Example fields of the invention include network communications and body monitoring systems. Example body monitoring systems include body worn health sensing systems, activity trackers and body worn sports performance systems. Networks of the invention can provide communications for any wearable device that needs to wireless communicate information around the body (e.g., wearable sensors, activity trackers, smartwatches, EEG headsets, etc.) with ultra-low-power consumption.
Medical devices and wearable consumer products have fundamental anatomically-driven size constraints that necessitate small form factors. Since most patients and consumers desire long battery life, and battery volume is limited by anatomy, one of the only ways to increase lifetime is to reduce the power of the underlying circuits. Even when wireless communications are limited in distance or duration to save power, the energy budget of the wireless communication components of the device can still dominate the overall energy budget of a wearable device.
Most efforts have therefore focused on providing higher performance wireless circuits. Low power, high performance wearable circuits tend to use expensive components. Ultra-low power radio circuits for example are available from IMEC as custom circuit designs, but depend up leveraging very small node low-power CMOS transceivers, e.g. a 7 Gbps 60 GHz transceiver IC implemented in 40 nm low-power CMOS. The cost of such wireless transceivers can substantially raise the price of a wearable component, and there is still a desire for reduced cost, area and power consumption wireless communications to improve wearable medical and consumer body monitoring devices.
One approach that turned away from merely improving the circuit efficient and power performance of conventional transceivers is an approach that uses the human body as a communication channel for electric fields via galvanic coupling. An e-textile approach was developed by one of the inventors and a colleague. See, P. P. Mercier and A. P. Chandrakasan, “A Supply-Rail-Coupled eTextiles Transceiver for Body-Area Networks,” IEEE J. Solid-State Circuits, vol. 46, no. 6, pp. 1284-1295, June 2011. Others have also used the human body as a communication channel for electric fields. Song, S. Lee, N. Cho, and H. Yoo, “Low Power Wearable Audio Player Using Human Body Communications,” in 2006 10th IEEE International Symposium on Wearable Computers, 2006, pp. 125-126. The eTextiles offers the lowest power consumption due to inherently low path loss, but leveraged dedicated clothing, which may not be practical or desirable in many applications.
Galvanic coupling typically employs two electrode pairs, which can be attached on the skin as the transmitter (TX) and receiver (RX) nodes. At the TX node, an electrical signal is applied differentially, inducing small currents that propagate across the entire body, some of which can be sensed by the RX. Thus, galvanic coupling acts much like a distributed wired connection across the body, and can thereby achieve a high level of security/privacy and good interference resiliency.
Another approach relies upon electric field human body communication and can be referred to as eHBC. J. H. Hwang, T. W. Kang, S. O. Park, and Y. T. Kim, “Empirical Channel Model for Human Body Communication,” IEEE Antennas Wirel. Propag. Lett., vol. 14, pp. 694-697, 2015. Such systems can have lower path loss compared to conventional far-field radios (e.g., Bluetooth, WiFi, Zigbee, LTE, etc.), and further benefit from lower-complexity multi-user access and security requirements due to limited broadcasting of energy. However, the improvement in path loss is not always large, especially when small, battery-powered devices are used, and thus the advantages of eHBC over conventional radios is still unclear. Additionally, galvanic eHBC systems have limited dynamic path loss degradation due to movement, and can be used to communicate with implants. However, due to the low conductivity of tissues found in the human body, galvanic eHBC has relatively large path loss compared to other approaches. B. Kibret, M. Seyedi, D. T. H. Lai, and M. Faulkner, “Investigation of galvanic-coupled intrabody communication using the human body circuit model.,” IEEE J. Biomed. Heal. informatics, vol. 18, no. 4, pp. 1196-206, July 2014.
