JP2023134369A - sensor system - Google Patents

Abstract

To provide a sensor system the SNR of which is improved.SOLUTION: A sensor system (1) comprises: a sensor (gas sensor 10) that has a light-emitting element (11) and a detection element (light-receiving element 12) that detects a signal based on light emitted from the light-emitting element; and a computation device (20) that computes one measurement value using the signal detected in an ON section and the signal detected in a plurality of OFF sections, where the ON section is a section in which the light-emitting element emits light, the OFF section is a section in which the light-emitting element does not emit light.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to sensor systems.

In recent years, detection devices include a light emitting part that emits infrared rays and a light receiving part that receives infrared rays that have passed through the gas to be detected (e.g., alcohol or carbon dioxide), and detect the concentration of the gas by using the infrared absorption characteristics of the gas. A non-dispersive infrared (NDIR) type gas sensor is being developed (for example, Patent Document 1).

Japanese Patent Application Publication No. 2004-271518

Here, in systems using sensors including NDIR gas sensors, further improvement in SNR (signal to noise ratio) is required.

An object of the present disclosure, made in view of such circumstances, is to provide a sensor system with improved SNR.

[1] A sensor system according to an embodiment of the present disclosure includes:
A sensor including a light emitting element and a detection element that detects a signal based on light emitted from the light emitting element;
The signal detected in the ON period and the signal detected in the plurality of OFF periods, where a period in which the light emitting element emits light is an ON period, and a period in which the light emitting element does not emit light is an OFF period. and an arithmetic device that calculates one measurement value using .

[2] As an embodiment of the present disclosure, in [1],
A duty, which is a ratio of the ON interval to the sum of the ON interval and the OFF interval, is 50% or more.

[3] As an embodiment of the present disclosure, in [1] or [2],
The calculation device calculates the measured value using the signals detected in the plurality of OFF periods before and after the ON period.

[4] As an embodiment of the present disclosure, in [3],
The number of the OFF intervals before the ON interval is the same as the number of OFF intervals after the ON interval.

[5] As an embodiment of the present disclosure, in any one of [1] to [4],
The calculation device calculates the measurement value by weighting each of the signal detected in the ON period and the signal detected in the OFF period.

[6] As an embodiment of the present disclosure, in [5],
The arithmetic device performs the weighting so that the influence on the signal due to variation in the object to be measured is canceled out.

[7] As an embodiment of the present disclosure, in any one of [1] to [6],
The detection element is a light receiving element, the sensor is an NDIR type gas sensor, and the measured value is a gas concentration of a gas to be detected.

[8] As an embodiment of the present disclosure, in any one of [1] to [6],
The detection element is a microphone, the sensor is a photoacoustic gas sensor, and the measured value is the gas concentration of the gas to be detected.

[9] As an embodiment of the present disclosure, in any one of [1] to [6],
The detection element is a light receiving element, the sensor is an optical pulse wave sensor, and the measured value is a pulse wave.

[10] As an embodiment of the present disclosure, in any one of [1] to [6],
The detection element is a light receiving element, the sensor is a distance measuring sensor, and the measured value is a distance to an object.

[11] As an embodiment of the present disclosure, in any one of [1] to [10],
A repetition period of the ON section and the OFF section is 1 second or less.

[12] As an embodiment of the present disclosure, in any one of [1] to [11],
The duty, which is the ratio of the ON interval to the sum of the ON interval and the OFF interval, is within the range expressed by the following formula (a), where k is the number of the plurality of OFF intervals.

[13] As an embodiment of the present disclosure, in any one of [1] to [11],
The duty, which is the ratio of the ON interval to the sum of the ON interval and the OFF interval, is within the range expressed by the following formula (b), where k is the number of the plurality of OFF intervals.

According to the present disclosure, a sensor system with improved SNR can be provided.

FIG. 1 is a diagram illustrating a configuration example of a sensor system according to an embodiment of the present disclosure. FIG. 2 is a diagram showing a configuration example of a gas sensor included in the sensor system of FIG. 1. FIG. 3 is a diagram illustrating the timing of the drive signal and the detection signal. FIG. 4 is a diagram illustrating ON intervals and OFF intervals used according to the number of OFF interval data. FIG. 5 is a diagram illustrating changes in parameters indicating improvement in SNR. FIG. 6 is a diagram for explaining weighting.

