An Open Torque-Controlled Modular Robot Architecture for Legged Locomotion Research (1910.00093v2)

Abstract: We present a new open-source torque-controlled legged robot system, with a low-cost and low-complexity actuator module at its core. It consists of a high-torque brushless DC motor and a low-gear-ratio transmission suitable for impedance and force control. We also present a novel foot contact sensor suitable for legged locomotion with hard impacts. A 2.2 kg quadruped robot with a large range of motion is assembled from eight identical actuator modules and four lower legs with foot contact sensors. Leveraging standard plastic 3D printing and off-the-shelf parts results in a lightweight and inexpensive robot, allowing for rapid distribution and duplication within the research community. We systematically characterize the achieved impedance at the foot in both static and dynamic scenarios, and measure a maximum dimensionless leg stiffness of 10.8 without active damping, which is comparable to the leg stiffness of a running human. Finally, to demonstrate the capabilities of the quadruped, we present a novel controller which combines feedforward contact forces computed from a kino-dynamic optimizer with impedance control of the center of mass and base orientation. The controller can regulate complex motions while being robust to environmental uncertainty.

• The paper presents a novel, low-cost actuator module for torque-controlled robots, achieving 2.7 Nm joint torque with a lightweight 150 g design.
• The paper introduces an innovative, sub-10 g foot contact sensor that reliably detects high-impact contacts during dynamic locomotion.
• The paper demonstrates a kino-dynamic impedance control strategy that achieves leg stiffness comparable to human performance with a dimensionless stiffness up to 10.8.

Open Torque-Controlled Modular Robot Architecture for Legged Locomotion Research

The paper introduces a novel open-source architecture for the development of torque-controlled robots intended for use in legged locomotion research. At the core of this architecture is a low-cost, low-complexity actuator module that incorporates a high-torque brushless DC motor along with a low-gear-ratio transmission, tailored for applications requiring both impedance and force control. The innovation extends to the implementation of a novel foot contact sensor optimized for legged locomotion involving substantial impacts.

The authors present a comprehensive design for a 2.2 kg quadruped robot, which is configured from eight identical actuator modules and four lower leg structures equipped with the proposed foot contact sensors. By leveraging standard plastic 3D printing combined with readily available off-the-shelf components, this robot is notably lightweight and economically viable, facilitating its broad adoption and duplication across various research facilities.

Key Contributions

1. Actuator Module Construction: The actuator module utilizes a brushless motor and a dual-stage timing belt transmission system with a 9:1 reduction ratio. This design prioritizes mechanical simplicity and accessibility, requiring minimal precision machining while favoring 3D printed and off-the-shelf parts. The module is designed to manage high torque outputs needed for dynamic locomotion with a reported maximum joint torque of 2.7 Nm while being lightweight at 150 grams.
2. Foot Contact Sensor: It is offered as an under 10-gram, robust alternative providing effective and reliable contact detection across a wide range of impact scenarios, crucial for robots that encounter hard impacts.
3. System Characterization: Impedance experiments demonstrate achieved leg stiffness similar to human leg performance under running conditions, with a dimensionless leg stiffness reaching up to 10.8. The system was validated both in static conditions and during dynamic drops to ensure reliability under various operating conditions.
4. Kino-Dynamic Motion Control: The paper outlines the use of a novel controller which marries feedforward contact forces derived from a kino-dynamic optimizer with impedance control strategies aimed at stabilizing the center of mass and base orientation.

Implications and Future Directions

The implications of this research extend across both practical and theoretical dimensions within the robotics field. Practically, the open-source nature of the design democratizes access to a sophisticated robotic platform, empowering a wider array of researchers to engage with and contribute to legged locomotion studies. The modular nature of the architecture encourages further customization, potentially leading to adaptations for diverse robotic applications, including manipulation and human-robot interaction scenarios. On a theoretical level, the successful application of impedance control through motor current sensing without external torque sensors provides an insightful exploration into simplifying complex control systems while maintaining performance.

This research underscores the potential for lightweight, cost-efficient robots in testing cutting-edge algorithms related to dynamic locomotion. Additionally, the researchers demonstrate that despite significant environmental uncertainties, the developed controller can adeptly regulate complex robotic movements, aspiring towards more autonomous operation over unpredictable terrains.

Looking ahead, enhancements might focus on improving the dynamic response and control precision of the robots, perhaps integrating learning algorithms to further refine sensorimotor capabilities. Continued development in sensor technology could enhance environmental interaction, allowing these robots to navigate even more complex terrains autonomously.

Overall, this work presents a substantive contribution to the robotic community by providing an accessible and effective platform for advancing legged robotic locomotion research. The potential for widespread usage and continuous development under an open-source model situates this as a valuable tool for ongoing exploration in compliance control and beyond.