development of a low-cost underwater manipulator

Development of a Low-Cost Underwater Manipulator

by

Lauren Alise Cooney

SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING INPARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

BACHELOR OF SCIENCEAT THE

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

June 2006

© 2006 Lauren Alise Cooney. All rights reserved

The author hereby grants to MIT permission to reproduce and to distribute publicly paperand electronic copies of this thesis document in whole or in part in any medium now

known or hereaftr created.

Signature of Author. ..... _.................... .Epartment of Mechanical Engineering

May 18, 2006

Certified by ...................................................... -- . ...- ..../'~ ~ John Leonard

,,ssociate Professor of Mechanical EngineeringThesis Supervisor

,CAccepted by ............... - ........... .. ..,................ ...

John H. Lienhard VProfessor of Mechanical Engineering

Chairman, Undergraduate Thesis Committee

MASSACHUSETTS INSTITUTEOF TECHNOLOGY

AUG 0 2 2006

LIBRARIES

ARCH!WvE,,

l

Development of a Low-Cost Underwater Manipulator

by

Lauren Alise Cooney

Submitted to the Department of Mechanical Engineeringon May 18, 2006 in Partial Fulfillment of the

Requirements for the degree of Bachelor of Science inMechanical and Ocean Engineering

ABSTRACT

This thesis describes the design, modeling, manufacture, and testing of a low cost,multiple degree-of-freedom underwater manipulator. Current underwater robotic armtechnologies are often expensive or limited in functionality. The goal of this research isto produce a multiple degree-of-freedom manipulator utilizing relatively inexpensive,commercial off-the-shelf servo motors. This project is designed for low-payload (< 0.5kg) and shallow depth operation on a small remotely operated vehicle.

A completed underwater manipulator has been built using the new servo housing design.Static and dynamic waterproofing techniques have proven satisfactory, offering a soliddesign for waterproofing of servo motors. Preliminary tests of the integrated servo armsystem indicate that the arm will operate successfully in the underwater environment.This design is anticipated to be used on an underwater vehicle in June 2006, as well as infuture undergraduate ocean engineering design subjects.

Thesis Supervisor: John LeonardTitle: Associate Professor of Mechanical Engineering

3

Acknowledgements

I would like to express my sincere appreciation to the following people that made thisproject possible:

· My advisor, Professor John Leonard.* The MIT ROV Team, in particular, Daniel Walker, Thaddeus Stefanov-Wagner,

and Kurt Stiehl. As well, thank you to the ROV Team's donors, who provided thefinancial support for this project.

* Mark Belanger and the Edgerton Student Machine Shop.* The MIT Towing Tank.* The Ocean Engineering Teaching Laboratory.* Zachary Gazak.* And finally, my family, for their love and support.

5

Table of ContentsABSTRACT 3Acknowledgements 5

List of Figures 8List of Tables 9Chapter 1 Introduction 11

1.1 Review of the State of the Art of Underwater Manipulators 111.2 Design Goals 131.3 Outline 15

Chapter 2 Mechanical Design 172.1 Motor Selection 172.2 Motor Housings 192.3 Linkages 202.4 Cost 222.5 Manipulator Arm Photographs 22

Chapter 3 System Modeling 253.1 Rigid Body Kinematics 253.2 Rigid Body Dynamics 263.3 Fluid Effects 28

3.3.1 Hydrostatics 283.3.2 Hydrodynamics 283.3.3 Previous Research on Hydrodynamic Modeling of

Underwater Manipulators 29Chapter 4 Performance Assessment 31

4.1 Control Architecture 314.2 Servo Characterization 324.3 Arm Characterization 32

Chapter 5 Conclusions 335.1 Summary of Contributions 335.2 Future Research 33

Appendix A:: Mechanical Drawings 35Bibliography 41

7

List of FiguresFigure 1.1: Schilling Robotics TITAN 4 [1] 12Figure 1.2: SeaBotix TJG300 Three Jaw Grabber [2 12Figure 1.3: MIT ROV Team 2004 Vehicle 14Figure 1.4: Gripper Assembly and Gearing [5] 15Figure 2.1: Arm with End-Effecter Defining Degrees-Of-Freedom 18Figure 2.2: Spring-Loaded PTFE Seal [6] 20Figure 2.3: Section View of Motor Housing 21Figure 2.4: Photograph of Inside of Motor Housing 22Figure 2.5: Photograph of Arm Assembly 23Figure 2.6: Photograph of Arm in Extended Position 24Figure 2.7: Photograph of Arm in Stowed Position 24Figure 3.1: Reference Frames 25Figure 4.1: Pulse Width Modulation 31

