JP3208538B2 - Photoplethysmograph - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoplethysmograph which measures a volume pulse wave such as a pulse wave of a living body.

[0002]

2. Description of the Related Art FIG. 3 is a configuration diagram showing a configuration of a conventional photoelectric volume pulse wave meter. In the photoplethysmograph shown in FIG. 1, the light emitting diode 2 emits light by being driven to blink by the oscillator 1 based on a sufficiently high frequency with respect to the pulse wave. The light emitted from the light emitting diode 2 and the external light 7 are transmitted (or reflected) through the living body 6 and enter the photodiode 3. When the photodiode 3 generates a current corresponding to the amount of incident light, the current is input to the current-voltage converter 4, converted into a voltage, and further input to the demodulator 5. The demodulator 5 extracts a signal corresponding to the light emitted from the light emitting diode 2 to remove a frequency component of the external light from the input voltage signal, and obtains a volume pulse signal.

In order to improve the S / N ratio, the gain of the current-to-voltage converter 4 as the first-stage amplifier must be set high, but if the gain is set high, the start-up of the photodiode 3 caused by the external light 7 will occur. The current causes saturation of the gain set in the amplifier (ie, the current-to-voltage converter 4). For this reason, there has been a problem that high gain cannot be set in the current-voltage converter 4. Therefore, as shown in FIG. 4, a closed circuit is formed in which the photodiode 3, a reverse bias high voltage power supply 13 for supplying a reverse voltage to the photodiode 3, and a load resistor 14 are connected in series. Further, by forming a circuit in which a capacitor 15 is connected between the connection between the photodiode 3 and the resistor 14 and the input terminal of the first-stage amplifier 16, the gain set in the first-stage amplifier 16 is prevented from being saturated. .

As another device, Japanese Patent Laid-Open No. 1-2
32928, JP-A-8-80288 and JP-A-9-80
What is disclosed in each of No. 70394 is known.
The apparatus disclosed in Japanese Patent Application Laid-Open No. 1-232928 uses a filter that transmits only transmitted light that detects a fingertip pulse wave to a light receiving unit, for a light receiving unit that detects light transmitted through a living tissue by a light emitting unit. In addition, detection of a change in the amount of light other than the pulse wave is suppressed.

The device disclosed in Japanese Patent Application Laid-Open No. 8-80288 discloses a light emitting unit (light emitting diode) for irradiating light to a part of a living body, and receiving light emitted by the light emitting unit through the living body. By using light having a wavelength of 300 nm to 700 nm, which is hardly affected by external light in a living body, as a light wavelength band of each element of a light receiving unit (phototransistor) to be used, even if there is no large-scale light shielding structure, it is possible to meet the usage conditions We try to relax the constraints.

Further, the device disclosed in Japanese Patent Application Laid-Open No. 9-70394 discloses a light-emitting unit (light-emitting diode) for irradiating light to a part of a living body, and boosts an output voltage of a battery and supplies the voltage to the light-emitting diode. The light emitting diode is intermittently illuminated to suppress power consumption, and the capacitive element suppresses fluctuations in power supply voltage due to the intermittent light emission.

[0007]

However, in the above-described conventional photoplethysmograph shown in FIG. 3 in which the circuit shown in FIG. 4 is formed, the dynamic range can be expanded. The operating range having linearity is reduced. Therefore, it is not suitable for an apparatus requiring linearity such as a photoplethysmograph. Further, since the positive electrode side of the high-voltage power supply is connected to one end of the photodiode to be mounted on the living body, it is necessary to take measures against electric shock and requires safety.

In the apparatus disclosed in Japanese Patent Application Laid-Open No. 1-232928, the wavelength range of light detection using a filter has little attenuation due to living tissue, and detects a pulse wave, that is, a change in the amount of hemoglobin in blood. Depending on the conditions that can be used, 6
A wavelength of 00 nm or more is used. For this reason, the influence of disturbance light such as a fluorescent lamp is small, but there is no filter effect for disturbance light including a wavelength of 600 nm or more such as sunlight.

[0009] In the light having a wavelength of 600 nm or more, the living tissue becomes a relatively good light guide, and a part of the light irradiated on the skin surface far from the light receiving portion is received, transmitted, reflected, and diffused. Getting to the department is well known. In order to suppress this, it is necessary to shield the area around the light receiving unit from light in an appropriate range.

