Modular Autonomous Robotic Fish
Tushar Mohan, Adnan Ahmed Salman, Indra Tjitra Salim
Engineering Product Development
Singapore University of Technology and Design
Singapore
[email protected]
Abstract— the paper is subjected to a novel design of an
autonomous biomimetic robotic fish. The novel approach
culminates in the usage of cable mechanics to mimic aquatic life
form in specificity the Koi fish. This paper presents a holistic
study and design architecture of the underwater robot. The
modularity and accessibility of the design set it apart from any
previous bio-mimicry experimentation or productization. This is
chiefly due to the use of a polycarbonate spine maneuvered by a
single servo motor and a separate module for the head to contain
the microprocessor controlling the sensory data. Reduction of
complexity for the common user and number of day to day
underwater applications without disrupting the ecosystem are
few of the real-world outcomes with the implementation of this
principle of robotic fish.
Keywords—underwater robotic fish; autonomous underwater
vehicle; modular robots
I. INTRODUCTION
Biomimicry has been the center of mass interest and
general intrigue since engineers started to adapt ways of
nature to solve complex problems. Evolution drives life-forms
to the best adaptation to the surrounding by natural selection.
Organisms inherit better structures and materials for survival
with improved healing, tolerance, resistance and with more
relevance to our design, locomotion. Underwater locomotion
is one of the challenges that engineers faced by implementing
biomimetic principles.
The concentration on the majority of research was to
achieve high speed, mechanical maneuverability and
efficiency. One of the pioneers of the mathematical models of
the swimming mechanism, Gray in the 1930's estimated the
driving power of a dolphin. He approximated the drag in his
calculations via a rigid model. He also made assumptions on
turbulent flow of the system. His model showed that the
power needed for the cruising dolphin exceeded the estimates
of muscle power output by 7 times. [1] Subsequently, the
reversed Karman vortex-street proposed a new vortex peg
mechanism, undulating mechanism along with vorticity
control mechanism. [2] Elongated-body theory along with
large-amplitude elongated body theory were developed to
analyse the propulsive mechanism. [3] The trio theories came
close to explaining the required power as opposed by the Gray
Paradox. These number of theories still requires numerous
assumptions for a suitable mathematical model which comes
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closest to encapsulating the swimming mechanics and the
energy requirements. Some of the assumptions include
resistive hydrodynamic models, 2D (subsequent 3D) waving
plate theory [4] and oscillating foil propulsion theories [5].
These theories in principle helps to design an artificial
propulsion system, the mechanical considerations of the head
and tail. Quasi-steady fluid flow requires current wake models
usually for the static water. Otherwise for modelling in
unsteady water environment the above theories and
assumptions are reformulated and optimized to derive more
dynamic models of the oscillating foils.
The next big challenge engineers today face are the
motion control methods. The three main facets of locomotion
for the any robotic biomimetic fish are cruising, hovering and
maneuvering. Cruising refers to swimming at constant
velocity, maneuvering is simplified term for any turns and
direction changing mechanism of the fish which includes
acceleration and deceleration and finally hovering corresponds
to the static positioning in a particular position in water.
Propulsion efficiency and fluid flow effect were subject of
intensified research during the 80's to obtain an efficient
cruising level. MIT and Draper Laboratories applied a
parameterized kinematic model during an experimentation.
Years ahead of the trials, RoboTuna, one of the world's
pioneering biomimetic fish was developed. [6] Albeit, the
accuracy and robustness were some keys issues still
unresolved
from
numerous
experimentation
and
implementation of the model. On the aspect of hovering and
maneuvering detailed hydrodynamic interaction models were
developed. Nonlinear control mechanism and fuzzy pectoral
fins control method were proposed on the basis of quasisteady fluid flow. Hovering at unsteady flow situation is yet to
be explored upon and during strong current flow in water mass
especially that of river research on this area is imperative. [7]
Early research work on propulsive theory focused
highly on the carangiform mechanism where the movement to
generate the waves of flexion is concentrated on the rear of
body and tail. Alongside RoboTuna from MIT, in Nagoya T
Fukuda developed another aquatic robot with a pair of fins
actuated by piezoceramics. Beijing University of Aeronautics
went further to design a first biomimetic anguilliform
locomotion system and a robot dolphin. The simple yet
efficient cable mechanics till then has not been implemented
until the same team under Triantfyllou at MIT had a third
improvement in store for RoboTuna.