8

List of Tables

Table 1.1: State of the Art of Underwater Manipulator Technology 13Table 2.1: Motor Torque Requirements 17Table 2.2: Manufacturer's Motor Characteristics [5] 19Table 3.1: Denavit-Hartenberg Parameters [8] 26Table 4.1: Servo Characteristics 32

9

Chapter 1 Introduction

The ability to operate in the subsea environment presents many opportunities and benefitsin research, commercial, and military endeavors. However, the added challenge ofworking underwater often results in technologies that are either very expensive or havelimited functionality, a trend largely reflected in underwater robotic arm technologies.The goal of this research is to develop a working five degree-of-freedom (DOF)underwater manipulator created from low cost, off-the-shelf parts.

1.1 Review of the State of the Art of Underwater Manipulators

Current underwater manipulator technologies largely fall into two categories: highfunctionality at high cost, or limited functionality at low cost. Robotic arms withmultiple degrees of freedom are typically designed for medium- to large-scale remotelyoperated vehicles (ROV's). These manipulators are often expensive, heavy, and large.For example, the Schilling Robotics TITAN 4, shown in Figure 1.1, is a popular deep seamanipulator employed on large ROV's working in the offshore industry. This arm ismade primarily out of titanium and is powered by hydraulics. Although the TITAN'sseven degrees-of-freedom are an attractive feature, due to its large weight and expense(76 kg in seawater, >$150,000), this class of manipulator is not feasible for smaller low-cost vehicles.

At the other end of the spectrum, lower cost manipulators commonly have much lessfunctionality. The SeabotixTJG300 Three Jaw Grabber, as shown in Figure 1.2 and asdescribed in Table 1.1, has only a gripping mechanism. Because of this limitedperformance, a greater dependence is placed on vehicle maneuverability in order toachieve complex manipulator tasks.

11

Figure 1.1: Schilling Robotics TITAN 4 []

Figure 1.2: SeaBotix TJG300 Three Jaw Grabber [2]

12

Table 1.1: State of the Art of Underwater Manipulator Technology

SeaBotix, Inc. [2] Inuktun Servies Schilling InternationalLtd. [3] Robotics Submarine

Systems, Inc. [4] Engineering Ltd.

[4]

Model TJG300 Micro Titan III s ISE 7FManipulator

DOF Gripper Gripper 6 plus gripper 6 plus gripper

Actuator Lead Screw N/A Hydraulic HydraulicDrive Mechanism Cylinders Cylinders

Jaw Closing 14 lbs 5 lbs. 1000 lbf 330 lbsForce

Price $1,995 $2,700 $149,500 $70,000-$250,000

1.2 Design Goals

The design of an underwater manipulator encounters challenges from the combinedcomplexities of subsea operations and robotic arm design, which are further intensifiedwhen trying to do so in a cost-efficient manner. The waterproofing of electronics andother sensitive elements is critical to a working system. System design is driven bydesired payload capacity and end-effecter positioning and speed, as well as additionalforces from static and dynamic fluid effects.

Design considerations for this arm are made with the anticipation of functioning on asmall remotely operated vehicle (12 x 12 x 12in) built by the MIT ROV Team to competein the national ROV competition organized by the Marine Advanced TechnologyEducation Center (MATE) and the Marine Technology Society (MTS) ROV Committee.

The 2006 MATE ROV Competition is to be held at the Neutral Buoyancy Laboratory atthe NASA Johnson Space Center in Houston, Texas. The competition takes place in afresh water pool, where the maximum operating depth is 40 feet. There are several tasksthat the ROV must complete within the 30 minute competition time limit. The ROVmust transport an electronics module weighing 1.5 kg underwater to the pool bottom, andsecure this module into a frame. The module door needs to be opened-requiring 1 N offorce-so that a 1" PVC probe weighing 0.3 kg underwater may be inserted into the porton the module.

