# TPMS under inflation measurement

Maynard Technician Ontario Posted   Latest

Hi everyone My question actually lies in the last 2 sentences, the rest is laying the ground work

I have a question concerning indirect TPMS measurements. I have never paid much attention to those systems, the ones with no sensors in the wheels. I always "assumed" it will be calculated off individual speeds via ABS sensors. One day when discussing the nuances of resetting these systems ( due to winter tires being installed) my co worker informed me that it doesn't matter if wheel and rim combo changes with an entire set at once, the light will almost certainly come on if the system is not re calibrated after the wheel exchange. I was quite resistant to that theory, thinking that it doesn't matter what size of rim and tire combo we install, the car will not know.

However, when i got to researching what is going on behind the scenes I was in for quite a surprise. Take the later Honda systems for instance. They have no pressure sensors, but they recommend pushing the button on the dash to re calibrate the TPMS system any time air has been added to a tire or any wheels or tires have been exchanged. There is apparently 2 systems embedded and algorithms are utilized to detect tire under inflation.

The first system seems a logical one to me "wheel speed differential" apparently compares RF to RR, and RF to LF, and RF to LR then math algorithms used to decide when rolling radius is affected enough to alert the driver through light on dash. However then there is another system which plays into the first, "Peak tire resonance".....and that is where my question lies. How is tire resonance (vibration?) measured? Is that still a function of the abs sensor pulses actually accelerating and decelerating in sync with tire vibration?

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Dmitriy Analyst
Ontario
Dmitriy

Maynard, there are patents by Infineon that present this way: Embodiments relate to indirect tire pressure monitoring systems (TPMSs) and methods that utilize anti-lock braking system (ABS) signals. In embodiments, information from the ABS Hall signal is obtained in analog form, before pulse forming. The information can be analyzed for a resonance frequency within the ABS sensor. In some…

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Maynard Technician
Ontario
Maynard

Thank you Dimitry, Yes I suppose that is how the Honda system works, For that matter i am interested just in general what goes on in those systems no matter what brand. So from reading through there.... I am not sure i came to a complete solution. There is suggestion that an analog signal is actually generated from the sensor BEFORE the sensor converts it to digital. And again my question then…

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Dmitriy Analyst
Ontario
Dmitriy

From the descriptions, the analog signal is the measurement of magnetic field, nothing else. Standard digital ABS sensors convert this analog signal into digital one by using a comparator, but this is not a very precise method. This patent describes at great length how to process the analog signal, and then output digital signal such that its frequency spectrum (as deciphered by the ABS module)…

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Maynard Technician
Ontario
Maynard

Wow, i didn't read nearly all that but with the bits I did read and the included pictures..... basically a "perfectly clean" sine wave projection is used as a theoretical template, then actual samples are collected from the "real" wave form and the inconsistency in their progression along the wave will be calculated and this is translated as a vibration/resonance. (but the template is produced…

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David Owner/Technician
Alberta
David

So, I understand the resonance is directly proportional to the tire circumference/diameter. This gives a natural resonance period for any particular speed. Therefore it is a way of measuring or verifying rotational velocity of the wheel. If you change the wheel outside diameter, the resonance will change and not match any other input signals as per the program. Very low speeds are unlikely to be…

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Dmitriy Analyst
Ontario
Dmitriy

Yes, this method becomes viable only at speed, and, even then, one would need to filter out bumps in the road... which means it would require some statistical processing rather than simple comparisons. The “reset” button in such systems instructs the algorithm to start statistical learning of what is ”normal“. Now, if the vehicle’s user just resets the TPMS light to make it go away, without…

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Maynard Technician
Ontario
Maynard Resolution

I am closing this post, i was notified its still open. Sorry i didnt realize that a close is expected. I really appreciate the participants discussion. I posted as an open ended question and i did receive enough information from the links Dimitry provided to give me a bit of an insider view on what is going on with the sensing devices. Something that comes to mind pretty heavy for me is " how…

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Thanks