Hereinafter, a sensor system according to an embodiment of the present disclosure will be described with reference to the drawings. In each figure, the same or corresponding parts are given the same reference numerals. In the description of this embodiment, the description of the same or corresponding parts will be omitted or simplified as appropriate.

<Sensor system>
FIG. 1 is a diagram showing a configuration example of a sensor system 1 according to the present embodiment. The sensor system 1 includes a sensor including a light emitting element 11, a detection element that detects a signal based on the light emitted from the light emitting element 11, and an arithmetic device 20 that calculates a measured value. Here, the detection elements and measured values in the sensor system 1 are not limited. In this embodiment, the description will be made assuming that the detection element is the light receiving element 12, the sensor is the NDIR type gas sensor 10, and the measured value is the gas concentration of the gas to be detected. The NDIR method uses the fact that the wavelength of infrared rays absorbed differs depending on the type of gas, and measures the gas concentration by detecting the amount of absorption. Gases to be measured include, for example, alcohol and carbon dioxide, but are not limited to these. In this embodiment, the infrared rays absorbed by the gas to be detected correspond to a signal based on the light emitted from the light emitting element 11.

Here, the sensor system 1 may have the following configuration. In the sensor system 1, for example, the detection element may be a microphone, the sensor may be a photoacoustic gas sensor, and the measured value may be the gas concentration of the detected gas. The photoacoustic method measures gas concentration by picking up the vibrations of gas molecules that absorb light as sound using a high-performance microphone. At this time, the vibration of the gas molecules that absorbed the light corresponds to a signal based on the light emitted from the light emitting element 11. Further, in the sensor system 1, for example, the detection element may be the light receiving element 12, the sensor may be an optical pulse wave sensor, and the measured value may be a pulse wave. A pulse wave is a waveform that indicates changes in blood vessel volume that occur as the heart pumps blood. Further, in the sensor system 1, the detection element may be the light receiving element 12, the sensor may be a distance measuring sensor, and the measured value may be the distance to the target object. The distance sensor measures the distance based on the time it takes for the light emitted from the light emitting element 11 to be reflected and received. Here, a proximity sensor that detects when a target object approaches may be used as the distance measurement sensor. Although the configuration of the sensor system 1 is not limited in this way, the following description will be made assuming that the sensor is an NDIR type gas sensor 10.

<Gas sensor>
In this embodiment, the gas sensor 10 includes a light emitting element 11 and a light receiving element 12. The gas sensor 10 may further include a light emitting element drive section 13 and a storage section 15.

(Light emitting element)
The light emitting element 11 outputs light including a wavelength that is absorbed by the gas to be measured. The light emitting element 11 may be, for example, a light emitting diode (LED) or a micro electro mechanical systems (MEMS) light source. In this embodiment, the light emitting element 11 is an infrared LED.

Here, the wavelength of the infrared rays may be 2 μm to 12 μm. The region of 2 μm to 12 μm has many absorption bands unique to various gases, and is a wavelength range particularly suitable for use in the gas sensor 10. For example, absorption bands exist for methane at a wavelength of 3.3 μm, carbon dioxide at a wavelength of 4.3 μm, and alcohol (ethanol) at a wavelength of 9.5 μm. In this embodiment, the gas to be detected contains alcohol, and infrared rays in a wavelength band including 9.5 μm are used.

(Light receiving element)
The light receiving element 12 is sensitive to a light band including wavelengths absorbed by the gas to be measured. As a specific example, the light receiving element 12 may be a quantum sensor such as a photodiode with a PIN structure. In this embodiment, the light receiving element 12 is a quantum infrared sensor.

(Light emitting element drive unit)
The light emitting element driving section 13 outputs a drive signal to the light emitting element 11 to cause the light emitting element 11 to emit light at a predetermined brightness. The light emitting element driving section 13 may cause the light emitting element 11 to emit light by, for example, constant current driving. Furthermore, the light emitting element driving unit 13 generates a drive signal (drive voltage) adjusted according to the environmental temperature based on the correction parameters acquired from the storage unit 15, and sends the temperature-corrected drive signal to the light emitting element 11. You can output it. Here, the correction parameters may be generated based on a known temperature correction method.

