Publications

Development Of A Prototype Retractable

Wheelchair Foot Tray

Kenneth D. Belliveau, Melissa L. Carreau

Mechanical Engineering Department

Worcester Polytechnic Institute, Worcester MA 01609

Abstract

A young woman with Dystonia is confined to a wheelchair and uses her feet to perform daily tasks of living. To accommodate such activities, she requires a work surface at her foot level. The goal of this project was to design such a work surface that is retractable and detachable from the user’s powered wheelchair and can be operated independently by the user. The team designed and built a portable foot tray system that attaches to the undercarriage of the woman's electric wheelchair. The prototype system was fastened to the wheelchair and evaluated for 7 weeks. The young woman used the device independently at home, school, and in physical and occupational therapy sessions.

Background

The device was designed for an active sixteen-year-old girl enrolled in public high school. She previously used a manual wheelchair and pulled herself around by "walking" the wheelchair with her legs. With the floor as a work surface, the woman became very adept at using her feet to carry out many daily tasks. Recently she was refitted with a powered wheelchair. Although the powered wheelchair has greatly increased her mobility, it is configured at a height such that her feet are positioned several inches above the floor, eliminating the option of using the floor as a work surface. She requested that her wheelchair be outfitted with a retractable, removable foot tray system at her foot level that she can operate independently and is available to her anywhere.

Statement Of The Problem

The young woman requires a work surface at her foot level. The problem was to design and fabricate a work surface she can operate independently, is retractable, and can be detached from and reattached to her powered wheelchair.

Rationale

Development of a portable, retractable wheelchair foot tray system will supply a work surface for the young woman to carry out daily tasks wherever she goes. Moreover, introducing a system that must be manipulated independently will increase the user’s motor skills and self-independence.

Design

The following major design goals were established. The user must be able to independently deploy the tray when her leg rests are swung away and then tuck it away when not needed. The surface area of the tray must be strong enough to support various objects and provide the user with enough room to carry out desired tasks. The device must easily detach from and reattach to the wheelchair using no tools and without irreversible modification of the wheelchair. Finally, the device must not interfere with the functionality of the wheelchair or any of its components.

Development

Figure 1. Computer Model of the Foot Tray Figure 2: Spring Design

The foot tray was modeled in Pro Engineer (Figure 1), and a physical prototype was fabricated. The foot tray consists of a flat Lexan surface attached to four pivoting legs that are attached to ball bearing slides. The configuration creates a parallelogram linkage that translates from underneath the wheelchair and rotates up to the user’s foot level. Torsion springs are mounted between the tray legs and surface. Analysis was performed to determine the torque required to rotate the system to various positions from 0 degrees (stowed position) to 90 degrees (deployed position). Torsion springs were selected that would assist in deploying the system (Figure 2). The tray features folding shutters that allow a large, flat surface to be stored in a compact area. When opened, the shutters increase the work surface area by 71%. The entire system is stored underneath the wheelchair and held in place by custom mounting brackets that feature quick release spring pins for easy removal of the unit. Four inputs are required to operate this device. The user grips a large handle, with her foot, to pull the tray forward from underneath the wheelchair. Next she pulls a small handle upward to activate the linkage, which brings the tray up to foot level. At this point the latch will automatically engage to lock the tray in its upright position. Finally, she opens the shutters to increase the tray surface area. The foot tray is a custom device designed to work specifically with the user’s Invacare® Ranger XTM Storm Series® wheelchair.

EVALUATION

Laboratory tests were carried out on all of the steps of deployment of the tray system. A force gage was used to measure the force required to perform each step. The first test was to measure the force required to "unlock" the tray and slide the system horizontally from underneath the battery box. This test was carried out at three different angles of force application - 0, 45 and 60 degrees. The average pulling force required was 9, 10 and 13 lbs. respectively. The next test was to measure the force required to activate the linkage. The average force required to rotate the linkage to the height that would allow the springs to take over was 3.44 lbs. The final test was to measure the force required to open the shutter tops. This test was carried out for two pulling positions 0 and 45 degrees. The average forces required were 3.4 and 2.7 lbs. respectively. All inputs required of the user were well within the limits of her ability.

