A couple of ideas regarding future NanoNotes

Werner Almesberger werner at openmoko.org
Tue Aug 10 07:15:53 EDT 2010


nebajoth wrote:
> Still, I think there is merit in Mark's take-a-step-back approach
> involving moving the battery forward to help counterweight the
> screen.

Thanks for the nice links !

The counterweight is something I did mainly for hack value and
because it looked like a useful project for learning more about
machining things useful in real life.

I agree that a proper solution means to re-engineer the case.
Moving the battery should indeed help. I would expect a torque gain
of about 2-3 mNm. (1)

(1) The battery weighs 18.5 g, the entire PCB 20.7 g. If we assume
    that about a third of the PCB's weight is in the area that will
    be displaced, that the battery's center of mass moves from 20
    mm to 50 mm from the axis, and that the displaced PCB area's
    center of mass moves from 50 mm to 20 mm (it'll be a bit more,
    but this is a rough estimate anyway), we get

    battery: 0.0185*9.81*(50-20)   = 5.446 mNm
    PCB:     0.0207/2*9.81*(20-50) = -3.046 mNm
    total:			     2.4 mNm

This should just be enough to keep the Ben from falling over on its
own. It still wouldn't feel very stable, though. Reengineering the
display to make it lighter (2, 3) and moving the feet a little
would further improve the situation.

(2) The LCD module and its board together weigh 26.6 g. The board
    alone is about 8.3 g. If we could find a display that connects
    to the main PCB directly, that would be weight loss of 30% !

(3) The plastic is quite heavy, too. We have: outer shell, inner
    structure (with the hinge), and the glossy plastic in front.
    The outer shell alone is about 11-12 g (measured on a pre-Ben
    case part that's all plastic, without the decorative metal.)

One problem with re-engineering the case is that it's expensive to
put into production. Someone also has to do the actual design.

- Werner




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