Push-pull electret transducer with controlled restoring force for low frequency microphones and energy harvesting

Provided is an energy harvesting system, including: a first electrode; a second electrode; a non-metalized mono-charged electret diaphragm disposed between the first and second electrodes; a base; a spring extending between the base and the electret diaphragm; and a rod in communication with the electret diaphragm and for manipulating a position of the electret diaphragm relative to the first and second electrodes.

FIELD

This disclosure is generally directed to the field of electret transducers for use as low frequency microphones and in energy harvesting applications.

BACKGROUND

The use of stored charge rather than piezoelectricity has gained substantial interest in small electronic devices. These devices generate power in the micro-watt range due to their small size and limited range of diaphragm motion. One might treat these devices as an electret microphone from the operation standpoint to determine the power generation ability. While these systems can be scaled up in size, the output power is still limited by the amount of movement of the device's diaphragm, and the charge level. Large area space-charge generators can produce average peak power terminated with one Meg Ohm resistor of up to 44 mW.

What is needed in the art, therefore, is an improved energy device that overcomes the limitations of low output power of prior art energy devices.

SUMMARY

In an embodiment, there is a push-pull electret transducer, comprising: a first electrode; a second electrode; and a non-metalized mono-charged electret diaphragm disposed between the first and second electrodes.

In an embodiment, there is a method of making a push-pull electret transducer, comprising: providing a first metal electrode; providing a fluoropolymer film over the first metal electrode; and introducing a negative charge into the fluoropolymer film.

In an embodiment, there is an energy harvesting system, comprising: a first electrode; a second electrode; a non-metalized mono-charged electret diaphragm disposed between the first and second electrodes; a base; a spring extending between the base and the electret diaphragm; and a rod in communication with the electret diaphragm and for manipulating a position of the electret diaphragm relative to the first and second electrodes.

In an embodiment, there is a single ended sensor, comprising: a base; a diaphragm; at least one spring extending from the base to the diaphragm; a metal backplate disposed between the base and the diaphragm; and an electret disposed between the metal backplate and the diaphragm.

In an embodiment, there is a mono-charged push-pull (PP) electret transducer comprising a spring for supporting a diaphragm, wherein the spring comprises a restoring force spring constant and wherein a resonance frequency of the transducer and a static pressure operating range of the transducer are pre-determined via adjusting the spring's restoring force spring constant. In an embodiment, the diaphragm comprises an electret diaphragm and a static position thereof is balanced between two electrodes for the push-pull arrangement configured as a microphone. In an embodiment, there is a microphone with variable restoring force. The microphone can have sensitivity in the 10-100 mV/Pa range, for example, depending on resonance frequency and electret charge level (e.g., charge density).

Advantages of at least one embodiment include an asymmetric static position of the mono-charged diaphragm. An advantage of at least one embodiment includes an improved output voltage for energy generation, such as, for example, up to 90% of the polarization voltage for centimeter range of the diaphragm motion.

Additional advantages of the embodiments will be set forth in part in the description which follows, and in part will be understood from the description, or may be learned by practice of the invention. The advantages will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following embodiments are described for illustrative purposes only with reference to the Figures. Those of skill in the art will appreciate that the following description is exemplary in nature, and that various modifications to the parameters set forth herein could be made without departing from the scope of the present invention. It is intended that the specification and examples be considered as examples only. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

Push-pull electret transducers have attracted a great deal of scientific attention as headphones and high frequency loudspeakers. Push-pull headphones have been on the market since the late 1990s. The basic principle is explained in the following; a diaphragm of two electrets contacted with their metal layers is sandwiched in the middle between two metal plates, which consists of a symmetrical system. This system can be widely used in high frequency loud speakers and headphones with relative high efficiency.

