OHIO University Mechanical Engineering Concept Design Foot Powered Wheelchair Team B-Ballin

Transcription

1 OHIO University Mechanical Engineering Concept Design Foot Powered Wheelchair Team B-Ballin Andy Fay Evan Gilliland Sam Hallam Haowen Huo Trace Lydick Kyle Sullivan 11/11/2011

2 1.0 Concept Generation 1.1 Problem Statement for Concept Generation 1.2 The problem that needs to be solved by the final product is multi-faceted. Our client, Ashley Stump, suffers from Cerebral Palsy which restricts the movement of the left side of her body. This condition makes full participation in her sports league very difficult as she has very limited maneuverability. For the needs of our client to be fully met, modifications to a wheelchair must be designed and created such that it can be powered and controlled exclusively with the right side of the client s body. This needs to be done with as little assistance from the arms as possible. The design needs to be such that it is easy to learn and use, while maintaining a suitable comfort level. The design should also allow the product to be easily maintained, using as many common parts as possible and not overly complex in function. Lightweight materials are also to be used to allow for easy transportation in and out of the client s vehicle. Finally, the product will need to be within the maximum dimensions allowed for wheelchairs by the ADA, and will follow the requirements set forth by ANSI. 1.2 Patent Search Patents were initially searched out in a way that encompassed several aspects of transportation. Focus was placed on anticipated individual sub-systems so that function, not final product was examined. An example of this can be seen with regard to the rear differential considered as a possible form of propulsion. After an exhaustive search was made with respect to wheelchairs with live axles, our attention focused toward other products of similar dimension that incorporated the technology. PROPULSION Rear Differential One of the first ideas that came to mind when considering wheeled propulsion was the type of technology already found in today s automobiles and ATV s. The rear differential fit this application because it allows for rotation to be translated to wheels, as well as simultaneously allowing for turning because of its ability for independent axle movement. Some of the patents found under this topic are the following:

3 Figure 1.1 Recreational Vehicle Locking Differential United States Patent NO. 7,018,317 B2

4 Figure 1.2 United States Patent No Treadle When looking for systems to power the wheelchair, a treadle was another idea that came to mind. The best treadle design encountered was Patent No This treadle design is such that very little foot movement is required to power the treadle. Little foot movement means that Ashley would not have her ankle cramp up after extended use of the wheelchair. Powered instead by the entire leg in small oscillating motions, the treadle can be used for an extended period of time

5 without worry of fatigue. By integrating this design into the wheelchair modification, Ashley would be able to use the product for a much longer time than if designed with an ankle powered variant of the treadle. Figure 1.3 Sewing Machine Treadle Patent No For transferring the pedaling motion of a treadle into useable motion, Patent No was also seen as a useful design. The patent is a child s toy treadle sewing machine that uses basic belt ratios to transfer the power. Since there is not a lot of design space under the wheelchair, a basic design will be utilized. By using a basic design less space is used while also keeping the product simplistic which in turn makes for a more maintainable product.

6 Figure 1.4 Toy Sewing Machine Treadle Patent No Nordic Track Another form of propulsion that was considered was a cable and pulley activated flywheel, similar to the device found in a NordicTrack Pro Skier. This flywheel device has application to our product because of its characteristics for easy rotation and available power application on both front strokes and back strokes; seen in the below patent.

7 Figure 1.5 NordicTrack Flywheel Patent number Figure 1.6 shows the gears hand crank wheelchair. A set of gears were attached to the wheel and the arm of the wheelchair, designed to be driven by hand. The drive gear is connected to the wheel sprocket via a third floating gear. This allows for consistent direction of hand motion to chair motion.

8 Figure 1.6. Hand crank wheelchair mechanism Patent No The ratchet-device, Figure 1.7, is allows continuous linear or rotary motion in only one direction, which prevent motion in the opposite direction. A ratchet contains a round gear or linear rack with teeth, and a pivoting spring-loaded finger called a pawl that engages the teeth. This design can be applied to a hand crank system, which can power the wheelchair by reciprocally pushing and pulling on the handle.

9 Figure 1.7. Ratchet-device and working theory STEERING Steering Casters Steering casters were looked at as a method of maneuvering the wheelchair. The steering casters fit the need of maneuverability for the wheelchair. One idea of using the steering casters is to attach a lever which will allow for easy and direct control of the wheelchair. Patents that exemplify similar ideas are listed below.

10 Figure 1.8 Caster Steering on a hospital bed attachment Patent No. US 6,874,800 B2 Figure 1.9 Swivel controls of casters allowing for external steering Patent No. US 6,874,800 B2

11 Figure 1.10 Caster steering system similar to shopping cart Patent No. US 6,302,421 B1 Figure 1.11 Frame shown with caster steering Patent No. US 6,321,878 B1

12 Bicycle Braking Device When searching for methods of stopping the wheelchair a simple and effective device is that used on bicycles. One patent for this option is patent application number US 2004/ A1 which uses small pads and allows for adjustment when the pads start to wear. The pads can be adjusted without tools which fits part of our requirements for simplicity. 1.3 Concept Generation Figure 1.12 Application No. 10/196,031 For the concept generation, the group implemented several different tactics before devising a way that actually worked to create a design plan. These concepts researched needed to take into consideration all of the needs gathered from the customer found in the revised needs statement. Initially, each team member took it upon himself to research existing modes of propulsion that were applicable to wheelchairs. After the findings, each team member presented the material researched at a weekly group discussion. Both pros and cons of each idea were discussed as a group, and the feasibility of these ideas was then determined. Though the process was effective, the ideas were ineffective, as most methods of wheelchair propulsion that already exist require almost exclusive use of the hands. The search was then broadened to all powering methods of various types of mechanisms. This resulted in the findings of the treadle, bicycle-powered golf cart, hand-crank wheelchair ratchet, foot-pedal scooter, etc. Though these designs are not directly related to wheelchairs, they were all applications that inhibited motion using mechanical systems. The problem with all of these ideas was that each one alone could not cover all of the very specific design specifications that needed to be covered. It was also discovered that propulsion was not the only sub-system that needed to be covered. The braking and steering sub-systems were very much a part of this design