13

In addition, there are design requirements created by the ROV itself. Low weight andsmall size of the stowed arm are essential in minimizing manipulator effects on vehicleoperations. The arm should also be capable of operating off of a bottomside powersupply rated at a nominal 12 V and 20 Ah. Topside communication with the arm will bethrough a fiber-optic tether. Of overarching concern for the arm is budget. The ROVTeam operates off of donations from both industry and academia. Keeping the arm cost-effective (<$500) would make it a feasible enterprise, as well as increase the viability ofreproduction.

Figure 1.3 shows the ROV Team's vehicle for the 2004 competition. The gripperassembly in Figure 1.4 was used on this vehicle to retrieve a weighted 2" PVC pipe off ofthe bottom of a swimming pool and to bring it up to the surface. Although this gripperdoes not have much in the range of maneuverability, it was acceptable for this mission, asthe only functional requirements were the ability of the ROV to land within graspingrange, and that the gripper could maintain a tight hold on the object.

Figure 1.3: MIT ROV Team 2004 Vehicle [5]

14

Figure 1.4: Gripper Assembly and Gearing [5]

For this year's competition, however, relying on the ROV's propulsion/control system toperform tasks such as plug the instrumentation probe into the module port could be timeinefficient and difficult. An underwater manipulator with high functionality wouldprovide several advantages for the ROV competition, and to underwater operations ingeneral. A multiple degree-of-freedom robotic arm would allow for more precise controlin manipulation tasks, as opposed to a vehicle with a single gripper which would requirehigh levels of hovering control.

1.3 Outline

This thesis documents the development of a low-cost underwater manipulator. Themechanical design process is described, including motor selection, housing design,manufacturing, and cost. Modeling of the system is then performed, for rigid bodykinematics and dynamics, and an evaluation of fluid effects. Preliminary performanceassessment, conclusions and suggestions for future research are discussed.

15

Chapter 2 Mechanical Design

This chapter describes the mechanical design and manufacturing of the manipulator.Component selection, development, and integration are discussed. Photographs of partsand the assembled arm are shown in Figures 2.4 through 2.7. A bill of materials is listedto illustrate the low-cost aspect of the design. Mechanical drawings for components canbe found in Appendix A.

The manipulator is made up of five direct serial drive revolute joints. The end-effecterwas not developed for the purposes of this thesis. The configuration of the manipulator issimilar to that of a human arm, with a two degree-of-freedom shoulder connected to thebase (01 and 0 ), a one degree-of-freedom elbow (03), and a two degree-of-freedom wrist(04 and 05) (Figure 2.1).

2.1 Motor Selection

Hobby servos are an appealing technology, as they are readily available with gear trainsand feedback positioning electronics at a relatively low cost. A new breed of roboticservos marks a departure from standard model airplane servos, as they boast high torqueand over 180 degrees of rotation.

Torque requirements for each motor are determined using a static moment balance, wherebuoyancy effects are considered and hydrodynamic effects are small at very low speeds.These calculations include the weight of an end-effecter and a 1 N weight. Table 2.1presents the torque requirements for the case of hydrostatically optimized motor housingsthat achieve neutral buoyancy.

Table 2.1: Motor Torque Requirements

Torque Requirement

In Air In Water

Motor 2 4.53 N-m 1.309 N-m

Motor 3 1.856 N-m 0.562 N-m

Motor 4 0.473 N-m 0.158 N-m

17

03

0f1

0 f Z6

04,L.

-1H

Figure 2.1: Arm with End-Effecter Defining Degrees-Of-Freedom

18

!, ___' ' :·· .r:..... .. .. .~ ~ ~ ~ ~ ~ ~~··

, " ,

t;NJV

Two servo motors produced by Hitec, Inc, make up the drive framework for themanipulator: the HS-805BB and the HS-755HB. The HS-805BB is used to drive thearm's shoulder and elbow joints, as these joints require more torque, but can also handlea larger size motor as the moment arm created about the base motor is minimal. TheHS-755HB motor drives the two wrist joints as they do not require as much load carryingcapacity.

