October 31, 2013

Robot Arm: Testing Construction Methods and More Design

I CADed up a 3D-printable test arm, to see how well adding carbon fiber over ABS plastic would work:

And then printed it in two parts on a Stratasys uPrint.  The two halves were superglued together.  The full part would have just barely fit diagonally on the uPrint's print bed, but I split it into two parts so that I could also print test pieces on my own UP! if needed.

I machined an aluminum end for it:

Next step was adding carbon fiber to the arm.  To compress the carbon and epoxy, I heat-shrinked the wet assembly:

Here it is after a bit of cleanup.  The heatshrink leaves a nice satin surface finish.

The test piece turned out to be pretty much a complete failure.  First, the epoxy I used did not fully cure.  It's the same batch of epoxy I used on my carbon fiber bike, so I'm not sure what the problem was.  Nick has had lots of problems with this epoxy recently as well.  Also, the epoxy did not bond whatsoever to the ABS plastic.  I was able to cut the carbon fiber and peel it off by hand after letting it sit for 4 days.  I think if I redesign the arm such that pretty much the entire surface can be covered in carbon, the carbon delaminating won't be an issue.

Other thoughts:  The whole assembly is much heavier than I would like.  The aluminum attachment point at the end should be smaller, and the printed walls could be made thinner.  This version was printed with 2 mm thick walls, but thinner would be fine with carbon fiber added.  The plastic serves two purposes, which are to provide a precisely sized mold for the carbon fiber, and interfaces for bearings.  It should not be taking much of the bending load, so the walls can be made as thin as possible.

Also, with newly acquired TechX funding, I've started ordering things like bearings, belts pulleys.  Most of the parts were found on SDP-SI, and then ordered on Ebay for significantly less cost.  To figure out the pulley sizes, I plugged in the moment of inertia figures for my arm CAD model in Solidworks to my gear ratio-optimizing script, and then found the closest sized pulleys I could actually buy.  Rather than using the single extremely large tooth-count pulley recommended by the script, which I would have to machine myself and would be awkwardly large in diameter, I have two-stage reductions with smaller pulleys.

The relevant moment of inertia

Here's a render of the partially-CADed belt reduction

The other robot-arm progress I've made is translating my Python optimization script over to MATLAB.  This was an exercise in using MATLAB, a way to double check my work, a faster alternative to my Python script, and a way to generate better plots.  To check that my extremely simple Python ODE integrator worked, I compared the results it got with the results generated by integrating the equations of motion with ODE45 in MATLAB.

On the left is the MATLAB result, and on the right the Python result with step size decreasing by a factor of 10 each time.

And a comparison of the charts generated:

The MATLAB code can be found here.

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