Over the past year, Aaed Musa built a robot dog called CARA, short for "Capstans Are Really Awesome". The design does not involve gears or pulleys like traditional ones. CARA uses something called capstan drives that move joints using ropes wrapped around rotating drums. This system has numerous advantages, such as zero backlash, high torque transparency, low inertia, low noise, and low cost.
After careful rope and drum sizing, the robot's joints use a capstan gear ratio of 8:1, which means the motor turns 8 times for 1 turn of the joint. It uses a 5-bar linkage leg design. This allows even load distribution across motors and a more compact structure. The leg has three joints in total. A 2 mm Dyneema DM20 rope is used. It has zero creep and hence won’t stretch over time. It is powered by Eaglepower 90KV BLDC motors and controlled by ODrive S1 FOC controllers with magnetic encoders to detect position. Magnets and 3D-printed parts are used for positioning and cooling.
A Teensy 4.1 microcontroller is the brain of the unit. Its legs are attached to a carbon fiber tube frame. CAN bus (Controller Area Network) is used to connect all motor controllers and communicate. The robot is controlled remotely through an 8-channel RC transmitter and walks using a trotting gait. It is capable of tilting its body in the opposite direction of the ground’s slope using the IMU sensor to stay stable. A smaller CARA that’s easier to build is planned for the future.