Table 2.2: Manufacturer's Motor Characteristics [6]HS-755HB HS-805BB

Stall Torque 13.2 kg*cm 19.8 kg*cm

Speed (no load) 0.23 sec/60 deg 0.14 sec/60 deg

Operating Angle ±90 deg ±90 deg

Operating Voltage 4.8 - 6.0 V 4.8 - 6.0 V

Current Drain (no 285 mA 830 mA

Dimensions 59 x 29 x 50 mm 66 x 30 x 58 mm

Weight 110g 152 g

Price 27.99 USD 39.99 USD

2.2 Motor Housings

The motor housings are constructed out of black nylon 6/6, chosen for its good tensilestrength, dimensional stability, machinability and relatively low cost compared to plasticsof similar performance level. Large pieces of stock, however, were more expensive thanseveral smaller pieces of stock with the same cumulative volume. Therefore, the housingis made up of two smaller equal sized blocks. A standard face seal O-ring groove createsthe static seal between the two housing components and the outside environment.

The dynamic seal for the motor shaft is provided by a spring-loaded PTFE seal (Fig 2.2).These seals employ U-Cup lip seal geometry using a canted coil spring that creates asealing force that is also energized under dynamic conditions. The ratings for these sealsare well within the operational limits for this project (pressure x velocity limit of 75,000psi-fpm). For deeper applications, an alternative dynamic seal would need to be applied.

19

71Figure 2.2: Spring-Loaded PI1E Seal L'J

Shaft seals prove difficult for hobby servos as a servo horn is directly connected to thefinal gear stage, leaving no shaft exposed to create a seal around. The final designconsists of a commercially available inline shaft adaptor connected to the servo horn,around which the shaft seal is created.

Plastic flanged sleeve bearings are utilized at both the output shaft and linkage pivots.The bearings chosen are of material Rulon LR, preferred for its low friction and becauseit does not absorb water. These bearings act as constraints in the radial direction and alsohandle radial and thrust loads. The bearings used at the linkage pivots also serve as a lowfriction pivot point.

2.3 Linkages

Linkages are constructed out of 3/8" clear polycarbonate, a material that exhibits goodstrength and relative ease of machining, yet low weight as compared to metals such asaluminum. Each motor driven linkage operates in parallel with a load-sharing linkageconnected to the motor housing via a freely rotating pivot. Linkages are secured to shaftsusing a dowel pin.

