[LAU] A crazy idea

Folderol folderol at ukfsn.org
Thu Mar 4 13:14:30 EST 2010

On Thu, 04 Mar 2010 10:27:43 +0000
Jonathan Gazeley <jonathan.gazeley at bristol.ac.uk> wrote:

> On 04/03/10 10:20, James Morris wrote:
> > On Thu, March 4, 2010 09:48, Jonathan Gazeley wrote:
> >
> >    
> >> 1. A sensor that can measure my cadence. A simple magnet switch that
> >> triggers once a revolution won't be enough to measure the cadence with
> >> sufficient resolution, since my cadence is usually between 50 and 80
> >> rpm. I would probably need to mount multiple magnets spaced equally
> >> around the chainwheel and have a single sensor on the frame. Then I have
> >> to get it to supply this information to my control program.
> >>      
> > Cycle computers which measure cadence work with a single magnet on the
> > crank and can measure cadence in that range and above. I think the fastest
> > RPM I ever got was (only a vague memory) around 120 (maybe?) when I was
> > being silly trying to see how fast I could pedal.
> >    
> Yes - I have a cycle computer that can measure cadence. However, I 
> typically pedal between 60 and 90 rpm which means my reading would only 
> be available for sampling every 0.7 - 1.0s. It would work, but in the 
> space of one second my cadence can vary a lot if I'm accelerating so the 
> sound of the rising revs would be jumpy.
> Also, if I suddenly stopped pedalling, it might take up to a second for 
> the sampler to register and kill the engine noise. I don't think it 
> would be a very satisfactory experience, which is why I was thinking of 
> mounting 10 or so magnets around the chainwheel, to be able to sample 
> the cadence every 0.1s.

Instead of directly trying to measure RPM, measure milliseconds per
rev. then invert it. Once you have your first reading, then start to
average over 2/3 revs. gives you a pretty fast responding figure with a
simple setup.

Also, use a short range photocell on the spokes (say 50mm). Easy to
mount, reliable results with minimal effort. As these can usually
respond up to 10-30kHz you won't have to worry about pedalling too
fast :)

Will J Godfrey
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.

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