Millions of industrial robots are active in Industry 4.0 factories around the world, improving productivity, enhancing quality, reducing costs, and supporting more flexible and sustainable operations. Given the importance of industrial robots, the International Organization for Standardization (ISO) developed standard 8373:2021, "Robotics Terminology," to define the terminology used in the robotics industry and provide a common language for discussing various types of robots and their applications.
The International Federation of Robotics (IFR) uses the key terms defined in ISO 8373:2021 and has identified six categories of robots based on their mechanical structure, including:
• Joints
Cartesian coordinates
Cylindrical coordinates
Parallel/Delta
• Polar coordinates
SCARA
This article reviews ISO 8373:2021, exploring the four key terms defining a robot, highlighting the necessity of reprogrammability, and the types and numbers of robot joints used by the IFR in determining robot classification. The article then delves into the details and nuances of various robot classifications and introduces typical robots from several manufacturers. In the process, the article also covers systems that do not meet all ISO requirements but are still referred to as robots.
ISO 8373:2021 defines an industrial robot as “an automatically controlled, reprogrammable, multipurpose manipulator capable of being programmed to operate three or more axes, and which can be stationary or mounted on a mobile platform for automation applications in an industrial environment.”
Reprogrammability is a key differentiating factor. Some industrial machines may have operating devices and multi-axis movement, enabling them to perform specific tasks, such as picking up bottles and placing them in boxes on a beverage filling line. However, if a machine can only be used for a single purpose and cannot be reprogrammed, then it is not a robot. ISO 8373 defines "reprogrammable" as "the ability to change programmed motions or auxiliary functions without requiring physical changes to the design."
Types and number of robot joints
ISO 8373 defines two types of robot joints:
• Prismatic joint, or sliding joint, is an assembly between two links that allows one link to move linearly relative to the other.
• A slewing joint, or rotary joint, is an assembly that connects two links, allowing one link to rotate relative to the other about a fixed axis.
The IFR uses these definitions, along with other ISO 8373 definitions, and has established six classifications of industrial robots based on their mechanical structure or topology. Furthermore, different robot topologies have different numbers of axes, and therefore different numbers of joints.
The number of axes is a key characteristic of industrial robots. The number of axes and the type of axes determine the robot's range of motion. Each axis represents an independent motion or degree of freedom. The more degrees of freedom a robot has, the larger and more complex the space it can move in. Some robot types have a fixed number of degrees of freedom, while others can have different numbers.
End effectors, also known as end tools (EOAT) or "multi-purpose manipulators" in ISO 8373, are another important component of most robots. End effectors come in a wide variety of types, including grippers, specialized machining tools (such as screwdrivers, paint guns, or welding torches), and sensors (including cameras). They can be pneumatic, electric, or hydraulic. Some end effectors can rotate, providing the robot with another degree of freedom.
The following section first introduces the IFR definition for each robot topology, and then discusses their functions and applications.
Articulated robots have three or more rotary joints.
This is a broad category of robots. Articulated robots can have ten or more axes, with six axes being the most common. Six-axis robots can move in the x, y, and z planes and perform pitch, yaw, and roll rotations, which allows them to mimic the movements of a human arm.
Their payload capacities also vary widely, ranging from less than 1 kg to over 200 kg. Their reach also differs significantly, from less than 1 meter to several meters. For example, KUKA's KR 10 R1100-2 is a six-axis articulated robot with a maximum reach of 1,101 mm, a maximum payload of 10.9 kg, and a pose repeatability of ±0.02 mm (Figure 1). This robot also features high-speed movement, short cycle time, and an integrated power supply system.
Figure 1: A six-axis articulated robot with pose repeatability of ±0.02 mm.
Articulated robots can be permanently mounted on the ground, walls, or ceilings, or mounted on tracks on the ground or in the air, on top of autonomous mobile robots or other mobile platforms, and can move between workstations.
They can be used to perform a variety of tasks, including material handling, welding, painting, and inspection. Articulated robots are the most common topology for collaborative robots (cobots) that work in conjunction with humans. While traditional robots operate in safety cages with safety barriers, collaborative robots are designed for close interaction with humans. For example, Schneider Electric's LXMRL12S0000 collaborative robot has a maximum reach of 1,327 mm, a maximum payload of 12 kg, and a pose repeatability of ±0.03 mm. To enhance safety, collaborative robots typically feature collision avoidance, rounded edges, force limiting, and lightweight design.
A Cartesian coordinate robot (sometimes also called a rectangular coordinate robot, linear robot, or gantry robot) has a manipulator with three prism joints whose axes form a Cartesian coordinate system.
Modified Cartesian coordinate robots have two prism joints. Nevertheless, they still do not meet the requirements of ISO 8373, which stipulates that they must be "programmable for three or more axes," and therefore, from a technical point of view, they are not robots.
