Indirect TPMS system

General
1

My old car (Qashqai J10 2009) does not fit a TPMS system. I am a lot of years old, and I can remember that, before valve-mounted TPMS (direct TPMS) were available, an indirect system based on ABS sensors was used.

Since I found information on my car’s CAN bus regarding the speed of each wheel and the steering wheel angle, I thought about implementing a basic indirect TPMS system, but I need help to do it.

I measured wheel speeds at various pressures and steering wheel angles, and came to the following conclusions:

  1. A low-pressure alarm should sound or be showed when the difference between the speeds of two wheels on the same axle is equal to or greater than 0.4 km/h. However, this difference must be maintained for 6 seconds, and during this time, the steering wheel angle must always be between -4 and 4°. This can be applied to both axles, and it is possible to determine which wheel has the lowest pressure because it is the one whith higher speed.

  2. If both wheels on the same axle were equally deflated, the previous system would not work. However, to solve this, and given that the tire pressures on both axles are different, an alarm could also sound or be showed when the speed difference between wheels on different axles is equal to or greater than 2 km/h. This difference must be maintained for 6 seconds, and during this time, the steering wheel angle must always be between -4 and 4°.

I have not knowledge enough to do this, but it might be not too difficult for an experienced forum member. So I hope.

2

Can be done with Triggers, but will be somewhat complicated and don’t know how reliable. But the basic idea:

  • Make a trigger that sets Dummy value to 1 when your condition is met and sets it to 0 when not.
  • Make another trigger that sets Dummy Timer to 0 when above dummy is 0
  • Make yet another trigger that checks if Dummy Timer is greater than 6, and use this trigger to activate warning.
3

Understood. It will be a good challenge for me.

Since all wheels will be in the same working conditions, and the measurements are not absolute, but relative to the other wheels, I’m optimistic about the reliability of the results.

It will certainly be a very basic system and will likely require further adjustments. I think I’ll spend some time looking into the calculation algorithm used by the manufacturers of these TPMS systems.

I’ll keep you informed. Thank you again and again.

4

I’m surprised with the results obtained using the following values:

  1. Speed difference between wheels on the same axle: => 0.4 km/h. I don’t use this value, but the one obtained directly before its conversion (v/175), that is, 0.4*175=70.

  2. Steering wheel angle: between -10 and +10º. Again, I don’t use this value, but the one obtained directly before its conversion (v/10), that is, -100 and +100.

  3. Time during which the above conditions must be maintained: =>4 seconds. I don´t use dummy timers for this purpose but a “DELAY”=4 available as a trigger parameter.

I still haven’t considered the possibility that both wheels on the same axle are deflated.

I’m sure this system is imperfect, and I haven’t tested it with more than 2 occupants, but it seems to be a good starting point (at least for my car), especially after reading several technical papers about the algorithms used by manufacturers of these systems, which are quite complex.

It would be nice if someone else would try this rudimentary system and share the results so that we can fine-tune it or discard it.