BACKGROUND

There have been several different implementations of 'intelligent' wheelchairs, which are able to detect obstacles in the environment and assist with navigation and obstacle avoidance (1,2,3). These wheelchairs are intended to aid wheelchair users who have difficulty driving a standard wheelchair due to visual impairments, fine motor limitations, cognitive challenges, and other impairments. These devices have focused on detecting upright obstacles, such as walls, furniture, and people. Unfortunately, many of them do not implement any way of detecting drop-offs such as curbs or descending staircases. Such drop-offs can pose a more serious risk of injury than upright obstacles. Those that do implement drop-off detection often use laser range finders (4). Laser range finders are very costly, on the order of several hundred dollars. The authors are working toward a more cost-effective smart wheelchair system that also includes the ability to detect drop-offs.

RESEARCH QUESTION

The Sharp GP2D12 (Sharp, Romeoville, IL) infrared sensor is able to detect objects at distances between 0.8 cm and 75.8 cm (5). This is consistent with the distance between the floor and the footrests of a wheelchair. Initial tests with various sensor angles indicated that the GP2D12 could discriminate between level ground and a drop-off at sensor angles between 45 and 60 degrees. Further tests were conducted to evaluate this sensor's potential for use as a drop-off detector.

METHOD AND RESULTS

To be useful, the sensor must be able to detect drop-offs far enough ahead to stop the chair. Tests indicated that a Quickie S-626 power wheelchair (Sunrise Medical, Carlsbad, CA) required 27 cm to come to a stop from a speed of 43.7 cm/s (1.43 ft/s). The GP2D12 was tested to determine how far away it could detect drop-offs. These tests were conducted outdoors on the edge of a 12.7 cm curb, and indoors on the edge of a 96.5 cm drop. In addition to two conditions of drop-off depth, this also represented two lighting conditions; which is important since infrared sensors are light sensitive. The GP2D12 was attached to the footrests of a Quickie S-626 power wheelchair in such a way that the sensor angle could be varied. The voltage response was measured when the wheelchair was on level ground. The wheelchair was then positioned at the furthest distance from the drop-off that resulted in a sensor reading at least 20% lower than the original value, and the distance from the drop-off to the wheelchair's front casters was measured. This test was repeated for two sensor angles (45o and 60o) under each lighting condition. The results, listed in Table 1, show that the detection distance is great enough to provide sufficient stopping distance.

Based on these preliminary results, the GP2D12 sensor was mounted on the power wheelchair. The GP2D12 sensor was mounted on cut wood. The wood was cut at a 45 degree angle and mounted on the footrests of the wheelchair. The sensor itself was mounted with two #6 3/4 inch screws. The result, without a user in the wheelchair, is a sensor angled 55 degrees from vertical and 18.4 cm from the ground. As this device is mounted on the footrests of a wheelchair, the weight of a user's leg and foot will affect the angle and the height slightly. We have found that a user of about 5' 7" and 150 lbs will move the sensor to about 50 degrees from vertical and about 17.15 cm from the ground, which nominally changed the voltage response of the sensor. The sensor angle may need to be readjusted should users with heavier legs attempt to utilize this mounting method.

Figure 1: Drop-off Detector mounted on an Invacare Action Arrow (Invacare, Elyria, OH)

The GP2D12 was powered by a 5 V voltage regulator (PT5101N, Texas Instruments, Dallas, TX) and monitored by a laptop computer (Gateway 2000, Poway, CA). Two Data Acquisition cards (DaqCard 1200 & 6024, National Instruments, Austin, TX) intercepted joystick signals, captured the GP2D12 voltage response, and sent output signals to the wheelchair motors. A Visual C++ (Visual C++, Microsoft, Redmond, WA) program was written to compare the GP2D12 voltage response with a given threshold and prevent the wheelchair from moving forward if the sensor's voltage response fell under the threshold. This threshold was selected to distinguish between level ground and a drop-off, based on results from previous tests.

On level ground and without a user in the wheelchair, the voltage response from the sensor was about 0.9 volts. When a user was seated in the wheelchair and had their feet in the footrests, then the voltage response increased to about 1.0v due to the change mentioned previously.

The wheelchair was placed on a 1.8 m x 2.4 m platform. With one of the investigators driving the chair, it was backed up as far as possible and he attempted to drive off the platform with full forward speed. The maximum distance traveled was 81.3 cm, and the resulting speed prior to drop-off detection was 43.7 cm/s. The platform was approximately 10 cm from the ground, and the GP2D12 sensor with a threshold of 0.6 volts was able to detect the drop-off.

To test less drastic drop-offs, 2.5 cm thick cubicle sidings were placed in front of the platform. With one cubicle siding, the effective drop-off was reduced to about 7.5 cm. During this trial, although the drop-off was detected in the sensor, it did not fall sufficiently below the threshold to stop the wheelchair. After modifying the threshold to 0.8 volts, the drop-off was detected, the threshold was met and the wheelchair stopped in time to prevent driving off the platform. To further test the drop-off detector, we placed another cubicle siding which reduced the height to about 5 cm. After running another trial, it was determined again that the drop-off was seen but did not meet the thresholds. Adjusting the threshold up to 0.9 volts allowed the system to detect the drop-off and prevent falling from the platform.