Aaed Musa’s Capstan Drive-Equipped Robot Dog Amazes Us
Aaed Musa has spent years developing capstan drives for robotics applications and put those drives to use building a robot dog called CARA.
There are many, many ways to apply mechanical advantage to the output of an electric motor to achieve desired speeds, torque, and precision. All of them have both advantages and disadvantages, but similar robots tend to have similar needs. That’s why we see a lot of robotic actuators with planetary gearboxes, cycloidal drives, and harmonic (strain wave) gearing. But though the “perfect” system hasn’t been found, because it can’t actually exist, Aaed Musa’s capstan drives are very appealing and to prove it, he built a robot dog called CARA.
The term “capstan drive” refers to its resemblance to the capstans you see on ships. The basic idea is that the rotating electric motor winds a rope or cable one way or the other, which forces the output shaft to rotate accordingly. But the ratio of the motor’s spool diameter and the output shaft’s pulley diameter can be very different, resulting in a gear reduction. Capstan drives have several advantages, including low cost, loose fabrication tolerances, smooth and quiet operation, very little backlash, and easy assembly.
And if you google “capstan drive,” the first result you’ll see is Musa’s website. That’s because he has been a pioneer in the development of maker-friendly capstan drives and has done more than anyone else to bring them to the attention of amateur and even professional roboticists.
CARA, which stands for “Capstans Are Really Awesome,” cements Musa’s reputation in this space. CARA is a robot dog with 12 actuated joints, each with its own 3D-printed capstan drive powered by an Eaglepower 90KV brushless DC motor. They have 8:1 reductions and are driven by ODrive S1 controllers with magnetic encoders.
A Teensy 4.1 microcontroller development boards sends commands to the ODrive S1 controllers through a CAN bus using an SN65HVD230-based Waveshare transceiver. The Teensy, in turn, receives movement commands from a basic Radiolink eight-channel radio transmitter and receiver set. A Sparkfun BNO086 IMU helps CARA sense its own orientation, relative to the Earth. Power comes from a 24V 3000mAh battery, which Musa says is probably too small for the application. But it is what he had on hand.
With that hardware, Musa was then able to develop walking gaits for CARA. Because each leg has three actuated joints, Musa had quite a bit of freedom to experiment and find an optimal gait.
The resulting movement is very impressive. CARA can walk with a lot of grace, compensate for surfaces with a high grade, and even jump — a feat that has become a necessity for showing off in the robotics world.
If you want to try Musa’s capstan drive design yourself, you can get the files through his Patreon — either by becoming a member or purchasing the files directly.