8 robotic grippers
TRANSCRIPT
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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