Working BB-8 Droid from Star Wars Episode 7

At Christmas 2015 I saw two products that were moving BB-8 Droids, but no LEGO one.
The LEGO system is capable of producing a working BB-8 Droid, so let's have one!

A BB-8 Droid consists of a sphere that can rotate in all directions, with a head on top that can balance and spin independently.  I chose to use Power Functions because it would be easier and cheaper than an EV3 kit; the droid has to fit a reasonable price point.

The LEGO system has a big challenge to make a smooth sphere.  The first prototype has begun with many #3 axle joints 32016 and 4M axles to make a 20cm diameter sphere.  #1 axle joints 32013 and 3-way axle joints 57585 have allowed the 8 segments to be filled.  Panel pieces allow some structure to be shown.  A pair of panels is used in each of 3 spaces in each of 8 segments.  The head has just a few axle joints for now but it will develop, perhaps to use panels.  In the final model it could use parts similar to the R2D2 head panels from the Droid Developer Kit.

Colours:
The first prototype used parts of the right shape to create the sphere with movement function.  The second prototype improves the sphere colours with available parts.  The final model would need to use the right colour pieces throughout, mostly white and orange.  White radar dishes cover each of 6 nodes where there is a z24 cog connecting the segment structures.  Smaller orange pieces would fill in between the white pieces to make the sphere.  The radar dishes and head parts may have stickers for the details; these should not go over any edges.  An alternative is to use sail material pieces to fill in the gaps, securing them with axle pins and bushes.

Movement Functions:
The first and second prototypes spin the casing on one axis.  One L-motor moves the sphere, another motor is waiting to be connected to another function.  Both motors add to the battery box counterweight, which works well enough.
It works better without the radar dishes because the sphere is smoother.  Further use of material pieces could help towards a smoother sphere.  I have tested fitting a piece of paper to the sphere; with the right cuts to fit the curvature, stronger material with printed patterns would be feasible.

Further functions:
Further prototypes may tilt a weight to steer the droid, or use a flywheel; spin up slowly and stop suddenly to spin the sphere.  The head may be held using 2-4 strong sewing magnets with a rotating and tilting support inside the sphere.  For the final LEGO product the magnets would need to be encased in a LEGO piece of a size too large to swallow.  2 small motors may be added to control the head functions.

Product comparison:
The smaller phone-controlled product retails at £130.  My model is currently cheaper and larger.  The aim is for the completed product to retail at under £100 but to have more educational value than other products.
The product would be driveable but programmability would be a later option.  The idea is to teach the user how the droid works, all about counterbalancing and other aspects of mechanical engineering.  The price point of £100 is similar to the spring flagship Technic set.

There would be the option to add an SBrick or EV3 kit to enhance the features, programmability and sound effects, customised operation, interaction and mapping.
An existing Power Functions sound effect brick could be programmed with some of the regular sounds.

If you would like to see a working LEGO BB-8 droid then please support this project.