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Electret Energy Harvester
December 7, 2011
The first
microphone I owned was a "
crystal" microphone. This was essentially a thin foil backed with a small patch of Rochelle salt (
potassium sodium tartrate, KNaC
4H
4O
6•4H
2O). This material is a
water-soluble piezoelectric, so it's easy to process to make microphones. It does have the disadvantage that it's
fragile, a quality I found after dropping my microphone on the floor.
Other inexpensive microphones use
magnetism to generate a
voltage by moving a
coil of wire in a
magnetic field by the principle of
electromagnetic induction. Sometimes, as in a
ribbon microphone, the wire "coil" is just a
conductive sheet. The better microphones of yesteryear were
condenser microphones in which the vibrating membrane was one plate of a
capacitor held at a high voltage.
The Neumann Model U 47 multi-directional condenser microphone.
This microphone was much beloved by record producers.
It's the microphone used in many Frank Sinatra and Beatles recordings.
(Photo by C.J. Sorg, via Wikimedia Commons))
One disadvantage of the condenser microphone is the need for a power supply to bias the capacitor plates. Furthermore, any
noise present in the biasing supply will be impressed on the microphone signal. These problems are overcome in
electret microphones.
Electret materials are the
electrostatic analog of
magnets; that is, they are materials that have a permanent
electric field, just as magnets have a permanent magnetic field. Electret microphones use an electret as the non-vibrating plate in a condenser microphone.
Microphones are
vibration energy-harvesters, since they convert vibration to a voltage signal. Piezoelectricity and electromagnetic induction are commonly applied to vibration energy-harvesting. An electret condenser microphone will also function as a vibration energy-harvester, but a more efficient device would use a
cantilever geometry. I described similar cantilever energy-harvesters in a
previous article (Cantilever Energy Harvesting, August 16, 2011)
That's the device investigated by
French scientists from the
Laboratoire d'électronique des technologies de l'information (CEA-Leti, Grenoble, France) and
Centre national de la recherche scientifique (CNRS).[1-3] The device structure, as shown in the figure, is very similar to that for piezoelectric vibration energy-harvesters, but without the piezoelectric element at the
highly-strained portion of the beam near the mounting point. That's replaced by an electret material below the
proof mass.
A cantilever beam vibration energy-harvester using an electret material. (Via arXiv Preprint Server, Ref. 3, Fig. 1).
The physics involved in such a device can be seen in the figure below. The electret has a constant charge,
Qi, and because of
charge conservation, this is equal to the sum of charges on the base electrode
Q1 and the cantilever electrode
Q2. Movement of the cantilever causes a change in the capacitance, which puts a voltage across the load
resistance, as described by the following equation:
∂V/∂t = Qi / (∂C/∂t)
Model of a cantilever beam vibration energy-harvester using an electret material.
(Via arXiv Preprint Server, Ref. 3, Fig. 3))
The French team's
theoretical analysis of such a device shows that with a vibration of just 0.1
g, or about a
meter per
second-squared, it's possible to harvest 30
microwatts/
gram of proof mass at cantilever
resonance; that is, when the beam is tuned to the
frequency of the vibration source.
Actual devices were found to be prone to problems caused by
parasitic capacitance, so only about 10 microwatts/gram were obtained
experimentally. The French team built a prototype harvester using a
silicon cantilever and a
Teflon (PTFE) electret that gave 17 microwatts for 0.2 g vibration.[3] The magnitude of harvested energy was comparable to that of other energy-harvesters. Unfortunately, this was at a loading of 210
megohms, so it's hard to see a practical application at this point.
References:
- S Boisseau, G Despesse, T Ricart, E Defay and A Sylvestre, "Cantilever-based electret energy harvesters," Smart Materials and Structures, vol. 20 no. 10 (October, 2011), Document No. 105013.
- S. Boisseau, G. Despesse, T. Ricart, E. Defay and A. Sylvestre, "Cantilever-based electret energy harvesters," arXiv Preprint Server, November 10, 2011.
- S. Boisseau, G. Despesse and A. Sylvestre, "Electret-based cantilever energy harvester: design and optimization," arXiv Preprint Server, November 10, 2011.
Permanent Link to this article
Linked Keywords: Microphone; crystal; potassium sodium tartrate; water-soluble; piezoelectric; fragile; magnetism; voltage<; solenoid; coil of wire; magnetic field; electromagnetic induction; ribbon microphone; conductor; condenser microphone; capacitor; Georg Neumann GmbH; Frank Sinatra; Beatles; Wikimedia Commons; noise; electret microphone; electret material; electrostatic; magnet; electric field; vibration; energy-harvester; cantilever geometry; French; scientist; Laboratoire d'électronique des technologies de l'information; Centre national de la recherche scientifique; deformation; strain; proof mass; arXiv Preprint Server; charge conservation; resistance; theory; gravitational acceleration; g; meter; second; microwatt; gram; resonance; frequency; parasitic capacitance; experiment; silicon; polytetrafluoroethylene; Teflon; PTFE; megohm.