PowerSource

I had a conversation with Jim Grothe, strategic marketing and business development for MEMs at Freescale, about power management for tire pressure monitoring systems (TPMS). These are the pressure sensor systems made up of a MEMs pressure sensor and an RF transmitter that goes on the wheel, and an RF receiver and processor that reside on the car frame/body and communicate the tire pressure information to the system processor. TPMS is a prime example of how car sybsystems are under very tight power constraints as electronics proliferate under the hood. Grothe emphasized, “Power is THE driving requirement, second only to the accuracy of the pressure sensor itself.”

Unlike most the rest of a car’s electronics that can use power from the main electrical system, the power for the tire-side sensor electronics comes from its own small battery, which has to last for at least 10 years without being replaced. That’s a painfully tight budget. I asked about energy harvesting systems that transform a small amount of the tire’s mechanical energy to electrical energy. Grothe said Freescale, along with several other companies is working on it, but energy harvesting isn’t practical for at least the near future for the automotive environment.

Another example of how power miserliness drives the TPMS architecture is that Freescale uses capacitive sensing rather than piezoresistive technologies (PRT) for the tire pressure transducer. Capacitive sensing is a low-power technique because you’re not generating a current to sense the voltage across the transducer. While a PRT transducer doesn’t require a lot of current, every little bit counts when you’re looking at ten years from a tiny battery.