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Based on the FEA results, a simple, single rubber-pocketed flexible gripper was designed and built. Feasibility experiments were performed to demonstrate and obtain an overallunderstanding about the capability and limitations of the gripper. It was found that objects with different shapes (cylindrical, prismaticand complex), weight (50g20kg.), and types (egg, steel hemi-spheres, wax cylinders, etc.) could be picked and placed without any loss ofcontrol of the object. The range of positioning error for two different part shapes (i.e., prismatic or cylindrical) was found to be 2090mm(translational) and 0.030.91 (rotational).r 2005 Elsevier Ltd. All rights reserved.Keywords: Gripper design; Strategies; Flexible; Selection; Robotic; Rubber1. IntroductionA gripper is an end-of-arm tooling used on robots forgrasping, holding, lifting, moving and controlling ofmaterials whenever they are not processed. Human handshave been the most common, versatile, effective anddelicate form of material handling. But, for repetitivecycles, heavy loads and under extreme environments,grippers had to be developed to substitute for humanhands. In the 1960s, after the emergence of modern robots,grippers replaced human hands on numerous occasions.Robot-gripper systems are found to be effective forrepetitive material handling functions in spite of theirinitial capital and ongoing maintenance expenses becauseof their reliability, endurance and productivity. However,the cost of grippers may be as high as 20% of a robotscost, depending on the application and part complexity 1.For manufacturing systems where flexibility is desired, thecost of a suitable gripper may even go higher since theyrequire additional controls, sensors and design needs withregards to being able to handle different parts.In the 21st century, under the influences of globalization,manufacturing companies are required to meet continu-ously changing demands in terms of product volume,variety and rapid response. flexible and reconfigurablemanufacturing systems (FMS and RMS) have emerged asa science and industrial practice to bring about solutionsfor unpredictable and frequently changing market condi-tions 2. In order to fully realize the benefits of RMS andFMS, the grippers, being one of the few direct contactswith the product at the very bottom of the manufacturingchain, must also be designed for flexibility.In the early days of robotic technology applications,most grippers were designed for dedicated tasks, and couldnot be revised for other shape, size and weight conditions.Later on, a variety of flexible gripper designs weresuggested to overcome such drawbacks. But their highcost was a barrier in addition to maintenance issues andlimitations to few materials and applications. Despite suchARTICLE IN PRESS front matter r 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.ijmachtools.2005.10.009?Corresponding author. Tel.: +7347637119; fax: +6465147590.E-mail address: mkocumich.edu (M. Koc -).drawbacks, cost effective flexible gripper designs havebeen always sought as a viable solution for agile materialhandling systems as an important element of the envisionedFMS and RMS. For example, assembly operations inmany industries make extensive use of dedicated grippersand fixtures. These are part-specific, and therefore, must bemodified or replaced when model changes are introduced.The cost of redesigning, manufacturing, and installingthese grippers and fixtures is substantial (on the order of$100 million per plant per year for automotive manufac-turers) and would be significantly reduced if a more flexiblealternative was developed.Inthispaper,followinganextensivereviewanddiscussion on different gripper types and design issues inthe first section, a flexible gripper design based on the useof compliant materials and internal pressure (i.e., inflatablerubber pockets) approach is introduced in the second part.This type of grippers conforms to the shape of an object bymeans of elastic gripping elements and pressurization withactive degrees of freedom. In the third section, the resultsof a parametric FEA study are presented to characterizethe performance of the selected configurations of theflexible gripper under different loading and part conditionsto determine the proper parameters setting and thematerial. Finally, in the fourth section, following theprototyping, feasibility tests conducted to characterize thelimits and capabilities of the flexible gripper are explained.2. Literature survey on gripper design and types2.1. Design methodology of grippersWright et al. 3 compared the grippers to the humangrasping system, and categorized the design requirementsof grippers into (a) compatibility with the robot arm andcontroller, (b) secure