increased functionality of an underwater robotic manipulator1
TRANSCRIPT
Increased Functionality of an Underwater Robotic Manipulator1
Benjamin Champion2School of Engineering
Deakin UniversityWaurn Ponds, VIC, AUS
Mo JamshidiCollage of Engineering Electrical Engineering
University of Texas at San AntonioSan Antonio, TX, USA
Matthew JoordensSchool of Engineering
Deakin UniversityWaurn Ponds, VIC, AUS
Abstract—Research into underwater robotic applications iscurrently a growing field. There are many challenges involved inunderwater robotics that are not present in other mediums, suchas how the harsh environmental conditions that this environmentinvokes onto the robot and any equipment that is attached to therobot. In this paper an attachment to an underwater gripper isproposed that adds another Degree Of Freedom to the system,thus allowing the gripper to move along the belly of the robot.Adding this functionality to the gripper has many advantages,some of which involve the robot being able to easily pass acollected object to another robot with minimal interference. Thisattachment is constructed using 3D printed parts, a waterproofedservomotor and a leadscrew to provide linear motion to acommercial gripper.
Keywords—Underwater robot, Gripper, 3D printed, Underwa-ter manipulator, ROV, AUV
I. INTRODUCTION
Robots are becoming more commonplace in the worldtoday. Land vehicles are still the most common form of robotthat is employed to accomplish task, such as the vacuumcleaning robots or mine detection robots. Robots that areable to fly through the air are also becoming a common sightand are used for many different applications such as in themilitary, aerial photography, etc. A growing area of researchis into applying robotics to the underwater domain as well.Underwater robotics is becoming popular for many reasons,as robots are able to stay in harsh environments to easilymonitor changes to the environment, as well as go to areasthat man cannot easily access. A good example of this is it ispossible to design robots to go down significant depths andraise very quickly if required, wheres a technical diver is onlyable to go down to depths of 300m, unless a submersiblevehicle is used [1].
When Underwater robotics is concerned, there are twobroad categories that the robots can fall into, these beingan Remotely Operated Vehicle (ROV) or an AutonomousUnderwater Vehicle (AUV). It is commonplace to use ROVsystems in the underwater domain, due to the challengesthat making a system that is entirely autonomous presents,such as trying to localize the robot without the use of aGPS system and accounting for the strong currents that
1 Work was supported, in part, by grant number FA8750-15-2-0116 fromAir Force Research Laboratory and OSD through NCS&T State University
2 On study leave at ACE Laboratories
can be present underwater. As communication to the robotsalso presents a major challenge, most of the robots that arecurrently used are tethered back to a boat on the surfaceof the water, making the teleoperation approach a viableoption as the robot is already limited to the length of the tether.
This paper presents a design for an underwater gripperattachment that will be used with an AUV robot, acting ina swarm. The purpose of this attachment is for the underwaterrobot to be able to grasp and carry an underwater object fromthe bottom of a pool, and then be able to transfer this object toanother robot to transport the object to another location. Thereasoning behind designing and constructing the attachmentto the commercially available arm is that the current armavailable for the robot used, the VideoRay Pro 3, is fixed inplace and therefore makes the collection process a complicatedproblem as the rest of the robot will interfere while the objectis being collected. For the same reasoning the extension to thearm is required for the robot to be able to transfer the objectto another robot without either of the robots interfering withthe process.
II. BACKGROUND
As robotics in the underwater domain has become morecommonplace, a need for a robot to be able to manipulateunderwater objects has become apparent. To achieve thisgoal, a lot of work has been conducted in the development ofunderwater grippers to allow the robots to accomplish a widevariety of different tasks, such as the retrieval of an airplanesblack box, for both ROV and AUV missions. Differentapproaches have been undertaken to try and overcome theunique challenges that the underwater domain introduces,such as by utilizing cables, hydraulics and magnets togenerate movement for various different applications [2]–[4].Custom grippers have also been developed to enable a ROVto collect specific objects such as cylinders [5].
