JP2008275504A - Sensing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sensing device capable of measuring without directly exposing a piezoelectric vibrator into gas or liquid which is a measured object and capable of stably sensing in a liquid phase or a gas phase. <P>SOLUTION: The sensing device includes the piezoelectric vibrator 10 excited at predetermined frequency; an oscillation circuit 20 electrically oscillating the piezoelectric vibrator 10; and an external capacity detecting unit 30 with a resistance 40 and an inductance element 50 connected in parallel or in series to form part of the oscillation circuit 20. The oscillation circuit 20 is grounded through the piezoelectric vibrator 10 and the external capacity detecting unit 30, and outputs a change of capacitance in liquid or gas as a change of phase or frequency. With such arrangement, sensing can be performed without directly exposing the piezoelectric vibrator 10 into gas or liquid which is the measured object, and this feature gives a large effect on various sensors. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a sensing device for physical quantity measurement and chemical measurement, and more particularly to a sensing device suitable as a flow rate sensor, pressure sensor, temperature sensor, vibration sensor, gas sensor, suspended particulate, chemical sensor, immunosensor, and the like. Is.

  In recent years, attention has been focused on a quartz crystal microbalance (QCM) called a microbalance using a crystal resonator which is a piezoelectric resonator. This QCM detects the mass of the surface generated by the substance adhering to the electrodes of the crystal resonator as a change in the resonance frequency of a frequency conversion element such as a crystal resonator. Since QCM is capable of mass detection of nanogram (ng) or less, it is applied to detection of trace substances in a wide range of fields such as medicine, biochemistry, food and environmental measurement as a biosensor and chemical sensor.

  For example, in some cases, a crystal resonator is used as a harmful chemical substance monitoring device in a gas phase, and an antibody that specifically binds to a chemical substance to be measured is formed on the surface of the crystal resonator. (For example, Patent Document 1) As a conventional oscillation circuit for a sensing device using a crystal resonator, a method of sustaining stable oscillation for a long time even when the fundamental natural frequency of the crystal resonator as a sensor increases. For example, an oscillation circuit has been proposed in which a plurality of crystal resonators having different fundamental frequencies are oscillated by the same circuit using an inverter composed of a high-speed CMOS (Patent Document 2).

  In order to use a crystal resonator as a sensor, it is desired to have a wide frequency variable region with respect to an oscillation frequency change caused by adhesion of a target substance to the surface of the resonator. As one method of expanding the variable range of the oscillation frequency, a variable capacitor is inserted in series with a crystal resonator as seen in VCXO (Voltage Controlled Crystal Oscillator), and the variable capacitor is adjusted by adjusting the voltage from the outside. There is a method of changing the oscillation frequency by changing the capacitance.

In addition, as a sensing device that does not directly use a crystal resonator as a sensor, a crystal resonator sealed in a case, an oscillation circuit, and a detection external unit that is part of an oscillation loop and includes a pair of conductor plates, And a sensing device that takes in a change in electrical impedance or admittance of the detection external unit into an oscillation loop and measures a change in phase or frequency output from the oscillation circuit has been proposed (Patent Document 3).
JP 2002-48797A. JP 2001-289765 A. Japanese Patent Laid-Open No. 2006-322887.

  However, in the sensing device as described above, the measurement substance is adsorbed on the surface of the vibrator, which is the oscillation frequency source itself, and the vibrator is directly exposed to the gas phase or liquid phase. The stability of the oscillation frequency is extremely inferior to a communication piezoelectric vibrator that uses the vibrator in a sealed manner. Furthermore, deterioration of the vibrator is inevitable.

  Further, in the method of expanding the variable range of the oscillation frequency by inserting a variable capacitance element such as the VCXO described above, a sufficiently wide variable region cannot be obtained for use as a sensing device.

  In Patent Document 2, only sensing data in the gas phase is disclosed in the embodiment, and it cannot be confirmed whether sufficiently stable oscillation is possible using the oscillation circuit shown in Patent Document 2.