RoboTuna II came close to implementing a
simplified cable-pulley system connecting to the controllers
but the complexity of the system increased by several folds
with 8 vertebrae and the system of cables used as tendons and
muscles. The team also focused on thunniform locomotion
emulating the motion of a tuna. This implied that the sideways
movement is in the tail mostly and in the peduncle (the portion
connecting the tail). This characteristic is only unique to tuna
which makes them high-speed long-distance swimmers. A
novel approach to simpler cable mechanics helps to simplify
the complexity to a large extent and adds a new level of
autonomy. This meets the realization of globalization of
robotic fish as a whole for numerous if not endless
applications.
despite the cable mechanics involved. In contrast the
simplicity of the cable mechanics adds to the accessibility and
fine-tuning of the tail, caudal fins, servo motor position and
the cables. Modular connections and the facilitations are
subsequently put forth. The next sub-section is the different
systems of mechanical structure and electronic systems. The
third section is regards control system architecture.
II. FISH DESIGN
The main objective of this paper is to showcase a
design of an autonomous underwater robotic fish, Meta-KOI,
which is highly modular and easy to deploy. The process to
achieve this includes tail actuation using cable mechanics,
head to tail communication, stability control, sensory circuit,
servo motor control and the microprocessor algorithms. The
foregoing brief is the introduction to the design and
fabrication of our robotic fish.
Biological background
Fishes propel themselves underwater using the thrust
generated by bending their bodies, producing a backwardpropagating propulsive wave. Selection of mechanical
structure, sensors and navigation techniques are crucial for the
design of a robotic fish. These considerations are diversified to
a colossal extent according to the numerous biological kind of
locomotive properties.
Figure 1: Comparison between Koi fish and Meta-KOI
The rest of the paper commences by discussing the
biological background of the design. Carangiform mechanism
inspired from the particular Koi was the concentration of the
research that followed. Structural analysis adheres to
showcasing the different parts and functionality of the fish.
The main two parts - head and tail - accommodates all the
components needed for autonomy and efficient propulsion in
steady water condition. The two portions connected by
neodymium magnetic designs are subjected to high modularity
Figure 2: (a) anguilliform,(b) subcarangiform and (c) carangiform to (d)
thunniform mode [10]
There are different ways to achieve this bending:
Anguilliform, subcarangiform, carangiform and thunniform
swimming (Refer to figure 1). Anguilliform swimming motion
can is achieved by numerous movements of muscles from
head to tail, whereas subcarangiform, carangiform and
thunniform swimming only requires the second half of the
body. [8] If the robotic fish has to emulate the motion of an eel
its design has to include more joints linking part from head to
tail that of a tuna (thunniform) where the propulsion is driven
mostly by the rear part of the tail and hence more focus has to
be put on the buoyancy for the latter biomimetic kind.
Therefore research on the carangiform swimming group of
fishes were more prevalent since the early days and thereby
this paper and the Meta-KOI regards more relevance to that
particular swimming type.
In fishes, muscle fibres are responsible for the tail
actuation supported by a flexible central spine. The tail
module of our robotic fish is inspired by this principle and is
composed of a polycarbonate central spine attached with
skeletal discs at equal spacing.
Structural Analysis
The robotic fish we designed is divided into two
segments: 1) Fish Head and 2) Fish Tail (figure 1). The
division of the body into two simplifies the implementation of
electronic circuits. The fish head houses the microprocessor,
gyroscopes, infrared sensors, CG mechanics and the battery.
The tail consists of the central spine made up of
polycarbonate, steel cables and a servo motor for actuation.
Both of these sections are held together using strong
neodymium magnets. It enables easy replacement of both the
parts upon damages.