grasping and holding of the objects,and (c) accurate completion of the handling task. Manyindustrial examples of grippers were also described, and theguidelines for gripper design were presented. Pham et al. 1summarized the strategies for design and selection ofgrippers in different application cases. In their study, thevariables affecting the selection of a gripper were listed as:(a) component, (b) task, (c) environment, (d) robot armand control conditions. The component variables includegeometry, shape, size, weight, surface quality and tempera-ture of objects to be handled. For reconfigurable systems,they divided these components into part families accordingto their shape and size. For the task variables, type ofgripper, number of different parts, accuracy, and cyclewere considered in addition to major handling operationssuch as pick, hold, move and place. For the right gripperdesign at the right place, all aspects should be considered,and multiple validation tests should be conducted. Toreduce this exhaustive effort, Pham et al. 4 developed anexpert system for selecting robot grippers. They built ahybrid expert system that employs both rule-based andobject-oriented programming approaches.2.2. Gripper types and classification by driving forceGrippers could be also classified with respect to theirpurpose, size, load, and driving force. Typically, grippermechanisms and major features are defined by their drivingforces. The driving forces for robot grippers are usuallyelectric, pneumatic, hydraulic; or in some cases, vacuum,magneto-rheological fluid and shape memory, etc.Grippers with electric motors have been used since 1960,abreast with robot technology. Many other grippersadopted motor driven mechanisms. Basically, this type ofsystems included step motors, ball screws, encoders,sensors and controllers. As the arms approach the object,distance, force, weight and slip are detected by sensors. Atthe same time, a controller regulates the force, speed,position and motion. Friedrich et al. 5 developed sensorygripping system for variable products. They used multiplesensors to measure the grasping force, weight and slip.Mason et al. 6 and Kerr et al. 7 presented thefundamentals of grasping with multi-fingered hands. Leeet al. 8 comprehensively reviewed the field of tactilesensing. For contact and slip, Tremblay et al. 9 consideredslip detection, and Howleg et al. 10 divided slip into fourstages; pre-slip tension, slip-start, post-movement, and stopto better analyze grasping of parts.Another way of actuating the robot gripper is throughpneumatic (or hydraulic) systems. Pneumatic systems havebeen developed because of their simplicity, cleanliness andcost-effectiveness. Warnecke et al. 11 and Wright et al. 3developed a soft pneumatic gripper which can handle softmaterials such as eggs. Ottaviano et al. 12 developedgrasp-force control in two-finger grippers with pneumaticactuation. They proposed force control in a two-fingergripper with a sensing system using commercial forcesensors. A suitable model of the control scheme has beendesigned to control thegrasping force.Experimentsshowed the practical feasibility of two-finger grippers withforce controlled pneumatic actuation 12. Lane et al. 13used hydraulic force for a sub-sea robot hand. They offerednaturalpassivecompliancetocorrectforinevitablepositioning inaccuracy with simple design and minimummoving parts. The gripper finger relied on the elasticdeformation of cylindrical metal bellows with thin con-voluted walls. The convolution ensured that the assemblywas significantly stiffer in the radial direction than thelongitudinal one. Therefore longitudinal extension wasmuch greater than radial expansion when subjected tointernalhydraulicpressure.Themodularfingertipcontained a variety of sensors and interfaces. The fingertip contact zone contains both a strain gage and apiezoelectric vibration sensor. Closed-loop position controlwas used. It was driven by hydraulic pressure measuredfrom sensors within each tube 13.Grippers based on vacuum forces are designed and usedmainly for deformable and lightweight part handling.Kolluru et al. 1417, for example, used suction-basedcontrol for handling limp material without distortion,ARTICLE IN PRESSH. Choi, M. Koc - / International Journal of Machine Tools & Manufacture 46 (2006) 135013611351deformation or damage. They developed a fixed-sizedgripper and also a reconfigurable gripper system withsuction units. A sensor-based control system based on thehierarchical control architecture controlled the operationof the robotic gripper system. Fixed dimension gripperswere developed for stacking dissimilar-sized panels ofclothes. A fuzzy controller computes the needed suctionand control depending on material