Shintemirov, A

Telegenov, K

Tlegenov, Y

Yedige Tlegenov

Kuat Telegenov

Almas Shintemirov

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An open-source 3D printed underactuated robotic gripper

Apollo (Cambridge)

An Open-Source 3D Printed UnderactuatedRobotic GripperYedige Tlegenov, Kuat Telegenov, Almas ShintemirovDepartment of Robotics and MechatronicsNazarbayev UniversityAstana, Kazakhstan, 010000Emails: yedige.tlegenov@nu.edu.kz,ktelegenov@nu.edu.kz, ashintemirov@nu.edu.kzAbstract—In this paper authors present an open source low-cost basic robotic end effector platform for facilitating researchon robotic grasping. The 3D design model of a three fingeredunderactuated robotic gripper is presented and manufacturedwith minimal number of 3D printed components and an off-the-shelf servomotor actuator. An underactuated finger, gear trainmechanisms and an overall gripper assembly design are describedin details followed by illustration and discussion of graspingof objects with various geometries. The presented open sourcegripper design will be released for downloading on the authors’research lab web-site www.alaris.kz and can be useful forrobotic researchers as a platform to build their own robotic endeffector solutions for research and educational purposes.I. INTRODUCTIONDevelopment of robotic end effectors that are employed forgrasping of a variety of objects is an active research area.Various robotic end effectors were developed for wide range ofapplications where reproducing the human hand functionalityis required [1]–[3]. However, in most of industrial and serviceapplications manipulation of objects with anthropomorphicrobotic hands is not required and three fingered graspingdevices are more sufficient [4]. Considering mechanical fingerdesigns, a lot of robotic grippers utilize individual actuationof each joint of the finger using a small high precision DCservomotors [5], [6]. This provides high number of control-lable degrees of freedom (DOF), which is very suitable forgrasping of complex shape objects. On the other hand, usageof multiple actuators in each finger mechanism results in highcost of the device and control complexity.A number of designs utilize pulley/tendon driven mech-anisms for wide range of robotics systems [7]. They havemany advantages such as low weight, less number of actuatorsand high degree of adaptability. Among disadvantages arelimitations in load carrying capacity and low wear resistance.Therefore, as an alternative, mechanical linkage systems areutilized for different designs of underactuated artificial fingers[8], [9].There are many simple designs of commercially availablegrippers for grasping cylindrical and spherical shape objects.For example, a 3-fingered concentric gripper [10] is designedfor relatively high payload industrial applications, but it isnot able to perform enveloping grasping. The reconfigurablegripper [11] with convex shape thin fingers is designed forpicking round objects and placing them in a tray with littlespace for movement.In general, many of the commercially available hands do notaccommodate extensive customisation of the design featuresfor attachment to different robotic arm platforms or integrationof additional sensors for research purposes [12]. To addresssuch problems 3D printing rapid prototyping technology isactively applied for manufacturing of low-cost robotic hands[13], [14]. Aiming to provide a basic robotic platform withminimal number of 3D printed components and off-the-shelfactuator for facilitating robotic research efforts, a 3D printedopen source tendon driven hand is proposed in [12]. State-of-the-art of the grasping in robotics recently presented in [15].To extend the choice of robotic end effectors freely availableto researchers, in this paper the authors utilize a linkagebased finger system and present an open source 3D printedunderactuated three fingered robotic gripper with a simpledesign and relatively higher payload property comparing tosimilar size tendon driven mechanisms. The 3D model of thegripper is created using the SolidWorks CAD software andwill be available for downloading from www.alaris.kz/.The organization of the paper is as follows. Section IIdescribes the design and manufacturing of the 3D printedthree-fingered robotic gripper. The grasping performance ofthe robotic gripper prototype is presented in Section III, whichis followed by the conclusions and future work in Section IV.II. DESIGN AND FABRICATION OF 3D PRINTED THREEFINGERED ROBOTIC GRIPPERA. Underactuated Finger MechanismUnderactuated fingers have less number of actuators thantotal DOFs and are widely utilized in design of variousrobotic hands for industrial [16] and service robotics [17].This is largely attributed to the relatively simple design of suchmechanisms comparing with fully actuated dexterous artificialfingers. At the same time, an underactuated finger mechanismshould ensure close wrapping of different shape objects dueto its adaptive grasping capability with one DOF actuation.In this work, the underactuated linkage mechanism is em-ployed for finger design described in [18], [19]. This designensures a simple low-cost mechanical system which wasone of the principles for finger design requirements [20].Fig. 1. Closing sequence of a 2-DOF underactuated finger.