tyre choices

[Philr]

You say 'front left transmits first' and then the others in order. But how does the sensor in the tyre know it's 'front left" and therefore has to transmit first? And it you move the tyre to front right, how does the sensor know it's not still 'front left'?

There's something basic I am missing.

Alan

It doesn't the main unit does because that sensor has a unique id and it used a set pattern for pairing, front left first, front right second and so on.

[PhilHornby]

I am still confused as to how it knows which axle and side it's on.

The usual, secretive world of automotive electronics

Although I found a suggestion that accelerometers are the key, I can't for the life of me come up with the maths you would need to use, to implement it. I can see how you might tell front from back ... but not left from right.
(It must be something to do with the first bend you drive round, generating lateral acceleration - would that be noticeably different between back-left and back-right - I dunno).

If I was to hazard a guess, I would suggest that they might be able to use some form of signal strength triangulation. Each of the TPMS sensors has its own ID, so they can be individually polled. If the receiver had multiple aerials, it could poll the sensors in ID order, and use triangulation to work out the relative position of the sensor to the receiver.

Some sort of UWB implementation could work, where time-of-flight is available. However, TPMS like PKES seems to be quite old technology, that has finally filtered down to mainstream vehicles.

This has got me itching to find out how it is actually done though, so I know what I'll be diving into when I next have a few spare hours.

It's been on my list of things-to-do for quite a while now (I want to be able to walk round the car and query the pressures, without having to drive it).

A while back, I acquired a standard alloy wheel from a crashed Tucson, to use as a spare. I'm pretty certain I went for a drive round the block with it on and saw its TPMS sensor reporting (without any sort of pairing). I subsequently had the tyre removed and refitted (I wanted an expert opinion on some damage to the wheel). While it was off, I rescued the TPMS sensor, for experimental purposes. Unfortunately, I've been somewhat side-tracked.

Here is a block diagram from that NXP document I linked to earlier :-

My expectation is/was that the wheel sensor effectively identified itself to the TPMS sensor, by modulating the 125KHz 'LF initiator' signal - but at the time of writing, I have zero evidence for that

[alan sh]

If there was some sort of sensor/receiver on each hub - that stays there when you move wheels - then that would make sense. But not if the sensor is with the wheel.

Alan

[pharaoh]

Our cars have a direct TPMS, so there is a sensor in each wheel that tells the cars computer that pressure is low in a particular tyre.

See this: from the internet: Direct TPMS: This system uses sensors mounted inside each tire. These sensors measure the air pressure directly and send this information to the car’s computer system. If the pressure in any tire drops below a certain threshold, the system triggers a warning light on the dashboard, indicating which tire is deflated.


Obviously the system is clever enough to determine which tyre has low pressure even if the wheels and tyres have swapped positions since being fitted in the factory.

[Indalo1]

[PhilHornby]

Obviously the system is clever enough to determine which tyre has low pressure even if the wheels and tyres have swapped positions since being fitted in the factory.

Yes, it's this "cleverness" we were pondering

It might just be as simple as querying the TPMS sensors sequentially. If it knows which wheel it queried, it can marry-up the TPMS sensor's Unique Id with a particular wheel.

[PhilHornby]

If there was some sort of sensor/receiver on each hub - that stays there when you move wheels - then that would make sense. But not if the sensor is with the wheel.

Somewhere near each wheel, is a tiny coil that emits a 125KHz magnetic signal - just like the PKES. Just like a fob, a TPMS responds with a UHF radio signal.

[alan sh]

If there was some sort of sensor/receiver on each hub - that stays there when you move wheels - then that would make sense. But not if the sensor is with the wheel.

Somewhere near each wheel, is a tiny coil that emits a 125KHz magnetic signal - just like the PKES. Just like a fob, a TPMS responds with a UHF radio signal.

That makes sense. So the car 'brain' tells the tiny coils (one at a time) to emit a signal. The TPMS near that will then respond. And the brain knows which wheel it is because the one nearest the coil will be the only one to respond.

[PhilHornby]

That makes sense. So the car 'brain' tells the tiny coils (one at a time) to emit a signal. The TPMS near that will then respond. And the brain knows which wheel it is because the one nearest the coil will be the only one to respond.

Something like that

[PhilHornby]

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