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DESIGN OF A UNIVERSAL SWITCH MOUNT

Aaron D. Little, Kamel S. Saidi, Joan-Marie Shouman The University of Texas at Austin, Austin, Texas

ABSTRACT

Individuals with various degrees of motor impairments use switches to control electrical devices. Mounting these switches, onto wheelchairs and other fixtures has proven to be a complicated and costly task. The authors of this paper developed a design for a switch mount for Rosedale School in Austin, Texas that solves many of the problems currently encoutered by school's staff. This design mounts securely to most of the fixtures found at Rosedale and can be reproduced at the school's own wood shop for a total cost of approximately $40. Therefore, four times as many switches can be provided to Rosedale's students than what is currently available.

BACKGROUND

A person with a severe motor impairment of the types characteristic of cerebral palsy will find it difficult to live and work independently. However, technological advances are being made which enable these individuals to participate more fully in the community. Among these advances are adaptive switches which can be used to control any number of electrical items and can be operated by any part of the body [1]. Example switching functions include control of wheelchair speed and direction, operation of communication aids, computers, environmental controls, and other devices.

A variety of switches are available to meet the needs of people with motor impairments. For example, persons who have spastic muscle movements generally require durable switches, whereas individuals with hypotonia need switches that are sensitive to a light touch [2]. Still others, namely people who are paralyzed, benefit from switches operated by light or air (ibid.). The varied nature of these motor impairments inevitably results in switches of different shapes and sizes as well as in the need to position the switches in different locations and angles.

STATEMENT OF THE PROBLEM

Clearly, the problems that arise are: 1) how to attach different switches to a variety of fixtures (i.e., wheelchairs, tables, tumbleforms, etc.) that a person with cerebral palsy uses and 2) how to position the switches into any number of configurations. Several different approaches have been taken to solve this problem.

Figure 1: The Rosedale Switch Mount

The most common commercially-available switch mounts are produced and/or distributed through AbleNet, a company that specializes in adaptive devices. These mounts are costly, heavy, difficult to adjust, and tend to slip from their original position during use. As a result, people also create their own means of mounting switches such as using velcro, cardboard, microphone tubing, tape, and various other materials available to them. Although home-made devices solve the problem of affordability, they are often neither durable nor sturdy. Furthermore, because of their crude appearance, such mounts can detract from an individual's ability to be successfully included and accepted in his/her community. The authors worked towards a solution to these problems by designing and developing an affordable and socially-acceptable switch mount that is durable, stable, easy to adjust and can be attached to a variety of objects.

METHODOLOGY

Armed with the original project statement supplied by Rosedale, the design team used traditional product design and development techniques, as explained by Ulrich and Eppinger [3] to create the Rosedale Switch Mount (RSM) shown in Figure 1. The team first gained a better understanding of the problem by conducting customer interviews at Rosedale. Through this process, the team was able to compile a list of tangible customer needs and to satisfy these needs with corresponding engineering requirements. Using Quality Function Deployment methods, a House of Quality (ibid.). was then created. The House of Quality illustrated the interrelationships between the customer needs and the engineering requirements and established their relative importance. The benchmarking information in the House of Quality also helped in constructing the list of target specifications.

The next phase of the design process involved a simplification of the primary task of the RSM and the determination of the energy, material, and information flows needed to accomplish this task. The design team divided the primary task into a series of basic functions, each having several possible solutions. The two primary functions of the RSM were clamping onto different surfaces and positioning the switch. The solutions were then combined into a multitude of possibilities to form the alternative designs. Using Ulrich and Eppinger's selection methodology (ibid.). the team selected several concepts.

DEVELOPMENT

Once the problem was thoroughly analyzed and concepts were selected, the authors built proof-of-concepts to determine the feasibility of their preliminary designs. First, the team reproduced a patent from 1950 (ibid.) which consisted of a series of joints, not unlike the vertebrae of a human, made out of wooden spheres with a steel cable drawn through them. The advantage of this concept rested in the fact that such an arm would be easy to use because the joints could be locked in one place by pulling the cable taut. But it was discovered that too much force was required by the user to lock the arm. Improvements could have been made by altering the surfaces of contact between each joint; however, doing so would have made the implementation more complex and difficult to manufacture. Therefore, this concept was rejected.