There is more than one way to configure three prism joints, and therefore more than one way to configure a Cartesian coordinate robot. In a basic Cartesian topology, all three joints are at right angles. One joint moves along the x-axis and is connected to a second joint that moves along the y-axis, while the second joint is connected to a third joint that moves along the z-axis.
While gantry topology is often used synonymously with Cartesian coordinate robots, the two are not entirely the same. Like basic Cartesian coordinate robots, gantry robots support linear motion in three-dimensional space. However, gantry robots are equipped with two base x-axis rails, a supporting y-axis rail spanning both x-axis lines, and a cantilevered z-axis connected to the y-axis. For example, Igus's DLE-RG-0012-AC-800-800-500 is a gantry robot with a working area of 800 mm x 800 mm x 500 mm, a maximum payload of 5 kg, a moving speed of up to 1.0 m/s, and a repeatability of ±0.5 mm (Figure 2).
Figure 2: A gantry robot with a workspace of 800 mm x 800 mm x 500 mm.
The manipulator of a cylindrical coordinate robot has at least one rotary joint and at least one prism joint, whose axes form a cylindrical coordinate system.
Cylindrical coordinate robots have a relatively simple and compact structure and a limited range of motion, making them easy to program. They are not common compared to more complex counterparts. However, they are particularly suitable for applications such as grinding, palletizing, welding (especially spot welding), and material handling, for example, loading or unloading semiconductor wafers into wafer cassettes in integrated circuit manufacturing operations (Figure 3).
Figure 3: This cylindrical coordinate robot has a rotary and prismatic joint.
Cylindrical robots typically move at speeds of 1 to 10 m/s and can be designed to handle heavy objects. These robots can be applied in the automotive, pharmaceutical, food and beverage, aerospace, electronics, and other industries.
Parallel/delta robots are a type of robotic arm with links forming a closed-loop structure.
Other robots (such as those with cylindrical or Cartesian topologies) are named for their motion patterns, while Delta robots are named for their inverted triangular shape. Delta robots have 2 to 6 axes, with 2-axis and 3-axis designs being the most common. Like 2-axis Cartesian coordinate robots, 2-axis Delta robots do not technically meet the requirements of ISO 8373 and cannot be considered robots.
Delta robots are designed for speed, not power. Mounted above the work area, they perform functions such as pick-and-place, sorting, disassembly, and packaging. Typically positioned above conveyors, they move parts along the production line. Grippers are connected to slender mechanical links. These links connect to three or four large motors on the robot's base. The other end of the links connects to a tooling plate on which the EOAT (Engineer Oriented Attack) is mounted.
The Igus RBTX-IGUS-0047 is an example of a 3-axis delta robot. It has a workspace diameter of 660 mm and a maximum payload of 5 kg. When handling a 0.5 kg load, it can perform 30 pick-up operations per minute, with a maximum speed of 0.7 m/s and an acceleration of 2 m/s². Its repeatability is ±0.5 mm (Figure 4).
Figure 4: Three-axis delta robot and controller (left)
A polar coordinate robot (spherical coordinate robot) is a manipulator with two rotary joints and one prism joint, whose axes form a polar coordinate system.
One of the rotary joints allows the polar robot to rotate about a vertical axis extending upwards from the base. The second rotary joint is perpendicular to the first and allows the robot arm to swing up and down. Finally, the prism joint allows the robot arm to extend or retract from the vertical axis.
Although polar coordinate robots have a simple structure, their use is limited by the following drawbacks compared to other topologies (such as joint coordinate, Cartesian coordinate, and SCARA robots):
• Spherical coordinates make programming more complex.
• Compared to other types of robots, their payload capacity is typically more limited.
They are slower than other robots.
The main advantages of polar coordinate robots are their large workspace and high precision. They can be used for machine tool operation, assembly work, material handling in automobile assembly lines, as well as gas welding and electric arc welding.
The name SCARA stands for "Selectively Compliant Arm for Robotic Assemblies," and it is a manipulator with two parallel rotary joints that provides compliance in a selected plane.
A basic SCARA robot has three degrees of freedom, with the third coming from a rotary end effector. An additional rotary joint can be added to a SCARA robot, bringing the total to four degrees of freedom, thus enabling more complex movements.
SCARA robots are typically used in pick-and-place or assembly applications requiring high speed and precision. For example, Dobot's M1-PRO is a 4-axis SCARA robot with a maximum working radius of 400 mm, a maximum payload of 1.5 kg, and a repeatability of ±0.02 mm. It features sensorless collision detection and drag-and-drop teaching programming capabilities, making it suitable for both collaborative and standalone use (Figure 5).
Figure 5: Four-axis SCARA robot with repeatability of ±0.02 mm
Summarize
All industrial robots comply with ISO 8373 and can be automatically controlled via reprogrammable, multi-purpose manipulators. However, not every design has a fixed number of axes for a specific topology. Delta and Cartesian coordinate robots have fewer axes than specified, while some SCARA robots have more axes than the IFR-specified number.
Figure 6: CRT-DMC600M Motion Control System