Other systems capacitively couple to the body. Capacitive eHBC systems also require two electrode pairs to generate differential signals around the human body, but their physical configurations are slightly different. With a capacitive couple, only one electrode should be directly placed on (or near) the skin to produce electric fields within the human body, while the other electrode should be placed facing outwards in order to capacitively couple to the environment. See, e.g., T. G. Zimmerman, “Personal area networks (PAN): Near-field intra-body communication,” M.S. Thesis, Massachusetts Inst. Technol., Cambridge, Mass., 1995. According to other researchers, this coupling mechanism can be modeled as distributed RC circuits if the operation frequency is low enough for electrostatic analysis. N. Cho, J. Yoo, S. J. Song, J. Lee, S. Jeon, and H. J. Yoo, “The human body characteristics as a signal transmission medium for intrabody communication,” IEEE Trans. Microw. Theory Tech., vol. 55, no. 5, pp. 1080-1085, 2007. However, as the frequency is scaled above tens of MHz, the power radiated by electrodes increases, and others have proposed a wave propagation model operating on the surface of human body. J. Bae, H. Cho, K. Song, H. Lee, and H.-J. Yoo, “The Signal Transmission Mechanism on the Surface of Human Body for Body Channel Communication,” IEEE Trans. Microw. Theory Tech., vol. 60, no. 3, pp. 582-593, March 2012. Such models have shown that capacitive coupling achieves a lower path loss than galvanic coupling.
However, the present inventors have recognized that capacitive eHBC systems suffer from a number of drawbacks. For example, they require large ground planes to increase environmental coupling and reduce path loss. This path loss also has high variability based on environmental conditions (and the availability of objects to couple to). Furthermore, since the IEEE established the 802.15.6 WBAN standard in 2012, eHBC has used 21 MHz as its operation frequency, yet capacitive coupling generally achieves the lowest path loss at higher frequencies. The present inventors have also identified that although capacitive eHBC can offer superior path loss compared to conventional far-field radiation, the variance caused by environmental effects and the poor conductivity of the human body further limit its utility.
An embodiment of the invention is a body area network that uses a near magnetic field for communications. A first coil is configured to be worn on a body portion of a human. A transmitter drives the first coil to generate a magnetic body field through the first magnetic coil. A second coil couples to the signal transmitted via the first coil matched to the magnetic field and first coil. A receiver for receiving the signal from the second coil.
A method of the invention for establishing network communications using the human body as a magnetic field includes associating a transmitter coil with a portion of a human body, the transmitter coil being configured to couple to a receiver coil in a near field of the human body; driving the transmitter coil to generate a magnetic near field around the human body; and coupling to the magnetic near field with a receiver coil.
A preferred embodiment is low power body area network. The network is applicable, for example, to wearable health monitoring systems and consumer systems, such as sports performance monitoring systems. A network of the invention leverages a magnetic field as the network medium. The inventors have determined that the body is an exceptional medium for propagating magnetic fields. A preferred system of the invention includes a body leveraged magnetic field and sensors that sense a health signal in one area of the body (heart rate, EEG, etc.), or an activity signal, such as acceleration, distance traveled. The sensor transmits the sensed signal via the magnetic network. A receiver placed elsewhere on or proximate the body, e.g. a smart watch worn by a person, a smart phone carried by a person or a computer device with a receiver in the vicinity of the person, is able to sense the signals transmitted by the sensor via coils at transmit and receive ends of the magnetic field pathway. Receivers in the vicinity of a person can include a computer in a medical text or a station along a sport performance route, for example, equipped with a magnetic coil and receive matched to the mHBC transmitter coil and transmitter carried on the person.
The network and systems of the invention provide excellent communications, and compare favorably to more traditional electric field communication systems. As another advantage, the magnetic field communication medium of the invention falls off very rapidly outside of the body. In preferred embodiments, the maximum range is a few meters, i.e., ˜2-4 meters. This can act as a privacy function for a user that lends more privacy to the user than electrical based systems. The inventors have determined that the present magnetic field system can be more reliable than E field or other wireless systems.