(a)(b)Fig. 2. 2D (a) and 3D (b) designs of the underactuated finger.Consider a 2-DOF and one degree-of-actuation (1-DOA) fingermechanism shown in Fig.1. A passive element, i.e. a springbetween the first and second phalanges is used for providingactuation of the second DOF of the finger [19]. The closingsequence of the underactuated finger is described as follows.Firstly, the finger moves as a rigid body from its initial positionsince no external force is acting on it. When the first phalanx ofthe finger is in contact with an object, the second phalanx startsmovement around a pivot point to complete a full wrapping.Same method can be applied to n-phalanx finger [19].Utilizing this principle, an underactuated finger mechanismis designed and presented in Fig. 2. The finger consists of twophalanges, two links, an extension spring and a worm wheel.Note that the worm wheel and link 1 are rigidly connected.The worm wheel transmits rotary motion to link 1 around its(a)(b)Fig. 3. 2D (a) and 3D (b) sketches of the gear train design for one finger.pivot point; subsequently link 1 transfers the motion to link 2.The extension spring allows the finger to behave as a singlerigid body during rotary motion around the fixed pivot. Whenthe first phalanx touches an object, the force produced by anactuator extends the spring which starts transferring motion tothe second phalanx only. Finally, both the phalanges contactan object that concludes a finger closing sequence.Grasping characteristics of the gripper can be modified bysetting various geometries of two actuation links of the fingers.For instance, changing the length of two actuation links ofthe finger can result in various dynamic outputs of the firstand second phalanges. The surfaces of the finger componentsare flat so that the printer support material can be removed(a) (b)Fig. 4. Arrangement of worm wheels connected to the actuating worm in 2D (a) and 3D (b) views.manually with little effort. All rotational joints of the fingerare connected via pivots with retaining rings at both sides, asshown in Fig. 2(b). The pivot 3D models are supplied with thegripper CAD model and can be used for manufacturing metalpivots using standard machine shop facilities.B. Gear Train DesignThe three fingers in the gripper are actuated by a singleactuator using a gear train transmission system. The 2D and3D representations of the single finger actuation is shown inFig.3. A servo actuator is fixed on the base and drives thedirectly connected worm gear. The worm transfers rotationalmotion to the set of worm wheels, which are connected toeach finger. All three fingers are driven simultaneously fromtheir initial position. Usage of the worm gear ensures non-backdriveability of the finger actuation mechanism. The set oftwo worm wheels are placed in a way that allows adjustmentof the rotational speed and torque transferred to a finger bychanging the gear ratios within the given distance betweengear rotational axes. Researchers can also change the actuatorand set up custom gear train speed/torque ratio depending ontheir need without modifying the overall gripper design.As rapid prototyping technology can impose limitations inproducing strong and precise miniature parts due to the lowresolution and/or printing material properties of 3D printers,the gear train design utilizes a minimal number of small sizeparts. The presented worm and the worm wheels are designedto allow easy 3D printing that is achieved with high geartooth module number. So one can easily replace to their owndifferent variants depending on custom gripper requirements.One should take note that the layer resolution of used 3Dprinter is up to 0.15 mm [21]. It is not possible to create smalland miniature gears with small teeth for the gear train. Moreadvanced and sophisticated 3D printer can be used instead.C. Three Fingered Robotic GripperThe three fingered robotic gripper consists of the three 2-DOFs underactuated fingers, a base, a palm, a gear train setand an actuator. The fingers are attached in a circular waywith 120 degrees between each other. This allows the wormwheels of the fingers to be driven from a single actuator viathe actuating worm as illustrated in Fig. 4. This type of layoutof the fingers is preferable for grasping spherical or cylindricalobjects of different sizes.The CAD model of the robotic gripper and a 3D printedassembled prototype are presented in Fig. 5. The prototype ismanufactured using 3D printing technology with ABS plasticand rubber materials. ABS is a strong, durable production-grade thermoplastic used across many industries, and it is anideal material for conceptual prototyping [22]. An additionaloff-the-shelf component, a Dynamixel MX-28 servomotor,is used as an actuator for robotic gripper prototype thateliminates necessary for complex electronic circuits and en-coders for motor position control. Control of the actuatorcan be performed directly using MATLAB [23], a C/ C++programming environment and/or the Robot Operating System(ROS) [24], that provide easy and straightforward integrationof the gripper with other robotic research setups. The outputtorque of Dynamixel MX-28 servomotor at 12V is 2.5 Nm[25].The exploded assembly view of the gripper design ispresented in Fig. 6. The