Patent Publication Number: US-2016242660-A1

Title: Vibration sensor and pulse sensor

Description:
TECHNICAL FIELD 
     The present invention relates to a vibration sensor and a pulse sensor which utilize the Doppler effect of an electric wave. 
     BACKGROUND ART 
     Until now, in order to detect a pulse of a human body, it has been necessary to perform sensing in a state where a sensor is brought into contact with a human body like in the case of a photoelectric pulse sensor and an electrocardiograph. 
     If a pulse of a human body can be detected without contact, there can be expected applications to goods for health maintenance or health care, watch sensing of a solitary senior people, and the like. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent No. 3057438 
     SUMMARY OF INVENTION 
     Technical Problem 
     There exists a technology in which an electric wave is used as a method for detecting, without contact, an activity condition of a human body. In Patent Literature 1, a sensor of a contactless type cardiopulmonary function monitor apparatus that uses an electric wave is disclosed. 
     The sensor disclosed in Patent Literature 1 is one called a Doppler sensor, which is a sensor that detects the presence or the like of a target object by utilizing the Doppler effect, as has been named. 
     The Doppler sensor disclosed in Patent Literature 1 uses arithmetic processing by a fast Fourier transformation and a computer to thereby make an apparatus large and expensive. Accordingly, in order to apply a contactless pulse sensor to inexpensive goods, further simplification and low price are desirable. 
     The present invention has been made in consideration of such a situation, and an object of the present invention is to provide a vibration sensor and a pulse sensor which can detect, without contact, a low-frequency vibration of an object to be detected such as a pulse of a human body by using an extremely simple and inexpensive circuit configuration. 
     Solution to Problem 
     In order to solve the above-described problem, an vibration sensor of the present invention includes a signal generator that generates a signal including a frequency component utilizable as an electric wave; and a band pass filter that is provided with a prescribed bandwidth and allows a signal of a frequency included in the bandwidth to pass from the signal generated by the signal generator. In addition, the vibration sensor includes a first RF amplifier that amplifies a signal obtained from the band pass filter; an antenna that emits the signal amplified by the first RF amplifier, as an electric wave; a directional coupler that is interposed between the first RF amplifier and the antenna. Furthermore, the vibration sensor includes a first mixer that multiplies a reflected wave output from the directional coupler and a progressive wave obtained from the band pass filter or the directional coupler; a second mixer that multiplies a reflected wave output from the directional coupler and a progressive wave obtained from the band pass filter or the directional coupler; and a differential amplifier that differentially amplifies an output signal of the first mixer and an output signal of the second mixer. 
     Advantageous Effects of Invention 
     According to the present invention, a vibration sensor and a pulse sensor which can detect, without contact, a low-frequency vibration of an object to be detected such as a pulse of a human body by using an extremely simple and inexpensive circuit configuration can be provided. 
     Problems, configurations and effects other than those described above will become clear by the following explanations of embodiments. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of the vibration sensor according to a first embodiment of the present invention. 
         FIG. 2  is a block diagram of the vibration sensor according to a second embodiment of the present invention. 
         FIG. 3  is an example of a circuit of a pulse wave generator. 
         FIG. 4  includes a wave form chart of pulse output from the pulse wave generator, a spectrum chart in the frequency region, obtained by subjecting the pulse output from the pulse wave generator to Fourier transformation, a frequency characteristic chart of a BPF, and a spectrum chart showing harmonic components passing through the BPF. 
         FIG. 5  includes a spectrum chart showing harmonic components passing through the BPF, and a spectrum chart showing reflected waves output from a directional coupler. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The vibration sensor according to an embodiment of the present invention is a Doppler sensor using an electric wave. That is, a target object is irradiated with an electric wave, and the change in frequency of the reflected electric wave is detected. 
     However, when a target object is close to an antenna, the resonance frequency of the antenna easily fluctuates depending on the position or movement of the target object. 
     In the vibration sensor according to the embodiment of the present invention, electric waves of a plurality of frequencies are extracted using a band pass filter encompassing the fluctuation range of the fluctuating resonance frequency, which are utilized for detecting a low-frequency vibration. 