Figure 2: The "Tinker Toy" Concept

The authors then developed a concept based upon Tinker Toys and built a preliminary prototype out of wood (see Figure 2). This concept impressed the design team by its simplicity and by the similarity of its different parts. While producing the prototype, the tolerances for the joints were hard to predict using common wood shop tools, therefore a jig was manufactured to build the hinge discs for the next prototype (see Figure 3). The next step was to select the materials and determine the dimensions for each part of the alpha prototype.

Figure 3: The Jig

Concurrently, the authors worked to solve the problem of attaching the switch mount to the fixtures used by the customer. The AbleNet clamp being used by Rosedale School at that time worked well but could have easily been improved with a better grip surface and an attachment for flat surfaces. Therefore, the team decided to purchase the AbleNet clamp and modify it to meet Rosedale's requirements. This decision not only eliminated any machining for the device, thus keeping the cost per unit low, but also enabled the staff members of Rosedale School to significantly improve the stability of their current switch mounts.

Developing the concept into a fully working alpha prototype involved engineering analysis, material selection, cost analysis, and safety issues. The critical function of the alpha prototype was the locking mechanism. Each joint had to lock three degrees of freedom: rotation of the extension, translation of the extension, and rotation of the hinge discs. It was important to select polycarbonate for the extensions in order to provide the flexibility needed to reduce the force at the joints and the clamp. The design team chose PVC for the joints because it was the most inexpensive and lightweight material that fulfilled the engineering and safety requirements. Rosedale School spends approximately $180 per switch mount and indicated that they would like to have the ability to purchase three times as many. Therefore, the authors decided to develop the product for under $60. As with any design, various safety concerns arose during the development of the alpha prototype; such as rough surfaces, sharp edges, and the possibility that the user's fingers could be pinched between the hinges. These issues were evaluated and resolved by the design team.

After testing the alpha prototype at Rosedale, the authors determined that more clamping force was required to keep the arm in place when a stronger students was using it. Because of the sharp edges on the circular knobs, the person adjusting the mount was not able to grip the knob tightly. Therefore, sufficient torque could not be achieved. As a result, bar knobs were implemented for the beta prototype. In addition, the team decided to use a stainless steel tube for the first extension in order to alleviate the excessive "bounciness" caused when using all polycarbonate extensions.

DISCUSSION

This paper presents the development of an affordable and reliable switch mount for people with dissabilities. The RSM clamps securely to wheelchairs, tables, desks and other fixtures and allows the switch to be configured to any desired position up to 26 inches away from the base. The final cost of the switch mount with the modifed clamp is approximately $40 and can be completely manufactured at Rosedale's wood shop. A fully illustrated fabrication/assembly plan and a user's guide have been provided to facilitate the implementation of the device. The authors have enabled Rosedale School to acquire four times as many mounts as they can currently afford which increases the ratio of switches to students in their classrooms. In addition, the device was designed with inclusion in mind, thus allowing it to be used at job sites and during social activities (without being obtrusive.) Furthermore, the Rosedale staff is enthusiastic about being able to produce more switch mounts in house and at an affordable price.

REFERENCES

[1] Cogher, L. et al. Cerebral Palsy : the Child and Young Person. Chapman & Hill Medical, 1992.

[2] Church, G. and S. Glennen. The Handbook of Assistive Technology. Singular Publishing, San Diego, 1992.

[3] K.T. Ulrich and S.D. Eppinger. Product Design and Development. McGraw-Hill, New York, 1994. [4] Tesmer, E. "Flexible Positioner." Patent number 2,510,198. U.S. Patent Office, Washington, D.C., 1950.

ACKNOWLEDGMENTS

The authors would like to express their sincere gratitude to the Rosedale School for all their assistance during the project. We are indebted to Drs. Rich Crawford and Kris Wood for the advice and guidance they provided and to the expert machinists at the Mechanical Engineering student machine shop for their help in building the jig.

Aaron D. Little Department of Mechanical Engineering The University of Texas at Austin ETC 4.150, Austin, Texas 78712-1063 alittle@mail.utexas.edu Design of a Universal Switch Mount