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Initial Design Concept
November 2018
Our design team chose to build a tendon-driven hand. Currently, there are similar tendon driven designs in the market, however, many of them can only perform the flexion and extension of the hand as one moving part. To increase the overall functionality of the hand, our hand will have control over three separate moving parts. Therefore, our design team kept their focus on a single finger. Once one finger is perfected, the rest of the hand can mimic these mechanisms. 
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Developing the first finger started with two designs. Both designs used the same mechanics for flexion but differed in terms of extension. To create flexion of the finger, a braided fishing line runs under and through the holes in the “bone” or part of the finger as shown in Figure 1. The line is connected to a motor that will be placed in the forearm. The motor pulls the line and the finger bends. Each part is held together with pins and micro bearings to allow the rotation and consists of 45-degree mating faces to create a 90-degree angle at the joint when the string is fully retracted.
Once the finger is fully flexed and the motor is released, we have considered two ways to extend the finger back to its’ straight position. The first method, shown in Figure 2, places elastic bands at the top of each “knuckle”. The elastic bands stretch when the finger is flexed creating tension in the bands. Once the motor is released, the finger extends.
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Figure 2. Top view with the elastic band method.
The second method, shown in Figure 3, replaces elastic bands with springs. Like the bands, when the finger is flexed, tension is built up in the springs, forcing the finger to extend once the motor is released.
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Figure 1. Bottom view.
Figure 3. Top view with spring method.
After brief testing of each design, we found that the elastic bands and string are not strong enough to fully extend the finger to its’ original position. As a result, we decided to combine each design, creating a spring and elastic band design that will work together to extend the finger. The reason behind this design will prevent the use of a second tendon-driven system and allows for easier customization.