     First Embodiment: Overall Configuration of the Vibration Sensor  101   
       FIG. 1  is a block diagram of a vibration sensor  101  according to a first embodiment of the present invention. 
     The vibration sensor  101  is divided into two constituents described below. 
     A first constituent is a constituent that transmits an electric wave being a progressive wave to a target object, and that receives and extracts a reflected wave reflected from the target object. The first constituent includes a pulse wave generator  102 , a band pass filter (hereinafter, abbreviated as “BPF”)  103 , a first RF amplifier  104 , a directional coupler  105  and a helical antenna  106 . 
     A second constituent is a constituent which generates a frequency difference signal from the progressive wave and the reflected wave, and which further extracts a vibration signal. The second constituent includes a second RF amplifier  108 , a third RF amplifier  109 , a first mixer  110 , a second mixer  112 , a first low-pass filter (hereinafter, abbreviated as “LPF”)  114 , a second LPF  115 , a differential amplifier  116  and a third LPF  117 . 
     The pulse wave generator  102 , which may also be referred to as a signal generator, generates a pulse signal of a comparatively low frequency. The frequency to be generated by the pulse wave generator  102  is, for example, 1 MHz. 
     The BPF  103  takes out a harmonic component from the pulse signal generated by the pulse wave generator  102 . The central frequency and bandwidth of the BPF  103  are, for example, 60 MHz±3 MHz. The BPF  103  can utilize a circuit configuration obtained by, for example, cascade-connecting LC resonance circuits. 
     The first RF amplifier  104  amplifies a signal of a harmonic component of a pulse signal having passed through the BPF  103 . 
     The signal of a harmonic component of the pulse signal 1amplified by the first RF amplifier  104  is input to an input terminal of the directional coupler  105  (“IN” in  FIG. 1 ). Then, the signal of a harmonic component of the pulse signal is supplied to the helical antenna  106  connected to an output terminal of the directional coupler  105  (“OUT” in  FIG. 1 ). 
     The directional coupler  105  is formed of a coil, a condenser and a resistance, and is a well-known circuit element to be used in a VSWR meter (Voltage Standing Wave Ratio) or the like. The directional coupler  105  can output an output signal proportional to the progressive wave and an output signal proportional to the reflected wave, respectively, on the basis of a progressive wave and a reflected wave included in a first transmission path. 
     The helical antenna  106  emits electric waves of a plurality of frequencies based on a signal of a harmonic component of a pulse signal. Then, the electric wave reflected by a target object is received by the helical antenna  106  to thereby generate a standing wave in the inside of the directional coupler  105 . 
     A signal that is proportional to a signal of an electric wave (reflected wave) input from the output terminal through the helical antenna  106  is output to an isolated terminal of the directional coupler  105  (“Isolated” in  FIG. 1 ). 
     A signal that is proportional to the signal of a harmonic component (progressive wave) of pulse signal input to the input terminal is output to a coupled terminal of the directional coupler  105  (“Coupled” in  FIG. 1 ). 
     The coupled terminal is connected to a ground node via a resistance R  107 . As to a resistance value of the resistance R  107 , a resistance value that is equal to an impedance of the directional coupler  105  and the helical antenna  106  is set. In many cases, the impedance of the directional coupler  105  and the helical antenna  106  is 50Ω or 75Ω. 
     The second RF amplifier  108  amplifies the signal of a harmonic component (progressive wave) of the pulse signal passing through the BPF  103 . 
     The third RF amplifier  109  amplifies a signal of an electric wave (reflected wave) output from the isolated terminal of the directional coupler  105 , and input from the output terminal through the helical antenna  106 . 
     The output signal of the second RF amplifier  108  is supplied to the first mixer  110  and is supplied to the second mixer  112  via an inverting amplifier  111 . 
     The output signal of the third RF amplifier  109  is supplied to the second mixer  112  and is supplied to the first mixer  110  via a buffer  113 . Note that, even though phases may be different between the output signal of the second RF amplifier  108  and the output signal of the third RF amplifier  109 , intended signals can be obtained from the first mixer  110  and the second mixer  112 . Accordingly, a buffer (non-inverting amplifier) may be used instead of the inverting amplifier  111 . 