The VCUUV project at MIT adapted the use of
traditional sensors such the six-axis Inertial Measurement Unit
Figure 3: Breakdown of the Meta-KOI structure
Figure 4: Prototype of Meta-KOI
for the purpose of navigation. Most research based designs
and trials seldom implemented image sensors, hydrophones,
infra-red sensors and ultrasonic sensors. This is chiefly due to
the waterproofing requirement of the biomimetic robots space
became a major constraint and in water the sensors never were
used as much as in the air autonomous robots. The usage of
one single HerculeX DRS-0201 robot servo with malleable
steel cables in this case saved an accountable amount of space
for the sensors to be located in the head portion.
Cable Mechanics
The tail is a simplified system of 8 vertebrae, 1.5 cm
of a central spine and HerculeX servo. The servo has a pulley
with steel cables of 2 mm diameter bounded till the 8th
vertebra from two sides of the tail. The malleability of the
steel cables enables the structure to achieve the carangiform
Koi movements during operation. The spine plays a crucial
alloy), a network of cables or a chain of servo motors with
synchronized programming for each.
Modularity Connector
Modularity and accessibility of the robots enables
one to open up the sensors and reconnect any other sensing
chip to the microprocessor and customize the system. Inspired
from MagSafe (popular MacBook Pro charger connectors), six
neodymium magnets are used to connect the head and tail
modules. With regards to the fish mechanism, in the unsteady
water medium Meta-KOI will be subjected to number of
uneven forces or drag. However, the magnetic strength of
neodymium magnets will ensure the structure intactness. The
modular design will ensure fast deployment to achieve
specific tasks. An example scenario will be to have multiple
Meta-KOI heads programmed for specific tasks. During
deployment the fish head can be easily replaced without
Figure 5: Different tail angles with cable mechanics
part during the locomotion. The material for the use of the
central spinal structure was polycarbonate and that is key to
produce the propulsive forces for the fish to swim. This in turn
generates the flexion waves that travel through the whole tail
structure. A net backward force is generated which pushes the
head and the tail forward. Thereby the caudal fin and the tail
generates thrust using lateral movements. The simplicity of
the mechanism is notable in the sense that the cables and the
polycarbonate spine are doing all the propulsive duties rather
than having a complex system of SMA [11] (shape-memory
having the need to open up any of the individual modules. In
order to prevent water seepage into the copper connector
setup, an O ring is used to seal the gap between head and tail
module.
Electronic Systems
The prototype is shown in figure. The two main parts
of the fish are further divided into various sections which are
responsible for specific functions. The breakdown of the fish
is shown in figure. Firstly, the sensor system consists of infrared sensors, gyroscopes and temperature sensor. All these
sensors are connected to a custom designed printed circuit
board which is located inside the fish head. The printed circuit
board is designed to fit perfectly inside the fish head to reduce
additional mechanical sensor mounts. Two sensors: Left and
Right are placed at the fish eye opening.
Secondly the control system consists of a
microprocessor, servo motor driver and the power supply. The
control system hardware is integrated with the sensor system
inside the fish head, such that it can be easily accessed for
troubleshooting, without disturbing the tail drive mechanism.
The tail actuation system consists of a high torque servo motor
with feedback capabilities, a central polycarbonate spine,
skeleton discs and a silicon caudial fin.
Figure 6: Head of Meta-KOI with neodymium magnetic connectors
III. CONTROL SYSTEM ARCHITECTURE
The brain of Meta-KOI is a microprocessor which is
enabled to acquire sensory data, process them and transmit the
output signal to the actuators.
the real motions of a fish.
The motions of the fish is divided into two main
groups: horizontal and vertical. Horizontal movement covers
the forward thrust, yaw left and yaw right. Vertical movement
Figure 7: Control architecture layers
In this paper, the microprocessor is an Arduino micro
which acts as the link between the sensory system and the tail
actuation. The software framework was designed to duplicate
Figure 8: Control algorithm flowchart
Figure 9: Forces acting on a fish
covers the pitch up, pitch down and the roll motion. Figure
explains the various forces acting on a real fish.
The system architecture comprises of three layers:
Sensing layer, Behaviour Layer and Actuation Layer. [12] The
behaviour layer forms the core of control system which gets
information from the sensor layer and feedback from the
actuation layer. The sensory layer gathers information from the
sensors and converts them into corresponding states and are
sent to the behavioural layer. The lowest layer is of the
actuation layer which process the data from the behavioural
layer and converts into servo commands. Five actuation
commands are designed, namely Swim Forward, Pitch up,
Pitch Down, Yaw Left and Yaw Right.
