1. Field of the Invention
The present invention relates to power transfer devices and, more specifically, to a wireless power transfer device.
2. Description of the Related Art
Wireless power transfer devices can be used to transfer power from a source to a load without requiring a wired connection between the two. They can also be used to transfer data wirelessly as well. Such devices are commonly used in situations where it is either impractical to use wired connections or potentially unsafe to do so. For example, many electric tooth brush systems use wireless power transfer to recharge the batteries in the tooth brush. Since the elements of the system are covered in non-conductive plastic, there is little chance of electric shock with such systems.
Modern digital devices, such as smart phones, tablets and the like, require frequent recharging. However, most such systems require the digital device to be plugged into a recharger. Because doing so is somewhat inconvenient, users often forget to recharge their devices.
Numerous wireless power transfer methods have been proposed and studied in the past for various applications. Specifically, wireless power transfer has been achieved using near-field coupling in several applications such as, RFID tags, telemetry and implanted medical devices. In addition, certain inductive coupling techniques have been reported to exhibit high power transfer efficiencies (on the order of 90%) for very short distances (1-3 cm). However, the efficiency of such techniques drops drastically for longer distances.
One type of wireless power transfer system employs a strongly coupled magnetic resonance (SCMR) method. A typical SCMR system employs an inductive transmitter loop and a spaced apart inductive receiver loop. Each loop resonates as substantially the same frequency. An alternating current source is used to excite the transmitter loop, which when resonating causes the receiver loop to resonate. The receiver loop is inductively coupled to a load and transfers power to the load as a result of its resonating.
Loop misalignment can result is a substantial decrease in efficiency. Conventional SCMR systems tend to be highly sensitive to the alignment between transmitter loop and receiver loop. The loops can be angularly misaligned, in which the loops exist on non-parallel planes. A greater angular difference in the planes results in lower power transfer efficiency. The loops may also be laterally misaligned, in which the loops may be parallel to each other but are on laterally spaced apart axes. Again, a greater distance between the axes results in a lower power transfer efficiency.
One approach to correcting SCMR's angular misalignment sensitivity employs tuning circuits. This method is generally not able to maintain high efficiency above 60° of misalignment. Also, tuning circuits add to the complexity of SCMR systems and they cannot compensate for large angular and radial misalignments as they cannot recover the lost flux density between transmitter and receiver. However, tuning circuits can be useful for compensating the effects of variable axial distance between the transmitter and the receiver.
Many digital devices require frequent data updating. One convenient time to update a digital device is during periods of non-use, such as when the device is being recharged.
Therefore, there is a need for a convenient wireless power transfer system that is efficient at longer distances.
Therefore, there is a need for a convenient wireless power transfer system that is efficient when the transmitter and the receiver are misaligned.
Therefore, there is a need for a convenient wireless power transfer system that facilitates both power transfer and data transfer simultaneously.
The disadvantages of the prior art are overcome by the present invention which, in one aspect, is a wireless power transmission system for transmitting power from a power source to a load that includes a planar source conductor configured to generate a first periodically fluctuating electromagnetic near field in response to an alternating current received from the power source. A planar resonant source element is coplanar with the planar source conductor and has a first resonant frequency. The planar resonant source element has a Q factor that is at a maximum at the first resonant frequency. The planar resonant source element is configured to resonate with a first oscillating current at the first resonant frequency in response to excitation from the periodically fluctuating electromagnetic near field generated by the planar source conductor. A planar resonant load element is spaced apart from the planar resonant source element and is configured to resonate at the first resonant frequency with a second oscillating current in response to excitation from the planar resonant source element. The planar resonant load element is configured to generate a second periodically fluctuating electromagnetic near field when resonating with the second oscillating current. A planar load conductor is electromagnetically coupled to and coplanar with the planar resonant load element and is configured to generate a current in response to the second periodically fluctuating electromagnetic near field.
