Tire pressure monitoring sensor teardown

[ivan747]

Not my video, but I thought it would be interesting to share.
On american cars it is required by law to have an electronic system that directly measures the pressure on the tires. Instead of having spinning wires, manufacturers have opted instead for small wireless devices fitted inside the wheel of the car. They usually transmit a unique ID to a receptor in the car on one of those unlicensed, free for all bands.

This guy took apart a RAV4 sensor, and the teardown was very interesting. He knows what he's talking about.


Definately deserves more views 
I'm not going to spoil what's actually inside, but, once you find out, you can get lots of datasheets and app notes with some googling with the right search terms. It's quite amazing that they manage to get so much functionality with so little power.

[SeanB]

With those do not use any fix-a-flat tyre inflator, or you will replace it very fast. The stuff in there kills them.

[mikeselectricstuff]

I've seen mention of systems that measure changes in the frequency of ABS sensor pulses to detect flats - seems like a better way to do it if it's reliable.

[SeanB]

I've seen mention of systems that measure changes in the frequency of ABS sensor pulses to detect flats - seems like a better way to do it if it's reliable.

The "new" Mini ( how they can call something bigger than most small cars a mini now is beyond me, I thought the idea was to have as small a car as possible while still having 4 wheels) does that, and while it does work it can be fooled in long sweeping curves, which will trigger the tyre pressure warning every so often. As well if all 4 lose pressure at roughly the same rate (through the valve caps or porous cast alloy rims) it will go very low without triggering the warning, even if all 4 tyres are at less than half the recommended inflation pressure. If there is a "warning system" most drivers will not actually look at the tyres, and those expensive Michelin run flats can have zero pressure in them and still look inflated at the rear with no load.

[ivan747]

With those do not use any fix-a-flat tyre inflator, or you will replace it very fast. The stuff in there kills them.

Having a car not come with a spare tire should be illegal. In my country the car can't pas its roadworthyness test without one.

[ivan747]

I've seen mention of systems that measure changes in the frequency of ABS sensor pulses to detect flats - seems like a better way to do it if it's reliable.

I think you're talking about indirect tire pressure measuring system. It basically senses the speed that each wheel is going at. If one tire is flat or looses pressure abnormally fast, it's gonna have effectively a smaller diameter than the rest, thus it's going to spin faster, so it triggers the warning. But it's kind of useless if your tires loose pressure all at the same pace.

[wreeve]

I've seen mention of systems that measure changes in the frequency of ABS sensor pulses to detect flats - seems like a better way to do it if it's reliable.

Yes I've retrofitted the BMW system to my old E39; connects to each ABS sensor and the instrument display and a simple push switch to "store" a default tyre pressure. Works really well. A lot cheaper than the wireless system that BMW used to use.

[retrolefty]

I've seen mention of systems that measure changes in the frequency of ABS sensor pulses to detect flats - seems like a better way to do it if it's reliable.

 It is very simple and reliable. My 2000 Buick used ABS sensor and would tell me at least one tire is lower that I could not tell by eye. Problem is then figuring which is the low one. 

Not sure what my 2014 Honda Accord uses.

[wreeve]

I've seen mention of systems that measure changes in the frequency of ABS sensor pulses to detect flats - seems like a better way to do it if it's reliable.

 It is very simple and reliable. My 2000 Buick used ABS sensor and would tell me at least one tire is lower that I could not tell by eye. Problem is then figuring which is the low one. 

Not sure what my 2014 Honda Accord uses.

Even more annoying was the fact the car knew and just didn't want to tell you 

[SeanB]

With those do not use any fix-a-flat tyre inflator, or you will replace it very fast. The stuff in there kills them.

Having a car not come with a spare tire should be illegal. In my country the car can't pas its roadworthyness test without one.

You don't have Ferrari's in the Dominican Republic? The latest models do not come with a spare tyre even as an option, and the only thing they come with is a wheel jack, a wheel nut spanner and 2 large Ferrari branded cans of fix-a-flat to inflate the tyre to something that will get you out of Dodge. Done to save mass and to streamline the vehicle by not having to find the space for a spare.

[ivan747]

With those do not use any fix-a-flat tyre inflator, or you will replace it very fast. The stuff in there kills them.

Having a car not come with a spare tire should be illegal. In my country the car can't pas its roadworthyness test without one.

You don't have Ferrari's in the Dominican Republic? The latest models do not come with a spare tyre even as an option, and the only thing they come with is a wheel jack, a wheel nut spanner and 2 large Ferrari branded cans of fix-a-flat to inflate the tyre to something that will get you out of Dodge. Done to save mass and to streamline the vehicle by not having to find the space for a spare.

There are a handful of privately imported Ferraris. AND it's explicitly stated in the Vehicle Transit law, that every vehicle in order to use public roads needs an additional, properly inflated spare tire. The ugly truth is that no road worthyness test actually gets done, you just pay at the relevant government institution or selected banks and they'll give you the sticker. Still, if you had an accident on a non compliant car that could be an advantage for other drivers to sue you.

If you imported your Ferrari on your own, no one would notice the lack of spare tire under normal conditions. Even if you got pulled over, the police can't enter and search inside your vehicle without a warrant. The lack of tire would have to be something in plain sight, for example an SUV without spare tire. You can inspect the whole SUV just by looking inside the window. So Ferraris get away with that loophole.

