A powerful, compliant actuator for legged robotics
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Specs:
OpenTorque is being used in the Blackbird bipedal robot. Check that project page for links to the resources and inspirations I used when designing OpenTorque.
Get the latest CAD files and instructions from the GitHub repository:
opentorque_v2.stepSTEP file of the assembly. STLs for the individual parts are available on the GitHub repo (https://github.com/G-Levine/OpenTorque-Actuator).step - 9.38 MB - 02/15/2019 at 22:27 |
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V2 actuator on the right:
The first OpenTorque v2 unit is built and working successfully, so I'm releasing the design files. You can get them at https://github.com/G-Levine/OpenTorque-Actuator. They're licensed under Creative Commons BY-SA 4.0, so feel free to modify them and use them in your own projects.
I'm currently prioritizing passive cooling because my walking robot will have a low duty cycle on the motors. As a result, this version is releasing with passive cooling only. Active air-cooling will come in a future update. I'd recommend it if you plan on building a robot arm, as the duty cycle on an arm is near 100%.
Other future updates will include support for the single-axis ODrive board (once it's released). This will allow daisy-chained power and data connections between actuators, greatly simplifying the wiring.
This is the new version of the OpenTorque actuator (still under development). I've made a number of design improvements:
These improvements are aimed at making it easier to build systems with large numbers of OpenTorque actuators. I'm using the new actuator for my Blackbird Bipedal Robot but it's also well-suited for quadrupeds, robot arms, exoskeletons, and so forth.
I tested the thermal performance at 30A, 60A, and 100A. This was done by setting the calibration current to the desired level, then disconnecting the encoder and having the motor search for the index pulse. This results in the heat being evenly distributed among the 3 phases, as described here: https://discourse.odriverobotics.com/t/testing-forced-air-cooling-with-a-5060-motor/483. I collected data from a thermistor epoxied into the motor windings, and the built-in thermistor on the ODrive board.
Conclusions:
I'm building a bipedal robot inspired by Agility Robotics' Cassie. It uses 10 OpenTorque actuators.
These actuators are backdrivable and able to absorb kinetic energy during the walk cycle. As a result of this, there's no need for the complex series-elastic linkages present in Cassie. Instead, each leg consists of a simple parallelogram linkage with an extra joint at the ankle.
Carbon fiber tubes are used for the linkages in order to keep inertia as low as possible. The total weight is in the ballpark of 30 pounds -- substantially lower than the 60-70 pounds that Cassie weighs. This is helped by the use of 3d-printed plastic rather than CNC aluminum parts. (All the structural parts will be printed out of NylonX.)
To control this robot, I'm going to use reinforcement learning. I'll create an OpenAI training environment with a simulation of the robot, then let the controller learn a stable walking gait on its own. This is a lot easier than programming a walking gait by hand, and it's already been done successfully on the Minitaur robot.
Printed parts:
Non-printed parts:
Instructions:
Hi Gabrael, I'm new to this, but I work for another bio-mimicry lab and we were hoping to incorporate the OpenTorque actuator into some of our research. What methods have you been using to control the motor or the software involved?
hello, what power supply do you use to test different A? have a link for the power supply?
thz Gabrael
Very nice project. How do you calculate a maximum torque of 80Nm? The manufacturer on Alibaba states a max torque of approx. 5Nm (no rpm given, but i assume it's the peak torque) so with a reduction of 8:1 40Nm would be peak? What are your test results?
In your components you list the "THK RA8008 cross roller bearing" which are currently being sold for $150, but in your directions for assembly you denote the much cheaper "6813-2RS bearings".
I see the design improvements mention the THK RA8008 cross roller bearing being an upgrade component but the cost is enormous! Maybe we can make a version of v2 which continues to use the 6813-2RS bearings.
Hey Gabrael, incredible work! I'm starting a print as I type this. Quick question, the large RA8008 bearing is quite expensive right now. Has the price just gone up since you designed this? Or was it always an expensive part? Any suggestions on the best place to purchase? Or is Alibaba it?
