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{{short description|Device at the end of a robot arm}}

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In robotics, anAn '''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 [[manipulator (device)|manipulators]], the end effector means the last link (or end) of the robot. At this endpoint, the [[tool]]stools are attached. In a wider sense, 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]]<ref name=":0" /> or the feet of a [[humanoid robot]], which are also not end effectors—theyeffectors arebut rather part of thea 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:<ref>{{cite book |last1=Monkman |first1=G. J. |last2=Hesse |first2=S. |last3=Steinmann |first3=R. |last4=Schunk |first4=H. |title=Robot Grippers |publisher=Wiley-VCH |year=2007 |isbn=978-3-527-40619-7 |page=62}}</ref>

# Impactive – jaws or claws which physically grasp by direct impact upon the object.

# Ingressive – pins, needles or hackles which physically penetrate the surface of the object (used in textile, carbon and glass fibre handling).

# Astrictive – attractive forces applied to the objects surface (whether by vacuum, magneto- or [[electroadhesion]]).

# Contigutive – requiring direct contact for adhesion to take place (such as glue, [[surface tension]] or freezing).

== Grippers ==

They are based on different physical effects used to guarantee a stable grasping between a gripper and the object to be grasped.<ref>{{cite journal | last1 = Fantoni | first1 = G. | last2 = Santochi | first2 = M. | last3 = Dini | first3 = G. | last4 = Tracht | first4 = K. | last5 = Scholz-Reiter | first5 = B. | last6 = Fleischer | first6 = J. | last7 = Lien | first7 = T.K. | last8 = Seliger | first8 = G. | last9 = Reinhart | first9 = G. | last10 = Franke | first10 = J. | last11 = Hansen | first11 = H.N. | last12 = Verl | first12 = A. | year = 2014 | title = Grasping devices and methods in automated production processes | url = | journal = CIRP Annals - Manufacturing Technology | volume = 63 | issue = 2| pages = 679–701 | doi = 10.1016/j.cirp.2014.05.006 }}</ref>

=== Categories ===

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 grip|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]]). Bernoulli grippers are a type of contactless grippers, namely the object remains confined in the force field generated by the gripper without coming into direct contact with it. Bernoulli grippers have been adopted in photovoltaic cell handling, [[silicon wafer]] handling, and also in the textile and leather industries.

Other principles are less used at the macro scale (part size >5mm), but in 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 between the gripper and the part to center, align and grasp the part, cryogenic grippers freeze a small amount of liquid and the resulting ice guarantees the necessary force to lift and handle the object (this principle is used also in food handling and in textile grasping). Even more complex are [[ultrasound|ultrasonic]] based grippers, where pressure [[standing wave]]s are used to lift up a part 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 (mainly cells). The laser gripper are known also as [[laser tweezers]].

When referring to robotic prehension there are four general categories of robot grippers, these are:<ref name=":0">{{cite book |last1=Monkman |first1=G. J. |last2=Hesse |first2=S. |last3=Steinmann |first3=R. |last4=Schunk |first4=H. |title=Robot Grippers |publisher=Wiley-VCH |year=2007 |isbn=978-3-527-40619-7 |page=62}}</ref>

A particular category of friction/jaw gripper are the needle grippers: they are called intrusive grippers and exploits both friction and form closure as standard mechanical grippers.

# Impactive –: jaws or claws which physically grasp by direct impact upon the object.

# Ingressive –: pins, needles or hackles which physically penetrate the surface of the object (used in textile, carbon, and glass fibrefiber handling).

# Astrictive –: attractive forces applied to the objectsobject's surface (whether by vacuum, magneto-, or [[electroadhesion]]).

