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The SDM Hand as a Prosthetic Terminal Device

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The SDM Hand as a Prosthetic Terminal Device: A Feasibility Study Aaron M. Dollar, Member, IEEE, and Robert D. Howe, Member, IEEE Abstract— In this paper we discuss the potential of applying our concept for a robotic hand fabricated via Shape Deposition Manufacturing (SDM) as a prosthetic terminal device. Experimental results with the hand have shown a level of robustness, adaptability, and other performance properties as yet unseen in a robotic hand. Besides reliable performance, the hand is durable, is produced using a molding process that allows both for inexpensive mass production as well as a realistic appearance without the need for a cosmetic glove, and incorporates a simple design that requires only a single actuator for the eight active degrees of freedom. All of these factors make it a good candidate as a basis for either a body-powered or externally-powered prosthetic terminal device that is realistic, functional, robust, and inexpensive. I. INTRODUCTION VER 10,000 major amputations of the upper extremities occur every year in the United States alone [1]. However, while technology has improved drastically, very few advances in prosthetic devices have been adopted by the amputee community in the last century. Most patients still choose hooks or other simple mechanisms as terminal devices for functionality during their every day lives, switching to a less functional, more cosmetic terminal device for social activities [1-4]. Much of the research in prosthetic hands in the past few decades has focused on externally powered, multifunctional anthropomorphic devices (e.g. [5-12]). In order to capture the dominant performance characteristics of the human hand, some of these devices can incorporate 17 or more actuators, placed in the body of the forearm and sometimes upper arm of the prosthesis. For this reason they are therefore not purely terminal devices and are limited in application to amputees requiring the respective level of prosthesis. Additionally, current limitations with interfacing these devices and their operator do not permit nearly the same number of independent control signals required to operate them. A few of these devices have been designed with a smaller number of actuators that are in fact modular as a terminal device and can therefore be used by the largest number of individuals (e.g. [10]). However, size, weight, and power requirements as well as current limitations with both myoelectric and body-powered methods of actuation limit the number of degrees of actuation that can reasonably be incorporated into a prosthesis. Indeed, current state-of-the-art commercial products are limited to 1 or 2 degrees of actuation. This work was supported in part by the Office of Naval Research grant number N00014-98-1-0669. A. M. Dollar is with the Harvard/MIT Division of Health Sciences and Technology and the MIT Media Lab, Massachusetts Institute of Technology, Cambridge, MA 02139 USA (phone: 617-253-2941; fax: 617-253-8542; e-mail: [email protected]). R. D. Howe is with the Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 USA (e-mail: [email protected]). For these reasons and others, there will always be a desire to maximize functionality of the terminal device while keeping the degrees of actuation small. In this paper, we describe the design of a robot hand that requires only a single actuator yet is able to reliably grasp target objects spanning a wide range of size, shape, and mass. The most interesting properties of the current implementation of the robot hand include: • Molded from polyurethanes that are lightweight and tough • Robust to impacts, other large loads • Hand is compliant when unactuated, rigid when actuated • Fingers are actuated via tendon cables • Only a single actuator is needed • Performs reliably without the need for sensory feedback • Passively adaptable to a wide range of target objects Besides good performance, the above properties lead to the following desirable attributes if constructed as a prosthetic terminal device: • Cable actuation fits naturally with body-powered methods of actuation • Single actuator can easily be incorporated into the palm of the hand for an externally-powered modular device • Fabrication via polymer molding allows for realistic appearance and no need for a cosmetic glove • Molding allows for easy, inexpensive mass production • Reduced number of actuators while retaining performance decreases the mass of the hand We begin this paper by describing the design of our four-fingered robot hand (Fig. 1) built using Shape Deposition Manufacturing (SDM) [13,14]. This process uses polymeric materials to simultaneously create the rigid links and O proceedings of the 2007 IEEE Iternational Conference on Rehabilitation Robotics,Noordwijk, Netherlands, June 12-15, 2007.Fig. 2. Details of finger parts and placement of components. Note that the sensors shown are not currently in use. compliant joints of the gripper, with embedded actuation components. In addition to simplifying the construction process, the result is an extremely robust gripper, fully functional after impacts and other large loads due to unintended contact. We then describe the performance of the hand, including the ability to grasp a wide range of common target objects, and end with a discussion of modifications that might be made in order to realize the hand as an effective prosthetic terminal device. Fig. 1. SDM Hand II. SDM HAND DESIGN In this section we describe the architecture of our robot hand as it was designed for use as a robot end-effector. Note that some of the design choices such as the non-anthropomorphic finger placement might need to be modified to make it appropriate for use as a prosthetic terminal device. These modifications, addressed in section IV, are not expected to adversely affect the performance of the hand. As mentioned previously, this paper presents evidence of the feasibility of incorporating the main features of our hand concept as a prosthetic terminal device, and not the specific design of such a mechanism. A. Finger design To provide both adapatability and robustness, the fingers and base of our hand, featuring passively compliant joints, were fabricated using polymer-based Shape Deposition Manufacturing (SDM) [13,14] (Fig. 1). SDM is an emerging layered manufacturing technique with which the rigid links and


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