Mechanical engineering undergrad Caden Kraft was dismayed on the price ticket for quasi-direct-drive (QDD) actuators — so set about designing his personal 3D-printed model, buildable for as little as $40.
“In my final SCARA [Selective Compliance Assembly/Articulated Robot Arm] arm venture, I had a really irritating expertise designing excessive torque/excessive accuracy gearboxes for the Nema 23 stepper motors I used,” Kraft explains. “Ideally for that venture I might have used fancy off the shelf servos or QDD (quasi-direct-drive) actuators just like the MIT Cheetah. The issue with these is that they often price ~$500-1000 every, which is one thing I am unable to actually justify for my initiatives. This led me to need to develop my very own QDD actuator with a heavy give attention to retaining the price low, each on the elements and instruments wanted to create one.”
Quasi-direct-drive (QDD) actuators mix a radial brushless direct-current (BLDC) motor with a gearbox discount contained in the stator, and are continuously seen in high-end quadrupedal robotic designs — together with MIT’s Cheetah. Their excessive value, although, retains them out of attain for many hobbyists, which is the place Kraft’s venture is available in.
Kraft’s homebrew QDD actuators are based mostly round a 3D-printed Halbach array, designed utilizing magnetic simulations to check 4 totally different configurations earlier than deciding on a closing design that features 42 N52 magnets as the principle pole pairs and 42 N50 magnets for the Halbach array. The planetary gearbox, in the meantime, makes use of an uncommon rounded design generated utilizing Eelco Hoogendoorn’s open supply pygeartrain instrument — expanded by Kraft to permit the design to be exported for 3D modeling and printing, producing rounded gear tooth to work round limitations of widespread 3D printers. Lastly, a 36-slot stator completes the design.
“I needed this actuator to have an built-in controller,” Kraft provides, “and to maintain prices down I selected to go together with the MKS X Drive Mini. It is a low price controller based mostly on the older ODrive 3.6 and has a inbuilt corridor impact encoder making it appropriate for integrating straight into the actuator. I printed all the elements out of PA6-GF for some further power however I most likely ought to have used simply regular nylon filament for the gears if I had to do that once more as a result of the glass additive may trigger abrasion over time.”
The design maximizes efficiency whereas minimizing price, and contains tweaks to work across the limitations of 3D printing. (📷: Caden Kraft)
In testing, the actuator was in a position to simply hit a peak holding torque of 14Nm — restricted solely by its energy provide. A alternative PSU revealed an precise higher restrict of 29.4Nm, properly above Kraft’s goal of 10Nm. “Even on this check there was no mechanical or electrical failure and it solely stopped as a result of the controller had reached the 50A present restrict I had set,” he notes. “Nevertheless, I obtained the sensation that I used to be reaching the sting of what each the coils and plastic gearbox had been able to, so I left it at that.”
The venture is documented in full on Kraft’s web site, with design information out there on GitHub below the permissive MIT license; Kraft estimates the invoice of supplies at $40, or $70 together with the price of the controller.