(Storage part)
The storage unit 15 may store a program that causes the processor included in the gas sensor 10 to function as the light emitting element driving unit 13 when the function of the light emitting element driving unit 13 is realized by software. Furthermore, the storage unit 15 may store correction parameters used to generate a drive signal for the light emitting element drive unit 13 and the like. The storage unit 15 may include one or more memories. The memory is, for example, a semiconductor memory, a magnetic memory, or an optical memory, but is not limited to these and may be any memory.

(mold resin)
The gas sensor 10 may have a configuration in which the above-mentioned components are packaged together with an optical member, as shown in FIG. In the gas sensor 10, at least one of a light emitting element 11 and a light receiving element 12 is sealed with mold resin. In the example of FIG. 2, the light emitting element 11 and the light receiving element 12 are sealed with a mold resin together with the storage section 15 and the IC 30. Here, the IC 30 includes one or more processors and realizes the function of the light emitting element driving section 13 in this embodiment.

(Light guide part)
Further, a reflecting section 17 is provided so as to reflect the light 18 emitted from the light emitting element 11 and make it enter the light receiving element 12 . That is, in this embodiment, the gas sensor 10 includes the reflection section 17 that functions as a light guide section that guides the light 18 from the light emitting element 11 to the light receiving element 12. In the example of FIG. 2, the reflecting section 17 is a concave mirror. The reflective surface of the reflective section 17 may be made of a metal with high reflectance, such as aluminum and gold.

(filter)
As shown in FIG. 2, in this embodiment, the gas sensor 10 includes a filter 16 that is provided on at least one of the light emitting element 11, the light receiving element 12, and the light guide section to limit the wavelength of the light 18. For example, when the gas to be measured is alcohol (ethanol), the filter 16 may be a bandpass filter that passes infrared rays in a wavelength band (typically around 9.5 μm) in which ethanol absorbs a large amount of infrared rays.

(diode structure)
In this embodiment, the light emitting element 11 is an infrared LED. Furthermore, in this embodiment, the light receiving element 12 is a quantum infrared sensor. At least one of the light emitting element 11 and the light receiving element 12 contains at least one of indium and gallium and at least one of arsenic and antimony as a material, and has a diode structure consisting of at least two types of layers, a P-type semiconductor and an N-type semiconductor. may have.

<Arithmetic device>
In this embodiment, the calculation device 20 includes a signal acquisition section 21 and a calculation section 22. The arithmetic device 20 may be a device including a processor that executes arithmetic operations, and may be realized by, for example, a computer. When the calculation device 20 is a computer, the signal from the gas sensor 10 may be acquired by a communication device included in the computer, and the gas concentration may be calculated by a CPU (central processing unit) or the like included in the computer. As another example, the computing device 20 may be realized by the IC 30 of the gas sensor 10. At this time, the gas sensor 10 is a device that integrates the arithmetic device 20, and can constitute the sensor system 1 by itself.

The functions of the signal acquisition section 21 and the calculation section 22 may be realized by software. For example, one or more programs may be stored in a storage device that can be accessed by a processor included in the arithmetic device 20. When the program stored in the storage device is read by a processor included in the arithmetic device 20, the program may cause the arithmetic device 20 to function as the signal acquisition unit 21 and the arithmetic unit 22.

(Signal acquisition section)
The signal acquisition unit 21 acquires at least a detection signal output from the light receiving element 12 and a drive signal for the light emitting element 11. The signal acquisition unit 21 outputs the acquired signal to the calculation unit 22.

(calculation section)
The calculation unit 22 calculates the gas concentration based on the signal acquired by the signal acquisition unit 21. For example, the calculation unit 22 calculates the amount of light received at a wavelength that is absorbed by the gas to be measured from the detection signal of the light receiving element 12, and compares it with the amount of light received when the gas to be measured is not present, thereby determining the gas concentration. can be calculated. Further, the calculation unit 22 executes calculation processing for improving the SNR, which will be described below.

