JPS61199832A - Pulse signal detector - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Object of the Invention] [Industrial Application Field] The present invention relates to a heartbeat signal detection device that measures a person's heartbeat using an optical sensor, and in particular to prevent malfunctions due to the influence of external light. This invention relates to a signal processing circuit for.

[Prior Art] For example, a reflective optical sensor having a structure in which a light-emitting element and a light-receiving element are arranged close to each other and the light-emitting axis of one light-emitting element and the light-receiving axis of the light-receiving element are oriented in the same direction is attached to a part such as a human finger. By making contact, an electrical signal corresponding to the heartbeat can be obtained. this is,
This is because the blood flow rate in that area changes periodically in synchronization with the heartbeat, and the light reflectance changes accordingly.

In this type of heartbeat detection device, when there is external light, it enters the light receiving element together with the reflected light, so the output level of the optical sensor is affected by the external light, so the external light level exceeds a predetermined level. In this case, correct detection results cannot be obtained. For this reason, commercially available portable heart rate monitors are equipped with a black light-shielding cover to prevent the influence of extraneous light, and the detector and finger are covered with the cover during measurement.

However, it is impossible to perform general work or drive a car while wearing such a cover.

In this type of detection device, if the light emitting element is driven by an AC signal with a relatively high frequency and an AC signal whose amplitude is modulated according to the heartbeat signal is obtained on the light receiving side,
It is known that the influence of external light can be reduced to some extent.

[Problems to be Solved by the Invention] However, when measuring outdoors where the external light is too strong, such as when used in a car, the external light causes the signal to reach a saturation level in the signal processing circuit.

As shown in FIG. 4, there are times when a signal appears in the demodulated output and times when it does not appear, which makes measurement impossible.

In order to prevent saturation of the signal level due to external light, the amplification degree of the signal processing circuit may be reduced, but if this is done, the amplitude of the heartbeat signal will also be reduced, making malfunctions more likely to occur.

SUMMARY OF THE INVENTION An object of the present invention is to provide a heartbeat signal detection device that can obtain a normal signal even when receiving strong external light.

[Configuration of the Invention [Means for Solving the Problems] In order to achieve the above object, the present invention uses light-emitting elements that have a relatively high frequency (for example, IKH) with respect to heartbeat signals.
z) In addition to driving with an alternating current signal SA, the light receiving circuit is provided with a phase shift circuit that causes a phase change of substantially 180 degrees with respect to the frequency of the signal SA, and the direct signal SB from the light receiving element and the output signal SC of the phase shift circuit are provided. are applied to each input terminal of the differential amplifier.

[Operation] When the light emitting element is driven with an alternating current signal SA, the signal SB outputted by the light receiving element is amplitude modulated by a heartbeat signal that changes at a period of 1 second, for example. An AC modulated signal component SBI with a waveform similar to that of the signal SA, and a DC component SB2 depending on the external light level.
and a DC bias component SB3. The signal SC is
The signal SB is phase-shifted by 180 degrees with respect to the frequency of SA, but its DC component (SB2 and SB3
corresponding to ), no matter what time you look at it, it is virtually the signal S.
It is on the same level as B.

Therefore, the DC level of the differentially amplified signal SB and SC is equal to zero. That is, the component of the external light is thereby canceled, and the influence of the external light does not appear on the output of the differential amplifier. Regarding the AC modulation signal component, since the signal SB and the signal SC are equal to each other with their polarities inverted, the output of the differential amplifier has a 2.
An AC modulated signal with an amplitude G times as large is obtained (G is the gain of the differential amplifier).

[Example] The present invention will be described in detail below with reference to one drawing.

FIG. 1 shows the configuration of one type of on-vehicle heart rate monitor implementing the present invention. Referring to FIG. 1, detectors SE1, SE2 .
Display device DSP, oscillation circuit OSC.

It is equipped with a driver DRV, a buffer BFF, a phase shift circuit PH8, a differential amplifier circuit DFA, a demodulation circuit DEM, an analog/digital converter, and a digital control circuit CPU.

