‘Shock’ energy produces power

Imec and the Holst Centre say they have made a micromachined harvester for vibration energy with a record output of 489 µW.

Measurements and simulation show that the harvester is also suitable for shock-induced energy harvesting in car tyres, where it could power built-in sensors. In a tyre, at 70 km/h, the new device can deliver a constant 42 µW, which is enough to power a simple wireless sensor node.

The harvester consists of a cantilever with a piezoelectric layer sandwiched between metallic electrodes, forming a capacitor. At the tip of the cantilever a mass is attached, which translates the macroscopic vibration into a vertical movement - putting strain on the piezoelectric layer and generating a voltage across the capacitor.

As piezoelectric material, aluminium nitride was chosen. The harvesters are packaged with a 6-in wafer scale vacuum packaging process.

The harvester has a record output of 489 µW when the vibrations closely match the MEMS’s resonance vibration, which in this case is 1011 Hz. Together with a motor vehicle partner, imec also validated the use of the harvester for use in car tyres.

These submit the harvester to regular shocks, depending on the vehicle’s speed and the characteristics and condition of the tyre. Each shock will displace the mass, after which it will start to ring down at its natural resonance frequency.

During the ring-down period, which depends on the quality factor Q of the harvester, part of the mechanical energy is harvested. It is shown that in this way, a constant power output 42 µW can be harvested at a speed of 70 km/h.

Micromachined vibration harvesters such as these are suitable devices for generating electricity from machines, engines and other industrial appliances which vibrate or undergo repetitive shocks. In these environments, they will power miniaturised autonomous sensor nodes, in situations where battery replacement is not sustainable or practical.