Design and feasibility tests of a flexible gripper based on inflatable rubber pockets.pdf

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 (50g20kg.), 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 2090mm (translational) and 0.030.91 (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 robots 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: (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. 1417, 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. 8590. 21 T. Yoshikawa, Passive and active closure by constraining mechanism, in: Proceedings of IEEE International Conference on Robotics and Automation, 1996, pp. 14771484. 22 A.S. Wallack, J.F. Canny, Planning for modular and hybrid fi xtures, in: Proceedings of the IEEE International Conference on Robotics and Automat