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UNIT III - Robot Effectors

Robot end effector

In robotics, an end effector is the device at the end of a robotic arm, designed to

interact with the environment. The exact nature of this device depends on the application of

the robot.

In the strict definition, which originates from serial robotic manipulators, the end effector

means the last link (or end) of the robot. At this endpoint the tools are attached. In a widersense, an end effector can be seen as the part of a robot that interacts with the work

environment. This does not refer to the wheels of a mobile robot or the feet of a humanoidrobot which are also not end effectorsthey are part of the robot's mobility.

End effectors may consist of a gripper or a tool. When referring to robotic prehension there

are four general categories of robot grippers, these are:

1. Impactivejaws or claws which physically grasp by direct impact upon the object.2.

Ingressive pins, needles or hackles which physically penetrate the surface of the

object (used in textile, carbon and glass fibre handling).3.

Astrictive suction forces applied to the objects surface (whether by vacuum,

magneto- orelectroadhesion).

4. Contigutiverequiring direct contact for adhesion to take place (such as glue, surfacetension or freezing).

They are based on different physical effects used to guarantee a stable grasping

between a gripper and the object to be grasped. Industrial grippers can be mechanical, the

most diffused in industry, but also based on suction or on the magnetic force. Vacuum cups

and electromagnets dominate the automotive field and in particular metal sheet handling.Bernoulli grippers exploit the airflow between the gripper and the part that causes a lifting

force which brings the gripper and part close each other (i.e. the Bernoulli's principle).Bernulli grippers are contactless gripper, namely the object remains confined in the force

filed generated by the gripper without coming into direct contact with it. Bernoulli gripper are

adopted in Photovoltaic cell handling in silicon wafer handling but also in textile or leather

industry.

Other principles are less used at the macro scale (part size >5mm), butin the last ten years they demonstrated interesting applications in micro-handling. Some of

them are ready of spreading out their original field. The other adopted principles are:

Electrostatic grippers and van der Waals grippers based on electrostatic charges (i.e.van der

Waals' force), capillary grippers and cryogenic grippers, based on liquid medium, and

ultrasonic grippers and laser grippers, two contactless grasping principles. Electrostatic

grippers are based on charge difference between the gripper and the part (i.e. electrostatic

force)often activated by the gripper itself, while van der Waals grippers are based on the low

force (still electrostatic) due to the atomic attraction between the molecules of the gripper and

those of the object. Capillary grippers use the surface tension of a liquid meniscus betweenthe gripper and the part to center, align and grasp the part, cryogenic grippers freeze a small

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amount of liquid and the resulting ice guarantees the necessary force to lift and handle theobject (this principle is used also in food handling and in textile grasping). Even more

complex are ultrasonic based grippers, where pressure standing waves are used to lift up apart and trap it at a certain level (example of levitation are both at the micro level, in screw

and gasket handling, and at the macro scale, in solar cell or silicon wafer handling), and laser

source that produces a pressure able to trap and move microparts in a liquid medium (mainlycells). The laser gripper are known also aslaser tweezers.

A particular category of friction/jaw gripper is the needle grippers: they are called intrusive

grippers and exploits both friction and form closure as standard mechanical grippers.

The most known mecanical gripper can be of two, three or even five fingers.

The end effectors that can be used as tools serve various purposes, such as spot welding in an

assembly, spray painting where uniformity of painting is necessary, and for other purposes

where the working conditions are dangerous for human beings. Surgical robots have end

effectors that are specifically manufactured for the purpose.

Mechanism of gripping

A common form of robotic grasping isforce closure.

Generally, the gripping mechanism is done by the grippers or mechanical fingers. Generally

only two-finger grippers are used for industrial robots as they tend to be built for specific

tasks and can therefore be less complex. The fingers are also replaceable whether or not the

gripper itself is replaced. There are two mechanisms of gripping the object in between the

fingers (for the sake of simplicity, the following explanations consider only two fingergrippers).

Shape of the gripping surface

The shape of the gripping surface of the fingers can be chosen according to the shape

of the objects that are to be manipulated. For example, if a robot is designed to lift a round

object, the gripper surface shape can be a concave impression of it to make the grip efficient,

or for a square shape the surface can be a plane.

