Progress on the bicep gear box.

Well, I redesigned the lid on the bicep gear box and got all the pieces printed out.  I discovered that to print big footprint pieces you need to recalibrate the print table when you do a big print.  That doesn't take a long time and appears to be necessary if you intend to print pieces with dimensions over about 90-100 mm.

In any case, I spent a little time upgrading the lid to the gear box so that it take much less after print hand working than it did before.  The involute profile gear and the worm gear are, of course, opposite what one has in the shoulder.  For a 3D printer, that is not a big matter.

I did notice that the axis in the gearbox for the worm gear is about 0.5-1 degree off true.  That doesn't cause any real trouble, but I will remedy that later on.

I slapped on the same servo that I'd modified for the shoulder and, of course, the leads into the potentiometer were backwards to what they needed to be.  :-)

All the same, the HS-805BB servo turned the drive shaft barrel easily without lubrication.  

I got notice from the vendor for the spare gear sets for the HS805BB that they will arrive either this Friday or next Monday or at latest onTuesday.  Of course, they'd promised to deliver them by this Friday when I ordered them.  :-D

This weekend, I hope to have the time to get test the servos on the lead screws that position the forearm and the bicep away from the body.

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.

Sorting out the Kinect skeleton feature

The Kinect has some very clever software and firmware which enables it to capture the skeleton patterns of up to two people at the same time.  I have been tracking the progress of the Kinect since it's introduction and bought a commercial, as opposed to an Xbox release, when it became available in 2012.  With the rapid completion of the Inmoov robot print here and the successful testing of the Mini Maestro 24 servo controller with the Hitech HS-805BB servos used to move the arms and shoulders, I began to delve into the Kinect SDK.

The Kinect can capture skeleton patterns in two modes; full body...

...and upper body {seated}

Extracting the Cartesian coordinates of the bones in three dimensions is straightforward.  Being a naturally suspicious sort, however, I decided to check to see how consistent the length of the individual bones actually was.  As I suspected, they vary.  This shouldn't be surprising considering the wickedness of trying to figure out the position of bones buried in flesh and muscle starting only with surface measurements of the body and on top of that making the calculations at a rate of thirty frames per second.

Just eyeballing the captured data it appeared that the bone lengths were varying wildly.  When I took a mean and standard deviation for each bone, however, I discovered that the calculation was remarkably consistent. I made a test data set using myself as the subject.  The measurements are in meters.

Mean SD Shoulder Left 0.217 0.017 Right 0.218 0.013 Humerus  Left 0.242 0.025 Right 0.241 0.026 Forearm Left 0.235 0.024 Right 0.235 0.026 Palm Left 0.089 0.034 Right 0.086 0.029

The numbers are not too bad.  The wrist/palm measurements are the worst.  I have extra code which I hope will give better results for the hands and fingers.

The next step will be to see if I can transform the coordinates into something more useful with the Inmoov by fixing the bone lengths to calibrated means.

Commissioning the Mini Maestro 24 servo controller

I managed to get the drivers installed and the Visual Basic 2010 example coding running on my PC and demonstrated that I could drive a small test servo from my keyboard.

My old ATX PC power supply seems to be able to handle the test servo without a lot of bother.  It is rated at 30 amps, so with luck I won't have to do what Gael did with batteries and a charger just yet.  In theory, this controller should be able to handle the whole Inmoov robot.  It will certainly handle one arm, I hope.

I order the HS-805BB servos.  They should be here by Friday.

Just discovered a spool of 250 yards of 8 lb monofilament.  That should work for the hand.