JP2006300742A - Chemical substance detection device using oscillation frequency adjusting system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a chemical substance detection device having no problem of cost or a circuit design even if a high-speed vibrator is used, by solving the problem of cost increase or designing wherein when an oscillation frequency band of a vibrator becomes a high frequency band exceeding several-ten MHz, a low-cost TTLIC used in common at present cannot be used as it is. <P>SOLUTION: This chemical substance detection device utilizing a piezoelectric oscillator is provided with: a piezoelectric oscillation circuit 7 for oscillating with a resonance frequency of the piezoelectric oscillator; and a fixed frequency oscillation circuit 9 for oscillating with a fixed frequency. In the device, an output frequency of the piezoelectric oscillation circuit is mixed with an output frequency of the fixed frequency oscillation circuit by an analog mixing circuit 11 so as to output a beat (resonance) frequency, and a beat component frequency is extracted by a beat extraction circuit 13, and then guided to a frequency counter circuit 15, thereby to count the frequency. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a detection device that detects gaseous chemical substances present in the air, and more particularly to a detection device that detects changes in the resonance frequency of a piezoelectric vibrator.

Piezoelectric vibrators called quartz vibrators have a piezoelectric property that the resonance frequency changes in proportion to the mass change when the mass changes. Conventionally, gaseous chemistry that exists in the air using this piezoelectric property. There are detection devices for detecting substances, and they are widely used for measuring work environments and the like.
As shown in FIG. 6, a typical quartz resonator as a sensor element of a detection device has a quartz plate 31 sandwiched between two electrode plates 33, and a sensitive film 35 (gas molecule adsorption) on each of the two electrode plates 33. And a lead 37 is connected to the electrode plate 33. The crystal resonator is disposed in the sensor cell while being held by the holder 39. When gas molecules are adsorbed, the mass of the crystal unit increases, causing a decrease in the resonance frequency. The relationship between the change in frequency and the mass of the adhering substance is expressed by the following equation (Equation 1) called the Sauerbrey equation. That is, the mass change on the electrode is proportional to the frequency change.

The detection limit of a crystal oscillator currently on the market is several ng cm-2, and it is called QCM (Quartz-crystal microbalance) because it is sensitive enough to measure even the adsorption of a monolayer of atoms.
Since most of the odor molecules are adsorbed, the odor is detected using the above-described apparatus.

  The conventional detection apparatus has an electrical configuration as shown in FIG. 7, and oscillates in the oscillation circuit of the QCM oscillation circuit system (sensor cell) 43 in which the above-described crystal resonator 41 is arranged, that is, the QCM oscillation circuit 45. The resonance frequency is directly measured by the high-speed counter circuit 47, and the calculation unit 49 performs calculation (digital) processing based on the measured count number to derive a change amount of the resonance frequency, that is, a difference frequency.

Japanese Patent Laid-Open No. 10-090152

The oscillation frequency of a crystal resonator was originally high, from several MHz to several hundred MHz, but recently, the resolution increases in proportion to the square of the fundamental frequency, and the tendency to use a higher-speed transducer is increasing.
However, since the upper limit of the operating speed of TTLIC that is widely used at present is about several tens of MHz, if a higher-speed vibrator as described above is used, the conventional element cannot be used, and it is used for another expensive element corresponding to high speed. Must be switched. In addition, since problems such as difficulty in synchronization due to high speed, heat generation, and generation of noise on the circuit board newly occur, circuit design becomes difficult.

  Therefore, an object of the present invention is to provide a chemical substance detection apparatus that does not cause problems in cost and circuit design even when a high-speed vibrator is used.

  In order to solve the above-mentioned problems, the invention of claim 1 is directed to a sensor cell having a piezoelectric oscillator having a gas adsorption film provided on a surface thereof, and a piezoelectric cell having a measurement target gas when the measurement target gas is supplied to the sensor cell. A piezoelectric oscillation circuit that oscillates at a resonance frequency, a fixed frequency oscillation circuit that oscillates at a fixed frequency, and an analog that outputs a beat (resonance) frequency by mixing the output frequency of the piezoelectric oscillation circuit and the output frequency of the fixed frequency oscillation circuit Chemical substance detection using an oscillation frequency adjustment method, which has a mixing circuit and a beat extraction circuit that extracts the beat component frequency from the beat frequency, and uses the beat component as a detection signal for the chemical substance that makes the adsorbed gas Device.

  According to a second aspect of the present invention, there is provided the chemical substance detection apparatus according to the first aspect, further comprising: a counter circuit that counts the beat component frequency; and the count number measured by the counter circuit is directly used as a differential frequency, It is a chemical substance detection apparatus provided with the arithmetic processing part which derives | leads-out the kind and density | concentration.

  According to a third aspect of the present invention, in the chemical substance detection apparatus according to the first aspect, the counter circuit that measures the wave number output from the fixed frequency oscillation circuit for the length of one wavelength of the beat component frequency is measured by the counter circuit. A chemical substance detection apparatus comprising: an arithmetic processing unit for deriving a difference frequency by dividing a fixed frequency by the counted number and deriving a gas type and concentration based on the derived difference frequency .

