The Cutting Device

Overview

Using a pair of scissors is difficult for a humanoid prosthetic hand, because it requires the coordination of multiple fingers to rotate the handle around a remote centre of motion. Using a crank-and-rocker mechanism, our cutting device produces a continuous cutting motion, and brings fiction to life by turning the prosthesis into a tool.

Design

Schematic diagram, annotated with key components.

Real terminal device being used to cut a sheet of tissue paper.

Mechanical Design

Motor rotation produces oscillatory motion of the rocker arm (actuated blade).

Control

One electrode reading muscle activity.
Tensing rotates the motor at a fixed rate.

Testing

Experiment - Cutting Circles

Used to evaluate both speed and accuracy.
Users must cut a 10 cm circle from a sheet of A4 paper.
Performance metrics: relative circularity and completion time.
Repeated 3 times.

Results

Task Performance

Participants using the Cutting Device completed the task over four times as fast as those using the humanoid hand: 83.2 seconds vs 351.6 seconds; a significant performance increase (p<0.001, Mann-Whitney U test).

Those using the Cutting Device were also significantly more accurate, with a relative circularity of 0.91 compared to 0.83 for the humanoid group (p<0.001, Mann-Whitney U test).

Participant 1 is a bilateral amputee, and completed the task using their left residuum to rotate the paper around the scissors. This meant that the task took longer to complete.

Compensatory Motion

Significant reduction in chest and upper arm motion (p<0.001, Mann-Whitney U test), and reduction in lower arm motion (p<0.01, Mann-Whitney U test) during use.

Participant 1 is a bilateral amputee, and completed the task using their left residuum to rotate the paper around the scissors. This greatly increased body motion.

Task Load

Significant reduction in median mental demand (MD, p<0.01, Mann-Whitney U test), physical demand (PD, p<0.01, Mann-Whitney U test), and frustration (Fr, p<0.05, Mann-Whitney U test), and significant increase in median perceived performance (Pe, p<0.01, Mann-Whitney U test) compared to the humanoid group.

Participant 1 is a bilateral amputee, and completed the task using their left residuum to rotate the paper around the scissors. This increased all aspects of task load.

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