On the other hand, since the light receiving section employs a conventional circuit configuration such as a CdS (cadmium sulfide) photoelectric cell, which is conventionally used, it may be a problem of saturation of the light receiving section due to disturbance light, particularly sunlight. is there. Therefore, the apparatus disclosed in the above publication requires a large-scale light shielding around the light receiving unit and a countermeasure against saturation of the light receiving unit in order to suppress the influence of disturbance light such as sunlight.

In the apparatus disclosed in Japanese Patent Application Laid-Open No. 8-80288, the wavelength range of 700 nm or less, at which the attenuation effect by the living tissue is large, that is, 300 nm to 700 nm.
By using the nm, even if disturbance light such as sunlight is irradiated on the surface of the living body, it is disturbed in the living body by utilizing the fact that the attenuation is large and the absorbance of blood hemoglobin is high. Pulse wave detection in which the influence of light is suppressed is performed. However, since the light emitted from the light emitting unit (light emitting diode) to the body surface also undergoes the same attenuation as the disturbance light, the light emitting diode needs to emit a large amount of light such that the disturbance light can be ignored. Needs power. Therefore, application to a portable device or the like is not practical.

Further, since the light emitting diode cannot secure a sufficient amount of light emission against disturbance light, the band for fixing the sensor has a simple light shielding function, and the light shielding of disturbance light is simplified. It is not only that the light shielding itself is unnecessary.

Although the light receiving amount of the phototransistor greatly changes depending on the thickness, position, or individual difference of the finger on which the light emitting diode and the phototransistor are mounted, the operating point of the phototransistor, that is, the light detection sensitivity and The dynamic range must be fixed at a certain point determined experimentally, and the determined operating point is not always optimal.

Further, in the device disclosed in Japanese Patent Application Laid-Open No. 9-70394, since the light emitting diode needs a large amount of light emission, power consumption is suppressed by means of intermittent light emission to realize a portable device. However, the need for disturbance light shielding, optimization of the operating point of the phototransistor,
There are problems such as limitations on the measurement site.

Accordingly, the present invention provides a photoplethysmograph capable of improving volume pulse wave detection sensitivity and its linearity, and performing high-sensitivity photoplethysmogram detection with a wide dynamic range. The purpose is to do.

[0016]

Means for Solving the Problems To achieve the above object,
Therefore, the photoplethysmograph of the present invention6Circumference of plethysmogram
Light-emitting element that emits light by blinking at a frequency higher than the wave number2
And this light emitting element2Light and external light emitted from7Is measured
Target organism6Receives the light transmitted and / or reflected by the
Light receiving element that generates a current corresponding to the amount of light3And the light receiving element
Child3Conversion means for converting the current generated by the voltage into a voltage and outputting the voltage
4And a converting means.4Or
From the voltage signal output from the2High by flashing
A high-frequency cutoff filter 9 for removing a frequency signal component;
High frequency cutoff filter9The voltage output from the
Reference voltage12Differential amplification means for amplifying the difference between
10And this differential amplification means10Output voltage to current signal
Convert to light receiving element3Means to subtract from the current generated
And characterized in that:

In the photoplethysmograph having such a configuration, a signal component caused by external light is removed or reduced from a signal output from the light receiving means. Therefore, the pulse wave can be measured accurately, and the linearity can be improved along with the expansion of the allowable range of external light, that is, the dynamic range.

[0018]

[0019]

[0020]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a block diagram showing an embodiment of a photoelectric volume pulse wave meter according to the present invention. This photoplethysmograph irradiates a measured portion of a living tissue with light by a light emitting element such as a light emitting diode, and transmits or reflects light transmitted or reflected by the measured portion of the living tissue by a light receiving element such as a photodiode. Detecting the change of the volume pulse wave of the living tissue based on the detected signal.
It includes a light emitting diode 2, a photodiode 3, a current-to-voltage converter 4, a demodulator 5, a current adder 8, a high-frequency cutoff filter 9, an error amplifier 10, and a resistor 11.

The light emitting diode 2 is provided as a light source for irradiating the living body 6 with light, and is driven to blink by the oscillator 1 which oscillates at a sufficiently high frequency with respect to the volume pulse wave signal.