In the end, dealers would rather not have the risk of test driving a non compliant car and potentially losing sales because of customer expectations. I don't see the spare tire becoming optional in this market until the law gets modified. I hope that soon they actually enforce the tests.

And remember, a Ferrari is a Ferrari, but a Toyota Corolla weights more than a thousand kilos. A spare tire would be merely 1% of weight, much less than even a child occupant. I'd rather have the spare.

[discomike]

While first generation indirect tire pressure monitoring systems (ITPMS) work as described above (detecting the difference in roll radius) newer ones work on measuring various oscillations characteristics in the tire. This is done using the normal ABS sensors using sufficiently high resolution of the time stamps. You then do some signal processing magic on them and find out that some oscillation frequencies are highly pressure-dependent. This way you can detect scenarios where all 4 tires deflate at the same rate as well.

The software has to keep a database of a lot of different tire-profiles and try to identify which one you put on for this to work since the oscillations are highly tire-dependent, and might not work that well with aftermarket tires. If you search for patents from on this you can find some interesting details on how the systems work.

The long rolling hills scenario is probably fooling the older type ITPMS systems because the road is banked enough so that you do not need to turn the steering wheel - normally if you're in a corner the system should not trigger.

If found some nice animations:


http://www.ibnm.uni-hannover.de/en/research/finite-element-algorithms/numerical-simulation-of-tire-rolling-noise-radiation/details/

As well as this poster explaing them (research relating to emitted noise but the principle for ITPMS is similar):
http://www.ibnm.uni-hannover.de/uploads/pics/projects/numerical-simulation-of-tire-rolling-noise-radiation.pdf


//Mike (currently working on integrating a 3rd party ITPMS system into our software)

[ivan747]

discomike, is that where tire noise comes from, as well? I get the feeling that well inflated tires are less noisy.

[SeanB]

Higher pressure, less rolling resistance, lower fuel economy. Too high and excessive wear in the middle. Lower pressure and more comfortable ride, but more heat generated in tyre and higher risk of it overheating and blowing out, and wear in the outer side of the tread is higher. Sort of a compromise all round, I try to keep the inflation pressure at the top end of the vehicle manufacturers range, not the range for the tyre ( the tyre is going to be rated higher than the vehicle, for safety), so that fuel economy is better.

[saturation]

As TPMS is mandatory in the USA there is nothing we can do about it.  However, it has some interesting problems not found in regular tire valves.

The valve stem appears sheared from the threads.  I read of more stem failures with TPMS since it was instituted.  In the old passive stem, failures were uncommon except in severe tire trauma like grinding on a curb.



A small electric current flows in the stem, its subject to electrolysis, which can damage the valve.  Salt water, during winter driving, bathes the valve surfaces and potentially can eat away the aluminum alloy.  A a simple solution is the valve caps are O ring sealed, and it costs as much as the one's without O rings.  It also pays to rinse your tires frequently as the cap doesn't protect the exposed stem, were this video's failure was.  Also, the cap is more mandatory for long TPMS life than optional.



As first time owners found out the hard way, using stylish metal caps can causes galvanic currents, typically the stylish cap is a steel alloy and the valve an aluminium alloy, and eat into the valve and or fuse the cap to the valve stem.  So, plastic is the only cap you should use on them.  This phenomena is independent of electrolysis.

A typical TPMS unit is about $30-60 each, per tire.  The ordinary stem is about $1-6 each.  Although the Li coin cell can last up to 10 years, and a tire about 5 years, if an owner failed to replace the TPMS units at 5 years, the TPMS could die before the next tire change so its best to replace the TPMS unit with each tire change.  That increases the cost of each tire change up to $60 per tire.


discomike@ great graphics, explains quite a lot in one view.

ivan747@ good video.  The construction is very robust and the maker, TRW, is a major OEM for many auto electronic and other safety systems.

Panasonic branded 2450 coin cell, quality battery, so it should give its expected mAH and shelf life.

[mc]

Standard tyre valves were never aluminium, I think they were brass, so corrosion was never an issue.

Everything now is about cost cutting and weight saving, and I suspect alloy valves are far cheaper and easier to make with a body/housing attached. I'd hazard a guess the stem is die cast, machined, plastic moulded around it to form the housing, and the internals/cover are secured in place. It's the only practical way you could cheaply mass produce items of this design.
Ye olde style valve is pretty crude in comparison, as it's just a flanged threaded tube which is cast in rubber, where the only critical tolerance are the thread sizes. An old multi spindle machine could churn the metal bits out for next to nothing.

The designers will most likely of counted on the aluminium lasting roughly the same length of time as the battery, but naturally, local environments and usage patterns will play a large part in what fails first.
As with all things in the motor trade, as parts become more common and in demand, prices will fall once the aftermarket gets their supply chain in place.

[saturation]

Yes, you are right mc.  I stand corrected on implying the older style valves were aluminum, they were good ole' brass.

Standard tyre valves were never aluminium, I think they were brass, so corrosion was never an issue.
..
As with all things in the motor trade, as parts become more common and in demand, prices will fall once the aftermarket gets their supply chain in place.