Field testing of the tray system consisted of a 7-week period of full time use at home and school. The woman could deploy and stow the tray independently and efficiently, requiring a mere 15 seconds to deploy the tray and 10 seconds to stow it. The tray was attached to the wheelchair in the morning at school and removed at the end of the day. The tray surface was large and strong enough to support her schoolbooks, homework, schoolwork, computer keyboard and trackball. The woman’s teacher felt that the attachment and removal of the tray was rather difficult for her to accomplish in a timely manner. During the evaluation period the user found that non-ADA ramps and sidewalks consistently hindered mobility. The tray system also bumped the wheelchair lift in the woman’s van. This could be compensated for by having the woman back up ramps, but this proved to be difficult due to her medical condition.

DISCUSSION

A prototype retractable wheelchair foot tray system has been designed, fabricated and tested in the laboratory and with the intended user. Laboratory force tests were carried out to measure the ease of use of the system. Field tests were carried out for a period of 7 weeks. The user could physically deploy and stow the tray independently and efficiently. The ball bearing slides and spring-loaded linkage made the system extremely easy to operate. The steps for deployment were easy for the user to understand and reproduce. The surface area supplied by the system was large enough to hold the required items needed for her daily tasks. Installing the tray beneath the undercarriage, while not optimal, provided the space necessary to store the large work surface. The storage location decreases the ground clearance, which limits the user to travel over smooth surfaces. Also, the user’s aids reported that removing the unit from the chair was cumbersome. Nonetheless, the tray served its stated purpose and has proven to increase the user’s self-reliance and functional independence.

ACKNOWLEDGEMENTS

A portion of this work was supported by NSF BRAD Grant #BES-9410501.

We would like to thank Professors Allen H. Hoffman and Holly K. Ault, and Gary M. Rabideau of the Massachusetts Hospital School for their guidance during this project.

Kenneth D. Belliveau, Mechanical Engineering Department

Worcester Polytechnic Institute, 100 Institute Rd, Worcester, MA 01609

508-831-5983, kenbell@wpi.edu

Development of a Prototype Retractable Wheelchair Foot Tray

Kenneth D. Belliveau

Melissa L. Carreau

Alternative Text Page

Figure 1:

Computer Model of the Foot Tray: Image showing a 3D rendering of the prototype foot tray created in ProEngineer. The configuration of the parallelogram linkage is shown in the figure by the ball bearing slides, the legs and the tray top. As attached, the ball bearing slides are 3 member-telescoping slides that make up the bottom of the parallelogram. The legs are mounted with pivot rods to the slides. The tray surface is mounted to the top of the legs through mounting blocks and shoulder screws. The deployed configuration is a rectangle. At either side of the tray top a shutter equal to half the width of the tray surface is attached. A large nylon handle is looped at the front of the tray surface. Small nylon handles are attached to the underside of the shutters, at their attachment point, so that in the deployed position the handles are located underneath the shutters. The device has a large flat bottom shield attached to the bottom of each ball bearing slide that protects the device when mounted on the wheelchair. The custom mounting brackets, described as C-channels, are shown attached to the outer member of the ball bearing slides. A special latch, which automatically locks the device in place, is shown attached to the right mounting bracket, and in the deployed position, having a "hook" locked in front of the right back leg. This is a physical stop, which prevents the rotation of the linkage while engaged. At the attachment point of the tray surface with the leg, a torsion spring is mounted through the pivot axis on each corner of the device.

Figure 2:

Spring Design: Two curves are plotted on the same axes. A blue parabolic curve describes the torque required by the foot tray linkage when rotated through angles ranging from zero (stowed position) to ninety (deployed position) degrees. A yellow line having a slope of -.02 describes the torque supplied by the torsion springs mounted between the tray legs and tray top over the same range of tray positions. The value of the yellow curve is equal to and then increasingly greater than that of the blue curve as the tray moves from its stowed to its deployed position. This data represents the weight of the tray being balanced in the stowed position and then at about 32 degrees of rotation the system becomes spring loaded. This indicates that due to the torsion springs, the tray user must supply a minimal amount of force to deploy the device. This data was used to choose the proper torsion springs for the intended design.

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