Force required to grip the object

Though there are numerous forces acting over the body that has been lifted by the

robotic arm, the main force acting there is the frictional force. The gripping surface can be

made of a soft material with high coefficient of friction so that the surface of the object is not

damaged. The robotic gripper must withstand not only the weight of the object but also

acceleration and the motion that is caused due to frequent movement of the object. To find

out the force required to grip the object, the following formula is used

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where:

is the force required to grip the object,

is the coeffecient of friction,

is the number of fingers in the gripper and

is the weight of the object.

But the above equation is incomplete. The direction of the movement also plays an important

role over the gripping of the object. For example, when the body is moved upwards, against

the gravitational force, the force required will be more than towards the gravitational force.

Hence, another term is introduced and the formula becomes:

Here, the value of should not be taken as the acceleration due to gravity. In fact, here

stands for multiplication factor. The value of ranges from 1 to 3. When the body is moved

in the horizontal direction then the value is taken to be 2, when moved against the

gravitational force then 3 and along the gravitational force, i.e., downwards, 1.

TYPES OF END EFFECTORS

The End Effector is the last part of the robot furthest from the base which interacts

with the environment. In the human arm comparison, the End Effector would be the

equivalent of the hand. The robot's task is to deliver the End Effector to the desired location

so that the End Effector may accomplish its task by either being a tool itself or by holding a

tool. There are many different types of End Effector available which depend wholly on each

application.

1. Grippers

Used to grasp and hold objects. e.g. loading and unloading, etc

2. Tools

End effectors designed to perform work on the part rather that to merely grasp

it.

e.g. spot welding, spray painting, etc.

Below are some types of End Effectors Loop Technology can utilise to provide your robotic

solutions:-

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VisionFor applications that require a vision system for inspection or guidance, the End

Effector will be or contain a camera. Many other End Effectors could incorporate acamera to give the operators a view of the robot at all times.

Welding Guns

Robotic welders have End Effectors available for each type of welding(i.e., MIG, TIG, Laser, Resistance, Gas etc). Each End Effector will have the welding

torch and method of applying the filler material where required.

GrippersThere are a few types of grippers and some can be custom designed if the object is a

particular shape or structure.

Here are the main types of grippers:-

o General Purpose (2 Jaw) Gripperfor applications requiring a high grip

force to moment ratio and positive pick and place.

o General Purpose (3 Jaw) Gripperfor applications requiring a gripper that

offers self centring of parts, high grip force to moment ratio and positive pick

and place.o High-Moment Gripperfor applications that apply extreme stress to the

gripper (e.g., applying high acceleration to heavy objects or extremely long,precisely positioned jaws).

o

Long-Stroke Gripperfor applications requiring longer stroke lengths.

o Miniature Gripperfor applications requiring small stroke lengths.

o Collet Gripper- for applications requiring precise picking and placing ofuniform cylindrical parts.

Composite Handling GrippersWe can now offer bespoke composite gripper designs which address

composite handling risks whilst improving manufacturing and quality standards.

Milling Spindles

Robots are carving out a niche in the field of CNC milling whereby their long

reach and comparative low cost provides a very flexible and large volume machining

platform.

Vacuum Cups

These can be used singularly or in multiple to provide a means of picking a part up

without obstructing the sides or bottom, making placement and/or further processes

easier. Vacuum cups can be used on plastic, metal, glass, wood, card and more.

Magnetic

A means of magnetically attaching a product to the End Effector for manipulation.

Cutting ToolDevices used to physically cut material with blades/saw devices or high pressure shotor laser cutting. High temperature tips can also be used to cut material which

automatically seals edges to prevent the material fraying.

Deburring Tool

A cylindrical file type device rotated like a drill and used to remove shards protruding

from an edge or feature in a material after it has been processed.

ScalpelsUsed by surgical robots.

DrillsLike a domestic drill with the advantage that the depth of the hole can be monitored

via the movement of the robot.

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Tool ChangersThis type of End Effector has multiple tools with a common interface so that they can

be changed to perform different processes.

Anti Collision

Devices that detect whether the end effector touches anything in its environment.