In another aspect, the invention is a device for transmitting power wirelessly that includes a source unit and a load unit. The source unit includes an alternating current power source, a source conductor element electrically coupled to the alternating current power source, and a resonant source element. The resonant element surrounds the source conductor element and is physically decoupled from the source conductive element. The conductive resonant element has a resonant frequency and has a maximum Q factor at the resonant frequency. The source resonant element is configured to resonate in response to the alternating current being applied to the source conductor element. The load unit includes a resonant load element, a load conductor element and a load. The resonant load element is spaced apart from and that is physically decoupled from the resonant source element. The resonant load element is resonant at the resonant frequency and has a maximum Q factor at the resonant frequency. The resonant load element is configured to resonate in response to resonance in the resonant source element. The load conductor element is disposed within the resonant load element and is physically decoupled from the resonant load element. The load is electrically coupled to the load conductor element. The load conductor element is configured to apply electrical power to the load in response to resonance in the resonant load element.
In yet another aspect, the invention is a method of transmitting power from a source to a load, in which an alternating current is generated at the source. The alternating current is caused to flow through a source conductor element. A periodic electromagnetic field resulting from the alternating current flowing through the source conductor element is inductively coupled to a resonant source element that surrounds the source conductor element. The resonant source element has a resonant frequency at a frequency at which the resonant source element has a maximum Q factor. The resonant source element is inductively coupled to a resonant load element. The resonant load element has a resonant frequency that is substantially the same as the resonant frequency of the resonant source element, which is a frequency at which the resonant load element has a maximum Q factor. A load conductor element is inductively coupled to the resonant load element, thereby inducing a current in the load conductor element. The current induced in the load conductor element is applied to the load.
These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure.
A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. Unless otherwise specifically indicated in the disclosure that follows, the drawings are not necessarily drawn to scale. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” Also, as used herein “Q factor” means the quality factor associated with a resonant circuit.
As shown in
The elements are typically made from conductive wires (such as copper) or conductive ink. In one embodiment, they can be formed by depositing a conductive material (such as a metal) on a substrate (such as a crystalline substrate) and then forming the elements through an etching process, or through using conventional lithographic techniques typically employed in circuit applications.
In one embodiment, the invention employs a wireless powering system based on a strongly coupled magnetic resonance (SCMR) method, which is discussed theoretically in
SCMR systems use resonant transmitters and receivers that are strongly coupled. Strongly coupled systems are able to transfer energy efficiently, because resonant objects exchange energy efficiently versus non-resonant objects that only interact weakly. A standard SCMR system consists of four elements (typically four loops or two loops and two coils) as shown in
The source element is connected to the power source, and it is inductively coupled to the TX element. The TX element exhibits a resonant frequency that coincides with the frequency, where its Q-factor is naturally at a maximum. Similarly, the RX exhibits a resonant frequency that coincides with the frequency where its Q-factor is naturally at a maximum. Furthermore, the load element is terminated to a load. The analysis that follows assumes that the entire system operates in air. Also, SCMR requires that the TX and RX elements are resonant at the same frequency in order to achieve efficient wireless power transfer.
The analysis that follows employs TX and RX elements that have an arbitrary number of helical loops. However, in the simple embodiment shown above, only a single loop is used. The TX and RX elements can be equivalently represented by a series RLC circuit. Helices are often preferred as TX and RX SCMR elements because they exhibit both distributed inductance and capacitance and therefore, they can be designed to self-tune to a desired resonant frequency, without the need of external capacitors. Also, external capacitors have losses, which in practice can reduce the Q-factor of the TX and RX elements and in turn decrease the efficiency of SCMR systems. Based on the equivalent RLC circuit of an SCMR system, its resonant frequency, fr, can be calculated, by following equation:
The resonant frequency, fr is also the operational frequency for the SCMR wireless powering system. The Q-factor of a resonant RLC circuit is given by:
Therefore, the Q-factor of a resonant helix (i.e., self resonant) can be written as:
where L, Rrad, and Rohm are the self-inductance, radiation resistance and ohmic resistance of the helix, which is for a short helix or solenoid (2r>h) are given by:
where μ is the permeability of free space, ρ is the helix's material resistivity, r is the radius of the helix, rc is the cross sectional wire radius, N is the number of turns (the simple single turn embodiment above uses N=1), f is the frequency, ηo is the impedance of free space and c is the speed of light, h is the height of the helix. It should also be noted that equations (3)-(6) are valid only when r<λ/6π.
