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 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 or the feet of a humanoid robot which are also not end effectors—they 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]

1. Impactive – jaws 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- or electroadhesion).
4. Contigutive – requiring direct contact for adhesion to take place (such as glue, surface tension or freezing).

They are based on different physical effects used to guarantee a stable grasping between a gripper and the object to be grasped.^[2] 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). 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 ultrasonic based grippers, where pressure standing waves 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.

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.

The most known mechanical 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 is force closure.^[3]

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 finger grippers).

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

$W= \mu F n$

where:

$\,W$	is	the force required to grip the object,
$\,\mu$	is	the coeffecient of friction,
$\,n$	is	the number of fingers in the gripper and
$\,F$	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:

$W= \mu F n g$

Here, the value of $\,g$ should not be taken as the acceleration due to gravity. In fact, here $\,g$ stands for multiplication factor. The value of $\,g$ 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.

Examples

The end effector of an assembly line robot would typically be a welding head, or a 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 cutters. The end effector on the 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.

Examples of end effectors
An example of a basic force-closure end effector  A spot welding end effector  A laser welding end effector  A repair and observation end effector in use in space (Canadarm2)  A highly sophisticated attempt at reproducing the human-hand force-closure end effector

See also

• Grapple (tool)
• Tongs
• Shadow Hand

References

1. Monkman, G. J.; Hesse, S.; Steinmann, R.; Schunk, H. (2007). Robot Grippers. Wiley-VCH. p. 62. ISBN 978-3-527-40619-7.
2. Fantoni, G., Santochi, M., Dini, G., Tracht, K., Scholz-Reiter, B., Fleischer, J., Lien, T.K., Seliger, G., Reinhart, G.,Franke, J., Hansen, H.N., Verl, A.,2014, Grasping devices and methods in automated production processes, CIRP Annals - Manufacturing Technology, Volume 63, Issue 2, 2014, Pages 679-701, ISSN 0007-8506, http://dx.doi.org/10.1016/j.cirp.2014.05.006.
3. "Robotics Grasping and Force closure" (PDF). pdf. FU Berlin. Retrieved 2014-03-20.

• Koren, Y. (1985). Robotics for engineers. McGraw-Hill. ISBN 0-07-035399-9