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Design of a low cost Thruster for an Autonomous Underwater Vehicle

Article · June 2009

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 Design of a low cost Thruster for an Autonomous
Underwater Vehicle
Srujana Eega, Student Member, Matthew A. Joordens, Member IEEE, Mo Jamshidi, Fellow, IEEE Member,
Autonomous Control Engineering (ACE) Center and ECE Department
The University of Texas
San Antonio, TX, USA
Email:e.srujana@gmail.com

Abstract- Brushless Dc Motor (BLDC) is widely used in and a computer fan grill is used as a finger guard over the fan
industries for various applications. The advantages of blades. Thrusters are controlled using Pulse Width Modulation
BLDC motor over the other types of motors made it a best technique. The robot has three vertical thrusters, two forward
choice for the design of low cost thruster. Presently the thrusters and one aft and two horizontal thrusters .The body of
remotely operated underwater vehicle uses bilge pump the robot is built with the DWV PVC pipe [1]. (Fig.1) The
motors to drive the vehicle under the water. Improvements robot works under the shallow water, at a depth of 9 feet.
are being done to make the vehicle autonomous and more
efficient to run deeper under the water. To satisfy such
improvements a more efficient thruster is essential and this
paper explores the design of efficient, low cost and less
weight thruster. The propeller is surrounded by the motor
fitted in plastic or carbon fiber. Rotor holds the propeller and
all electronics are embedded in plastic or carbon fiber. The
mathematical model, design, controlling techniques of the
motor and the future work are reported.

Keywords: Thruster, Brushless Dc Motor

I. INTRODUCTION
Underwater vehicles are always driven by the thrusters and
they being very expensive units raise the overall cost of the
vehicle. Currently a huge research is going on in the area of
underwater vehicles (robots) all around the world. System of Fig. 1. Initial Prototype with all thrusters [1]
Systems incorporating underwater vehicles along with air and
land vehicles to working together has the potential to increase III. NEW DESIGN
effectiveness over individual robots or swarms in any of the
three environments. Swarms of low-cost vehicles in each The aim is to build a low cost autonomous underwater
environment can be used to cover more area with less cost and vehicle that is targeted at a depth more than 1 meter. The
lower risk and higher fault tolerance than individual vehicles, electronics of the robot are pretty much the same as the initial
since loss of a few vehicles in a large swarm does not lead to design except the changes like making it autonomous instead
the failure of the entire mission as it would for a single controlling it remotely, need for more efficient thrusters and
expensive vehicle. If each robot is costly and if the application the chassis is built using the PVC pipes as they are strong
needs around 10 to 15 vehicles then the overall cost of the enough to withstand the pressure under the water. So the most
project is more. So there is undoubtedly need for low cost interesting and the challenging task is the design of the
underwater vehicles. The costly part of the robot is thruster. thruster. The thrusters used initially are good for shallow
An efficient, less weight, very small in size thruster is not less water. So a better thruster is needed. The features of the new
$3000. An autonomous underwater vehicle with six thrusters design are it should have more efficiency, should be really
will have to spend around $15000 exclusively for thrusters. A small in size and less in weight, simple water proofing system
thruster that costs less than $1200 would definitely be a good and have large staring torque. Thrusters are very expensive
commercial product. To design a more efficient, small in size, units. As each unit being very expensive, the underwater
less weight and low cost thruster is a challenging task. vehicle with 6 thrusters would raise up the cost of the overall
vehicle. The best design with the required features will be
II. INITIAL PROTOTYPE achieved with the right choice of the motor to drive the
thruster and very effective way of controlling the motor. With
It is a remotely operated underwater vehicle (robot). Bilge many advantages over the other types of the motor, Brushless
pump motors are used to drive the thruster. They are modified Dc motor best fits to serve the purpose. The Characteristics of
in way that computer fan is attached in the place of impeller
the BLDC, its speed Vs torque characteristics and the design are encapsulated in the plastic. The power available is 186W.
specifications are described in detail. It is a 24 pole stator, 6 poles Rotor and three phase motor
which rotated from 15-7.5 mechanical degrees. The magnetic
IV. BRUSHLESS DC MOTOR field intensity of the magnets of thickness 1.5mm is 11,400 H.
Thrusters are classified as hydraulic thruster systems and The basic calculations torque, back emf, and force between
electrical thruster systems. Electrical thruster systems are two poles are:
mostly used due to the recent advances in the PM magnet
motors [2] and one such motor is Brushless Dc motor. BLDCs
are in great demand for many industrial applications. BLDC
have linear speed Vs torque characteristics. (Fig. 2) It has a
wound stator, a permanent magnet rotor assembly, and
internal or external devices to sense rotor position. The rotor
assembly may be internal or external to the stator. The
combination of an inner permanent magnet rotor and outer
windings offers the advantages of lower inertia. Some more
advantages of the BLDC are:

