[Ovmsdev] indirect TPMS in app

Michael Balzer dexter at expeedo.de
Mon Jan 11 05:40:46 HKT 2021

I read something about this being typical for all (or many?) VW 
vehicles, apparently they don't have dedicated TPMS sensors but derive 
the tyre "health" from the ESP sensors.

If that's correct, there is no way we will ever be able to map the 
values to actual temperatures & pressures.

So… how about adding new relative TPMS metrics & warning/alert 
indicators to be used alternatively to the current absolute temperatures 
& pressures?

We could add these now as vectors, and replace the previous single 
metrics for temperatures & pressures by vectors as well. That would 
reduce the metric payload size on both MQTT and the WebSocket transport, 
and also adapt nicely to vehicles with more or less than four wheels.

Proposal: numbering scheme rows front to back, left to right per row. 
For four wheels:

v.t.pressure            fl,fr,rl,rr    [kPa]
v.t.temp         fl,fr,rl,rr    [°C]
v.t.health              fl,fr,rl,rr [%]
v.t.alert              fl,fr,rl,rr    [0/1/2]

The alert vector can use the same scheme as with the battery cells, i.e. 
0 = OK, 1 = warning, 2 = alert.

The MP message "W" can be replaced by a new one. Proposal: Code "Y", data:

  * number of wheels
  * pressure values + staleness
  * temp values + staleness
  * health values + staleness
  * alert values + staleness

We can continue producing "W" messages for old clients in the field.

Anything missing?

If that's OK for all, I can make the necessary changes.


Am 09.01.21 um 19:35 schrieb sharkcow:
> Thanks for your input, Michael.
> I agree that we could use the relative metrics to roughly estimate the
> current assumed pressure from a reference value (which would have to be
> updated every time a user presses the TPMS reset button...).
> But: I'm afraid the correlation is not linear, probably not even
> reproducible. Also, we have two pressure-related values per wheel, while
> OVMS for now has a pressure and a temperature value.
> So... I think I'd still prefer two relative indicators for the e-up in
> the app. I can try to work on implementing a custom MP record.
> sharkcow
> Am 08.01.21 um 18:33 schrieb Michael Balzer:
>> Sharkcow,
>> as we don't have a new capabilities message scheme yet, conditional
>> blocks are the way it works right now in the App.
>> The meanings of standard metrics and message data fields should not be
>> changed. Introducing custom metrics and a custom MP record is a possible
>> solution.
>> But having custom solutions for common things like this is a PITA. Maybe
>> we can work something out to convert the readings into the standard
>> metrics?
>> For example: if you get a "health percentage", we could introduce a
>> configuration for the "100%" health pressure level, so we can get an
>> absolute value from that.
>> Regards,
>> Michael
>> Am 08.01.21 um 15:12 schrieb sharkcow:
>>> I have now figured out how to read and partly how to interpret the
>>> metrics of the indirect TPMS on the e-up.
>>> We get a "tyre emergency" and a "pressure diffusion" value for each
>>> wheel.
>>> Since the values are relative, I would like to transmit them to the app
>>> as raw data and only display a percentage (100% means everything is 
>>> ok).
>>> Would it be ok to just introduce an if-statement in the app to detect
>>> whether the vehicle type is an e-up and change the default displays
>>> accordingly?
>>> sharkcow
>>> PS: I feel this value (incl. a useful alert value) would be very
>>> beneficial: I just found out that I had lost quite some pressure over
>>> the 2 years I own the vehicle without TPMS reacting. The metrics on OBD
>>> showed a nicely measureable change however, which could easily be used
>>> to have OVMS notify me. (Yes, I should have checked the pressure
>>> regularly, but hey: what's a TPMS for?!)
>>> _______________________________________________
>>> OvmsDev mailing list
>>> OvmsDev at lists.openvehicles.com
>>> http://lists.openvehicles.com/mailman/listinfo/ovmsdev
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Michael Balzer * Helkenberger Weg 9 * D-58256 Ennepetal
Fon 02333 / 833 5735 * Handy 0176 / 206 989 26

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