November 29, 2019

Building All The Robots

Here's a photo-dump of some pictures from assembling the herd of Mini Cheetahs.

150 Stators:

150 Front actuator housings with output bearing installed:

50 upper legs.  The upper link was consolidated from 3 parts to 2 for the new robots:

For assembly I got these fancy Wera adjustable torque screwdrivers, so I (and people helping assemble) could repeatably torque all the screws:

A handful of the 450 planet gears with needle bearings installed:

150 rings gears with OD's post-machined to size.  Ready to be pressed into the housings:

A sea of sun gears:

Actuator with stator and gearbox installed:

I made several minor design mistakes I had to fix.  Some holes in the sheet metal were 1mm off, so I made this fixture for re-machining the holes.  Small mistakes like this sunk a huge amount of time, because each on has to be fixed between 50 and 150 times.  Fortunately everything was salvageable.

Most of the actuators with stators and planetary gearboxes installed:

Part of one robot body with computer, CAN interface, IMU, RC receiver, and power supply:

The glorious actuator cabinet:

Computers and CAN breakouts:

Power supplies, body center-plates:

Belts installed, ready for tensioners and outer half of the leg clamshell.

A mostly assembled leg:

A nearly complete robot, waiting for the last few wire-management steps and final body assembly:

One robot during electrical and software testing, before final assembly:

Half a robot's worth of actuators:

Final assembly step of attaching the legs to the body:

Robot with top panel removed:

A pair of robots ready for final testing:

Most of the new robots lined up on the lab treadmill:


  1. I just love these detailed pics!

    Could you share how you decide on belt tension and how do you properly measure it? What's the center distance between the pulleys compared to belt length - how much slack do you introduce? During any of the stressing motions (e.g. backflips) did you experience the belts jumping teeth?

    1. There have been no belt-skipping or breaking problems. The robot has tensioners, so I left just a little bit of slack and adjusted the tensioners by ear. The Gates Poly Chain Design Guide has tension information for 8 and 14mm pitch belts, but not 5, but you could probably scale their data appropriately. The gates guide also shows several techniques for measuring tension.

      For this application it's important to use the belts with kevlar tensile members, not fiberglass, as they're stiffer, stronger, and handle shock loading better.

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  3. "Small mistakes like this sunk a huge amount of time" I really agree with that.
    Now I'm following your path to build a such an masterpiece.
    I will share you after I finish this job ^ ^.

  4. Hello Ben,
    very impresionant, was a big editing job that took you several weeks, an accelerated video editing will have been interesting, I noticed you change the design for the actuator, now you add a new big ball bearing and close the planet gears (grease), but I was wondering if you removed 6702 that was inside the rotor,
    one last question the PSU is still for a Kobalt 24V batterry only and if we can find PSU's eagle PCB design
    If not good luck in your projects, and thank you for your blog

    1. Yes, I removed the 6702 bearing now that there are two bearings directly supporting the planet carrier.

      Sorry, I don't have the PSU design posted anywhere right now.

  5. Any chance of doing a YouTube video with parts list for the legs?

    1. I didn't film much of the assembly process. Plus I don't really like youtube as a documentation medium - it's just free video hosting to me.

  6. Hi Ben,
    how are you doing?

    I was just wondering, could you by any change share with us, what kind of motors did you use for the Mini Cheetah robot? That would be awesome, since I want to do some experiments with them and build some kind of robot eventually.

    One more question, did you buy all of the planetary gears on the internet or are they custom made?

    Thank you so much what you're doing and keep up the good work.

  7. Hi, Great jobs.
    Where did you buy the gear sets? Misumi gears are two expensive. If you know any cheaper vendor please let us know.