The general function of the project is to make areas that are difficult to navigate more accessible to people with limited mobility by creating a device designed to assist manual wheelchair users in ascending and descending inclines and declines. The general specifications include cost, weight, waterproofing, width, and International Organization for Standardization (ISO) specifications and testing procedures. A lot of the specifications are based on the International Organization for Standardization standards for electric and manual wheelchairs ISO 7176. The ISO is a highly reputable organization for standards and is the source that will be used to ensure the entire device is safe and accessible for users. The general specifications incorporate aspects of the design and testing that overlap among subteams. This includes things such as waterproofing all subsystems (Specification G3) as well as other overlapping specifications. These overlapping specifications can be seen in the table below (Table 1).
Table 1 and the following tables have a distinction between target minimum value, target maximum value and constrained maximum/minimum value. This is to distinguish between the target values that the device is trying to achieve (Target maximum and minimum value) and the constrained max/min that would cause the device to violate a key function or ISO standard. The target values are intended to be met, but if they are not met a serious redesign may not be required. However, if a constrained value is not met, a redesign should be considered and discussed because the device is not meeting a previously agreed constraint. In addition to numerical values, the constrained values include answers such as “yes” and “no” to show if the specification must be met. The maximize/minimize/target/constraint column found in the table shows whether the target values should be maximized, minimized, meet a target value, or if the specification is constrained and must meet a value, standard, or function.
Table 1: General Design Specifications and Metrics
| # | Metric/Specification | Target Minimum Value | Target Maximum Value | Unit | Maximize /Minimize/ Target/Constraint | Constrained Max/Min or Yes/No |
| G1 | Total cost of device | 0 | $2,500 | USD | Minimize | |
| G2 | Total added weight of the device | 0 | 25 | lbs. | Minimize | |
| G3 | Waterproofing all parts of the device | Constraint | Yes | |||
| G4 | Maximum added width | 0 | 4 | in. | Minimize | |
| G5 | Static stability full assembly testing | Costraint | Yes | |||
| G6 | Dynamic stability full assembly testing | Constraint | Yes | |||
| G7 | Brake effectiveness full assembly testing | Constraint | Yes | |||
| G8 | Obstacle climbing ability full assembly testing | Constraint | Yes | |||
| G9 | Power and control systems full assembly testing | Constraint | Yes | |||
| G10 | Batteries and charges full assembly testing | Constraint | Yes |
G1. Specification G1 involves the maximum total cost of the device. The device cost will not exceed $2500 to stay within competitive pricing of prior art (Table 1, Current State of the Art). This will put the device in the same price range as the Firefly 2.5 [5] and E-Motion [1] and significantly below the SmartDrive MX2 Power Assist [11], and the SMOOV One [9], all of which are existing motor assist devices on the market (Table 1, Current State of the Art). Cost of manufacturing, and cost of parts is always considered in determining the design of the device to ensure this specification is met.
G2. Specification G2 sets the total weight of the add-on device to a target maximum of 25 lbs. This means that the total weight of all the parts being added to the wheelchair should not exceed 25 lbs. Setting the total added weight to 25 lbs allows the user to push the wheelchair when the device is not in use and attached the device to the wheelchair with minimal added strain [2]. Similarly to Specification G1, Specification G2 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 in the Current State of the Art, 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). To achieve this specification, lightweight materials are being used where applicable such as aluminum while also balancing Specification G1 to keep costs low.
G3. This specification involves weatherproofing all parts of the device. To achieve this the device should withstand weather including snow, and rain based on ISO 7176-9 standards and testing methods [12]. 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. ISO 7176-9 specifies the requirements and test methods to determine the effects of different climatic events for electric wheelchairs [12]. Standard ISO 7176-9 will be used to test the device and assess the device’s ability to withstand different weather changes (Specification G3). To aid in weatherproofing, the device’s electrical components are being waterproofed and stainless steel and aluminum are the main materials used to deter corrosion and allow the device to operate in most outdoor environments.
G4. The added width of the device is defined as the width the device extends outward from the current width of the wheelchair. It should not exceed the specified value of 4 inches to allow the device and wheelchair to pass through an ADA regulated doorway [12]. Specification G4 is derived from the width of the standard manual wheelchair (26 inches) and the standard width of a doorway (36 inches) [12]. Adding a width of 4 inches at maximum (Specification G4) 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. To reduce added width of the design every component has been sized and positioned to fit within the original footprint of the wheelchair.
G5. Specification G5 is defined as the static stability testing method for the wheelchair with the device attached. Specification G5 ensures the device passes the static stability testing for wheelchairs set by the ISO under the standard ISO 7176-1 [12]. Both Specification G5 and G6 ensure that the device will not make the wheelchair unsafe while it is and is not moving. This testing will be carried out when the device prototype is fully assembled.
G6. This specification defines the testing method for dynamic stability as defined by ISO 7176-2 [12]. ISO 7176-2 sets the standards for determining dynamic stability of the wheelchair and is intended to be followed [12]. Testing Specification G6 using ISO 7176-2 requires a full assembly prototype of the device attached to a wheelchair. This specification will be taken into account during the testing stage of our design, which will occur after we complete our first prototype.
G7. Specification G7 defines the testing method for brake effectiveness as set by ISO-7176-3 [12]. Specification G7 focuses on safety and derives from ISO 7176-3, which specifies the test methods and effectiveness of brakes for manual and electric wheelchairs [12]. The current design allows for some electrical braking of the wheelchair from the add-on device which will be tested using the ISO 7176-3 standards once the prototype is assembled.
G8. Specification G8 constrains the device to the standards and testing method of ISO 7176-10 determining the obstacle-climbing ability of electrically powered wheelchairs [12]. Specification G8 is justified by ISO 7176-10 [12]. It specifies the test methods for determining the ability of the device and wheelchair to climb and descend obstacles [12]. This standard heavily covers the intended goal of the device. The testing as defined by ISO 7176-10 will be completed on a full assembly prototype.
G9. Specification G9 determines the requirements and testing method as set by ISO 7176-14 for the power and control system of electric wheelchairs, and it states the maximum speed of the wheelchair, 9.32 mph [12]. This constraint will be used to confirm that the device’s control and power system meet the requirements of the standard. The maximum speed will be handled by the precision of the speed being controlled by the Electro-Mechanical Integration Team. This also overlaps with Specification P4 below for the propulsion team which ensures the propulsion system does not violate ISO 7176-6 which will limit the maximum speed of an electric wheelchair. This testing will be conducted on the full assembly prototype.
G10. The wheelchair design must not violate the ISO 7176-25 requirements for batteries and chargers [12]. This requirement defines that for lead acid batteries and chargers, the rated input voltage should be no greater than 250 Vac and the nominal output voltage should be no greater than 36V. Specification G10 ensures that the device’s batteries meet the requirements of this standard. This was taken into account when choosing the battery. We have currently chosen a 48V lithium ion battery. While this goes over the nominal voltage, this is not a lead acid battery, which is the one specified in this ISO requirement. This standard also defines the testing method for batteries and battery chargers intended for use with electrically powered wheelchairs [12]. These testing methods will be used to test the full assembly prototype and ensure the batteries and chargers are safe and up to ISO standards. ISO 7176-31 is a standard under development concerning lithium based battery technology for electric wheelchairs. If this is published before the completion of the project, it may be used to further define this specification.