(arithmetic processing)
Here, the light emitting element 11 emits light for a predetermined time according to a drive signal from the light emitting element driving section 13, passes through a period in which it does not emit light, and then emits light again for a predetermined time. FIG. 3 is a diagram illustrating the timing of the drive signal output from the light emitting element drive unit 13 and the detection signal output from the light receiving element 12. As shown in FIG. 3, an ON period and an OFF period are determined according to the drive signal. The ON section is a section in which the light emitting element 11 emits light. Further, the OFF period is a period in which the light emitting element 11 does not emit light. In this embodiment, during the operation of the light emitting element 11, the ON period and the OFF period are repeated at a period T. In the example of FIG. 3, the period T is 200 ms, but is not limited to a specific value. Further, the ON period and the OFF period may be different periods as in the example of FIG. 3, or may be the same period (for example, 100 ms each). The ON section and the OFF section are sometimes simply written as ON and OFF in the figures. Further, in the example of FIG. 3, the drive signal and the detection signal are shown normalized with the maximum value being 1. Regarding the detection signal, an integral interval I _ON and an integral interval I _OFF are determined with 0.5 as a reference. The detection signal is integrated in the integration interval _ION , and the amount of light received when the light emitting element 11 is emitting light is calculated. Further, the detection signal is integrated in the integration interval I _OFF , and the amount of light received when the light emitting element 11 is not emitting light is calculated. The error R due to the transient response of the detection signal will be described later.

Here, by reducing noise in the sensor system 1, the SNR can be improved. An example of noise is dark current. Due to the influence of dark current, the amount of received light does not become zero even in the OFF period. Conventionally, in order to remove the influence of dark current, correlated double sampling (correlated double sampling) is used to calculate the difference between the data signal and the reference signal, using the amount of light received in the OFF section in period T as the reference signal and the amount of light received in the ON section as the data signal. CDS (correlated double sampling) was sometimes performed. However, there is also an instantaneous change in dark current, and there is a limit to the improvement of SNR with conventional methods. The sensor system 1 according to the present embodiment has a higher SNR than the conventional technology by calculating one measurement value using a signal detected in an ON period and a signal detected in a plurality of OFF periods. can be improved.

FIG. 4 is a diagram illustrating ON intervals and OFF intervals used according to the number of OFF interval data. When the number of OFF interval data is 1, it corresponds to conventional correlated double sampling. In this embodiment, the calculation device 20 sets the number of OFF interval data to 2 or more, calculates a moving average of the amount of light received in those OFF intervals, and calculates the difference between the amount of received light in the ON interval and the value of the calculated moving average. do. For example, when the NDIR gas sensor 10 is mounted on a vehicle to measure alcohol, it is necessary to measure when the user blows into the sensor, so a fast response is required for the ON section. On the other hand, since the OFF period is a period in which no measurement is performed, quick response is not required, and it is possible to eliminate the influence of instantaneous changes in noise by averaging a plurality of OFF periods with different timings.

Here, the numerical value attached to the period T in FIG. 4 is expressed as a negative value before the reference and a positive value after the reference, with the period T used for calculation of the amount of light received in the ON period as a reference (0). In the example of FIG. 4, the calculation device 20 calculates the measured value using signals detected in a plurality of OFF periods before and after the ON period. However, the calculation device 20 may perform the calculation using a plurality of OFF periods before the ON period or a plurality of OFF periods after the ON period. Furthermore, in the example of FIG. 4, the number of OFF sections before the ON section is the same as the number of OFF sections after the ON section. Noise such as dark current often changes almost linearly before and after the ON period. Therefore, by using a plurality of OFF intervals before and after the ON interval, it is possible to cancel out the approximately linear change in noise and more accurately calculate the noise at the time of the ON interval. Furthermore, by setting the same number of OFF sections before and after the ON section, the effect of canceling out the linear variation described above can be enhanced.

As described above, the ON period and the OFF period are arbitrarily determined, but the duty, which is the ratio of the ON period to the sum of the ON period and the OFF period (period T), affects the improvement of the SNR. Furthermore, the number of OFF sections (the number of OFF section data) used by the calculation device 20 to calculate one measurement value affects the improvement of the SNR. First, the duty is expressed by the following equation (1).