# Contigutive –: requiring direct contact for adhesion to take place (such as glue, [[surface tension]], or freezing).

TheyThese arecategories baseddescribe on differentthe physical effects used to guaranteeachieve a stable graspinggrasp between a gripper and the object to be grasped.<ref>{{cite journal | last1 = Fantoni | first1 = G. | last2 = Santochi | first2 = M. | last3 = Dini | first3 = G. | last4 = Tracht | first4 = K. | last5 = Scholz-Reiter | first5 = B. | last6 = Fleischer | first6 = J. | last7 = Lien | first7 = T.K. | last8 = Seliger | first8 = G. | last9 = Reinhart | first9 = G. | last10 = Franke | first10 = J. | last11 = Hansen | first11 = H.N. | last12 = Verl | first12 = A. | year = 2014 | title = Grasping devices and methods in automated production processes | url = http://orbit.dtu.dk/en/publications/grasping-devices-and-methods-in-automated-production-processes(ec5df835-f404-40c9-afd3-c2fb2ea6f4aa).html| journal = CIRP Annals - Manufacturing Technology | volume = 63 | issue = 2| pages = 679–701 | doi = 10.1016/j.cirp.2014.05.006 | url-access = subscription }}</ref>

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

Industrial grippers canmay beemploy mechanical, the most diffused in industrysuction, but also based on suction or on the magnetic forcemeans. Vacuum cups and electromagnets dominate the automotive field and in particular metal sheet handling. [[Bernoulli grip|Bernoulli grippers]] exploit the airflow between the gripper and the part, thatin causeswhich a lifting force which brings the gripper and part close each other (i.e. theusing [[Bernoulli's principle]]). Bernoulli grippers are a type of contactless grippers, namely; the object remains confined in the force field generated by the gripper without coming into direct contact with it. Bernoulli grippers have been adopted in photovoltaic cell handling, [[silicon wafer]] handling, and also in the textile and leather industries.

Other principles are less used at the macro scale (part size >5mm), but in the last ten years, theyhave demonstrated interesting applications in micro-handling. Some of them are ready of spreading out their original field. The otherOther adopted principles areinclude: Electrostatic grippers and van der Waals grippers based on electrostatic charges (i.e. [[van der Waals' force]]),; capillary grippers and; cryogenic grippers, based on a liquid medium, and; ultrasonic grippers; and laser grippers, the latter two contactlessbeing contactless-grasping principles.

Electrostatic grippers areuse based ona 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 theof atomic attraction between the molecules of the gripper and those of the object.

Capillary grippers use the surface tension of a liquid meniscus between the gripper and the part to center, align and grasp thea part,. cryogenicCryogenic grippers freeze a small amount of liquid, andwith the resulting ice guaranteessupplying the necessary force to lift and handle the object (this principle is used also in food handling and in textile grasping). Even more complex are [[ultrasound|ultrasonic]] based grippers, where pressure [[standing wave]]s are used to lift up a part 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 ablesufficient to trap and move microparts in a liquid medium (mainly cells). TheLaser laser grippergrippers are known also as [[laser tweezers]].

A particular category of friction/jaw grippergrippers areis thethat of needle grippers:. theyThese are called intrusive grippers, and exploitsexploiting both friction and form -closure as standard mechanical grippers.

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. They are very helpful according to many sources

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

==Gripper Mechanism==

A common form of robotic grasping is [[robotic force closure|force closure]].<ref name=fub20140320>

{{cite web |title=Robotics Grasping and Force closure |url=http://ipvs.informatik.uni-stuttgart.de/mlr/marc/teaching/11-Robotics/11-grasping.pdf |work=pdf |publisher=FU Berlin |accessdate=2014-03-20 }}{{dead link|date=April 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><!-- there appears to be no WP article on force closure so leaving a link to source the concept. -->

=== Gripper Mechanismmechanism ===

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.{{citation needed|date=March 2014}}

A common form of robotic grasping is [[robotic force closure|force closure]].<ref name="fub20140320">{{cite book | last1=Lynch | first1=Kevin M. | last2=Park | first2=Frank C. | title=Modern robotics: Mechanics, planning, and control | date=2017-05-25 | publisher=Cambridge University Press | isbn=978-1-107-15630-2 | oclc=983881868}}</ref><!-- there appears to be no WP article on force closure so leaving a link to source the concept. -->

Generally, the gripping mechanism is done by the grippers or mechanical fingers. Generally only twoTwo-finger grippers aretend to be used for industrial robots asperforming theyspecific tendtasks toin beless-complex builtapplications.{{citation forneeded|date=March specific2014}} tasksThe andfingers canare therefore be less complexreplaceable.{{citation needed|date=March 2014}}

The fingers are also replaceable whether or not the gripper itself is replaced.{{citation needed|date=March 2014}}

There are two mechanisms of gripping the object in between the fingers (for the sake of simplicity, the following explanations consider only two finger grippers).