Systems of the invention use transmission and reception coils, one or more of which being configured to wrap around a portion of the body or configured to be parallel to a portion of a body such as a patch with coils that are substantially parallel to a portion of a body that carries the patch, such as an arm, wrist, head, the chest etc. Devices that naturally wrap-around anatomy (e.g., smartwatches and headbands) can naturally leverage the preferred mHBC technique for a wide range of wearable and medical applications. The coils can be packaged in commonly worn items such as wrist bands, watch bands, arm bands that hold devices, headbands, leg bands, or apparel. The coils are driven to produce a magnetic field and a resonantly coupled to each other to act as a transmitter and receiver coil to establish the body area network. Preferred embodiments have been demonstrated via both simulation and measurements that show that resonantly coupled magnetic coils of the invention can achieve at least 20 dB lower path loss across the human body than both far-field radios and eHBC systems. Energy stays primarily in the magnetic near-field. The permeability of biological tissue is low. Thus, propagation and broadcasting is limited, but the path loss is low. Thus, the preferred mHBC technique can enable low-power transceivers that retain the security and privacy benefits of conventional eHBC systems.
Preferred embodiments of the invention will now be discussed with respect to the drawings. The drawings may include schematic representations, which will be understood by artisans in view of the general knowledge in the art and the description that follows. Features may be exaggerated in the drawings for emphasis, and features may not be to scale.
One of the coils 12 acts as a transmitter (Tx) coil and generates magnetic energy. The other of the coils 14 acts as a receiver (Rx) coil and receives magnetic energy. The coils 14 can be worn on the body or can be associated with a device that is off the body but within the near field communication zone, such as a medical device near a patient or a sports monitoring device that a person will be nearby or pass. The first and second coils 12 and 14 can be conjugate matched with small capacitors be ˜100 pF at 21 MHz. The generated magnetic fields travel freely through biological tissue, which enables a much lower path loss than both galvanic and capacitive eHBC approaches. The
The wavelength of the magnetic field can be selected to balance privacy and sensing concerns. For example, the near-field region (wavelength/(2 pi)) at 21 MHz is 2.3 m, which ensures transmitted energy stays primarily in the magnetic near-field, thereby limiting propagation and broadcasting. Keeping the energy in the near field protects privacy, while extending the field permits communications with more distant magnetic coils. Preferred transmission frequencies are in the range of 1-100 MHz. Preferred transmission frequencies for maximizing security are in the range of from 20 to 50 MHz, while preferred transmission frequencies for maximizing the distance between transmitter and an off-body receiver coil are in the range of from 10 to 30 MHz. Generally, shortening the wavelength with higher frequencies extends power toward further field, while lengthening the wavelength with lower frequencies reduces power away from the near field.
Simulations were conducted to validate the preferred
To ensure electromagnetic simulations produced realistic results, dielectric properties of biological tissue must be considered. Although a fully-modeled internal body structure is possible, the complexity of the resulting geometry would have required prohibitively long simulations. Instead, the dielectric properties of internal tissues were averaged over the body, weighted by their respective volumes, to reduce the number of dielectric layers. The dielectric properties themselves are shown in
On the other hand, since the permeability of most biological tissue is similar to air, magnetic fields can travel much more freely through the human body, as seen in
Simulations were conducted. The simulations were also validated via measurements with an actual human subject, using the experimental set up and positions shown in
Unlike eHBC systems, which cannot use wall-powered devices due to ground loop coupling, magnetic fields are reference-free, and thus an Agilent E5071C vector network analyzer (VNA) could be used. The same two types of coils as described above for simulations were fabricated for the measurement experiment using 10 AWG enameled copper wires with PVC tubes for insulation.
Unlike conventional eHBC electrodes that are difficult to match due to the need for prohibitively large inductors, mHBC coils can be conjugate-matched using small capacitors. Matching is not necessary, but can be used to optimize performance. This implies that mHBC systems can be easily reconfigured to operate at any frequency below the self-resonant frequency of the coil. Thus, rather than measuring S21 at a small handful of frequencies after manually tuning resonant capacitors, the full set of S-parameters were measured and used to estimate the maximum available gain (MAG)—the path loss of the system assuming perfect matching. To obtain accurate measured data, each measurement was performed three times and the S-parameters were averaged prior to MAG computation. To validate the choice of using MAG as a metric, S21 measurements were also taken with through-hole capacitors placed on the coil feeding ports to resonate at 21 MHz. In this case, S21 measurements at 21 MHz matched MAG results within 8.7%.
