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A BODY POWERED REHABILITATION ROBOT

Sean Stroud, Whitney Sample, Tariq Rahman Applied Science and Engineering Laboratories (ASEL) University of Delaware/Alfred I. duPont Institute Wilmington, Delaware USA

Abstract

This paper describes a wheelchair mounted, gravity balanced, mechanical arm whose end point is controlled and powered by a functional body part of the user, via Bowden cables. Two important design features are adaptability to different user input sites and ability to provide external power augmentation. The salient design features of such a system and the development of a prototype are described.

Background

There are currently a handful of rehabilitation robots commercially available. One of these is the Manus [1] manipulator. This robot is an electric motor powered, wheelchair mountable device which allows several modes of user control, such as joint-level and Cartesian end-position. DeVar [2] is a work station robotic system which can be used either in a structured or unstructured environment through voice control. While these rehabilitation robotic systems can be fairly easy to use, they are costly (in the $40,000 range), prone to frequent electro-mechanical failures, and provide only visual feedback to the user. The Helping Hand, Handy 1, and Magpie are three lower- cost rehabilitation robots. The Handy 1 [3] is primarily used as a feeding device through switch control of pre-programmed movements, which limits its functionality. The Helping Hand [4] is a wheelchair mounted, electric motor powered, robotic arm which is controlled by the user at the joint level, via a joystick. The Magpie [5] is a body powered and controlled, mechanical robot. It couples spoon motions (output) to the user's foot motions (input) through Bowden cables. The great advantages of this system are design simplicity, low cost, extended physiological proprioception (EPP), and force reflection. The project proposed in this paper combines the simplicity of the Magpie with the functionality of a higher degree of freedom device.

Problem Statement

The goal of this work is to develop a technologically simple, wheelchair mounted manipulator to allow a person with no or very little arm function to interact with his surroundings. It has been shown that a system which compliments visual feedback with sensory channels is superior to visual feedback alone [6]. The intended population that would benefit from such a device has physical disabilities such as spinal chord injury, multiple sclerosis, and cerebral palsy. The following four features highlight the design objectives.

Intuitive and easy to use - The inputs of the user should map in an integrated manner to the outputs of the manipulator: a proportional, three dimensional position mapping of the user's input position signal to the position of the arm's gripper is desired. A direct connection between the user interface and arm facilitates a system which is easy to use since proprioception and force reflection are naturally built into the control system.

Modular - The system will be modular in two senses. First, the arm will accept several different user inputs. These inputs depend upon the available user body motions, which to a large extent, depend upon the user's disability. For example, if the best available user input is from the head, the arm needs to accommodate whatever interface is designed for head input. Alternatively, if the best user input is from his hand, the arm needs to accommodate whatever interface is deigned for hand input. Another way the system needs to be modular is in its ability to accept power assist units. In this way, if the user can not supply sufficient power to the interface to directly cause the arm to move, power amplifier modules will be added to specific joints to assist the user in operating the arm. The issue of desiring minimal user input power naturally leads to the requirement of arm gravity compensation throughout its range of motion.

Cost - The high cost/usefulness ratio of most rehabilitation robots makes their use very limited. It is the goal of this project to maintain a simple design philosophy so costs can be kept at a minimum.

Aesthetics - The arm is designed to geometrically and functionally resemble a human arm. The interface unit will be designed as unobtrusive as possible and the cable routing will be neat.

Prototype Development

To date, two arm prototypes have been designed and constructed and two interface units have been designed, one for the head and the other for the hand. As a start for constructing a prototype, head input was chosen. A schematic diagram of the main system components, namely the arm (without a gripper) and head interface unit, both attached to a wheelchair, is shown in Figure 1.

One of the design objectives was to have an end-point controlled, mechanical linkage which resembles the human arm. To facilitate this objective, a spherical coordinate system {qA1, qA2, rA3} was chosen for the arm with an extra degree of freedom (qA4) added to kinematically couple head input to arm motion. A detailed drawing of the arm is shown in Figure 2. A direct mapping exists between the yaw (qA1 Æ qI1), pitch (qA2 Æ qI2), and roll (qA4 Æ qI4) axes of the arm and head interface, while a proportional mapping is present between the linear, horizontal motion of the user's head (corresponding to the qI3 revolute joint of the head interface) and the radial (rA3) motion of the arm. Radial motion of the arm is achieved through the use of three pulleys. As seen in Figure 2, many links and joints run alongside the main arm beams; the purpose being to constrain the arm to move in its designed coordinate frame, {qA1, qA2, rA3, qA4}. Although not shown for the purpose of clarity, Bowden cables connect the arm's coordinate frame to the head interface unit. In order to minimize friction, thereby reducing the power required by the user to operate the system, the GORE- TEXTM RideOnTM Derailleur Cable System is used for Bowden cables. The four bar linkage design of the arm's main beams is to allow gravity compensation (not shown) of the mechanism throughout its full range of vertical motion as discussed in [7]. An additional design feature is the constant vertical orientation of the distal link, thus giving the user drinking capability.

