Chapter 6: Robot Design and Mechanics - "Types of Joints"

Abstract:

There are several types of joints used in robotics, including hinged, prismatic, rotational, ball, cylindrical, orthogonal, and revolute joints. Hinged joints 

• Also known as pivot joints, these joints only allow movement in one plane, similar to a door hinge.
• The human elbow and knee are examples of hinged joints.

Prismatic joints 

• These joints are also known as linear motion joints.
• There are two types of prismatic joints: single-stage and telescoping.
• Single-stage joints have a moving surface that slides along a fixed surface.

Rotational joints 

• These joints allow rotation around an axis, similar to the shoulder joint.
• Rotational movement is measured in degrees.

Ball joints 

• These joints allow multi-directional movement.
• Ball joints are often used in applications that require flexibility and articulation.
• They are commonly used in automotive steering systems.

Cylindrical configuration 

• This configuration provides two linear and one rotary motion.
• It has a cylindrical work envelope and a good work area.

Orthogonal joints 

• Also known as O-joints, these joints use input and output links to achieve desired motion.

Revolute joints 

• These joints are also known as hinge or pin joints.
• They allow the rotation of one rigid body.

So let's explore the Chapter 6 in detail 

6.1 Introduction

Robotics is a multidisciplinary field that integrates mechanical engineering, electrical engineering, computer science, and artificial intelligence to design and develop automated systems. The mechanical design of a robot plays a crucial role in its functionality, efficiency, and application. One of the fundamental aspects of robot mechanics is the type of joints used, as they define the degrees of freedom (DOF) and movement capabilities of the robot.

This chapter explores different joint types, including revolute, prismatic, and spherical joints, along with other essential mechanical aspects that contribute to the overall design of robots.

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6.2 Importance of Joint Selection in Robot Design

Joints in a robotic system determine the movement and flexibility of the robot. The type of joints used affects:

• Degrees of Freedom (DOF): The number of independent movements a robot can perform.
• Workspace: The volume in which the robot can operate.
• Stability and Precision: The mechanical constraints that impact control and accuracy.
• Structural Complexity: The design and manufacturing considerations for building the robot.

Selecting the right joint type is critical for ensuring that the robot meets its intended application, whether it is for industrial automation, medical surgery, or humanoid robotics.

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6.3 Types of Joints in Robotics

6.3.1 Revolute Joint (Rotational Joint)

A revolute joint allows rotational motion around a fixed axis, similar to a hinge. It provides one degree of freedom (DOF) and is the most common joint in robotic arms.

Characteristics:

• Allows rotational motion around a single axis.
• Commonly used in robotic arms and manipulators.
• Provides high precision when combined with encoders and actuators.

Applications:

• Industrial Robots: Used in robotic arms for manufacturing and welding.
• Humanoid Robots: Emulates human-like motion, such as shoulder or elbow movements.
• SCARA Robots: Used in pick-and-place applications.

Example:

A robotic arm with a revolute joint at the elbow enables bending movement similar to a human arm.

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6.3.2 Prismatic Joint (Linear Joint)

A prismatic joint allows linear motion along a single axis, providing one degree of freedom. It is commonly used in robots that require straight-line movement.

Characteristics:

• Allows sliding motion along a straight path.
• Provides linear displacement rather than rotation.
• Typically used in robotic positioning systems and elevators.

Applications:

• Cartesian Robots: Used for precise pick-and-place operations.
• Automated Guided Vehicles (AGVs): Used in robotic lifting mechanisms.
• Medical Robotics: Helps in robotic surgery where controlled linear motion is needed.

Example:

A robotic gripper moving forward and backward to grasp an object uses a prismatic joint.

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6.3.3 Spherical Joint (Ball-and-Socket Joint)

A spherical joint allows rotation in multiple directions, similar to a human shoulder joint. It provides three degrees of freedom, enabling complex movements.

Characteristics:

• Allows motion in three rotational axes (pitch, yaw, and roll).
• Used in robots that require flexible movement.
• Provides a large range of motion compared to other joint types.

Applications:

• Humanoid Robots: Provides mobility in shoulder and hip joints.
• Medical Prosthetics: Enables natural movement for artificial limbs.
• Inspection Robots: Used in flexible robotic arms for inspection and maintenance.

Example:

A humanoid robot's shoulder joint using a spherical mechanism for arm rotation.

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6.4 Other Types of Joints in Robotics

6.4.1 Cylindrical Joint

A cylindrical joint allows a combination of rotational and linear motion along a single axis. It provides two degrees of freedom (DOF).

Applications:

• Used in robotic manipulators for grasping and assembling.
• Common in cylindrical-coordinate robots.

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6.4.2 Planar Joint

A planar joint allows movement in a two-dimensional plane, providing two translational and one rotational DOF.

Applications:

• Used in robots that require flat-surface movement, such as conveyor belt systems.

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6.4.3 Universal Joint

A universal joint enables rotation in two perpendicular axes, often used for flexible coupling between components.

Applications:

• Used in robotic drive shafts and mobile platforms.

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6.4.4 Screw Joint

A screw joint converts rotational motion into linear motion, similar to a threaded bolt mechanism.

Applications:

• Used in robotic actuators and positioning systems.

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6.5 Comparison of Joint Types

Joint Type	Motion Type	Degrees of Freedom (DOF)	Applications
Revolute Joint	Rotational	1	Robotic arms, humanoids
Prismatic Joint	Linear	1	AGVs, pick-and-place robots
Spherical Joint	Multi-rotational	3	Humanoids, medical robots
Cylindrical Joint	Rotational + Linear	2	Industrial manipulators
Planar Joint	2D Translational + Rotational	3	Conveyor belt robots
Universal Joint	Multi-axis Rotation	2	Drive shafts, flexible couplings
Screw Joint	Rotational to Linear	1	Precision positioning systems

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6.6 Considerations in Joint Selection

When selecting joints for a robot, engineers must consider:

1. Application Requirements: Whether the robot needs rotational, linear, or multi-axis movement.
2. Degrees of Freedom (DOF): Higher DOF provides flexibility but adds complexity.
3. Load Capacity: The joint should withstand the weight and force applied during operation.
4. Precision and Accuracy: Critical for applications like surgery or semiconductor manufacturing.
5. Material and Durability: Ensures longevity in harsh industrial environments.
6. Actuation and Control: Integration with motors and sensors for efficient movement.

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6.7 Conclusion

Robot joint design plays a crucial role in determining the movement, flexibility, and efficiency of robotic systems. Revolute, prismatic, and spherical joints are the most commonly used types, each serving specific applications. Additional joint types like cylindrical, planar, universal, and screw joints further expand the design possibilities for specialized robotic applications.

By understanding the mechanics of these joints and their applications, engineers can design robots that are optimized for various industries, from manufacturing and healthcare to space exploration and autonomous vehicles.