· Better speed Vs torque characteristics

· High dynamic response
· High efficiency
· Long operating life
· Noiseless operation
· High speed ranges

Fig. 3. Basic Design of the Thruster

Back EMF = p max flux ∙ sin (rotor position ∙ φ) ∙ (Dφ/Dt)

Where
p is the pole pair,
D is the diameter of the stator

Torque Tind = (AG/ µ)*Bloop *Bstator*sin theta

Where,
G is the geometry of the coil
A is the area of the coil
µ is the permeability of the stator material
Fig. 2. Speed Vs Torque Characteristics of BLDC [3]
Npulses = NPnm
BLDC is electronically commutated. Each commutation Here N is the number of phases.
sequence has one of the windings energized to positive, the
second winding is energized to negative and the third winding Force between two poles,
is non-energized. BLDC is usually driven as unipolar or F = µ*q1m*q2m/ (4*pi*r^2)
bipolar. Motor driven as unipolar gives high speed and the
motor driven as bipolar gives large starting torque. Where,
The current design is the driven as bipolar as large starting qm1 and qm2 are the pole strengths (SI units: Newton)
torque is required and the motor runs from 0 rpm to maximum r is the separation (SI units: meter)
rpm.
VI. WATER PROOFING SYSTEM
V. CONCEPTUAL DESIGN OF THE THRUSTER To attach the thrusters to the robot is the hardest part. As the
Fig. 3 shows the outline of the thruster. The Central part is water and electronics do not mix, all the thrusters with their
the propeller, and the blades of the propeller are protected by electronics must be water proofed carefully especially to avoid
the thin fair duct. Rotor holds the propeller. All the electronics cavitation. The initial prototype uses simple method to attach
which include stator windings, rotor with permanent magnets the thruster to the body of the underwater vehicle. This is done
with stainless steel bolts with a hole drilled through the axis.
With an o ring the bolt connects the propeller guard to the three different ways to run the robot. Three vertical thrusters
body. The electrical wires go through the centre of the bolts and 1 aft made of BLDC control the depth of the robot and
with a tube over the bolt and wire to waterproof it. The tube is two horizontal thrusters with the same motors control the
clamped to the bolt and to the cable. (Fig. 4) [1] motion of the robot. The motor characteristics should also
match the characteristics of the propellers.

PWM
PIC18F4550 H-BRIDGE

HALL SENSORS

Fig. 4. Cross section of the connection of the thruster to the Fig. 5. Block diagram representing the control of the
body of the robot BLDC

VII. MOTOR CONTROL TECHNIQUE A, B, and C are three phases of the Brushless Dc Motor. The
position of the rotor is detected by the hall sensors and the
BLDC makes use of two types of control technique, information is fed as feedback to the PIC controller and the
sensorless control and sensor control. In the sensorless control position of the motor is controlled using PWM technique.
the rotor position is detected form a feedback method which
employs zero cross detection circuit which senses the back VIII. MATHEMATICAL MODEL OF THE MOTOR
emf and the rotor position is estimated. In this method back
emf generated is in the opposite direction to the supply voltage The typical mathematical model of the Brushless Dc motor is
of the winding. Though it simplifies the motor construction, as represented by the following equations. [4]
the back emf is very large in value proper circuit protection is
needed. Heat sink can be used for safety. Integrated gate di1 1
biased transistor acts as a heat sink. = ( Ri1 + v1 - e1 )
dt L
In sensor control, the rotor position is controlled using hall di2 1
sensors, they record the position of the rotor as the rotor
= ( Ri2 + v 2 - e2 )
dt L
passes the each hall sensor. Current design employs the sensor
di3 1
control of the BLDC. = ( Ri3 + v3 - e3 )
dt L
This position of the rotor is controlled used PWM (Pulse
Width Modulation Technique). PWM is generated using 1
PIC18F4550 microcontroller. The microcontroller uses a Te = (e1i1 + e2 i 2 + e3 i3 )
special platform to program it, MPLAB. It is easy to program w
and simple to debug the code onto the hardware. The load on
the motor is expected to be 18 Kg. The motor is controlled R is the resistance, i1, i2, and i3 are the line currents and v1,
using H-bridge on PIC microcontroller board. Two H-bridges, v2, and v3 are the voltages for three different windings and Te
with a maximum frequency of 20 KHz, are used for the represents the back emf of the motor where e1, e2, and e3 are
control of the motor where each of them can control single the back emfs of the windings. As the motor is controlled by
phase and multiple phase of that pole. Each thruster has its the H-Bridge, the objective is to describe the behavior of the
own control board and proper power management is done motor connected in Y and it has four states. H-Bridge controls
using the PIC microcontrollers as the robot is expected to the motor in 4 different states, OFF, ON, RIGHT AND LEFT
sustain under the water for 2 hours. The supply voltage is 24v directions.
Dc and the supply current is 9 A. The control of the motor
with feedback is shown in the figure.5. The motor are used in Current work of the project involves running the model in
MATLAB/SIMULINK and tests the speed Vs torque
characteristics and performance characteristics with the
mentioned specifications for the motor design. Once the model Placement of the magnets and aligning at right positions with
is simulated and required characteristics are achieved, the proper air gap is required. Different steps of manufacturing
design is good to be manufactured and the different steps in rotor are shown in (fig.8)
the manufacturing process are described in section IX, in
detail. GLUEING MAGNETS