The photodiode 3 is a light emitting diode 2
Emitted light and external light 7 are transmitted (or reflected) through the living body 6
Then, a current corresponding to the amount of incident light is generated.

The current-voltage converter 4 converts the input current into a voltage, amplifies the converted voltage with a predetermined gain, and outputs the amplified voltage.

The demodulator 5 demodulates the output signal output from the current / voltage converter 4 based on the oscillation signal of the oscillator 1. That is, a signal corresponding to the light emitted by the light emitting diode 2, that is, a volume pulse signal is output.

The current adder 8 inputs the output current from the photodiode 3 to one input terminal, inputs the output current from the resistor 11 to the other input terminal, and converts the current input to these input terminals. Add and output.

The high-frequency cutoff filter 9 removes a high-frequency component signal from the input output signal of the current-voltage converter 4.

The error amplifier 10 inputs the output signal of the high-frequency cutoff filter 9 to the minus (-) input terminal side, inputs the reference voltage 12 to the plus (+) input terminal side, and inputs to these terminals. A difference between the two voltages is obtained, and a voltage obtained by amplifying the difference is output. This output signal is output as an error signal, converted into a current via the resistor 11, and then input to the other input terminal side of the current adder 8. The reference voltage is a value at which the operating point of the current-voltage converter 4 does not saturate (that is, a value at which the gain does not saturate) when a current corresponding to the amount of light emitted by the light emitting diode 2 is converted into a voltage and amplified. Is set to

The processing operation of the photoplethysmograph having the above configuration will be described. The light emitting diode 2 is oscillated from the oscillator 1, and is driven to blink and emits light based on an oscillation signal having a frequency sufficiently higher than the pulse wave signal. The emitted light is incident on the living body 6, and after being reflected or transmitted through the living body 6,
The light is incident on the photodiode 3. The external light 7 is incident on the living body 6, and is reflected or transmitted through the living body 6, and then is incident on the photodiode 3. The photodiode 3 receives the light reflected or transmitted through the living body 6, and generates a current corresponding to the amount of the received light. This current is input to one input terminal of the current adder 8.

However, since nothing is input to the other input terminal of the current adder 8, the input current itself is output from the current adder 8 and input to the current-voltage converter 4.

The current-voltage converter 4 converts the input current into a voltage, amplifies the converted voltage with a predetermined gain, and outputs the amplified voltage. The output signal includes a current signal (signal of a high frequency component) generated by the photodiode 3 due to light emission of the light emitting diode 2 and a current signal (DC signal) generated by the photodiode 3 due to external light 7. Component signal).

The output signal from the current-voltage converter 4 is input to the high-frequency cutoff filter 9 and the demodulator 5.

The high-frequency cutoff filter 9 removes a frequency component (high-frequency component) caused by the blinking of the light emitting diode 2 from the input voltage signal and outputs the signal. Therefore, this output signal is a signal mainly composed of the DC component caused by the external light 7.

When the output voltage output from the high-frequency cutoff filter 9 is applied to one terminal of the error amplifier 10, the error amplifier 10 applies the voltage to the reference voltage already applied to the other input terminal. Is obtained, and a voltage obtained by amplifying the difference is output. This output signal (ie, error signal)
Is converted into a current through the resistor 11 and then input to the other input terminal of the current adder 8.

Then, the current adder 8 adds the current input through the resistor 11 and the current generated by the photodiode 3 and outputs the result to the current-voltage converter 4. The current generated by the photodiode 3 includes a current (high-frequency component current) generated by the light emission of the light emitting diode 2 by the photodiode 3 and a current generated by the external light 7 by the photodiode 3 (Current of a low frequency component), the current adder 8 calculates the resistance 1
1 to output a current obtained by removing the current (DC component current) input thereto, that is, a high frequency component current corresponding to the light emitted by the light emitting diode 2 blinking.

Accordingly, the current-to-voltage converter 4 converts a high frequency component current resulting from the light from the light emitting diode 2 into a voltage out of the current output from the photodiode 3, amplifies the voltage, and amplifies the voltage. 5 is output.

The demodulator 5 demodulates the input voltage signal into a volume pulse wave signal indicating a volume pulse wave based on the oscillation signal from the oscillator 1, and outputs the signal.

