What is a robot joint exactly?

Robot joints enable movement in robots by connecting two rigid links. The type of joint determines the range and nature of movement possible. 

These joints allow robots to complete tasks with various levels of flexibility and dexterity. The most advanced versions can even allow bots to operate at superhuman levels. 

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Rotary joints: Movement around an axis

Rotary joints, as the name suggests, enable rotational movement around an axis. These joints allow robots to twist and turn, which is essential for mobility and manipulation.

There are several types of rotary joints out there: 

• Single-axis rotation (revolute joints). The most common type is the revolute joint, which provides rotation around a single axis. Robotic arms typically have more than one revolute joint, allowing them to bend at the elbow and move from side to side. 
• Multi-directional mobility (spherical joints). For more complex movement, spherical joints provide rotation in multiple directions. These joints have three perpendicular axes of rotation, similar to a ball and socket in your shoulder. Spherical joints are useful for robotic hands, which allows them to maneuver objects with skill. 
• Combined rotation and sliding (cylindrical joints). Cylindrical joints enable both rotation around an axis and sliding along that axis. They’re useful when rotation alone is not enough, such as in robotic fingers or legs. The combination of revolute and prismatic joints in cylinders allows for grasping, walking, and other coordinated movements that require controlled sliding and rotation.

We’ll get into each of these types of robotic joints in more detail below. 

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Linear joints: Straight-line motions

Linear joints enable robots to move in a straight line. These joints typically consist of a movable element that slides or glides along a set of rails or guides. 

The most common types are prismatic and slider joints:

• Prismatic joints allow for sliding motion in one direction. An example is a drawer — it can only move in and out along one axis. Industrial robots often use prismatic joints to extend and retract robotic arms. This allows the arm to reach farther without needing a larger base.
• Slider joints enable two plates to slide over each other in a plane. They’re commonly found in X-Y tables that move in two dimensions. These joints give robots a wide range of motion within a flat surface. 3D printers frequently use slider joints to control the print head. By moving it in the X and Y axes, the print head can access the entire print bed.

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Twisting joints: Enabling rotation

Twisting joints, known as revolute joints, allow for rotation around a single axis. They’re one of the simplest yet most useful types of robot joints. 

How twisting joints work 

A twisting joint has two links connected by a pin, serving as the rotation axis. One link stays fixed, while the other rotates around the pin. Many twisting joints can rotate a full 360 degrees, but often have limits built in for specific uses. 

The joint is powered by an actuator, like an electric motor, which provides the force needed for rotation.

Applications of twisting joints 

Twisting joints are all around us in robotics and automation. 

They’re often found in robotic arms and hands, and are a perfect fit for tasks that involve:

• Assembly: Screwing components together, precisely placing parts, or any task that needs a rotating motion.
• Packaging: Opening jars, twisting caps, or rotating items for optimal placement in boxes.
• Machining: Rotating workpieces for cutting/milling, operating grinding tools, or polishing surfaces.

Examples of twisting joints

Here are some specific examples of real-world robots and their use cases:

• RO1 by Standard Bots (machine tending): This collaborative robot is known for its flexibility. Its twisting joints allow it to carefully and precisely position both small and large components during machine tending operations. 
• ABB IRB 6700 (auto assembly): This industrial robot utilizes multiple twisting joints to precisely tighten bolts on car chassis, ensuring consistent quality throughout the assembly line.
  
• Yaskawa Motoman GP8 (food processing): Equipped with a specialized gripper, this robot uses twisting joints to swiftly remove lids from jars prior to filling on a high-speed production line.

Revolute joints: Single-axis rotation

Revolute joints, also known as hinges, have a single axis of rotation. Think of a door hinge allowing a door to swing open and closed. Revolute joints are simple but essential, enabling elbows, knees, and fingers to bend.

Design of revolute joints

A revolute joint consists of two links connected by a pin, which acts as the joint’s axis of rotation. 

