View Single Post
  #72  
Old March 14th 19, 01:04 AM posted to rec.bicycles.tech
Ralph Barone[_4_]
external usenet poster
 
Posts: 853
Default GPS Units = Show road steepness?

Radey Shouman wrote:
Radey Shouman writes:

Mike A Schwab writes:

Here is a great article by Sheldon Brown showing how bicycle
cyclometers work. When bladed spokes came out, some units would
register twice the distance at slow speed up to 6 mph 10 kph, so it
can give you an idea of how fast it can register the magnetic field
closing and opening a reed switch in the pickup.

https://www.sheldonbrown.com/cyclecomputer-magnet.html


That does seem to show that, at least at slow speeds, on a high-end
computer, every reed switch pulse is used to compute a new speed. It's
not clear whether the computer fails to register a double pulse at
higher speeds, or that the internal algorithm changes. Either one is
possible.


Oops, posted in haste, and less than half right.

The identified cause was a double pulse on the reed switch signal, the
symptom was, at lower speeds, "a speed readout varying erratically
between 7 and 12 MPH", which was roughly twice the true speed.

The computer worked as expected at higher speeds. The most likely
explanation for that is that analog signal conditioning circuitry
filtered out the double pulses when closer together. A conceivable but
less likely explanation is that software either deliberately or by
accident de-bounced the signal.

Since it worked consistently at high speed, I'm guessing that the
de-bouncing happened sometimes at low speed and sometimes not. If this
wasn't true then I don't have a good explanation.

1) Obviously speed was not computed by the dividing rollout distance by
elapsed time between pulses. In that case the speed reading would
alternate between almost right and very high. Linear filtering would
result in a speed that was much too high. Using the most recent value
would result in a speed that was usually almost right.

2) The speed could have been computed by multiplying the number of
pulses by the rollout distance and dividing by the combined elapsed
time.

3) The speed could equally well have been computed by dividing rollout
distance times number of pulses in a fixed time, by the fixed time, and
then filtering.

4) It's certainly possible that Shimano used some algorithm I'm not
familiar with.

Looking at the output would have given a clue: For (2) we would expect
the speed shown to be either about right or about double. For (3) we
would expect the speed to vary between about right and about double,
with values in between.

So what did "varying erratically" mean? I favor "taking many values
between", but "oscillating irregularly between approximately the same
two values" is not impossible.


.... or when the magnet passed the reed switch at a slow rate of speed, the
switch didn't open or close cleanly, but it did when the magnetic field
changed more quickly.

Ads
 

Home - Home - Home - Home - Home