The U.S. National Highway Traffic Safety Administration (NHTSA) requires a tire pressure monitoring system (TPMS) on all cars and light duty trucks as of Sept. 1, 2007. To meet the requirements of the Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act of 2000, if any tire’s pressure drops below 25 percent of its required pressure rating while driving, the system must illuminate a malfunction indicator light (MIL). While the regulation applies only to vehicles with a gross weight of less than 10,000 lb, heavy trucks can also benefit from accurate tire pressure monitoring. Without the regulation’s requirement for monitoring during driving, an alternate approach eliminates the battery in every tire.
Using Surface Acoustic Wave (SAW) technology, Honeywell Sensing & Control in cooperation with Michelin North America developed a pressure and temperature sensor that does not require batteries. The SAW sensor uses a piezoelectric ceramic substrate with three aluminum electrodes that provide pressure and temperature measurements as well as a reference value. The bidirectional Interdigital Transducer (IDT) provides either a mechanical output with an applied voltage or a voltage output with an applied mechanical signal. To power the sensor, the sensor antenna converts the radio frequency signal to a voltage with a frequency determined by the spacing of stripes and temperature. To get the mechanical signal, the cover of the sensor housing acts as a diaphragm that applies a force to flex the resonator substrate, which changes the SAW frequency proportional to the applied pressure.
The sensor weighs about 3 gm and mounts with the radio frequency identification (RFID) tag and antennae in a rubber patch weighing 11 gm. Dubbed the eTire II, the unit consumes about 1 mW.
Radio frequency transmission of the sensor data to a Hand-Held reader or a Drive-By reader allows the driver or supervisor to easily obtain measurements accurate enough to detect slow leaks. The wireless technology allows the measurement of pressure and temperature from inside the tire to help truck fleet managers accurately monitor tire pressure for improved fuel efficiency and extended tire life.