10 robotic gripper control

7/29/2019 10 Robotic Gripper Control

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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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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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DoF = 3 Dof = 4

Point contact with friction Linear contact without friction


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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

10 robotic gripper control

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