Automation and Robotics

The Industrial Robot

The development of the industrial robot can be traced back to the work done in the United States at the Oak Ridge and Algonne National Laboratories to develop mechanical tele-operated manipulators to handle nuclear material. It was realised that by the addition of powered actuators and a stored programme system, a manipulator could perform autonomous, repetitive tasks frequently found in manufacturing industry. The sequence of events that led Joseph Engleberger and George Devol to form Unimation Inc (now Staubli-Unimation). and start the production of the PUMA robot are well recorded. Even with the considerable advances in sensing systems, control strategies and artificial intelligence the basic-shop floor industrial robot is not significantly different from those of the initial concepts. The industrial robot can be considered to be a general purpose reprogrammable machine tool, moving an end effector, which either holds components or a tool.

The main trade association in the United Kingdom is British Robot Association which has links to many sites of interest.

The functions of a robot are best summarised by considering the following definition of the industrial robot:

For the purpose of this course a robot is considered to be a specific application of a manipulator. A manipulator is a system with a number of joints and links that can be controlled (either position or speed) within three dimensional space.

Depending on the type of robot and the application, the mechanical structure of a robot can be divided into two parts, the main manipulator and a wrist assembly. The manipulator will position the end effector while the wrist will control its orientation.

The work envelope is the space that can be reached by the end of the robot arm. All interaction between the robot, and other machines, parts and processes must take place within the work envelope.

The structure of the robot consists of a number of links and joints, a joint will allow relative motion between two links. Two types of joints are used, a revolute joint to produce rotation and a linear prismatic joint. To achieve complete control of the end effectors position and orientation a minimum of six joints are required. The basic robot arm has three joints, this allows the tool at the end of the arm to be positioned any where in the robots working envelope. Even though there are a large number of robot configurations that are possible, only five configurations are commonly used in industrial robotics:

Polar The linear extending arm is capable of being rotated around the horizontal and vertical axes.
Cylindrical cylindrical robot The linear extending arm can be moved vertically up and down around a rotating column.
Cartesian and Gantry: cartisian robot Three orthogonal sliding or prismatic joints.
Jointed Arm jointed arm Three joints arranged in an anthropomorphic configuration.
Selective Compliance Assembly Robotic Arm, SCARA scara Two rotary axes and a linear joint.

The Robot Wrist

wrist To orientate the tools, three additional joints are require, these are normally mounted at the end of the arm in an assemble termed the wrist: it is conventional to define the joints of a wrist roll, pitch and yaw. The arm and wrist give the robot the required six degrees of freedom, that permit the tool to be positioned and orientated as required by the task.

Comments and observations

The selection of a robot is a significant problem to the design engineer and depends on the task to be performed. One of the earliest applications was to operate in the foundry, the environment being considered to be a hazard to a human operator, due to the noise, heat and fumes from the process. This is the classic type of application where a robot is used to replace workers because of the hazards.

Other reasons include, repetitive work cycles, difficult or hazardous materials to be moved, and the requirements for multi-shift operation. The robots that have been installed in manufacturing industry are normally employed in one of four application groups; material handling, process operations, assembly and inspection. To control a robot so that it is capable of performing the required task, all the joints need to be accurately controlled.

  1. The work volume depends on the actual joint limits of motion.
  2. A rotary motion is normally quicker that the equivalent linear motion. However to achieve linear motion using rotary joints, a co-ordinated move is required.
  3. Not all robotic applications require 6 axes, for example paint spraying, this requires a five axis robot as spraying is symmetrical about the roll axis.

R M Crowder
January 1998