For the TEK Design Project (11), our team has been tasked with developing and integrating accessibility modifications to our target’s mobility scooter.
Dwarfism is a condition caused by a genetic mutation that results in diminutive stature, restricting mobility in situations where height is a critical factor. This project aims to assist an adult woman possessing this disability by implementing a series of modifications to her accessibility scooter. The proposed solution is expected to accommodate ergonomic seating on the device, storing/removing the rain cover, and propping up the user’s feet. As the client anticipates moving out of her current residence in the foreseeable future, the goal is to provide more universal support of quality-of-life.
Priorities:
Secondary requests:
As a preliminary guide, the client was requested to provide product information for her accessibility scooter. From this, the voltage ratings for the DC motor and rechargeable battery were determined to inform measures for simplifying the device’s power supply process. Currently, it is anticipated that the voltage from the lift will need to be stepped up to that of the 24 V motor.
Beyond this, our team investigated the operation of the lifting mechanism to edify the approach to modifying the rain cover.
The lift on the back of the car uses a line that leads directly from the battery terminals to the hydraulics pump. Right off the bat, we need to check that there is a fuse or some implementation of a safety device to ensure that there’s no potential for a violent short circuit. Secondly, we will need a step-up device to jump the voltage from the battery from 12 to 24 volts because this is the voltage that the mobility charges on. The astute minded have caught that the batteries would charge while the car is off, which would kill the battery on the car. So, to prevent this, behind the voltage stepper would need to be a relay connected to a fuse that is only active while the engine is running.
For the stair/footrest design, we were able to extract a SolidWorks reference model from a slider mechanism commonly used in office drawers that we felt we would be able to integrate into a sliding stair mechanism. However, after some cost analytics and measuring, we decided to deviate from this route and choose a static stair design that clamps onto the tie-down bar located under the seat of the mobility scooter.
In this segment of the concept design, the objective was to address the difficulty of removing the rain cover from the mobility scooter.
For the first design, it is proposed to put tie-down holes in the client’s rain cover to affix it to the lift via carabiners that loop into four hooks. This will require drilling into the base to enable the insertion of eyebolts to attach said hooks.
The second design entails creating a structure that is reminiscent of a children’s play tunnel, the base of which is a solid platform that the scooter can be parked onto. Additionally, there is a vertical zipper located 90 degrees away from the opening to create an exit point.
For this portion of our design, we wanted to integrate a folding stair mechanism that allows the user to hook a set of folding stairs onto the scooter for better accessibility to reach the seat.
In the first stage (upright position), the stair is in its folded position, where it is hooked onto the scooter.
In stage two (unfolded position), the stair will be unhooked in its unfolded position, where there will be two support bars to set the stair at a specific angle. These support bars will have a static revolute at one end, and a sliding revolute at the other end.
For this concept design, our team wanted to implement an elevated foothold for the user in order to provide more support when maneuvering the mobility scooter.
On the first image, we have a desk-drawer style footrest that will slide out whenever the user wants to use the scooter. It will have a stopping and lock mechanism in order to set it in place while the footrest is in the extended position.
On the second image, we have a storage container style footrest, that doubles simply as a footrest and a storage crate.
Despite deviating from our original decision matrix, we still plan on combining some of the preceding ideas.
Our team’s choice for overall concept design is split into three sections.
For our first section, we decided to merge the footrest and folding stair designs into a more compact, efficient design. After analyzing and measuring various parts of the mobility scooter, we realized that our sliding stair design would not be practical given the scooter’s dimensions. Therefore, we are implementing a two-step setup with a revolute on the bottom member.
For our second section, we decided to do some slight modifications to an already existing part of the scooter. There is a canopy that covers the mobility scooter whenever it is resting on its lift, in order to prevent weather damage. In order to combat the struggle of removing and replacing this cover, we decided to implement a drawstring where the current elastic is and create a folded seam which will attach to a beam on the lift, in order to guide the canopy on and off the lift.
For our final section, we plan on adapting an electrical relay system in order to efficiently charge the mobility scooter while it is attached to the vehicle. In order to prevent the car battery from dying, there will be a relay implemented to prevent any power flow while the car is not running. We will wire the charger from the car’s battery in its on state to the scooter, in order to make sure it is charged whenever the scooter needs to be in use (i.e. public outing).
For each design parameter, we selected the elements that would best help our client and stay robust over time. In turn these options will increase the convenience of the mobility scooter for our client to the point she can use it anytime she wants.
The footrest combining with the stairs allows for direct and easy access to the seat, prompting natural movements to get seated.
Charging will be easy as plugging in the scooter before driving, so it can charge on the trip. The tripping hazards for our client and her neighbors is nonexistent for this solution.
