WO2002060215A2

WO2002060215A2 - Wireless battery-less microphone

Info

Publication number: WO2002060215A2
Application number: PCT/US2002/001497
Authority: WO
Inventors: Haruhiko H. Asada
Original assignee: Massachusetts Institute Of Technology
Priority date: 2001-01-26
Filing date: 2002-01-17
Publication date: 2002-08-01
Also published as: WO2002060215A3

Abstract

A passive microphone (110), which is wireless and battery-less, contains a resonant sensor circuit (112) and a reactive sensor (114) sensitive to vibration may be remotely coupled to a receiver circuit (130). The sensor (114) is a passive condenser microphone wherein vibration of the diaphragm alters the condenser capacitance and the resonant frequency of the resonant sensor circuit (112). The passive microphone is directly or indirectly attached to the person and is coupled to a remotely disposed reader (120) carried by the person and then is transmitted to a remote receiver (130). The passive microphone sensor (114) exposed to vibrations alters the resonance frequency of a sensor circuit (112) containing the sensor (114). Wireless coupling of the sensor circuit (112) to a reader circuit (122) changes the impedance of the reader circuit (122) that is detectable upon sequentially exciting the reader circuit (122) over a range of frequencies that includes the resonance frequency.

Description

Wireless Battery-less Microphone

Field of the Invention The present invention pertains to a method and apparatus for capturing sounds and vibration with wireless and battery-less microphones.

Background of the Invention

As the aged population rapidly grows, healthcare is becoming one of the most critical problems faced by society. Mobile telephony, e.g. cellular phone and personal handy-phone system (PHS), is expected to be a key technology for expanding healthcare services beyond the traditional hospital-based services. Using mobile telephony, a patient can be connected to a service network of physicians and caretakers anywhere at any time. In turn, the physicians and caretakers can monitor the patient's condition, both physiological and behavioral, remotely and continually, in the home as well as in hospitals. This would expedite and streamline the patient's recovery process, improve the quality of life, prevent disastrous and costly failures, and promote preventive and behavioral medicine.

Sensors for detecting vibration and acoustic signals from the body include stethoscopes and microphones. Stethoscopes, however, monitor physiological sounds at only a single location and must typically be held in place by a clinician. Microphones for health monitoring typically use cables connecting the microphones to a control box, an arrangement that may be cumbersome.

Summary of the Invention

In accordance with preferred embodiments of the invention, there is provided a passive microphone and a reader. The passive microphone has a sensor for converting vibrations into variations in a resonant frequency of a sensor circuit, while the reader remotely detects the variations in the resonant frequency of the sensor circuit and generates an electrical signal related to the variations in the resonant frequency of the sensor circuit.

In accordance with alternate embodiments of the invention, in the conversion of vibrations into variations in a resonant frequency of a sensor circuit, the sensor may vary a reactive component of the sensor circuit, and, more particularly, a capacitive component, thereby varying the resonant frequency of the sensor circuit. The reader may be inductively coupled to the sensor circuit. Moreover, the sensor may be attached to a person, such as by coupling to the skin of the person.

In accordance with another aspect of the invention, a method is provided for monitoring a person. The method has a first step of attaching a passive microphone, either in direct or indirect contact with the patient. Indirect contact may include attachment to the clothes of the patient. The method has the further steps of wirelessly coupling the passive microphone to a remotely disposed reader carried by the patient and transmitting the signal from the reader to a remote receiver. In accordance with still another aspect of the invention, a method is provided for remotely detecting vibrations. Exposure of a passive microphone sensor to vibrations alters the resonance frequency of a sensor circuit containing the passive microphone sensor. Wireless coupling of the sensor circuit to a reader circuit alters the impedance of the reader circuit. Excitation of the reader circuit over a range of frequencies encompassing the resonance frequency of the sensor circuit identifies the current resonance frequency, and, consequently, the current vibration of the passive microphone sensor.

Brief Description of the Drawings The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which:

FIG. 1 is a schematic description of the relationship between a passive microphone, a reader, and a remote receiver in the transmission of vibrational information remotely in accordance with embodiments of the invention. FIG. 2 depicts an application of a wireless microphone in accordance with embodiments of the invention;

FIG. 3 contains equivalent circuit diagrams of the sensor circuit and reader circuit of FIG. 2, in accordance with an embodiment of the present invention;

FIGS. 4a-4c depict the time variation of the frequency of the input voltage to the read circuit, the voltage generated in the read circuit in response to the read circuit input voltage, and the processed read circuit voltage; and

FIG. 5 shows a spectrum of the voltage generated in the read circuit exhibiting a dip corresponding to the instantaneous effective capacitance of the passive microphone sensor, in accordance with an embodiment of the present invention.

Detailed Description of Specific Embodiments In accordance with preferred embodiments of the present invention, a passive microphone captures sounds and vibrations and transmits the signals wirelessly to a reader without battery power. The reader subsequently transmits the signals wirelessly to a receiver. One application, described without limitation, is that of hands-free mobile phone operation, where the passive microphones are attached to the clothes, or directly to the skin of the wearer, and the voice signals are wirelessly transmitted to the mobile phone of the wearer. More advanced applications include wearable health monitoring, where the microphones attached to the patient detect vital signs and other behavioral signals.