     In this way, each of the first mixer  110  and the second mixer  112  outputs a multiplication signal of the progressive wave and the reflected wave. Here, for example, a dual gate FET or the like is utilizable as the first mixer  110  and the second mixer  112 . 
     The output signal of the first mixer  110  is supplied to the first LPF  114 . The first LPF  114  outputs a signal of difference in respective frequencies of the progressive wave and the reflected wave among the multiplication signals of the progressive wave and the reflected wave which are output from the first mixer  110 . 
     In the same way, the output signal of the second mixer  112  is supplied to the second LPF  115 . The second LPF  115  outputs a signal of difference in frequencies of the progressive wave and the reflected wave among the multiplication signals of the progressive wave and the reflected wave which are output from the second mixer  112 . 
     Each of the output signal of the first LPF  114  and the output signal of the second LPF  115  is input to the differential amplifier  116 . The differential amplifier  116  including an operational amplifier outputs an output signal of the first LPF  114  and a signal obtained by removing a noise component from an output signal of the second LPF  115 . 
     The output signal of the differential amplifier  116  is supplied to the third LPF  117 . The third LPF  117  removes an alternating-current component having a comparatively high frequency from an output signal of the differential amplifier  116  and allows a low-frequency signal exhibiting a pulse of a human body to pass through. 
     Second Embodiment: Overall Configuration of Vibration Sensor  201   
       FIG. 2  is a block diagram of a vibration sensor  201  according to a second embodiment of the present invention. 
     The differences of the vibration sensor  201  shown in  FIG. 2  from the vibration sensor  101  shown in  FIG. 1  are that the input terminal of the second RF amplifier  108  is connected to the isolated terminal of the directional coupler  105 , and the input terminal of the third RF amplifier  109  is connected to the coupled terminal of the directional coupler  105 , respectively, and that a buffer  202  instead of the inverting amplifier  111  is connected between the second RF amplifier  108  and the second mixer  112 . Note that, although the output signal of the second RF amplifier  108  and the output signal of the third RF amplifier  109  have different phases from each other, intended signals can be obtained from the first mixer  110  and the second mixer  112 . Accordingly, an inverting amplifier may be used instead of the buffer  202 . 
     That is, in the vibration sensor  201  according to the second embodiment of the present invention, the second RF amplifier  108  amplifies the reflected wave, and the third RF amplifier  109  amplifies the progressive wave. 
     Specific Example of Pulse Wave Generator  102   
     The pulse wave generator  102  that is common to the first embodiment and the second embodiment generates a pulse signal shown in  FIG. 4A  to be described later. Many circuits and apparatuses are considered as a method for generating such a pulse signal, and an example is shown in  FIG. 3 . 
     Each of  FIG. 3A  and  FIG. 3B  is a circuit example of the pulse wave generator  102 . 
       FIG. 3A  is a block diagram of a microcomputer  301 . A CPU  302 , a ROM  303 , a RAM  304  and a serial interface  305  are connected to a bus  306 . These days, a one-chip microcomputer that is inexpensive and excellent in usability can be easily obtained. Such one-chip microcomputer can easily generate a pulse signal as shown in  FIG. 4A  by writing a program in the ROM  303  being a built-in flash memory. 
       FIG. 3B  is a circuit diagram of an oscillation circuit using a quartz oscillator and a waveform shaping circuit using a mono-multi. 
     One end of a quartz oscillator  312 , one end of a resistance R  313  and one end of a condenser C  314  are connected to an input terminal of a NOT gate  311 . 
     The other end of the resistance R  313  is connected to the output terminal of the NOT gate  311 . 
     One end of a resistance R  315  and the input terminal of a mono-multi  316  are connected to the output terminal of the NOT gate  311 . 
     The other end of the quartz oscillator  312  and one end of a condenser C  317  are connected to the other end of the resistance R  315 . 
     Each of the other end of the condenser C  314  and the other end of the condenser C  317  is connected to the ground node. 
     That is, a signal generated by the oscillation circuit constituted by the NOT gate  311  and the quartz oscillator  312  is frequency-controlled by the quartz oscillator  312  and is waveform-shaped into a pulse signal whose duty ratio has been adjusted by the mono-multi  316 . 
