Industrial Robots

Abstract

This chapter provides more detail on industrial robots commencing with the accepted definition. The various configurations are introduced, including articulated, SCARA, cartesian, and parallel or delta. The typical applications and market shares for each configuration are discussed. The key issues regarding robot performance, including working envelope and repeatability, are discussed together with the main points to consider when selecting robots. This includes a review of the typical contents of a robot data sheet. The benefits that robots can provide are also discussed, both for system integrators and end users. This includes the 10 key benefits that robots can provide for a manufacturing facility.

Keywords

Robot configuration

Articulated

SCARA

Cartesian

Parallel

Delta

Working envelope

Repeatability

Chapter Contents

2.1 Robot Structures   21

2.1.1 Articulated Arm   22

2.1.2 SCARA   24

2.1.3 Cartesian   26

2.1.4 Parallel   27

2.1.5 Cylindrical   28

2.2 Robot Performance   28

2.3 Robot Selection   31

2.4 Benefits of Robots   33

2.4.1 Benefits to System Integrators   34

2.4.2 Benefits to End Users   35

Reduce Operating Costs  35

Improve Product Quality and Consistency  36

Improve Quality of Work for Employees  36

Increase Production Output Rate  36

Increase Product Manufacturing Flexibility  37

Reduce Material Waste and Increase Yield  37

Comply with Safety Rules and Improve Workplace Health and Safety  37

Reduce Labour Turnover and Difficulty of Recruiting Workers  38

Reduce Capital Costs  38

Save Space in High-Value Manufacturing Areas  38

An industrial robot has been defined by ISO 8373 (International Federation of Robotics, 2013) as:

An automatically controlled, reprogrammable, multipurpose manipulator programmable in three or more axes, which may be either fixed in place or mobile for use in industrial automation applications.

Within this definition, further clarification of these terms is as follows:

 Reprogrammable - motions or auxiliary functions may be changed without physical alterations.

 Multipurpose - capable of adaptation to a different application possibly with physical alterations.

 Axis - an individual motion of one element of the robot structure, which could be either rotary or linear.

In addition to these general-purpose industrial robots, there are a number of dedicated industrial robots that fall outside this definition. These have been developed for applications such as machine tending and printed circuit board assembly and do not meet the definition because they are dedicated to a specific task and are therefore not multipurpose.

As mentioned in Chapter 1, the first application of an industrial robot was at General Motors in 1961. Since that time, robotic technology has developed at a fast pace and the robots in use today are very different to the first machines in terms of performance, capability, and cost. There have been various mechanical designs developed to meet the needs of specific applications, which are described below.

These different configurations have resulted from the ingenuity of the robot designers combined with advances in technology, which have enabled new approaches to machine design. The most significant of these was the introduction of electric drives to replace the use of hydraulics and the increasing performance of the electric drives, providing increased load-carrying capacity combined with high speed and precision.

Initially, hydraulics was used as the primary motive power. Hydraulic power was capable of providing the load-carrying capacity necessary for the early spot welding applications in the automotive industry. However, the responsiveness was poor and the repeatability and path following capabilities limited. For the first installations the robot technicians were required to start work early to turn on the robots, so they were warmed up prior to production starting, to ensure the robots performed repeatably from the first car body to welded.

Pneumatics were used to provide a low cost power source; however, this again could not achieve high repeatability due to the lack of control available. Hydraulics were also used for the early paint robots because electric drives could not, at that time, be used in the explosive atmosphere of the paint booth, caused by the use of solvent-based paints. Painting, by the nature of the application, carrying a spray gun with a 12 inches wide fan, about 12 inches from the surface, did not require the repeatability and control necessary for other applications; therefore, this proved to be a successful application for robots.

Electric drives of various different types have been used. DC servo motors were initially the most prevalent. These however had limited load-carrying capacity, which did initially provide constraints for the use of robots for spot welding applications due to the weight of the welding guns. Stepper motors were also utilised for high precision, low load-carrying applications. Once AC servo motors became available these took over the majority of applications. Their performance has continually increased providing better control, high repeatability, and precision as well as high load-carrying capacity. AC servo motors are now utilised in almost all robot designs.

2.1 Robot Structures

An industrial robot is typically some form of jointed structure of which there are various different configurations. The robot industry has defined classifications for the most common and these are:

 Articulated

 SCARA

 Cartesian

 Parallel (or Delta)

 Cylindrical.

These structures and their benefits are described in more detail below. The structures are achieved by the linking of a number of rotary and/or linear motions or joints. Each of the joints provides motion that collectively can position the robot structure, or robot arm, in a specific position. To provide the ability to position a tool, mounted on the end of the robot, at any place at any angle requires six joints, or six degrees of freedom, commonly known as six axes.

The working envelope is the volume the robot operates within. This is typically shown (see Figure 2.1) as the volume accessible by the centre of the fifth axis. Therefore, anywhere within this working envelope the robot can position a tool at any angle. The working envelope is defined by the structure of the robot arm, the lengths of each element of the arm, and the motion type and range that can be achieved by each joint. The envelope is normally shown as a side view, providing a cross-section of the envelope, produced by the motion of axes 2-6 and a plan view then illustrating how this cross-section develops when the base axis, axis 1, is moved. It should also be noted that the mounting of any tools on the robot will also have an impact on the actual envelope accessible by the robot and tool combined.

The first robot, a Unimate, was designated as a polar-type machine. This design was particularly suited to the hydraulic drive used to power the robot. The robot (Figure 2.2) provided five axes of motion; that is, five joints that could be moved to position the tool carried by the robot in a particular position. These consisted of a base rotation, a rotation at the shoulder, a movement in and out via the arm, and two rotations at the wrist. The provision of only five axes provided limitations in terms of the robot's ability to orientate the tool. However, in the early days, the control technology was unable to meet the needs for six axes machines.

2.1.1 Articulated Arm

The most common configuration is the articulated or jointed arm (Figure 2.3). This closely resembles the human arm and is very flexible. These are normally...