actuator is mounted using bolts(a) (b)Fig. 5. Three fingered gripper CAD assembly model (a) and a 3D printed prototype (b).Fig. 6. Exploded assembly view of the presented three fingered gripper.Fig. 7. Grasping of different objects using the presented three fingered gripper.on a base with a circular shape and flat bottom. The baseshape and attachment holes for mounting the gripper can befurther created in a CAD program or easily drilled dependingon robotic arm used. The worm is fixed to the actuator’soutput shaft at one side and to the palm at the other end.The worm axis is coincident with the symmetry axis of thegripper palm. The three fingers are fixed to the palm viapivots (shafts) with retaining rings and rotate about their pivotpoints. The palm and the base are connected to each othervia three aluminium standoffs fixed by bolts from both sides.The standoffs with various length can be used to accommodatealternative gear trains. The palm and the fingers have spacingsfor thin rubber cushions. These outer cushions are designed forreducing slippery between the gripper and a grasping objectand can be 3D printed using any rubber-like material [26]. Ifneeded, the cushions can be replaced by tactile sensors withminor modification of the finger and palm designs. The overallweight of the gripper with current configuration is 500 grams.III. GRASPING PERFORMANCEThe effective grasping of objects with different shapes andsizes is one of the important gripper design requirements. Thepresented underactuated gripper prototype ensures adaptivepassive grasping and full wrapping of grasping objects. Fig.7illustrates the performance of the robotic gripper in graspinga number of objects with different shapes. As shown in thefigure during the grasping of an egg shaped ball all the threegripper fingers and their phalanges move as single rigid bodiesand their passive elements are not active.On the other hand, the springs in each finger provide thesecond DOF when grasping a spherical shaped ball: as allthe first phalanges of the fingers touch the ball, the secondphalanges envelop the object and complete grasping. Thesame grasping pattern is observed in grasping short cylindricalobjects such as a bearing block.Small objects are normally grasped by second phalanges orfinger tips only, performing the spherical pinch grasp.Executing grasping of complex shape objects with strongasymmetric geometry, all the three fingers wrap of the objectwith different degree of bending depending on the objectshape. For example, in the case of grasping of a cup anda joystick objects, one finger moves as a single rigid bodywhile the others actuate the second DOF and envelop theobject. Thus, it is shown the gripper can successfully graspobjects with different shapes and sizes and can used withfurther customisation for research and educational purposes.The maximum gripping force at the tip of the robotic gripperprototype was experimentally estimated as 9 N. However, dueto non-backdriveability of the gripper actuating mechanism,the gripper fingers may resist much larger forces that theyactually exert [27].IV. CONCLUSIONThis paper presents authors’ design of an open source3D printed underactuated robotic gripper. The gripper designmodel and an experimental prototype are introduced anddiscussed in detail. It is shown that the the presented roboticgripper with one actuator meets design objectives in terms of:• a simple mechanical structure of the gripper due to usageof a four-bar linkage mechanical system for finger designand a single actuation;• a low cost due to utilizing of a single off-the-shelfactuator and 3D printing technologies;• relatively high payload comparable with similar sizetendon driven robotic end effectors.The 3D model of the gripper is created using the SolidWorksCAD software and will be available for downloading fromthe authors research lab website www.alaris.kz/. Similarto [12], the open source gripper design aims to overcomeconstraint of the rigid, closed designs of existing robotic hands.The simple three fingered robotic gripper design releasedopen-source for public use would allow researchers modifyingthe design for both research and educational purposes.Future work includes design and implementation of the pre-sented end effector prototype with embedded sensing elementssuch as tactile sensors for force feedback capabilities anda depth camera for object recognition to facilitate researchon autonomous grasping of different shape objects. The grip-per prototype will also be mounted on an industrial robot-manipulator for evaluating grasping performance in real-lifescenarios.REFERENCES[1] “Shadow Dexterous Hand Technical Specification,”2013. [Online]. Available: http://www.shadowrobot.com/wp-content/uploads/shadow dexterous hand technical specificationE1 20130101.pdf[2] S. C. Jacobsen, E. K. Iversen, D. Knutti, R. Johnson, and K. 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Available:https://www.google.com/patents/US5108140

https://www.repository.cam.ac.uk/bitstream/handle/1810/290756/Tlegenov%20et%20al%20-%20An%20Open-Source%203D%20Printed%20Underactuated%20Robotic%20Gripper.pdf?sequence=3&isAllowed=y

An open-source 3D printed underactuated robotic gripper

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Apollo (Cambridge)