     [Operation of Vibration Sensor  101 ] 
     Hereinafter, operations of the vibration sensor  101  will be explained with reference to  FIG. 4A ,  FIG. 4B ,  FIG. 4C ,  FIG. 4D ,  FIG. 5A  and  FIG. 5B . 
       FIG. 4A  is a wave form chart of a pulse signal output from the pulse wave generator  102 . The horizontal axis of the wave form chart represents time and the vertical axis represents a voltage. As shown in  FIG. 4A , a waveform that has a small duty ratio and is close to an impulse contains a lot of harmonics, and thus such a waveform is desirable for the vibration sensor  101  of the embodiment. 
       FIG. 4B  is a spectrum chart in the frequency region obtained by subjecting the pulse signal shown in  FIG. 4A  output from the pulse wave generator  102 , to Fourier transformation. The horizontal axis of the spectrum chart represents a frequency, and the vertical axis represents a voltage. As shown in  FIG. 4B , the pulse signal includes a plurality of harmonics having frequencies of integral multiples relative to the fundamental wave. 
       FIG. 4C  is a frequency characteristic chart of the BPF  103 . The scale in  FIG. 4C  is adjusted to that in  FIG. 4B , and thus the horizontal axis of the frequency characteristic chart represents a frequency and the vertical axis represents a voltage. 
       FIG. 4D  is a frequency distribution chart of signals having passed through the BPF  103 . The scale in  FIG. 4D  is also adjusted to that in  FIG. 4C , the horizontal axis of the frequency characteristic chart represents a frequency and the vertical axis represents a voltage. 
     As shown in  FIG. 4C , the BPF  103  allows a component of a specific frequency to pass through among harmonic components included in the pulse signal. Then, as shown in the frequency distribution chart in  FIG. 4D , in the harmonic component of the pulse signal having passed through the BPF  103 , frequency components or the like having a cutoff frequency or less including the fundamental wave are removed from the pulse signal. 
       FIG. 5A  shows enlarging the frequency axis (horizontal axis) of the frequency distribution chart in  FIG. 4D , which is a spectrum chart representing harmonic components of the pulse signal having passed through the BPF  103 . 
       FIG. 5B  is a spectrum chart showing the reflected wave output from the directional coupler  105 . 
     Now, as shown in  FIG. 5A , harmonic components of the pulse signal having passed through the BPF  103  are assumed to be five signals centering on 60 MHz. Five signals are f 1 =58 MHz, f 2 =59 MHz, f 3 =60 MHz, f 4 =61 MHz and f 5 =62 MHz in ascending order of frequencies. These five signals are amplified by the first RF amplifier  104 , and the amplified signals are emitted as electric waves from the helical antenna  106  via the directional coupler  105 . 
     However, since the frequency characteristic (bandwidth) of the helical antenna  106  is narrow, any one or approximately two of signals of f 1  to f 5  are emitted as electric waves from the helical antenna  106 . 
     Then, electric waves emitted from the helical antenna  106  are reflected by a target object and input to the directional coupler  105  through the helical antenna  106 . Signals of these reflected waves are, for example as shown in  FIG. 5B , any of f 1 ′=58.1 MHz, f 2 ′=59.1 MHz, f 3 ′=60.1 MHz, f 4 ′=61.1 MHz and f 5 ′=62.1 MHz in ascending order of frequencies. In the example, it is assumed that the frequency of the reflected wave has shifted by 100 kHz from the progressive wave by the Doppler effect. 
     Any of f 1  to f 5  and any of f 1 ′ to f 5 ′ are input to the first mixer  110  and the second mixer  112 , and are subjected to multiplication. Then, the first mixer  110  and the second mixer  112  output signals obtained by adding respective frequencies and signals obtained by subtracting respective frequencies. 
     Signals obtained by adding a frequency include, for example, f 1 +f 1 ′, f 2 +f 1 ′, . . . and f 5 +f 1 ′ when a reflected wave includes f 1 ′. 
     When the reflected wave is f 2 ′, the signals include f 1 +f 2 ′, f 2 +f 2 ′ . . . and f 5 +f 2 ′. 
