Tratamiento de rescate

Once all four subsystems were created and assembled, the entire system was tested to measure the following specifications: terrain and incline capability, maximum speed on asphalt and turf, range, time required to attach the device to the wheelchair, and weight. Significant results are included below, and raw data from testing can be found in Appendix H.

Terrain and Incline

Off-road capability was an important goal for the device, so it was tested on a variety of surfaces, and, as recorded in Table 8, its performance was qualitatively rated as “incapable,” “poor,” “satisfactory,” or “very good.”

Table 8: Performance on various terrain rated as either: Incapable, Poor, Satisfactory, or Very Good. Surface Performance

Asphalt/Concrete Very Good Cobblestone Very Good Grass Very Good Artificial turf Very Good Hard-packed dirt Very Good Gravel Satisfactory 8% Incline Very Good 23% Incline Very Good Speed Bump Very Good

The attachment was able to propel the wheelchair with “Very Good” performance over every surface tested except for gravel, over which the drive wheel experienced occasional slipping but was overall “Satisfactory.” This project only sought to design for traveling up a grade of 8%, which is a standard maximum incline for wheelchair ramps, but the device had no difficulty propelling the wheelchair and occupant up a 23% incline (the steepest slope that could be found

for testing). The terrain and incline testing showed that changes made to the design of the second prototype succeeded in addressing the insufficient torque and traction of the first prototype.

Maximum Speed

The maximum speed of the wheelchair when propelled by the device was determined by measuring the time required to travel 17.5 ft. on a flat surface. Starting and ending points were marked, and the operator started the wheelchair far enough behind the initial mark to provide sufficient time to accelerate to maximum speed. This test was repeated three times on two different surfaces, asphalt and artificial turf, and the averaged results are shown in Table 9 with the target design specifications and the marginally acceptable values.

Table 9: Average maximum speed on asphalt and turf compared to target and min/max acceptable values. Surface Min (mph) Target (mph) Max (mph) Measured (mph) Asphalt 4.5 5 5.5 _3.5 Artificial Turf 3 4 5.5 _3.3

The device did not meet the target specifications for speed on either asphalt or turf, although the speed on turf was within the acceptable range. Due to the limited time available for redesigning and fabricating the second prototype, we had to use sprockets that were readily accessible even though they did not provide the ideal gear ratio to achieve the target speed. However, after testing the device we found that 3.5 mph was a sufficient top speed, and that going any faster would likely be disconcerting or even unsafe for the occupant. Thus, although the design specifications were not met, the final performance was considered satisfactory.

Range

The range was tested by completely charging the battery and then operating the device

continuously until the battery’s built-in monitoring system cut power once the lowest acceptable voltage of 33V was reached. A field of artificial turf was chosen for the range test because it

approximately every 5 minutes throughout the test (shorter intervals at the beginning and end of the test) in order to generate a battery discharge graph, which can be seen in Figure 45.

Figure 45: Battery voltage during range test on artificial turf, starting at 40.6V and ending at 33V after 161 minutes

The battery voltage began at 40.6V and ended at 33V after 161 minutes, and the voltage drop was similar to the discharge behavior of typical lithium iron phosphate batteries, which

experience a very slow linear decrease for the majority of discharge except for a rapid drop-off at the very beginning and end. Given a measured speed of 3.3 mph on artificial turf, the time

traveled (161 minutes) was used to calculate an approximate range of 8.9 miles on turf (the range would likely be slightly greater on asphalt due to its smaller rolling resistance). This is well above the target range of 5 miles, suggesting a future prototype could potentially utilize a smaller battery that would be less expensive and also lighter.

Time to Attach Device

securing the rod with pins, placing the battery in the sling hanging from the handlebars, and securing several electrical connections. We originally intended to conduct experiments with volunteers attaching the device while both seated in the wheelchair and standing behind it, but the second prototype proved to be too heavy and unwieldy for most people to attach to the chair from a seated position, so data was only collected for standing tests. On average participants took 42.7 seconds to attach the device, which was slightly above the target of 30 seconds, but well under the maximum acceptable time of 60 seconds. While by no means completely unsuccessful with regards to usability, the power-assist device would need significant improvement in this category in order to compete on the market.

Weight

The user interface was designed to be semi-permanently mounted to the user’s wheelchair, so only the trailing arm assembly and battery would need to be taken on and off. The trailing arm weighed 26 pounds, nearly meeting our target weight of 25 pounds and well under the maximum acceptable weight of 35 pounds. Despite meeting the target specification, the trailing arm was still difficult for users to handle when seated in the chair, so for future prototypes the ideal weight would likely be set at a lower value, perhaps around 15 pounds. Although the battery was an additional 14 pounds, it was a separate component in the second prototype and didn’t add to the weight of the trailing arm, which made the attachment process easier.