IX. MANUFACTURING OF THRUSTER

Manufacturing the overall unit involves certain steps. (Fig.6)

MAGNETIZING

STATOR

TURNING YOKE
ROTOR

PROPELLER PRESSING IN TAPE

MAGNET

EMBED MOTOR IN
PLASTIC Fig.8 Manufacturing of the Rotor [5]

Propeller used for the current design is made of fiber and

ATTACH PROPELLER TO once the motor characteristics match with the propeller,
ROTOR propeller can be attached to the rotor running through the shaft
and it has to be made complete water proof. Finally for the
motor with all electronics to be embedded in the plastic,
WATER PROOF THE
plastic has to be molded in such a way that motor fits in the
SYSTEM
plastic rim and is completely water proof.
Fig.6 Overall design of the unit X. CONCLUSION
Thruster design involves manufacturing of the motor initially. The final research platform will result thruster being very
Stator and Rotor manufacturing in turn involves few steps. efficient, effective in cost and small in size, comprises of easy
Design of stator is shown in (fig.7) [5] water proofing system, complete control board which includes
the microcontroller for PWM technique and the H-Bridges for
WINDING COILS driving the motor, and complete manufactured design with
theoretical and simulated results and values. The construction
technique and the control technique are simple and the cost
CONNECTING WIRE estimated for the overall design which includes all the
ENDINGS electronics and the manufacturing cost is around $1200.

SOLDERING WIRES XI. FUTURE WORK

Determine the hydrodynamic characteristics of the thruster

PUNCHING LAMINATIONS
before manufacturing using the finite element analysis
method. Determine the design optimization techniques and
also if any tradeoff between different design parameters. And
run the autonomous underwater vehicle in depth more than 1
STACKING LAMINATIONS meter, driven with the new thrusters.

REFERENCES
LAMINATION STACK
INSULATION [1] Joordens, M.A., "Design of a low cost underwater robotic
research platform," System of Systems Engineering, 2008.
Fig.7 Manufacturing of the Stator SoSE '08. IEEE International Conference on , vol., no., pp.1-6,
2-4 June 2008
[2] Abu Sharkh, S.M.; Harris, M.R.; Stoll, R.L., "Design and
performance of an integrated thruster motor," Electrical
Machines and Drives, 1995. Seventh International Conference
on (Conf. Publ. No. 412), vol., no., pp.395-399, 11-13 Sep
1995
[3] Yedamale, Padmaraja, "Brushless DC (BLDC) Motor
Fundamentals," Microchip Technology Inc., Chandler, AZ,
Appl. Note AN885, DS00885A, 2003.
[4] Muresan, P.P., Forrai, Al., Biro K.Á., "Mathematical
modelling and control of brushless dc drives- unified
approach", Proceedings of the 6th International Conference
OPTIM '98, Brasov (Romania), 1998, pp. 557-563.
[5] Van Hoek, J.C.M.; Ackermann, B., "Manufacturing
aspects of small brushless DC motors," Permanent Magnet
Machines and Drives, IEE Colloquium on , vol., no., pp.4/1-
4/4, 5 Feb 1993

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