     In the same way, cases when a reflected wave includes f 3 ′, . . . and a reflected wave includes f 4 ′ follow the above, and when a reflected wave includes f 5 ′, the signals include f 1 +f 5 ′, f 2 +f 5 ′ . . . and f 5 +f 5 ′. 
     Signals obtained by subtracting a frequency include, for example, |f 1 −f 1 ′|, |f 2 −f 1 ′| . . . and |f 5 −f 1 ′| when a reflected wave includes f 1 ′. 
     When a reflected wave includes f 2 ′, the signals include |f 1 −f 2 ′|, |f 2 −f 2 ′| . . . and |f 5 −f 2 ′|. 
     In the same way, cases when a reflected wave includes f 3 ′, . . . and a reflected wave includes f 4 ′ follow the above, and when a reflected wave includes f 5 ′, the signals include |f 1 −f 5 ′|, |f 2 −f 5 ′| . . . and |f 5 −f 5 ′|. 
     The lowest frequency among signals output from the first mixer  110  and the second mixer  112  is |f 1 −f 1 ′|, |f 2 −f 2 ′|, |f 3 −f 3 ′|, |f 4 −f 4 ′| and |f 5 −f 5 ′|. In the cases of  FIG. 5A  and  FIG. 5B , all frequencies of these signals are 100 kHz. These signals have only a component resulting from the shift of frequency by the Doppler effect, and all frequencies become identical. 
     When a target object is close to an antenna, the resonance frequency of the antenna easily fluctuates depending on the position or action of the target object. Consequently, even when an electric wave is emitted from the antenna by using a signal of a single frequency, the signal generates mismatch with the resonance frequency of the antenna, and a reflected wave cannot be correctly received. 
     Therefore, the vibration sensor according to the embodiment of the present invention utilizes electric waves of a plurality of frequencies using a band pass filter encompassing the fluctuation of the resonance frequency. As a result, even when the resonance frequency of the antenna fluctuates, any one or approximately two of signals of a plurality of frequencies coincide with the bandwidth of the antenna and the reflected wave can be received. 
     When the reflected wave can be received, the presence and/or fluctuation state of the target object can be detected by taking out, by using a mixer, the frequency difference between the reflected wave and the progressive wave generated by the Doppler effect. 
     Note that 60 MHz is a frequency that allegedly matches most easily with the blood flow of a human body. 
     Both the vibration sensor  101  according to the first embodiment of the present invention and the vibration sensor  201  according to the second embodiment remove noise of an in-phase component included in a signal by using the differential amplifier  116 . Furthermore, these sensors also remove noise of a high-frequency component by causing the signal to pass through the third LPF  117 . As a result of removing these noises, the vibration sensor  101  and the vibration sensor  201  of the embodiment can detect vibration from a weak fluctuation generated on electric waves by vibration of a target object, without using an expensive apparatus such as high-speed Fourier transformation. 
     Application examples as described below are possible in embodiments having been described above. 
     (1) In the above-described embodiments, the helical antenna  106  has been used, but the type of antennas is not limited to this. There can be used any antenna having an open end, such as a dipole antenna, a ground plane antenna, and a meander line antenna. Furthermore, even a loop antenna not having an open end is utilizable although gain lowers. 
     (2) In place of the pulse wave generator  102 , a circuit that generates white noise may be used. 
     (3) The vibration sensors  101  and  201  of the embodiments detect, without contact, a low-frequency vibration of an object. An object to be detected is not limited to a specific object as long as it is an object that can cause a change in the distribution constant of an antenna by approaching the antenna. Accordingly, the vibration sensors  101  and  201  can also be used as a pulse sensor. 
     (4) The vibration sensors  101  and  201  according to the embodiments can be applied to various applications. For example, there can be expected an application as a driver dozing preventing apparatus that detects a driver&#39;s doze by locating a pulse sensor on a driver&#39;s seat of a car. In addition, variations can also be given to game development by locating a pulse sensor on a game machine and detecting a pulse of a player to thereby give analogy of the excitation degree. Furthermore, it is also possible to realize a pulse detection device that detects a pulse, when providing a desk and a chair at a corner of a room by using a pulse sensor together with a human detection sensor, and immediately when detecting that a human has sat on the chair. The pulse detection device can detect a pulse more simply and in a shorter time than a conventional blood pressure gauge. 
