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International Journal of Machine Tools accepted 19 October 2005 Available online 5 December 2005 Abstract In this paper we present feasibility test results of a fl exible gripper design following a literature survey on various types design and control strategies of the existing grippers A fl exible gripper based on the use of compliant materials i e rubber with pneumatic infl ation was designed analyzed built and tested Parametric FE analyses were conducted to investigate the effects of process and design parameters such as rubber material pressure initial jaw displacement and friction Based on the FEA results a simple single rubber pocketed fl exible gripper was designed and built Feasibility experiments were performed to demonstrate and obtain an overall understanding about the capability and limitations of the gripper It was found that objects with different shapes cylindrical prismatic and complex weight 50g 20kg and types egg steel hemi spheres wax cylinders etc could be picked and placed without any loss of control of the object The range of positioning error for two different part shapes i e prismatic or cylindrical was found to be 20 90mm translational and 0 03 0 91 rotational r 2005 Elsevier Ltd All rights reserved Keywords Gripper design Strategies Flexible Selection Robotic Rubber 1 Introduction A gripper is an end of arm tooling used on robots for grasping holding lifting moving and controlling of materials whenever they are not processed Human hands have been the most common versatile effective and delicate form of material handling But for repetitive cycles heavy loads and under extreme environments grippers had to be developed to substitute for human hands In the 1960s after the emergence of modern robots grippers replaced human hands on numerous occasions Robot gripper systems are found to be effective for repetitive material handling functions in spite of their initial capital and ongoing maintenance expenses because of their reliability endurance and productivity However the cost of grippers may be as high as 20 of a robot s cost depending on the application and part complexity 1 For manufacturing systems where fl exibility is desired the cost of a suitable gripper may even go higher since they require additional controls sensors and design needs with regards to being able to handle different parts In the 21st century under the infl uences of globalization manufacturing companies are required to meet continu ously changing demands in terms of product volume variety and rapid response fl exible and reconfi gurable manufacturing systems FMS and RMS have emerged as a science and industrial practice to bring about solutions for unpredictable and frequently changing market condi tions 2 In order to fully realize the benefi ts of RMS and FMS the grippers being one of the few direct contacts with the product at the very bottom of the manufacturing chain must also be designed for fl exibility In the early days of robotic technology applications most grippers were designed for dedicated tasks and could not be revised for other shape size and weight conditions Later on a variety of fl exible gripper designs were suggested to overcome such drawbacks But their high cost was a barrier in addition to maintenance issues and limitations to few materials and applications Despite such ARTICLE IN PRESS 0890 6955 see 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 mkoc umich edu M Koc drawbacks cost effective fl exible gripper designs have been always sought as a viable solution for agile material handling systems as an important element of the envisioned FMS and RMS For example assembly operations in many industries make extensive use of dedicated grippers and fi xtures These are part specifi c and therefore must be modifi ed or replaced when model changes are introduced The cost of redesigning manufacturing and installing these grippers and fi xtures is substantial on the order of 100 million per plant per year for automotive manufac turers and would be signifi cantly reduced if a more fl exible alternative was developed Inthispaper followinganextensivereviewand discussion on different gripper types and design issues in the fi rst section a fl exible gripper design based on the use of compliant materials and internal pressure i e infl atable rubber pockets approach is introduced in the second part This type of grippers conforms to the shape of an object by means of elastic gripping elements and pressurization with active degrees of freedom In the third section the results of a parametric FEA study are presented to characterize the performance of the selected confi gurations of the fl exible gripper under different loading and part conditions to determine the proper parameters setting and the material Finally in the fourth section following the prototyping feasibility tests conducted to characterize the limits and capabilities of the fl exible gripper are explained 2 Literature survey on gripper design and types 2 1 Design methodology of grippers Wright et al 3 compared the grippers to the human grasping system and categorized the design requirements