Wheelchair Control Simulator

Designers: Drew Narayan
Client Coordinators: Robbin A. Newton, Lenox Baker Children’s Hospital
Supervising Professor: Dr. Larry N. Bohs

A wheelchair control simulator has been designed for Lenox Baker Children’s Hospital. This device allows children to practice using the controls typical to a motorized wheelchair while a therapist monitors their progress. The wheelchair control simulator comprises a joystick, a processing and display unit, a video monitor, and a custom motorized car with a CCD camera mounted to the front. As a child drives the car through a maze, sensors on the car detect when the car hits a wall. The number of hits, the time moving, and the elapsed time are combined into a running score that motivates the child and provides feedback to the therapist. A four-section LED display indicates the time moving, total time, overall score, and number of hits to each of the four sensors. The monitor shows the camera’s view to provide the child with the sense that they are sitting in the car. This device can be used by therapists to assess whether a child will be able to safely operate a motorized wheelchair, while at the same time providing an entertaining game for the child.

How this project helped
Each year about 50 children at Lenox Baker Children’s Hospital could benefit from motorized wheelchairs. Because such wheelchairs are expensive and potentially dangerous, therapists must assess whether a child could benefit from a motorized wheelchair, and if so whether they are ready to try driving one. The wheelchair control simulator allows children to practice “driving” while the therapist monitors their driving skills with a running score. Experience to date with the simulator indicates that two modifications could greatly improve its usefulness. First, using radio control rather than a wire tether would improve the car’s maneuverability, since the car has a limited range and sometimes does not turn properly. Second, the switches that sense hits to walls could be more sensitive and reliable. These improvements may be addressed in a future design.

The wheelchair control simulator has five main components: a joystick, a processing and display unit, a motorized car, a monitor, and a maze, as shown in Figure 1. The joystick, a modified Radio Shack Avenger 700, controls the movement of the motorized car. The aviator style handle of the joystick is replaced with a 1.75inch diameter ball to simulate a standard wheelchair joystick, which was not used because of cost. The joystick is attached to the processing and display unit using a 15-pin D-subminiature connector.

The core of the processing and display unit (Figure 2) is the Z180 microprocessor on the SmartCore Z1 Board (Zworld Engineering, Davis, CA). The microprocessor program was developed in Dynamic C using the SmartCore Development Board (Zworld). The microprocessor receives input from touch sensors and a display mode selector switch via a 74LS373 latch. Output from the processing and display unit appears on a 4-section LED display, each section comprising two 7-segment digits. This unit serves several functions: to control the movement of the motorized car, to process information from the touch sensors, to control the display, and to provide a power supply. The unit converts the nonlinear output from the joystick into an amplified, linear input for the motors of the car. This is accomplished using a dual analog to digital converter (ADC0844), low pass digital filters, a dual digital to analog converter (DAC0890), and two push-pull current amplifiers. To process information from the touch sensors, a software loop on the SmartCore reads data continuously from the input register (74LS373). The microprocessor counts a hit only if it registers on five consecutive loops. This prevents counting a hit more than once.

The microprocessor also continuously monitors the signals from a three position rotary switch and displays either: “Time Moving and Total Time”, “Hits” (the number of hits on each of the four touch sensors), or “Overall Score and Total Time”. A real time clock on the Z1 board keeps track of the total elapsed time, and the following equation is used to compute the score:

Overall Score = (1000- 10 * (total hits)) * (time moving)/(total time).

The motorized car is constructed from Legos with all pieces secured with epoxy to provide durability. Separate motors drive each of the rear wheels of the car so that a tight turning radius is achieved. The speed and direction of each motor is controlled by altering the amount and polarity of the voltage and current applied to the motor, thereby allowing the car to move in any direction. Each motor is geared down three times to reduce speed and increase torque. Touch sensors are attached to each of the four corners of the car and a CCD camera is bolted in the interior of the car. The car is equipped with bumpers to protect the camera, and a wire tether connects the car to the processing and display unit. The tether contains a total of nine wires: one for each of the ground sensors, two for the CCD camera, one for each of the two motors, and a common ground.

A standard five-inch black and white television monitor attaches to the processor to display the output of the CCD camera. Alternatively, the processor and display unit can be attached to any television, using the Channel 3 and 4 modulator that is included in the unit. The maze is made of a 4″x4″ pegboard base with 1″x4″ wooden blocks, each with two protruding dowels on the bottom to attach to the base. The wooden blocks may be placed in any desired orientation by the therapist depending on the child’s skill level.

The total cost of the wheelchair control simulator was approximately $830.00.

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