A push-pull system100of an embodiment is comprised of two electrodes101connected to a circuit105, such as a rectifying circuit, and a non-metalized polymer diaphragm103that has a mono-charge in the kV range and sandwiched between the two electrodes101as shown inFIG. 1. While not limited to any particular method, measurement of the mono-charge at 1.5 kV on 25 μm FEP has been found to be stable even at temperatures of 100° C. degree.

An asymmetrical electret transducer200according to an embodiment is shown inFIG. 2and comprises a non-metalized mono-charged electret diaphragm103which may comprise a polymer as described above. The push-pull electret transducer comprises: a first electrode101′; a second electrode101″; and a non-metalized mono-charged electret diaphragm103disposed between the first and second electrodes. The push-pull electret transducer may further comprise at least one spring214, wherein the at least one spring214contacts the electret diaphragm—e.g., the diaphragm103can be suspended between the first electrode101′ and second electrode101″ by the at least one spring214. The push-pull electret transducer200may further comprise a supporting frame counterbalance210, wherein the supporting frame counterbalance210contacts the electret diaphragm103, for example, at attachment points216and wherein the at least one spring214opposes a force218applied against the electret diaphragm103by the frame counterbalance210. In an embodiment, the electret diaphragm103is disposed between the supporting frame counterbalance210and the at least one spring214. In an embodiment, a change in pressure on the supporting frame counterbalance causes the electret diaphragm103to move between the at least two electrodes101′ and101″. In an embodiment, movement of the electret diaphragm103results in a charge rearrangement on the two electrodes.

The diaphragm may comprise any suitable material. For example, in an embodiment, the diaphragm comprises a polymer. The polymer may be selected from a fluoropolymer. For example, the fluoropolymer may comprise fluorinated ethylene propylene (FEP) or polytetrafluoroethylene (PTFE).

As shown inFIG. 2, the diaphragm103comprises a non-metalized mono-charged electret diaphragm and is suspended between two electrodes101′ and101″ by at least one spring214that extends between base212and a supporting frame counterbalance210(e.g., a seismic mass). A change in pressure on the supporting frame counterbalance210causes the diaphragm103to move resulting in charge rearrangement on the two electrodes (plates)101′ and101″. When the restoring force equilibrant position is centered between the two plates101′ and101″, the output voltage (e.g., to rectifier circuit105connected to101′ and101″) is proportional to the force218on the counterbalance. Transducer200thus provides a linear system by which sound and vibration measurements can be performed. For energy generation, an asymmetrical equilibrant position is more efficient for transferring maximum charge between the two electrodes as shown in the figure (e.g., a gap of length t1between electrode101′ and diaphragm103, such as a first surface of diaphragm103, can be larger than a gap of length t3. between electrode101″ and diaphragm103, such as a second opposing surface of diaphragm103, or vice-versa). This system is adaptable to both static and changing pressure levels by selecting the proper spring constants for the static pressure operating range, for example depth operating range in water can be predetermined. The mono-charge transducer200of the embodiments also makes it possible for a relatively large gap (e.g., of length t1and/or t2) between electret and metal plate electrode(s) allowing maximum charge transfer to circuit105. With this kind of transducers, electret-metal plate electrode spacing in the orders of a few cm is possible.

Accordingly, in an embodiment of a push-pull transducer200as described above, a static position of the electret diaphragm103is balanced between the two electrodes101′ and101″ such that push-pull electret transducer is configured as a microphone. As such, the at least one spring214can provide a variable restoring force to support the mono-charged electret diaphragm103between the first and second electrodes101′,101″. Accordingly, the push-pull electret transducer may be configured as a microphone for use in water. Additionally, the push-pull electret transducer200may have sensitivity in the range of from about 10 to 100 mV/Pa. Further, for the push-pull electret described above, wherein the mono-charged electret diaphragm103may have an asymmetric static position.