SCMR requires that both RX and TX helices also exhibit maximum Q-factor at their resonant frequency fr, in order to achieve maximum wireless power efficiency. This can also be seen by the equation for describing the efficiency of an SCMR system derived in at it operation frequency fr as follows:
where KTX_RX is the mutual coupling between the RX and TX helices and where QTX and QRX are the Q-factors of the RX and TX helices, respectively. If the TX and RX helices are identical, then their Q-factors are equal i.e., QTX=QRX=Q; therefore equation (7) can be written as:
Equation (8) shows that in order to maximize the efficiency of an SMCR system, the operation frequency fr must be equal to the frequency fmax, where the Q-factor is maximum. In what follows, the maximum Q-factor of a resonant helix is derived. The Q-factor of a resonant helix can be expressed in terms of its geometrical parameters using (3)-(6) as:
The maximum Q-factor, Qmax, and the frequency, fmax, where Qmax occurs, can be derived from (9) using calculus as:
Based on the above discussion, an SCMR system requires that
fr=fmax (12)
which can be written based on (10) as:
Therefore, (13) shows that the geometrical parameters of a helix can be appropriately chosen so that the helix has maximum Q-factor at a chosen frequency, fr. For example, if the parameters fr, rc, N and ρ are specified by a designer, (13) can be solved for the radius of the maximum Q-factor, rmax, as follows:
Next, the helices are analyzed using (10), (11) and (14) to study the behavior of the maximum Q-factor, Qmax, versus the electrical length of the helix (Cdev/λQ
where Ldev is the length of the helix (device), λmax is the wavelength corresponding to fmax given by (10). Specifically, optimum SCMR loops with N=1 are designed in the frequency range 100 KHz≦5≦GHz for four values of the cross-sectional radius, rc=0.01, 0.1, 1.0 and 10 mm. The material of the helices is assumed copper and for each pair of fmax and rc, the optimum r is calculated by (14). Then Qmax from (11) is plotted in
In what follows the global maximum Q-factor of the helix, QGmax, is formulated. First, the local maximum Q-factor, QLmax, is derived by substituting (10) into (11):
Using again calculus, we can find out that the global maximum for the Q-factor occurs when:
This result shows that the ratio between the helix radius, r, and the cross-sectional radius, rc, must be approximately 9.52 in order to achieve the maximum Q-factor. This ratio is also independent of frequency and material.
Also, by substituting (17) into (16) we can write the global maximum for the Q-factor as:
Therefore, if a helix is designed to operate at the global maximum Q-factor it will yield the maximum possible wireless efficiency for the corresponding SCMR system. In order to verify the global maximum design of (17), we assume that an arbitrary ratio of r/rc=t, and solve (13) and (17) to obtain the r and rc given the number of turns, N, and the desired frequency of operation, fo:
Based on (19) and (20), SCMR systems were designed and simulated in Ansoft HFSS for different ratios r/rc (2≦t≦50) and assuming the number of turns, N=5, distances, l1=l3=2 cm, l2=10 cm (see
The following are guidelines for designing helical TX and RX elements of SCMR wireless powering systems. An SCMR system based on helices will not be optimal unless the spacing, s, is picked so that the helices exhibit the appropriate capacitance in order to resonate at the desired operating frequency of the system. The spacing, s of an SCMR helix is an important parameter that should be picked to ensure optimal wireless power transfer efficiency. The capacitance formula for closely wound helix is as follows:
where r is the radius of the helix, rc is the cross sectional wire radius, ε0 is the permittivity of free space, s is the spacing between adjacent turns of the helix, Ct is the total distributed capacitance of the helix, and t is the thickness of the insulation coating.
The capacitance formula of (21) is valid when s/2rc≦2 and t<<s−2rc. In order to resonate the helix at a desired frequency f, the spacing between two adjacent turns, s, can be adjusted to provide the required capacitance calculated from (1) as:
Then equation (21) can be solved for the spacing, s, as follows:
Equation (23) is valid when s/2rc≦2 and t<<s−2rc. Therefore, the spacing, s, can be adjusted using (23) independently from the other geometrical parameters to achieve the necessary capacitance and without affecting the frequency where a short helix or solenoid (2r>h) exhibits maximum Q-factor since (13) shows that the fmax does not depend on s.