The rain cover will be easy to pull over the scooter. With one side unzipped it will simply pull straight over, without excessive reaching for our client.
Steps
After initial measurements, we decided that we could efficiently integrate the footrest and folding step into one another. We knew that the most important aspect of this scooter was its accessibility, so we had to let go of some other good ideas to make room for the primary design aspect. After a post measurement, we realized that we could take advantage of a preexisting bracket that swivels with the seat. This Bracket would allow a permanent footrest to swivel with the chair. This would give our client the room to turn around the stairs to sit down and a place for them to rest their feet; two birds, one stone. The step that folds down for our client facilitates getting onto the vehicle, after which point it can be retracted with a magnetic telescoping pole that connects to a nut epoxied to its surface. The geometry of the bottom step accommodates use as a footrest in this configuration. Beyond this, grip tape is applied to the top of each step to minimize the risk of slips and falls.
Charging
It’s important to our client and us that their scooter is fully charged when they need it, so we are integrating a passive charging system into their car, which allows the car to charge the 24 Volt Scooter Battery ONLY when the engine is running. To achieve this, the following modifications are made to the existing electrical system attached to the mobility scooter lift affixed to the rear of the car. Running from the positive lead that runs directly from the battery to the lift (which has a 30 Amp fuse) would be a 12-volt relay rated for 20 Amps. This relay would be flipped by a fuse that is only active when the car is on, like a phone charging port. The relay would consume 0.96 Watts from the 90 that are available to phone charging to keep the relay flipped and the scooter battery charging. Once this relay was flipped, the 12 volts coming from the battery would be stepped up to 24 volts, then fed into a CC/CV Buck converter to supply a steady 2 A supply at 24 volts. Then, this would output to the scooter battery to be charged. And of course, a weatherproof case for these components is provided to diminish, if not eliminate, electrical hazards.
Canopy
We ended up taking a closer look at the canopy and found some differences with our initial understanding of the design. The existing elastic band was only attached to half the canopy; a buckle system is sewn onto the other half, and the lift may not be high enough for the previous idea of a pulley system. So after reconsideration, it was decided that we should remove the elastic band, expand on the buckle system to loop around the entire canopy (making up for the elastic), and sew on two zippers on one end to help take apart the canopy for removal and covering.
When conducting structural analysis, developed stresses and deflections are of salient interest to ensure safety for human use.
Toward this end, a FEA simulation was run on the entire stair assembly within Inventor. The selected material was 6061-T6 aluminum alloy, the support bar was designated as ground, and the applied force was a 300 lb. (~ 1334 N) point load at the center of the bottom step. Moreover, each revolute on the platform was configured as a pin, and the mesh quality was set to high at a moderate density.
Ultimately, the maximum deflection was 0.08695 in. at the edge of the bottom stair, which is negligible for our purposes. This suggests the system is very rigid and should be sufficiently sturdy for use by the client.
For maximum protective capabilities, weatherproof insulation for our relay circuit is obligatory. Toward this end, we designed and 3D-printed a housing that is stored on the lift. Consequently, a sustainability study was pursued to obtain insight into the effects of environmental degradation over time on the product. While no simulation can perfectly encapsulate this behavior, along with the influence of related factors like material properties and extrusion quality, a baseline estimate is still valuable as a reference point.
That being said, our assessment consisted of two separate calculations using AgentCalc’s “Polymer Aging Rate Calculator,” which considers parameters ranging from base service life to mechanical stress factor and uses a simplified exponential decay model. As controls, we assume the average UV index, temperature, and relative humidity levels in Cookeville, Tennessee.
Beyond this, worse-case criteria are given for base life and UV protection to account for peak structural imperfections. For the exterior made of thermoplastic polyurethane (TPU), this resulted in a projected life of about 2.7 years. Conversely, the interior shell, which is made of acrylonitrile styrene acrylate, has an estimate of 2.1 years, an improvement on the base life of 2 years. This can be attributed to the 70% UV protection per the lower range for absorption for unmodified TPU.
Ostensibly, these estimates are not promising. However, the intent was to provide a temporary solution until the client can relocate to a residence where she can take the scooter indoors for charging, thereby removing the need for the relay. Since the client anticipates moving within the next year, this is more than sufficient.
It’s very important that modifications to the canopy don’t ruin the integrity of its weatherproofing.
Canopy Material Selection:
Zippers- We chose a size #10 molded tooth zipper made with DuPont Delrin® acetal resins using a locking slider. Size #10 zippers are used for projects such as boat covers and tents. Delrin molded tooth zippers are UV and corrosion-resistant. The locking slider will resist unzipping when pulled perpendicular.
