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Tyson Skinner’s Individual Wiki Report

Date: November 29, 2006

Author: Tyson Skinner	Team: Robot Hand Mechanics

Tyson’s Home Page

Tyson’s Problem definition

• To design and build a robotic finger to demonstrate the usability of Shape Memory Alloy (SMA) as well as create a lightweight prosthetic device. As you can see from last years finger design, there were issues of friction, poor construction, and inconsistent force application to the joint. In order for this idea to be implemented into prosthetics the design will have to be very efficient and lightweight, therefore the issues of last years design will need to be resolved in order to have a functional prosthetic device that demonstrates the usability of SMA.

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Requirements and Specifications

• Demonstrate a Possible Use of SMA Wires
  • SMA is a newly available alloy with unique characteristics. It is very usable and could be used for many various applications. In order to show its usability the new alloy is being implemented into a finger as muscle fibers.
• Anthropomorphic (human-like) Finger Design
  • In order for the SMA actuated finger to be usable it will need to be accepted by society. In order to do this it will need to resemble that of a human finger so that people are not afraid of the fingers use.
• Reduce Friction
  • The reduction of friction will enable the finger to be very usable and efficient since it will be implemented into human use
• Have Predictable Design
  • Last years design team used knots tied in the tendons to connect the tendons, but since the design will be implemented into human use, the finger assembly will need to be very predictable.

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Concept Generation and Sketches

Perspective Finger Sketch

By: Tyson Skinner

• There were many inital concepts for the finger design which can be seen in the concept mind map. There were U-channel member designs, similar to that of last years design, circular member designs and square member designs, all of which had a wide range of variations. The general size of the finger members were based off of the average human finger size as well as the sketches of last years design. The square member internal and external bone concepts were sketched and can be seen by selecting the links below.
  • Tyson’s Mind Map
  • Internal Bone Finger Concept
  • External Bone Finger Concept

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Initial Down Selection and Design Refinement

Internal Bone Finger Concept	External Bone Finger Concept

• After completion of a Design Selection Matrix the two concepts that were determined to be superior to all other concepts generated were the Refined Internal Bone Finger Concept and the Refined External Bone Finger Concept. These two concepts would be able to produce all of the needed specifications and give a human like design as desired.

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Final Selection, Supporting Calculations, and Assembly Drawing

Solid Model Finger Assembly	CFP Finger Assembly

• Critical Function Prototype (CFP) Finger Assembly Design
  • In order to determine the final design the Design Selection Matrix was further evaluated and the External Bone Finger Concept was determined to be the best design. The design was further refined to the CFP Finger Assembly Solid Model Design and was created to produce the CFP Finger Assembly. This concept was able to produce the needed specifications and was also easily created using materials and parts readily available, therefore requiring only minimal machining. The drawings to create the finger assembly can be accessed from the link below.
    • Complete Drawings for the Each Finger Joint, Including Cam Dimensions
  • Since the reverse biasing of the finger was also a crucial part in the finger design, the general shape of the cam was critically evaluated. When the design of the cam was decided, exact dimensions were still needed. In order to do this a Cam Design Spreadsheet was created in order to calculate the moment arm of the cam for both flexion (Cam Design Flexion Model) and extension (Cam Design Extension Model) at every orientation as the joint is bent. By applying a cam design, a varying application of force can be achieved that is tailored to the characteristics of SMA contraction. Namely, SMA wire can apply more force at the beginning of the wire contraction, and reduces as the contraction continues. This will give a reverse biasing effect, which will also help to prolong the life of the SMA wire. The cam also implemented a better tendon attachment method that is very predictable and can easily control the lengths of the tendons.
  • Cam Design Drawings and Calculations
    • Cam Design Flexion Model
    • Cam Design Extension Model
    • Cam Design Spread Sheet Calculations

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Lessons Learned

• For the most part the finger design was very successful. From the creation of the CFP a few improvements for further design was learned.
  • Since the needed force was not gained as determined by Team Robot Hand Mechanics CFP, the cam dimensions will need to be reevaluated in order to give the forces needed.
  • The material purchased, clear acrylic, for the finger members was too brittle and new material will need to be purchased for the new designs.

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Conclusions

• The CFP of the finger was very beneficial. A few areas of improvement were identified and can be continued to be refined as the design process continues.

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