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Author Topic: Tire Inflation Sensors - How do they know?
Atlanta Jake
Xboxing Day


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My Ford Windstar has a warning light on the dash board that lights up to warn me whenever one of the tires gets low. I have removed/changed the tires on this vehicle, and find no connection to the wheels except for the standard lug nuts. So could someone please tell me how the car knows when there is an issue with one of the tires?

Is it some sort of comparison between the suspension in all wheels, where deviation in one wheel triggers the light? I don't see how this could work reliably when different terraine could cause any or all of the wheels to bear differing loads.

This is just one of those little things that makes me go Hmmmmm....

Jake

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Posts: 1366 | From: Atlanta, Georgia | Registered: Sep 2001  |  IP: Logged | Report this post to a moderator
TrekkerScout
Deck the Malls


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It's most likely the indirect measuring system that is described by the National Highway and Traffic Safety Administration.

quote:
The indirect measurement systems are designed for use with the anti-lock brake system (ABS) and compare the relative wheel speed of one wheel to another. Wheel speed correlates to tire pressure since the diameter of a tire goes down slightly with low tire pressure.

Posts: 306 | From: Tacoma, WA | Registered: Sep 2005  |  IP: Logged | Report this post to a moderator
Atlanta Jake
Xboxing Day


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Hmmm... 'Tis possible, I suppose.

But I thought that I recalled a time when the light came on when I started the car, but before I had put it in gear (said light not being lit upon parking the car). But to be honest, I don't remember for certain.

Thanks for the link and the info, TrekkerScout! I'll check it out.

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Posts: 1366 | From: Atlanta, Georgia | Registered: Sep 2001  |  IP: Logged | Report this post to a moderator
rwolff
I'm Dreaming of a White Sale


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I saw (in a magazine targeted toward auto mechanics - article about pitfalls of new technology) that it's done through a module in the wheel (tire valve is part of the module). There's a nonreplaceable (most likely lithium) battery in the module (so the module needs to be replaced every 10 years or so), and it communicates with the car's on-board computer via a short-range radio link.

Implications of this:

- When you rotate your tires, you need to tell the system (for the one in the article, a sequence of key actions, followed by putting a ring magnet around the valve stems in a particular order) which module is on which wheel.

- Some use the valve stem as an antenna. If it came with a plastic dust cap, you can't use a metal dust cap. If it came with a metal one, you can't use a plastic one.

- If you get a set of "winter rims" for your snow tires, you need another set of moudules.

- Anyone changing the tires needs to be careful not to damage the module in the process (i.e. prying on the bead in the wrong location). Some "old-time" practices are also BAD NEWS - when I got new tires for my car (local chain store), they deflated the old tires quickly by cutting off the valves (new valve included with installation). On my car, it's OK, but on a car with these sensors, the modules would need to be replaced (and they typically have a 3-figure price tag - each).

Note: I've been reading Snopes and its message board for a while. This is my first post - and I signed up because I wanted to answer this question.

Posts: 18 | From: Ontario, Canada | Registered: Dec 2005  |  IP: Logged | Report this post to a moderator
Troberg
Angels Wii Have Heard on High


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Some loose thoughts on the subject:

I see no need for a battery. There is a lot of spinning motion in a wheel, that should be usable to get some current and store in a capacitor or rechargable battery.

I doubt the dust cap will make any difference at all if the valve stem is used as an antenna. It would only change the length of the antenna a few millimeters, not more than if you don't tighten it enough. Antennas are not that sensitive. I think the constant spinning and reflections from the rim would make a bigger problem.

Personally, I think checking the rotation speed against the other wheels is easier (although it will probably fail if driving in circles).

There is probably as many solutions to this problem as there are car manufacturerers...

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/Troberg

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GenYus
Away in a Manager's Special


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It could be set up so that no battery is required for the system. It could use a variant of the RFID tags used in many places. The car sends a signal every so often to each wheel. The sensor in the wheel used the power from the radio signal to send back a signal depending on if the tire pressure is good or bad.

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rwolff
I'm Dreaming of a White Sale


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Troberg, here are a few points:

quote:

I see no need for a battery. There is a lot of spinning motion in a wheel, that should be usable to get some current and store in a capacitor or rechargable battery.

While there is spinning motion, in order to capture this energy you'd need to have your generator assembly connected to both the wheel and something that isn't spinning. With ultra-low power circutry, it's easier to just use a lithium (long shelf life, high energy density) battery.

quote:

I doubt the dust cap will make any difference at all if the valve stem is used as an antenna. It would only change the length of the antenna a few millimeters, not more than if you don't tighten it enough. Antennas are not that sensitive.

You're assuming it works as a "longwire" (untuned) antenna. Tuned antennas are much more efficient (important when you don't have much power going into them), and they are tuned by making their length a precise fraction of the wavelength of the signal they're broadcasting (quarter and half wave antennas are fairly common). A couple millimetres (note for non-metric people: "meter" is a device, "metre" is a unit for measuring length) can detune the antenna for a high-frequency signal.


quote:

Personally, I think checking the rotation speed against the other wheels is easier (although it will probably fail if driving in circles).

Or if the tires on all 4 wheels aren't identical (including wear history). Direct pressure measurement is immune to errors caused by replacing a damaged tire with a new one (possible different brand/model, definitely different wear history), putting snow tires on only the drive wheels (different brand/model), and other "mix and match" tire situations.

I did a search on "tire pressure sensors" and came up with the following:

This one has a picture of a wheel with the sensor installed, and one procedure for telling the car which sensor belongs to which wheel. Looks like I overestimated the price of the sensors.

Here is the take on another brand - better toss that can of "fix-a-flat".

This tech article has descriptions of both methods (sensors and "compare the wheel rotation speeds to each other"), with benefits and pitfalls of both.

Here are part 1 and part 2 of the article I mentioned in my first post.

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Wizard of Yendor
Markdown, the Herald Angels Sing


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quote:
Originally posted by rwolff:
(note for non-metric people: "meter" is a device, "metre" is a unit for measuring length)

Meter is the standard spelling in the US. (Yes we have a seperate spelling for a unit we barely use.)
Posts: 2352 | From: California | Registered: May 2001  |  IP: Logged | Report this post to a moderator
   

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