Planned Approach – Design Metrics and Constraints

The specifications identified are derived from the design objectives, which have been informed by research on the largest areas of improvement for wheelchair users. Various engineering metrics will inform the specifications, which are found in Table 4. Initially, general specifications, functions, and constraints were developed, but as subteam work has developed, subteam specific metrics emerged. Some measurable quantities identified are the velocity, acceleration, weight of the device components, weight the device and wheelchair can support, cost, and maximum incline grade the wheelchair is designed to climb. Metrics that stem from this involve maximum and unassisted achievable velocity on level ground and specified incline or decline (Specifications 6, 7-12).  The unassisted achievable velocity is the lower bound of the maximum speed achievable by the device and controller with no user input for acceleration or deceleration. The range of maximum grade was derived from the range that most electric wheelchairs are rated to maneuver [14]. Maximum and unassisted achievable velocities were based on The International Organization for Standardization (ISO) standards of the maximum velocity of electric wheelchairs and the average speed of manual wheelchairs (ISO 7176) [15, 16]. Another metric includes waterproofing the housing to ensure the device will be operable in different weather phenomena  (Specification 5). All specifications are given in Table 4. Testing for waterproofing of components will take direction from ISO 7176-9, which describes climatic testing of electric wheelchairs [15]. The weight and cost of the attachment is another important metric to consider throughout the design process (Specification 1 & 2). Cost and weight specifications were based on the cost of typical wheelchair add-ons and the maximum weight that the wheelchair is rated to carry, which can be seen in Table 1 [17]. These complement design objectives stated above and are seen in conceptual drawings (Appendix B). Subsystem specifications will be further developed as prototyping and modeling continue.

 Table 4: Design Specifications and Metrics

Considerable justification goes into each specification value. Specification 1 involves the maximum cost of the device. The maximum cost of the device will not exceed $2,500 (USD). This will put the device in the same price range as the Firefly 2.5 [19] and E-Motion [20] and significantly below the SmartDrive MX2 Power Assist [21], and the SMOOV One [22], all of which are existing motor assist devices on the market  (Table 1). Similarly, Specification 2 was determined by putting the maximum added weight of the device components within the range of other motor assist devices currently on the market. As seen in Table 1, the range of added weight of the four prior art is 13.5 lbs – 35 lbs. 25 lbs is currently the specified added weight as it is in the middle of the prior art range (Specification 2).  Specification 3 is based on the average distance an American walks in a day [23]. An average American walks around 1.5 to 2 miles per day [23]. This distance was used to determine the distance the battery of the device should be able to accomplish with no user effort (Specification 3). 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 (Specification 4). The device needs to be able to withstand different weather phenomena such as snow and rain to allow the greatest accessibility and utility of the device. The International Organization for Standardization (ISO) has a standard ISO 7176-9 which specified the requirements and test methods to determine the effects of different climatic events for electric wheelchairs [15]. Standard ISO 7176-9 will be used to test the device and assess the device’s ability to withstand different weather changes (Specification 5). Specification 6 was based on the maximum grade electric wheelchairs are rated to maneuver on 8.3% to 12.5% grade [14]. Specification 8-12 used the average weight of an American man (198 lbs) as the weight of the user for each specification [24].  The maximum speed for the device on level ground, a 8.3% -12.5% grade incline, and a 8.3% -12.5% grade decline is set at 9.32 mph (Specification 7, 9 & 11). While it is unlikely that the device will reach this speed especially on an incline or level ground, this speed is set by the International Organization for Standardization (ISO) as the maximum speed for electric wheelchairs according to ISO 7176-6 [15]. The unassisted achievable speed speed of the device on level ground (Specification 8)  was determined by using the average walking speed of an adult [18]. This is used to ensure the user is at a safe speed, but not too slow to keep pace with additional foot traffic. This same unassisted achievable speed was used for the maximum grade decline (Specification 11) to keep the user within the same safe operating level but this speed can be increased or decreased based on the user’s comfort. The unassisted achievable  speed of the device on an incline was set at 2.25 mph (Specification 9), which was based on the average speed of a person who uses a manual wheelchair. 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.  