So, one link remains stationary, while the other rotates around the pin. Also, revolute joints can rotate through a full 360 degrees, although the rotation is often limited for specific applications. 

The joint is also driven by an actuator.

Applications of revolute joints

Robotic arms and hands:

• Grabbing and manipulating objects: Multiple revolute joints in a robot's fingers, wrist, and arm allow it to grasp items of various shapes and sizes, and move them with precision.
• Precision assembly: The controlled rotation provided by revolute joints enables robots to carefully place and align delicate components during intricate assembly processes.
• Material handling: Revolute joints allow robotic arms to pick, place, rotate, and reposition objects throughout a manufacturing or logistics environment.

Examples of revolute joints

Some well-known examples of robots using revolute joints include:

• The robotic arms installed on the International Space Station to help astronauts.
• Robotic arms used in automobile manufacturing for painting, welding, and parts handling.
• Humanoid robots like Atlas, ASIMO, and Sophia, where revolute joints provide realistic arm, hand, and leg movements.

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Spherical joints: Multi-directional mobility

Spherical joints are one of the most versatile types of robot joints. They open up movement in multiple directions at once — rotation, swiveling, and pivoting. These highly mobile joints provide robots with super-high levels of skill. 

How they work

Spherical joints use ball and socket mechanisms that allow the joint to spin, swivel, and pivot freely in any direction. The ball, attached to one link, rotates within a socket, which is attached to the adjacent link. This construction gives spherical joints their signature multi-axial movement abilities.

Applications

Industrial robot arms: Many industrial robot arms integrate spherical joints in their shoulders, elbows, and wrists for:

• Maneuvering payloads in complex or tight spaces.
• Reaching around obstacles with increased dexterity.
• Performing intricate tasks with a wider range of motion.

Robotic grippers and hands Spherical joints are frequently found in robotic grippers and hands, providing:

• Enhanced dexterity for grasping objects of various shapes and sizes.
• The ability to manipulate objects with more freedom of movement.

Humanoid robots: Some humanoid robots utilize spherical joints to achieve highly articulated movements mimicking human joints:

• Ball-and-socket joints like those in the hips and shoulders provide a more natural and fluid range of motion.

Examples

• KUKA KR 210 R2700 (Industrial Painting Robot): This industrial robot features spherical joints that enable a precise and expansive painting range, even in challenging areas.
  
  
• Fetch Robotics Freight500 (Logistics Robot): Its arm employs spherical joints for flexible movement when picking and placing items in a warehouse environment.
  
  
• Boston Dynamics' Atlas (Humanoid Robot): This cutting-edge humanoid robot relies heavily on spherical joints for its remarkable agility and natural-looking motions.

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Cylindrical joints: Combined movements

A cylindrical joint allows motion in two degrees of freedom, linear motion along an axis and rotation around that same axis.

Think of it as a revolute joint that can slide up and down. This combination of rotation and translation allows for more complex movements and a greater range of motion.

Applications

There are some of the applications where these robot joints excel: 

• Prosthetic limbs and exoskeletons to restore natural movement.
• Surgical robots that need precise control and navigation.
• Exploration robots traversing uneven terrain. The ability to slide and rotate helps them adapt to the environment.
• Entertainment robots and animatronics that need to move and gesture in a very lifelike fashion.

Examples of cylindrical joints include:

• The human elbow joint is a great example of a cylindrical joint in nature. Your elbow can bend and straighten (rotate) as well as slide inward and outward (translate) along your arm. Advanced robotic arms like Standard Bots’ RO1 have cylindrical joints at the “elbow” to mimic human range of motion.
• Cylindrical joints are also commonly used in robotics for the “shoulder” joint, where the robot arm attaches to the body. This allows the entire arm to rotate in a circle as well as slide up and down for maximum flexibility and extension.

Summing up

And those are the 6 key types of robot joints that enable robot arms to move — and groove.

So, the next time you see a robot in action, take a moment to appreciate the ingenious joints that allow it to function just like us — or better!