Buckle System- The final materials include a high-strength polyester webbing with an anodized aluminum buckle, both designed to hold up to moisture and UV.
For the fabrication of our stair/footrest design, we decided to go with a simple and lightweight aluminum alloy for the material. Originally, we wanted to go with steel, but the weight and stress analysis quickly led us to choosing a lighter material for the stairs.
10/31
We ordered some longer bolts (due to the fact that the ones we had prior were barely threaded about halfway through the nut) and some grip tape, that way the stairs were secured better and provided better user stability overall.
11/14
The static top stair prototype was fully printed in multiple sections which were then heat welded together to get our initial reference on the scooter.
11/17
The bottom stair prototype was attached to the existing configuration, and it was demonstrated that all clearances were acceptable for the stair/footrest.
11/24
Once we got the prototype model fit, we presented the corresponding drawings to the shop manager, thus beginning our machining process.
12/5
The fabricated stairs were retrieved from the shop. Upon inspecting the components, we noticed some minor clearances issues relating to improper welds. Aid was sought from shop personnel to complete cutting, smoothing, and welding our stairs, clamps, and other parts. Moreover, a layer of grip tape was applied to the top static stair.
12/9
All machining errors were resolved, and the stairs were fully assembled.
12/12
Following our complete design review, a dremel was utilized to eliminate the sharp edges on the stairs. Moreover, a counterbore hole was drilled through the bottom step to feed a rope through. Here, said rope was knotted, and a soldering iron was used to make a hole near the other end to screw it into one of the armrests. This provided a built-in support to rotate the bottom step about the pivot.
Steps
Testing for the portable stairs was conducted with respect to loading capacity, range of motion, and safety. Where the first is concerned, several of our team members, ranging from approximately 140-200 lb., individually verified that the steps could withstand their weight. In doing so, various angles for entering and exiting the vehicle were examined to gauge the holistic ergonomics of the setup. While there was a noticeable amount of deflection, this was somewhat expected given the way the stairs are attached. Aside from this, the grip tape also served the intended purpose, providing a slip-resistant base when stepping on or off the scooter.
As for the second criterion, the design intent, barring initial manufacturing errors, was fully realized for the final iteration of the stairs. Compared to its 3D-printed predecessor, the aluminum stairs are more suitable for the client in the footrest configuration, as the pivot for the folding step was raised after we noted that her feet were rolled outward slightly to meet the surface of the prototype. Additionally, the stairs retracted and extended as anticipated, avoiding interference when folded in and reaching sufficiently low to the ground when folded out.
Finally, the safety of our portable stairs was found during our complete design review to be limited by several sharp edges near the corners. This was swiftly addressed by using a dremel to round surfaces.
Charging
When testing our passive charging system, we initially experienced issues troubleshooting the cessation of current delivered by our 12 V source despite the scooter battery not being fully charged. Eventually, it was discovered that some of our test loads (e.g., resistors) and DMM equipment were faulty; upon substituting them for more functional counterparts, the circuit began working as expected. After this point, the only adjustment to this deliverable was the installation of an auxiliary 12 V wire with an in-line fuse in the client’s vehicle.
Steps
Charging
Canopy
At the start of this endeavor, our team anticipated that our efforts would be focused solely on the task outlined by our client: modifying a motorized scooter pursuant to a more ergonomic driving experience, along with ease of entry and exit. However, this quickly evolved into an eclectic mixture of sub-projects to address the broader issues impeding their quality of life. From designing a passive charging system for use while traveling to replacing the tough elastic in the client’s rain cover with zippers and a cloth drawstring, this act of service to a valued member of the Cookeville community was truly fulfilling.
That said, this project was not without its challenges, due in no small part to the extensive amount of time that was spent deciding on a stair design. Without doubt, the ample, yet reasonable, criticisms we received on our first idea of sliding platforms strongly hinted at the many, many Motion-gen Pro simulations, FEA studies, and CAD sessions that were in our future. Even so, we persevered, eventually receiving the green light for our final design and commencing the bulk of the work with a little over a month prior to the deadline. By comparison, our ancillary efforts were realized with much less difficulty: the rain cover alterations were completed in the span of a few days over Thanksgiving Break, and troubleshooting for the charging circuit was done a couple of weeks later. Although this timeline was certainly more condensed than we would have liked, it was also a testament to our effective coordination as a team.
But of course, the true silver lining was knowing that our client’s needs were met to the best of our collective ability. After all, leveraging the totality of one’s moral and technical capabilities to develop solutions that substantively impact others’ lives is the way of true engineers. We may have yet to earn that title, but this deeply rewarding experience is nonetheless paradigmatic of the professionals we hope to become, uplifting the community one project at a time.
