Robot end effector: Difference between revisions

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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 not end effectors but rather part of a robot's mobility.

End effectors may consist of a gripper or a tool. bb

When referring to robotic prehension there are four general categories of robot grippers:<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>

== Grippers ==

=== Categories ===

When referring to robotic prehension there are four general categories of robot grippers:<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>

# 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 fiber 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).

These categories describe the physical effects used to achieve a stable grasp 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>

Industrial grippers may employ mechanical, suction, or magnetic means. Vacuum cups and electromagnets dominate the automotive field and metal sheet handling. [[Bernoulli grip|Bernoulli grippers]] exploit the airflow between the gripper and the part, in which a lifting force brings the gripper and part close each other (using [[Bernoulli's principle]]). Bernoulli grippers are a type of contactless grippers; 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 in the textile and leather industries.

Other principles are less used at the macro scale (part size >5mm), but in the last ten years, have demonstrated interesting applications in micro-handling. Other adopted principles include: 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 being contactless-grasping principles.

Electrostatic grippers use a charge-difference between gripper and part ([[electrostatic force]]) often activated by the gripper itself, while van der Waals grippers are based on the low force (still electrostatic) of 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 a part. Cryogenic grippers freeze a small amount of liquid, with the resulting ice supplying 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]] 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 sufficient to trap and move microparts in a liquid medium (mainly cells). Laser grippers are known also as [[laser tweezers]].

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

=== Gripper mechanism ===

The end effectors that can be used as tools serve various purposes, including spot-welding in an assembly, spray-painting where uniformity of painting is necessary, and other purposes where the working conditions are dangerous for human beings. Surgical robots have end effectors that are specifically manufactured for the purpose.

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. -->

==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. -->

Generally, the gripping mechanism is done by the grippers or mechanical fingers. Two-finger grippers tend to be used for industrial robots performing specific tasks in less-complex applications.{{citation needed|date=March 2014}} The fingers are replaceable.{{citation needed|date=March 2014}}

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The shape of the fingers' gripping surface can be chosen according to the shape of the objects 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. For a square shape, the surface can be a plane.

=== ForceLevels requiredof to grip an objectforce ===

Though there are numerous forces acting over the body that has been lifted by a robotic arm, the main force 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 by 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>

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

| &nbsp;is&nbsp;

| the coeffecientcoefficient of friction and

|-

| <math>\,n</math>

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| the number of fingers in the gripper.

|}

</dd></dl>

A more complete equation would account for the direction of movement. For example, when the body is moved upwards, against 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 journal |title=Grasp planning to maximize task coverage |journal=The International Journal of Robotics Research|volume=34|issue=9|pages=1195–1210|doi=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, including spot-welding in an assembly, spray-painting where uniformity of painting is necessary, and other purposes where the working conditions are dangerous for human beings. Surgical robots have end effectors that are specifically manufactured for the purpose.

==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 other tool used in surgery. Other possible end effectors might be machine tools such as 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}}

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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 ==

* [[Grapple (tool)]]

* [[Prehensility]]

* [[Tongs]]

* [[Shadow Hand]]