Attaching a gripper directly to a robot limits the robotsability to accomplish any different tasks. To overcome thisinconvenience, as well as to let the robot collect objectsfrom potentially hard to reach places, underwater roboticarms have been developed for both ROVs and AUVs. Thereare many challenges presented when designing and using
robotic arms for underwater applications, such as effectivelyusing teleoperation to control a many degrees of freedom(DOF) arm [6], as this becomes a major problem due to thechallenging environment presented, as well as how a roboticarm needs to be protected from its surrounding environment.A majority of the arm designs that have been presentedin literature are for ROV operation, as remotely operatedrobots are still the norm in the underwater domain. Differentmethods of providing movement to the underwater armshave been investigated, such as by using stepper motors[7], [8], servo motors and by using hydraulic muscles [9].Robotic arms that dont conform to the traditional rigid bodyconstruction have also been investigated as they could behighly advantages in the complicated underwater environment[10]. It has also been noted that the ability to water proofthe arm needs to be considered, as many of the componentsused in electronics are not able to function in the underwaterdomain. To waterproof the components, many differentmethods can be used. The most common method is to simplyuse a sturdy casing material to enclose the components of thearm, and then use a seal, such as a rubber O-ring, at the joins[7]. Other methods have been presented though, such as byemploying a silicone sleeve to act as a barrier between theinternals of the robotic arm and the environment [10].
All of the researched methods are relatively expensive toimplement, either using expensive hardware or having tocustom machine parts to be able to water proof the system. Theproposed design for an linear extender shows an inexpensiveand simple method of adding linear movement to a preexistingunderwater gripper to allow the robot to collect and hand-offobjects in an underwater setting.
III. DESIGN AND CONSTRUCTION
When constructing artifacts to be placed under the surfaceof the water, there are many different challenges that needto be overcome. When working within the constraints of apreexisting vessel, some of these requirements become vastlyimportant. The buoyancy of the constructed object needs tobe considered, as if the object is too buoyant it will floatand therefore become a hindrance to the craft that is attachedto, wheres conversely if the object is not buoyant enough itwill sink and again become a hindrance. The object must alsobe able to be easily mounted onto a preexisting system sothat it does not hinder the craft in any way. In the case ofthis design, it is also required to be water proof as thereare electronics involved that could potentially be destroyedif water is introduced into them. To simplify the design, theinclusion of the commercially available gripper for the Video-Ray Pro 3 robot was included. By using this preexisting zeroDegree Of Freedom (DOF) gripper, the challenges involvedin waterproofing the end effector have been removed, andthe only challenge left is to give the gripper the ability toextend and retract, to enale the robot to be able to easilycollect and pass off objects. Unfortunately, the device that this
robot is attached to, the VideoRay Pro3, does not allow for thereprogramming of its internal control unit. To overcome thisissue, as well as to increase the sensing ability of the unit,a separate underwater box has been mounted to the top ofthe unit as shown in figure 1. This underwater box containssensors to improve the robots ability to locate itself in space,such as an IMU, as well as providing a microcontroller thatcan be used to control the underwater grippers linear motion.
Fig. 1. Robot with Underwater box and Gripper attachments
A. Gear Box
The first challenge that needs to be overcome is convertingthe angular motion that a motor produces, into the linearmotion that is required to enable the arm to travel along thebelly of the robot. This is accomplished by utilizing a simple1:1 gear box, as shown in figure 2. This gearbox consists of agear attached directly to the shaft of a servo motor. The secondgear within the gear box is attached to the back plate via ametal bearing. This second gear is able to be separated intotwo halfs, and then two nuts are able to be placed within thegear. Using these nuts, a threaded rod can be screwed throughthis gear. When the this gear is turned by the motor the nutswill want to move down the shaft of the threaded rod. Asthey are unable to do so as the gear is fixed in place via itshousing, the threaded rod will move instead, thus allowing theunderwater gripper to travel forwards and backwards along thebottom of the robot, relative to the direction of spin of the gearand in turn the motor. The back half of the outside casing ofthe gearbox is able to be mounted directly to the robot to keepit in place using screws into the predefined holes on the base ofthe robot, with the front half of the casing being screwed to thefront of this. The front plate serves two purposes, it containsanother bearing to ensure that the second gear does not moveout of place but is still able to free turn, whilst also ensuringthat if the gear did somehow become detached from the servo,it would be contained. This allows thee reasoning behind thefailure to be determined, as well as the help remove the robotfrom producing any waist into the environment. The front plate
also prevents the gear from spinning down the threaded rodinstead of the threaded rod moving.
