OrbitDrive Actuator

OrbitDrive is a compact and modular actuator made for dynamic robots. It consists of a frameless brushless DC motor (BLDC), a  planetary gearbox, and an incremental magnetic encoder for precise, closed-loop control.
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This is an ongoing project and will be constantly being delayed as my full-time job as a mechanical engineer soaks up my time and energy :). However, I will still try to make improvements to this actuator and update this page.

Functional V1.0 actuator, which is a (mostly) 3D-printed version.

The previous design was a mostly 3D-printed version and worked fairly well. As shown in the videos below, the plastic version can exert momentary torque @ up to 5 N*m and can run @ a maximum speed of ~300 rpm.

I am building an all-metal version with simplified stack-up and (hopefully) improved performance. Please see the following sections for details.

Inspiration & Design Novelty

This project was inspired by an open-source actuator designed by Ben Katsz at MIT: https://dspace.mit.edu/handle/1721.1/118671
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Compared to other clone designs from other makers as well as oversea manufacturers, I tried developing my design as independently as possible. The original Mini Cheetah actuator and most other clone designs use a outrunner BLDC drone motor similar the T-Motor U8, Whereas my design uses a frameless in-runner motor from the same manufacturer:

I believe using an In-runner BLDC motor has the following advantages:
- Reduces rotational inertia by some degree
- keep the moving parts of the actuator in the center - reduce complexity
- reduced size with similar torque-to-weight ratio
- components are readily available and easy to acquire.
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V2.0 Building Process

The most difficult part to source is the all-metal planetary gearbox, as I need a specific reduction ratio in a pre-defined small space. Luckily, after some relentless search, I found a Nema 11 stepper motor gearbox that suits my need. With some modifications, this gearbox fits perfectly within my frameless motor.

Teardown of 5 : 1 planetary gearbox from Stepperonline.

The V2.0 actuator is held together by 2 metal brackets, and their precise locations are defined by 4 precision shoulder screws which situate perfectly in the groves of the stator. Each bracket has a large-diameter ball bearing at the center which locate the rotor and the planetary gearbox assembly at the center of the actuator. The ring gear from the gearbox is modified and press-fitted in to the upper bracket.

Exploded view of V2.0 stack-up. Parts that are made from machined aluminum are highlighted.

I prototyped the top bracket and the bottom bracket using a Kent CNC Knee Mill (A Bridgeport-style milling machine with 3-axis CNC). I modeled the parts so that all critical dimensions for the bearing and the motor locating screws can be machined in the first operation. After the first operation, the mounting holes were tapped with M2 threads.

machining-pocket-1, machining-roughing-1

Tapping the part carefully with M2 threads.

I ended up machining a locating fixture anyway to clamp the parts upside-down for a second operation to face away the remaining stock and to add some non-critical geometry. The fixture is a rectangular piece of stock with all 6 sides faced on the mill to increase clamping force and ensure flatness. The tapped M2 holes can now be used to secure the stock to the fixture.

Machining fixture for top/bottom bracket

Finished top bracket

I also machined the rotor adapter, which will be press-fitted into the hollow rotor ring and will hold the sun gear of the gearbox. The part was first turned on a lathe, then clamped onto the Kent mill with v-blocks to finish up the operation. In the future, I would like to set up a collet on the lathe so that I can perform all the operations using the same work-holding to ensure concentricity. The sun gear was then press-fitted to the adapter with retaining compound.

Rotor Adapter

V1.0 Prototyping Process

The previous version is designed to be fully 3D-printed, with the exception of the rotor adapter, which I decided to make out of aluminum since plastic press-fit parts could not withstand the amount of torque being transmitted between the rotor and the planetary gearbox.

Exploded view of V1.0 stack-up

Building the 3D-printed version