20

Motor Outpu

Spring-Load

Spring-Load

Shaft Attach

Servo Horn

servo

O-Ring

Motor Housii

Bearing

Linkage Pivot Rod

Figure 2.3: Section View of Motor Housing

21

~~~~~~~~~ I~~~~~~~l

�lr

Ro~rirnI " I ~ I I

I

./

2.4 Cost

Table 2.3 presents the cost of materials used in the final manipulator design, notincluding the cost of an end-effecter or PIC microcontroller for control. As can be seen,the total cost is significantly less as compared to even the single degree-of-freedomgrippers listed in Table 1.1.

Table 2.3: Bill of MaterialsUnit Price Quantity Total Price

IMotors IHitec HS-755HB 1/4 Scale Karbonite Gear Servo U | $27.99 | 21 $55.98Hitec HS-805BB Mega 1/4 Scale 2BB Servo U $39.99 3 $119.97

MaterialsBlack Nylon 6/6 $114.80 $114.80Clear Polycarbonate Sheet $23.79 1 $23.79

Other Aluminum Shaft Attachment $9.95 5 $49.75Sleeve Bearing $1.88 10 $18.80

Spring-Loaded PTFE Seal PTFE, 1/16" Width, 1/4" Shaft Dia, 3/8" Seal Od $8.00 5 $40.00Unshielded 22AWG UL2464 3 COND, 100" $31.75 1 $31.75

Grand Total: $454.84

2.5 Manipulator Arm Photographs

Figure 2.4: Photograph of Inside of Motor Housing

22

. . :'. . j, - T...., i z �..i' , , S� , , ��4 M.-

11-4 10MOIX10- Aft

Figure 2.5: Photograph of Arm Assembly

23

: I I

-:17 .1.111 1.11111%

link--. I .- ..,

Figure 2.6: Photograph ot Arm in Extendaea osition

Figure 2.7: Photograph o0 Arm in Stowea Posltlon

24

Chapter 3 System Modeling

This chapter presents the kinematic and dynamic analysis of the manipulator. In addition,fluid effects are defined and discussed.

3.1 Rigid Body Kinematics

The system in consideration can be idealized as 5 revolute joints, each rotated at an angle0i with respect to the orientation of its connecting serial linkage of length fi (Fig 3.1).The Forward Kinematic Equations are used to describe the location and orientation of theend-effecter in the fixed vehicle reference frame 0- x0yoz0. For the purposes of thisproject, the vehicle is considered to be stationary within the fixed earth reference frame.The position of the end-effecter is described by the three coordinates [Xe, ye, Ze].

I r;-11

03 04 05

t2 t3 t4

Figure 3.1: Reference Frames

Forward Kinematic Equations:

Xe = {2 cos82 + e3 cos(02 + 3 )+ f4 cos(0 2 +3 + 04 ) + (w4+ 5 )sin(02 +03 + 4 )}cos01

'e = { 2 cos 0 3 cos( 2 + 0 3 )+ e4 cos( 2 +03 + 04 ) + (w4+

5 )sin(0 2 + 03 + 04 )}sin0 1

Ze = *1 +f2 sin0 2 + 3 sin(02 +0 3 )+ e4 sin(02 +03 +04 )-(w 4 +/I 5 )cos(0 2 +03 +04)

25

Table 3.1: Denavit-Hartenberg Parameters 8]

i ai ai d i i

1 O -90 ° e' a0

2 E2 0 0 02

3 t 3 0 0 03

4 W4 -90° '4 04

5 i5 0 0 05

where:a i Link length, distance from Z i to Zi+ along Xi;ai Link twist, angle from Zi to Zi+, about Xi,d Link offset, distance from Xi_, to Xi along Zi, and0 i Joint angle, angle between Xi-, to Xi along Zi.

Homogeneous transformation Ai:

Soi Cai Soi ai ai coi

Soi COi Ca i -Ci Sa i aisOio S ai Cai

0 0 0 1

3.2 Rigid Body Dynamics

Characterizing the motion of the arm within the fixed vehicle reference frame is criticalto understanding arm dynamics. These equations can become quite complicated. For thepurposes of this research, primary interest is with regard to large motions of the arm.Therefore, in the analysis in this section, rotations in 01, 02 and 03 only will be considered,and the joints at 04 and 05 will be rigid.

The Jacobian matrix J describes the relationship between joint angular velocity, i and the velocity of the end-effecter in the reference frame:

26

K'

8t)s I Zm I tQ C'I ~

( 0 ( 0 ( c c( cO8Q Q (Z-Q t CZ --' I ( ( ( 0 0 (

I -I ( CO cO IIcC; rg ~ ~OII I

r

CI

o Cc ) 0++ C

++CD C DL. :, + ) +

+O " ' +o o 0L

ICD + ~ 2C+'D + + +

+ + C

(_ . r Co CN

-I- + C' +>

+ + +C4 0

,Co - +_o CD CO '

*q r (C r

+0a C o+ + 0@4 C)o CO C @,

c) Co COC")

0 I I a'

@ a'- '

o

C) C@(NJ

C'D

I I

--.

I

('14

'O5

0

r +

o

+

r 0

+ C-

+-N

4D 1a',+

+ C

+IC-- C-

04

04

+

(CNr,

Co

+

+

+(f)

o0.C

C'q

<- (" e1

I II

I I

C)

3.3 Fluid Effects

Underwater operation adds both static and dynamic complexity. The hydrodynamicmodeling of underwater manipulators is quite complex, therefore, discussion in thissection is aimed more at an overview of the hydrodynamic effects and their possibleimpact on performance, rather than computation of exact values.

3.3.1 Hydrostatics

