This was a monumental change to the field, basically all the humanoids you see now started with this. To someone outside the field:

1. Boston dynamics pioneered hydraulic actuators (for their humanoid Atlas) and some really complex motors for their quadrupeds (I don’t think they ever explained their design)

2. There was a whole research line on Series Elastic Actuators that was pursued in the quadrupeds from Europe, famously Anymal from ETH Zurich.

The main problem they were trying to solve was high torque density with back drivability. If you wanted large torque, you could use a high gear ratio but then you lose back drivability and the walking becomes really stiff. MIT Cheetah basically custom built a motor with a really large radius that just output a really large torque. So to summarize, we had hydraulics, a really complex motors + spring mechanism, a really complex large motor to solve this problem and they all required huge amounts of money and complexity ( arguably MIT cheetah was simple but I heard thermal management was a big headache, and the entire robot cost millions)

Ben Katz, then at MIT, showed that you could use regular BLDC Motors, heavily used in drones with a small gear ratio, like say 1:10 or 1:20 and get a really good torque density, backdrivability tradeoff. It started around the above blogpost, in the coming year as a part of his masters thesis, he build the MIT Mini Cheetah, that was also the first quadruped I saw that performed a backflip, (maybe Boston dynamics was earlier and so it was the second then), and that basically started the goldrush into his actuator tech that we call Quasi Direct Drives.

Looking now 10 years later, in some ways he was vindicated, all humanoids and quadrupeds are made with this actuator now, Unitree is basically QDDs through and through. But the same faults we saw then, still hobble it now. It just does not generate enough torque, its position control isn’t milli degree precise, and so it never took off in industrial robot arms, though many tried (Berkeley Open Arms for one).

[1] The author of the posted article, Ben Katz, designed the MIT Mini Cheetah during his master’s program. His thesis has become very popular in the robotics community as a reference for actuator design, and it is freely available online.

Thesis: A low cost modular actuator for dynamic robots

https://dspace.mit.edu/entities/publication/b85069e2-f1cd-47...

https://www.youtube.com/watch?v=nmViQ1T03Rk

[2] Unitree, LIMX Dynamics, and other Chinese robotics companies may also have referenced or been influenced by his actuator design in the evolution of their own actuators.

https://www.militarytimes.com/industry/techwatch/2026/08/18/...

[3] Ben, tear down a Chinese version of the Mini Cheetah.

https://robot-daycare.com/posts/2022-11-02-mini-cheetah-clon...

https://www.youtube.com/watch?v=t5SHUWb6kH4&list=PL8H6WijZPO...

Next step is to measure inertias and friction (via Ino-load) and then you can start optimizing gear ratios. For a purely inertial load ideal gearing is inertial match. For a complex mix of inertial and friction loads one can simulate things in a fairly straightforward fashion and find optimum gear ratios but remember to iterate on winding temperature - resistivity of Cu changes about 0.4%/°C. Fun stuff!

Ben's writeup of the mini-cheetah actuator remains one of the best resources to learn about robot actuator design and control