20_Finger Length Tests

One of the Project Launch Review Members emphasized that our device would need to accommodate the change in finger surface length as the user’s fingers curl and extend.  Team member’s Devin and Patrick had been considering this in their system concepts, but we decided to perform a rough initial test to determine the general change in length on the surface of a finger between curled and extended positions.

I conducted the test on my hand, with the results below in Table 1.  In order to measure the change in length of the surface of the finger between curled and extended positions, I tied a fine string to the tip of a finger and pulled it tight over the top of my finger being sure that it ran along the center of my finger, not falling to the side.  Next, I marked on the string the point at which it met the base knuckle of the finger.  This was done for each finger position and the distance between the two points was measured.  Additionally, the length of the finger was measured, and the experiment was repeated for each finger (1-4, index-pinky) excluding the thumb.

Table 1: Single Run Results of Finger Surface Length Change Test

We’ve noticed that the change in length of the “pinky” surface was not significantly smaller than the longer fingers as we thought it would.

 

Thinking about what causes the change in length: the joints in the finger can be approximated as cylindrical pins with a diameter.  As the finger rotates about the cylinder joints, more of the surface area of the cylinders are exposed on top, requiring anything that conforms to this surface (such as skin or our device) to increase in length as the finger curls to accommodate this added area.  The finger’s actual length through its center some not change though.