In static analysis of underwater bodies, both the gravitational force acting on the bodymass and buoyancy force must be considered. Archimedes's principle states that thebuoyant force is equal to the weight of the fluid displaced. Therefore,

Fbuoyancy = Pwater submerged g

Fgravity = mbodyg

FtotaI = Fbuoyancy -Fgravity

3.3.2 Hydrodynamics

Added mass is the effect of fluid inertia in the environment on a moving body. This isnot a phenomena that extends to above water operations, however, the effect is vastlymore significant subsea as the density of water is much more comparable to the densityof the body of interest. The added mass coefficient is dependent on body geometry andmotion.

Fluid viscosity also creates drag and lift forces on a body. Drag acts in parallel with theflow velocity on the body:

Fdrag = pU 2A Cd (Re)

where p is the fluid density, U is the flow speed, A is the projection of the body in thedirection of flow, and Cd is the drag coefficient which is dependent on Reynold'snumber:

Re = plUID'U

where gt is the dynamic viscosity of the fluid. The lift force acts normal to flow velocity:

Flif1= pU2ACI (Re,a)

where the lift coefficient C1 is a function of Reynold's number and a angle of attack.

28

For a cylinder in non-uniform flow, the total added mass and drag forces can be foundusing Morison's equation, which relates the inertial and drag forces:

dF =-C, * pA Udl -C * pWIUIUdl

3.3.3 Previous Research on Hydrodynamic Modeling of Underwater Manipulators

The complexity of hydrodynamic modeling of underwater manipulators is largely due toa lack of understanding of the 3-dimensional hydrodynamic effects of manipulatorsystems. To be considered are flow effects around rotating joints and linkages in closeproximity, and their dependency on rapid movement and unsteady motions.

It has been determined through experimental research that using constant added mass anddrag coefficients for a manipulator body are not sufficient for complete analysis [9]

Leabourne and Rock [10] expanded on this research, and found the drag and added masscoefficients to be dependent on elbow angle in their two degree-of-freedom arm model.In these tests, the maximum value of the drag coefficient for the manipulator model wasfound to be 3.

29

Chapter 4 Performance Assessment

This chapter describes the performance of the arm design presented in previous chapters.The overall arm control architecture is discussed, and preliminary testing of motors andthe arm are assessed.

4.1 Control Architecture

Hobby servos are controlled using pulse width modulation (PWM). The servo angle isregulated by varying the signal pulse width, where 1.0, 1.5, and 2.0 msec prescribe motorrotation angles of -90 deg, 0 deg, and +90 deg respectively.

Signal

50 Hz Period 1 .0 - 2.0 msec Pulse Width* -

Tine

Figure 4.1: Pulse Width Modulation

The control system and user interface for the manipulator were developed by members ofthe ROV Team. The arm is operated using a master-slave system. The master is topside,and is of identical kinematic design to the actual manipulator. Each joint of the masterhas a position potentiometer that measures the motion of the operator as the master ismoved to simulate the position of the actual arm. The desired position is chosen inparallel with real-time video feed of the manipulator's location.

The control system employs two PIC microcontrollers operating in closed loop, one-waycommunications control. The topside PIC reads in the potentiometers in the joints of themaster arm, converts the values into motor angles and sends the info down a fiber optictether. The PIC on the bottom side receives the data, converts the motor angle and sendsthe signal to the motors.

31

4.2 Servo Characterization

An HS-805 servo was tested using a PIC outputting PWM. The motor was enclosed inits waterproof housing for both air and water tests. Maximum slew rate was determinedby measuring the time for the motor shaft to rotate clockwise, and averaging this rateover 10 trials. Power consumption without load was measured as the current drawobserved under no load, with a voltage limited power supply at 6.0 V. Maximum powerconsumption with load was the largest current draw observed over the testing period,under voltage limited power supply of 6.0 V. These characteristics are shown below inTable 4.1. As can be seen, the max slew rate and power consumption without load arefairly close to those expected by the manufacturer's specifications. The higher measuredvalues are expected, as the motor shaft, even under no load, is experiencing additionaltorque due to the spring-seal. These discrepancies are not large enough to be cause forconcern with regards to housing and shaft seal effects on motor performance.

