Lockable Finger Design

Employer: Shirley Ryan AbilityLab / RIC

Role: Resarch Engineer III

This design came out of my work on the RIC Arm and body powered VO/VC Hand. I was the primary inventor for this design.

Prosthesis users lack the same proprioception available in anatomical hands and limbs. This requires users to use visual feedback to understand where their prosthesis is in space when manipulating and interacting with their environments. Prosthetic hands are notorioulsy bad at blocking line of sight due to their size and opacity. Being able to move digits "out of the way" by locking them in the flexed position improves visual feedback and allows for easier navigation of more dexterious tasks such as a two finger pinch between the index finger and thumb.

The first implementation of this design utilized a simple latch on the knuckle of each digit coupled with a light spring which biased the digit straight. While this worked great, it was not very cosmetic to have latches on the back of the hand.

The second generation of this design aimed to make the locking feature self contained in the 4-bar linkage of the finger design. This was accomplished via a slot, two torsion springs, and a bi-stable 4-bar linkage design.

• The slot meant that locking fingers did not impact the actuation of the rest of the hand

• The bi-stable linkage system meant that in one state, the fingers where actuated and driven by user input but when pushed through a singularity and into the flipped state, the fingers where locked in a flexed position.

• The two springs held different roles depending on the state of the 4-bar linkage. In locked mode, one spring drove part of the mechanism into a hard stop, maintaining the locked position. In the unlocked state, one spring biased the finger open while the other spring moved the linkage through singularity and into another state.

Locking a finger:

• Flex the finger by pressing the distal linkage past the end of the range of travel (i.e. once the fingers are driven closed by the user, bump the distal linkage against a table edge, pressing it further into flexion). This will push the linkage system through singularity.

Unlocking a finger:

• Press down on the finger linkage, this will move the linkage to the point of singularity. When you release pressure on the finger linkage, both springs are acting to drive the finger into extension and the digit will spring open.