porosity, weight, robotspeed, and travel distance.Rong et al. 18 presented flexible fixtures based on theuse of phase-changing materials that change the phasefrom liquid to solid upon application of electricity orelectromagnetism (known as magneto-rheologicalMRfluid). Bertetto et al. 19 made a two-degree of freedomgripper actuated by shape memory alloy (SMA) with aflexure hinge in micro scale. They designed NiTi SMAgripper and the test showed to be able to reach designperformance. The SMA has some drawback when it isapplied to high working bandwidth, because of its thermalresponse 19.2.3. Control algorithm development for grippersA basic role of the gripper is grasping an object securely.For this purpose, many researchers have investigated onconstraining algorithms. Asada et al. 20 presented formclosure grasping by a reconfigurable universal gripper.They applied the form closure concept to grasping awork-piece. They analyzed kinematic conditions for totallyconstraining an object which is assumed to be piece-wisesmooth. An efficient algorithm for examining the totalconstraint was also devised. The gripper had reconfigurablefingers consisting of a multi-degree of freedom linkmechanism. It had the capability of changing the config-uration of individual fingers so that the finger tip could belocated at an appropriate position depending on the shapeof the work-piece 20. Yoshikawa et al. 21 provided aunified theoretical framework for grasping and manipula-tion by robotic grippers and hands as well as for fixingworks by fixtures and vises. They introduced the concept ofpassive closure and active closure. Passive closure wasdivided into passive form closure and passive force closure.They studied conditions for these closures to hold an objectsecurely 21. Wallack et al. 22 developed an algorithm forplanning planar grasp configuration for the modular vise.Brown et al. 23 expanded that work to produce a 3-Dfixture and gripper design tool. They described severalanalyses, and added a 2-D algorithm, 3-D grasp qualityandgeometricloadinganalyses.Theyshowedsomeapplications and potential uses of their code in an agileassembly line 23. Qian et al. 24 presented an efficientalgorithm for computing object poses in a modular fixturegripper. They introduced efficient algorithms for com-puting poses of given objects. The computer programshowed universal application, running fast and accurateresults 24.After gripping, alignment of a part for assembly ormachining purpose is needed. Kaneto et al. 25 showed apractical procedure for achieving an enveloping grasp. Itconsists of several phases including approach, lifting,grasping and coordinate phases. The experiment alsoshowed the effectiveness of the proposed grasping proce-dure. Zhang et al. 26 showed an alignment of parts duringgrasping using a standard parallel-jaw gripper. They usedfour gripper point contacts that will align the part in thevertical plane as the jaws close. The algorithm for partalignment includes toppling, accessibility, and form-closureanalysis 26.As mentioned in the gripper design section, grippersshould be compatible with the robot and controller. Thecontrol system could vary according to its purpose,constraining algorithm, environment, and condition. Itcould be an independent and subordinate system based onthe purpose. The control system could also vary based onthe sensors and actuators that are adopted. For example,Kolluru et al. 15 used vision and optical sensors for agripper system. In their case, the gripping process iscomposed of two steps. First, the gripper is coarselypositioned by means of a vision sensor. Second, finealignment is measured and controlled using an opticalARTICLE IN PRESSFig. 1. Conceptual models of flexible gripper designs with rubber pockets, movable/adjustable jaws and additional pin locators, etc; (a) three rows ofvertical rubber pockets, (b) hemispherical rubber pockets, and (c) single rubber pocket design. In all cases, note the multiple holes and pins on the upperplate.H. Choi, M. Koc - / International Journal of Machine Tools & Manufacture 46 (2006) 135013611352sensor array. For an integrated robotic gripper system,they also suggested hierarchical control architecture andfuzzy logic formulation. For manipulating payloads withmultiple robots, Sun et al. 27 described an approach tonon-model based controls of multi-robot systems.2.4. Flexible gripping strategiesIn terms of accomplishing flexible gripping tasks, fivedifferent strategies were suggested by Pham and Yeo 1 toachieve the flexibility in a cost-effective manner. The firstkind of gripper gains its flexibility from a number ofnotches on the gripping surfaces so that objects withvarious shapes can be handled. Obviously, notching ofgripper fingers is only suitable for the parts of similar