Designers: Mark Palmeri, Brian Pullin and Ethan Fricklas
Client Coordinator: Jodi Petry, Lenox Baker Children’s Hospital, DUMC Supervising
Professor: Dr. Laurence N. Bohs
Ryan is a 16 year old male high school student who suffers from Duchenne Muscular Dystrophy. This disease is characterized by a progressive weakening and deterioration of muscles, and has limited the mobility of Ryan’s arms and legs. He is therefore restricted to a powered wheelchair during the school day. Desks and tables that are currently available to Ryan for writing and eating are not easily accessible from his wheelchair. We have designed and built a retractable desk that Ryan can move into position by himself.
The desk is mounted to the back of his chair on two telescoping rails. A gearmotor rotates the desk from its storage position behind the chair, over Ryan’s head, and into place in front of him. A linear actuator extends and retracts the telescoping rails to clear his head and to clear the ground when the desk is stored behind the chair. Control switches attached to Ryan’s armrests activate the motor and linear actuator, both of which use the wheelchair’s existing two 12VDC batteries. The desktop is made of transparent polycarbonate, and can support over 10 pounds of books. Rubberized elbow areas prevent Ryan’s arms from slipping off of the desktop surface, while the remaining smooth surface allows his hands to slide freely.
SUMMARY OF IMPACT
Ryan’s ability to transport his desk on his chair, and to move it into and out of position, greatly increases his independence. He can use the desk for doing schoolwork and reading, for using his laptop computer, and for entertainment purposes such as playing video games. Jodi Petry, Ryan’s occupational therapist at Lenox Baker Children’s Hospital (Duke University Medical Center), had the following quote regarding the project:
“This project fulfills a need that Ryan has had for about a year. He has struggled with the fact that he has had to spend more time in a wheelchair and that aspects of his school environment, such as desks, are not fully accessible to him. This wheelchair attached desk decreases his dependence on others to modify the environment for him, allowing him retrieve, or only have to ask for items to be placed on his desk. This allows him to get at school related tasks faster. He can once again move about carrying items with greater weight than what he was able to manage on his lap.”
TECHNICAL DESCRIPTION
Ryan’s desk (Figure 1) is designed to swing into place over his head. This approach was taken because of space constraints and the lack of stationary mounting points on the sides of his powered wheelchair. The storage position for the desk is behind the chair (Figure 2). When activated, it swings up and over Ryan’s head (Figure 3), and down onto the armrests in front of him (Figure 1). Retraction of the desk takes place in reverse along the same path.
A 12VDC permanent magnet gearmotor rotates the desk. It produces a maximum continuous torque of 500in-lb, and supports an overhang load of over 200lb. The motor is powered from one of the two 12V batteries on Ryan’s wheelchair. It mounts to the left seat post of the chair, using a 1/4” thick stainless steel mounting bracket. The speed of the motor is reduced by a commercial DC motor speed controller kit (Kitsrus kit #67) (Figure 4). The speed controller uses pulse-width modulation to reduce the desk’s rotational speed to approximately 2 rpm.
The motor is connected to the bracket – and the bracket to the seat post – with 1/4” bolts. Washers on both sides uniformly distribute the clamping force, and a locking washer on the nut side to prevent loosening during use of the desk. A specially designed coupler, made of 1/4” aluminum, connects the motor shaft to the telescoping rails. This coupler is attached to the inside of the end of the telescoping rail using two 1/4” hex bolts with locking washers. Two set-screws along the keyway and one set-screw on the shaft secure the coupler to the motor shaft.
The extension and retraction of the desk along the telescoping rails is accomplished using a 24VDC linear actuator, capable of a thrust load of 300lb. This actuator runs at half-speed on 12V using the second battery on Ryan’s wheelchair.
The weight of the desk is sufficient to force the rotation of the motor, even after power is removed. A brake is therefore used to prevent further rotation of the desk when the controls are off. The brake is a 24VDC holding brake from Warner Electric (model ERS-57). When it is desired to rotate the desk, a voltage is applied to the brake which releases internal springs and allows it to turn. When the voltage is removed, the springs activate and the brake locks into position.