  In Patent Document 3, only the equivalent circuit of the sensing device is described in the best mode for carrying out the invention, and the specific circuit configuration is not clarified. High-precision sensing is performed by the sensing device shown in Patent Document 3. I can't check if is possible.

Further, in Patent Document 3, the oscillation frequency, frequency stability, circuit configuration, and circuit constants of the crystal resonator used in the examples are not clarified. In addition, the experimental results show that the relative permittivity change using liquid ethyl alcohol (relative permittivity ε _s = 25) and water (relative permittivity ε _s = 80) as detection objects is detected as a frequency change. However, it is considered that these two liquids have an extremely large relative dielectric constant among various liquids and can be easily detected, and it is not possible to confirm from this example whether a highly accurate dielectric constant measurement is possible with the sensing device shown in Patent Document 3. .

  The present invention has been made to eliminate the drawbacks of the prior art described above, and can oscillate while maintaining high stability by sensing the vibrator without directly exposing it to the gas or liquid containing the target substance. Provided is a high-precision sensing device that includes a possible oscillation circuit and can be used regardless of whether it is in a liquid phase or in a gas phase.

  In order to achieve the above object, a sensing device of the present invention includes a piezoelectric vibrator excited at a predetermined frequency, an oscillation circuit that electrically oscillates the piezoelectric vibrator, and a resistor and an inductance element connected in parallel. An external capacitance detection unit that is connected in series and forms part of the oscillation circuit, and the oscillation circuit is grounded via the piezoelectric vibrator and the external capacitance detection unit, and is electrostatically charged in a liquid or a gas. A change in capacitance is output as a change in phase or frequency.

  Further, in the sensing device of the present invention, when the oscillation frequency deviation of the piezoelectric vibrator is expressed by a multi-order function having the capacitance of the external detection unit as a variable, the capacity and the minimum value at which the multi-order function takes a maximum value. The external detection in the section between the capacitance having the maximum value and the capacitance having the minimum value has a characteristic that the rate of change of the frequency deviation with respect to the capacitance is 3.84 ppm / pF or more. It is preferable to be able to output a frequency change that is extremely sensitive to changes in the capacity of the unit.

  In the sensing device of the present invention, it is preferable that the piezoelectric vibrator is a crystal vibrator.

  In the sensing device of the present invention, it is preferable that the piezoelectric vibrator is a SAW vibrator.

  In the sensing device of the present invention, it is preferable that the piezoelectric vibrator is a Lamb wave vibrator.

  According to the present invention, it is possible to obtain highly stable frequency data by using the piezoelectric vibrator while being sealed in a metal package or a ceramic package without exposing it to the measurement atmosphere. The sensitivity of identifying chemical substances and measuring physical quantities can be dramatically improved.

  Conventional sensing devices oscillate and measure with the vibrator exposed, so external stress applied to the vibrator cannot be ignored, and frequency measurement data includes frequency fluctuations due to external stress. The reliability was low. On the other hand, the sensing device of the present invention can use a vibrator sealed in a package with a high degree of sealing, and includes an external detection unit for impedance measurement made of a highly reliable material. Therefore, the measurement reliability can be drastically improved because the change in impedance taken in is measured as a frequency change.

  Further, the structure of the external detection unit can be freely selected according to the measurement object.

  FIG. 1 is a diagram illustrating an example of a sensing device 100 according to an embodiment of the present invention. The sensing device 100 is a sensing device that includes a piezoelectric vibrator 10, an oscillation circuit 20, an external detection unit 30, a resistor 40, and an inductance element 50. The oscillation circuit 20 is grounded via the piezoelectric vibrator 10 and the external detection unit 30, and the external detection unit 30 is connected in parallel with the resistor 40 and the inductance element 50. Thus, since the resistor and the inductance element are connected in parallel or in series with the external detection unit 30, the oscillation circuit 20 can be configured easily.