13 as well, and further research was done. Closer observation at the revised needs statement was done so that efficient sub-systems could be found to solve the problem. Group brainstorming sessions were utilized and all types of l ideas, traditional and radical, were thrown discussed. This process was very similar to the IDEO process discussed during class. Many ideas were not applicable, but the discussions lead to thinking of products on the market that could be a suitable solution. This resulted in the finding of various sub-systems dealing with methods of propulsion, maneuverability and braking. Within the findings of each sub-system, one original concept covering all the bases of the design specifications could be found. Posted below in Figure 1.14 is the list of ideas that were thought of during the brainstorming session: Figure 1.14

14 The ideas generated in the brainstorming sessions, were then explored and researched further by the team, with the stipulation that human power be the propulsion force. This enabled the design search to be simplified, with the results of the search and subsequent patents found in the above section Concept Screening When the customer was asked as to what modifications were wanted, no specifics as to how the wheelchair was to be modified were given. Any concepts that would improve the client s situation were acceptable. This allowed for an open approach to modify the wheelchair, as long as it would be powered by the client s right leg. The product has no real market outside of this specific application so it can be customized specifically for the client s needs. All concepts considered in Figure 1.14 were examined with the customer s needs in mind. These needs led to the elimination of a few subsystems. An example of which was the electronic steering foot pedal. It was clear that the foot pedal steering control was not in the spirit of maintaining a sense of physical participation on the part of the client. This was ruled out early on for these reasons as well as being a very expensive piece of equipment. 2.2 Data and Calculations for Feasibility and Effectiveness Analysis From our initial specifications and pair wise comparison chart (Table 2.1) it was determined that the most important specifications were maneuverability and safety. For maneuverability the measureable aspects would be the turning radius and the turning speed. For safety the measurable aspects would be the overall speed of the wheelchair, the stopping distance, and the safety factor.. In general all of the design options fit the specifications, so in order to narrow the search for the perfect system, feasibility and effectiveness were also considered. Propulsion A worst case scenario for the estimation of force applied to move a wheelchair is 67N to maintain a velocity of 1m/s. The rolling resistance could be found from this by applying basic physics equations. These equations include: F = m a (1) F f = μ N (2) M = F d (3)

15 The basic free body diagram for the wheelchair is as follows: Figure 2.1 Knowing that the force to push the chair forward shouldn t be any more than 67N, it can be reasoned that the opposing force is the friction involved in chair. The mass of the client and the mass of the chair total to be 130 lbs, or about 59 kg. This translates to a normal force, calculated from Equation (1), of 579N. Equal and opposite reactions tell us that the resistive force is equal to the applied force. Therefore, F friction = 67N and application of Equation (2) returns an overall coefficient of friction of u = 0.12.

16 The base numbers calculated here were then used to evaluate the effectiveness of each propulsion subsystem. Because full analysis of the all subsystems has yet to be completed, presentation of completed calculations will be withheld until final submission. 2.3 Concept Development, Scoring and Selection To then narrow the number of subsystems for steering and propulsion, four considerations were in mind: difficulty of application, cost, space, and learning curve. The different ideas for the subsystems were then ranked against one another to narrow the number of ideas down. By narrowing the pool of subsystems to implement into the project, the best ideas could be brought together shown in Tables 2.1 and 2.2. The subsystems were ranked from best to worst on a scale of 1 to 5, with 1 being the best. Table 1.1 Propulsion Treadle Hand Crank Nordic Track Differential Team votes Total Table 2.2 Front steering casters/braking Maneuvering Differential system Individual Brakes Team votes Total The propulsion subsystems that were ranked against one another included the modified hand crank, treadle, Nordic Track, and differential. For the steering subsystems, the design ideas ranked against one another were the front steering casters with braking system, individual brakes, and differential. The differential was looked at as being used for both propulsion and steering due to how it would be integrated into the design, being able to both power and steer the wheelchair. Using the aforementioned considerations without any calculations to confirm the results, the selected subsystems were the modified hand crank and steering with braking system. It was thought that the differential would require a complete overhaul of the wheelchair by adding axels to the chair. There was also limited space with which to work, not allowing additions to be easily made. There would have been a significant weight addition to the wheelchair, thus decreasing its portability. The treadle was discarded due to the lack of reverse

17 and that there could potentially be safety issues with the pedal flapping around as the wheelchair is powered. The Nordic Track idea was decided to require a lot of space which would make the wheelchair unwieldy. There was also an injury risk to the knee as the required motion while sitting is very uncomfortable. The modified hand crank idea was chosen due to its flexibility with an option of reverse being plausible due to the ratcheting of the cranks. The hand cranks could also be easily modified to be used by feet. This idea would cut down on extreme changes to the wheelchair thus reducing cost and time. The steering casters allow for the control of the wheelchair to be similar to that of a car; the braking system allows for a complete stop of the wheelchair. The individual brakes were not chosen because by relying completely on braking power for turning; the brakes would need to be replaced regularly, increasing the cost of maintenance. 3.0 Conclusion The next step for the project will be to determine the dependent calculations of the design choices. This will be done during the winter break when possession of the donor chairs is gained. These chairs will provide us with the dimensions of the chair that will actually be modified, such that further assumptions will not have to be made. Once the dependent calculations for the design choices have been completed, the decision of subsystems to be used, propulsion, maneuvering, and braking, will be determined.