FIG. 1 illustrates elements of one embodiment of the invention. Wireless microphone 100 comprises a passive microphone 110 and a reader 120. Passive microphone 110 comprises a sensor circuit 112 that, in turn, comprises a sensor 114. Vibrations to sensor 114 alter the electrical characteristics of sensor circuit 112. These electrical characteristics are detected wirelessly and electrically by reader circuit 122 and alter its electrical performance. The electrical performance of reader circuit 122 is next communicated wirelessly to receiver 130.

Passive microphone sensors are passive condenser microphones embedded in a sensor circuit with wireless links to a reader. As shown in FIG. 2, one embodiment comprises passive microphones attached to the skin to detect voice sounds (210), to detect vibrations of the cartoid artery (220, 222, and 224), and to monitor the neck and chest (231, 232, 233, 234, and 235) with reader 240 hung around the neck.

FIG. 3 illustrates schematically the interaction between embodiments of a passive microphone 110 and a reader 120. Sensor circuit 112 comprises a condenser microphone sensor 315 and a coil 330 with a small resistance 325. (Since passive microphone 110 has no battery, it is a passive device.) When condenser microphone sensor 315 is exposed to acoustic waves 350, a diaphragm 355 of sensor 315 moves, thereby altering the capacitance, C_m, presented by condenser microphone sensor 315 to sensor circuit 112. This causes a change in the resonant frequency of sensor circuit 112, the RLC circuit formed by coil 330 with inductance L₂, and condenser microphone sensor 315 with capacitance C_m.

Unlike the standard condenser microphone in which the diaphragm motion is transformed into a voltage drop across the capacitor, passive microphone 110 converts the diaphragm motion to a change in the resonant frequency, f_res. Reader 120 hung from the neck detects this resonant frequency wirelessly through inductive coupling 375 between coil 330 in sensor circuit 112 and coil 355 in reader circuit 122. Reader circuit coil 355 is placed in the near field, i.e. less than (wave length)/(2π)) so that an inductive coupling with sensor circuit coil 330 may be formed. In other words, the two coils form a type of transformer in the air with mutual inductance M. Reader 120 has a voltage source 370 providing a high frequency oscillatory voltage, u₀. Through the inductive coupling, this induces a current 265 (i ) in sensor circuit 112. The impedance of sensor circuit 112 viewed from reader circuit 340 is maximum when the frequency of oscillatory voltage 370 (uo) agrees with the resonant frequency, f_res, of sensor circuit 112. In turn, this changes current 360 (ii) of reader circuit 340, which is detected by the voltage drop across resistor 345 (Rj.). To increase the sensitivity of the voltage drop, reader circuit 340 is made into a resonant circuit. Namely, capacitor 350 ( ) creates a resonance so that the voltage drop at coil 355 (Li) cancels with that of capacitor 350 ( ) at the resonant frequency. To find the resonant frequency f_reS( the excitation frequency of reader circuit

340 is swept from its minimum value, f_nun, to its maximum f_max. FIGS. 4a-4c show the process of reading the resonant frequency of sensor circuit 112. In FIG. 4a, saw- toothed waveform 410 illustrates periodic sweeping of the excitation frequency superimposed on acoustic waveform 420. In FIG. 4b, waveform 430 shows that when the excitation frequency of reader circuit 340 matches the resonant frequency of sensor circuit 112 f_re_S) a sharp reduction in reader circuit current 360 (ii) is observed and manifests itself in a sharp drop in voltage across resistor 345 (Ri). Peak detection circuit 347 in reader circuit 340 identifies this timing and converts the dips in resistor 345 voltage to diaphragm 355 displacement of the condenser microphone sensor. The series of step changes in waveform 440 in FIG. 4c show the sampled data output from reader circuit 340.

In the derivation of the governing equations for reader 120 - passive microphone 110 combination, sinusoidal current 360 (ii) of frequency -yin reader circuit coil 355 induces current 365 (i₂) in sensor circuit coil 330 through mutual inductance M, yielding the following relationship.

jωM • z. = jύL₂ ^■ i₂ + R₂ • i₂ i₂

JωC_m Likewise, reader circuit 340 satisfies

1 u₀ = R.-_j + jωL. • i_j -I j oM • i₂ jωC₁ where u₀ is the voltage of source 370. The total impedance of sensor circuit 112 viewed from reader circuit 122 is given by ω²M²

_JT - jωL₂ +R₂ +-

This transformed sensor circuit impedance varies as the capacitance of the passive microphone sensor 315 changes and the frequency of the reader circuit excitation is varied. When the excitation frequency matches the sensor circuit resonant frequency, a sudden increase in the transformed sensor circuit impedance Z occurs. To magnify the effect of this impedance change on the reader circuit current 360 (ii), the impedances of the reader circuit coil 355 and the reader circuit capacitor 350 are selected to cancel with each other. This cancellation yields the reader circuit current 360 of ii = uo / (Ri + Z_τ), the source of the voltage drop across resistor 345.