  Further, the oscillation circuit 20 of FIG. 1 is configured by a general Colpitts transistor oscillation circuit. C11, C12, C13, and C14 are capacitors, R11, R12, R13, R14, and R15 are resistors, and TR11 and TR12 are npn transistors. The oscillation circuit 20 is grounded via the piezoelectric vibrator 10 and the external capacitance detection unit 30, and outputs a change in capacitance in liquid or gas as a change in phase or frequency. This can dramatically improve the reliability of measurement. The oscillation circuit 20 may be any of a Pierce type transistor oscillation circuit, a Clap type transistor oscillation circuit, a Butler type transistor oscillation circuit, a Hartley type transistor oscillation circuit, and a gate type inverter oscillation circuit, and can operate stably as an oscillation circuit. I just need it. The oscillation circuit 20 is not limited to these types of circuits, and the oscillation circuit shown in FIG. 1 is merely an embodiment. Vcc represents a power supply voltage, and OUT represents an output terminal.

  FIG. 2 is a diagram showing an equivalent circuit 200 of the sensing device 100 according to the embodiment of the present invention. The piezoelectric vibrator 10 includes an equivalent series inductance L1, an equivalent series capacitance C1, an equivalent series resistance R1, and an interelectrode capacitance C0. An equivalent series inductance L1, an equivalent series capacitance C1, and an equivalent series resistance R1 are connected in series, and these are further connected in parallel to the interelectrode capacitance C0. An equivalent circuit of the oscillation circuit 20 is indicated by a load capacitor Cci connected in series with the negative resistance Rci.

FIG. 3 is a diagram showing an equivalent circuit 300 obtained by equivalent circuit conversion of the external detection unit 30, the resistor 40, and the inductance element 50 of the equivalent circuit 200. The combined series capacitance Cx connected in series with the combined series resistor Rx is obtained by converting the external detection unit 30, the resistor 40, and the inductance element 50 in series equivalent circuit. Rx and Cx are represented by the equations (1) and (2), respectively.

Qa and ωa in the expressions (1) and (2) are expressed by the expressions (3) and (4), respectively.

In the equations (1) to (4), ω is the angular frequency of the piezoelectric vibrator 10, La is the inductance of the inductance element 50, Ra is the resistance of the resistor 40, and Ca is the capacitance of the external detection unit 30. Furthermore, the synthetic capacitive impedance CL was obtained by synthesizing the synthetic series resistance Ra, the synthetic series capacitance Ca, and the load capacitance Cci. CL is expressed by equation (5).

FIG. 4 shows a result of simulating the characteristics of the frequency deviation DL when Ca of the sensing device 100 according to the embodiment of the present invention is varied using the equivalent circuit as described above. The frequency deviation DL of the simulation was calculated using equation (6). The simulation conditions were Ra = 1.5 kΩ, La = 8.2 μH, C0 = 1.5 pF, C1 = 4.6 fF, ω = 2π × 4.915 MHz, Cci = 70 pF.

Γ in equation (6) is expressed by equation (7).

  Subsequently, the simulation results were compared with the characteristics using actual circuits. FIG. 4 also shows the results of measuring the characteristics of the frequency deviation DL when Ca of the sensing device 100 according to the embodiment of the present invention is varied using an actual circuit. The circuit constants for the AT-Cut crystal resonator are 4.915 MHz: C0 = 1.5 pF, C1 = 4.6 fF, R1 = 45Ω, L1 = 230 mH, Ra = 1.5 kΩ, La = 8.2 μH, C11 = C14 = 0.1 μF, C12 = C13 = 131 pF, R11 = R15 = 820Ω, R12 = 7.5 kΩ, R13 = 6.8 kΩ, and R14 = 2.4 kΩ. When the oscillation frequency deviation of the piezoelectric vibrator 10 is represented by a multi-order function having the capacitance of the external detection unit 30 as a variable by simulation, the piezoelectric vibrator 10 takes a capacity and a minimum value where the multi-order function takes a maximum value. It has the characteristic that the rate of change of the frequency deviation with respect to the capacitance is 3.84 ppm / pF or more. Since the rate of change of the frequency deviation with respect to the capacitance depends on the resistance Ra of the resistor 40, it is possible to adjust the rate of change of the frequency deviation by changing Ra. Accordingly, the piezoelectric vibrator 10 can output a frequency change that is extremely sensitive to a change in the capacity of the external detection unit 30 in a section between a capacity having a maximum value and a capacity having a minimum value.

