Caden Kraft's 3D-Printable Quasi-Direct-Drive Actuators Cost as Little as $40 Each

Mechanical engineering undergrad Caden Kraft was dismayed at the price tag for quasi-direct-drive (QDD) actuators — so set about designing his own 3D-printed version, buildable for as little as $40.

"In my last SCARA [Selective Compliance Assembly/Articulated Robot Arm] arm project, I had a very frustrating experience designing high torque/high accuracy gearboxes for the Nema 23 stepper motors I used," Kraft explains. "Ideally for that project I could have used fancy off the shelf servos or QDD (quasi-direct-drive) actuators like the MIT Cheetah. The problem with these is that they generally cost ~$500-1000 each, which is something I can't really justify for my projects. This led me to want to develop my own QDD actuator with a heavy focus on keeping the cost low, both on the parts and tools needed to create one."

Quasi-direct-drive (QDD) actuators combine a radial brushless direct-current (BLDC) motor with a gearbox reduction inside the stator, and are frequently seen in high-end quadrupedal robot designs — including MIT's Cheetah. Their high price, though, keeps them out of reach for most hobbyists, which is where Kraft's project comes in.

Kraft's homebrew QDD actuators are based around a 3D-printed Halbach array, designed using magnetic simulations to test four different configurations before settling on a final design that includes 42 N52 magnets as the main pole pairs and 42 N50 magnets for the Halbach array. The planetary gearbox, meanwhile, uses an unusual rounded design generated using Eelco Hoogendoorn's open source pygeartrain tool — expanded by Kraft to allow the design to be exported for 3D modeling and printing, generating rounded gear teeth to work around limitations of common 3D printers. Finally, a 36-slot stator completes the design.

"I wanted this actuator to have an integrated controller," Kraft adds, "and to keep costs down I chose to go with the MKS X Drive Mini. This is a low cost controller based on the older ODrive 3.6 and has a built in hall effect encoder making it suitable for integrating directly into the actuator. I printed all of the parts out of PA6-GF for some extra strength but I probably should have used just normal nylon filament for the gears if I had to do this again because the glass additive might cause abrasion over time."

In testing, the actuator was able to easily hit a peak holding torque of 14Nm — limited only by its power supply. A replacement PSU revealed an actual upper limit of 29.4Nm, well above Kraft's target of 10Nm. "Even in this test there was no mechanical or electrical failure and it only stopped because the controller had reached the 50A current limit I had set," he notes. "However, I got the feeling that I was reaching the edge of what both the coils and plastic gearbox were capable of, so I left it at that."

The project is documented in full on Kraft's website, with design files available on GitHub under the permissive MIT license; Kraft estimates the bill of materials at $40, or $70 including the cost of the controller.