Fig. 2. Gearbox Assembly
B. Slider
The next component that needs to be developed is a wayfor the preexisting arm to travel up and down the belly of therobot, via the motion provided by the lead screw, on a fixedpath. To achieve this, a simple solution has been developed thatconsists of three parts, a shaft for the arm to travel down anda clamp at both ends of the gripper, one which will push thearm down the shaft and is attached directly to the lead screw,and the other one that is simply designed to prevent the armfrom coming out of the channel. The shaft is depicted in figure4. The lead screw is able to push the arm down the shaft viathe clamp at the rear. This clamp is able to be split into twohalfs, with the bottom half consisting of nothing but a bufferto keep the arm in the shaft. The top half contains a third nutthat the lead screw is able to be screwed into. This nut holdsthe lead screw in place, whilst also enabling the transfer offorce from the led screw into the clamp and therefore causingthe arm to travel in a linear plane. To prevent the gripperfrom being able to rotate about, a grub screw was insertedinto the top of the clamp responsible for pushing the gripper.Unfortunetly it was discovered that this grub screw was notsufficient to prevent rotations of the arm, due to the grubscrewbeing threaded into plastic which was able to be deformed justenough to allow rotation, as well as the arm being completelycircular and sooth, meaning that the grub screw was unable toget sufficient grip on the gripper. To overcome this problem,Gorilla Glue was used to stick the gripper to the into thebottom half of the rear clamp. It was determined that if thearm needed to be removed from the clamp entirely, the bottomhalf of the clamp could be removed by destroying it if needed,and then replaced by simply re-printing the component.
C. Limit Switch
To be able to track where the gripper is along the base ofthe robot, a limit switch has been added. This limit switch
Fig. 3. Slider with lead screw and gripper
serves the same purpose as that of a CNC machine or a 3Dprinter, it allows for the device to know when the arm is atthe end of the shaft. This introduces many advantages that thesystem did not previously have, mainly being that the robotcan initialize the position of the gripper as the arm can traveldown the shaft until the limit switch is activated to get thegripper into a known position at start up. The position thatthe gripper is currently at along the base of the robot can alsobe calculated. This is achieved by simply knowing the speedin which the gripper is able to travel at and multiplying it bythe time that is has been traveling for in a certain direction.This can also be achieved without the use of a limit switch,but the switch allows for the removal of drift, as every timethe switch is activated the position of the gripper can be reset.The inclusion of drift is expected to become an issue in thisenvironment, as the deeper the robot travels it is expectedthat time for the arm to travel the length of the robot willchange significantly.
To be able to use the limit switch underwater, a microswitch with an arm and a roller is used. To be able to usethis device underwater, the switch was covered in two separatepieces of heat shrink, and then the edges of the heat shrinkwas glued together using silicone. Two pieces of heat shrinkwere used simply because a single piece was not available thatwas large enough to accommodate the switch without causingthe switch to activate, but was too large that a significant
amount of bunching would happen while the heat shrink iscollapsing causing the seal at either end of the heat shrink tobe compromised. The reasoning behind using heatshrink thathas not been heated and therefore shrunk was for the flexibilityof the material. It was found that if the heat shrink was shrunk,it became considerably stiffer, which hindered the operationof the limit switch as either this shrinking would cause theswitch to be locked into an on state, or requiring more forcebeing required to press the switch which could cause the armto pause momentarily while it generated enough force to doso.
Fig. 4. Limit Switch assembly
D. Servo Motor
A continuous drive servo motor (servo) was chosen for thisapplication to enable the movement of the arm. Brushed andbrushless motors were considered as viable solutions to powerthis device. It has been shown that both brushed and brushedmotors that are commonly used to power land rovers willstill function underwater. The biggest drawback with thesemotors is that these motors do contain bearings, gearboxs andwinding’s that will corrode upon continuous contact to water,as well as the various lubricants that are required to keepthem functioning correctly being washed away. This processwill be significantly faster in different types of water, suchas fresh water vs salt water. This problem can be overcomein a variety of ways, such as by purchasing motors that donot rely upon these bearings or lubricants, as well as having
all of their components tightly sealed against corrosion. Asthese motors are generally considerably more expensive thana standard motor, they were discounted as a viable cheepoption. A standard motor can be disassembled and all of theparts manually coated with a protective layer and in the caseof a brushed motor, a sacrificial anode can be used to preventcorrosion of the brushes, but it was determined that this wasnot necessary as a servo motor would be significantly easierto water proof.