Here, "m" is the period of the ON section. Further, "n" is the period of the OFF section. For example, if the ON period and the OFF period are each 100 ms, the duty is 50%.

Further, P _SNR, which is a parameter for relatively comparing SNR, is expressed by the following equation (2). A larger value of P _SNR indicates that the SNR is improved.

Here, "k" is the number of OFF sections. For example, when the number of OFF section data is 10 (see FIG. 4), "k" is 10. Further, the period T is expressed by the following equation (3).

From equations (1), (2), and (3), the P _SNR can also be expressed by the following equation (4).

Here, when the period T is a constant, the duty at which the P _SNR is maximum is expressed by the following equation (5) with k≧2.

At this time, the maximum value P _{SNR_MAX} of P _SNR is expressed by the following equation (6). Here, the period T is a constant.

Further, the duty at which the P _SNR is maximum when k=1 is 1/2, which is 50%. At this time, the maximum value of P _SNR is √T/2. Setting the number of OFF section data to two or more has a greater effect on improving the SNR than when the number of OFF sections is one. When the SNR is expressed by Equation (4), the duty that improves from √T/2 (the maximum value of SNR when the number of OFF section data is 1) is within the range expressed by Equation (7) below.

FIG. 5 shows changes in the P _SNR value with respect to the number of OFF section data for each duty. As shown in FIG. 5, for example, when the number of OFF section data is 10, even if the duty exceeds 50%, the SNR is improved more than when the duty is 50%. When the duty exceeds 50%, the detection time in the OFF period becomes shorter than that in the ON period in one cycle T, leading to a relative decrease in the accuracy of data in the OFF period. However, in this embodiment, the data of the OFF interval is given as the average of a plurality of OFF intervals, so sufficient data accuracy can be ensured. When the number of OFF section data is 10, even if the detection time in the OFF section is shorter than the ON section in one cycle T, the SNR is improved compared to when the duty is 50%.

Here, as shown in FIG. 5, even if the number of OFF interval data is less than 10, by setting an appropriate duty, the SNR can be improved more than when the duty is 50%. For example, when the number of OFF section data is 4, the SNR can be improved by setting the duty to 60% or 70%. For example, when the calculation device 20 calculates a measured value using OFF intervals before and after an ON interval, if the number of OFF interval data increases, it takes time from acquiring the data of the ON interval to completing the calculation. In other words, it takes time to wait for the data of the OFF period obtained after the ON period. Therefore, it is preferable that the sensor system 1 adjusts the duty and the like based on the required time to output the measured value, the required SNR improvement, and the like.

When the number of OFF section data is 2 or more and the SNR is expressed by equation (4), the duty range in which the SNR is improved more than when the duty is 50% is expressed by equation (8) below. From the viewpoint of improving the SNR, the duty is preferably within the range of formula (8).

Here, the calculation device 20 may calculate the measurement value by weighting each of the signals detected in the ON period and the signals detected in the OFF period. As shown in FIG. 3, the integral interval _ION does not coincide with the ON interval, and a deviation occurs. Further, the integral interval I _OFF does not coincide with the OFF interval, and a deviation occurs. The deviation occurs due to the transient response of the detection signal, and is affected by fluctuations in the object to be measured (in this embodiment, the gas to be detected). The error R shown in FIG. 3 indicates the influence on the detection signal due to variations in the measurement target. The error R is caused by the detection signal in the ON period being shifted to the OFF period due to the influence of fluctuations in the object to be measured. In other words, the amount of received light corresponding to the error R should be counted in the ON period, not in the OFF period. It is preferable that the calculation device 20 performs weighting and calculates the measured value so that such an error R is canceled out.

