Automated Desk Mounted on Wheelchair

Designers: Ethan Fricklas and Brian Alonso
Client Coordinators:
Supervising Professor: Dr. Larry N. Bohs

The client is a 17-year-old male with Duchenne Muscular Dystrophy who uses a power wheelchair. A senior in high school, the client currently uses a small lap tray that must be place in front of him for reading and writing.  To be more independent, the client desires a desk that he can move into place automatically without assistance.  This project involved modifications to the 2000 senior design project “Wheelchair Desk”. Chiefly, the previous design did not provide sufficient torque to lift the desk reliably.  In addition, numerous safety features were incorporated. The Automated Desk mounts to the power wheelchair and is actuated with two switches.  One switch rotates the desk from its storage position behind the chair, over the client’s head, and into working position in front of him.  The other switch controls a linear actuator that extends and retracts the desk to provide head clearance and suitable storage and working positions.  Several sensors provide for safe operation and help automate desk motion.  An electronic brake prevents the desk from moving when the rotational motors are not active.

Evaluation of the completed design on the client’s wheelchair indicated that all functions of the desk work as intended. Using two switches, the client can independently move the desk into a workable position, and retract it to a storage position. Nevertheless, further work is necessary to improve the overall appearance, to provide further clearance in two positions, and to build a printed circuit board for the electronics. The nature of this project is such that the client will rely on it heavily.  For this reason, future development is required to address remaining issues such as product reliability and comprehensive safety testing.

The desktop is constructed from clear ¼” polycarbonate, with a steel reinforced under-frame.  This assembly attaches to telescoping rails for extension and retraction using a 24VDC linear actuator (Dayton 2506). In the initial design, the desk and actuator were to be rotated from the storage position to the armrests by a single 12VDC, 6 RPM gearmotor (DaytonÒ 1L474).  This motor, located on the left-hand side of the wheelchair, can produce 500 lb-in of torque, which is insufficient to reliably rotate the desk.  In addition, the motor speed was deemed too fast. To solve these problems, a second motor was added to the right side of the wheelchair, and a pulse-width-modulated speed controller integrated into the electronics to adjust the motors to the desired speed.

A custom mounting plate was machined out of ¼” steel and welded together so that the brake, extra motor, and actuator (Figure 1) all mount together to the right side of the wheelchair frame.  On the left side, a ¼’ steel mounting bracket was fabricated for the single motor. All fasteners are Grade 5 bolts and nylon insert locknuts, to prevent loosening due to vibration.

A number of sensors and electronic logic circuits ensure safe operation of the desk.  Along the linear actuator, upper and lower limit magnetic reed switches sense full extension and retraction of the desk.  These switches are closed when contacted by a magnet mounted to the actuator shaft.  Five mercury tilt switches are orientated at various positions on the actuator side of the desk (Figure 2).  The combinations of signals from these switches at any given time are processed by the electronic logic circuit and dictate the position of the desk.  The logic is designed to prevent the desk from ever touching the client or his wheelchair.  For example, if the desk is rotated back from the working position, the actuator must be fully extended or the desk will not move beyond a certain limit.  Additionally, ribbon switches placed along the upper and lower edges of the desk (Figure 1) will stop all motion if the desk comes into contact with an object.

The device is controlled with two remote single-pole double-throw switches, one for motor forward and back, and one for actuator extend
and retract.  The electronics are housed in a resilient plastic enclosure.  The circuitry also contains a power conservation relay to
minimize current draw when not in use, which significantly increases the time required between charging of the wheelchair’s batteries.

The cost of materials added to the project this year was approximately $375.

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