A Picowatt Energy Harvester
- PDF / 244,361 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 71 Downloads / 248 Views
A Picowatt Energy Harvester Joe Evans1, Johannes Smits2, Carl Montross1, and Gerald Salazar1 Radiant Technologies, Inc., 2835D Pan American Fwy NE, Albuquerque, NM 87107 USA 2 Scaldix B V Stationsstraat 13 4331 JA Middelburg, The Netherlands
1
ABSTRACT The authors describe an energy harvester circuit fabricated with integrated thin ferroelectric film capacitors on a silicon substrate. The harvesting mechanism is a folded double-beam cantilever with proof masses at both end points. Interdigitated electrode capacitors are located at the three points on the folded cantilever that are expected to experience maximum bending moment and should produce up to 5V as a function of external vibration. The die has the dimensions of 1.6mm on a side and is designed to be mounted in a TO-18 package transistor-style package. Due to its small size, the self-contained piezoelectric MEMs device should produce 50 picowatts in a 1g vibration environment while occupying little space. THEORY The authors are fabricating an energy harvesting device with piezoelectric bimorph cantilevers to convert ambient vibration into electrical energy. The device consists of a single double-beam cantilever with a cross-bar tip mass connecting the ends of the two cantilevers, each of which has an interdigitated electrode piezoelectric capacitor at the point of maximum bending. A second single-beam bimorph is placed in the gap between the two beams of the double-beam cantilever. The single-beam cantilever will have a different resonant frequency than the double-beam cantilever and will help capture more energy from the ambient vibration without increasing the overall size of the device. The theory describing energy capture by a piezoelectric bimorph follows. We calculate the voltage developing at the electrodes of a piezoelectric bimorph vibrating with amplitude a0 at the clamping point in a direction perpendicular to the length. One of the boundary conditions for the solution of the differential equation is that the clamping point undergoes a sinusoidal motion with an amplitude a0. All other boundary conditions are those of free bimorphs [1]. The solution of the Euler Bernoulli equation gives the local deflection in the x direction along the length of the cantilever as:
(1) We use here the conventions c=cos kL, s=sin kL, ch= cosh kL, sh= sinh kL as described in [1]. The shape of the vibrated bimorph near resonance is shown in Figure 1.
105
Figure 1. The deflection profile of the cantilever near resonance. Note that the cantilever has a high radius of curvature near the clamping point and is nearly straight at the free end. The piezoelectric capacitor used as the scavenging engine will gain capacitance linearly with its length but will receive minimal piezoelectric distortion near the free end of the cantilever. Consequently, the authors elected to restrict the length of the piezoelectric capacitors to the region of maximum bending so that each cantilever will operate a higher energy to capacitance ratio. To find the charge induced between the electro
Data Loading...
