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:
[email protected] Abstract- Brushless Dc Motor (BLDC) is widely used in industries for various applications. The advantages of BLDC motor over the other types of motors made it a best choice for the design of low cost thruster. Presently the remotely operated underwater vehicle uses bilge pump motors to drive the vehicle under the water. Improvements 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.
and a computer fan grill is used as a finger guard over the fan blades. Thrusters are controlled using Pulse Width Modulation technique. The robot has three vertical thrusters, two forward thrusters and one aft and two horizontal thrusters .The body of the robot is built with the DWV PVC pipe [1]. (Fig.1) The robot works under the shallow water, at a depth of 9 feet.
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 Systems incorporating underwater vehicles along with air and land vehicles to working together has the potential to increase effectiveness over individual robots or swarms in any of the three environments. Swarms of low-cost vehicles in each environment can be used to cover more area with less cost and lower risk and higher fault tolerance than individual vehicles, since loss of a few vehicles in a large swarm does not lead to the failure of the entire mission as it would for a single expensive vehicle. If each robot is costly and if the application needs around 10 to 15 vehicles then the overall cost of the project is more. So there is undoubtedly need for low cost underwater vehicles. The costly part of the robot is thruster. An efficient, less weight, very small in size thruster is not less $3000. An autonomous underwater vehicle with six thrusters will have to spend around $15000 exclusively for thrusters. A thruster that costs less than $1200 would definitely be a good commercial product. To design a more efficient, small in size, less weight and low cost thruster is a challenging task. II. INITIAL PROTOTYPE It is a remotely operated underwater vehicle (robot). Bilge pump motors are used to drive the thruster. They are modified in way that computer fan is attached in the place of impeller
Fig. 1. Initial Prototype with all thrusters [1] III. NEW DESIGN The aim is to build a low cost autonomous underwater vehicle that is targeted at a depth more than 1 meter. The electronics of the robot are pretty much the same as the initial design except the changes like making it autonomous instead controlling it remotely, need for more efficient thrusters and the chassis is built using the PVC pipes as they are strong enough to withstand the pressure under the water. So the most interesting and the challenging task is the design of the thruster. The thrusters used initially are good for shallow water. So a better thruster is needed. The features of the new design are it should have more efficiency, should be really small in size and less in weight, simple water proofing system and have large staring torque. Thrusters are very expensive units. As each unit being very expensive, the underwater vehicle with 6 thrusters would raise up the cost of the overall vehicle. The best design with the required features will be achieved with the right choice of the motor to drive the thruster and very effective way of controlling the motor. With many advantages over the other types of the motor, Brushless Dc motor best fits to serve the purpose. The Characteristics of
the BLDC, its speed Vs torque characteristics and the design specifications are described in detail. IV. BRUSHLESS DC MOTOR Thrusters are classified as hydraulic thruster systems and electrical thruster systems. Electrical thruster systems 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: · · · · · ·
are encapsulated in the plastic. The power available is 186W. It is a 24 pole stator, 6 poles Rotor and three phase motor which rotated from 15-7.5 mechanical degrees. The magnetic field intensity of the magnets of thickness 1.5mm is 11,400 H. The basic calculations torque, back emf, and force between two poles 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] BLDC is electronically commutated. Each commutation sequence has one of the windings energized to positive, the second winding is energized to negative and the third winding is non-energized. BLDC is usually driven as unipolar or bipolar. Motor driven as unipolar gives high speed and the motor driven as bipolar gives large starting torque. The current design is the driven as bipolar as large starting torque is required and the motor runs from 0 rpm to maximum rpm. V. CONCEPTUAL DESIGN OF THE THRUSTER Fig. 3 shows the outline of the thruster. The Central part is the propeller, and the blades of the propeller are protected by the thin fair duct. Rotor holds the propeller. All the electronics which include stator windings, rotor with permanent magnets
Npulses = NPnm Here N is the number of phases. Force between two poles, F = µ*q1m*q2m/ (4*pi*r^2) Where, qm1 and qm2 are the pole strengths (SI units: Newton) r is the separation (SI units: meter) VI. WATER PROOFING SYSTEM To attach the thrusters to the robot is the hardest part. As the water and electronics do not mix, all the thrusters with their electronics must be water proofed carefully especially to avoid cavitation. The initial prototype uses simple method to attach 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 body. The electrical wires go through the centre of the bolts with a tube over the bolt and wire to waterproof it. The tube is clamped to the bolt and to the cable. (Fig. 4) [1]
three different ways to run the robot. Three vertical thrusters and 1 aft made of BLDC control the depth of the robot and two horizontal thrusters with the same motors control the motion of the robot. The motor characteristics should also match the characteristics of the propellers. A
PIC18F4550
PWM
H-BRIDGE
B C
HALL SENSORS
Fig. 4. Cross section of the connection of the thruster to the body of the robot
Fig. 5. Block diagram representing the control of the 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 information is fed as feedback to the PIC controller and the position of the motor is controlled using PWM technique.
