Twisted String Actuator for Robotic Hand Grasping

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Solution Overview

Problem

Existing robotic hands struggle with precision grasps, as underactuation can perform poorly in tasks requiring accurate control of fingertip positions and limited contact points, while also being inadequate for handling unknown objects effectively.

Innovation Solution

A robotic finger assembly with a twisted string actuator system, featuring a motor-driven rotor, twisted strings, and non-circular pulleys or cams, allowing for antagonistic operation to maintain tension and enable precise control of finger movements, combined with selective joint locking and electroadhesive skin for enhanced grasping capabilities.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If underactuated fingers are used for power grasps, then the robotic hand can self-adapt to wrap around unknown objects, but the control precision of fingertip positions deteriorates

Engineering Contradiction:

Improveself-adaptation to unknown objectsVSAvoidfingertip position control accuracy

Core Design Contradiction:

Adaptability or versatilityVSMeasurement precision

Solution Approach 1:

The finger is divided into multiple segments (proximal phalange, intermediate phalange, distal phalange) with independent joints, allowing selective actuation and control of different segments to achieve both adaptation and precision

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The system dynamically switches between underactuated mode for power grasps and precision control mode for delicate tasks, with the ability to lock/unlock joints selectively to change the degree of freedom and control characteristics

Inventive Principle:

Principle #15Dynamics

2Reliability

If underactuated fingers are used for power grasps, then the robotic hand can grasp objects effectively, but the performance in precision grasps with limited contact points deteriorates

Engineering Contradiction:

Improvepower grasp effectivenessVSAvoidprecision grasp performance

Core Design Contradiction:

ReliabilityVSEase of operation

Solution Approach 1:

The finger assembly can dynamically change its actuation mode between underactuated and fully actuated states, allowing it to perform reliably in both power grasps and precision grasps by adjusting the degree of freedom as needed

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The system changes the operational parameters by selectively locking and unlocking joints, transforming the finger from an underactuated state (good for power grasps) to a fully actuated state (good for precision grasps)

Inventive Principle:

Principle #35Parameter changes

3Reliability

If twisted string actuators with non-circular pulleys are used, then the system can maintain tension in both actuators during antagonistic operation, but the device complexity increases

Engineering Contradiction:

Improvetension maintenance in twisted stringsVSAvoidpulley or cam mechanism complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

Non-circular pulleys or cams with asymmetric profiles are used to vary the effective radius during rotation, enabling the system to maintain tension in both twisted strings during antagonistic operation by compensating for length changes

Inventive Principle:

Principle #4Asymmetry

Solution Approach 2:

The non-circular pulley or cam acts as an intermediary mechanism between the motor and the twisted string, transforming rotational motion into controlled linear motion while maintaining tension through its varying radius profile

Inventive Principle:

Principle #24Intermediary (Mediator)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

Enables both power and precision grasps, allowing the robotic hand to adapt to various objects by self-adapting to unknown shapes and maintaining precise control, with improved resistance to shock and overload, and the ability to re-grasp objects efficiently.

Implementation Method 1

a twisted string comprised of a pair of cords. One end of the twisted string is attached to the rotor and an opposite end of the twisted string is coupled to the finger skeleton. The cords are twisted about each other for a first section of the twisted string and untwisted for a second section of the twisted string

Methodology Applied

Scientific EffectTwisted string actuation:

Implementation Method 2

The non-circular shape of the pulleys is adapted to keep both twisted strings in tension throughout a range of the rotary motion produced by the motor

Methodology Applied

Scientific EffectGeometric constraint: Geometry

Data Source

PatentUS9272425B2Twisted string actuator systems

Publication Date: 2016.03.01 SRI INTERNATIONAL

AI summary

A twisted string actuator system includes a motor generating rotary motion of a rotor and a twisted string comprised of a pair of cords. One end of the twisted string is attached to the rotor and an opposite end of the twisted string is coupled to a load. The cords are twisted about each other for a first section of the twisted string and untwisted for a second section of the twisted string. A cord guide is fixedly disposed between the cords. The first and second sections of the twisted string are on a first side and second side, respectively, of the cord guide. Rotary motion of the rotor in one direction operates to twist the pair of cords on the first side of the cord guide while pulling a portion of the pair of cords from the second side of the cord guide into the first side.