The brake mounts to the chair using a 1/2″ thick aluminum bracket and 1/4″ bolts. An axle mounted on the chair-end of the actuator, and at a right angle to the actuator axis, connects to the center of the brake. The desk is prevented from rotating when this axle is locked into position by the brake. The axle is attached to the actuator using a 2-piece bracket that clamps around the base of the actuator shaft. 5/16” bolts with locking washers join the two halves of the bracket together. Care was taken not to over-tighten this bracket, because the shaft of the actuator is made of sheet metal, and is easily deformed by any non-symmetric radial loading. These pieces are centered underneath the motor of the linear actuator to provide symmetric weight distribution. A second 2-piece bracket connects the shaft of the actuator to one of the telescoping rails. The rail attaches to the bracket using low-profile 1/4” bolts.
The telescoping rails are high-quality commercial file cabinet rails, built to support heavy weights without binding. Ball bearings in the rails reduce resistance to sliding, which is desirable since extension and retraction by the linear actuator is only applied to one rail. Three pieces in each rail slide into one another, with the smallest inner rail guided along a series of ball-bearings. The inner rail can also be removed from the rest of the pieces by disengaging a plastic clip. This clip is essential to the quick release mechanism of the desktop. The desktop mounting pieces are attached to these inner pieces, which means that removal of both inner rails allows the entire desktop to be removed as a unit. The inner rail on the actuator side is connected to the actuator using a 1/2” x 1/4” aluminum rod and 1/4″ bolts. The attachment to the rail is made using a wing nut for quick release, and the attachment to the actuator occurs through the hole in the end of the extending shaft.
Since the motor and the actuator are controlled by separate switches, safety sensors and logic circuitry are provided to ensure that the desk is fully extended before it passes over Ryan’s head. A reed switch is mounted along the inside of the telescoping rail connected to the actuator and a magnet is placed along the actuator shaft. The mounting positions are set so that the magnet trips the reed switch as the desk reaches the fully extended position. If the desk is fully extended, then it is allowed to rotate freely. If it is not fully extended, the desk may not rotate forward past vertical (directly behind Ryan’s head), or reverse past 45? above horizontal. Three mercury tilt switches are used to determine the angle of the desk. These metal-encased switches are secured with set screws into a housing situated on the outer actuator mounting piece. A logic circuit (Figure 4) connects to these switches and the reed switch to control the operation of the gearmotor and actuator.
The desktop is constructed of clear polycarbonate for durability, and also to allow Ryan to see what is below and in front of him. The desktop is equipped with a cutout for his motion joystick (allowing him to move his chair while the desk is in place), a groove along the perimeter of the surface to prevent items such as pencils from rolling off, and rubberized elbow rests to cushion his elbows and keep them from slipping off of the desktop. A protruding lip can also be attached to the inner edge of the desktop so that books and papers will not slide off.
Since gearmotor that rotates the desk only attaches to one of the arms, the desk itself provides the link for rotation of the actuator-side arm. The polycarbonate desk itself is not strong enough to bear this twisting load, so a desk-reinforcing bracket is implemented. This bracket also helps prevent any buckling of the desk as heavier books are placed on its surface, and the elbow areas are further reinforced since they bear the greater load under Ryan’s weight. The reinforcing bracket is made of 1/2” square steel tubing, silver-soldered at all joints. The desktop surface is attached over this bracket using #4-40 machine screws that extend through the tubing to nuts on the bottom side. The inner section of each telescoping rail is attached to the bracket using #10-24 machine screws and cap nuts. 1/2” nylon spacers are included in this connection to align the rails with the width of the desk.
To ensure that the desk stops if it encounters an obstacle while rotating, 24″ ribbon switches are mounted parallel to each rail on both top and bottom sides, using aluminum sleeves mounted to the rails. If any of these four switches is contacted, a logic circuit stops the motor from rotating. The cost of the project is approximately $1000.00.