     In the above-described embodiments, there has been explained the vibration sensor  101  in which an electric wave (progressive wave) is emitted from an antenna by using signals of a plurality of frequencies, an electric wave (reflected wave) reflected from a human body being a target object is taken out using the directional coupler  105 , and a frequency difference signal between the progressive wave and the reflected wave is extracted using a mixer. 
     The BPF  103  connected to the pulse wave generator  102  has a bandwidth encompassing the fluctuation range of the resonance frequency of the helical antenna  106 . Accordingly, even if the resonance frequency of the helical antenna  106  fluctuates by means of a target object approaching the helical antenna  106 , one or a number of a plurality of frequencies passing through the BPF  103  can pass through the bandwidth of the helical antenna  106 . Consequently, the vibration sensor being a Doppler sensor using an electric wave of a low frequency can be realized. 
     In the vibration sensors  101  and  201  of the embodiments, frequencies of signals to be handled are as low as approximately several dozen to several hundred MHz, as compared with conventional Doppler sensors. Consequently, the price of the circuit element is inexpensive. Furthermore, since frequencies are low, the implementation of circuits including the BPF  103  and the directional coupler  105  is easy. In addition, since frequencies of signals to be handled are low, only a small amount of electric power is consumed. 
     Moreover, the vibration sensor  101  of the embodiment has an extremely small circuit scale, as compared with conventional Doppler sensors. 
     One transistor is required for each of the first RF amplifier  104 , the second RF amplifier  108  and the third RF amplifier  109 , and the buffer  113  and the inverting amplifier  111 . 
     One dual gate FET is also required for each of the first mixer  110  and the second mixer  112 . 
     One inexpensive one-chip microcomputer is required for the pulse wave generator  102 . 
     The vibration sensors  101  and  201  of the embodiments do not require Fourier transformation and complicated data processing, differently from the technology disclosed in Patent Literature 1. 
     In this way, the vibration sensors  101  and  201  of the embodiments can be implemented with semiconductor elements being active elements having a number less than ten in total. Accordingly, they can be manufactured inexpensively and be easily mass-produced. 
     Hereinbefore, although embodiments of the present invention have been explained, the present invention is not limited to the above-described embodiments, and includes other modifications and application examples as long as they do not deviate from the gist of the present invention described in the claims. 
     For example, the above-described embodiments are those in which the configuration of apparatuses and systems are explained in detail and in concrete terms for simply explaining the present invention, and embodiments are not necessarily limited to those provided with all configurations having been explained. Additionally, it is possible to substitute a part of the configuration of an embodiment with the configuration of another embodiment, and furthermore, it is also possible to add the configuration of another embodiment to the configuration of an embodiment. Moreover, it is also possible to perform addition of another configuration, elimination or substitution, for a part of the configurations of the respective embodiments. 
     In addition, a part or the whole of the above-described respective configurations, functions, processing units or the like may be realized with hardware, for example, by designing these through the use of an integrated circuit. Furthermore, the above-described respective configurations, functions or the like may be realized with software for interpreting and executing a program that causes a processer to realize respective functions. Information such as a program for realizing the respective functions, tables and files can be stored on a volatile or nonvolatile storage such as a memory, a hard disk, or an SSD (Solid State Drive); or on a recording medium such as an IC card or optical disk. 
     Moreover, control lines and information lines indicate lines that are considered to be necessary for explanation, and do not necessarily indicate all control lines and information lines in products. Actually almost all configurations may be considered to be mutually connected. 
     REFERENCE SIGNS LIST 
       101 : vibration sensor,  102 : pulse wave generator,  103 : BPF,  104 : first RF amplifier,  105 : directional coupler,  106 : helical antenna,  108 : second RF amplifier,  109 : third RF amplifier,  110 : first mixer,  111 : inverting amplifier,  112 : second mixer,  113 : buffer,  114 : first LPF,  115 : second LPF,  116 : differential amplifier,  117 : third LPF,  201 : vibration sensor,  202 : buffer,  301 : microcomputer,  312 : quartz oscillator,  316 : mono-multi