Tyre Pressure Monitor Sensors

JimB said:

An interesting solution, but square boxes don't make very good pressure vessels.

Consider the second box:
Outside dimensions 130 x 80mm
Assume a wall thickness of 5 mm
Inside dimensions 125 x 75mm
Convert to good old imperial measurements
Inside dimensions 5 x 3 inches
Inside area of lid is 15 sq inches.
Pump it up to 30psi to simulate the tyre pressure
Force trying to separate lid from box is 450 pounds.
Box is made from diecast aluminium, a material not known for its elastic properties.

Please don't pressurise this thing near me.

JimB

Click to expand...

The tensile strength of diecast aluminum is around 4700 to 6000 psi, so that shouldn't be a problem. And the fatigue strength of a standard M4 screw is about 13000psi (multiplied times 4 bolts!), so that's not a problem either. Even if it did blow, the small volume of pressurized air inside it wouldn't send parts of shrapnel very far at all, or with any significant velocity.

hyedenny said:

The tensile strength of diecast aluminum is around 4700 to 6000 psi,

Click to expand...

Hyedenny, I'm not arguing with you, well, I am, but from a novices point of view .. .. … surely there's a dimension to consider that's not mentioned there .. .. the tensile strength of the aluminium will be directly related to the thickness of the material ??

S

Yes and no!
No, because "Tensile strength" is a property of a material and is related to force per cross sectional area. It does not change. (it can change if the sample thickness is sufficiently small -- where material crystal grain size can start to affect a sample's properties --intrinsic vs extrinsic values -- but that's obviously at a very small sample size and irrelevant to this conversation).
And, yes, the final strength would depend on the thickness of this container, which we don't know. It would have to be thinner than a minimum of 0.09" for it to fail with 30psi, something I'm assuming to be thicker -- mea culpa!
But again, even if it were to fail, it would hardly even make much more than a "pop."

JimB said:

From other reading that I have been doing on these things, I think the pressure sensing element is some kind of piezo-electric device which produces a voltage when it is subjected to pressure.
The pressure sensitive device being integrated into custom IC package.

JimB

Click to expand...

Indeed, you are correct Jim.

I removed (butchered) the lid from the custom IC to reveal what appears to be exactly that:


Since there was not much else to go on from there, the custom IC was removed, to reveal yet more potting:


This was scraped away carefully (but not enough to preserve the bond wires) to reveal the tracks and a die:


Here's a close-up of the die:


And a zoomed-in view:


Getting back to MM's problem, if he were using this kind of sensor (High level), maybe applying some mechanical force to the lid of the custom IC could trick it into displaying the required pressure?

I did record a signal using SDR#, prior to butchering the IC, and then viewed it in Audacity, but did not spend much time trying to ID the type of modulation. Maybe I can post some screenshots if someone would like to have a go?

Mickster said:

Indeed, you are correct Jim.
I removed (butchered) the lid from the custom IC to reveal what appears to be exactly that:

Click to expand...

Well the next step, if you fancy a challenge, is to connect a voltmeter to that lump and put some pressure on it to see if it develops a voltage.

JimB

Well as part of my daughters post doctoral research she needed to be 'clean room certified', so she was required to construct a solar cell - or more accurately a wafer of solar cells. She got to keep the resultant wafer, and it's pretty impressive. Anyway, the reason for mentioning it is that her husband works as a technician at the same University, and is currently working for an Electronics department - part of which involves manufacturing integrated circuits. He was telling me about accelerometers, and how the silicon is etched away to make a tiny piece that moves under acceleration - I can't say I'd ever considered how such devices worked. Presumably pressure sensors work in a similar way, with part of the silicon deforming under pressure.

This link, although pretty old now, details the LIS331DLH accelerometer used in the iPhone4.

https://www.memsjournal.com/2010/12/motion-sensing-in-the-iphone-4-mems-accelerometer.html

EDIT: Another interesting link regarding MEMS devices:
https://www.maximintegrated.com/en/app-notes/index.mvp/id/5418

It kinda blows your mind to see what we can do at such a scale.

Musicmanager
There is likely to be a receiver somewhere in or close by the spare wheel well of your vehicle, so for the best signal, you should find that and put your device next to it. As you are using the spare wheel well for your conversion, you may have to relocate the receiver anyway.

That's a weird setup, but if it works, it works.
Good luck.

