JP3483432B2 - Reference value generator - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to CO2 detection, and more particularly to generation of a reference value for use as a reference signal of a sensor to be used. 2. Description of the Related Art In a CO2 sensor, a turbidity sensor, an oxygen deficiency sensor and the like, a sensor signal has a drift with time, and the drift is corrected using a reference value. For example, Japanese Patent No. 2507079 discloses that the maximum value of the output of the solid electrolyte CO2 sensor is sampled every day and used as a reference value. In the solid electrolyte CO2 sensor, the output increases when the CO2 concentration decreases, and this corresponds to the minimum value of the CO2 concentration. In Japanese Patent No. 2,507,099, the maximum value of the sensor output is allocated to 350 ppm of CO2, which is the concentration of CO2 in the atmosphere. However, the output of the CO2 sensor is liable to be influenced by various kinds of noises other than the drift, and it is questionable to use the maximum value of the sensor output as a reference value. From such a viewpoint, the inventor studied sampling of a reference value using data reflecting a longer-term sensor signal instead of the instantaneous value of the sensor output. That is, the phenomenon that the CO2 concentration is minimized and approaches the CO2 concentration in the atmosphere is almost a daily cycle, and it is considered that the reference value should not be generated from an instantaneous signal. [0004] The inventor created a histogram of the sensor signal over a period of about one day, and the peak of the histogram was the highest in CO.
2 It was thought that this corresponded to the situation where the concentration was low. However, as a result of the experiment, it has been found that in a building or the like in which the CO2 concentration is controlled by air conditioning, a peak of the histogram may occur at a point where the CO2 concentration is 800 ppm or the like. An object of the present invention is to generate a reference value corresponding to an almost clean environment even when the maximum frequency of a sensor signal occurs in a situation where the environment is polluted. According to the present invention, a CO2 sensor for detecting CO2 in an environment is provided, and the output of the sensor is learned in a time-series manner to use the sensor as a reference signal for the sensor. In a reference value generating device configured to generate a value, a means for creating a histogram from a time series of the output of the sensor, and searching for a plurality of peaks in the created histogram from the side where CO2 has a low concentration. , A minimum density peak detecting means for extracting the first peak and setting it as a reference value. The sensor is a CO2 sensor, particularly a solid electrolyte CO2 sensor. The histogram is formed, for example, on a one-day cycle, but may be determined in accordance with the cycle of environmental fluctuations. For example, the histogram is created over a period of about one hour to one month, preferably about four hours to one week. According to the present invention, a histogram is created from the time series of the output of the CO2 sensor. A histogram often has a plurality of peaks, and using the peak with the highest frequency results in an unexpected concentration as a reference value. Therefore, the histogram is searched from the side where the CO2 concentration is low, and the first peak is used as a reference value. In this manner, even when a plurality of peaks exist in the histogram, a reference value for a clean environment can be generated. FIG. 1 to FIG. 5 show an embodiment using a solid electrolyte CO2 sensor 2. FIG. The type of the CO2 sensor is arbitrary, and in addition to the solid electrolyte CO2 sensor 2 used in the embodiment, an NDI
RCO2 sensor or C using resistance value of metal oxide semiconductor
An O2 sensor or a CO2 sensor using the capacitance of a metal oxide semiconductor may be used. Each of these sensors has a signal drift and needs to be corrected by a reference value. FIG. 2 shows the structure of the CO 2 sensor 2.
Is a sodium ion conductor, 6 is a working electrode made of a mixture of gold and an alkali carbonate or a mixture of gold and an alkaline earth carbonate, and 8 is a reference electrode made of gold or the like. Reference numeral 10 denotes an alumina substrate, 12 denotes a heater such as a platinum heater, and 14 denotes a sealing glass for sealing the reference electrode 8. This CO2 sensor 2 is known. Returning to FIG. 1, reference numeral 20 denotes a thermistor for correcting the ambient temperature. The thermistor 20 is connected to the CO2 sensor 2 so that the response does not shift between the CO2 sensor 2 and the thermistor 20.
