Editorial Feature

Building an End-Effector for Robotic Control

An end-effector is the device at the robotic arm end that is designed for interactions with the environment. The nature of the end-effector is based on the use of the robot. In strict terms, the end-effector is the last link or the end of the robot. Tools are fitted at this endpoint. The wheels that make up the feet to a humanoid robot are not end-effectors, they simply aid the robot’s mobility.

Types of end-effectors include:

  • Mechanical grippers
  • Negative pressure (vacuum)
  • Magnetic
  • Hooks
  • Ladles (scoops liquid or powder)
  • Others (electrostatic).

The gripper may have two, three, four or even five fingers. Four general categories of robotic grippers include:

  • Impactive – claws or jaws that grasp physically by direct impact upon the object.
  • Ingressive – needles, pins or hackles that penetrate physically the object surface used in carbon, textile and glass fiber handling.
  • Astrictive – suction forces applied to the surface of the object by magneto, electro or vacuum adhesion.
  • Contigutive – needs direct contact to enable adhesion such as glue, surface tension or freezing.

Research

Verma A et al (2011) published a study on the end-effector position analysis of SCORBOT-ER Vplus Robot. This paper details the kinematic analysis of SCORBOT-ER Vplus robot arm that is used for performing successful robotic manipulation tasks in its workspace. The SCORBOT-ER Vplus is a vertical articulated robot with five degrees of freedom and all the joints are revolute as is demonstrated in the following video:

This study used the Denavit-Harbenterg (D-H) model of representation to model robot links and joints along with a 4x4 homogeneous matrix. This is a safe and reliable robotic system designed for training and laboratory applications. This system enables students to gain practical and theoretical experience in automation, robotics and control systems. For a set of joint parameters, the MATLAB 8.0 is used for solving this mathematical model.

Toklu E et al (2011) identified the need for a novel range of end-effectors, which is suitable for non-rigid products and introduced a new non-contact gripping device. The operation of the end-effector is based on the principle of generating a high speed fluid flow between the product surface and the end-effector hence creating a vacuum that levitates the product. Finite element analysis and optimization methodology are used to discuss the lifting forces and conditions. The end-effectors were optimized to lift non-rigid food materials such as jelly blocks.

The results showed that the non-contact end-effectors developed based on numerical analysis results are highly successful and these are applied to handle non-rigid and rigid materials with smooth surfaces. A vacuum is caused by the radial flow through the nozzle. The vacuum created is capable of lifting materials with porous surfaces and structures covered by viscous substances.

Current Applications

End-effectors are designed to suit a range of applications as detailed below:

  • Welding robots normally perform spot welding. A spot welder is mounted to the robot and the robot is used to position the welder at each weld point.
  • Assembly robots are commonly used for assembling especially in manufacturing setups. Here a robot will normally have a vacuum cup or a pneumatic gripper to move parts through the process.
  • Painting or spraying robots use a small spray gun as the end-effector. They have special equipment to ensure that no sparks are generated. They are mostly equipped with a coat that prevents paint from collecting on the arm surface. The coat is a consumable and changing the same is quite easy. In this application, the robot’s reach is quite critical as the robot should be capable of spanning large distances.
  • NASA has two robots that are popular, each of which have distinct end-effectors. Dextre has a retractable socket drive and built-in grasping jaws. The end-effectors of NASA’s most recent humanoid Robonaut are very similar to human hands.
  • The end-effector of a dispensing robot is a hot glue gun. It is important to maintain the surface velocity and also the angle of application.
  • Inspection robots use vision systems, lasers, or other types of measurement tools as an end-effector. The path or velocity of the robot is not critical here but the end-effector needs to be positioned correctly for maximum performance.
  • Material handling robots range from palletizers to machine loading and unloading robots.
  • Material removal robots may include those that pick a part and present it to a cutter and robots that wield a material removal device or a cutter.
  • In the medical sector, the tele-operated da Vinci medical robot has tool tips that include robotic scissors, robotic forceps and a robotic scalpel.

Future Developments

Companies such as Comau Aerospace are developing advanced end-effectors, which combines fastener insertion and drilling operations into a single unit. It is possible to mount the same onto a robot or other machines.

The end-effector is lightweight as well as reliable, robust and accurate. All required components are housed onboard to create a single, turnkey solution end-effector with a number of functional packages.

Sources and Further Reading

  • Allyson Pulsipher and Kasey Greenland. End Effectors. 2007.
  • Toklu E and Erzincanli F. Investigation of flow and vacuum lifting force on a non-contact end effector for robotic handling of non-rigid material. Scientific Research and Essays. 2011;6(29):6152-6161.
  • Owens Design – End Effector Selection and Design.
  • Verma A and Deshpande V.A. End-Effector Position Analysis of SCORBOT-ER Vplus Robot. International Journal of Advanced Science and Technology. 2001;29:61-66.

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