Waterproofing the servo motor was a very simple process.100% silicone that was purchased from a hardware store wasused to coat the entire outside of the servo. This preventedany leaks from getting into the servo itself, and it wasdetermined that the internals of the servo would not haveto be accessed unless the entire motor was destroyed, thussealing the unit up was a viable option. As silicone cannotbe used to seal up the point where the shaft protrudes fromthe servo motor, due to the rotation of the servo motorcausing the silicone to tear, a different approach had tobe used. To overcome this problem, a rubber O-ring wasplaced over the shaft, and then silicone was used to holdit in place. As the servo horn, or in this case the gear, isplaced into the servo shaft it compresses this O-ring andtherefore creates a watertight seal. This method of sealingthe servo motor has currently been tested up-to a depth of 3m.
Another major advantage of using the servo motor for thisapplication is the amount of space that is able to be saved. Thisis a major advantage to reduce the impact that the attachedequipment will have on the pre-existing system. If either abrushed or brushless motor is used to generate the angularmotion required to move the lead screw, either a motor driveror an electronic speed controller would be required to drivethe motor. As the artifact is designed to be an attachmentto a pre-existing robot, the amount of space that the motorand control unit takes up must be kept to a minimal. Due tothe small form factor of the servo motor, and the fact thatthe servo has an inbuilt motor driver, this problem is simplysolved. Feed back can also be removed, as the servo motor hasan on-board PID controller so it will try and move the armat the desired speed without any intervention. Another majoradvantage to using the servo motor is that it only requires 5Vto power the unit. As the microcontroller that is being used tocontrol the device also runs off the same voltage requirements,a single voltage regulator can be implemented to power bothof the systems. A servo motor also enables the unit to onlyconsume a small amount of power, when compared to someother options such as stepper motors, as only minimal powerwill be drawn from the servo motor while it is not moving.This could potentially enable a system similar to this to beplaced on a robot containing an enclosed power source, aswell as reducing the draw from the above water power sourceused to power the currently equipped robot.
E. Construction
As Can be seen in the preceding figures, 3D printingusing Polylatic Acid (PLA) filament was the main methodused for constructing the presented artifact. As the FusedDeposition Modeling (FDM) technique is used by the printerused to generate the parts, having air trapped within themodels causing them to be very buoyant was a significantpossibility. TO overcome this, the parts were printed with100% infill, thus ensuring that they were able to add as muchweight to the system as possible, as the designs themselveswill naturally float. The main reasoning behind using thismethod to construct the artifacts was that 3D printing was acheep way of generating parts that could be custom mounteddirectly onto predefined points on the surface of the robot.Using this method also ensures that the generated parts havethe least amount of interference possible with the operationof the robot. This also allowed for the rapid design andimplementation of the parts, as the time that is required toget the parts from a CAD model into a physical object isrelatively small.
As mentioned throughout the Servo Motor section, acombination of silicone and a rubber O-ring was used towaterproof the servo motor. It was found that this was acheep and easy way to solve the space and size restrictionsthat using other methods added into the system.
Each of the generated parts were able to be fitted directlyonto the robot by using blots into the predefined mountingholes on the robot. This is holds many advantages as it meansthat no modification to the actual robot was required to attachthe parts and different attachments can be easily added andremoved as required to increase the functionality of the robot.
IV. RESULTS
Several tests have been preformed using the actuator, andit has been found that the designed gripper system is able tofunction sufficiently. The speed in which the gripper is ableto extend and retract was tested. It was found that it takesthe gripper 20 seconds to travel 3.5cm, meaning that is ableto travel at a speed of 0.175cm/sec. As this is designed foran autonomous system it was determined that this speed isacceptable for this application.
A depth test using the constructed gripper was also con-ducted. The gripper was taken down to a depth of 3m, andthen told to both extend and retract. It was found that theactuator was able to preform both these applications, thusproving that the waterproofing of the servo and the limit switchwas functional, and that the implemented setup was able tofunction as desired. A foam torpedo, approximately the samelength as the ROV, was collected and brought back up to thesurface. Currently this was done by manually in-putting thecommands into the robot, but this proves that the robot shouldbe able to accomplish the same task in autonomously as well.