In selecting parameter values, several requirements and performance goals are considered:

1) Bandwidth and Sampling Rate. For a signal bandwidth corresponding to acoustic signals (20 kHz), the sampling rate should exceed 40 kHz. 2) Selection of the Excitation Frequency Band. Since inductive coupling is used, the distance between the sensor circuit 112 and the reader circuit 122 should be within the near field, i.e. less than λ/2π, where λ is the wavelength of the electromagnetic radiation corresponding to frequency ω. For the range to be 30 cm or longer, the excitation frequency should not exceed 150 MHz. Electromagnetic interference with the human body and loss associated with the interference are factors in selecting the excitation frequency. Evidence suggests that frequencies lower than 300 MHz have no significant interference (Rhee, S. W., Yang, B. H., and Asada H., Artifact-Resistant, Power-Efficient Design of Finger-Ring Plethysmographic Sensors, IEEE Trans. Biomedical Engineering, Vol. 48, No. 7, 795-805, 2001). Another requirement to consider in selecting the frequency is resolution and accuracy in measuring the resonant frequency. Increasing the range of the sweep frequency and increasing the average frequency so that more cycles of waves are involved in a single sweep improve these measures. If 500 cycles of waves are needed during a sweep, the average excitation frequency should exceed 20 MHz. The range of the excitation frequency is established by assuming a variation within 4% of the average frequency. Among the available frequency ranges approved by FCC in the United States and satisfying these requirements is the frequency range 26.565 ~ 27.405 MHz.

3) Coil Size and Inductance. The efficiency of larger coils must be balanced against size constraints. For a radius of the sensor circuit coil 330 of 5 mm and a radius of the reader circuit coil 355 of 5 cm, 10 turns of the sensor circuit coil 330 has an inductance L₂ of 5J8 μH, and 30 turns of the reader side coil 355 has an inductance Li of 520.8 μH. When the coils are placed 30 cm away, the mutual inductance M becomes 7.71 nH.

FIG. 5 shows a simulation result for the above parameter selection. Gaussian noise is added to the system. Sensor condenser microphone 315 is assumed to vary its capacitance between 5.85 pF and 6.15 pF. Parameters of other circuit elements are selected from their specifications and data sheets. Observed voltage 510 across reader circuit resistor 345 is readily distinguishable above the noise.

The process of reading out the passive microphone output has several features. First, although the magnitude of the voltage drop varies depending on the mutual inductance M, the acquired data of the passive microphone output may not directly be influenced. Peak detection circuit 347 detects only the time at which the excitation frequency agrees with the resonant frequency of the sensor circuit. Although the magnitude of the voltage across resistor 345 (Ri) varies depending on the relative location between reader coil 355 and the sensor circuit coil 330, the timing does not change. Also, use of a saw-toothed waveform resembles the basic principle of pulse width modulation and analog to digital conversion, an inherently digital and robust process.

Finally, passive microphone 110 does not require a battery, making the passive microphone very simple, small, and potentially disposable. A passive microphone is preferably less than a few millimeters in diameter and less than 5 mm in length. A passive microphone sensor is a condenser microphone well suited for miniaturization through micro-electromechanical systems (MEMS) technology. With a diameter of less than 2mm, a passive microphone sensor can be easily attached to the skin with adhesive. In addition, the small size allows long term attachment by minimizing adverse effects due to air blockage (skin can breath through the uncovered surrounding area) and dislodging by motion of the body.

Distributed placement of passive microphones at various spots around the neck and chest permits acquisition of additional information - phase distribution along with a magnitude distribution over the body. This may advantageously suppress acoustic noise, distinguish diverse sources of signals, and reduce motion artifact. Having more sensor units may increase the reliability of the system as well.

The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.

Claims

I claim:

1. A wireless microphone system, comprising: a. a passive microphone containing a sensor for converting vibrations into variations in a resonant frequency of a sensor circuit; and b. a reader for remotely detecting variations in the resonant frequency of the sensor circuit and for generating an electrical signal related to the variations in the resonant frequency of the sensor circuit.

2. The wireless microphone system of claim 1, wherein the sensor varies a reactive component of the sensor circuit to vary the resonant frequency.

3. The wireless microphone system of claim 2, wherein the reactive component is a capacitive component.

4. The wireless microphone system of claim 2, wherein the reader is inductively coupled to the sensor circuit.

5. The wireless microphone system of claim 1, wherein the sensor is attached to the skin of a person.

6. A method for monitoring a person, comprising the steps of: a. attaching a passive microphone in direct contact or indirect contact with a person; b. wirelessly coupling the passive microphone to a remotely-disposed reader carried by the person; and c. transmitting a signal from the reader to a remote receiver.

7. A method for remote detection of vibrations, comprising the steps of: a. exposing a sensor to vibrations; b. altering a resonance frequency of a sensor circuit containing the sensor; d. wirelessly coupling the sensor circuit to a reader circuit; e. sequentially exciting the reader circuit over a range of frequencies that includes the resonance frequency of the sensor circuit; and f . detecting changes in the impedance of the reader circuit.