The Challenges in Building Humanoid Joints Part 3: the Archimedes Drive
To conclude this series of articles on humanoid robot joints, we will now focus on the future and solutions when it comes to technical difficulties. Humanoid robots are still facing issues in terms of software and hardware. Here we will focus only on the hardware solution, especially when it comes to actuator performance.
The development of robotics often encounters limitations with traditional actuators, which combine motors and gearboxes. Recognizing these challenges, the introduction of the Archimedes Drive represents a significant innovation. This newer technology surpasses the constraints of conventional designs by offering improved precision, efficiency, and adaptability. It signals a promising shift towards more advanced and capable robotic systems, opening new possibilities for technological progress in robotics.
1. Enhanced Balance: through Precise Movements
With zero backlash, the Archimedes Drive significantly improves the balance of humanoid robots by enabling more precise and controlled movements. Its innovative design with flexroller allows for exact positioning and smoother motion without lost motion, essential for maintaining stability during complex tasks or when navigating uneven terrain. This precision ensures that the robot can execute finely tuned adjustments in real-time, crucial for keeping its balance in dynamically changing environments.
The direct and precise response of the Archimedes Drive also reduces the need for complex control algorithms to compensate for mechanical play or inaccuracies. This simplification not only makes the system more robust but also more efficient, as control efforts can be minimized, leading to lower energy consumption and faster response times.
2. Weight Efficiency: Optimal Torque without Added Bulk
When designing actuators for robotics, it’s crucial to consider two types of torque: nominal torque and security torque. Nominal torque is the regular operating torque that the actuator uses to perform its everyday functions. Security torque, on the other hand, is essential for ensuring the actuator can withstand unexpected stresses, such as those encountered during accidents or shocks. This requires the actuator to handle higher torque values than usual to survive potential impacts without damage.
Traditionally, when selecting an actuator that includes a conventional gearbox, engineers must choose an actuator with a higher torque capacity to ensure it can cope with these shocks. This often means opting for a larger, heavier actuator. However, this requirement for increased size and weight can conflict with the design goals for humanoid robots, where compact and lightweight joints are preferred, particularly in areas like the wrist and elbow where space and weight are critical constraints.
The Archimedes Drive offers a unique solution to this problem. Unlike traditional gear-based actuators, the Archimedes Drive can enter a sliding regime under extreme torque or shock situations. This drive is composed of rolls that simply slide when subjected to excessive force, rather than locking up or breaking, as gears might in conventional actuators. This innovative feature, the over-torque protection, means that the rolls can absorb and dissipate energy without transmitting catastrophic forces to the gearbox itself.
Thanks to this sliding regime, there is no need to select an actuator with a larger size to account for security torque with the Archimedes Drive. Engineers can focus on choosing an actuator based solely on the required nominal torque, simplifying design considerations and enabling the integration of smaller, lighter actuators into humanoid robots. This advantage makes the Archimedes Drive particularly suitable for applications where space efficiency and weight reduction are crucial, enhancing the overall performance and durability of humanoid robotic systems.
Below you can see a demonstration of our DELTA-15 Archimedes Drive being overtorqued. Once this happens, the drive acts as a friction brake, and when the extra load is removed it recovers its normal operating regime.
3. Enhanced Sensory Feedback: A transparent drive.
When an actuator is both highly efficient and backdrivable, it ensures that changes at the output (such as forces or torques encountered) are accurately and promptly reflected to the input side, we talk then about transparency in the system. This characteristic is particularly crucial in the context of humanoid robots.
- Transparent mechanism: In engineering and robotics, a transparent mechanism often implies that the system’s behaviour can be predicted and effectively controlled, as there is a clear relationship between input actions and output responses.
For instance, if a humanoid robot equipped with a highly efficient and backdrivable actuator encounters an obstacle, the actuator facilitates the immediate transmission of the torque experienced back through the system. This capability allows the robot to “feel” and react to external interactions accurately, enhancing its ability to interact with its environment safely and effectively ( you can read more on this topic here). In contrast, actuators with lower efficiency lose a significant amount of energy between the input and output.
For example, consider an actuator with only 40% efficiency: If the robot’s limb strikes an object, the actuator would only transmit 40% of the encountered torque back to the control system. This situation requires the system to perform additional calculations to estimate the missing 60% of the torque, potentially leading to inaccuracies and miscalculations. Furthermore, with such low-efficiency actuators, the robot would need additional sensors to accurately measure the actual torque encountered. These extra sensors increase system complexity, cost, and the potential for sensor-related errors. They also add to the maintenance and calibration requirements of the robotic system.
Conversely, with an Archimedes Drive, which typically operates at around 90% efficiency, if a robot encounters a force, about 90% of that force is conveyed back through the actuator. This high level of efficiency dramatically simplifies the system’s task of determining the total torque, as it only needs to infer about 10% rather than 60%. This reduction in guesswork and computational demand leads to fewer errors and a more reliable and responsive robotic system. Additionally, the higher efficiency of the Archimedes Drive reduces the need for additional sensors to track torque levels. With fewer sensors required, the system becomes simpler and less prone to sensor-related errors, enhancing the overall robustness and reliability of the robot.
The Archimedes Drive’s high efficiency and backdrivability not only enhance the robot’s interactive capabilities but also improve its overall performance by reducing the likelihood of errors associated with force and torque measurements. This leads to a more streamlined, cost-effective, and easier-to-maintain robotic system.