As an example, assume that the duty is 70% and the number of OFF section data is 8. The upper diagram in FIG. 6 shows the initial weighting performed by the arithmetic unit 20. Since it is necessary to cancel the dark current by weighting, a positive value is used in the ON period, and a negative value is used in the OFF period. The arithmetic device 20 weights the signal detected in one ON period by +1. In addition, the calculation device 20 calculates the weighting for each of the signals detected in the eight OFF sections by dividing the negative value of the weighting 70/30 (2.333) when the duty is 70% by 8 -0 It is set at .292. Here, it is assumed that the error R shown in FIG. 3 is 9.6% with respect to the integral value of the amount of light received in the ON section and the OFF section. The arithmetic device 20 further performs adjustment regarding the error R. As shown in the lower diagram of FIG. 6, the arithmetic device 20 adjusts the weighting in the ON period so that it increases by a total of 9.6% from the initial state. Specifically, the arithmetic device 20 adds weighting of +0.012 to each of the eight ON period signals. Furthermore, for the eight OFF sections, the error R per one OFF section is 1.2%. The arithmetic unit 20 performs adjustment by adding a weighting of -0.028, which is 1.2% of -2.333, to each of the eight OFF sections. In other words, the weighting of -0.292 for each of the eight OFF period signals is adjusted by -0.028, resulting in -0.320. In this way, the arithmetic device 20 further improves the SNR by adjusting the weighting of the ON interval and the weighting of the OFF interval so that the influence of fluctuations in the measurement target (error R) becomes zero in total. can be done.

As described above, the sensor system 1 according to the present embodiment can improve the SNR with the above configuration.

Although the embodiments of the present disclosure have been described based on the drawings and examples, it should be noted that those skilled in the art can easily make various changes or modifications based on the present disclosure. It should therefore be noted that these variations or modifications are included within the scope of this disclosure. For example, functions included in each component can be rearranged so as not to be logically contradictory, and a plurality of components can be combined into one or divided. Embodiments according to the present disclosure can also be realized as a program executed by a processor included in the device or a storage medium recording the program. It is to be understood that these are also encompassed within the scope of the present disclosure.

1 sensor system 10 gas sensor 11 light emitting element 12 light receiving element 13 light emitting element drive section 15 storage section 16 filter 17 reflection section 18 light 20 arithmetic unit 21 signal acquisition section 22 arithmetic section 30 IC

Claims (13)

A sensor including a light emitting element and a detection element that detects a signal based on light emitted from the light emitting element;
The signal detected in the ON period and the signal detected in the plurality of OFF periods, where a period in which the light emitting element emits light is an ON period, and a period in which the light emitting element does not emit light is an OFF period. A sensor system comprising: an arithmetic device that calculates one measurement value using the . The sensor system according to claim 1, wherein a duty, which is a ratio of the ON period to a total of the ON period and the OFF period, is 50% or more. The sensor system according to claim 1 or 2, wherein the calculation device calculates the measured value using the signals detected in the plurality of OFF periods before and after the ON period. The sensor system according to claim 3, wherein the number of the OFF intervals before the ON interval is the same as the number of the OFF intervals after the ON interval. 3. The sensor system according to claim 1, wherein the calculation device calculates the measured value by weighting each of the signal detected in the ON period and the signal detected in the OFF period. 6. The sensor system according to claim 5, wherein the arithmetic unit performs the weighting so that influence on the signal due to variation in the object to be measured is canceled out. 3. The sensor system according to claim 1, wherein the detection element is a light receiving element, the sensor is an NDIR gas sensor, and the measured value is a gas concentration of a gas to be detected. 3. The sensor system according to claim 1, wherein the detection element is a microphone, the sensor is a photoacoustic gas sensor, and the measured value is a gas concentration of a gas to be detected. The sensor system according to claim 1 or 2, wherein the detection element is a light receiving element, the sensor is an optical pulse wave sensor, and the measured value is a pulse wave. The sensor system according to claim 1 or 2, wherein the detection element is a light receiving element, the sensor is a distance measuring sensor, and the measured value is a distance to a target object. The sensor system according to claim 1 or 2, wherein a repetition period of the ON section and the OFF section is 1 second or less. 1 or 2, wherein the duty, which is the ratio of the ON interval to the sum of the ON interval and the OFF interval, is within a range expressed by the following formula (a), where k is the number of the plurality of OFF intervals. 2. The sensor system according to 2.
1 or 2, wherein the duty, which is the ratio of the ON interval to the sum of the ON interval and the OFF interval, is within a range expressed by the following formula (b), where k is the number of the plurality of OFF intervals. 2. The sensor system according to 2.

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