General Specifications

The general specifications include cost, weight, waterproofing, width, and International Organization for Standardization  (ISO) specifications and testing procedures. Many are based on the ISO, 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. These overlapping specifications can be seen in the table below (Table 3). 

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 minimum or maximum that would cause the device to violate a critical function or ISO standard. The target values were intended to be met, but if they were not met a redesign may not be required. However, if a constrained value was not met, a redesign should be considered and discussed because the device was not meeting a  critical constraint required for target goals to be met. In addition to numerical values, constrained values require 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. Modifications that would be made if this project continued due to failure to meet specifications is discussed in Section 6.

 Table 3: General Design Specifications and Metrics

1.     Specification G1 involves the maximum total cost of the device. The device cost shoul not have exceeded $2500 to stay within competitive pricing of prior art (Table 1).  This puts 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). Cost of manufacturing, and cost of parts were considered in determining the design of the device to ensure this specification was met.
2.        Specification G2 sets the total weight of the add-on device to a target maximum of 25 lbs. Specifically, the total weight of all the parts being added to the wheelchair should not have exceeded 25 lbs. Setting the total added weight to 25 lbs allows the user to push the wheelchair when the device is not in use, but attached to 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 (Section 3), the range of added weight of the four prior art is 13.5 lbs – 35 lbs. 25 lbs is the specified added weight as it is in the middle of the prior art range.  To try to achieve this specification, lightweight materials were used where applicable such as aluminum while also balancing Specification G1 to keep costs low.   
3.        The device should 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 would test the device and assess the device’s ability to withstand different weather changes. To aid in weatherproofing, the device’s electrical components were waterproofed. Stainless steel and aluminum were the main materials used to deter corrosion and allow the device to operate in most outdoor environments. 
4.        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 have exceeded 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 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. Any added width limits the accessibility of the device. To reduce added width of the design components of the propulsion system have been sized and positioned to fit within the original footprint of the wheelchair. The control interface is the only component that extends past the original footprint of the wheelchair and was tested to see if it meets this specification the result of which is discussed below.
5.       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 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 was carried out when the device prototype was fully assembled. Results of this test are discussed below.
6.       ISO 7176-2 sets the standards for determining dynamic stability of the wheelchair and is intended to be followed [12]. Testing using ISO 7176-2 would require a full assembly prototype of the device attached to a wheelchair. This specification was not tested because of time constraints and complexity of ISO 7176-2 procedures.
7.       Specification G7 focuses on safety and is derived 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. This specification test was planned but not performed.
8.        Specification G8 constrains the device to the standards and testing method of ISO 7176-10 which determines the obstacle-climbing ability of electrically powered wheelchairs [12]. It specifies the test methods for determining the ability of the device and wheelchair to climb and descend obstacles [12]. This standard covers the intended goal of the device. The testing as defined by ISO 7176-10 was planned but not performed during the project duration. 
9.         Specification G9 determines the requirements and testing method as set by ISO 7176-14 for the power and control system of electric wheelchairs. It also states the maximum speed of the wheelchair, 9.32 mph [12]. This constraint would be used to confirm that the device’s control and power system meet the requirements of the standard. The maximum speed would have been 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 of this specification was not planned because of the complexity of testing procedures.
10.       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]. This specification test was not completed during the project duration.