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I am building my own Quasi Direct Drive actuator and looking for information/ design guidelines for building a High Torque Motor similar to the MIT Mini Cheetah below. Specifically I cannot seem to find any info on winding patterns used in High Torque designs and preferred Slot/Pole configurations since most I found are for your standard BLDCs. I am wondering if anyone here has industry or research experience building actuators for High Torque, High backdrivability applications. Any help would be appreciated!

MIT Mini Cheetah Teardown Video

Edit: Found a spreadsheet that covers the information I was interested in -> https://docs.google.com/spreadsheets/d/1AZ2w6lbniuLydnSUgLaUv4zhjWA-wICHkOnHHVaU8Mg/edit#gid=352296252

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    \$\begingroup\$ Powwer = Torque x Speed, so high torque means low speed. There is no much difference between motors as you call them high torque VS. standard. What does high backdrivability mean? \$\endgroup\$ Oct 12, 2021 at 6:25

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Backdrivability of geared drives is poor due to friction in the gearbox.

So for high torque, choose a high pole count inner or outer motor design, which translates into low RPM/V.

Although, it appears the Blind Cheetah 3 uses a special gear-chain driven, low-mass compact motor.

So without specs, your question is unanswerable.

i.e. size, torque, speed, inertia, cost and backlash specs, we're kind of "spinning our rotors. "

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    \$\begingroup\$ I'd like to add, backdrivability is null if ther's a worm gear in the train, as in most high reduction factor \$\endgroup\$ Oct 12, 2021 at 6:48
  • \$\begingroup\$ I guess I made a poorly formulated question. Lets say I was designing an actuator and I wanted it to be QDD and <24 reduction factor. In my approach it would be most logical to begin with what options I have for first designing the motor part of the actuator like the pole/slot configuration and its corresponding winding pattern that has already been tested. After that I will see what reduction my gear box can be to achieve some values of desired torque, but these options all stem from the initial possibility of the pole/slot and winding config. Lets put backdrivability aside for now. \$\endgroup\$ Oct 12, 2021 at 20:44
  • \$\begingroup\$ Yes without all the specs a design is not possible. youtube.com/watch?v=YsSm65DAcCg \$\endgroup\$ Oct 12, 2021 at 22:01
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For high torque applications you want to maximize motor radius over axial length. See my previous post.

How to select BLDC motor for high torque applications

In addition, you want to use the highest energy product magnets possible (Referred to as BHmax). The energy product is generally proportional to motor torque. The competing issue is that the higher energy product magnets you select the less thermally stable the magnets are. So you have to balance temperature vs torque. I think most motor designers usually use something close to a N45SH grade magnet material.

In terms of winding patterns there are formulas you can use. I would recommend Duane C. Hanselman book: "Brushless Permanent Magnet Motor Design 2nd Edition". This book provides an appendix with many different winding combinations and the associated harmonics that you would definitely find useful.

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  • \$\begingroup\$ Thank you, this exactly the starting point I was looking for! \$\endgroup\$ Oct 19, 2021 at 16:51

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