These can also be combined with some other types of End Effector functionality. Compliance (Alignment) Devices

Compensate for fixture misalignments, positioning errors, shifting parts, and/or

variances in part tolerances by providing the robot with a little freedom in movement

so placement of parts allows the finding of its best fit position. Applications that could

benefit from using a Compliance Device include machine loading, machine

unloading, assembly and insertion operations.

Screw Drivers / Spanners

Effectors which can place and tighten screws, nuts and bolts. Spray Guns

For the application of paint and other such materials.

Adhesive Applicators

Brushes or sprays adhesive onto surfaces. The tips of these End Effectors are

normally heated to avoid the glue drying in situ.

Sanders / Polishers / FinishersThese will have varying degrees of abrasives to remove surface imperfections or

marks.

MECHANICAL GRIPPERS

Pneumatic grippers are mechanical devices that are really common in automated

manufacturing processes. Combined with a robot they can do many robotic applications such

as: pick-and-place,machine tending orassembly.We will talk about how they work

generally, including the pros and cons.

Usual pneumatic grippers work using a piston to open and close their fingers/jaws. Air is

compressed in the chamber of the piston and, according to the piston size, when it has enoughpressure the air will make the piston move. The simple formula to remember is:

Pressure X Surface = Force

Even if these robot grippers all work with the same principle, they may have different

configurations:

Angular gripper

The piston in this kind of gripper will make the jaws move around a rotating axis in order to

make it close. So it uses a lever to make the fingers move and grasp a part. Here is a picturethat explains it all.

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In blue, you can see how the air makes the piston move. This type of gripper can provide a

fairly large gripping force. However, one gripper can only grip a limited size variation ofparts. Other versions of this gripper can include a third or fourth jaw.

Parallel gripper

The next version of pneumatic end effector that we will talk about is the parallel gripper. In

this configuration the gripping surface of the jaws remains parallel throughout all their

movement. This kind of gripper can also include a third or fourth jaw for gripping unusual

parts.

Pros and cons

The pneumatic end effectors are low cost devices that are simple to use and implement. With

any type of pneumatic gripper, jaws can be design to fit different kinds of pieces. However,

this is also a limitation for this type of device, since it takes different jaws to grip verydifferent pieces. Moreover, using air is another limitation, because as a compressible fluid,

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Gear and Rack Grippers:movement of input due to gear motion which makes connecting linksto go in motion to make gripping action at the output link.

Cam-actuated Grippers:reciprocating motion of the cam imparts motion to the follower, thus

causing fingers to produce a grabbing action. A variety of cam profiles can be employed-

constant velocity, circular arcs, harmonic curves etc.

Screw-driven Grippers: operated by turning screw, in turn giving motion to connecting links

and thus giving griping motion to output. Screw motion can be controlled by motor attached.

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Rope & Pulley Grippers:motor attached to the pulley makes the winding and unwindingmotion of rope in turn it set gripper action into motion via connecting link.

Fig 5: Rope & Pulley Grippers

GRIPPER FORCE ANALYSIS

The fig. shows the linkage mechanism and dimensions of a gripper used to handle a work

part for a machining operation. Suppose it has been determine that the gripper force is to be

25 lb. What is required is to compute the actuating force to deliver this force of 25 lb.

Due to symmetry of the gripper consider only one half of the mechanism.

Moments :-

25(4cos 15) = F sin 45(1.5cos15) + Fcos45(1.5sin15)

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96.6 = F(1.0246+0.2745) =1.2991F

F = 74.4 lb

The actuation force applied to the

plunger to deliver this force of 74.4 lb to eachfinger.

So,

Fa = 2 X 74.4 X cos45

Fa = 105.2 lb

OTHER TYPES OF GRIPPERS

1. Vacuum Cups

2. Magnetic Grippers

3.

Adhesive Grippers

4. Hooks, Scoops and other miscellaneous devices

Vacuum cups, also called suction cups

Requirements on the objects

to be handled are that they be

flat, smooth and clean, to

form satisfactory vacuum.

Gripper made of some elastic

material i.e. rubber, plastic ,

etc.

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Suction cups can be used to grip large flat surfaces. The cups are,

- typically made of soft rubber or plastic

- typically round, or oval shapes

A piston operated vacuum pump (can give a high vacuum), or a venturi valve (simpler) can

be used to generate the vacuum.

