Editorial Feature

Basic Robotics: What is a Robotic Manipulator?

Topics Covered

Introduction
Kinematics of a Robotic Manipulator
Robotic Manipulator Arm Configuration
Wrist Configuration
Applications
References

Introduction

Industry-specific robots perform several tasks such as picking and placing objects, movement adapted from observing how similar manual tasks are handled by a fully-functioning human arm. Such robotic arms are also known as robotic manipulators. These manipulators were originally used for applications with respect to bio-hazardous or radioactive materials or for use in inaccessible places.

A series of sliding or jointed segments are put together to form an arm-like manipulator that is capable of automatically moving objects within a given number of degrees of freedom. Every commercial robot manipulator includes a controller and a manipulator arm. The performance of the manipulator depends on its speed, payload weight and precision. However, the reach of its end-effectors, the overall working space and the orientation of the work is determined by the structure of the manipulator.

Kinematics of a Robotic Manipulator

A robot manipulator is constructed using rigid links connected by joints with one fixed end and one free end to perform a given task (e.g., to move a box from one location to the next). The joints to this robotic manipulator are the movable components, which enables relative motion between the adjoining links. There are also two linear joints to this robotic manipulator that ensure non-rotational motion between the links, and three rotary type joints that ensure relative rotational motion between the adjacent links.

The manipulator can be divided into two parts, each having different functions:

Arm and Body – The arm and body of the robot consists of three joints connected together by large links. They can be used to move and place objects or tools within the work space.

Wrist – The function of the wrist is to arrange the objects or tools at the work space. Them structural characteristics of the robotic wrist includes two or three compact joints.

Robotic Manipulator Arm Configuration

Manipulators are grouped into several types based on the combination of joints, which are as follows:

  • Cartesian geometry arm – This arm employs prismatic joints to reach any position within its rectangular workspace by using Cartesian motions of the links.
  • Cylindrical geometry arm – This arm is formed by the replacement of the waist joint of the Cartesian arm with a revolute joint. It can be extended to any point within its cylindrical workspace by using a combination of translation and rotation.
  • Polar/spherical geometry arm – When a shoulder joint of the Cartesian arm is replaced by a revolute joint, a polar geometry arm is formed. The positions of end-effectors of this arm are described using polar coordinates.
  • Articulated/revolute geometry arm - Replacing the elbow joint of the Cartesian arm with the revolute joint forms an articulated arm that works in a complex thick-walled spherical shell.
  • Selective compliance automatic robot arm (SCARA) – This arm has two revolute joints in a horizontal plane, which allow the arm to extend within a horizontal planar workspace. The TH650A SCARA Robot by TM Robotics is a great example to demonstrate pick and place functionality of robotic manipulators:

Video courtesy of TM Robotics.

Wrist Configuration

The two main types of wrist design include:

  • Roll-pitch-roll or spherical wrist
  • Pitch-yaw-roll.

The spherical wrist is more common because of its mechanically simpler design. It has 6 degrees of freedom, and consists of a Hooke shoulder joint followed by a rotary elbow joint.

Applications

Some of the major applications of robotic manipulator are discussed below:

  • Motion planning,
  • Remote handling
  • Micro-robots
  • Humanoid robots
  • Machine tools.

References


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