sizeand weight. Another flexible gripper concept is based oninterchangeable gripper fingers. This method is moreflexible and reliable than notching method when thegripper is equipped with a finger changing apparatus anda standard set of fingers. Another strategy is to change thegripper itself. This method can be used when a singlegripper cannot handle a whole set of parts with differentsizes, geometries and weights. These grippers need variouschanging systems for locking and unlocking grippers. Thismethodismore complexandexpensivethanotherstrategies. Finally, use of multiple grippers was alsosuggested. Multiple grippers are attached on revolving orsliding mechanisms. This technique could reduce timecomparing to gripper changing technique. But, it is obviousthat the pay load increases in proportion to the numbersof grippers. Therefore, it is more suitable for lightpayload applications such as electronics and small precisemachining.Universal grippers are also suggested to be a viableway of handling a wide range of objects with differentshapes and weight 1. The universal grippers are groupedinto two categories: active and passive grippers. Passivegrippers automatically conform to the shape of the objectsby means of gripping elements which are elastic or havepassive degrees of freedom. It was reported that, withpassive gripping, it is difficult to ensure the precise positionof the gripped object with respect to the robots coordinateARTICLE IN PRESS00.511.522.533.544.550100200300400500600700Strain (%)Stress (MPa)100% modulus1.7 MPa300% modulus2.9 MPaFig. 3. Flow stress curve of neoprene rubber material used in the FEanalyses 27.Table 1Coefficient of friction of neoprene rubber with different part materialsMaterialCoefficient of static friction on rubberWax0.5970.01Aluminum0.6670.01Steel0.6970.01Fig. 4. FE model, initial conditions (DD- Initial distance btw jaws) andmeasurement (Dy-vertical displacement of part during gripping).Fig. 2. FE models of two conceptual flexible gripper designs: (a) singlerubber pocket and (b) multiple rubber pockets.H. Choi, M. Koc - / International Journal of Machine Tools & Manufacture 46 (2006) 135013611353system. In one specific case 28, the gripper surface iscovered with a membrane comprised or cubic cells filledwith compressible fluid. The cells are separated by a flexiblebut non-elastic material. After the initial contact is made,the robot applies a force to simultaneously compress andlift the part. A shear force will be introduced by thisparticular manner of applying the force. When the liftingforce and gripping force are increased, the distortion willalso increase until the friction force is large enough to liftthe part. At this point, the distortion will reach itsmaximum. After that the distortion will decrease whenthe cells are further compressed by increasing the grippingforce because of the tendency of fluid inside the cells tomaximize its volume. The magnitude of the distortion ismonitored with shear force sensors. Thus, the point wherethe distortion decreases is the point at which it is time topick up the object. Key idea of this kind of gripper is toimitate the basics of a human hand. Although thesegrippers are so amazing and can ideally handle any kindsof part, at least in short term, active grippers are expensive,unreliable, and not a good fit for industrial use.3. Design, analysis and fabrication of a flexible gripperbased on inflatable rubber pocketsIn order to complement the efforts and realize the fullbenefitsof reconfigurableand flexible manufacturingsystems, we made an attempt to design, prototype andvalidate a flexible gripper system based on pneumaticallyinflated rubber pockets concept. In this section, design andanalysis of the flexible gripper are presented in detail.The requirements for the desired flexible gripper systemcan be summarized as follows: it should be (1) able handleparts with different shapes, sizes and weights, (2) durable,(3) highly accurate and repeatable in terms of placementpositioning (less than 100mm), (4) cost-effective, and (5)easy to implement and maintain. The targeted part sizeenvelope was 60 (w)?70 (d)?90 (h)mm, and the weightrange was between 50g (i.e., wax) and 3Kg (steel).We adopted the pneumatically inflatable rubber pocketsthat are compliant to different shapes in order to handledifferent shapes, size and weight. Fig. 1 illustrates variousconceptual designs generated based on this idea. In general,for all conceptual designs, rubber pocket(s) are embeddedARTICLE IN PRESSTable 2Factors and their levels considered in the FE analyses (Ppressure, DDinitial distance btw jaws, and Wpart weight), and response (Dy)Factors and levelsResponseInitial distancebtw jaws (DD,mm)Internal pressure(P, kPa)Weight of part(W, kg)3.0150.06Vertica