Anthromorphic Underactuated Hand with 15 Joints M E S R O B 2 0 1 - - PowerPoint PPT Presentation

M E S R O B 2 0 1 5 A U T H O R S : E . M A T H E S O N , Y . A O U S T I N , E . L E C A R P E N T I E R , A . L E O N , J . P E R R I N T H A N K S T O B E N J A M I N G A U T I E R F O R T H E V I D E O .

Anthromorphic Underactuated Hand with 15 Joints

Matheson et al, 8-10 June MESROB 2015

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CONTENTS

 Introduction  Underactuated Systems  Mechanical Model  Simulation  Conclusion

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INTRODUCTION

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 What are prosthetic devices?

 Replaces missing or injured part of the body

 Why do we need them?

 Perform everyday tasks, regain independence  Ability to work, ability to interact socially

 What are the main challenges involved?

 Replicating human motion  Producing natural control – the human body is a vastly

superior system to any we have in robotics

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INTRODUCTION

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 Development of a prosthetic hand

 Similar to human hand size  Ability to grasp different objects – natural conformity  Simplicity of movements for control

 Challenges

 Complexity of a human hand: 20

DOF in the fingers and thumb alone, 19 muscles, 17 joints, 20 bones

 Potential Solution

 Underactuated mechanism: less

actuators than independent joint variables

Picture courtesy of AIST [11]

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INTRODUCTION

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 IRCCyN Hand

 4 fingers, 1 thumb  15 DOF  6 Motors  Flexion and extension motion only

for fingers and thumb

 Supination and Pronation for the

thumb

 Design and construction by

Alpes Instruments, Grenoble

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UNDERACTUATED SYSTEMS

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 Performance Metrics: Grasping Ability

 Determined by the mechanical design: however, the design

process is not well understood for underactuation

 Control during grasping is limited – a simulation is necessary

to understand the behaviour during grasping

 Grasp-stability metric: Contact Forces

 Positiveness: All contact forces must be positive (compressive)  Distribution: Isotropic force applications – each contact point

applies an equal force

 Magnitude: Different criteria such as minimum or maximum

allowable contact force in any configuration

 Resultant Direction: Designing the contact force to be in a

specific direction e.g. Toward the palm

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UNDERACTUATED SYSTEMS

 Mechanical methods of achieving underactuation

 Differential mechanisms  Self locking mechanisms  Compliant mechanisms

 Benefits

 Intrinsically conform to the shape of the object  Less actuators = lower cost, mass and profile  Robust to collisions

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MODEL: Transmission System

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 Cable / Pulley (flexion) and spring (extension)

transmission system

 Pulleys are free to rotate – introducing compliance

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MODEL: Joint Structure

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 Finger:

Thumb: Hand:

 3 revolute joints 3 revolute joints

15 joints

 Axes all parallel 1 perpendicular, 2 parallel Tree-structure

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Model: Finger Motion

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 Effect of the pulley radii

 Finger radii:  With tuning: [0.004, 0.0035, 0.003]

• [0.001, 0.003, 0.004]

 Thumb radii:  With tuning: [0.004, 0.0025];

[0.001, 0.004]

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MODEL: Workspace

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 The workspace can be increased by solving an

• ptimisation of design parameters (e.g. Pulley radii)

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MODEL: Force Analysis

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 When is an underactuated system stable?

• At static equilibrium.

“A grasp is stable if and only if the finger is in static equilibrium ”

• Birglen et al. (2008) [1]

 The phalanges in contact with an object have a

positive (or zero) corresponding contact forces.

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MODEL: Contact Force

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 i) Ideal Grasping Sequence  ii) Ejection sequence (Birglen et al.[1])

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MODEL: Contact Force (Birglen et al)

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 Analysis of whether an under-actuated finger is able

to generate an external wrench onto a fixed object

 Defines two matrices - completely describe the

relationship between the input torque of the finger actuator and the contact forces on the phalanges

 First we correlate input and output virtual power of

the kinostatic system

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MODEL: Contact Force (Birglen et al)

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MODEL: Contact Force (Birglen et al)

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 Compare input and output virtual work

 τ- input torque vector (actuator and springs)  ω – vector of angular joint velocities  ξi – twist of ith contact point on ith phalanx  ζi – wrench of ith contact point on ith phalanx  Ki – stiffness of spring at joint i

          ∆ − = ∆ − = =

3 3 3 2 2 2

θ τ θ τ τ τ K K

a

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MODEL: Contact Force (Birglen et al)

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 Assumptions

 One contact point on each phalanx  Contact force is in the plane  Only revolute joints present

          =

y i x i i i

v v ω ξ

• ωi – angular vel. of ith phalanx
• vi – linear vel. in x, y at ith

contact point

• fti – tangential force
• fi – normal force
• τi – torque applied by ith

phalanx

          =

i i ti i

f f τ ς

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MODEL: Contact Force (Birglen method)

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 T – depends on transmission mechanism used to

propagate actuation torque to the phalanges

 r1,2 are the radii of pulleys at joints 2 and 3

        − = 1 1

2 1

r r T T

thumb

a

Tω θ =

.

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MODEL: Contact Force (Birglen method)

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 Forces at the contact point

 Here, τ is torque applied at each joint (originally from the

motor and then that from the springs/ geometric setup)

 Note: for the thumb, force is planar (just from

flexion/ extension) rather than spatial (a component from pronation/ supination).

 Singularities in J or T will lead to cases where no contact forces

can be produced.

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SIMULATION

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 Single Point Contact

 Finger , Phalanx 3 Contact

Force: 18.75

 Thumb, Phalanx 3 Contact

Force: 1.9948

 Two Point Contacts

 Finger, Phalanx 1 Contact Force:

35.3254

 Finger, Phalanx 3 Contact Force:

18.75

 Thumb, Phalanx 3 Contact

Force: -6.3827

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SIMULATION

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 Three Point Contact

 Finger, Phalanx 1 Force: 24.1233  Finger, Phalanx 2 Contact Force: 17.5636  Finger, Phalanx 3 Contact Force: 18.75  No contact with the thumb

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SIMULATION

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 Non-homogenous object

 Finger 1, Phalanx 1 Contact Force: 32.88  Finger 1, Phalanx 2 Contact Force: -24.37  Finger 1, Phalanx 3 Contact Force: 18.75  Finger 2, Phalanx 3 Contact Force: 18.75  Finger 3, Phalanx 3 Contact Force: 18.75  Finger 4, Phalanx 1 Contact Force: 34.61  Finger 4, Phalanx 2 Contact Force: -27.09  Finger 4, Phalanx 3 Contact Force: 18.75  No contact with the thumb

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CONCLUSION

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 Underactuated prosthetic hands show valuable

promise as useful and efficient tools for patients in the future

 Simulation of this hand in Matlab shows adequate

contact forces can be generated for cylindrical

• bjects weighing less than 0.5 kg

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Perspectives

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 Control these hand with signal of the nervous central

system.

 An on-line intramuscular EMG (iEMG)

decomposition was proposed with a Markov Model.

 From the excitation signal to the input actuator:

A muscle model. A matrix moment.

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Perspectives