In the rapidly evolving field of humanoid robotics, the focus often lies on artificial intelligence (AI) and machine learning algorithms. However, the mechanical components that enable these robots to mimic human movements are just as crucial, and one such essential component is the rod end (also known as Heim joints). These components are key to the articulation of humanoid robots, enabling fluid, human-like movements in various joints. Among the different types of robotic joint configurations, the rotary + linkage configuration is the most common application for rod ends.
What is the Rotary + Linkage Configuration?
Below is a diagram demonstrating humanoid robot various foot mechanism.
Humanoid robots typically utilize a combination of rotary and linear actuators to optimize their body balance and motion inertia. The joints in these robots primarily exhibit three core configurations, with rotary + linkage being the primary configuration where rod ends are used.
Rotary + Linkage Configuration: This involves a rotary actuator combined with a linkage mechanism. The system translates the rotary motion of a motor into a specific trajectory of either straight-line or oscillating movement. It provides flexibility and dynamic performance, crucial for humanoid robot joint articulation.
Rotary + Linear Configuration: This system integrates a planetary roller screw or ball screw driven by a motor to directly convert rotational motion into high-thrust linear movement.
Rotary + Rotary Configuration: This configuration relies on a “motor + gearbox” system to achieve pure rotational output. It is commonly used in joints that require large angular rotations, such as the shoulder or hip joints. The gearbox ensures high torque density and a compact layout.
Among these, the rotary + linkage configuration is the most widely used for joint articulation in humanoid robots, especially in controlling bending movements like knee and ankle joints.
Why the Rotary + Linkage Configuration is Ideal for Rod Ends?
Below are two diagrams demonstrating how the rotary + linkage configuration is implemented in humanoid robots.
Source: Agibot
The rotary + linkage configuration has unique advantages that make it particularly well-suited for humanoid robots. Here’s why it stands out as the main application for rod ends:
Compact Structure: The rotary + linkage configuration offers a compact design, making it ideal for small spaces where large movement ranges are required. This allows for efficient use of space in humanoid robots.
Dual Functionality: The system serves both as a drive and a load-bearing component, simplifying the overall structure and reducing the number of parts needed. This streamlines the robot’s design and reduces its complexity.
Controlled Trajectories: With the rotary + linkage mechanism, the motion trajectory is highly controllable, enabling more precise, human-like, non-uniform motion. This is essential for robots that need to perform complex and varied tasks.
High Torque and Reaction Force Tolerance: The rigid structure of the linkage system can withstand significant torque and reaction forces, which are important for humanoid robots that need to maintain stability and perform various movements under different conditions.
Cost Efficiency: Compared to other configurations, the rotary + linkage system tends to be more cost-effective, making it a preferred choice for many robotic applications.
Application of Rod Ends in the Rotary + Linear Configuration
Below is a diagram demonstrating the rotary + linear configuration used in the XPeng Iron Robot’s arm, leg, and thigh.
While the rotary + linkage configuration is the main application for rod ends in humanoid robots, the rotary + linear configuration also uses rod ends in some parts. This configuration integrates rotary and linear motion degrees within the same drive unit, making it suitable for tasks requiring high load, stability, flexibility, and perceptual feedback.
The rotary + linear system has several notable advantages:
Space Efficiency: The ability to use longer and stronger actuators in a compact space leads to better output force. This is crucial for humanoid robots that need to lift heavy objects or maintain balance during dynamic movements.
Self-locking Capabilities: The linear actuator in this system can be designed to self-lock, enhancing energy efficiency and stability during operation.
Enhanced Functionality: The system’s design allows for improved thrust in the upper arm (mimicking biceps), more freedom in hand movement, and reduced interference from the forearm in visual feedback systems, improving the robot’s visual positioning accuracy.
When Rod Ends Aren’t Needed: The Rotary + Rotary Configuration
Below is the structural layout of the Agibot X1 humanoid robot, illustrating how a fully rotary-based joint architecture stacks multiple joint motors to deliver multi-degree-of-freedom motion.
On the other hand, the rotary + rotary configuration typically does not require rod ends. This setup consists of two or more rotary actuators (usually servos) connected in series to form a multi-degree-of-freedom joint. Each actuator controls one rotational degree of freedom, with fixed constraints between the actuators.
The main advantage of the rotary + rotary configuration is its ability to provide high degrees of freedom, enabling the construction of composite joints with 2-3 degrees of freedom. This configuration allows each degree of freedom to have independent rotational control, offering high decoupling in motion.
However, the rotary + rotary configuration also comes with some drawbacks:
High Power Consumption: Multiple motors running simultaneously increases power consumption, which can be inefficient, especially in energy-sensitive applications.
Increased Inertia: The combined motor system leads to higher rotational inertia, which can negatively affect the dynamic performance and speed of movement.
Conclusion
Rod ends are crucial components in humanoid robots, especially in configurations like the rotary + linkage system, where they serve as both a driving and load-bearing mechanism. Their ability to provide controlled, flexible, and human-like movements makes them indispensable in achieving high-performance, dynamic motion in humanoid robots. With the demand for efficient, cost-effective solutions in robotic articulation, the role of rod ends is only expected to grow as humanoid robotics continues to advance.
