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Stanley - the capstan based quadruped

A maker friendly capstan based BLDC driven quadrupedal robot. Based on mjbots electronics

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Goal:
My goal is to get as close to the performance of the MIT mini cheetah as possible while keeping the cost down and the entire build accessible to the average maker. The use of Capstan Drive's allowed me to lower the price of the entire build to a BOM cost of less than 2400USD, where MIT mini cheetah style robots tend to cost upwards of 6000USD to build. Two fabrication techniques are being used - FDM 3D printing which almost every maker now has access to, and 2D milling in FR4 - which is less available, but it is cheap to order these parts in a local shop (this should add 300-400usd to the BOM price). Drivers used are mjbots moteus r4.5, the main computer is a Rpi4. The motors are 90KV 8308's.

Files:
All software I write will be open-source on GitHub
The mechanical design of the leg is available to my 2nd tier Patreons
The mechanical design of the entire robot will soon be available to buy as a package. Patreons will get a discount = the total amount pledged.

The current stage of the project:

Done:

  1. Design a single capstan reducer and build a test stand for it - perform tests and improvements
  2. Design and build a 2Dof leg - with the knee and hip joint
  3. Write a python script for communication with the moteus r4.5 controllers over the USB-CAN_FD adapter
  4. Solve inverse kinematics and scripts for jumping and various other demonstations.
  5. Perform testing on the 2Dof leg - continuous jumping for 1h - inspect, iterate.
  6. Design and build a 3Dof leg - adding the ab/ad joint
  7. Perform testing on the 3Dof leg - continuous jumping for 1h between randomly chosen points.
  8. Arrive at the final version of the leg
  9. Design the chassis
  10. Print a mockup of the chassis before ordering the "real thing" in FR4 to verify the design 

To do:

  1. Get FR4 parts cut for the entire robot
  2. Print and assemble the remaining 3 legs
  3. Assemble the frame
  4. Connect electronics
  5. Addapt Mjbots controll software from quad A1 to work on Stanley
  6. //If'll manage to do that// switch the RPi4 to an "UP board" and run the MIT mini cheetah controller

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Discussions

marazm wrote 7 days ago point

Jaki ma udzwig ten robot. Da się nim przenieść powiedzmy lekarstwa, albo wodę? wystarczyłby np. 2 litry wody + kanapka. Wtedy można było by to coś wykorzystać do transportu a ludzie zapewne chieli by zając się programowaniem. Gotowe moduły mógłbys sprzedawać jak Józef Prusa swoje drukarki

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Jamie McLaughlin wrote 01/09/2021 at 10:35 point

Amazing project!  Are you planning on making the models available at some point?  I'd love to experiment with the design.

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Damian Lickindorf wrote 3 days ago point

Please read the "Files:" section of the Description

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David Greenberg wrote 01/08/2021 at 17:50 point

I really like your design. Do you have any posts I could read about why you chose to use capstans over wobblers?

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Damian Lickindorf wrote 01/08/2021 at 18:21 point

Wobblers?! 

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David Greenberg wrote 01/08/2021 at 19:01 point

Haha, it's a funny name I've heard is slang for cycloidal gearboxes, another small way to get a big reduction. I know that harmonic drives (which now sums up the 3 compact low-to-zero backlash gearboxes I know of) can be annoying due to the constantly changing flexion of one component, which can drive up costs. I think yours is the first capstan quad I've seen, and I'd love your thoughts on why capstans over cycloidal drives. I can send you some links to cycloidal drive quads if you haven't seen that design.

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Damian Lickindorf wrote 01/09/2021 at 01:16 point

@David Greenberg You could compare against many things, cycloids aren't even a worthwhile comparison cause the range of reduction ratios they offer has very little overlap with what u get with capstans. First the ratio - you need less than 1:9 to have a proper Quasi Direct Drive actuator, to achieve the torque transparency required to apply advanced controllers like the MIT mini cheetah one. (this is not true for all scales, but at this scale it is). So here cycloids are out of question already - just because of the ratio - they are also bad for many other reasons I won't get into. The only worthwhile comparison is to the planetary gearboxes in the actuators used in MIT mini cheetah and all similar bots. A planetary reducer is a better solution, it might be the best available - but only if you can fabricate your bot in metal. I decided to limit myself to 3D printing - and when we consider 3D printed planetary gearboxes - they would need to be really bulky, contain a lot of bearings and would wear quickly, plus will never be truly 0 backlash. A Cable drive can be as efficient as a planetary (also as torque transparent) - and at the same time, it does not suffer because of the limits of 3D printing. There are no contact spots that are under loads as high as the loads on teeth of a planetary gearbox. Thanks to this a highly torque transparent yet lightweight and robust to high dynamic loads can be manufactured using only 3D printing.

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David Greenberg wrote 01/09/2021 at 01:20 point

Your reasoning makes a lot of sense. If you have any references on other downsides of cycloids, I'd be interested in reading, especially as to whether the downsides are fabrication or control related.

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Damian Lickindorf wrote 01/09/2021 at 01:54 point

@David Greenberg the downsides are mainly low efficiency, and high massif higher efficiency is attempted (you could easily fit 50 bearings into a cycloidal gearbox if you want all contact to be rolling contact). I have experience making cycloids, they are good for robotics, even cobot-style robotics - but legs need much more than that in terms of torque transparency.

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Larock1234 wrote 01/08/2021 at 09:00 point

This is amazing! I had a very similar design in my mind for a long time, but never had the time to realize it. I'm an engineer working in haptics where capstan drives are used quite often and I am conviced that they are very nicely applicable to quadruped robots. So I am really stoked to see your amazing implementation of this kind of transmission. Looking already forward to seeing your next progress update! 

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ekaggrat singh kalsi wrote 01/08/2021 at 00:29 point

really cool work . !i wish i could afford to make one :(

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Damian Lickindorf wrote 01/08/2021 at 02:04 point

I did my best to go as low with the price as i could, while still making a true high performance BLDC based quadruped. Below ~2400usd you need to go with hobby servos or dynamixels. There are also cheaper bldc options if you're willing to solder your own ESC's - take a look at the open dynamics initiative 

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Dan Maloney wrote 01/07/2021 at 17:24 point

Nice design, I appreciate the attempt to keep it affordable.

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Damian Lickindorf wrote 01/08/2021 at 02:04 point

Thanks :D was trying my best! 

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