Imec and Holst Centre have created a micromachined harvester for vibration energy. The harvester has a record output power of 489µW when the vibrations closely match the MEMS’ resonance vibration (1011Hz in this case). Based on measurements and simulation, the device is ideal for shock-induced energy harvesting in car tires, where it could power built-in sensors. In a tire, at 70km/h, the micromachined harvester can deliver a constant 42µW, which is enough to power a simple wireless sensor node.
The new micromachined harvester features 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. This strains the piezoelectric layer and generates a voltage across the capacitor. AlN (aluminum nitride) was selected as the piezoelectric material. The harvesters are packaged with a 6-inch wafer scale vacuum packaging process. The micromachining production process is compatible with low-cost mass-production fabrication.
Imec teamed with an automotive partner to validate the harvester for use in car tires. The harvester are subjected to regular shocks, which are dependent on the car’s speed and the characteristics and condition of the tire. Each shock will displace the mass, after which it will start to ring down at its natural resonance frequency. During this ring-down period, which depends on the quality factor Q of the harvester, part of the mechanical energy is harvested. As a result, a constant power output as high as 42µW can be harvested at a speed of 70km/h.
Micromachined vibration harvesters are ideal devices for generating electricity from machines, engines and other industrial appliances which vibrate or undergo repetitive shocks. In these environments, they will power miniaturized autonomous sensor nodes, in situations where battery replacement is not sustainable or practical. Harvesters will allow sustainable monitoring on a massive scale.