Discussion

Although the general system layout has been thought- out and addressed through the help of consumer input, several issues still need to be resolved. It is envisioned that, with head input, the user will move the gripper (attached to the arm's end) by a head interface unit which fits in the user's mouth. When the gripper approaches the vicinity of the user's face for tasks such as eating, the user can disengage from the head interface. The user will operate the gripper through movements in the tempo-mandibular joint. The following issues still need to be addressed.

Gripper - A gripper and its connection to the interface unit needs to be designed and built to allow the user to manipulate items in his environment.

Safety - Large forces applied to the arm transmit large forces and torques to the human interface and hence, the user's head. A means to avoid this occurrence needs to be devised.

Donning/Doffing- Since this system will be secured to a wheelchair, adjustability for allowing the user to easily engage/disengage from the system is required.

Adjustability - To allow recalibration of the system and to allow the robot to fit many users, variables such as link lengths and cable connections need to be adjustable.

Power assistance - Although the design will try to reduce friction as much as possible, the user may not have sufficient strength to completely body power the arm. To compensate, mechanical power amplifier modules are currently being investigated to determine if they can be designed into the system.

Locking - The user needs the ability to lock the arm in position when moving heavy loads or needs to use his input site for a purpose other than operating the arm.

Summary

This paper discussed how a mechanical arm could be used by a person with a physical disability to allow them to more fully interact with their surroundings. The advantages of the proposed system, namely, low cost and ease of use were highlighted. Presently, prototype designs for the arm and multiple input units are under development. During development, the system will undergo evaluation on such tasks as eating and page turning. Evaluation criteria will include items such as, completion time, required effort, fatigue, and subjective comments.

References

H. H. Kwee and J. J. Duimel, "The Manus Wheelchair- Borne Manipulator: System Review and First Results", Second Workshop on Medical and Healthcare Robot ics, Newcastle Upon Tyne, U.K., September, 1989.

Van der Loos, H. F. Machiel, Hammel, J., & Leifer, L. J. (1994). "Devar transfer from r&d to vocational and educational settings". In D. C. Bacon, T. Rahman, & W. S. Harwin (Eds.), Fourth International Conference on Rehabilitation Robotics (pp. 151­156).: ASEL.

Mike J. Topping, "Some Early Experience Gained in the Placement of 50 Handy 1 Robotic Aids to Eating", 1992 International Conference on Rehabilitation Robotics, Keele University, Staffordshire, England, September 15-16, 1992.

Sheredos, S. J. et al., "The Helping Hand Electro- Mechanical Arm", Proceedings of the RESNA `95 Annual Conference, Vancouver, Canada, June 9-14, 1995. pp. 493 - 495.

MAGPIE - Its Development and Evaluation, Internal Report: Oxford Orthopaedic Engineering Centre, Nuff ield Orthopaedic Centre, Headington, Oxford, England, 1991.

B. Hannaford, L. Wood, D.A. McAffee, and H. Zak, "Performance Evaluation of a Six-Axis Generalized Force Reflecting Teleoperator", IEEE Transactions on Systems, Man, & Cybernetics 21(3): 620-633, 1991.

Rahman T., Ramanathan R., Seliktar R., Harwin W., "A Simple Technique to Passively Gravity Balance Articu lated Mechanisms", Transactions of the ASME - Jour nal of Mechanical Design, vol. 117, December, 1995.

Acknowledgements

Work on this project was performed at the University of Delaware Center for Applied Science and Engineering, located at the Alfred I. duPont Institute, in Wilmington, DE, with support from the United States Department of Education, National Institute on Disability and Rehabilitation Research, grant number H133E30013, Rehabilitation Engineering Research Center on Rehabilitation Robotics and the Nemours Foundation.

Internet address: stroud@asel.udel.edu