Harmonic Drive
Description
en.wikipedia.org/wiki/Harmonic_drive
Harmonic drives have an elliptically deformable inner gear (the flex spline) that is meshed into a ridged outer gear (the circular spline) using an elliptical horn ( the wave generator) that attaches to a motor. As the wave generator moves it deforms the flex spline and changes its contact points with the circular spline. Every full rotation of the wave generator moves the flex spline by only by the difference in the number of teeth in the flex spline and the circular spline. This design has 32 teeth in the flex spline and 34 teeth in the circular spline. See more details:
projectbiped.com/projects/harmonic-drive
Check out the video of the drive in action:
youtube.com/watch?v=vF3HHH-Et4c
Instructions
projectbiped.com/prototypes/fobo
The main technical addition to the basic harmonic drive design is the stabilizer which attaches to both the flex spline and the circular spline to constrain the output force to the radial axis. This reduces sheering forces on the flex spline that would otherwise destroy it quickly.
I went through around 25 iterations for the design for the flex spline and the current version has stood up to about 3 hours of use without breaking. Some tweaking of the printer settings might make the flex spline more durable. I need your help to make this design better so that everyone can have access to this great technology.
The main parts needed for the drive are:
2 radial ball bearings
1 standard sized continuous servo
48 bbs
1 radial potentiometer
A full parts spread sheet that includes pricing and links to vendor web sites is available here:
docs.google.com/spreadsheet/ccc?key=0Ai_h1KTMNaWNdE8xRU5jdWNGSlhWRlNQRWlXej A0NXc#gid=0
(note that some of the parts must be purchased in greater volume than necessary for a single build)
Printing the parts should take about 2.5 hours depending on your printer setting. The tolerances are really important for this build, so some tweaking of the settings might be necessary to get a happily meshing harmonic drive (be patient and edon’t despair, it is definitely possible).
Modify the servo to be continuous if it isn’t already. A good tutorial on how to do this can be found here:
societyofrobots.com/actuators_modifyservo.shtml
Assembly of the drive should take about around 20 minutes. Step by step animated instructions can be found here:
projectbiped.com/projects/harmonic-drive
Testing the drive can be done with any microcontroller that can control a servo. I used an Arduino Uno to control the Harmonic Drive. Follow the Servo/Knob example that comes with the Arduino development environment (for both the wiring and software).
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Made one out of PLA
&
amp; putting it together. Pretty sure it will self destruct, the prints didnt come out great. will try again.
Nice, your parts look pretty good from the photo! I have a sizable pile of destroyed gears that were rejects from my printer parameter tweaking;-) I'll be interested to hear if the PLA is too ridgid to flex properly. I image that there are some adjustments of the wall thickness/printer settings that must make it possible ... after all they make the flex spline out of metal in commercial Harmonic Drives ;-) Good luck and let me know how it turns out!
This is something I need to try. I have a 2 questions, do you think mounted with a servo MG995 there would be enough torque to levitate my "hand robot" at the elbow and shoulder level (http://www.thingiverse.com/thi..., the whole arm should weight around 1.2kg. Also I was looking at your software FOBO Poser which seems to be a super nice piece of work, and I was wondering if it could be adapted to control my above project.
Thanks for your time.
Cool hand! The MG996 used in the drive is rated at 10 kg/cm (which I haven't verified), so with the 16 to 1 gear ratio you should get 160 kg/cm of torque. I imagine that the mechanical friction in the system will reduce this substantially, but I haven't measured the output. I will say that I'm not able to keep the drive from turning with my bare hand (which is not the case with just the servo).
One super nice feature about the harmonic drive is that it keeps its position without power. This could be great for your arm ... you wouldn't have to burn a lot of power holding a specific position.
The Poser software definitely could be modified to work with your arm. All of the source code i
s available on the site for download. I'd be happy to give some assistance/pointers ... just let me know.
Awesome! I guess you have tested some different types of plastic for the spline, which
gives the best results?
This is an animation showing how the harmonic drive works (from the Harmonic Drive wikipedia page).
License
More from jdow
Amazing!
Could you please post a link to the source blender files? This is just begging for a NEMA17 stepper motor implementation!
Printing a copy now on an accelerated 'bot. Lets see how the print turns out!
Sorry, I thought I had already posted the Blender file. The file is now posted! Let me know if you have any questions about the organization of the blender file. All of the parts are visible on the first layer and then most of the other layers have a single part.
A stepper motor version would be great ... that would have some crazy torque. Please share if you come up with something cool!