  According to a fourth aspect of the present invention, there is provided the chemical substance detection apparatus according to the first aspect, further comprising a frequency voltage conversion circuit for converting the frequency of the beat component into a voltage.

  A fifth aspect of the present invention is the chemical substance detection apparatus according to any one of the first to fourth aspects, wherein the piezoelectric oscillator is a crystal oscillator.

  According to the chemical substance detection apparatus of the present invention, even if the oscillation frequency band of the vibrator is in a high frequency band exceeding several tens of MHz, an inexpensive TTLIC that is currently used can be used as it is. The above problem can be solved.

FIG. 1 shows an electrical configuration of the chemical substance detection apparatus 1 according to the first embodiment of the present invention.
In the QCM oscillation circuit system 5 (sensor cell) using the crystal resonator 3 as a sensor element, the QCM oscillation circuit 7 is connected to a power source (not shown), and the gas molecules (the sample gas supplied to the sensor cell 5) When an example) is adsorbed, the QCM oscillation circuit 7 oscillates at a resonance frequency that changes in accordance with the amount of adsorption. The fixed frequency oscillation circuit 9 is also connected to a power source (not shown), and oscillates at a fixed frequency in a state where the chemical substance concentration is 0 (zero), that is, no odorous gas molecules are adsorbed to the crystal unit 3 at all.

The output terminals of the QCM oscillation circuit 7 and the fixed frequency oscillation circuit 9 are connected to an analog mixing circuit 11, and the analog mixing circuit 11 outputs the QCM oscillation circuit 7 as shown in the output waveform of FIG. The beat (resonance) frequency is output by analogally mixing the QCM oscillation frequency and the fixed frequency output from the fixed oscillation circuit.
The beat extraction circuit 13 receives the beat frequency output from the analog mixing circuit 11 and extracts the beat component frequency therefrom. When the QCM oscillation frequency is lowered by 300 Hz due to the adsorption of gas molecules, the beat component frequency becomes a sine wave of 300 Hz as shown in FIG.

  The low-speed counter circuit 15 is a counter circuit whose upper limit of the operation speed that is widely used at present is about several tens of MHz. The beat component frequency is guided to the low speed counter circuit 15 as it is, and the frequency is measured for 1 second. When the QCM oscillation frequency is lowered by 300 Hz, the number of counter circuits is 300.

Then, using the count number measured by the low-speed counter circuit 15 as a difference frequency as it is, the arithmetic processing unit 17 calculates the amount of adsorption based on the difference frequency in the same manner as in the past, and calculates the amount of gas passing through the sensor cell. Based on this, the chemical concentration is calculated.
Specifically, from the change in the resonance frequency measured by the low speed counter circuit 15, the adsorbed chemistry is compared with data on the type and concentration of the chemical substance to be analyzed (adsorption object) in advance in a storage element (not shown). Estimate the type and concentration of the substance.
Then, the estimation result is displayed on the display unit 19.

The personal computer (PC) 21 has an interface that can be connected to a CPU, a storage element, a display unit such as a liquid crystal monitor, and a measurement device such as a counter circuit for collecting and analyzing data. In this embodiment, the personal computer 21 constitutes the arithmetic processing unit 17 and the display unit 19.
Data is transferred between the measurement device and the personal computer 21 based on a transfer method based on the RS-232C standard.

  According to this detection apparatus 1, since the measured number of counter circuits can be used as a difference frequency as it is, it is not necessary to perform special (digital) arithmetic processing thereafter.

A chemical substance detection apparatus according to a second embodiment of the present invention will be described.
The measurement principle of this chemical substance detection apparatus is the same as that of the chemical substance detection apparatus 1, and the measurement method in the low-speed counter circuit 15 is different. Therefore, only the differences will be described.
Until the beat component frequency (waveform) as shown in FIG. 3 is output, the beat component frequency is the same as in the first embodiment, but in the first embodiment, the beat component frequency is directly measured by the low-speed counter circuit 15. On the other hand, in the second embodiment, measurement is performed using a fixed frequency output.

Specifically, as shown in FIG. 4, at the same time as the wave of the beat component frequency rises from 0, measurement of a fixed frequency, for example, a square wave number of 20 MHz is started, and the measurement is stopped when one wavelength has passed.
Next, the difference frequency is first calculated by the arithmetic processing unit.
If the count is 66,667, the difference frequency is estimated to be 20 MHz ÷ 66,667 = 299.999 Hz,
If the count number is 66,666, the difference frequency is
It is estimated that 20 MHz ÷ 66,666 = 300.003 Hz.
Thereafter, data can be handled in the same manner as in the first embodiment.

