The automotive Tire Pressure Monitoring System, Part 4: Issues

Sensor-based monitoring systems for tire pressure can warn of potential problems, but there’s much more to the story than just a pressure-measurement system.

The TPMS adds a highly specific safety-related feature to cars. However, as with many presumably good ideas, the “devil is in the details”, and that’s especially true for the TPMS. There are many issues associated with the TPMS, among them:

• The RF link is very low power to extend battery life. Due to the weak signal and the general automotive RF environment, it is subject to internal and external EMI-induced errors.
• The very weak RF signal can be blocked if something metallic (such as lunch in an aluminum-foil wrapper) is put in the wrong place, which varies by car; for some cars, putting lunch under the driver’s seat can block and cause the loss of the signal.
• The battery in the TPMS unit in the tire has a life of only 5-10 years depending on various factors and is not replaceable, so the entire module must be replaced. In reality, the units for all four tires should be replaced simultaneously, so we’re looking at several hundred dollars. The replacement must be the same type and frequency as the originals, of course.
• If you have to replace a tire due to failure or routine wear, the same replacement scenario applies to end-of-battery. The TPMS module itself can also be damaged by hitting a curb or pothole.
• Not surprisingly, the sensor and entire TPMS need to self-calibrate after module replacement, keeping things steady for a few hours to a few days. Going on the highway and heating up the tires – and thus increasing their pressure – too soon after replacement can corrupt the calibration process. The TMPS includes a temperature sensor in each module to incorporate a temperature reading into the algorithm, which computes pressure from the send variables.
• Transient shifts in tire temperature such as occur when going from a heated garage to a cold outside can also cause false readings; again, the temperature reading must be used.

To provide additional insight to the driver, most cars allow the driver to invoke a console display that shows each tire’s pressure (Figure 1). This is more information than just the low-pressure indicator.

While this is useful, most drivers don’t understand that a displayed value can easily “chatter” by ±1 PSI (or the least significant digit, if readings are in bar units). To minimize this occurrence, the TPMS software adds some hysteresis to the displayed reading to stabilize it and minimize unneeded confusion or worry. Also, the observed pressure will legitimately increase by several PSI as rolling friction heats the tire; this, too, can cause confusion.

Finally, as with most data systems, it is possible to lose synchronization or ID for an individual TPMS unit or need to reset or recalibrate the sensor. Obviously, this must be done via a wireless link. Mechanics can even buy a specialized instrument which reads, resets, and resyncs the TPMS units (Figure 2).

Conclusion

TPMS is another safety-related feature in a car and has definite benefits. However, as with all add-ons, it adds up-front cost and incurs significant replacement cost, and is another thing that possibly fails (both hard failure and intermittent). Successful implementation of TPMS is a significant engineering challenge which designers and vendors have met through advanced ICs and packaging techniques.

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References

TPMS Background and History

• Schrader TPMS Solutions, “Schrader History”
• CarID, “What Is The History Of Tire Pressure Monitoring Systems?”
• Gregg’s Automotive, “The Story Behind Tire Pressure Monitoring System (TPMS)”
• TMPS Direct, “A Brief History of TPMS”
• Wikipedia, “Firestone and Ford tire controversy”

TPMS Operation

• Tirebuyer, “What is TPMS (Tire Pressure Monitoring System)?”
• Wikipedia, “Tire-pressure monitoring system”
• Inertion, “The Better Way to Fill Your Tires”
• TiresPlus, “Tire Pressure Monitoring System (TPMS)”
• TMPS Direct, “TPMS Sensor Batteries”
• Priority Tire, “TPMS 101: What Is it and How it Works?”
• Car and Driver, “A Tire-Pressure Sensor Can Save You From Having a Blowout”
• How Stuff Works, “How Tire Pressure Monitoring Systems Work”
• Mechanic Base, “How to Reset Tire Pressure Light (TPMS) – By Car Model”
• Autel, “MaxiTPMS TS401”
• Subaru 2010, “How much does change in tire pressure affect tire circumference?”

TPMS ICs and Components

• Melexis, “World’s smallest TPMS solution”
• NXP, “Tire Pressure Monitoring Sensors”
• NXP, “TPMS Selector Guide”
• NXP, “A Global Solution for Tire Pressure Monitoring Systems”
• Infineon, “Tire Pressure Sensor (TPMS)”
• Infineon, “SP400-11-11 Automotive Tire Pressure Monitoring Sensor”
• Infineon, “SP40PLUS Tire Pressure Monitoring Sensor”