End Effector | Robot End Effector | 5+ Types and Important Characteristics

End Effector | Robot End Effector | 5+ Types and Important Characteristics

Robotic Gripper

Image Source: “Grip” (CC BY-NC 2.0) by jev55

The subject of discussion: Robot End Effector and its characteristics

What is Robot End Effector?

End Effector | End Effector definition

A Robot End effector or a Robotic Gripper is a mechanical part attached to the end of the robot arm hardware that is intended for direct interaction of environment and adjacent. The purpose of this mechanical part is subject to the robot’s application in the world. In the case of a serial manipulator, the robotic gripper usually lies in the hardware’s last link. The end effector is also called Gripper, and it is analogous to the hand of a human body.

This is different from the wheels or legs of mobile robots in that the latter is used to facilitate the mobility of the robots only. But robot end effector is application-specific in nature and contains varied designs to accommodate varying purposes for manipulating an object.

Robot End Effector Design

End Effector position

The end effector is generally designed to be attached at the end of the robot manipulator. Hence the term ‘End of Arm Tooling’ is also used in appropriate cases. This facilitates direct contact of the end effector with the environment. Therefore, manipulation of an object takes place through the gripper in accordance with the application of the robot. These are often custom-tailored to fit special processes requirements, other than the ones that are generally used.

Types of End Effectors in robots

Robot End-effector is classified into four general types based on physical effect usage to achieve a stable grasp amongst the gripper and the object to be grabbed.

  1. Impactive Gripper: These are jaws or claws that exhibit physical grasping directly impacting the object.
  2. Ingressive Gripper: These are sharp-point surfaces like pins, needles or hackles which exhibit physical penetration inside the surface of the object. Applications can be seen in textile, carbon and glass fibre handling.
  3. Astrictive Gripper: These grippers apply attractive forces to the surface of the object using a vacuum, magneto- or electroadhesion.
  4. Contigutive Gripper: These grippers require direct contact to exhibit adhesion, such as glue, surface tension or freezing.

End Effector example

Mechanical End Effector | Mechanical Gripper End Effector

end effector
Basic Force Closure End Effector
Image credit :Chojitsa at English Wikipedia, brightness changed by an unknown user, Endeffector, marked as public domain, more details on Wikimedia Commons
End Effector | Robot End Effector | 5+ Types and Important Characteristics
End-Effector
Image credit :Alexgace, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

Robot End Effector gripper | Different types of end effectors

Some grippers are categorized by their principle of operation. A few of them have been briefly discussed below:

Bernoulli Gripper

It applies Bernoulli’s principle by exploiting the airflow between the gripper and the object to be grasped. This generates a lifting force that brings the gripper and the object closer without letting them come into direct contact with each other. Hence, Bernoulli Gripper is a contcatless gripper. The applications of Bernoulli gripper can be seen in photovoltaic cell handling, semiconductor manufacturing industries, and in the textile industry.

Electrostatic Gripper

 It exploits the characteristics of electrostatic charge by utilizing a charge-difference between the gripper and the object. The gripper itself usually activates this charge-difference.

Van der Waals Gripper

Van der Waals gripper exploits the low electrostatic force amongst the gripper and the elemental objects.

Capillary Gripper

Capillary grippers utilize a liquid meniscus’s surface tension between the gripper and the object towards its orientation and grasping.

Cryogenic Gripper

Cryogenic grippers create ice by freezing a small amount of liquid, which is then used to supply the obligatory force to lift and grip the object. The application of cryogenic gripper can be seen in food handling and in textile grasping.

Ultrasonic Gripper

Ultrasonic gripper are complex in nature than the above categories, which exploit pressure standing waves to levitate up a part and enclose it at a certain level. Lifting can be seen both at the micro-level and the macro level. Micro-level levitation is evident in screw and gasket-handling. In contrast, macro-scale lifting can be seen in a photovoltic cell or Si substrate handling and in laser source.

Intrusive Gripper

This a specific category of jaw grippers that utilizes friction force for grasping objects. One example of an intrusive gripper is that of a needle gripper. These are termed intrusive grippers because they exploit both frictions and enclosing characteristic as that of standard mechanical grippers.

The most typical type of mechanical gripper is multi-fingered grippers containing two, three or even five fingers.

The end effectors can be widely used in the tooling industry for applications like spot-welding in an assembly, spray-painting to conform uniformity in the application of paint, and other situations where safeguarding human interests become essential. Surgical robots, too, have end effectors that are customized as per the requirements of the procedure.

How do End Effectors work?

A robot end effector is basically the business end of the robot. If it is not there, a robot is mostly of no use because it is devoid of the equipment that performs the main function in order to serve a particular purpose. For example, an articulated robotic arm is usually programmed to a reach particular location within its workspace. Still, without an end effector’s availability, it cannot perform the operation that it is assigned to, thereby making its existence redundant.

Although this main equipment for tooling is custom designed for every purpose, the basic underlying working principle remains more or less the same. Hence it is of utmost importance that we understand how a robot end effector works. This will not only aid us in the design process of the equipment but also help us in choosing the right end effector for our purpose. The robotic gripper is physically mounted on the wrist of the manipulator, followed by the attachment of the power connections. The power connections can be hydraulic, pneumatic or electric in nature.

The main force component is generated at the base link, which produces motion. This motion is then transferred link by link until the robotic manipulator’s extreme periphery, where the gripper is attached. This case is valid if a single actuator powers the base’s robotic manipulator. But it can also lead to malfunctioning and structural failure even with the slightest deviation from the force limits of the individual linkages.

