Thursday, March 7, 2013

Left Shoulder Progress

Sadly, my consultancy is in a parlous state thanks to a perfect storm of crazy circumstances, so my thoughts and time have been almost completely consumed in the past month in doing everything I can to make sure that my contributions are not part of the problems my clients are facing. Over the weekend and in the odd hour this week, however, I've turned back to my project to get the left shoulder, arm and hand of Gael Langevin's Inmoov working.

I have been working on the Kinect motion capture part of the project in recent weeks while my broken UP! 3D printer was properly diagnosed by Brian Quan at X-objects and the warranty replacement parts dispatched.  On Tuesday, a full extruder head assembly arrived which enabled me to put the printer back into service.

Just before the UP! failure last month, I decided to redesign the worm gear box that is used so extensively in the shoulder.  My main objective was to create a smoother running gearbox that required less modifications after coming out of the printer to work.  Initially, I was going to do a radical redesign of the gearbox.  Eventually, however, sanity returned and I decided to make as few mods to Gael's gearbox design as I could so that other people building Inmoov could, if they wanted, use my modified box as well.

Aside from some minor mods to let the box work more smoothly with the involute profile gear and matching worm gear which I published in Thingiverse, the major problem that I wanted to address was the difficulty in assembling the box and especially in mating the worm gear to the servo.

Gael screwed the worm gear onto the servo's nylon turntable.  When I tried that, the screw heads clashed with the gearbox.  I found that when I put the screws in from the nylon turntable side, however, I had ample clearance without disturbing the basic gearbox configuration.

In this configuration it became a simple matter just to plug the servo into the back of the nylon turntable.

Placing the screw heads on the backside of the turntable did, however, create one minor clashing problem.

I had to place the screws on the next line of holes in on the turntable instead of the outside ring as Gael did.  Originally, the gearbox had a rather angular opening to accommodate the servo drive shaft.  I had to open that up a bit to let the screw heads pass properly.  This was no big matter.

Here you can see the nylon turntable and screw heads in the actual modified gearbox.

In a brief correspondence with Gael he mentioned that the weight of the arm was going to be critical to its successful operation.  That got me to worrying that we would eventually have to increase the torque to the gearboxes.  A search revealed several other servos which had much higher torque ratings available at prices either near or not too far from what the Hitech HS-805BB cost me.

I noticed that the original gearbox design depended heavily for stability on the strength of the servo box attached with screws to the gear box.  To lessen that dependence I strengthened the frame containing the gearbox so that more powerful servos could be used.

Finally, I did a little paring on the gearbox to give better clearance to the involute profile gear and the connector between the worm gear and the servo turntable.

The top to the gearbox will require some superficial modifications as well.  I used a Dremel tool with a sander to make them for this exercise and will apply the mods to the STL of the top later on.

Overall, the modified gearbox is no great departure from Gael's original conception.  I checked and it is not substantially bigger than the original and does not clash, as best as I can determine, with the bicep/shoulder assembly.

At that point I was ready to mate the servo into the gearbox and test the ensemble.  Unfortunately, I missed trimming the gear stop from the main drive gear, something that Gael very specifically showed me how to do in this pic in his assembly instructions.

That little omission cost me two stripped gears in the servo when I fired it up.  Fortunately, replacement gear sets for the HS-805BB cost about $10, so it was no big tragedy.  I cannibalized replacement gears from one of the other servos.  At that point, I discovered that what Gael thought was left and what I thought was left were two very different things.  Once I swapped the leads on the potentiometer, the gearbox behaved brilliantly!

Note that the gearbox runs smoothly without grease.  Mind, I intend to grease it when I put it in service, but for now, it doesn't need lubrication.


  1. Great work, I'm very glad you are stepping in the the gearbox design. It is true that the screw clashing was a problem, I had solved by using small wood screws, allowing the head to come inside the turntable. Strengthening the servo mount is a judicious idea if one uses higher torque servos, although it doesn't seem to be fragile at the moment once the servo is tightly mounted. Sorry to hear, my left isn't what you had in mind for left.
    May I add the link of your blog on InMoov's forum, your work could be of great help for others?

    1. I would be honored for you to link my blog to the InMoov forum! I worked on the core team of the open source Reprap 3D printer project for five years and know very well how fast development in disruptive technology can proceed with a community of motivated developers such as you have attracted to your InMoov project.

    2. Also, you asked in Youtube if I had used the modified involute profile gear and worm gear combination in this work. Yes, I certainly did. It was, necessary, to modify the involute profile gear to include the slot for the potentiometer. Just now I updated the most recent gear to the Thingiverse entry that you've seen before.

  2. Perfect, I have created a InMoov collection on Thingiverse, and have included your derivatives in it.

  3. You know, it might be safer to do that when I have the whole shoulder/arm assembly working so we can be sure that my derivatives are really worthwhile. :(

  4. Well, maybe you are right, but it's already done so I'm not going to take it off. It wouldn't be cool.

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  6. Gael: I hope you understand that the better I get to understand your InMoov design, the more impressed I am with your talent.