The sensor 2 is disposed inside a housing (not shown). As a result, the thermistor 20 receives heat from the CO2 sensor 2 and is heated to, for example, about 100.degree. The temperature of the thermistor 20 fluctuates in conjunction with the ambient temperature, and the fluctuation of the ambient temperature is measured by the thermistor 20. 22 is a buffer amplifier for amplifying the output of the CO2 sensor 2 (electromotive force between the electrodes 6/8), and 24 is a differential amplifier. 26 may not be a sensitivity adjustment amplifier, and 30 is a microcomputer for signal processing. In the microcomputer 30, 32
Is a bus, 34 is an AD converter, 36 is a temperature correction unit, which stores a reference temperature TSTD for temperature correction, temperature-corrects the electromotive force EMF of the CO2 sensor 2 with the output of the thermistor 20, and generates a temperature-corrected electromotive force EMF. * The temperature correction is performed for a change from the reference temperature TSTD, and the reference temperature TSTD is changed, for example, every day. Reference numeral 38 denotes a memory for storing a histogram of EMF *, for example, a histogram of EMF * for the past day. EMF * used for histogram
Is 20 minutes, for example, and the interval may be increased or decreased according to the capacity of the memory 38. Further, the histogram memory 38 may store, for example, an EMF * histogram for the past week instead of the past one day. Reference numeral 40 denotes a DA converter, which applies a reference voltage C for differential amplification to the positive input side of the differential amplifier 24 so that the difference between the output of the CO2 sensor 2 and the reference voltage C is AD-converted by the AD converter 34. . Reference numeral 42 denotes a minimum density peak detecting means, which detects CO2 from the peaks (generally, a plurality of peaks) in the histogram.
The EMF * of the peak corresponding to the minimum concentration is extracted, and a reference EMF is generated. Further, an offset is added to the reference EMF to obtain a reference voltage C output from the DA converter 40. Reference numeral 44 denotes a CO2 detector which literally calculates and outputs the CO2 concentration, or outputs EMF * as a reference EMF or a reference EMF.
In comparison with + H (H is a constant) or the like, a control signal such as ventilation is generated. FIGS. 3 and 4 show examples of the created histogram. The vertical axis is frequency, and the horizontal axis is the temperature-corrected electromotive force EM.
F *. Since EMF * increases when the CO2 concentration decreases, the vicinity of the maximum value of EMF * is a candidate for the reference value. 3 and 4 show data in a building subjected to air-conditioning control using the CO2 sensor 2 of the embodiment, and FIG. 3 shows three peaks. In FIG. 4, a third peak exists at a position apart from the first and second peaks. The third peak, which is the largest peak in FIG.
ppm. This is because air conditioning is performed so that the CO2 concentration becomes about 800 ppm. Therefore, the minimum density peak detecting means 42 converts the histogram into an EMF *
Search from the side where EMF is larger, and extract the peak with the largest EMF *. FIG. 5 shows the operation of the CO2 detecting device according to the embodiment. Explaining the conversion of the electromotive force to the reference EMF and other variables, the AD converted electromotive force EMFAD is decoded by equation (1), and the effects of differential amplification are removed, and equations (2) and (3) are used. Correct the temperature. EMF = EMFAD + (CK) (1) EMF: The AD converted electromotive force EMFAD is converted to the original electromotive force using the reference voltage C for differential amplification (EMF * STD + small constant J) and constant K. Reconstructed electromotive force EMF * = EMF−A · ΔT (2) ΔT = T−TSTD (3) EMF *: EMF obtained by temperature correction of EMF A: Primary coefficient ΔT of temperature dependency of CO 2 sensor The difference between the temperature T of the thermistor 20 and the reference temperature TSTD. The minimum density peak is obtained from the histogram. In the histogram, points with a frequency of 2 or more are candidates,
The search is started from the side where EMF * is large, and a decrease in frequency is detected. If there is a maximum value in the data before the frequency decreases, it is determined as the minimum density peak.