Implementation
In order to control the tail movement, a very simple
closed loop control was designed. A linear relationship
between the angle of motor shaft and angle of fish tail was
established. This was done to have a simple feedback loop
using the information from the motor encoder. To process
these data, various electronic circuits and components were
used:
a) An Arduino micro was used as the microcontroller.
This processed the analog signals from the sensors and
converted them into states. It also processed the data from
gyroscopes and adjusted the centre of gravity required for
each swimming pattern. After processing all these
information, it then generated the output signals (pwm signals)
to control the servo motor.
b) A Herkulex servo motor was used to actuate the tail.
This is a smart servo which has a resolution of 0.325o and is
capable of detecting internal changes like temperature and
power supply. It is also capable of sending feedbacks about
current position, speed, load and temperature.
IV. CONCLUSIONS
This paper showcases the design of an autonomous
underwater robotic fish, Meta-KOI, which is highly modular
and easy to deploy. The design includes tail actuation using
cable mechanics, head to tail communication, stability control,
sensory circuit, servo motor control and the microprocessor
algorithms. In future works, the focus will be to develop a
better software such that it can adapt with the underwater
environment. In addition, battery lifespan will be one of the
major goal for the next phase of the design in order to produce
more reliable autonomous robot. Further research on
underwater charging will be performed to improve the
capabilities of the Meta-KOI. Finally, the major focus would
be to improve the waterproofing of the joint connector
between the head and the tail part.
ACKNOWLEDGMENT
The authors would like to thank Dr. Mohan Rajesh
Elara for his supervision on this project, SUTD-MIT IDC
(International Design Centre) for funding this project and
Chew Wei Liang, Dabin Lee for their contribution to the
project.
REFERENCES
[1] Measurement of hydrodynamic force generation by
swimming dolphins using bubble DPIVJ. Exp.
Biol. 2014 217:252-260.
[2] RAMIRO GODOY-DIANA, CATHERINE MARAIS,
JEAN-LUC AIDER and JOSÉ EDUARDO WESFREID
(2009). A model for the symmetry breaking of the reverse
Bénard–von Kármán vortex street produced by a flapping foil.
Journal of Fluid Mechanics, 622, pp 23-32
[3] Lighthill M J. Aquatic animal propulsion of high
hydromechanical efficiency. J. Fluid Mech. 1970,
vol.44, 265-301
[4] Wu T Y. Swimming of a waving plate. J. Fluid Mech.
1961, vol.10, 321-344
[5] Oscillating foil propulsion system US 6877692 B2
[6] Triantafyllou M S, Triantafyllou G S. An efficient
swimming machine. Scientific American, 1995, 272(3): 64-70
[7] Triantafyllou M S, Barrett D S, Yue D K P. A new
paradigm of propulsion and manoeuvring for marine vehicles.
Trans. Soc. Naval Architects Marine Eng. 1996, vol.104, 81100
[8] P. W. Webb, “Is the high cost of body caudal fin
undulatory swimming due to increased friction drag or inertial
recoil?” J. Exp. Biol., vol. 162, pp. 157–166, 1992.
[9] J. J. Magnuson, “Locomotion by scombrid fishes:
Hydromechanics, morphology and behavior,” in Fish
Physiology Vol. VII Locomotion, W. S. Hoar and D. J.
Randall, Eds. New York: Academic, 1978, pp. 239–313.
[10] C. C. Lindsey, “Form, function and locomotory habits in
fish,” in Fish Physiology Vol. VII Locomotion, W. S. Hoar and
D. J. Randall, Eds. New York: Academic, 1978, pp. 1–100.
[11] Bending continuous structures with SMAs: a novel
robotic fish design C Rossi, J Colorado,W Coral and A
Barrientos Centre for Automation and Robotics, Universidad
Polit´ecnica de Madrid, Madrid, Spain.
[12] Biologically Inspired Behaviour Design for Autonomous
Robotic Fish Jin-Dong Liu∗, Huosheng Hu Department of
Computer Science, University of Essex, Colchester CO4 3SQ,
UK.