In this example, the detectors SEI and SF3 are fixed on the steering wheel of the automobile, and the display DSP is mounted on the pad part in the center of the steering wheel. Each detector SEI, SE2 consists of a light emitting diode placed in its center and four phototransistors placed around it, all of which have their optical axes directed above the steering wheel.

The oscillation circuit ○SC generates a sine wave signal of IKHz in this example. The driver DRV current-amplifies the signal from the oscillation circuit ○SC and uses the amplified signal to trigger the detector SEl.
and energizes the light emitting diode of SF3. Buffer B
The FF supplies power to the phototransistors of the detectors SEI and SF3, and converts the current flowing through the phototransistors into voltage.

When the driver touches the detector SEI or SF3 with his finger, the light emitted from the light emitting diode of each detector is reflected by the blood in the finger, and the reflected light reaches the phototransistor. The level of this reflected light changes in synchronization with the driver's heartbeat. In other words, the light emitting diode is l K H
Since it is energized by the sine wave of z, the output of the phototransistor contains I
A KHz AC signal is obtained.

The phase shift circuit PH5 is a circuit that generates a phase delay of 180 degrees with respect to an IKHz signal. Differential amplifier DFA amplifies the level of the difference between the output signal of buffer BFF and the output signal of moving circuit PH8. The decoder circuit OE M has the function of a low-pass filter, and extracts a relatively low-frequency heartbeat signal component contained therein from the IKHz modulation signal.

The digital control circuit CPU includes a digital circuit such as a microcomputer, and converts the analog heartbeat signal obtained from the output of the demodulation circuit DEM into an analog/digital converter AD.
The heart rate is converted into digital information via C, the period of the heartbeat is calculated based on the digital information, the heart rate is calculated, and the result is displayed numerically on the display device DSP.

FIG. 2 shows details of the configuration of the analog signal processing circuit portion of the apparatus shown in FIG. 1. Referring to FIG. 2, the detector S
The light emitting diodes EI and SF3 are connected in series to the output terminal of the driver DRV, and the eight phototransistors are connected in parallel to the input terminal of the buffer BFF. In this example, the phase shift circuit PHS includes an operational amplifier,
It is constructed by connecting two stages of phase adjustment circuits in series, including an integrating circuit consisting of a resistor and a capacitor. The differential amplifier circuit DFA is composed of an operational amplifier and a plurality of resistors.

FIG. 3 shows signals VA of each part shown in the circuit of FIG. 2.

An example of waveforms of VB, VC, and VD, external light level, and electrocardiogram is shown. The circuit operation will be explained with reference to each episode.

The energization level of the light emitting diode of each detector SEL, SE2 changes at a regular cycle of IKHz like the signal VA. When a person's finger or the like is facing the detection surface of the detector SEI or SF3, a portion of the light emitted from the light emitting diode is reflected by the finger, especially the blood in the blood vessel, and the reflected light is received by the phototransistor. be done. Since the blood flow rate in the blood vessels fluctuates in accordance with the heartbeat, the reflectance caused by this fluctuates in synchronization with the heartbeat.

Therefore, the intensity of light received by the phototransistor is
As shown in the signal VB, it fluctuates in a sinusoidal manner with a period of IKHz, and the amplitude of the sine wave fluctuates periodically with a period of about 1 second in synchronization with the heartbeat.

However, since the current flowing through the phototransistor includes DC components such as the dark current of the phototransistor itself and the photocurrent due to external light, the voltage of the signal VB is at least 0 or more, and the average level of the signal, that is, the DC The level changes depending on the level of extraneous light.

If this DC level fluctuation causes the highest level of the entire signal to exceed the saturation voltage near the power supply voltage (Vcc in this example), the output of the circuit will be affected.

It becomes impossible to obtain an IKHz signal, and the heartbeat signal cannot be demodulated.

However, the signal level before amplification is small.

