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Tyre Pressure Monitor Sensors

Thread starter #1
Hi Guys

Does anybody know how these things work ?

TP Sensor.jpg

I'm interested in how the sensor gets a pressure reading from the valve body.

Anybody ever played with one ?

S
 

throbscottle

Well-Known Member
#2
I've not seen one of those in the flesh, but have looked inside the pressure sensor on an electric tyre pump. If I remember correctly the pressurised and non-pressurised sides are separated by a board with a strain gauge on it. I suspect the tyre version has a calibrated chamber on the non pressurised side, if it's anything similar. No clue how they transmit the readings though.
 

rjenkinsgb

Active Member
#3
The sensor is just attached to the valve stem as a convenient mounting & is located inside the tyre so directly exposed to the pressure.
It will almost certainly use an "Absolute" pressure transducer. You can see the sensor pressure port just to the right of the screw, a small stub tube.

There are many different makes and designs, but all will have a vent hole or tube in to the pressure transducer somewhere - it's very clear in this photo:
https://www.worldcarparts.co.uk/med...-Benz-Sprinter,-Viano,-Vito,-Mixto-VDO-11.jpg
 
Thread starter #4
Hi Guys

Thanks for that .. .. I'm not too concerned about how it transmits but I'd expect it to be a fairly straightforward RF send and receive .. ..

.. .. .. however, the info about a pressure transducer is very useful … would you expect there to be some kind of diaphragm inside that stub tube ? If so, maybe I can fix something in the hole to imitate a tyre pressure.

Let me explain a bit of what I need to do .. ..

I've just bought another car with a 4.7 litre petrol engine which I'm going to convert to run on LPG as is my habit. The LPG tank will fit in place of the spare wheel. I've never had a car with TPMS before and it didn't cross my mind that replacing the spare wheel with a fuel tank would cause a problem, but I get a constant error message and an annoying chime on the dashboard so I want to position a sensor in the car, without the wheel attached that will tell the TPMS system the spare wheel is fine ! Simples !

Looney Tunes ?

S
 

Inquisitive

Super Moderator
#6
Is it possible that the software reads 5 tyres/tires and its a simple matter of saying no we only have 4 tyres. Can a auto repair shop tweak the setting?
 

JimB

Super Moderator
Most Helpful Member
#7
You should be able to fit the extra valve in to a small IP68-rated metal metal enclosure (or something similarly gastight) and just pump that up to the required pressure.
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. :eek:

JimB
 
Thread starter #8
I'm glad you thought of that JimB !!

Also, wouldn't the metal case detriment the transmission signal ?

Inq .. the content of the software is a closely guarded secret there is no chance of such a simple step.

If I understand this correctly, inside that short tube hole there is a diaphragm which uses strain gauges to convert the pressure to a resistance and hence a voltage. There ought to be some way of presenting enough physical pressure to the diaphragm to achieve a result without puncturing it ?

Thanks for all the contributions .. .. .. .

S
 

JimB

Super Moderator
Most Helpful Member
#9
Also, wouldn't the metal case detriment the transmission signal ?
Yes it would somewhat.

inside that short tube hole there is a diaphragm which uses strain gauges to convert the pressure to a resistance and hence a voltage.
Not necessarily strain gauges in the conventional sense, but some kind of pressure sensor.
All you have to do is fudge the output of the sensor to simulate a pressure.

At its most crude, this could be by driving a screw into the hole to act on the diaphragm.
Sounds like a good way to completely bust the thing!

JimB
 
Thread starter #10
Not necessarily strain gauges in the conventional sense, but some kind of pressure sensor.
I was quoting from an article on pressure transducers I've just been reading .. ..

At its most crude, this could be by driving a screw into the hole to act on the diaphragm.
Sounds like a good way to completely bust the thing!
I guess that would depend on the screw somewhat .. .. ..

Aforementioned article says that the 'strain gauges' manipulate an output voltage by varying a resistance; maybe there's a way of substituting a variable resistance with a potentiometer .. .. .. perhaps a big screw IS a better option !

S

 

JimB

Super Moderator
Most Helpful Member
#11

alec_t

Well-Known Member
Most Helpful Member
#12
If I were doing this I'd try to find a blunt-ended non-metallic rod (cocktail-stick?) to fit the sensor aperture and apply gentle finger pressure to deflect the transducer and see whether that fools the monitor. If it does, then a more permanent screw-adjustable pressure applicator could be rigged up.
 
#13
Good point on the metal causing screening.
Some tyre sensors I looked at pass the sensor antenna through the valve stem tube seal, but no guarantees.