The design has additional constraints that have been developed through research and discussion. For example, the constraints of this design stem from ADA regulations [4] and ISO standards [15], which provide American and international standards for wheelchair design, which are seen as appropriate standards to follow. The ISO in particular is highly regarded as having appropriate constraints and standards across various fields [15]. Table 5 shows the constraints developed from the ISO regarding physical constraints such as maximum speed or testing guidelines and standards with static and dynamic stability as well as other standards (Constraints 1, 2, 6-11). Additionally, Table 5 shows constraints involving maximum weight derived from the rating of the allowable weight of the wheelchair used for prototyping and similar wheelchairs (Constraint 3) [17]. The electrical components have specific constraints that inform the design, which includes keeping the heat transfer of electrical components at a safe operating level (Constraint 5) [25]. The emergency stop that will be used as a fail-safe in the design also needs to meet the requirements of commercial-grade emergency stops (Constraint 4) [26]. 

Table 5: Device Constraints

The constraint values are also justified through evidence. Constraint 1 is derived from the width of the standard manual wheelchair (26 inches) and the standard width of a doorway (36 inches) [4]. Adding a width of 4 inches at maximum (Constraint 1) would make the width of the wheelchair and device 30 inches, which would still allow a wheelchair user enough space to pass through a standard doorway easily. Any added width limits the accessibility of the device. Many of the other constraints were set by the ISO’s standards for wheelchairs to ensure the device is safe and up to rigorous standards. Constraint 2 sets the maximum speed of the wheelchair at 9.32 mph, which derives from ISO 7176-6. This sets the maximum speed for an electric wheelchair at 9.32 mph [15]. Constraint 3 set the maximum weight the device and wheelchair supports at 300 lbs. This is derived from the manufacturer specifications of the weight the wheelchair that is used for testing and prototyping can support [17]. Constraint 4 considers the emergency-stop button that will be implemented in the user interface being up to the standards of other commercially used emergency stops [26]. An additional concern of the device is the heat transfer from the electrical components. For this reason, the maximum operating temperature of the electrical components and transmission is set at 158 °F. This derives from the standard temperature ratings of electronic devices which set the upper limit of temperature at 158 °F [25]. Constraint 6 ensures the device passes the static stability testing  for wheelchairs set by the ISO under the standard ISO 7176-1  [15]. Similarly, ISO 7176-2 sets the standards for determining dynamic stability of the wheelchair and is intended to be followed according to Constraint 7 [15]. Both Constraint 6 & 7 ensure that the device will not make the wheelchair unsafe while it is and is not moving. Constraint 8 is also focused on safety and derives from ISO 7176-3, which specifies the test methods and effectiveness of brakes for manual and electric wheelchairs [15]. This is important to test because the main means of aiding users on declines will be through caliper brakes, which is described in Section 5.2.1. Constraint 9 is justified by ISO 7176-10. It specifies the test methods for determining the ability of the device and wheelchair to climb and descend obstacles [15]. This standard heavily covers the intended goal of the device. Constraint 10 determines the requirements and testing method as set by ISO 7176-14  for the power and control system of electric wheelchairs [15]. This constraint will be used to confirm that the device’s control and power system meet the requirements of the standard.  ISO 7176-25  specifies the requirements and test methods for batteries and chargers used in electric wheelchairs [15]. Constraint 11 ensures that the device’s batteries meet the requirements of this standard.  Design decisions have been made to address the constraints as appropriate.

Currently, the specifications, functions, and constraints are fluid as more information develops regarding industry standards and is received from talks with stakeholders, which will lead to further design development.  In addition, interactions between subsystems will be further understood as designs develop. Overall, the current specifications and constraints are an efficient measure of what the project needs to accomplish.