were taken at a frame rate of 500 fps and an exposure time of 100psec. Other parameters were the same. The experimental setup is shown in Figure 4-8. Sliding block Scuba Fish High Speed Camera Figure 4-8: The experimental setup for the 2D PIV with the Scuba Fish. 37

38 PIV methods Since the goal of the experiments was to investigate the swimming performance of the original Scuba Fish bang bang controller and the adjusted ramp controller, three different sets of experiments: bang-bang and two ramp inputs with different frequency settings, were conducted for two different laser placements: the bottom of the tail and the middle of the tail Results After the data was collected, all sets were processed with DaVis with the same processing, masking, and postprocessing parameters. In addition, the body speed and the tail motion was tracked with Digitalizing Tools package in order to determine the Strouhal number. Moreover, for all of the experimental results presented below, the same parameters for towing and caudal fin motion were used. Overall, for all cases it has been observed, that the body of the fish creates additional vortex shedding at the tail attachment, which interacts with the vortices created by the caudal fin Bang Bang Results The original bang-bang values, identified in the design stage of the Scuba fish, were tested. The experiments have shown that the wake structure created by the Scuba Fish is similar to the one created by the HEXBUGTM Aquabots. As the caudal fin starts and stops moving, it creates vortex pairs, as can be seen in the Figure 4-8 below. The evaluated average Strouhal number for the bottom of the tail is St = 0.24, and for the middle plane the St = 0.25, which is in the range of a live fish. 38

39 ) ) a) T!= 1.l44 se' T2= sec T2= sec Vorticity (s b) TI = sec Vorticity (s 1 T2= sec T3= sec Figure 4-9: The time series velocity field for bang bang caudal fin control. Figures a) are the vortex patterns created at the bottom of the caudal fin, St = 0.24; figures b) are the vortex patterns created at the middle of the caudal fin, St = Scuba Fish creates a similar vortex pair pattern as the Aquabots in Figures PWM Results Next, the setup with the ramping signal was tested. Unlike the results in the bang bang case, the wake has a different structure. First of all, there are no start-stop vortices. Instead, negative vortex is created by the tail motion in addition with a small vortex pair at the maximum amplitude of inclination. The plots are presented below. For this case, even though the caudal fin pwm ramp parameters stayed the same, for the middle plane experiments the frequency of the tail beat was smaller than for the bottom plane experiments. Therefore, the resulting Strouhal number was 0.14 for 65msec time delay in between duty cycle changes and 0.15 for 70msec time 39

40 delay which is lower than fish-live Strouhal number. However, the resultant images show that the wake structure varies minimally between the bottom and middle planes cases. a) d) a) d) 0 9, T sec Tl=0.700 sec b) e) T2 O 66Osec T2 O00 sec C) ) T3 =0.70se T=.90 S. Figure 4-10: The time series of the vorticity fields for two different frequency parameters for the pwm ramp control for the bottom plane of the caudal fin. The time sequence a),b),c) is for the time delay 65msec, which results in the frequency of 1.22 Hz and the Strouhal number of 0.2. The time sequence d),e),f) shows the results for the time delay of 75msec, which results in a caudal fin frequency of 1.17Hz, and a Strouhal of

41 * a) 0 d) C 0 8~ * i5~1~9 G~ TI sec T1 = 0.400'sec b) e) ),? C T2 = sec T sec c) f) 0. 0 CC T3 = sec T3 =0.638 sec Vorticity ( LE) Vorticity ( EI) G Figure 4-11: The time series of the vorticity fields for two different frequency parameters for the pwm ramp control for the middle plane of the caudal fin. The time sequence a),b),c) is for the time delay 65msec, which results in the frequency of 1.03 Hz and the Strouhal number of The time sequence d),e),f) shows the results for the time delay of 75msec, which results in a caudal fin frequency of 0.96Hz, and a Strouhal number of Conclusions and Future Work Multiple PIV experiments were conducted with the Scuba Fish, in order to investigate the hydrodynamics of the robotic fish. Firstly, a bang-bang input signal was tested, which is the 41

42 same caudal fm motion control as used in HEXBUGTM Aquabots, that results in a discrete tail beat behavior. The results of the PIV have shown that the wake structure of the bang-bang input has a similar start-stop vortex structure to the previously determined the structure in the experiments with the Aquabots. Secondly, a proposed method of a pwm caudal fin motion control was tested in order to analyze the wake structure created by a more continuous movement of the caudal fin. In this case, the tail beat created single vortex ring and a small vortex pair. This shows that the wake structure can be changed by adjusting the caudal fin motion control method, and therefore, other strategies need to be investigated in order to achieve a behavior similar to live fish. The experiments have also shown, that there are multiple aspects that can be improved in order to achieve a more real fish-like swimming. First of all, the body and the mount to the sliding block of the Scuba fish can be adjusted to have a more streamline shape. This would allow to eliminate the creation of additional vortex patters that can be seen next to the tail joint. Moreover, in order to make sure that the Strouhal number is in the range of the live-fish swimming, the setup values for the pwm ramp method have to be recalibrated in order to have a more consistent design. 42