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Robotic Systems(10)
Dr Richard Crowder
School of Electronics and Computer Science
Revised: 18 November 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.
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Generalised Flow of Assembly
Reach
Assemble
Grip
Release Transport
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Frames of Reference
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Holding a Spherical Object
1. Pure enclosing without clamping
2. Partial form fit combined with
clamping force3. Pure force closure
4. Holding with vacuum
5. Using magnetic field
6. Adhesive media
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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.
The term gripper is also used in cases where no actualgrasping, but rather holding of the object as e.g. in vacuumsuction where the retention force can act on a point, line or
surface.
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Function
Temporary maintenance of a definite position and orientation of theworkpiece relative to the gripper and the handling equipment.
Restraint against static (weight), dynamic (motion, acceleration ordeceleration) or process specific forces and moments.
Determination and change of position and orientation of the objectrelative to the handling equipment through the use of the wrist axes.
Specific technical operations performed with, or in conjunction with,the gripper.
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Comparison with Human Hands
Articulation
Robotic systems have fewer DoF
Dexterity
In general dexterity is lower Sensing
Fewer sensing capabilities, in many cases limited to touch
Handling Capability
Wide range of handling capabilities (weight, size) Environmental Constraints
Far more robust (temperature, pressure, corrosive fluids)
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Attributes Dexterity
Balance between number of figures and task requirement
Hold sites
Enough to ensure force closure exists
Sensing
To detect object is being held correctly full dexterity
Compliance
To ensure force sensors operate correctly, and does not degrade robotpositioning
Compatibility
Small as possible the robots handling capability will include the gripper
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Theory and Control of Gripping
An unconstrained body has six degrees of freedom, if thebody is brought into contact with a rigid body, the motionwill be constrained. The manner of the constraint dependson the type of contact.
Eight types on contact are possible.
However we need to develop tools to study griping theDH approach is point based, we need a vector approach Screw Geometry.
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Screw Geometry
Consider a screw thread
Pitch
TwistForce: A zero pitch wrench is a pure moment,
Infinite pitch wrench is a linear force
Motion: A zero pitch twist is a pure rotation,
Infinite pitch twist is a linear motion
A twist is a motion caused by a wrench
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Wrench
For a rigid body to be in equilibrium the vector sum ofallforces acting on it must be zero.
Consider a force vector F and a position vector, the moment
M is the vector product (about the origin of r) :
A wrench is defined as:
FrM
F
Fr
F
MW
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..
For a gripper in equilibrium,
Pitch (p) is defined as
0W
FFFM
:
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Types of Contact - 1
DoF = 0 Dof = 1
Planar contact with friction
(aka glue)
Line contact with friction
(aka revolute joint)
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Types of Contact - 2
DoF = 2 Dof = 3
Soft Finger contact area is large
enough motion around the contactnormal, i.e. rolling is ok, but slidingis not
Planar without friction
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Types of Contact - 3
DoF = 3 Dof = 4
Point contact with friction Linear contact without friction
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Types of Contact - 4
DoF = 5 Dof = 6
Point Contact with 0ut friction Unconstrained motion
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Number of Fingers
If we have a finger without friction, only one normal forceper finger can be applied
Hence either no fewer than 6 frictionless contacts arerequired, or fewer finger with friction are required however this may not be force or form closed.
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Form and Force Closures
Widely discussed in robotic literature
A planar body requires at FOUR frictionless contacts to be fullyrestrained, or SEVEN in a 3D case this is termed form closure.
Form closure can be considered to be a complete restraint
Force closure exists when it is in equilibrium with any arbitrarywrench
A grasp is considered to be form closed if it is force closed with
frictionless contacts
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Grip analysis
Complex for multifingered devices.
Requires a clear understanding of both the object and thefingers
Does the object only have contact at the finger tips.
Is the object in contact with the palm or fingersegments.
Can the fingers be back driven.
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Form-Closure with underactuated hands
Form-closure depends on assumption that contact pointsare fixed in space.
Not-possible with an underactuated hand as the positions
of individual finger segments are not known.
Underactuated hands can have unstable grasps i.eejection of the object form the grip.
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Simple Gripper two fingered jaw
WFF 21
0)( 21 FlFxl
F1
F2
W
l
x
Balance forces:
Balance moments
On solving:
x
xlWF
x
WlF
)(2
1
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Multi-fingered Gripper
X
Y
Z
F1
F2
F3
Unit Cube
F1 = 1
F2 = 2
F3 = 1
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Force F1.
X
Y
Z
F1
0
1
0
0
0
1
0
0
0
0
0
0
0
0
1
0
0
1
1
1
1
1
1
1111
F
F
F
MW
F
MFfr
0
0
0
0
1
1
01
1111
F
MFfr
Note this solution is also possible
and could be considered
more geometrically correct
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other forces.
0
2
0
2
00
0
0
0
0
1
2
222
W
Ffr
X
Y
Z
F2
F3
1
0
0
0
1
1
00
0
11
3
3
33
W
F
fr
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hence
1
1
0
2
1
2
W
1
1
0
2
1
2
stabilityW
1. The mass of the object not included2. The wench is not zero hence the cube will
move
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Pitch
2:1
1
1
0
1
1
0
:
1
1
0
2
1
2
:
FFFM
The unit vector of the wrench is that of the force, namely
1
10
2
1F
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Point of action
Noting that r is a position vector of any point of the line ofaction of the force, we can generalise the moment equationto:
Hence
rpFrM
23
23
2
1
1
0
1
1
0
1
1
0
2
1
2
r
pr
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.final solution
The last equation cannot be uniquely solved, unless weimpose the condition F.r = 0.
Hence
1
1
23
r
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Applications of a gripper/hand
Explore object
This is termed haptics a significant research field
Restraining an object
Fixturing
Manipulating an object relative to a hands through the useof the fingers
Dexterous Manipulation
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Summary
Considered the basic structure of a robotic hand.
Grasp configurations
Introduction to the control and determination of forces
Considerable body of literature:
Robotic grasping and contact: A review (2000)
Antonio Bicchi, Vijay Kumar . Proceedings of IEEEInternational Conference on Robotics and Automation
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Other types of end effectors
Where robots are not required to manipulate the object, but act directlyon the workpiece using a tool in place of the end effector. Applicationsinclude:
Welding
Spray painting
Material applications
Assembly Operations
Palletizing and Material Handling
Dispensing Operations
Laboratory Applications
Water Jet Cutting
.
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Welding
Arc, MIG, TIG, Laser and SpotWelding
Increasing speed, quality andthroughput.
High quality welding is easilyrepeatable with robots.
Safer and more cost-effective
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Spray Painting
More correctly termed coating
Contour following and evencoating.
Safety
Liquid and power paints
No problems with repetition
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Assemble
Wide range of applications
Ranging from PCBs to cars
-
This application is the use ofa robot to place a windscreen