First Prototype: 7-Bar Retractor

<< Problem Statement / Background

Design

The goal for the first prototype was to create a mechanism with 1 degree of freedom, which could be driven by a single rocker attached to an electric motor. Our vision was to accomplish this with a collection of four-bar linkages. The design process began by trying to imagine a four-bar necessary to simply close the finger, as shown below. In all cases the primary design for our mechanisms was done via a graphical synthesis approach.

With all synthesis methods, there are free choices available to the designer to constrain the number of solutions. For this first four-bar, we chose to size the output rocker to the original finger actuator link length, and to size the crank as small as possible while still being capable of rotating the output greater than 100 degrees. These choices were made to both retain original gripper functionality, and minimize volume and weight added.

The next step in the design of the first prototype was to mount the previously mentioned closure four-bar to a pivoting link and external crank to allow for the entire finger to swing away from the wrist. The free choices in this case were the pivot link length and crank length. The pivot length was chosen to be half the length of the finger, to allow for retraction of the entire length of the finger when the pivot swings through 180 degrees. The crank was chosen to be a short as possible while still being able to actuate through the complete extension and closure of the finger. An example of the graphical solution is shown to the right-bottom.

The solution became a seven-bar, 2-DOF linkage, with many toggle positions in the active range of motion. The extra degree of freedom was constrained with linear springs, as shown at right, and the toggle positions were managed via physical guides and hard stops built into the links and supporting wrist structure. The 3D printed model of the first prototype can be seen below.