The types of box I picked were rather small and with extremely thick walls, with pressure in mind; I would not consider a normal thin-wall die cast box, or anything bigger than essential for the sensor body to fit.
 

throbscottle

Well-Known Member
#14
If you could detect the output of the sensor perhaps you could find some way to reproduce it and do away with the actual valve altogether. Shouldn't be a problem if it's a 1 way signal.
Where do these things get their power from?
 
Thread starter #15
If I were doing this I'd try to find a blunt-ended non-metallic rod (cocktail-stick?) to fit the sensor aperture and apply gentle finger pressure to deflect the transducer and see whether that fools the monitor. If it does, then a more permanent screw-adjustable pressure applicator could be rigged up.
Yes Alec .. I think that is more or less where I'm likely to end up with this … .. .. . although the size of the whole thing suggests to me the distance between 0psi and 40psi at the transducer is probably no more than a bee's knee, so that may be easier said than done. Also, the sensors work out at £40 a piece so a slip would be costly.

My initial thought was to detect the signal and duplicate, but RF friend has presented me with some parameters for success that I'm not going to be able to access. They get their power from a coin cell - if you look at the picture in post #1 you can clearly see the RH end of the sensor is almost circular .. .. allegedly lasts for 10 years !

I've just collected the car back from the local garage .. .. the mechanic there had a novel suggestion .. .. 2 sensors in one of the rear wheels ! I ned to think about that !

Thanks for your help

S
 

Mickster

Well-Known Member
#16
They get their power from a coin cell - if you look at the picture in post #1 you can clearly see the RH end of the sensor is almost circular .. .. allegedly lasts for 10 years !
Indeed they do and this one was sacrificed for our education. The cell powering this TPMS sensor is a CR2450A, which is a 3V cell, but only reads 2.32V now. I do not know how old this sensor is, but it came from a Lexus LS460.

I forgot to take a before pic, so this one is from Amazon:
LS TPMS.jpg
https://fccid.io/PAXPMV107J


The hole below the valve stem is what allows air in and out of the tyre, and the one which looks to be like a hole below that, is actually blind. There is a cover on the bottom, which reveals a totally encapsulated (white silicone potting) assembly - there is no orifice to any kind of sensing element. Peeling back the plastic and removing the silicone potting reveals this:

IMG_1062.jpg IMG_1063.jpg

"US 71 A42XJ" does not turn up any meaningful results for me, but a guess would probably say that it is the sensor (EDIT - FCC-ID link above notes it's a custom IC), because apart from jellybean passives, the only other components are what appears to be an RFID coil and a 314.680 MHz SAW resonator. (Check the FCC link for more info)

Other ones, which I have taken apart in the past, DO have an orifice which goes directly to the sensing element, and IIRC, they looked something like a BMP085;
https://www.digikey.com/product-detail/en/bosch-sensortec/BMP085/828-1005-1-ND/1987010

Regards.
 

Pommie

Well-Known Member
Most Helpful Member
#17
Many years ago my kids entered a competition to produce an air powered vehicle. A car valve was fitted to the top of a 2ltr coke bottle. The bottle was pumped up to 150psi. Could be a nice simple solution. Drill correct hole in bottle lid, fit valve and pump up.

Mike
 

JimB

Super Moderator
Most Helpful Member
#18
there is no orifice to any kind of sensing element.
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
 

Mickster

Well-Known Member
#19
For members interested in TPMS, the following does not solve MM's problem, but it is just a little more info.

There are 3 types of TPMS system:
Sensorless. (Not strictly true)
Low level.
High level.

"Sensorless" systems do not use a TPMS sensor in the wheel, but instead use the ABS wheel speed sensor, to compare wheel speeds. This is the slowest type of measurement, because there are a few factors which need to be taken into consideration in order for a decision to be made. The radius of a tyre will change, depending upon the pressure in that tyre, so a tyre with a lower pressure will be seen to rotate faster by the ABS wheel speed sensor. Temperature can affect the tyre pressure, so where a vehicle has been sitting can become a factor, since a vehicle may have been parked with one side directly in the sun, and the other in shade. Because of scenarios such as this, a period of driving time has to occur, to allow all tyres to reach the same temperature. This can take up to 20 minutes, before a warning light illuminates. In general, the tyre with a low reading is not indicated and all tyre pressures must be checked.

"Low level" systems use a sensor in the wheel, and can indicate which tyre has low pressure, but nothing more.

"High level" systems also use a sensor in the wheel, but the actual pressure for each tyre is displayed on the instrument cluster.

The Low and High level systems have the benefit of alerting a driver to a low pressure before a journey is started.



I am going to remove the gold lid from the custom IC at some point (maybe later today), and have a look at what is inside. Anything interesting will be posted in this thread.
 

JimB

Super Moderator
Most Helpful Member
#20
Mickster, you really must stop this, I am rapidly running out of "Informative" stickers ! :D

JimB
 

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