DESIGN REVIEW #1
DESIGN & PRODUCTION OF AN ALL – TERRAIN WHEELCHAIR
AGGRESSIVE DESIGNS
Andrew Baigrie
Connor Henley
Jeff Lance
Miguel Tovar
University of Michigan
College of Engineering
ME450Capstone Design Course
Fall Term 2006
Executive Summary
DESIGN & PRODUCTION OF AN ALL-TERRAIN WHEELCHAIR
Andrew Baigrie•Connor Henley• Jeff Lance• Miguel Tovar
Of the 1.5 million Americans who use manual wheel chairs, roughly 600,000 are working age adults [1]. Currently, manual wheelchairs are not capable of any form of off-road travel without significant road harshness and external assistance. This paper presents the engineering scope of work necessary for a manual wheel chair redesign. The purpose of this report is present concept generations, selection methods and our Alpha Design that bests meet the customer and technical specifications.
We began by benchmarking current manual wheelchairs in order to determine what attributes customers seek from a wheelchair. After developing an understanding of our product and redesign goals, we focused on evaluating our redesigncomponents. A Quality Function Deployment chart was used to quantify the merits of our potential redesigns. From our QFD chart, we determined the characteristics that could use the greatest improvement were the suspension, a 5th wheel, and the use of a recumbent seat.
After determining what characteristics could use the greatest improvement, we began to generate conceptual designs. Our product was decomposed into the two main design drivers: the suspension and the frame. For each driver five different concepts were generated and evaluated using Pugh Concept Selection matrixes. For the suspension, springs are the optimal dampening device for our application. For the frame, a single suspension design with a curved frame was selected.
The frame will be built using 1 inch outer diameter aluminum with a 1/8 inch wall thickness. The design will incorporate a detachable fifth wheel on the back of the chair, and two shocks design to dampen the vibrations from the two drive wheels. The seat will also be tilted back at a 5 degree angle to create a more comfortable chair.
The plan we developed for our project is an aggressive one. We plan on having a working prototype to test by the end of November. Construction is labor intensive; and we estimatethree weeks is required for manufacturing the chair. This drives completion and verification ofthe design concept by the end of October.
TABLE OF CONTENTS
Introduction ...... 4
I. Engineering Specifications ...... 5
Customer Requirements ...... 5
Engineering Characteristics...... 6
Quality Functional Deployment ...... 7
II. Concept Generation ...... 8
III. Concept Selection ...... 11
III. Alpha Design ...... 13
IV. Project Plan ...... 15
V. Informational Sources ...... 16
VI. Problem Analysis ...... 17
VII. Conclusions ...... 18
VIII.References ...... 19
IV.Appendixes ...... 20
Single Suspension Area Concepts ...... 20
Dual Suspension Area Concepts ...... 22
INTRODUCTION
DESIGN & PRODUCTION OF AN ALL-TERRAIN WHEELCHAIR
Andrew Baigrie•Connor Henley• Jeff Lance• Miguel Tovar
Of the 1.5 million Americans who use manual wheel chairs, roughly 600,000 are working age adults. Despite significant design improvements in the last few decades, commercially available wheelchairs still lack the ability to provide users safe and affordable off-road accessibility. Typical wheelchair users are unable to access mountain trails and other off-road paths without significant after-market alterations or enduring hazardous passages with frequent assistance from others.
The wheel chair manufacturing industry has historically been dominated by large manufacturing firms. The firms have primarily focused the development of their “sport” chairs on increasing speed and maneuverability on hard surfaces. Currently, no marketplace offerings exist for a manual chair designed for extensive off-road usage. Wheelchair users must either use aftermarket alterations or powered chairs whose design and performance typically resemble small all-terrain vehicles. A manual off-road wheelchair would provide users the ability to travel in limited access areas with increased independence.
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Customer research and competitive and lateral benchmarking were performed to develop an understanding of our product and redesign goals. A Quality Function Deployment chart was used to quantify the merits of our potential redesigns based on customer attributes. From our QFD chart, we determined the key focus areas for engineering: the suspension, a 5th wheel, and the use of a recumbent seat. The goal of this project is to design and prototype a wheelchair based on our customer attributes for off-road use.
I. ENGINEERING SPECIFICATIONS
CUSTOMER REQUIREMENTS
Both competitive and lateral benchmarking and customer surveying were preformed to determine the most significant attributes for a wheelchair design in our target market. From this research, we developed the following customer requirements.
Fig. 1Customer requirements for an all terrain wheelchair.
Additionally, our primary customer requested recumbent seating and easy transportation and storage. The customer was then asked to rank, in order of importance, attributes of wheelchair designs. These rankings are reflected in the QFD chart shown on page 7.
Currently, all commercially available manual wheelchairs still lack the ability to provide users safe and affordable off-road accessibility. The closest competitive product to an all-terrain design is the Quickie XTR. The XTR has limited suspension to reduce road harshness, but lacks the ability to traverse any type of rugged terrain and does not have recumbent seating for user comfort.