Table 4.1: HS-805 Servo Characteristics

Manufacturer'sSpecifications Motor in Air Motor in Water

Max Slew Rate 180 Deg / 0.42 Sec 180 Deg / 0.54 Sec 180 Deg / 0.57 Sec

Power Consumptionw/o Load 4.98 Watts 5.64 Watts 5.88 Watts

Max PowerConsumption w/Load N/A 11.22 Watts 12.66 Watts

4.3 Arm Characterization

The tests performed with the assembled arm consisted of controlling only the first andsecond motors, one at a time. The arm was assembled using the first four motor housingsand the two linkage pairs as seen in Figure 2.5. The motor that was being tested wasgiven a PWM signal to control the position of the motor shaft. Due to time constraintsand technical difficulties, the abilities of these motors to provide sufficient torque wasonly observed qualitatively. Both motors were able to drive their successive linkages andattached motors without stalling. The slew rate did not appear to vary significantly fromtests with no load.

32

Chapter 5 Conclusions

This chapter concludes the thesis by summarizing the contributions of this research andmaking recommendations for future work.

5.1 Summary of Contributions

This thesis presents the foundation for a low-cost, several degree-of-freedom underwatermanipulator for a small ROV. The primary achievement is the design of a waterproofservo motor housing consisting of static and dynamic seals. Several of these housingswere manufactured and proven to be watertight, and to have minimal effect on motorperformance. Kinematic and dynamic analysis was performed, as well as the discussionof fluid effects for the manipulator. In addition, a preliminary manipulator arm wasassembled, which required the design and manufacturing of motor shafts, linkages, andsupports. Preliminary underwater tests of the motor housings and of the assembledmanipulator were successful, and encourage the continued development of the arm.

5.2 Future Research

In order to make this arm ready for field operation, the motor housings should be testedto maximum designed depth. If more extreme operating depths are desired, a newpressure compensation system must be developed, such as the oil regulating bladdersystem described by Di Pietro for the ROV JASON manipulator [11]. An end-effecter iscurrently being developed for the arm. Continued arm characterization needs to beperformed using the end-effecter with various payloads.

The primary drawbacks to the current motor housing design are cost of material andmanufacturing time. A large portion of the nylon stock used to create the housing ishollowed out and discarded during manufacturing, which is an inefficient use of time andmoney. Using a readily available, inexpensive, waterproof housing would be asignificant improvement on manufacturing and cost. Another interesting avenue wouldemploy inexpensive DC motors with positioning electronics. DC motors would reducecomplexity in the shaft seal and possibly result in more compact motor housings.

33

Appendix A

0.400

3.080

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Bibliography

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[2] "LBV Grabber." Product Literature. SeaBotix, Inc. 15Apr. 2006<http://www.seabotix.com/products/pdf_files/LBV_GrabberA4.pdf>.

[3] "ISL MicroManip." Product Literature. RoboProbeTecnologies. Inc. 15Apr. 2006<http://www.roboprobe.com/DOWNLOADS/D 13_IK_MICROMANIP.PDF>.

[4] Yuh, J, and M West. "Underwater Robotics." Advanced Robotics 15 (2001): 609-639.<http://www.ingentaconnect.com/search/article?title=underwater+robotics&titletype=tka&year_from= 1998&yearto=2005&database= 1 &pageSize=20&index=5>

[5] Stanway, M. et al. "ROV Castor: Design of a Multi-Mission Remotely OperatedVehicle." 2004.

[6] "Giant Scale Servos." Product Literature. Hitec RCD USA Inc. 03 Feb. 2006<http://www.hitecrcd.com/homepage/product_fs.htm>.

[7] "FlexiSealTM." Product Literature. Parker Seals. 10 Mar. 2006<http://www.parker.com/packing/cat/english/5315_FlexiSeal.pdf.

[8] Craig, John J. Introduction to Robotics. 2nd ed. Reading: Addison-WesleyCompany, Inc, 1989.

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