  As a result of the measurement, it was confirmed that the frequency deviation DL when Ca is varied in the sensing device 100 according to the embodiment of the present invention changes extremely steeply when Ca is 100 pF or more and 150 pF or less. In particular, it has been found that the actual circuit has an extremely steep change in the frequency deviation DL when Ca is 111 pF or more and 120 pF or less, and it has been confirmed that high accuracy measurement can be performed by performing sensing using this portion.

  Below, the effectiveness as a sensing device of the present invention is explained in detail using an example shown in a figure. However, the components, types, combinations, shapes, relative arrangements, and the like described in this embodiment do not limit the scope of the invention only to that unless otherwise specified, and are merely examples. In addition, the piezoelectric vibrator is an AT-cut quartz crystal vibrator, an inverted mesa-type AT-cut quartz crystal vibrator, an SC-cut quartz crystal vibrator, a SAW (Surface Acoustic Wave) vibrator, a Lame mode (contour vibration mode) crystal vibrator or a Lamb wave Any of crystal units may be used.

  FIG. 5 shows a gas sensing experimental apparatus used in the example. The gas sensing experimental device 400 includes a gas collection bottle 61, a sensing cell 62, a detection target gas 63, a gas generator 64, a nitrogen gas cylinder 65 for gas flow, a frequency counter 66, a personal computer 67, and a sensing device. 100 is a gas sensing experimental apparatus. Nitrogen gas introduced from the nitrogen gas cylinder 65 into the gas generator 64 through the piping tube vaporizes the liquid of the detection target gas 63 by the bubbling method. The generated gas 63 to be detected is introduced into the sensing cell 62 through a piping tube.

  Measurement data measured by the sensing device 100 is recorded by the personal computer 67 via the frequency counter 66. The mixed gas of nitrogen and detection target gas discharged from the sensing cell 62 is filtered through the gas collection bottle 61 and exhausted. The sensing device 100 is a sensing device that includes the piezoelectric vibrator 10, an oscillation circuit 21 including an oscillation circuit 20, a resistor 40, and an inductance element 50, and an external detection unit 30. The external detection unit 30 is a parallel plate capacitor in which two aluminum plates (90 mm × 150 mm) are arranged in parallel. The capacitance of the external detection unit 30 in air was set to 115 pF by appropriately adjusting the distance between the electrode plates of the two aluminum plates. Acetaldehyde was selected as the detection target gas 63, and nitrogen gas flowed at 102 mL / min. The temperature in the experimental apparatus was always kept constant at 21 ° C.

  The following experimental procedure was performed. While the sensing device 100 was operated, nitrogen gas was introduced into the sensing cell 62 for 20 minutes alone. Thereafter, acetaldehyde generated by introducing nitrogen gas again into the gas generator 64 was introduced into the sensing cell 62 for 10 minutes. Thereafter, nitrogen gas alone was again introduced into the sensing cell 62 for 30 minutes.

  FIG. 6 shows a graph of the results of the gas sensing experiment. The sensing device 100 did not respond with nitrogen gas alone, but it responded sensitively when acetaldehyde was introduced, and recorded a maximum frequency change of 6.1 ppm (30 Hz). When nitrogen gas was again introduced again 20 minutes after the introduction of acetaldehyde, it was observed that acetaldehyde was discharged from the sensing cell 62 and the oscillation frequency returned to the original state.

