Spoke sensor for bicycle

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Joe Gwinn

The ones I've had just used an encapsulated reed switch on the fork with a magnet on a spoke. It woke up the processor as well as providing a rotation signal.

This was a few years back though.

Hmmm, but will that work at really low speed (like when pushing the bike uphill)? It is an idea to do it this way (I did not even know the name of the method so the idea is more than welcome), yet what I was thinking was more in the line of changing some oscillation frequency (thus detecting the spokes also "at DC").

People often talk about cell phones as if they had universal coverage. That is true for 99.9% of where people spend their time. But that 0.1% of the time, like biking in the woods, you are out of cell phone range. So any solution needs to not depend on a cell phone connection. If you can use the GPS in your phone without a cell connection, great! But I haven't found such a program as yet. Maybe I just haven't dug deep enough.

Oh where I bike there is coverage allright, and I think I had seen references to some apps for biking. Looks like the only thing making me think of that "project" I am unlikely to ever even start is the sensor design.... (I am vague because I am not so sure myself why I keep thinking of this every now and then for 4 years and still have nothing in use when I bike :).

On a sunny day (Tue, 14 Jun 2022 18:51:21 +0300) it happened Dimiter_Popoff snipped-for-privacy@tgi-sci.com wrote in <t8aapr$ab6$ snipped-for-privacy@dont-email.me:

Or you could make something like my gm_pic2:

It uses GPS and writes to SDcard or EEPROM where you were at any time, logs radiation there too There is software to replay the trip on google maps. After all the GPS talk here .. Simple maaz using the space and time between data points will allow you to find speed and distances. A GPS module and a Microchip 18F14k22 + EEPROM is all you need. Oh and a battery... Now been working 24/7 for 8 years... As clock mostly. No tinkering with your bike needed. Also logs trips per bus or train.

Is the GPS accuracy/latency of the speed measurement close to that of a car's speed indicator?

I can't get Alltrails to do much offline. It does have a numeric display that shows your lat/long, but that's not much use. To do anything else it's asking me to log in! That's hard to do without a connection. I guess people download stuff in advance of a hike or whatever... or they just use their data connection, when they have one.

A magnet secured to a spoke, and a reed switch, makes a durable sensor, but is somewhat clumsy. A sensor of the metal spoke is hard, because spokes aren't all magnetic, there's stainless steels. Have you considered a reluctance sensor on a gear-cluster tooth? There's some DC insensitivity, unlike with Hall sensors, but maybe a low-speed signal dropout isn't going to be a bother.

A reluctance sensor is a coil of wire between a long weak-ish magnet (you don't want to attract metal bits) and a tooth's circular trajectory. Into a high impedance, it generates a peak/valley two-lobed pulse at a tooth passage, where the volt-seconds of the peak and the volt-seconds of the valley are constants (so voltage goes low when speed is slow). A set/reset flipflop makes a good debouncer for that.

You might consider putting a magnet not on a spoke, but inside the tire; a button-size rare earth magnet won't show, but a Hall sensor of the non-hysteresis type should pick it up easily. I'd put the magnet center-of-tread, and affix the sensor near the bottom bracket.

There are at least two reasons for the magnet. One is that it triggers a reed contact on the other side. This allows the whole speedometer to not have to be turned off by hand for battery conservation. It simply does that on its own and when the reed switch changes state again it turns itself back on. So you can just park your bike and walk away. Well, maybe not in a large city because then it gets stolen.

The second reason is that this system works down to very low speeds. There isn't actually a lower limit and it will still clock the miles correctly.

I can already smell an engineer's solution coming up. Huge sensor, multiple bus cables, a laptop mounted to the handlebar ... :-)

Then, later in the afternoon during an all-day ride, there will be a low battery warning, followed by a dark screen.

Why do we always have to make things so complicated? When I grew up speedometers didn't even need any electrical power. They just worked.

"Close" is probably the best you can claim. Each 1 second update is a second old by the time you get it from the GPS. The accuracy can be there, but it depends on the constellation. With some xx feet of accuracy, each calculation can have errors that are significant if you are not moving fast. 30 mph is 44 fps, so 14 foot accuracy (what I'm seeing now and a typical value) is significant. GPS measurements are typically filtered. That's one reason why your car navigation can prompt you rather late sometimes (or early). It is hard for a GPS to know what lane you are in, but can tell if you are on the parallel access road and not the main highway.

Does it have to be magnetic? A conductor moving in a magnetic field creates a current which also creates a magnetic field which can be detected, no?

a car speedometer is only required to be -0 to +10%

Aaaah, the stainless steel is a game changer of course. Had not thought of that at all - though I know the spokes are not rusty (some at the end where they are tightened to the wheel but not all and not much). And it is not like they have not seen water, 1-2 months ago getting back after waiting for a torrential rain to subside I was nearly in submarine mode ...

Putting the magnet underneath the tire is an idea. Of course I might lose it if I have a flat and repair it on the road (you won't believe how often I was getting a flat until I got my latest tires, they claim to have some Kevlar and I had just one flat in a year or two (can't remember if I got them last year or the year before, almost replacement due now). But then I have a decent supply of coin magnets, too (say 1mm thick/10mm diameter)...

The Hall sensors I have "in the drawer" are some with hysteresis though, they need such a magnet really close (within 1mm if not less) to work, would need some other kind but that should be the easiest part. But well, it looks I am not going to start the project any time soon. And I did download some biking app to the phone, this makes the project's prospects even more bleak...

A nonmagnetic gear tooth will (in motion) disturb a magnetic field, and make a pulse. The eddy current response, though, is less than a ferromagnetic material's, and has a time decay of its own, added to the coil's DC insensitivity.

An IR (modulated? polarized?) source, aimed at a retroreflector on a spoke, has good range and sensitivity, at a cost of some power usage. Rejecting ambient light and dirt are other issues there.

As the COTS units are "super cheap", one has to assume you want to do something MORE -- or differently...

You need to consider your goal. If you simply want to know your "physical displacement" on the globe, then periodic fixes from a GPS (with straightline interpolation of route traveled) is a crude approximation. Advantage: you likely are already carrying a cell phone that could give you that data. Disadvantage: GPS accuracy varies (from moment to moment) as does reception. Likely won't tell you you're moving at all if your pedaling in a tight circle!

A variable reluctance sensor noting the passing of one or more "slugs" on the wheel gives you an idea of rotation -- if you aren't concerned with direction of rotation (software to filter out "jitter" if you're stopped and rocking forwards and backwards on a "sense point"). A magnet and reed switch can do comparable.

Optical ON THE WHEEL is likely not practical due to weather conditions.

All of these have issues with proximity of sensor -- an out of true wheel (wobble) could complicate things.

Personally, if I saw some added value to rolling my own, I'd opt for a "vintage" speedometer's pickup:

transferring motion through a "cable-in-cable" to a "head unit" that has been gutted. Mount a toothed gear on the end of the cable shaft *in* the head unit and sense rotation optically or variable reluctance. Quadrature detector if you want to be able to note direction of movement for FINE sensitivity.

Electronics mounted on a little board that also supports the inductive/optical sensors -- display, controls, etc.

This lets you move the electronics (and display) away from the wheel -- which is likely kicking up mud, water, etc.

how would that make any sense compared to the simple and ultra reliable reed and magnet?

Rube Goldberg would be proud