As part of enhanced car safety, cars manufactured in the US are required to build in a tire pressure monitoring system (TPMS), something that will soon also be mandatory in Europe. Now, Panasonic and Belgian company Imec have teamed up to develop an electrostatic energy harvester that can power these systems using vibration and noise from the tires themselves without the need for another power source.
"For user comfort and cost limitations, TPMS requires a compact and long lasting electrical power supply that does not need to be replaced during the lifetime of the tire," according to information provided to Design News by harvester developers from both Panasonic and Imec, who started work on the project about two and a half years ago. "There are a plenty of mechanical vibrations in a tire environment. Therefore, vibration energy harvesters, which act by converting ambient mechanical energy into useable electrical energy, are a potential solution for powering TPMS."
Rob van Schaijk, research and development manager for sensors and energy harvesters at Imec, told us how the harvester, which is about 1 cm
The harvester will be mounted on the inner liner of the tire. The device will be excited by the shocks and noise generated in the tire. With every rotation of the tire a shock will excite the device; noise will continuously excite the device. No match between vibration frequency and resonance frequency is needed with shock excitation.
This type of design is comparable with the same vibration used to cause a reaction in a tuning fork, van Schaijk added. The team chose an electrostatic method for designing the harvester rather than piezoelectric, the other typical method used for small microsystem-based energy harvesters.
The device is comprised of a stack of three wafers bonded together, with the central wafer containing a mechanical resonator made of a proof mass and springs etched in silicon, researchers said. This proof mass supports a corrugated electret on its bottom side, obtained by Corona charging of a SiO2/Si3N4 stack.
The bottom and top wafers are made of glass with the bottom supporting a set of two metallic electrodes that are connected to a load circuit. The top wafer is used as capping to protect the device and to allow vacuum encapsulation.
Benefits of this type of design include high power output, especially in a shock-based application like this one, and intrinsically better reliability due to smaller displacement. "The mass displacement in the piezoelectric energy harvester is often much larger and this hampers the reliability," van Schaijk said.
Device designers also developed the harvester with mass production in mind, basing it on a SiO2-Si3N4 electret compatible with CMOS processing, thus allowing low-cost production for large volumes. van Schaijk said:
The technology used to make this harvester are compatible with integrated circuit (IC) processing. This means that the harvesters can be made in standard clean rooms without compromising other processing in that clean room, including CMOS.
Moreover, the device was designed with materials that are well known to IC manufacturers. "No new or strange materials are used in the harvester. The polarization source for the harvester is based on an electret with silicon oxide and silicon nitride as material," he said, adding that it would likely will be another year and a half of product development before the harvester might be ready for commercial production.
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