The performance of the preferred mHBC was also benchmarked across a range of distances for single-turn and 4-turn coils in
The mHBC system of the invention has also been modeled mathematically and compared to an eHBC system. An eHBC can be generalized to an ideal dipole that is known as a Hertzian electric dipole and is illustrated in
where I0 is the magnitude of wire current, d is the dipole length, k0 is the wave number (2π/wavelength in free space), μ0 is the permeability of free space, ϵ0 is the permittivity of free space, n0 is the radiation impedance in free space, and ω is the angular frequency (2π frequency). In equations (1a) and (1b), 1/k0r, (1/k0r)2, and (1/k0r)3 terms are defined as the radiation, induction, and quasi-static portion, relatively. Generally, these three definitions can be classified as the far-field or near-field portion according to their dominant region, for example, the radiation (or real) power derived from 1/k0r term in both (1a) and (1b) dominates in the far-field region (k0r>1) and the reactive (or imaginary) power by (1=k0r)2 and (1=k0r)3 terms can be stored in the standing (or non-radiating) wave of the near-field region (k0r<1).
The human body has finite relative permittivity (ϵr), (1a) and (1b) can be rewritten with k(=k0√{square root over (ϵr)}) and η(=η0√{square root over (ϵr)}) as follows:
Equation (2b) verifies that the high permittivity of human tissues degenerates the near-field portion of E-field pattern, as a result, the E-field coupling between two capacitive antennas shows a disadvantage when the physical communication channel is biological tissues. Although (2a) and (2b) exclude the negative effect caused by the conductivity of biological tissues, it is enough to analyze the field intensity inside the human body that provides quite high relative permittivity and ignorable conductivity (σ) at lower frequency where the HBC physical channel usually operates. (ϵr≈80, σ≈0:15 S/m) at 21 MHz).
Analysis for the complex Poynting vector (power density) can address the degradation of near-field coupling in the human body more apparently as shown below.
On the other hand, the magnetic dipole by an ideal loop current can take an advantage on the near-field coupling in the high permittivity material, unlike the electric dipole. This benefit can be observed in the following E and H-field pattern of the magnetic dipole illustrated in
Equation (4b) describes that the high permittivity of a medium enhances both the radiation and induction portion of H-field generated by a magnetic dipole unlike an electric dipole that shows the degenerating of quasi-static near-field portion in the high permittivity medium. The complex Poynting vector (power density) verifies this benefit of a magnetic dipole source inside a high dielectric material.
The invention leverages the recognition of the inventors that the best way to utilize a magnetic source for the commutation in the body area (<˜2 m) is to deploy the magnetic resonance coupling between the coils because this coupling mechanism successfully employs the near-field portion that diminishes sharply by the distance. A data transfer system using the magnetic resonance coupling can be simply modeled as shown in
In (6), when the geometry of both coils provides certain resistances, only mutual inductance (M) decides the channel Therefore, exploring the influence of the human body on the mutual inductance is imperative for utilization of the magnetic coupling for body-area networks. Here, the mutual inductance is defined as how much the magnetic field flux generated by the current of the primary coil can flow through the inner dimension of the secondary coil as described below.
Equation (7) verifies that the mutual inductance is proportional to the averaged magnetic flux density over the inner area of the secondary coil when the current in the primary coil and the geometry of the secondary are fixed.
The systems of the invention include coils that can be conveniently wrapped around a portion of human anatomy.
