FXTH87 and FXTH87E acceleration tolerance are very large.

[tedwu1]

Hi friend,

I use FXTH87 and FXTH87E for same TPMS projects. It is as a car start if the X or Z axis acceleration exceeds +-6g. I use X/Z compensated values to calculate them. I set the values of X=410 (6g) X= 102 (-6g) and Z=307(6g) Z=205(-6g) for FXTH87. And I set values X=362 (6g) X= 148 (-6g) and Z=292 (6g) Z=213(-6g) for FXTH87E. The X-axis acceleration uses an offset of 7, and the Z-axis acceleration uses an offset of 6 to get the compensation value. I put the two TPMS in the machine for a spin. Its diameter is about 14 inches. The FXTH87 TPMS sends car start data at 80 rpm and the FXTH87E TPMS sends data at 160 rpm. Why the result is so much different. Am I doing something wrong? Can anyone guide me? Thank you in advance.

Best wishes

Ted Wu

[TomasVaverka]

Hi Ted,

Let me apologize for the delay in answering your question, we were experiencing some issues on certain areas of the NXP.com website including this community webpage.

Here is the answer I have received from our TPMS expert Camille S-J.

First of all, I would like to highlight that it is the accelerometer orientated in the radial direction which is the most suitable for motion detection. The accelerometer in the tangential direction is generally only slightly influenced by centrifugal acceleration, which does not allow to detect motion in a reliable manner.

Sampling only the accelerometer in the radial direction (versus both accelerometers) allows to save power.

Then, to understand why motion is detected at these wheel speeds, I would recommend to start by looking at the acceleration measured by each axis when the wheel is not moving, and the TPMS placed at each of the four positions below.

So, place the TPMS in each of the four positions and send out via RF the value of X and Z axis. This will be the first step to understand what is happening when the wheel starts moving.

Best regards,

Tomas

PS: Feel free to reach out to Camille to discuss it in more details if needed.