Two types of mechanisms used in two-finger gripping account for the shape of the surface to be gripped, and the force required to grip the object.

=== 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.

The shape of the fingers' 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 forFor 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

=== Levels of force ===

:<math>F= \frac{ma}{\mu n}</math>

Though there are numerous forces acting over the body that has been lifted by thea 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 toby frequent movement of the object. To find out the force required to grip the object, the following formula is used

:<math display="block">F= \frac{ma}{\mu n}</math>

where:

{|cellspacing="0" cellpadding="0" style="margin-left:1.5em;"

<dl><dd>

{|cellspacing="0" cellpadding="0"

|-

| <math>\,F</math>

Line 59 ⟶ 58:

| <math>\,\mu</math>

| &nbsp;is&nbsp;

| the coeffecientcoefficient of friction and

|-

| <math>\,n</math>

Line 65 ⟶ 64:

| the number of fingers in the gripper.

|}

</dd></dl>

ButA themore abovecomplete equation iswould incomplete.account Thefor 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 that towards the gravitational force. Hence, another term is introduced and the formula becomes:

:<math display="block">F= \frac{m(a+g)}{\mu n}</math>

Here, the value of <math>\,g</math> should be taken as the acceleration due to gravity and <math>\,a</math> the acceleration due to movement.

For many physically interactive manipulation tasks, such as writing and handling a screwdriver, a task-related grasp criterion can be applied in order to choose grasps that are most appropriate to meeting specific task requirements. Several task-oriented grasp quality metrics<ref>{{cite webjournal |title=Grasp planning to maximize task coverage |urljournal=http://journals.sagepub.com/The International Journal of Robotics Research|volume=34|issue=9|pages=1195–1210|doi/abs/=10.1177/0278364915583880|year=2015 |last1=Lin |first1=Yun |last2=Sun|first2=Yu|s2cid=31283744 }}</ref> were proposed to guide the selection of a good grasp that would satisfy the task requirements.

== Tools ==

The end effectors that can be used as tools serve various purposes, such asincluding 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. They are very helpful according to many sources

The end effector of an assembly -line robot would typically be a [[Welding|welding head]], or a [[Spray painting|paint spray gun]]. A [[surgical robot]]'s end effector could be a [[scalpel]] or othersother toolstool used in surgery. Other possible end effectors aremight be machine tools, likesuch as a [[drill]] or [[milling cutter]]s. The end effector on the [[Canadarm|space shuttle’sshuttle's robotic arm]] uses a pattern of wires which close like the [[aperture]] of a camera around a handle or other grasping point.{{Citation needed|date=July 2013}}

==Examples==

The end effector of an assembly line robot would typically be a [[Welding|welding head]], or a [[Spray painting|paint spray gun]]. A [[surgical robot]]'s end effector could be a [[scalpel]] or others tools used in surgery. Other possible end effectors are machine tools, like a [[drill]] or [[milling cutter]]s. The end effector on the [[Canadarm|space shuttle’s robotic arm]] uses a pattern of wires which close like the [[aperture]] of a camera around a handle or other grasping point.{{Citation needed|date=July 2013}}

{{gallery

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|width=160

|height=140

|lines=4

|File:Endeffector.png|An example of a basic [[robotic force closure|force-closure]] end effector

|File:Robotworx-spot-welding-robot.jpg|A spot welding end effector

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== See also ==

{{Portal|Robotics}}

* [[Grapple (tool)]]

* [[Prehensility]]

* [[Tongs]]

* [[Shadow Hand]]

* [http://rhgm.org IEEE RAS TC on Robotic Hands, Grasping and Manipulation]

* [http://journals.sagepub.com/doi/abs/10.1177/0278364915583880 Robotic grasping for manipulation tasks]

==References==

{{Reflist}}

* Koren, Y. (1985). Robotics for engineers. McGraw-Hill. {{ISBN|0-07-035399-9}}

[[Category:Robotic manipulation]]