16.1: Project Proposal

Introduction

Human hands have been highlighted in robotics, and researchers and engineers constantly try to replicate anthropomorphic hands with various actuation means. However, due to technological constraints and current knowledge, the human hand is extremely difficult to replicate due to its complexity with all the tendons, bones, and muscles required to move the 27 degrees of freedom. In the attempts to replicate the human hands, the cost has increased, and the designs haven’t become any less complex. Our project will attempt to go down another path: lower the price as much as possible, reduce the number of actuators to a singular motor, and maintain as much functionality as possible.

Problem Statement

Currently, robotic hands, such as prosthetic hands, which sparked the idea for this project, are very expensive, and this high cost comes primarily from the difficulty with manufacturing. They also require a very time- and labor-intensive development cycle in which the hardware and software must be optimized to perform their intended function. This primary pain in the development is from the design of the hand enclosure, which requires very high levels of tolerancing and a ton of research to fit the tiny drive motors and tensioning cables. Another issue is with the mechanism itself. Due to the use of tensioning cables, frequent re-tensioning and calibrations are required, adding to the cost and complexity of the hand.

We aim to design and manufacture a cheap robotic hand based purely on linkages and compliant/dampening designs. It will have one actuator and be able to grasp small objects, such as a small ball or a pencil.

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.

Moving on to the next section of the project, we need to focus on the “palm” of the hand. The palm’s sole purpose is to carry the compliant mechanism, enabling us to actuate five fingers with one motor. We propose a 4-bar kinematic chain with two grounded transition links connected with a spring link (which can be represented as two links). The spring will allow the fingers to comply, stopping when touching an object.

Another approach would be adding a compliant component to the end of the finger mechanism where the range of motion for the finger mechanism is to be limited allowing only the compliant mechanism to finish the closing motion of the hand. This enables a softer and more adaptive hand that can hold different objects.

Proposed scope

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.

Preliminary Design

Preliminary design of the fingers

We will be referring to the finger joints by their anatomical name. Figure 1 will show what the names of the joints are. Also, the crossing 4-bar mechanisms are 1 DOF based on the Gruebler equation: M = 3(4-1) - 2(4) = 1

Figure 1: Graph showing and labeling finger joints with their anatomical name.

Crossing 4-bar mechanism from MCP to PIP Joints

Figure 2 shows the planned preliminary design for the mechanism between the MCP and PIP joint, with an initial length. The actual lengths are still to be determined.

Figure 2: Preliminary design of the MCP - PIP crossing 4-bar mechanism

Grashof Condition

6.3 + 28.2 < 9.5 + 27.3

34.5 < 36.8

Class I Grashof: Crank-rocker

Crossing 4-bar mechanism from PIP to DIP Joints

Figure 3 shows the planned preliminary design for the mechanism between the PIP and DIP joint, with an initial length. The actual lengths are still to be determined.

Figure 3: Preliminary design of the PIP - DIP crossing 4-bar mechanism

Grashof Condition

6.0 + 24.8 > 9.4 + 20.1

30.8 > 29.5

Class II Grashof

Various ideas for achieving compliance within the hand.

Figure 4: shows the side profile of the pawm and finger mechanism. This is one potential design where the finger mechanism will close to a percentage of fully grasping then the compliant link will snap the finger close.

Figure 5: shows another potential configuration where the figure mechanism stays the same without the added compliant part. (4-bar kinematic chain) The palm shown has the linear actuator linked to a dampening kinematic chain that drives the finger mechanism.