BLDC makes use of two types of control technique, sensorless control and sensor control. In the sensorless control the rotor position is detected form a feedback method which employs zero cross detection circuit which senses the back emf and the rotor position is estimated. In this method back emf generated is in the opposite direction to the supply voltage of the winding. Though it simplifies the motor construction, as the back emf is very large in value proper circuit protection is needed. Heat sink can be used for safety. Integrated gate biased transistor acts as a heat sink. In sensor control, the rotor position is controlled using hall sensors, they record the position of the rotor as the rotor passes the each hall sensor. Current design employs the sensor control of the BLDC. This position of the rotor is controlled used PWM (Pulse Width Modulation Technique). PWM is generated using PIC18F4550 microcontroller. The microcontroller uses a special platform to program it, MPLAB. It is easy to program and simple to debug the code onto the hardware. The load on the motor is expected to be 18 Kg. The motor is controlled using H-bridge on PIC microcontroller board. Two H-bridges, with a maximum frequency of 20 KHz, are used for the control of the motor where each of them can control single phase and multiple phase of that pole. Each thruster has its own control board and proper power management is done using the PIC microcontrollers as the robot is expected to sustain under the water for 2 hours. The supply voltage is 24v 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
VIII. MATHEMATICAL MODEL OF THE MOTOR The typical mathematical model of the Brushless Dc motor is represented by the following equations. [4]
di1 1 = ( Ri1 + v1 - e1 ) dt L di2 1 = ( Ri2 + v 2 - e2 ) dt L di3 1 = ( Ri3 + v3 - e3 ) dt L Te =
1 (e1i1 + e2 i 2 + e3 i3 ) w
R is the resistance, i1, i2, and i3 are the line currents and v1, v2, and v3 are the voltages for three different windings and Te represents the back emf of the motor where e1, e2, and e3 are the back emfs of the windings. As the motor is controlled by the H-Bridge, the objective is to describe the behavior of the motor connected in Y and it has four states. H-Bridge controls the motor in 4 different states, OFF, ON, RIGHT AND LEFT directions. 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 is simulated and required characteristics are achieved, the design is good to be manufactured and the different steps in the manufacturing process are described in section IX, in detail.
Placement of the magnets and aligning at right positions with proper air gap is required. Different steps of manufacturing rotor are shown in (fig.8) GLUEING MAGNETS
IX. MANUFACTURING OF THRUSTER Manufacturing the overall unit involves certain steps. (Fig.6)
MAGNETIZING
STATOR
TURNING YOKE
ROTOR
PROPELLER
EMBED MOTOR IN PLASTIC
ATTACH PROPELLER TO ROTOR
WATER PROOF THE SYSTEM
Fig.6 Overall design of the unit Thruster design involves manufacturing of the motor initially. Stator and Rotor manufacturing in turn involves few steps. Design of stator is shown in (fig.7) [5]
PRESSING IN TAPE MAGNET
Fig.8 Manufacturing of the Rotor [5] Propeller used for the current design is made of fiber and once the motor characteristics match with the propeller, 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, plastic has to be molded in such a way that motor fits in the plastic rim and is completely water proof. X. CONCLUSION
CONNECTING WIRE ENDINGS
The final research platform will result thruster being very efficient, effective in cost and small in size, comprises of easy water proofing system, complete control board which includes the microcontroller for PWM technique and the H-Bridges for 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 estimated for the overall design which includes all the electronics and the manufacturing cost is around $1200.
SOLDERING WIRES
XI. FUTURE WORK
WINDING COILS
PUNCHING LAMINATIONS
STACKING LAMINATIONS
Determine the hydrodynamic characteristics of the thruster 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 meter, driven with the new thrusters. REFERENCES
LAMINATION STACK INSULATION
Fig.7 Manufacturing of the Stator
[1] Joordens, M.A., "Design of a low cost underwater robotic research platform," System of Systems Engineering, 2008. 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.4/14/4, 5 Feb 1993
