Propulsion Team Specifications

The purpose of the propulsive subsystem are the following functions: to be able to propel, slow down, turn, and stop a user in a wheelchair on level ground, inclines, and declines. Below is a list of metrics and specifications used by the Propulsion Team when designing the device. These are based on both knowledge of prior art, and research of speeds of wheelchair users and people who do not regularly use wheelchairs. Note that all speeds in the table below are being calculated based on the average weight of an American man, 198 lbs [9]. This was a conservative design choice because the weight of an average American man is greater than the average weight of an American person. 

Table 4: Propulsion Team Specifications and Metrics

1.       The range of the device on a full battery with no user effort is the allowable distance the user can travel in the device with 0% effort input from the user before the battery runs out of charge. The value of 1.5 miles was chosen based on the average distance an American walks in a day, which was determined to be around 1.5 to 2 miles per day [8]. This distance was used when determining the necessary amp hours of the battery, and it was used to determine the distance the battery of the device should be able to accomplish with no user effort (Specification P3). 
2.       The range of the device on a full battery with 50% effort from the user is the allowable distance the user can travel in the device with effort input from the user 50% of the time, until the battery runs out of charge. This range of 12 miles was chosen because it is comparable and competitive to other prior art  (Table 1). To ensure that the device is competitive with other motor assist devices on the market, the range of the battery should also be able to achieve a range of 12-15 miles at 50% user effort. The 50% user effort is defined as the motor running at half the speed, thus the user has to put in effort equivalent to 50% of the full speed motor. This value was used when determining the necessary amp hours of the battery. 
3.       Specification P3 defines the grade of the incline or decline our device should be able to safely traverse. It was created based on the maximum grade of “hand-propelled wheelchair ramps,” which is 8.3%. The maximum grade for an electric wheelchair is 12.5%. At minimum, the device needs to be able to allow the user to navigate the maximum ramp built for a manual wheelchair. However, the grade at which the device can operate was maximized to help the user navigate steeper inclines and declines [13]. The maximum grade was used to determine the necessary motor horsepower. 
4.       The maximum possible speed of the device on level ground, an incline or a decline, is the maximum speed of the user when the device is in use, which includes any input from the user. Specification P4 is based on the International Organization for Standardization (ISO) Standard 7176-6, which states that the maximum speed for electric wheelchairs is 9.32 mph (15 km/hr) [12]. This has not been used in any of our current calculations; however, in accordance with the ISO standard, when testing and using the prototype, it will not push the wheelchair at a speed exceeding 9.32 mph. 
5.      The unassisted achievable speed is defined as the speed the motor is able to propel the user without any user assistance. It is the lower bound of the maximum speed achievable by the device and controller with no user input for acceleration or deceleration. On flat ground, the user should be able to travel 3.0 mph without any user input [9]. This value was chosen because it is the average walking speed of an adult [3]. This is used to ensure the user is at a safe speed, but not too slow to keep pace with additional foot traffic. This specification was taken into consideration when choosing the motor horsepower.  
6.       The unassisted achievable speed on an incline is defined as the speed the motor is able to propel the user without any user assistance on an 8.3-12.5% grade, as defined in Specification P5. It is the lower bound of the maximum speed achievable by the device and controller with no user input for acceleration or deceleration. The unassisted achievable speed of the user on an 8.3%-12.5% grade incline should be 2.25 mph. This is the speed of the user without any user input. The value was chosen based on the average speed of a person who uses a manual wheelchair [6]. While this is slower than the unassisted achievable speed for the flat ground and a decline, this is an achievable speed that would allow a wheelchair user to safely navigate an incline. This specification was taken into consideration when choosing the motor horsepower. 
7.       The unassisted achievable speed on a decline is defined as the speed the motor is able to propel the user without any user assistance on an 8.3-12.5% grade, as defined in Specification P5. This is the speed of the user without any user input. This value was determined by using the average walking speed of an adult [3]. This is used to ensure the user is at a safe speed, but not too slow to keep pace with additional foot traffic. This unassisted achievable speed was used for the maximum grade decline (Specification P3) to keep the user within the same safe operating level but this speed can be increased or decreased based on the user’s comfort. As with Specification P6 and P7, this value was used when calculating the motor horsepower. Specification P6 was more important when calculating the motor horsepower because gravity can assist in the speed of the user, while on flat ground it is solely dependent on the motor in the device. 
8.       The curb height the device can overcome is the maximum curb the device will be able to go over while going up and down the curb.  The device should be able to overcome a 6 inch curb or bump up or down in the road. The device should be able to overcome inconsistencies in the road to allow for greater accessibility and use on different surfaces and roads. A 6 in curb is the standard curb height, so the device should be able to allow the device and wheelchair to overcome this without overextending the spring or putting too much added strain on the user [7]. This maximum curb height was used when determining what spring we should use. Our spring is able to extend the correct amount to allow the device to overcome a 6 in curb.  
9.       The device should be able to stop the wheelchair and the user on at least a 5% grade decline or incline. The maximum grade of most pedestrian facilities and public access routes is limited to a 5% grade at maximum[7]. No user intervention means that the person using the wheelchair would not have to aid in stopping by gripping the wheels or using the wheelchairs built in braking mechanism. An extension spring mechanism is the current mechanism in place to allow the device to break using the motors of the add-on device. The spring ensures a normal force at the point of contact going forwards and backwards. Static analysis was done to design a spring that would allow the device to stop on a grade between 5% and 8.3% further testing would need to be done to further specify the braking capabilities.
10.      The device is able to go backwards on level ground, which means the user will be able to propel the wheelchair backwards without the use of their hands on the wheels of the wheelchair. The ability to go backwards was chosen due to safety concerns regarding the user’s hands on the wheels while also being propelled by the device, and because it increases the accessibility of the wheelchair and device. Using static calculations on an inclined plane the device is able to come to a complete stop without user input so with the addition of the spring the device should be able to go backwards on level ground as well. Going backwards on an incline or decline may also be possible but may add additional safety concerns so should be addressed further at a later point in time. 
11.     The device is capable of steering on level ground, inclines and declines, which means the user should not have to use their hands on wheels of the wheelchair to complete a turn. The ability to steer was chosen due to concern regarding the safety of the user’s hands on the wheel of the wheelchair while being propelled by our device. To achieve this, the propulsion system includes two identical housings and motors that can operate independently at different speeds and control steering of the user to some extent. The extent of the turning radius and other specifications relating to steering have not yet been determined.