In an embodiment, there is a method for making a push-pull electret transducer. Generally, the method can include providing a first metal electrode; providing a fluoropolymer film over the first metal electrode; and introducing either a positive or a negative charge into the fluoropolymer film. A second metal electrode may be provided over the fluoropolymer film. In an example, the introducing of the negative charge is by electron bombardment. In an implementation of such a method, a plain FEP or PTFE Teflon film (no electrode) is tightly held in a designed circular metal frame. Before corona charge, two metal plates are prepared as the electrodes. The bottom metal plate and Teflon film are put together into a corona chamber. The needle voltage and grid voltage can be set at −10 kV and −2 kV, respectively. In the corona charge process, electron bombardment introduces negative charge into Teflon film by grid voltage of corona and consequently the compensated positive charges are generated at the bottom of metal plate. After the charging is complete, the top metal plate electrode is placed on top of the frame to prevent the decay of mono-charge in the air.

In an implementation of the method, sample push-pull monocharge electret systems S1-S4were formed and a retained induction charge on the top side of FEP monocharge electret samples, as measured with an Isoprobe Electrostatic Voltmeter were plotted as function of time. Results are shown in the graph ofFIG. 3. From the data, it is observed that the induction voltages on the samples are stably around −1200 V and shows no decay after three weeks.

A monocharge electret sample was also thermally treated in an oven at 100° C. for 72 hours, the induction voltage in the monocharge FEP electret sample did not deteriorate. The monocharge samples were found to be stable in the condition where the monocharge electret sample was kept in the middle of enclosed top and bottom metal electrodes with a distance around 1 cm. Thus, such push-pull monocharge electret systems according to embodiments described herein may be used for transducer and energy harvesting applications.

A push-pull transducer with a mono-charge electret may be adapted for use as an energy harvesting device, e.g., as a sensor, that can operate at any pre-determined static pressure. A resonant frequency of such a push-pull transducer can be below that of a normal electret microphone (about 10 kHz). Accordingly, for low frequencies, additional gain in sensitivity can be realized. In other words, according to an embodiment a push-pull transducer comprises a mono-charge electret in the center of two metal plate electrodes. An induction charge can, therefore, be generated at the top and bottom metal plate electrodes to balance the mono-charges. Following such an up and down movement of the mono-charge electret, the metal plate electrode closer to the mono-charge electret layer (e.g., a film) can generate more charges than opposite metal plates. A differential potential created by the transducers of the embodiments is largely dependent on the charge difference across two metal plates. If we collect these charges using a rectifier circuit, the voltage in the order of a few hundred volts is easily obtained for energy harvesting devices.

While not limited to any particular theory, it is believed that Gauss's law can be applied for the electric displacement of the interface between the mono-charge electret and metal plate electrode of a device of an embodiment, for example, that as shown inFIG. 2and yielding:

σm⁢⁢1=t2ɛ2⁢σ0+t3ɛ3⁢σ0t1ɛ1+t2ɛ2+t3ɛ3,(Eq.⁢1)
where t1, t2, t3are the gap between mono-charge electret diaphragm103and top metal electrode101′, the thickness of the mono-charge electret diaphragm103, and the gap between mono-charge electret diaphragm103and bottom metal electrode101″, respectively; ε1, ε2and ε3are dielectric constant at the gap between mono-charge electret diaphragm103and top metal electrode101′, of the mono-charge electret diaphragm103, and the gap between mono-charge electret diaphragm103and bottom metal electrode101″, respectively; σ0is the charge density; and σm1and σm2are the induction charge mono-charge at top and bottom metal electrode plates101′ and101″, respectively.

Similarly, the induction charge at bottom metal plate will become:

Accordingly, the induction charge can be estimated in two extreme cases. In case I, the monocharge electret is very close to top metal plate, such as t1=0.5 mm, t2=20 μm and t3=3 mm. If we set the dielectric constant ε1=ε3=1; ε2=2, the calculated σm1and σm2are 0.86σ0and 0.14σ0, respectively. In case II, if the monocharge electret is very close to the bottom plate, such as t1=3 mm, t2=20 μm, t3=0.5 mm, the calculated σm1and σm2are 0.14σ0and 0.86σ0, respectively. From this calculation, it is known that following the movement of the mono-charge electret, the closer metal plate will generate more charges than opposite metal plates. Thus, for the application of energy harvesting using this push-pull system with mono-charge electret, the efficiency of the entire system is only dependent on the ratio of generated charge over the input mechanical energy in the system. Thus, various systems can be designed depending on specific application requirement.

