JPH08292144A - Liquid viscosity sensor - Google Patents

Abstract

PURPOSE: To obtain a small-scaled lightweight liquid viscosity sensor with good measuring accuracy usable in a system (inline) by using an impedance bridge circuit making a circuit element of quartz oscillators and capacity elements. CONSTITUTION: The liquid viscosity sensor is constituted by an impedance bridge circuit making a circuit element of quartz oscillators for liquid viscosity measuring X1, X2 and capacity elements (capacitor) C1, C2. And a coefficient of viscosity (viscosity) of liquid specimen is measured by measuring the Q values of the quartz oscillators X1, X2 by the impedance bridge circuit. In other words, when alternating voltage of a few volts is applied between contact points 0-1 of the impedance bridge circuit, alternating voltage proportional to the Q values of the quartz oscillators X1, X2 produces at a resonance point of the bridge circuit between contact points 2-3. Direct voltage proportional to the Q value can be obtained by detecting and rectifying it. In addition, when the circuit elements are integrated on the same substrate, scattering of each circuit element is lessened.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid viscosity sensor used for measuring the viscosity of a liquid, and more particularly to a liquid viscosity sensor which can be used for measuring the viscosity of a low-viscosity liquid having a relatively small viscosity. Also, the present invention relates to a liquid viscosity sensor used for in-line liquid viscosity measurement in a small space, for example, which enables measurement of light oil viscosity in a fuel injection pump of a diesel engine from general measurement of liquid viscosity.

[0002]

2. Description of the Related Art In measuring the viscosity of a liquid, there are two methods or devices for performing absolute measurement: a method using the outflow of a liquid from a thin tube based on Poiseuille's law, and a method using a sphere in a liquid based on the use of Stokes' law. To measure the falling speed, to use the resistance of the liquid to the rotation or vibration of the disk or cylinder in the liquid, or to compare and measure the viscosity of the standard liquid, a small amount of liquid is applied to the thin tube. There are Ostwald viscometer, Washburn Williams viscometer, Ubbelohde viscometer, etc., which compare the time required to pass.

[0003] For example, a mechanical vibration type liquid viscosity measuring apparatus utilizing vibration of an object in a liquid is disclosed in Japanese Patent Laid-Open No. 1-2093.
As disclosed in Japanese Patent Publication No. 38-38, a vibrating piece is vibrated in a liquid tank, and the viscosity of the liquid is measured from the attenuation amount of the vibration amplitude due to the viscous resistance of the liquid. However, devices employing these measurement methods have a large measuring section and a large amount of liquid to be tested, and are therefore limited to use as stationary measurement devices.

In recent years, system automation has been advanced in many fields. Since the automation of the system is controlled based on various physical property values used in the system, various sensors for measuring the physical property values have been developed and used. In order to automate a system including a liquid, a small, lightweight, and inexpensive liquid viscosity sensor suitable for in-line measurement of viscosity, which is one of the important physical property values of a liquid, is required.

Attempts have been made to manufacture a small-sized viscosity sensor by using an ultrasonic transducer, a surface acoustic wave element, or the like made of a piezoelectric material as a viscosity sensor. Among the liquid viscosity sensors using such an ultrasonic vibrator, those using a quartz thickness sliding vibrator have features such as simple structure, high strength, and good stability characteristics against temperature change. To have
In particular, its practicality as a viscosity sensor for low viscosity liquids is expected. However, in the case of the conventionally proposed method of measuring the viscosity from the resonance frequency fr of the vibrator in the liquid, the decrease in resonance in the liquid is large, and it is difficult with a detection method using a simple oscillation circuit. There is a need for admittance measurement using an expensive measurement device such as a network analyzer, which has been an obstacle in configuring an inexpensive sensor system. Furthermore, the measurement of the resonance frequency fr as the admittance real part maximum point is the resonance intensity when the sensor is in the liquid, and therefore has a disadvantage that the resonance peak becomes broad and the measurement variation of fr becomes large.

[0006]

SUMMARY OF THE INVENTION In view of the above, it is an object of the present invention to provide a small, lightweight, and highly accurate liquid viscosity sensor which can be used in a system (in-line). It was done as.

[0007]

According to the present invention, there is provided a liquid viscosity sensor comprising at least one crystal oscillator for measuring liquid viscosity and an impedance bridge circuit having a capacitive element as a circuit element. I will provide a.

Preferably, a pair of vibrators provided separately on the same crystal substrate can be used as a pair of circuit elements opposed to each other in the impedance bridge circuit. As a circuit element of the bridge circuit, two pairs of vibrators having the same quartz substrate can be used. In this case, the admittance and resonance frequency of each circuit element, that is, the vibrator, It is adjusted by adjusting the magnitude of the mass load of the part. The adjustment of the mass load of the electrode portion, that is, the adjustment of the admittance and the resonance frequency of the vibrator, can be easily performed by using an electrode material having a different specific gravity, or changing the thickness of the electrode. In the present invention, the viscosity (viscosity) of the test liquid is determined as a value corresponding to the Q value by measuring the Q value of the crystal oscillator for measuring the liquid viscosity incorporated as a circuit element by the impedance bridge circuit. A detection method can be adopted. When the real part of the admittance of the crystal resonator is measured, it shows a Lorentz-type resonance peak near the resonance frequency, and the frequency at the maximum point is the resonance frequency fr. Then, the value of the admittance real part at this point is represented by G
-max, between G-max and the viscosity η and density ρ of the liquid in which the oscillator is immersed,

(Equation 1) (Where K is a proportionality coefficient and can be determined by measuring G-max for a standard solution with known ρ and η). Therefore, if the density ρ of the test liquid is known, the viscosity η of the test liquid can be obtained by measuring G-max.

