[Olpc-open] Foot power revisited...

[Mike Dawson]

Dear All,

We have done some rough experiments on some Afghan kids around here
with weights to see how much force they are capable of generating.  We
don't yet know how long they can sustain this for and how much that
might change if they are doing that regularly...

We made a scale, loaded some weights onto one side, and asked the
child to use their foot to make it go up and down.

We found that a 7 year old boy here can lift 7-8kg 25cm at 40rpm which
gives us 11W or so.

If we lose 50% or so to the efficiency of the dynamo, rectifier, etc.
we are looking at getting something like 5W...  Which seems like we
are getting close - we could say something like get fit, work out, and
then you can use the processor etoys etc. a bit more...

We should have one of the handcranks coming tomorrow - I was thinking
to see if we can connect our sewing machine to that instead - and see
how much we can get out of it...

Another interesting question from the cost point of view is if we
assume that this energy is coming from rice, bread, etc. what is the
cost of that and how does it compare to solar for example?  Though in
the classrooms solar is quite difficult as aforementioned given that
there is normally something overhead (building, tent etc) blocking the
sun...

Thanks to everyone who has contributed to this discussion and I look
forward to seeing where this is going to go.

I am going to be going back to Jalalabad (where the first deployment
school in Afghanistan is) and I can see if we can find some companies
that might be able to produce something along these lines and how much
it will cost...

Regards from Kabul,

-Mike

2009/1/14 Mike Dawson <mikeofmanchester at gmail.com>:
> Hi All,
>
> Thanks for the quick replies back - we do indeed have to consider how
> much force a child can comfortably generate over a sustained period of
> time.
>
> I guess the question is more to do with the strength of the muscles
> than the weight - and that comes back to health.
>
> The other thing I am wondering is what might be the difference between
> something that is more pedal based such as the designs shown and
> something that is more foot pump based such as the Weza...  I am sure
> that kids around here when they are cycling to school are generating
> considerbly more than 20W to move the bike around here, not to mention
> the 200W they must have to use for their brain to work around the
> traffic in Kabul.
>
> What if got some kids around and measure how much force they can
> comfortably generate?  Maybe that would be a sensible experiment
> direction?   Maybe we can work out a training program for the kids ;)
> ??  Good bye childhood obesity issues...
>
> Then I would also be very interested in another calculation - how much
> is that food going to cost?  These days 1 kg of rice I have not heard
> of in the city for less than $1 USD...  This one might be tricky...
> Yet the problem with solar under a tent, class room etc. and the price
> of copper speaks for itself.
>
> I guess we could do something like this - if we get a traditional
> style scale and put a weight of however many kg on it - we can then
> see how well the children can move the weight by placing their foot on
> the other side of the scale...  That could give us some proper
> parameters, what do you guys think?
>
> Thanks/Regards,
>
> -Mike
>
>
>
> 2009/1/14 Benjamin M. Schwartz <bmschwar at fas.harvard.edu>:
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>> Mike Dawson wrote:
>>> So the force required to generate 9W (e.g. for charging a little and
>>> running the laptop):
>>>
>>> 9 = 0.15F / 2
>>> F = 120N = 12.23kg (given gravity = 9.81N per kg)
>>>
>>> Which does not seem to be an unreasonable amount...
>>
>> I applaud your effort, and I look forward to seeing the results.  Your
>> calculation is a bit iffy, but the correct biomechanics are messy, and
>> probably not terribly informative.
>>
>> The thing to remember is: the average 7-year-old American girl only weighs
>> 24 kg, so 12 kgf is a lot.  It's probably about like climbing stairs,
>> which, if you've ever climbed a lot of stairs, you know is very tiring.
>> What really killed OLPC's human-power initiative was this realization:
>> that a bunch of healthy well-fed young adult first-world men do not have a
>> good sense for how much power a Rwandan child can comfortably generate.
>> (At age 13, or 16, the story is very different.)
>>
>> Nonetheless, I think this project is tremendously important.  I recommend
>> that you consider designs like
>> http://www.mattshaver.com/bikegen/index.htm, especially if bicycles are
>> readily available in your area.   This approach uses the rear wheel of the
>> bicycle as a friction gear to spin the driveshaft at the very high RPM
>> typically required by generators.
>>
>> Software improvements have been steadily decreasing the effective power
>> draw of the XO, and I expect that these improvements will continue, making
>> alternative power sources more and more effective.
>>
>> - --Ben
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