Unlike the near-field portion described in (8a) and (8b), the far-field portion in (9a) and (9b) forms a transverse electromagnetic (TEM) wave that E and H field are coupled with each other in terms of the radiation impedance η and the radiation direction vector {right arrow over (r)}. Therefore, the common EM wave theory is still available for the far-field portion by the magnetic dipole even inside the human body.
The placing of a human arm model in the air creates the boundary condition at the interface between the air and the human tissue while the EM radiating wave generated by a magnetic dipole is propagating inside the body.
In order for the total reflection (|Γ⊥|=1) to happen at the boundary, cos θt in (11a) should be zero or imaginary. With this condition, the critical angle (θc) can be derived by (10) as below.
For example. when the magnetic dipole by the primary coil is placed at the center of the cylindrical human arm model which has ϵr=80, the critical incident angle is calculated as 6.4°, which means the total reflection happens if the radiation angle from the source (θs) is smaller than the critical radiation angle (θsc=90°−θs=83.6°) as depicted in
The total reflection at the human body's boundary helps the far-field propagation deliver the data signal further by holding the real EM wave power inside the biological tissues. It is necessary to make an analytical model of the magnetic flux density (B) averaged over the secondary coil dimension (A2=πα2) placed at z=R to verify enhanced mutual inductance by this phenomenon.
where where n is the natural number (n=1, 2, 3 . . . ). When the far-field H-field generated by the primary coil is given as (9b), the averaged magnetic flux density passing through the secondary coil placed at z=R in the air can be derived as shown below On the other hand, the averaged magnetic field density with the cylindrical human arm model can be described as below.
On the other hand, the near-field portion described in (8a) and (8b) is stored in the standing wave rather than radiated as the radiating wave. Also, since the E and H field are not coupled with each other in this portion, the boundary condition derived from the far-field radiating wave properties cannot be applied to this standing wave. Therefore, for the analysis of the near-field portion's behavior at the body-air interface, it is more suitable to employ the boundary condition for time harmonic EM fields given below
{right arrow over (x)}×{right arrow over (E)}
i
={right arrow over (x)}×{right arrow over (E)}
t (15a)
{right arrow over (x)}·ϵ{right arrow over (E)}
i
={right arrow over (x)}·ϵ
0
{right arrow over (E)}
t (15b)
{right arrow over (x)}×{right arrow over (H)}
i
={right arrow over (x)}×{right arrow over (H)}
t (15c)
{right arrow over (x)}·μ
0
{right arrow over (H)}
t
−{right arrow over (x)}·μ
0
{right arrow over (H)}
i
={right arrow over (J)}
s≈0 (15d)
where {right arrow over (Js)} is the vector of the surface current density. Equation (15a) and (15c) imply that the tangential components ({right arrow over (y)} and {right arrow over (z)} components of E and H field) are continuous across the interface, but (15b) and (15d) indicate that both sides of the interface can have the difference of the normal component {right arrow over (x)} from each other. However, since the E field in (8a) does not include the normal component and the surface current density) ({right arrow over (Js)}) can be assumed as zero, the near-field portion of the EM field by the magnetic dipole does not show the discontinuity at the boundary. Therefore, the magnetic flux density can be calculated without the consideration on the boundary condition by the human body as shown below.
The mathematical models therefore confirm the better path loss performance of the invention compared to eHBC that was also shown simulations and measurements. The mathematical models also provide guidance to artisans for adjusting system parameters to optimize preferred parameters, e.g., security, transmission power. For example, the modeling can be useful when estimating the maximum gain of mHBC using Eqn (6) and (7). As discussed above, the gain is decided by B averaged with given coil geometry (deciding the R, L and A) and the current (power budget of application).
While specific embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims.
Various features of the invention are set forth in the appended claims.
The application claims priority under 35 U.S.C. § 119 and all applicable statutes and treaties from prior provisional application Ser. No. 62/208,881, which was filed Aug. 24, 2015.
Filing Document | Filing Date | Country | Kind |
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PCT/US2016/048200 | 8/23/2016 | WO | 00 |
Number | Date | Country | |
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62208881 | Aug 2015 | US |