Those having a large MF * are defined as minimum density peaks. In FIG. 3 and FIG. 4, the search is started from the side where EMF * is large, and the first peak is set as the minimum concentration peak. The conversion from the minimum concentration peak to the reference EMF is performed by equation (4), and the conversion from the reference EMF and the EMF * at each time point to the CO2 concentration is performed by equation (5). Reference EMFNew = Minimum concentration Peak + A · △ (TSTDnew−TSTDOld) (4) Reference EMF: Temperature corrected reference electromotive force reference EMFNew New reference electromotive force TSTDNeW: New reference temperature (maximum temperature and minimum temperature at histogram creation (last day)) TSTDOLD: The oldest reference temperature minimum concentration Peak: The peak reference EMF-EMF * with the largest EMF * in the peak of the histogram = B · Ln (PCO2 / PCO2STD) (5) B: Conversion between electromotive force and CO2 concentration Coefficient PCO2: CO2 concentration PCO2STD: Reference CO2 concentration (400 ppm) The electromotive force of the CO2 sensor 2 is unstable for about several hours from the start of operation of the heater 12, and waits for eight hours after the power is turned on. After eight hours, for example, the temperature of the thermistor 20 at that time is set as a reference temperature TSTD, and EMF * at that time is set as a reference EMF. Next, the histogram memory 38 is cleared, and creation of a histogram is started. In the DA converter 40, a reference voltage C is generated, and the reference voltage C and the CO2
Are differentially amplified and AD converted by an AD converter 34 to obtain an electromotive force EMFAD. Next, the temperature correction unit 36
The MFAD is restored to the EMF, and the temperature is corrected using the difference ΔT between the thermistor temperature and the reference temperature TSTD at that time, to obtain a temperature-corrected electromotive force EMF *. Due to the differential amplification, for example, the accuracy of AD conversion is improved by about 16 times. From the difference between the reference EMF and EMF *, the CO2 concentration is determined according to equation (5).
Therefore, the CO2 concentration is obtained by the CO2 detection unit 44 and output. Here, the output of the microcomputer 30 is C
The O2 concentration may be output as a control signal for, for example, turning on / off a ventilation fan without directly outputting the concentration. In the embodiment, if the peak corresponding to the minimum CO 2 concentration is set as a reference value corresponding to 400 ppm of CO 2, the variance of the detection result (the sum of squares of the difference between the known concentration and the detection result) in CO 2 of a known concentration is: Set the maximum value of EMF * to CO2 350
It was about 1/2 of what was assigned to ppm.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a reference value generator according to an embodiment; FIG. 2 is a cross-sectional view of a CO2 sensor used in the embodiment; FIG. 3 is a characteristic diagram showing an example of a histogram of the CO2 sensor; FIG. 4 is a characteristic diagram showing an example of a histogram of a CO2 sensor. FIG. 5 is a flowchart showing the operation of the embodiment. DESCRIPTION OF SYMBOLS 2 CO2 sensor 4 sodium ion conductor 6 working electrode 8 reference electrode 10 alumina substrate 12 heater 14 sealing Stop glass 20 Thermistor 22 Buffer amplifier 24 Differential amplifier 26 Sensitivity adjustment amplifier 30 Microcomputer 32 Bus 34 AD converter 36 Temperature correction unit 38 Histogram memory 40 DA converter 42 Minimum density peak detection unit 44 CO2 detection unit

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shinichi Saku 2-12-19 Shibuya, Shibuya-ku, Tokyo Yamatake Inside Honeywell Co., Ltd. (72) Inventor Kazumasa Inaba 2-12-19 Shibuya, Shibuya-ku, Tokyo Yamatake Inside Honeywell Co., Ltd. (72) Inventor Kazuyuki Otsuka 1-3-5, Senba-Nishi, Minoh-shi Figaro Giken Co., Ltd. (72) Inventor Takashi Yamaguchi 1-3-5, Senba-Nishi, Minoo-shi Figaro Giken Inc. ( 72) Inventor Takeshi Nakahara 1-3-5, Senba-nishi, Minoh-shi Figaro Giken Co., Ltd. (56) References JP-A-3-162659 (JP, A) JP-A-3-226665 (JP, A) JP-A-5-249907 (JP, A) JP-A-1-292242 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 27/00-27/49 G01N 33/00-33 / 46 G01N 21/64

Claims (1)

(57) [Claim 1] A CO2 sensor for detecting CO2 in the environment is provided, and the output of the sensor is learned in a time series to use as a reference signal of the sensor. Means for generating a reference value, a means for generating a histogram from the time series of the output of the sensor, and searching for a plurality of peaks in the generated histogram from the side where the concentration of CO2 is low. A minimum density peak detecting means for extracting the first peak and setting it as a reference value.