of grippers into a compatibility with the robot arm and controller b secure grasping and holding of the objects and c accurate completion of the handling task Many industrial examples of grippers were also described and the guidelines for gripper design were presented Pham et al 1 summarized the strategies for design and selection of grippers in different application cases In their study the variables affecting the selection of a gripper were listed as a component b task c environment d robot arm and control conditions The component variables include geometry shape size weight surface quality and tempera ture of objects to be handled For reconfi gurable systems they divided these components into part families according to their shape and size For the task variables type of gripper number of different parts accuracy and cycle were considered in addition to major handling operations such as pick hold move and place For the right gripper design at the right place all aspects should be considered and multiple validation tests should be conducted To reduce this exhaustive effort Pham et al 4 developed an expert system for selecting robot grippers They built a hybrid expert system that employs both rule based and object oriented programming approaches 2 2 Gripper types and classifi cation by driving force Grippers could be also classifi ed with respect to their purpose size load and driving force Typically gripper mechanisms and major features are defi ned by their driving forces The driving forces for robot grippers are usually electric pneumatic hydraulic or in some cases vacuum magneto rheological fl uid and shape memory etc Grippers with electric motors have been used since 1960 abreast with robot technology Many other grippers adopted motor driven mechanisms Basically this type of systems included step motors ball screws encoders sensors and controllers As the arms approach the object distance force weight and slip are detected by sensors At the same time a controller regulates the force speed position and motion Friedrich et al 5 developed sensory gripping system for variable products They used multiple sensors to measure the grasping force weight and slip Mason et al 6 and Kerr et al 7 presented the fundamentals of grasping with multi fi ngered hands Lee et al 8 comprehensively reviewed the fi eld of tactile sensing For contact and slip Tremblay et al 9 considered slip detection and Howleg et al 10 divided slip into four stages pre slip tension slip start post movement and stop to better analyze grasping of parts Another way of actuating the robot gripper is through pneumatic or hydraulic systems Pneumatic systems have been developed because of their simplicity cleanliness and cost effectiveness Warnecke et al 11 and Wright et al 3 developed a soft pneumatic gripper which can handle soft materials such as eggs Ottaviano et al 12 developed grasp force control in two fi nger grippers with pneumatic actuation They proposed force control in a two fi nger gripper with a sensing system using commercial force sensors A suitable model of the control scheme has been designed to control thegrasping force Experiments showed the practical feasibility of two fi nger grippers with force controlled pneumatic actuation 12 Lane et al 13 used hydraulic force for a sub sea robot hand They offered naturalpassivecompliancetocorrectforinevitable positioning inaccuracy with simple design and minimum moving parts The gripper fi nger relied on the elastic deformation of cylindrical metal bellows with thin con voluted walls The convolution ensured that the assembly was signifi cantly stiffer in the radial direction than the longitudinal one Therefore longitudinal extension was much greater than radial expansion when subjected to internalhydraulicpressure Themodular fi ngertip contained a variety of sensors and interfaces The fi nger tip contact zone contains both a strain gage and a piezoelectric vibration sensor Closed loop position control was used It was driven by hydraulic pressure measured from sensors within each tube 13 Grippers based on vacuum forces are designed and used mainly for deformable and lightweight part handling Kolluru et al 14 17 for example used suction based control for handling limp material without distortion ARTICLE IN PRESS H Choi M Koc International Journal of Machine Tools a three rows of vertical rubber pockets b hemispherical rubber pockets and c single rubber pocket design In all cases note the multiple holes and pins on the upper plate H Choi M Koc International Journal of Machine Tools a selected conceptual model b assembled gripper and c gripper attached to a robot H Choi M Koc International Journal of Machine Tools vol 2 Symposium on Management and Economics vol 3 Symposium on Manufacturing Systems vol 4 Manufacturing Science of Compo sites vol iiip Atlanta GA USA ASME 1988 pp 85 90 21 T Yoshikawa Passive and active closure by constraining mechanism in Proceedings of IEEE International Conference on Robotics and Automation 1996 pp 1477 1484 22 A S Wallack J F Canny Planning for modular and hybrid fi xtures in Proceedings of the IEEE Interna
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