Two views of a motor are shown. On the left, a ring of copper-wire-wound stator arms is visible inside a ring of magnets. Inside this, a planetary gearbox is visible, with three mid-sized gears surrounding a small central gear. On the right, the same motor is shown, but with the internal components mostly covered by a black faceplate with brass inserts.

A Budget Quasi-Direct-Drive Motor Inspired By MIT’s Mini Cheetah

It’s an unfortunate fact that when a scientist at MIT describes an exciting new piece of hardware as “low-cost,” it might not mean the same thing as if a hobbyist had said it. [Caden Kraft] encountered this disparity when he was building a SCARA arm and needed good actuators. An actuator like those on MIT’s Mini Cheetah would have been ideal, but they cost about $300. Instead, [Caden] designed his own actuator, much cheaper but still with excellent performance.

The actuator [Caden] built is a quasi-direct-drive actuator, which combines a brushless DC motor with an integrated gearbox in a small, efficient package. [Caden] wanted all of the custom parts in the motor to be 3D printed, so a backing iron for the permanent magnets was out of the question. Instead, he arranged the magnets to form a Halbach array; according to his simulations, this gave almost identical performance to a motor with a backing iron. As a side benefit, this reduced the inertia of the rotor and let it reverse more easily.

To increase torque, [Caden] used a planetary gearbox with cycloidal gear profiles, which may be the stars of the show here. These reduced backlash, decreased stress concentration on the teeth, and were easier to 3D print. He found a Python program to generate planetary gearbox designs, but ended up creating a fork with the ability to export 3D files. The motor’s stator was commercially-bought and hand-wound, and the finished drive integrates a cheap embedded motor controller. Continue reading “A Budget Quasi-Direct-Drive Motor Inspired By MIT’s Mini Cheetah”

DIY Robotic Actuator Built For Walking Robots

[Aaed Musa] has built a variety of robots over the years, but found off-the-shelf servos to be underwhelming for his work. Thus, he set out to build a better actuator to support his goals of building a high-performance walking bot in future.

[Aaed] decided to try and build a quasi-direct drive actuator, similar to those used in MIT’s agile mini Cheetah robot. It consists of a powerful brushless DC motor driving a 9:1 planetary gear reduction built with 3D printed parts, which provides high torque output. It’s designed to be run with an ODrive S1 motor controller with encoder feedback for precise control.

The actuator weighs in at a total of 935 grams. It’s not cheap, with the bill of materials totaling just under $250. For your money, though, you get a responsive robotic actuator with a hefty holding torque of over 16 Nm, which [Aaed] demonstrates by having the actuator shake around some dumbells on a long lever arm.

Walking robots have exploded in popularity ever since Spot hit the scene. We’ve seen everything from complex builds to super-simple single-servo designs.

Continue reading “DIY Robotic Actuator Built For Walking Robots”