Therefore, according to this plethysmograph, external light 7
(I.e., a current having a substantially DC component), the current input to the current-to-voltage converter 4 can be mainly composed of the signal resulting from the light emission of the light emitting diode 2.

That is, since the electromotive current of the photodiode 3 caused by the external light 7 can be removed, the saturation of the gain set in the current-voltage converter 4 due to the electromotive current is suppressed, and The range, that is, the dynamic range can be expanded. FIG. 2 shows the output characteristics of the photodiode 3 in which the dynamic range is expanded. FIG. 2 shows that the operation is performed on the dotted line indicated by reference numeral 20, that is, the dotted line from the current I1 to the current I2.

As described above, according to the present embodiment, the current from the current adder 8 is calculated based on the current (high-frequency component and low-frequency component current) generated by the photodiode 3.
A current from which the current (low-frequency component current) input via the resistor 11 is removed, that is, a high-frequency component current corresponding to the light emitted by the light emitting diode 2 is output. The saturation of the gain set in the voltage converter 4 can be suppressed. This is The rewritable larger tolerance i.e. the dynamic range of the ambient light 7 linear
To improve the arity means that it is Rukoto.

According to the present embodiment, the allowable range of the external light can be designed independently of the gain set in the current-voltage converter 4. The reason is that the output voltage of the error amplifier 10 provided independently of the current-voltage converter 4 is converted into a current converted via the resistor 11 (electromotive current of the photodiode 3 due to external light), This is because the electromotive current of the photodiode 3 caused by external light is canceled by the current adder 8.

Note that the present invention is not limited to the above embodiment, and various modifications and applications are possible. For example, in the above embodiment, a light emitting diode and a photodiode are illustrated as the light emitting element and the light receiving element, however, the light emitting element and the light receiving element are arbitrary, and for example, a phototransistor or the like may be used.

In the above embodiment, the photodiode
From the current signal output by the mode 3
Signal component by subtracting the
The effect was removed or reduced, but the photodiode 3
A circuit for extracting a signal component caused by external light from the output signal of the above, a circuit for subtracting the extracted signal component from the output signal of the photodiode 3, and the like can be arbitrarily changed and applied. .

According to the photoplethysmograph of the present invention, the dynamic range can be set so as to utilize the entire output characteristics of the photodiode even in a low-voltage drive circuit. The reason for this is that the output current of a general photodiode is about several mA at the maximum, and the generation of the current necessary to cancel the output current is sufficiently possible even in a low-voltage drive circuit. .

[0044]

As described above, according to the present invention,
By the signal processing, the signal component caused by the external light is reduced or suppressed, the improvement of Rutotomoni linearity to expand the allowable range i.e. the dynamic range of the external light can FIG Rukoto.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a configuration of an embodiment of a photoplethysmograph according to the present invention.

FIG. 2 is a diagram showing output characteristics of a photodiode provided in the photoplethysmograph of FIG. 1;

FIG. 3 is a configuration diagram showing a configuration of a conventional photoelectric volume pulse wave meter.

FIG. 4 is a circuit diagram showing a circuit used in the conventional photoplethysmograph shown in FIG.

[Description of Signs] 1 oscillator 2 light-emitting diode 3 photodiode 4 current-voltage converter 5 demodulator 8 current adder 9 high-frequency cutoff filter 10 error amplifier 11 resistance

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) A61B 5/02-5/0295 G01J 1/00-1/60 G01J 11/00 H03K 17/74-17 / 98

Claims (1)

(57) [Claims] 1. A light emitting element which blinks and emits light at a frequency higher than the frequency of a volume pulse wave of a living body, and receives light emitted from the light emitting element and external light transmitted and / or reflected by a living body to be measured. A photoelectric volume plethysmograph having a light receiving element for generating a current corresponding to the amount of received light, and a converting means for converting the current generated by the light receiving element into a voltage and outputting the voltage; A high-frequency cutoff filter that removes a high-frequency signal component caused by the blinking of the light emitting element from the voltage signal to be output; and amplifies and outputs a difference between a voltage output from the high-frequency cutoff filter and a preset reference voltage. And a means for converting the output voltage of the differential amplifier into a current signal and subtracting the current signal from the current generated by the light receiving element.