The biggest obstacle to vibration harvesters today is their extreme sensitivity to resonant frequency. A vibration harvester can output as much as 200-times the energy when operating exactly on its resonant frequency, compared with being just a few cycles off resonance. As a result, companies crafting vibration harvesters today are working with their customers to precisely match their application's vibes, as well as manufacturing their own piezoelectric materials that mitigate the effect of resonance.
For instance, in July, Advanced Cerametrics Inc. (Lambertville, N.J.) will announce a vibration harvester using its proprietary piezoelectric fibers that can generate milliwatts of power for remote sensors, wireless transmitters and industrial monitors.
"The key to how it works, is that, even while in sleep mode, it is harvesting vibrations to build up charge on a capacitor," said Farhad Mohammadi, director of research at Advanced Cerametrics. "When the capacitor's voltage gets up to 3.5 [volts], it comes out of sleep mode and turns the application's circuits on--for instance, to wirelessly transmit telemetry data then go back to sleep."
Advanced Cerametrics Inc. was already supplying the Department of Defense with a custom version of its harvester to mine the vibrational energy at the precise resonant frequency of specific pieces of military equipment. But its new line of commercial harvesters is less sensitive to resonant frequency, the company claims, giving it the latitude to serve a variety of applications, from powering industrial wireless sensors to consumer devices, such as pedometers.
To make its piezoelectric fibers less sensitive to resonant frequency, and more ruggedized for a wider variety of applications, Advanced Cerametrics manufactures its own piezoelectric elements, called Bimorphs. Bimorphs are made by spinning 10-micron piezoelectric fibers together with rayon, then embedding them into epoxy. Three Bimorphs power the first harvester model in Advanced Cerametrics new commercial line.
Other companies manufacturing their own piezoelectric materials include Mide Technology Corp, (Medford, Mass.), which earlier this year announced its line of commercial vibration harvesters that are currently being evaluated by several customers, including a railroad that intends to wirelessly transmit data to the operator about the force being applied by brakes on its trains.
"We use a piezoelectric beam with a tip mass, but have encapsulated the fibers in Kapton [a DuPont polymer], making our harvester much more rugged and less sensitive to resonant frequency," said Mide Technology's CEO, Marthinus van Schoor.