As described above, when this detection device is used, the measurement time in the low-speed counter circuit 15 is equivalent to one wavelength of the beat component, and therefore it takes only 1/300 second. When the detection apparatus 1 according to the first embodiment is used, since it took 1 second, the measurement time can be greatly shortened.
In addition, the measurement error is ± 1 Hz, which is ± 0.003 Hz or less, which is very high accuracy.

A chemical substance detection apparatus 25 according to a third embodiment of the present invention will be described.
A part of the electrical configuration of the chemical substance detection device 25 is the same as that of the chemical substance detection device 1, but the beat extraction circuit 13 is not connected to the low speed counter circuit 15 but is connected to the frequency voltage conversion circuit 27. .
That is, the beat component frequency output from the beat extraction circuit 13 is guided to the frequency voltage conversion circuit 27 and converted into an analog voltage signal proportional to the frequency. Since the beat component frequency changes continuously in response to changes in the sample gas concentration, the continuous voltage signal is used as an alarm signal for alarm devices (alarm lamps, alarm buzzers) and as a control signal for switchgear. it can. For example, an alarm is triggered when the voltage exceeds a certain level, or the door is closed.

Although the embodiment of the present invention has been described in detail above, the specific configuration is not limited to this embodiment, and the present invention can be changed even if there is a design change without departing from the gist of the present invention. included.
For example, since the gas to be measured is rarely composed of one component, conventionally, a plurality of QCM oscillation circuit systems each having a sensitive film with slightly different characteristics are attached to each other, and output from a plurality of QCM oscillation circuit systems. Each frequency was counted by a high-speed counter circuit, and after the arithmetic processing unit collected those data, the composition of the gas to be measured was identified, and then the concentration of the constituent gas was calculated. When the QCM oscillation circuit system is disposed, the analyzer of the present invention may share one fixed frequency oscillation circuit.
The output of the voltage signal is not limited to the analog output of the third embodiment, and as shown in the first and second embodiments, it is digitally output as an on-off signal from the arithmetic processing unit, and a switchgear, You may use as a control signal of a notification apparatus or a home electric apparatus.

  The chemical substance detection apparatus of the present invention can respond to a demand for improvement in detection performance immediately because a conventional TTLIC can be used even if a high-speed vibrator is used.

1 shows an electrical configuration of a chemical substance detection apparatus according to a first embodiment of the present invention. The output waveform in the detection apparatus of FIG. 1 is shown. It is explanatory drawing of the measuring method of the frequency in the detection apparatus of FIG. It is explanatory drawing of the measuring method of the frequency in the chemical substance detection apparatus which concerns on the 2nd Embodiment of this invention. The electrical structure of the chemical substance detection apparatus which concerns on the 3rd Embodiment of this invention is shown. It is a structural diagram of a crystal resonator. The electrical structure of the conventional chemical substance detection apparatus is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Chemical substance detection apparatus 3 ... Crystal oscillator 5 ... QCM oscillation circuit system 7 ... QCM oscillation circuit 9 ... Fixed frequency oscillation circuit 11 ... Analog mixing circuit 13 ... Beat extraction circuit 15 ... Low speed counter Circuit 17 ... Processing 19 ... Display unit 25 ... Chemical substance detection device 27 ... Frequency voltage converter 31 ... Crystal plate 33 ... Electrode plate 35 ... Sensitive film 37 ... Lead 39 ... Holder 41 ... …… Sensor cell

Claims (5)

  A sensor cell having a piezoelectric oscillator having a gas adsorption film on its surface, a piezoelectric oscillation circuit that oscillates at the resonance frequency of the piezoelectric oscillator when a measurement target gas is supplied to the sensor cell, and oscillates at a fixed frequency A fixed frequency oscillation circuit, an analog mixing circuit that outputs a beat (resonance) frequency by mixing the output frequency of the piezoelectric oscillation circuit and the output frequency of the fixed frequency oscillation circuit, and a beat extraction that extracts the beat component frequency from the beat frequency A chemical substance detection apparatus using an oscillation frequency adjustment method, characterized in that a beat component is used as a chemical substance detection signal for adsorbed gas.   2. The chemical substance detection apparatus according to claim 1, further comprising: a counter circuit that counts the beat component frequency; and the count number measured by the counter circuit is directly used as a difference frequency to derive a gas type and concentration. A chemical substance detection apparatus comprising: an arithmetic processing unit.   2. The chemical substance detection apparatus according to claim 1, wherein a counter circuit that measures a wave number output from a fixed frequency oscillation circuit for a length of one wavelength of a beat component frequency, and a fixed frequency with a count number measured by the counter circuit. A chemical substance detection apparatus comprising: an arithmetic processing unit for deriving a difference frequency by deriving and deriving a gas type and concentration based on the derived difference frequency.   The chemical substance detection apparatus according to claim 1, further comprising a frequency voltage conversion circuit that converts the frequency of the beat component into a voltage. 5. The chemical substance detection apparatus according to claim 1, wherein the piezoelectric oscillator is a crystal oscillator.

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