Sunday, November 7, 2021

What is a robot end effector?

Robot 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 not end effectors but rather part of a 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:
  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 fiber handling).
  3. Astrictive: attractive 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).

Industrial grippers may employ mechanical, suction or magnetic means.

Vacuum cups and electromagnets dominate the automotive field and metal sheet handling.

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, two 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 ultrasonic 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 sufficient to trap and move microparts in a liquid medium (mainly cells).

Laser grippers are known also as laser tweezers.

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, 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.

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