The level of the signal VB appearing at the output of the buffer BFF never reaches the saturation point6. Also, what is the amplification degree of the phase shift circuit PH8? Therefore, the level of the output signal VC also does not reach the saturation point.

Signal VB and signal VC are 180% for IKHz signal.
Therefore, for the IKHz signal component, VB and VC can be seen as inverted signals, so if they are amplified by a differential amplifier with a gain of 1, the output terminal will have a phase difference of .

A signal component of 1 KHz with twice the amplitude of the input (No. 1) is obtained. However, when looking at the DC components of the signal VB and the signal VC, that is, the average level component.

Both levels are virtually the same. Therefore, if two things of the same level are differentially amplified, the result will be zero. In other words, the DC component is canceled by the differential amplifier,
Its output does not have a direct current component.

Therefore, the signal VD at the output terminal of the differential amplifier @DFA contains only the 1 KHz AC component modulated by the heartbeat signal. However, in this example, the positive voltage line vcC and the ground line are connected to the power supply line of the operational amplifier of the differential amplifier DFA, so the negative signal component is cut. Since this signal VD does not include a DC component due to external light, even if the amplification degree of the differential amplifier DFA and subsequent amplifiers is set high, the signal level will not be saturated when the level of external light is large. do not have.

When the signal VD is passed through the demodulation circuit DEM, high-frequency components are averaged, and low-frequency signal components, that is, components of the heartbeat signal, are extracted as fluctuations in the average level. That is the signal VE. The signal VE is as shown in FIG.
Even when the external light level increases, a pulsed heartbeat signal synchronized with the electrocardiogram waveform is obtained as the signal VE.

The heartbeat cycle can be determined by A/D converting this signal and measuring the time from the peak of the heartbeat pulse to the next peak. The reciprocal of the heartbeat cycle per minute becomes the heartbeat rate to be output to the display DSP.

In addition, in the above embodiment, the phase shift circuit PH8 is
Although a circuit that generates a phase difference of 180 degrees with respect to a Hz signal is used, a circuit that generates a phase difference that is an odd multiple of 180 degrees can be considered to have substantially the same function as this circuit.

Further, in the above embodiment, the light emitting element is energized by a sine wave, but a waveform including harmonics, such as a triangular wave, a sawtooth wave, a rectangular wave, etc. can also be used. However, in that case, 18
It is difficult to configure a phase shift circuit to generate a phase difference of 0 degrees. Of course, if a low-pass filter is provided at the output of the buffer BFF to pass only frequency components below the fundamental wave.

As with the previous embodiment, a simple configuration is sufficient.

[Effects] As described above, according to the present invention, the influence of extraneous light can be removed by the signal processing circuit, so heartbeat detection can be performed outdoors, such as inside a car, without having to attach a light-shielding cover to the sensor or finger. Become.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of one type of heart rate detection device embodying the present invention. FIG. 2 is an electrical circuit diagram showing details of the analog signal system of FIG. 1. FIG. 3 is a timing chart showing an example of waveforms of signals at each part in FIG. FIG. 4 is a waveform diagram showing a conventional example. SEI, SF3: Detector ○SC: Oscillation circuit DRV: Driver BFF: Buffer (light receiving circuit) P) (S: Phase shift circuit DFA: Differential amplifier circuit DEM: Demodulation circuit DSP: Display

Claims (2)

[Claims] (1) At least one light-emitting element; at least one light-receiving element disposed near the light-emitting element; an oscillation circuit that generates an alternating current signal with a cycle shorter than the cycle of a heartbeat signal; a light-emission energizing circuit that energizes; a light-receiving circuit that obtains an electric signal from the light-receiving element; an input terminal is connected to an output terminal of the light-receiving circuit; a phase shift circuit that generates a phase difference of degrees; a differential amplifier circuit whose input terminals are connected to the output terminal of the light receiving circuit and the output terminal of the phase shift circuit;
and a demodulation circuit whose input terminal is connected to the output terminal of the differential amplifier circuit. (2) The heartbeat signal detection device according to claim (1), wherein the oscillation circuit is a sine wave excavation circuit.