8 Robotic Grippers

7/29/2019 8 Robotic Grippers

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Robotic Systems(8)

Dr Richard Crowder

School of Electronics and Computer Science

Last revised September 2011


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Robotic end effectors

The end effector is the element of the robot that interfaceswith the environment, and can either be a gripper or a tool.

In a wider sense, an end effector can be seen as the part of a

robot that interacts with the environment. Using this moregeneral definition, the wheels of a mobile robot or the feetof a humanoid robot are also end effectors.

Introduction to the Human Hand and Robotic Grippers


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Clarification

Grippers are subsystems of handling mechanisms whichprovide temporary contact with the object to be grasped.They ensure the position and orientation when carrying andmating the object to the handling equipment.


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Gripper Classification

Impactive jaws physically grasp by direct contact withthe object.

Ingressive pins, needles or hackles which physically

penetrate the surface of the object (used in textile, carbonand glass fibre handling).

Astrictive suction or other forces applied to the objectssurface (vacuum, magneto or electroadhesion).

Contigutive requiring direct contact for adhesion to takeplace (such as glue, surface tension or freezing)


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Human Hand

Pinch

Power


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Classification of human hand grips

Cylindrical hollow Tip Hook

Three fingered Palm Tong


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Grasp Task planning

The choice of a grasp for any given object should reflect therequirements of the task to be performed. To be able to move ormanipulate an object, the grasp must be robust to external

disturbances, and must be designed so that appropriate forces can beapplied to the object without excessive effort.

from http://graphics.cs.cmu.edu/nsp/projects/hands/hands.html


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Design Constraints

One goal of grasp choice for robotics is to choose contact points thatguarantee properties such as force- or form-closure.

Many efficient algorithms have been developed to address thisproblem, but most of these algorithms focus on grasps having a

minimal number of contact points. Increasing the number of contacts can dramatically improve the

quality and flexibility of grasps that are constructed.

Force production capabilities of the human hand can be computed ifmaximum muscle forces and tendon moment arms are known.

Although the tendon structure of the hand is complex, as suggested inthe figure below, estimates for the required information can be readilyobtained from the biomechanics literature


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Fingers

1: Number of Tools 2: Grip possibilities


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Number of fingers Zero should be understood as movement of the arm joints only.

Two: widely used in industrial applications, very simple, but only effective with adefined object.

Three: can give very flexible operation.

Four, Five: only required if the gripper is required to be anthropomorphic. Note thelittle finger does not contribute significantly to a hands gripping capability.

The addition of the fifth finger makes negligible contribution to industrial applications

About 90% of the grips involved in industrial applications can be realized with threefinger hand.

All human fingers do not possess the same strength. The middle finger is the strongest

one and the little finger the weakest. The strength potential is distributed as follows:index finger 21%, middle finger 34%, ring finger 27%, and little finger 18%.


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Typical Robotic Hands/Grippers


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Finger Actuation

Pneumatics

Hydraulics

Electric Motor

Tendon

Solid linkages


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Stanford/JPL Hand

Salisbury: Kinematic and Force Analysis of Articulated Hands

in Robot Hands and the Mechanics of Manipulation, MIT 1985


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Tendon Driven Finger - Bologna


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Comments

Tendon drives, while simple to design are difficult to commission andcan be complex.

Concept ofUnderactuation

More DoF than actuators

An underactuated robot is defined as a manipulator with oneor more unactuated joints.

Underactuated or self-adaptive fingers use passive elements(the most common of which are springs) in the design of theirunactuated joints.

Delegate the control of the finger from the electronic board to themechanical structure


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Tendon driven Finger


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Connected differential mechanism

FROM FLAPPING WINGS TO UNDERACTUATED FINGERS AND BEYOND, A BROAD LOOK TO SELF-ADAPTIVE MECHANISMS Birglen,

Proceedings of the 1st International Workshop on Underactuated Grasping, UG2010 August 19, 2010, Montreal, Quebec, Canada

Video
http://d/Teaching/Robotic%20Systems/131_Larouche_Lionel.avihttp://d/Teaching/Robotic%20Systems/131_Larouche_Lionel.avi


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Southamptons Whole Arm Manipulator


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Finger Mechanism

Tip

Middle section

Lower section

G

F

CE

D

Link 2

I

B

J

Link 1

K

A

Bell crank

Tip

Middle sectionLower section

Link 1

Link 2A

SliderEquiliser bar

Link B

Link ADrive

output

Equalizing bar

mechanism

Basic finger mechanism


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15 September 1999 CLAWAR 20

3 DOF finger

Curling, where displacement of control link 1, curls the twoupper finger segments about joint C.

Bending, where displacement of control link 2, bends all the

three finger segments about joint A.

Rotation, of the whole module about its central axis


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Ingressive Gripper

Designed to handle nonrigid material

textiles

Felt

carpets


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Ingressive Grippers

Compressed air


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Astrictive Grippers

Vacuum system

Sheet metal

GlassWood

Plastics

Compact discs


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Vacuum Gripper

Fth

Typical calculation

Fth = (m(g+a))s

Fth is the theoretical holding force requiredm is the load mass

g is the acceleration due to gravity

a is the system acceleration

s is the safety factor (>1.5)

Suction force is a function of (a) the pad rating and (b) The number

of pads used in an application. Also the suction force is not applied

instantaneously as the vacuum system has to evacuate all the pads

and associated pipework


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Contigutive Gripper

Mainly used microassembly applications due to the small force available

Documents

8 Robotic Grippers