Musicmanager said:

Hi Mickster

It's funny you should mention that .. .. ..

According to the Service Manual I have there are only 3 Transponders .. .. one each in Front wheel arches and one in Right rear wheel arch; Even more confusing .. the LF transponder does the Left Front wheel sensor; the RF transponder does the Right rear wheel sensor; and the RR transponder does the Right Front wheel sensor.

It then goes on to explain that the Left rear wheel sensor does not need a transponder because the system concludes the signal cannot be from anywhere except the Left rear wheel !

No mention of the spare wheel anywhere.

I'll order the bits tomorrow and see if I can make it work. If it does, who cares which transponder goes where and it can read any sensor it likes, as far as I'm concerned !!

Thanks

S

Click to expand...

Is it possible one of the existing TPMS sensors is faulty?

Inquisitive said:

Is it possible one of the existing TPMS sensors is faulty?

Click to expand...

Probably not !

I have a signal from all 4 roadwheel and also an error message that the spare wheel pressure is low - but the spare wheel sensor is missing.

S

The reason I asked is my vehicle only has 4 sensors, the spare does not have one. I had a TPMS error and it turned out to be my right rear tire. I replaced the sensor and the error is gone. Hence my query.

Hi Inq

Thanks for that … .. .. being the case, I'll get a diagnostic check done, maybe, it's a problem I do have hiding a problem I don't .. if you get my meaning ?

S

Most North American cars have a spare tire or "space saver" type that is slightly smaller and only good for lets say a couple hundreds miles. They don't contain the sensor. The spare is only meant to get you to safety or a repair shop to fix the tire issue. Not wishing you bad luck or anything but I hope its only a bad sensor/dead battery in an existing tyre, like mine was. This would alleviate the necessity of engineering a work around.

Link to space saver tire https://en.wikipedia.org/wiki/Spare_tire

Here below you see a regular tire and a space saver spare compared.



Took my vehicle to a big box market tire shop they had the reader for the sensor and were able to determine which tire had the bad sensor.

So, what happens when you get the tires rotated? Are they all mixed up now?

I heard somewhere that you need to reset the system and then add sensors in the right order. You manipulate the air pressure to "order" the sensors.
In that process, can you eliminate the spare.

I can't answer for other vehicles, but in my case the spare is a full alloy rim and according the manufacturer my VIN code confirms there should be a tyre sensor fitted which has to be manually programmed to the car, unlike the roadwheels.

As for rotating the tyres, the system checks the roadwheel sensor signal every 10 minutes or so and would recognise the sensor to the new location, so there would be no 'mix up' . Equally, if a tyre and sensor is renewed, so long as it is fitted as a roadwheel, it will identify and reset itself at the next sensor check.

The manufacturer also told me that my car is configured for 5 sensors - 4 roadwheels and a spare - but it can be re-configured for 4 roadwheels only using a diagnostic tool; the local Main Stealer, however, replied that they don't know how to do that !

S

The guys that replace the ECM's/BCM's (This was a GM car circa 2000) can sometimes mess up.

Interestingly the 'New" BCM module was bad and one feature wasn't programmed

The non-programmed feature was the TPMS. Don;t ask me why I checked. The radio acts as a limited options programmer for the car.
It can reset the TPMS (wheel rotation sensor based) and the oil change light,
It can configure the headlamp behavior e.g. How long to leave the headlight on after turning off the ignition and the auto-lock/unlock behavior.

The installer can tweek the oil interval on install. He said some options are "one time programmable" and I think that was the VIN#. I think other things can be told to sync to the new VIN like the radio. The software to manipulate the locks is restricted to those that possess locksmith credentials and car dealerships. So, a typical non-dealer shop might call some guy with a mobile business.

The guy the shop used actually came out to the house. When it came time to replace the BCM the second time, the shop said it had to be done at our shop for "insurance reasons" even though he was willing to do it in the driveway. It was inconvenient for everyone. The mobile repair place - he would of had the ability to just about come anytime he was free. the shop owner - it kept a bay in use for 3 days. Free transportation. Inconvenience for me.

The repair shop and the mobile shop misunderstood each other, so everyone lost.

not sure if this has been mentioned yet, but just putting the pressure sensor in a box won't work. The pressure sensors generate their electricity with the movement of the tire going around, it is what charges the sensor to be able to transmit the signal.

System knows spare wheel won't transmit because it is not charged since it is not spinning.