This video chronicles the year-long development of "Cara," an advanced robot dog distinguished by its unique capstan-driven joints, an acronym for "CAPstands Are Really Awesome." The creator explains the engineering advantages of capstan drives, such as zero backlash and high precision, and details the iterative process of optimizing their 8:1 gear ratio. The build covers the selection of a five-bar linkage leg design, integration of high-torque brushless motors and Field-Oriented Control (FOC) drivers, and extensive software development for motor tuning, inverse kinematics, and gait generation, ultimately favoring a smooth cycloidal step trajectory. Structural components are largely 3D printed from durable materials like polycarbonate and PET carbon fiber, reinforced by carbon fiber tubes. A significant challenge addressed was the robot's stability; this was achieved by incorporating an Inertial Measurement Unit (IMU) and developing complex inverse kinematics that allow Cara to dynamically adjust her body tilt to maintain balance. The video concludes with successful demonstrations of Cara's fluid and responsive movements, highlighting the potential of rope-driven robotics, and announces plans for future upgrades and an open-source, smaller version.
Building Cara: The Rope-Driven Robot Dog
0:00Introduction of Cara: Cara is introduced as the creator's fourth and most dynamic robot dog, uniquely featuring capstan drives for its joints, standing for "CAPstands Are Really Awesome."
0:58Capstan Drive Principle: Capstan drives are explained as rope-driven speed reducers offering virtually zero backlash, low noise, and low cost, making them highly suitable for robotics. They utilize Daea DM20 rope, known for its negligible creep rate.
2:02Achieving Precise Gear Ratios: The challenge of attaining a perfect 8:1 gear ratio was resolved by calculating the "effective diameters" of the drums, which account for rope thickness, and by interpolating data from arbitrary test drives.
3:44Leg Design and Actuators: The robot uses a five-bar linkage leg design for efficient load distribution and uniqueness, incorporating an additional degree of freedom. Joints are powered by high-torque Eagle Power 90 KV brushless motors and controlled by ODrive S1 FOC controllers for precise actuation.
5:14Software Development for Movement: Key programming steps included tuning motor controllers for compliance, implementing a homing sequence for joints, deriving complex inverse kinematics for accurate foot placement, and developing stepping trajectories. A cycloidal trajectory was ultimately chosen for smoother takeoff and landing compared to triangular or rectangular steps.
7:11Full Robot Hardware and Structure: Cara's "brain" is a TinC 4.1 microcontroller, controlled via an RC remote, and powered by a 24V drill battery. The robot's design emphasizes lightness, compactness, modularity, and durability, utilizing 3D-printed parts from high-strength polycarbonate and PET carbon fiber (with sponsorship from Bamboo Lab), along with carbon fiber tubes for the frame.
15:08Addressing Early Failures: An initial outdoor test led to a snapped upper leg link. This was addressed by redesigning and beefing up weak parts, and by reducing step length to manage the robot's momentum (Cara weighs 31 lbs/14 kg).
16:08Implementing Dynamic Stability: The most challenging aspect was achieving stability. This involved integrating an Inertial Measurement Unit (IMU) to measure the robot's tilt and deriving a new set of complex inverse kinematics equations that enable Cara to dynamically shift her body weight and tilt to maintain balance on uneven surfaces or when external forces are applied.
17:51Successful Demonstrations: Following stability implementation, Cara showcases fluid and responsive movements, including walking in various directions and effectively recovering balance when pushed or operating on uneven terrain.
21:21Future Plans: The creator plans further upgrades for Cara and aims to develop a smaller, open-source version of the robot complete with a step-by-step build guide.