Bryan’s Wiki Report
| Date: November 29, 2006 | Tendon Attachment |
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| Author : Bryan Adams | Team : Robot Hand Mechanics |
Bryan’s Problem definition
The purpose of this years design team is to improve upon last years finger design by reducing the overall size and friction of the previous finger design. The overall goal is to create a finger that can generate the force needed to model the human hand and be implemented into a whole hand device. The design should showcase the capabilities of Shape Memory Alloy (SMA) wires to produce the range of motion similar to that of a human hand. My initial part in this years team was to develop a thumb joint that could be implemented into a palm device. This task was eliminated in order to first improve upon the initial finger and transmission of the previous year’s design. My new task for Team Robot Hand Mechanics was to redesign the tendon attachment method for the redesigned finger.
Requirements and Specifications
- The customer requirements for our team were defined as:
- Demonstrate a Possible Use of SMA Wires
- Anthropomorphic (human-like) Finger Design
- Reduce Friction
- Have Predictable Design
- For further details on each of these customer requirements, please refer to our project proposal.
- In order to significantly improve upon last years design the tendon attachment method must be more secure and accurate. Last year’s team used a basic knot to attach the tendon to the reverse bias point and that allowed for too much movement of the tendon. This could potentially allow multiple tendons to become entangled and not function as they were intended. Additionally this redesign is to effectively utilize the reverse bias capabilities of the SMA spring that this year has chosen to use.
The requirements of a tendon attachment are three fold:
- Maximize the reverse bias capabilities of the SMA spring
- Minimize movement of the Tendons once attached
- Allow for easy attachment to the reverse biased cam
Each tendon attachment point needed to be carefully designed to maintain a force of 10 newtons at the tip of the newly designed finger while the finger was in Flexion. This was governed by the customer requirements and specifications to create a hand that was anthropomorphic or human like. Back To Top
Bryan’s Concept generation and sketches
|Solid model thumb concept|
The initial concepts that were generated were based on my initial assignment of developing a thumb. The thumb is a complicated portion of the hand and it needed to be simplified in order to incorporate it into a functioning palm device. The basis for the conceptual models is simplicity. The idea of Team Robot Hand Mechanics is to design a finger that can be implemented multiple times. The final finger design would only need a simple modification of an angle put in to the connecting joint in order to represent an anthropomorphic thumb. The initial concepts of the thumb and the various attachment points can be seen below.
With a change in direction my new focus became the actual method of attaching the tendons to the finger such that the overall requirements and specifications could be met. My Initial concept generation consisted of a dowel pin with a set screw to secure the Tendon. This concept was further explored and refined into a cam device.
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| Pin with set screw concept | reverse bais cam concept |
Initial downselect & design refinement
There were several options that could significantly improve upon last years design. These ideas are presented in the design selection matrix. Through several meetings with our Advisor and as a teem it was decided that the best tendon attachment device for the finger would be the reversed biased cam. The initial concept of the cam design was presented by Rob Poulsen A reversed bias cam would utilize set screws in conjunction with a crimp attached to the tendon to secure the tendon to the cam. This would allow for a single attachment point that would effectively use the reverse biased characteristics of the SMA spring. Additionally the single attachment point would simplify the design for ease of attachment.
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| Cam with Crimp and set screw |
Final selection, supporting calculations or data, and assembly drawing
Based on the initial concept of the reverse bias cam various calculations were made to effectively determine the correct shape. The shape of the cam was critical to its design in order to effectively utilize the characteristics of the SMA spring. Tyson Skinner presented some basic calculations in a spreadsheet format to determine the appropriate shape of the cam. His expertise in solid modeling enabled him to design the reverse bias cam used in our critical function Prototype. Tyson’s calculations and schematics can be found below. Additionally Rob Poulsen and my self went through several calculations to determine if the cam design would present the proper load at the finger tip of 10 newtons. The equations used in our calculations can be found below. Schematics of the the basis for our calculations can be found on Rob Poulsen’s wiki report.
- Tyson Skinner’s Cam Design Drawings and Calculations
- Rob and Bryan’s Calculations
Lessons Learned
- Further research has determined that the original calculations to transmit the proper force to the end of the finger were incorrect. The correct calculations are torque calculations that would actual yield a cam that was too large to incorporate into an anthropomorphic finger design given the capabilities of the SMA springs that we are currently using.
- Through further research it was determined that the actual force that the human finger can apply is closer to 40 newtons. This is much greater then the calculations of Last year’s team, having only 10 newtons of force. Due to our current findings the 40 newtons needed will not be possible with our current SMA springs. Below is a link to the report that describes the loads that can be applied by the human finger.
- Measurement and prediction of single and multi-digit finger strength
- Finger Force Paper
- Angela Di Domenico, Maury A. Nussbaum, Source: Ergonomics; 12/15/2003, Vol. 46 Issue 15, p1531–1548, 18p
- Finger Force Paper
Conclusions
Several conclusions have been made by the development of the reverse bias cam:
- The current SMA Springs will not supply the needed force to accurately model the human hand.
- New calculations for SMA springs will need to be done to determine if a new spring can be made to provide the needed force.
- A new cam will need to be designed to be able to transmit the appropriate force to the finger.
Special Thanks
- Evan Coombs for all the hard work into creating and editing the team’s wiki and creating a killer mold for the CFP.
- Tyson Skinner for the awesome solid modeling on the finger and cam.
- Eric Youssefi for the help with solving the force equations on the finger for the cam design.
- Rob Poulsen for sacrificing his awesome palm device this semsester to focus on our critical function prototype.