V. CONCLUSION AND FUTURE WORK
The presented paper has shown that it is possible to generatea waterproof linear extender for underwater applications on alow budget. It has been shown that the constructed artifactwas able to function at a depth of 3m, and should thereforebe able to accomplished the proposed goal of object retrievaland hand off.
There are many different thing that could be incorporated toimprove this system. It was found after the generation of theparts that PLA is not the best choice of material for underwaterapplications. It has been found that PLA degrades ratherquickly when exposed to a highly moisture filled environment[11]. To overcome this, either reprinting that parts using ABSplastic should be trialed, or by making the parts out of someother non-corrosive material, such as powder coated metal.Further depth tests should also be preformed to determine themaximum depth that this system is able to function at. Testswill also be conducted to determine how well this system willfunction when trialed in an autonomous setting, in both thecollection of the underwater object, as well as the ability forthe arm to be used to hand-off the object to another robot inthe same system.
REFERENCES
[1] D. Richardson, Encyclopedia of Recreational Diving. PADI, 2008.[2] H.-T. Su, T.-L. Tang, and W. Fang, “A novel underwater actuator driven
by magnetization repulsion/attraction,” in Micro Electro MechanicalSystems, 2009. MEMS 2009. IEEE 22nd International Conference on.IEEE, 2009, pp. 1051–1054.
[3] J. Bemfica, C. Melchiorri, L. Moriello, G. Palli, and U. Scarcia, “A three-fingered cable-driven gripper for underwater applications,” in Roboticsand Automation (ICRA), 2014 IEEE International Conference on. IEEE,2014, pp. 2469–2474.
[4] G. Bartolini and M. Coccoli, “Discontinuous control of an underwatermanipulator by a simplex of constant control vectors,” in Decision andControl, 1999. Proceedings of the 38th IEEE Conference on, vol. 4.IEEE, 1999, pp. 3242–3347.
[5] K. Ishizu, H. Nakayama, N. Sakagami, M. Shibata, S. Kawamura,S. Matsuda, and A. Mitsui, “Preliminary experiments of a human-portable underwater gripper robot for dexterous tasks,” in OCEANS2014-TAIPEI. IEEE, 2014, pp. 1–7.
[6] B.-H. Jun, P.-M. Lee, and J. Lee, “Manipulability analysis of underwaterrobotic arms on rov and application to task-oriented joint configuration,”in OCEANS’04. MTTS/IEEE TECHNO-OCEAN’04, vol. 3. IEEE, 2004,pp. 1548–1553.
[7] H. Shim, B.-H. Jun, H. Kang, S. Yoo, G.-M. Lee, and P.-M. Lee,“Development of underwater robotic arm and leg for seabed robot,crabster200,” in OCEANS-Bergen, 2013 MTS/IEEE. IEEE, 2013, pp.1–6.
[8] R. Gad, G. Naik, and N. Aralgedad, “A design of 2-dof gripper circuitfor deep-sea objects,” in Oceans’ 04 MTS/IEEE Techno-Ocean’04 (IEEECat. No. 04CH37600), 2004.
[9] K. K. Ku, R. Bradbeer, K. Lam, and L. Yeung, “Exploration for noveluses of air muscles as hydraulic muscles for underwater actuator,” inOCEANS 2008-MTS/IEEE Kobe Techno-Ocean. IEEE, 2008, pp. 1–6.
[10] T. Zheng, D. T. Branson III, R. Kang, M. Cianchetti, E. Guglielmino,M. Follador, G. A. Medrano-Cerda, I. S. Godage, and D. G. Caldwell,“Dynamic continuum arm model for use with underwater robotic ma-nipulators inspired by octopus vulgaris,” in Robotics and Automation(ICRA), 2012 IEEE International Conference on. IEEE, 2012, pp.5289–5294.
[11] G. Yew, A. M. Yusof, Z. M. Ishak, and U. Ishiaku, “Water absorptionand enzymatic degradation of poly (lactic acid)/rice starch composites,”Polymer Degradation and Stability, vol. 90, no. 3, pp. 488–500, 2005.