Introduction
One of the most striking examples of the gulf between human ingenuity and the designs of nature is the great degree of effort and technical knowledge an engineer needs to mimic even the simplest of natural mechanisms. There is perhaps no better example of this fact than the human hand: an assemblage of skin, muscles, tendons, ligaments, cartilage and bone that is capable of remarkably complex and precise motion. When looking at the vast majority of robotic hands, one of the first design realities that becomes apparent is the need for many different actuators to accomplish a product that resembles the real thing. The human hand has 27 degrees of freedom, so it is not hard to see why the amount of actuators required can easily become unwieldy when trying to replicate it. In this conundrum, the simplicity of flesh is lost, and the manufacturing cost can easily become prohibitive. With our project, we aim to explore another path: a robotic hand with as few actuators as possible.
Problem Statement
Many robotic hands designed today make use of numerous small actuators in combination with tensioning cables, which must be precisely toleranced and calibrated. The highly time- and labor-intensive development cycle required is one of the main reasons for their high cost. As part of our goal of minimizing the amount of actuators we use, motors and pistons must be replaced with mechanically interconnected linkages. Even with only one motor, we would like to be able to flex and extend our robotic fingers fully. If time permits, we would also like to animate the wrist.
Additionally, reducing the amount of actuators used is a goal that is intrinsically at odds with the goals of precision and delicacy. There is no purpose in simplifying your design if it cannot accomplish its intended function. We would like our product to have the required force to firmly grasp a small object without using so much force that the object is damaged. As in a real human hand, we want our robotic hand to be able to grasp objects of differing shapes and orientations. To do so, we will need to build in some inherent compliance to our mechanism so that our fingers can close tight around our object no matter what direction it is oriented in.
Ultimately, the problem we face is to find a middle ground between design simplicity and operational precision.
Mechanism
The primary mechanism that will actuate the project will be the crossing 4-bar linkage, which will act as a crank, giving us complex motion. Each finger will be constructed by two colocated crossing 4-bar linkages, enabling the finger to open and close all three joints, covering the flexion and extension movement of the finger. However, with this design, we are planning to forfeit abduction and adduction movements of the fingers to minimize the complexity.
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Our plan for producing this robotic hand includes researching the anatomy of the human hand, existing linkage designs for tension-driven robotic hands, experimenting with various linkage designs, software optimization, prototyping, testing, and improvements. We aim to complete our set objective of creating a pure linkage-based hand that can at least grasp some objects. This will likely include a lot of test designing, kinematic simulations, and prototyping. If possible, we would like to perfect the design and functionality of the hand by adding more range of motion and sensor-driven features in the future.
Additionally, we would like to put effort into the form factor of our hand. We want it to be sleek and aesthetically pleasing, and for it to connivingly model the human hand.
Preliminary Design
Preliminary design of the fingers
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