High-torque actuator for legged robots

g-levine · June 20, 2018, 10:48pm

If you haven’t seen this already, here’s a seminar by the founder of Agility Robotics (the company that built Cassie) about the hardware and controls for walking. They’re using series-elastic actuators with real springs, but you could get it to work for direct- or quasi-direct-drive actuators by programming them to act like springs.

Richard_Parsons · June 21, 2018, 2:32am

Apparently the efficiency of spur gears and helical gears is about the same according to this source. However its not clear from the book if this is only true for systems that are well lubricated and so this may not be true for your situation.

Still, if you end up sticking with 3D printed gears it may be worth comparing the efficiency at some point. If the herringbone design is roughly the same efficiency then it would make sense to stick with that design due to its other advantages like lower noise, higher load capacity, less backlash etc.

g-levine · June 21, 2018, 3:35am

Interesting. I didn’t like the herringbone gears since they complicated the assembly process and required splitting the ring gear into 2 parts, but a helical gear could be a good compromise.

g-levine · June 22, 2018, 1:58am

Updated the design with helical gears and active cooling. The duct on the back is sized for one of these frameless cooling fans: https://www.amazon.com/PLA09215B12H-GeForce-GTX760-TiTANX-Graphics/dp/B01M65LJFT

Here’s the STEP file: https://drive.google.com/open?id=1iAIbOn_ztodxXoGs9c7sHcxVPcMHvLLC

madcowswe · June 28, 2018, 9:00pm

8 posts were split to a new topic: How to read the torque of the motor?

g-levine · June 28, 2018, 9:25pm

Finished printing everything out and assembled it. Now it’s time to run some tests.

I’m going to put a thermistor in the windings and see how much current it can take before overheating. I think it can plausibly take 120 amps of peak current thanks to the cooling fan.

Is it safe to put 120 amps through the MOSFETs? I assume I should put a cooling fan on the board, too.

madcowswe · June 28, 2018, 9:32pm

I think 120 should be safe for short bursts. You need to turn up the motor.config.requested_current_range to 120A for the current sense amplifier gain range to let you go that high.
Yes put a fan to the board and keep an eye on the fet temps. Do you have a thermal camera?

madcowswe · June 29, 2018, 2:05am

3 posts were split to a new topic: Thermistors on the ODrive

einzeln · June 29, 2018, 2:07pm

Hi, I am doing electronics and firmware for the same purpose. I have done something like this before, a single stage 1/4-1/5 gear ration cyclic reduction, and later some timing belt based harmonic reduction. Now I hope to join you with your project as Open source development in electronics.

g-levine · June 30, 2018, 3:55am

I made a video showing off some of the features:

STLs, BOM, and assembly instructions coming soon.

einzeln · June 30, 2018, 4:03am

Great potential to do the equivalent to ANYmal robot drive:

Have you begin ROS integration? Looks like OD has no working ROS drive on web i could find at the moment.

g-levine · June 30, 2018, 4:16am

What’s involved in making it work with ROS? I’m going to make a physics model for use in OpenAI/Mujoco reinforcement learning, and I think you could easily import that model into ROS.

einzeln · June 30, 2018, 4:27am

Ahh, I see. I thought you will use this motor as a ROS node to test manipulation trajectory.

g-levine · June 30, 2018, 11:42pm

I set up a Hackaday page for this project. Follow here for future updates:

Valify · August 15, 2018, 7:34pm

This seems like an great fun project! Great work @g-levine
Just got 10pcs of the Turnigy Multistar 9235-100KV and have started printing the parts for the Actuators