Advancement in engineering with the usage of indigenous actuators at every joint has produced more flexibility towards using a robotic manipulator. Every link can generate more power individually, and the robot end effector can exploit the complete power from the actuator attached at the wrist. This enables the end effector to lift heavier objects and better grasp unstructured environments.

Robot Drive Systems and End Effectors

The robot drive systems are responsible for supplying power for the whole robot’s operation. The robot’s speed, load-carrying capability, and efficiency are all determined by the drive mechanism. In order for the manipulator’s body, arm, gesture, and wrist to execute the expected move, the movements of the individual joints must be properly controlled. The drive device that powers the robot does this job.

The most commonly used robot drive systems in industrial applications are discussed below.

Electric Drive

Electric drive systems can move robots at high speeds or with high electricity. This type of robot can be operated using either DC servo motors or DC stepping motors. It can be used with both rotational and linear joints. Small robots and precise systems can benefit from the electric drive system. Most notably, it has improved precision and consistency. This device does have one disadvantage: it is significantly more expensive. The Maker 110 robot is an example of this kind of drive mechanism.

Hydraulic Drive

Hydraulic drive systems are designed especially for massive robots. It is capable of deliver more power or speed than electric drive systems. Both linear and rotational joint might benefit from this drive mechanism. Rotary vane actuators generate rotary motions, while the hydraulic pistons produce linear motion. The most important downside of this drive is hydraulic oil leakage. ‘Unimate 2000 series robots’ is an example of a hydraulic drive system robot.

Pneumatic Drive | Pneumatic End Effector

Pneumatic drive systems are particularly well-suited to small robots with < 5o freedom. It has the potential to have high precision and speed. The rotary actuators can act on this drive mechanism to achieve rotary movements. The piston may also be used to provide translational gestures for sliding joints. As compared to hydraulic drive, this mechanism is less expensive. This method’s downside is that it would not be suitable for quicker operations.

Electric and hydraulic drive systems are the most widely used two types of drive systems. A detailed discussion can be found here.

End Effector Force

Gripper Mechanism

Various forces are working throughout the body of a robotic manipulator. The dominating force in this list is the friction force because that is what determines how hard or soft the grip should be to prevent any possible damage to the object.

The grip of the end effector should be strong and flexible enough to withstand the weight of the object and also handle the motion and acceleration produced by the continuous movement of the object. Hence it is imperative to calculate the amount of force required by the gripper to grasp an object.

The formula to find the force required by the robot end effector for the necessary grip on an object, the following formula is used:

 {F= \frac{ma}{\mu n}}

where

where

where F= force required to grip the object,

m= mass of the object,

a= acceleration of the object,

µ= coefficient of friction,

n= number of fingers in the gripper

The above equation is more of a generalized form and thus deemed incomplete in a variety of situations. To make it fit for a more realistic environment, another term is introduced that can be seen in the modified equation below. This will take care of the fluctuations in the force of gravitation that occur concerning the direction of the movement. For example, the upward movement of the object against gravity requires more force in the gripper than the downward motion of the object towards gravity.

 {F=\frac {m(a+g)}{\mu n}}

Here, g is acceleration because of gravity, and a is the acceleration because of object movement.

A task-related grip criterion can be used to pick grasps that are most suitable for fulfilling basic task specifications for certain visually interactive manipulation tasks, such as writing and screwdriver handling. Several task-oriented grasp consistency criteria have been proposed to aid in the evaluation of a strong grasp that meets the specifications of the task.

Is the Robotic End Effector multi functional?

Robotic gripper can perform more than one task through design and manufacturing. For example, household robots are aimed at helping old and disabled people or anybody with restricted mobility, for that matter. Hence they must be capable of mapping the environment, move to desired locations and also grab the necessary objects.

On the other hand, industrial robotic manipulators used in the automation industry can have end effectors capable of grasping and picking up objects. That also can be used as a piece of tooling equipment. The ‘end of arm tooling’ is highly justified in these cases because the robot end effector serves the purpose literally depicted by the name.

In the case of a surgical robot, the robot end effector is custom designed and manufactured to pick up the surgical equipments from desired locations, manipulate them at the region being operated and also perform the actual procedure using those instruments.

Hence, a single robotic gripper can be designed and manufactured for multiple tasks and operations through thorough research and careful study of the precise movements to be generated.

Robot Arm Gripper

End Effector | Robot End Effector | 5+ Types and Important Characteristics
Artaic Mosaic End Effector, Image Credit: Eba1968Artaic Mosaic Fabrication Robot, CC BY 3.0

Robot arm end effector

To read about Robot am design and its utilization click here.

Magnetic Gripper | Magnetic end effector

End Effector | Robot End Effector | 5+ Types and Important Characteristics
Artisentinal Specifications, Image Credit: LIVSMED, ArtisentialCC BY-SA 4.0

Pick and Place Gripper | Pick and place end effector

A robot in the form of a serial manipulator having a pick and place end effector or gripper typical has a multi fingered configuration. The number of fingers in the end effector depend on the shape, size and weight of the object that is supposed to be grasped. A detailed discussion can be found here.

About Esha Chakraborty

End Effector | Robot End Effector | 5+ Types and Important CharacteristicsI have a background in Aerospace Engineering, currently working towards the application of Robotics in the Defense and the Space Science Industry. I am a continuous learner and my passion for creative arts keeps me inclined towards designing novel engineering concepts.
With robots substituting almost all human actions in the future, I like to bring to my readers the foundational aspects of the subject in an easy yet informative manner. I also like to keep updated with the advancements in the aerospace industry simultaneously.

Connect with me with LinkedIn - http://linkedin.com/in/eshachakraborty93

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