The surfaces should be large, smooth, clean.

The force of a suction cup depends on the effective area of the vacuum and the difference

in the vacuum, and air pressures.

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e.g.

Advantages and features of suction cup gripper

Require only one surface of the part for grasping

Applies uniform pressure distribution on the surface of the part Relatively a lightweight gripper

Applicable to a variety of different material.

MAGENETIC GRIPPERS

Magnetic grippers are used extensively on ferrous materials. In general, magnetic

grippers offer the following advantages in robotic handling operations

Variations in part size can be tolerated

Pickup times are very fast They have ability to handle metal parts with holes

Only one surface is required for grippingThe residual magnetism remaining in the work piece may cause problems. Mother potential

disadvantage is the problem of picking up one sheet at a time from a stack. The magnetic

attraction tends to penetrate beyond the top sheet in the stack, resulting in the possibility thatmore than a single sheet will be lifted by the magnet.

Magnetic grippers can use either electromagnets or permanent magnets. Electromagnetic

grippers are easier to control, but require a source of dc power and an appropriate controller.When the part is to be released, the control unit reverses the polarity at a reduced power level

before switching off the electromagnet. This procedure acts to cancel the residual magnetism

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in the work plece ensuring a positive release of the part. The attractive force, P of anelectromagnet is found from

Permanent magnets do not require an external power and hence they can be used in

hazardous and explosive environments, because there is no danger of sparks which mightcause ignition in such environments. When the part is to be released at the end of the

handling cycle, in case of permanent magnet grippers, some means of separating the part

from the magnet must be provided.

Advantages

Pickup very fast

Variation in the part size can be tolerated

They can handle metal part with holes

Require only one surface for gripping

Disadvantages

Include the residual magnetism remaining in the work-piece

Picking up only one sheet from stack

ADHESIVE GRIPPERS

An adhesion gripper is a robot end effector that grasps objects by literally sticking to them. In

its most primitive form, this type of gripper consists of a rod, sphere, or other solid object

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covered with two-sided tape. Velcro can also be used if the object(s) to be grasped arelikewise equipped.

A major asset of the adhesive gripper is the fact that it is simple. As long as the adhesive

keeps its stickiness, it will continue to function without maintenance. However, there are

certain limitations. The most significant is the fact that the adhesive cannot readily bedisabled in order to release the grasp on an object. Some other means, such as devices thatlock the gripped object into place,must be used.

Hooks, Scoops and other miscellaneous devices

TOOLS AS END EFFECTORS

In most of the robot application in which a tool is manipulated, the tool isattached directly to the robot wrist. In these cases the tool is the end effector.

E.g. :-

Spot-welding tools

Arc-welding tools

Spray painting tools

Rotating spindles for operation such as:

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Drilling

Routing

Wire brushing

Grinding

Liquid cement application for assembly

Heating torches

Water jet cutting tool

ROBOT/END EFFECTOR INTERFACE

Important aspect

Physical support during the work cycle

Power to actuate must be supplied through the interface

Control signal to actuate

Feedback signal

High reliability of the interface

Protection against the environment

Overload protection in case of disturbance during the work cycle

Physical support during the work cycle

Achieved by the mechanical connection between the end effector and

robot wrist.

Ideally, three characteristics :-

1. Strength2. Compliance

3.

Overload ProtectionStrength

Its ability to withstand the forces associated with the operation of the end effector.

Compliance

Its refers to the wrist sockets ability to yield elastically when subjected to a force. In effect, it

is opposite of rigidity. Remote Center Compliance (RCC) devices have been designed to

provide high lateral compliance for centering the object relative to the hole.

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Overload Protection

When some unexpected event happens to the end effector such as a part becoming stuck in a

die or a tool caught in a moving conveyor. Whatever the cause, the consequences involve

possible damage to the end effector or maybe even the robot itself.

To eliminate this potential damage

Use Breakaway Feature in the wrist socket.

Sensor to indicate, so somehow take preventive action to

reduce further overloading.

Power and Signal Transmission

Pneumatic

Electric

Hydraulic

Mechanical