ENGINEREERING CHARACTERISTICS
The engineering characteristics were derived from the customer attributes. These characteristics are the driving factors in a new design to best meet the customer’s needs. They are summarized in the table below:
Characteristic / Current Design / TargetForce Required to Move / 12 lbs / 12 lbs
Horizontal/Vertical Location of Center of Mass / 20/20 in / 20/18 in.
Weight / 35 lbs / 25 lbs
Suspension Travel / 0 / 2 in.
Wheel Base/Track / 38/18 in. / 40/18 in.
Table 1: Engineering characteristics for all-terrain wheelchair.
The relationship between the customers’ attributes and the engineering characteristics are shown on page 7 in the QFD chart. The QFD is a tool that was used to translate customer requirements into design requirements. The QFD allows for quantitative ranking of design requirements that drive the design process of the project. For our project, the most important engineering design goals are: decreased road harshness, a rotating 5th wheel, and a recumbent seat.
Fig.2: QFD chart for all terrain wheelchair.
II. CONCEPT GENERATION
The driver design areas for our project can be decomposed into two major areas: suspension and frame.
DESIGN DRIVER: SUSPENSION
Three types of suspension were researched for our project application. These types were coil over-springs, leaf springs, and scissor dampening. Some examples of these components are provided in the concept selection matrix, shown as Fig. 9 on page 11.
DESIGN DRIVER: FRAME
Numerous drivers exist for the design of our frame. The impact of these drivers varied throughout our brainstorming sessions, as we shifted our focus priorities. The majority of concept brainstorming was performed on chalk boards, but sample sketches of some preliminary design concepts are provided in Appendix A on page 21.
Fig. 3: Benchmark of current market wheelchair.
The datum for our project (concept 1) is shown above. After-market installation of a fifth wheel increases the off road mobility; however, road harshness and comfort are main determents to user satisfaction. Additionally, the chair has insufficient ground clearance and poor maneuverability and aesthetics.
Figure 4: Four competing designs
From our preliminary design, we selected four promising designs to develop further and improve in CAD models. These competing designs are shown above in figure 4, and were evaluated in the concept selection matrix shown as Fig. 10. on page 12 to select the Alpha Design.
Fig. 5: Concept two introduces a curved frame design and rear suspension.
Concept two, shown above highlights a design class with rear suspension and fixed structural based front end. This design is structurally robust and strongly constrained to attempt to direct the dampening.
Fig. 6: Concept three uses rear suspension with slip joints and long curved structural bars.
Concept three highlights a design that also is based on a rear suspension that utilizes a slip joint to increase the dampening range. The structure is driven by large curved tubing.
Fig. 7: Concept four uses front and rear suspension and the structure is based on a single curved bar.
The design of concept four is driven by having dual suspension areas. The dual suspension areas create a structure which is supported by a single curved bar.
Fig. 8: Concept five uses only straight tubing and a single suspension area.
This design is driven by the use of only straight aluminum members. The use of straight bars mandates a single suspension area based on weight and manufacturing and disassembly constraints.
III. CONCEPT SELECTION PROCESS
DESIGN SELECTION: SUSPENSION
Subsystem: Empty bucket / 1 / 2 / 3 / 4Road Harness / 0 / + / + / +
Volume requirements / 0 / 0 / - / -
Production Costs / 0 / 0 / - / 0
Safety / 0 / + / - / +
Durability / 0 / 0 / 0 / 0
Weight / 0 / 0 / - / -
Sigma + / 0 / 2 / 1 / 2
Sigma - / 0 / 0 / 4 / 2
Sigma S / 6 / 4 / 1 / 2
Rank / 3 / 1 / 4 / 2
Fig. 9: Pugh chart for suspension subsystem.
Based on the Pugh chart developed for the suspension subsystem, using springs as the damping element is the optimal design. A spring based suspension will allow a significant reduction in road harshness without compromising safety or additional space. The design has minimal additional cost and significantly increases the functionality of the overall system.
DESIGN SELECTION: FRAME
Subsystem: Frame / 1 / 2 / 3 / 4 / 5Road Harness / 0 / + / + / + / +
Weight / 0 / 0 / 0 / 0 / -
Production Costs / 0 / 0 / 0 / - / 0
Ease of Manufacture / 0 / 0 / + / - / -
Aesthetics / 0 / + / + / + / -
Ease of Disassemble / 0 / + / + / + / 0
Sigma + / 0 / 3 / 4 / 3 / 1
Sigma - / 0 / 0 / 0 / 2 / 3
Sigma S / 6 / 3 / 2 / 1 / 2
Rank / 4 / 2 / 1 / 3 / 5
Fig. 10: Pugh chart for frame subsystem.
Based on the Pugh chart developed for the frame subsystem, design number three is the optimal design. Design three improves over the datum frame in four customer requirements without compromising performance in the other requirement areas of concern. Design three offers a significant aesthetic improvement, while dampening road harshness and allowing for easier manufacturing and disassembly.
III. ALPHA DESIGN
Fig. 11: Alpha design combined with electronic model of primary customer to demonstrate fit.