For example, a push pull energy harvesting system is shown inFIG. 4A. This system can be implemented in an enclosed chamber. Generally, the mono-charge electret may be supported with a metal frame which is supported by a spring at the bottom and includes a plastic rod on the top. When the plastic rod pushes up and down the electret frame, different amount of compensated charge will be generated at the top and bottom plates. A rectifier circuit can be used to collect the charge from two metal plates.

In other words, in an embodiment, for example, as shown inFIG. 4A, there is an energy harvesting system400, comprising: a first electrode101′; a second electrode101′; a non-metalized mono-charged electret diaphragm103disposed between the first101′ and second101″ electrodes; a base412; at least one spring414extending between the base412and the electret diaphragm103; and a rod411in communication with the electret diaphragm and for manipulating a position of the electret diaphragm relative to the first and second electrodes. As described above, manipulation of the rod411generates an amount of compensated charge at the first electrode101′, the second electrode101″ or both the first101′ and the second101″ electrodes. In an implementation, the rectifier circuit (not shown) is electrically connected to the first electrode and the second electrode, wherein the rectifier circuit receives the charge generated at the first electrode, the second electrode or both the first and the second electrode. As described above, the energy harvesting system400can further comprise a supporting frame410for supporting the electret diaphragm103, such as at attachment points416, and a first end of the spring414contacts the base412and an opposing, second end of the spring414contacts the supporting frame410. Additionally, the rod411comprises a first, moveable end and a second end connected to the supporting frame410.

In another example, a transducer may be realized for use in deep ocean where it allows corresponding large electret film deformation in electrical field, then generate large amplitude acoustic signals. A single ended sensor with pre-determined static pressure range is shown inFIG. 4B. In this system, the electret is part of the backplate so that the choice of the diaphragm material can be more flexible. Accordingly, in yet another embodiment, there is a single ended sensor400′, comprising: a base412; a diaphragm103; at least one spring414extending from the base to the diaphragm103, for example, at attachment points416; a metal backplate420disposed between the base412and the diaphragm103; and an electret401disposed between the metal backplate420and the diaphragm103. In an implementation, the electret401comprises part of the backplate420. In an example, the electret401comprises a metalized FEP electret. Accordingly, such a single ended sensor400′ can have a pre-determined static pressure range.

While the present teachings have been illustrated with respect to one or more implementations, alterations and/or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. For example, it will be appreciated that while the process is described as a series of acts or events, the present teachings are not limited by the ordering of such acts or events. Some acts may occur in different orders and/or concurrently with other acts or events apart from those described herein. Also, not all process stages may be required to implement a methodology in accordance with one or more aspects or embodiments of the present teachings. It will be appreciated that structural components and/or processing stages may be added or existing structural components and/or processing stages may be removed or modified.

Further, one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases. Furthermore, to the extent that the terms “including,” “includes,” “having,” “has,” “with,” or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” The term “at least one of” is used to mean one or more of the listed items may be selected. Further, in the discussion and claims herein, the term “on” used with respect to two materials, one “on” the other, means at least some contact between the materials, while “over” means the materials are in proximity, but possibly with one or more additional intervening materials such that contact is possible but not required. Neither “on” nor “over” implies any directionality as used herein. The term “about” indicates that the value listed may be somewhat altered, as long as the alteration does not result in nonconformance of the process or structure to the illustrated embodiment.

Terms of relative position as used in this application are defined based on a plane parallel to the conventional plane or working surface of a workpiece, regardless of the orientation of the workpiece.