Ah, you and I are noticing the same thing.
I bought a bunch of the other components before realizing this. I wish I'd made a cost spreadsheet first.
Maybe we can/should fork the repo and modify the design to revert back to the 6813-2RS's
Hi, what type of microcontroller do you use for controlling odrive, the MIT Cheetah is using nucleo boards with mbed RTOS, what is your suggestion?
Currently I’m using a Raspberry Pi with the preempt-rt patch. I’m planning to move to a Jetson Xavier later.
Hi Gabrael, for your force control you only use the current of the actuator or do you have some extra sensing ?
Just the current. The gearbox is transparent enough that it doesn’t need any extra sensors for force control.
Ok, what kind of motor is used. Or are you making your own with electromagnetic coils? Also, how does this attach/what part of it turns? I am thinking to maybe use this same concept slightly modified for a wheel drive on a vehicle. I am thinking you could put this inside of an individual wheel, and it would drive itself.
It uses an off-the-shelf brushless motor intended for heavy-lift quadcopters.
The mounting points are on the actuator housing. The output turns with the planet carrier.
Hey Gabrael, thanks for the great project!
Regarding your comment about using cross-roller bearing to make the gearbox more compact - are those simply thinner or will you also be changing the design in a significant way?
could this possibly be thinned, so it is not as thick, and would fit better into exoskeletons?
Yes, with steel gears. Hardened steel gears would be much thinner than plastic gears for the same load rating. They'd be heavier and add about $100 to the cost of each actuator, but if you want maximum compactness it would make sense to use them.
How thin could it get, from the back to a half inch shaft extending from it?
10 to 15mm thinner, depending on which gears you choose. The next version of the actuator will incorporate a cross-roller bearing, which will reduce the thickness by 10mm.
This looks really awesome, this is exactly what i have wanted to do since i first saw Atrias and then Cassie. My only concern is the actuators on the knees, i know there is where the highest load are and it's very convenient to put the actuator directly on the joint. But this will have a BIG impact on the acceleration of the legs, more mass will have a big negative impact on how fast the legs can move from A to B. Have you tried to figure out a mechanical linkage to put the actuator for the knee at the hip level or at least as far from the feet as possible?
To quote Colin Chapman, if you want more speed "Simplify, then add lightness”.
The actuator on the knee isn’t for the knee, it’s connected to the ankle with a linkage. The torque requirements for the ankle are actually quite small (0 torque when the foot is on the ground), so it will be possible to use a much smaller motor there.
The ankle actuator for the final design will either be a direct-drive 9235 motor (no gear reduction) or a 5008 motor with a 4:1 reduction.
Ok, great! also, just noted that i made the comment in the wrong project, sorry.
This is what we are discussing: https://hackaday.io/project/160882-blackbird-bipedal-robot
This image: https://cdn.hackaday.io/images/8728261530673941165.jpg
Now i see that its quite obvious that it is the ankle actuator that is placed on the knee.
Very nice! I just order the motor so I can build one! :-) thanks for sharing!
Hi Gabrael, your progress video seems really promising. I wanted to ask you, have you noticed any wear on the gears, or worsened backlash over time? I want to integrate your design into my robot arm project... I'm still tossing up whether to use a cycloidal direct drive design, or the OpenTorque actuator with a belt drive to the joint.
I haven't noticed any wear or increased backlash on the gears. Just make sure to use nylon and not ABS.
this looks to have real potential, cant wait to see how it develops!
Gabrael Levine
Petar Crnjak
Pavel Surynek
Ted Huntington
Hello Gabrael, your project is really very interesting.
I am currently developing a project with a BLDC motor, and with an Odrive v3.5 controller, which I need to control the torque of the motor. To do some initial tests and get to know the Odrive controller, how can I program it to handle the torque control?
Thank you very much Gabrael and Congratulations!