Figure nine shows a side view and an isometric view respectively of the chosen design concept. The frame will be made out of 1 inch outer diameter aluminum tubing with 1/8 inch wall thickness. The design will incorporate five wheels; two front castors, the two drive wheels, and a fifth wheel in the back. The fifth wheel on the back of the chair should make it easier to traverse over objects and rough terrain. The fifth wheel will be attached to the main frame using slip joints, and locked in place with cotter pins. Making the fifth wheel detachable will make it easier to store and transport the chair.
Fig. 12: Isometric and side view of Alpha Design.
The back of the seat will have a slip joint located slightly above where the fifth wheel attaches to the back of the chair. The slip joint allows for a detachableseat back and therefore easier transport from the decreased height of the chair once it’s removed. The arm support will also be detachable using a slip joint.
The chair has a wheelbase of 16 inches from the front castors to the drive wheels, and a wheelbase of 20 inches from the drive wheels to the fifth wheel when the chair is tilted back on the fifth wheel. The wheel track is 18 inches measured at the drive wheels.
The drive wheels transmit the majority of the vibrations during use andour new design will include two shocks for dampening the vibrations from the two drive wheels. The bottom end of the shock will be mounted to rear portion of the lower support for the chair, and the other end will be mounted to a trailing arm as shown in figure 10. The shock will be mounted at a 7.5 degree angle with relation to the baseline four wheel vertical position. When the chair is tilted back on to the fifth wheel the shock will be at a 7.5 degree angle in the other direction.
The seat back and bottom form a ninety degree angle, but the whole seat is tilted at a 5 degree angle. This will help relieve some of the pressure that is placed on the lower back and make the ride more comfortable.
A mesh fabric seat back will be formed across the two bars that make up the vertical portion of the seat back. The seat bottom will be a platform that is designed to accommodate the user’s traditional cushion.
IV. PROJECT PLAN
As shown in figure 11 below, the timeline for this project is very tight. In order to have a quality prototype finished in time for the design expo, we will need to adhere to a very aggressive schedule. The schedule is defined by several important milestones.The next big milestone is our second design review on October 28th. By the time this review comes around we will have selected one design and completed the specifications and analysis we will need to build the prototype. Also by this time we will have determined and obtained the materials that will be necessary for the build. Construction of the prototype will begin immediately following the second design review and must be completed before our third design review on November 23rd.
Since we plan to build a new product from scratch, our critical path is easily defined. We need to have our prototype constructed by the beginning of our Thanksgiving break on November 23rd to allow for adequate field testing and to meet our third design review requirements. We feel that we will need at least three weeks to build our prototype. Therefore, we must have a final design chosen, verified and ready for build by October 25th.
The design process has been driven mainly by Jeff Lance and Miguel Tovar, who have used their technical knowledge to verify the chosen design concept. Obtaining the necessary parts and determining the best manufacturing process will be conducted by the entire group using our combined knowledge. The fabrication process will become more detailed and roles more clearly defined when we actually begin the build.
Budget considerations will likely be the main driver in our design and production. The currently allotted budget of $400 represents a fraction of our anticipated project cost. John Norton has secured web space for an online reference for potential donors.
Fig. 13: Project plan.
V. INFORMATIONAL SOURCES
Primary users provided the baseline and target data from which design benchmarking and brainstorming began. Our primary customer contact provided the initial data through the introductory interview. Specifically, she gave a general overview on current wheelchairs usability and potential areas for improvement. Additionally, she provided introductions and assisted in obtaining feedback from other users to increase our knowledge about the target population.
Current wheelchair designs were examined and evaluated for benchmarking purposes. Data was obtained by hands-on testing and reference material that was found through product literature (owners’ manuals), advertisements, and online forums and sales outlets. This information broadened our knowledge database of current production chairs and after market enhancements. Specially, we focused on all-terrain and other athletically targeted wheelchairs (rugby, basketball, etc.). The benchmarking exercises allowed us to generate the data that drives our engineering specifications.
While there have been a few attempts for traditional manual wheelchairs to expand their road handling, we have found no constraining patents in our searches. In particular, we have focused our search effort on rear suspensions and fifth wheel designs, without finding any possible conflicting patents. The only applicable patents in our project will be if we purchase patented items for use in our prototype.
Our current informational gaps center around the optimal manufacturing process for our chair. As we move forward in the project, we will have to connect with manufacturing and machining specialists depending on the final prototype design and assembly. While at Michigan, our team has excellent contacts in this area from internships and Solar Car. These contacts will prove invaluable in providing technical references once we enter the prototyping stage.
VI. PROJECT ANALYSIS
The major characteristics that drove the design were: the optimal method for adding suspension, a frame design incorporating aesthetics and the correct seating position, and the addition of a fifth wheel.
The suspension design was driven by optimizing the location and method of application. The driver for our application was attempting to soften the road harshness on our primary customer’s lower back. Therefore, we selected a design where the primary dampening occurred under the rear portion of the seat. The method of damping was selected by using a Pugh Concept Selection Matrix.