The relative dielectric constant of acetaldehyde (gas) used in this experiment is 1.0213 at 20 ° C., and the relative dielectric constant of nitrogen (gas) is 1.000548 at 20 ° C. The difference in relative dielectric constant between the two gases is only 0.0208, and the above experimental results show that the sensing device of the present invention can detect a very small difference in relative dielectric constant. In Patent Document 3, as a result of an experiment in which a change in relative permittivity using liquid ethyl alcohol (relative permittivity ε _s = 25) and water (relative permittivity ε _s = 80) is regarded as a frequency change, liquid ethyl alcohol is obtained. It is shown that the change in frequency is about 2500 Hz while the difference in relative dielectric constant from alcohol is 55. However, as described above, the sensing device of the present invention can detect a relative dielectric constant difference that is about 1/2650 smaller than the measured relative dielectric constant difference described in Patent Document 3. It was shown to have high sensing ability.

  By configuring the sensing device as described above, it is possible to easily perform high-accuracy relative permittivity measurement regardless of whether it is in liquid or in gas. These features have a great effect on the field of sensors where the quartz crystal adopted adopts QCM (micromass measurement using a quartz crystal), and it is expected to develop technology in these fields.

It is a circuit diagram showing the sensing device concerning the present invention. It is an equivalent circuit diagram showing the sensing device according to the present invention. It is the circuit diagram which equivalent circuit-converted a part of equivalent circuit diagram showing the sensing apparatus based on this invention. The figure which shows the result of having simulated the change of the oscillation frequency of the piezoelectric vibrator when changing the capacity | capacitance of the external detection unit of the sensing apparatus based on this invention, and the experimental result when changing the capacity | capacitance of an external detection unit. It is. It is an experimental apparatus figure of the gas sensing experiment using the sensing apparatus which concerns on this invention. It is a figure which shows the change of the oscillation frequency when conducting a gas sensing experiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Piezoelectric vibrator 20 Oscillation circuit 21 Oscillation circuit 30 External detection unit 40 Resistance 50 Inductance element 61 Gas collection bottle 62 Sensing cell 63 Gas to be detected 64 Gas generator 65 Nitrogen gas cylinder for gas flow 66 Frequency counter 67 Personal computer 100 Sensing device 200, 300 Equivalent circuit of sensing device 400 Gas sensing experimental device C11, C12, C13, C14 Capacitor R11, R12, R13, R14, R15 Resistor TR11 and TR12 npn transistor Vcc Power supply voltage OUT Output terminal La Inductance element Inductance Ra Resistance (Synthetic series resistance)
Ca External sensing unit capacity (combined series capacity)
L1 Equivalent Series Inductance C1 Equivalent Series Capacitance R1 Equivalent Series Resistance C0 Interelectrode Capacitance Rci Negative Resistance Cci Load Capacitance Rx Composite Series Resistance Cx Composite Series Capacitance ω Angular Frequency CL of Piezoelectric Vibrator CL Synthetic Capacitive Impedance DL Frequency Deviation

Claims (5)

A piezoelectric vibrator excited at a predetermined frequency;
An oscillation circuit for electrically oscillating the piezoelectric vibrator;
An external capacitance detection unit in which a resistor and an inductance element are connected in parallel or in series, and forms a part of the oscillation circuit;
With
The sensing device, wherein the oscillation circuit is grounded via the piezoelectric vibrator and the external capacitance detection unit, and outputs a change in capacitance in liquid or gas as a change in phase or frequency.   When the oscillation frequency deviation of the piezoelectric vibrator is expressed by a multi-order function having the capacity of the external detection unit as a variable, a capacity between the capacity at which the multi-order function takes a maximum value and the capacity at which a minimum value is taken. The rate of change of the frequency deviation with respect to the frequency is 3.84 ppm / pF or more, and is extremely sensitive to the capacitance change of the external detection unit in the interval between the capacitance having the maximum value and the capacitance having the minimum value. The sensing device according to claim 1, wherein a change in frequency can be output.   The sensing device according to claim 1, wherein the piezoelectric vibrator is a quartz crystal vibrator.   The sensing device according to claim 1, wherein the piezoelectric vibrator is a SAW vibrator.   The sensing device according to claim 1, wherein the piezoelectric vibrator is a Lamb wave vibrator.

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