As shown in
One embodiment, as shown in
One approach to making such a three-dimensional structure is shown in
As shown in
In other embodiments, multiple source and resonator elements can be employed to tune the system to more than one different frequency. Such embodiments can facilitate, for example, the transfer of both power and data from the source to the load. This ability may be useful in such situations as when it is desirable to charge a cell phone (or other type of digital device, such as a tablet) which updating some of the data stored on the device. For example, one embodiment, as shown in
In another embodiment, as shown in
The above described embodiments, while including the preferred embodiment and the best mode of the invention known to the inventor at the time of filing, are given as illustrative examples only. It will be readily appreciated that many deviations may be made from the specific embodiments disclosed in this specification without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is to be determined by the claims below rather than being limited to the specifically described embodiments above.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/658,596, filed Jun. 12, 2012, the entirety of which is hereby incorporated herein by reference. This application also claims the benefit of U.S. Provisional Patent Application Ser. No. 61/658,636, filed Jun. 12, 2012, the entirety of which is hereby incorporated herein by reference. This application also claims the benefit of U.S. Provisional Patent Application Ser. No. 61/662,674, filed Jun. 12, 2012, the entirety of which is hereby incorporated herein by reference.
Number | Name | Date | Kind |
---|---|---|---|
5227720 | Gross | Jul 1993 | A |
6960968 | Odendaal | Nov 2005 | B2 |
7741734 | Joannopoulos et al. | Jun 2010 | B2 |
8400019 | Joannopoulos et al. | Mar 2013 | B2 |
8421274 | Sun et al. | Apr 2013 | B2 |
9070505 | Saitoh | Jun 2015 | B2 |
20090015075 | Cook | Jan 2009 | A1 |
20090079268 | Cook | Mar 2009 | A1 |
20110304217 | Yamamoto | Dec 2011 | A1 |
20120161537 | Kamata | Jun 2012 | A1 |
20130307468 | Lee | Nov 2013 | A1 |
Entry |
---|
Sample, et al., “Analysis, Experimental Results, and Range Adaption of Magnetically Coupled Resonators for Wireless Power transfer,” Feb. 2011, IEEE Transactions on Industrial Electronics, vol. 58, No. 2, 544-554. |
Sample et al. “Analysis, Experimental Results, and Range Adaptation of Magnetically Coupled Resonators for Wireless Power Transfer”; IEEE Transactions on Industrial Electronics, vol. 58, No. 2, Feb. 2011. |
Endo et al. “Proposal for a new resonance adjustment method in magnetically coupled resonance type wireless power transmission”; Microwave Workshop Series on Innovative Wireless Power Transmission: Technologies, Systems, and Applications (IMWS), 2012 IEEE MTT-S International; IEEE; May 10-11, 2012. |
Zhang et al. “Wireless Power Delivery for Wearable Sensors and Implants in Body Sensor Networks”; 32nd Annual International Conference on the IEEE EMBS Buenos Aires, Argentina, Aug. 31-Sep. 4, 2010. |
Notification of Related Application: The following US Patent Applications, naming common inventors and commonly owned, disclose related subject matter: U.S. Appl. No. 13/916,121, filed Jun. 12, 2013; U.S. Appl. No. 13/916,168, filed Jun. 12, 2013; and U.S. Appl. No. 13/916,200, filed Jun. 12, 2013. |
Unpublished version of Sample et al. “Analysis, Experimental Results, and Range Adaptation of Magnetically Coupled Resonators for Wireless Power Transfer”; downloaded from http://sensor.cs.washington.edu/pubs/WREL-paper.pdf. |
Number | Date | Country | |
---|---|---|---|
20130328408 A1 | Dec 2013 | US |
Number | Date | Country | |
---|---|---|---|
61658596 | Jun 2012 | US | |
61658636 | Jun 2012 | US | |
61662674 | Jun 2012 | US |