One of Portelligent's analysts had the presence of mind to gather up the damaged tire pressure monitor (TPM) from his Toyota during a recent service center visit. Although no electronics had failed, the threaded stem area for the
Schrader valve had been broken off, forcing replacement of the entire sending unit. A new TPM set my colleague back more than $100. Nonetheless, we did salvage a neat piece of kit to examine in an Under the Hood teardown.
Car manufacturers have long recognized the benefits of properly inflated tires. Better driving control, reduced rollover risk, improved fuel economy and reduced tire wear are all made possible by proper inflation. High-end cars have been using TPMs for years. The U.S. Transportation Recall Enhancement, Accountability and Documentation (Tread) Act mandates a TPM in pretty much any 2008-model vehicle. Projections for new automotive unit sales mean 60 million to 80 million units annually for TPM modules in the United States alone.
There are two techniques for keeping an electronic eye on tire inflation. The older, "indirect" methods sense variations in wheel rotation speed (an under-inflated tire usually shows up as faster angular rotation, measurable by the car's existing anti-lock braking system and traction control system). Although this method is cheaper, the Tread Act requires a level of observability not met by the indirect approach, making necessary "direct" methods that provide in situ pressure monitoring for each tire independently.
The TPM module examined here resides within each tire or wheel of the vehicle, implementing a direct monitor by providing the car's central electronics with a pressure reading by tire and over a wireless link. Specifically, this TPM module is made by TRW in a joint venture with Michelin called Entire.
TRW's wireless link for the module operates in the 315-MHz band, the RF communication path eliminating the need for complicated schemes and thereby cutting costs.
Using an internal coin-cell battery with up to 10 years of life, the assembly probably conserves power through timed readouts and/or by polling from the car's central electronics, with sleep cycles in between. A high-quality lithium battery from Panasonic (#BR2450A) helps the device withstand the harsh environment inside the wheel.
In keeping with design for harsh environments, the module uses a tough PPA-GF33 plastic resin for its top and bottom shells, the top shell holding the metal inflation stem and the bottom shell nothing but a glue-on cover to protect the electronics. A 2-D bar code laser etched on the shell casing carries a unique serial number to link the sensor to the target vehicle. Clearly, the pressure reading from the module must also encode the serial number in each pressure transmission.
Inside the module, pressure readings are obtained in a single-package, two-chip component (#SP311) from SensoNor. A crystallographically etched silicon MEMS transducer implements a silicon "drum head" strain gauge to sense pressure through an open port on the #SP311's package topside. Signals from the MEMS device are conditioned and processed in a part from NXP (#PCH7371) that likely contains a temperature sensor for calibrating out the effects of heat- and cold-cycling.
To send the determined pressure state and serial number of the tire's sensor to the car's central electronics, an Infineon #TDK5101 315-MHz transmitter chip modulates outputs from the #SP311 with either the amplitude-shift or frequency-shift keying (ASK/FSK) supported in the 10-pin device. A wire-wound antenna hangs directly from the #TDK5101 for transmission, and a 9.84375-MHz crystal from Hong Kong X'tals supplies timing for the sensor/transmitter pair, providing MCU clock and radio base frequency.
The Toyota setup doesn't identify exact pressure or even indicate which tire(s) might be low. Toyota seemed to think an "idiot light" warning was enough. In any case, this module sells for $100-plus, but probably costs $10 to manufacture in volume.
Behind the scenes I want to acknowledge the Portelligent staff who help with Under The Hood, now six years running in EE Times and TechOnline. A word of thanks goes to Bill, Bob, Chad, Claire, Dave, Jeff (both of you), Nathan, Stacy, Tim, Ted, Tom and everyone else who contributes to our daily efforts to "look inside" for these articles and for the far more complex client deliverables you'll find at www.teardown.com. Check it out!
About the author
 David Carey is president of Portelligent (www.teardown.com), a TechInsights company. The Austin, Texas, company produces teardown reports and related industry research on wireless, mobile and personal electronics.
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