The peak frequency fr of the real part of the admittance is
The measured value varies due to disturbance from the test liquid (non-uniform temperature distribution, turbulent flow, etc.). However, since the measured variation of G-max is relatively small, the liquid viscosity η is determined based on the measured value of G-max. By measuring, higher detection resolution can be obtained. The method of measuring liquid viscosity using G-max gives higher resolution than using fr measurement,
Direct measurement of G-max requires expensive measuring equipment such as a network analyzer. The present invention proposes the use of an impedance bridge circuit having a crystal oscillator for measuring liquid viscosity and a capacitor as a circuit element. In addition, a method of measuring the Q by focusing on the physical quantity Q related to G-max using the impedance bridge circuit is proposed.

That is, at the resonance point of the vibrator, Q = 2
πfrL / R, and since G-max = 1 / R, Q
And G-max have a relationship of Q = 2πfrL · G-max. Here, the amount of change in L due to the change in viscosity of the liquid is R
Q = K'G-max
(K ′ is a constant). As a desirable impedance bridge circuit, as shown in FIG. 1, two opposing crystal units X ₁ and X ₂ and two capacitors C ₁ and C ₂ having a capacitance equal to the equivalent parallel capacitance of the unit. Configure. Here, one of the two crystal oscillators serving as circuit elements can be replaced with a capacitor. However, in this case, the viscosity detection sensitivity is reduced by half, and therefore, two oscillators are used as opposing circuit elements. It is desirable to do.

In FIG. 1, when an AC voltage of several volts is applied between the contacts 1-0 of the impedance bridge circuit,
An AC voltage proportional to the Q value of the crystal oscillator is generated between the contacts 2-3 at the resonance point of the bridge circuit. By detecting and rectifying this, an output of a DC voltage proportional to the Q value can be obtained. In addition, by performing synchronous detection using the input signal as a reference signal, it is possible to obtain information on the phase change of the vibrator.
By adding an L circuit, various information such as the half width of resonance and the resonance frequency can be detected. Therefore, the detection circuit using the impedance bridge is also effective in increasing the versatility of the sensor.

The impedance bridge circuit can preferably be constituted by two crystal units and two capacitors. When an impedance bridge circuit is configured using each of the circuit elements manufactured separately, the variation in the characteristics of each element tends to increase, and therefore, the variation in the characteristics of the sensor tends to increase.

According to the present invention, as a method for eliminating the above-mentioned disadvantages, two sets of vibrators having the same parallel capacitance and different resonance frequencies are formed on the same quartz substrate, and these are used as circuit elements. It is proposed to construct an impedance bridge. FIG. 2 shows an equivalent circuit of the crystal resonator. Here, Cp is an electrical capacitance component determined by the substrate dielectric constant, thickness, and electrode area, and L ₁ , C ₁ , R ₁
Denotes a dynamic circuit element for describing the piezoelectric vibration of the crystal. L ₁ is increased by increasing the mass load of the electrodes of the quartz oscillator, the resonance frequency fr is reduced. As described above, in the present invention, a preferred impedance bridge circuit is configured by using two sets of oscillators having the same parallel capacitance and different resonance frequencies as circuit elements. For example, it is manufactured by the following method.

On the front and back surfaces of the same quartz substrate, four sets of opposed circular electrodes are provided separately from each other to form four quartz oscillators. Here, each electrode has the same radius,
Therefore, the parallel capacitance Cp of each transducer is equal. However, the electrodes of the two opposing transducers have different mass loads between the respective sets, and therefore, the resonance frequency fr of the transducer is different between the respective sets. The two sets of vibrators (circuit elements) obtained as described above are arranged such that the vibrator of the other set is separated from the resonance zone near the resonance frequency of one set of vibrators (circuit elements). It functions as a simple capacitor. Therefore, an impedance bridge circuit having a configuration as shown in FIG. 1 can be obtained by combining the two sets of vibrators (circuit elements) in a bridge shape. In addition,
The change of the mass load at the electrode portion of the vibrator (circuit element) may be changed, for example, by using different types of electrode materials, for example, Al and Au, or by further changing the electrode thickness. Can be done.

[0015]

The liquid viscosity sensor configured as described above using a quartz crystal vibrator and an impedance bridge circuit is a high-resolution liquid viscosity sensor in a low-viscosity region, and is a small and small-variation sensor. I will provide a.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The Q value of a vibrator can be measured from the frequency characteristics of the real part of admittance. Specifically, the frequency indicating the maximum value of the resonance peak is represented by fr,

(Equation 2) Δf which is the difference between the frequencies f ₁ and f ₂ (f ₁ <f ₂ )
= F ₂ −f ₁ , it can be measured as Q = fr / Δf.
When the dynamic inductance component of the vibrator is L and the dynamic resistance component is R, Q = 2π at the resonance frequency fr.
frL / R, and G-max = 1 / R. Here, since the amount of change in L due to the change in viscosity of the liquid is very small compared to the amount of change in R, Q = 2πfrLG-max =
K · G-max (K: constant). Therefore, G-max was measured as a substitute value equivalent to Q. Also, the resonance frequency f
r was similarly measured as the frequency giving G-max.

As the test liquid, a light oil whose viscosity and density are known in the temperature range of 20 to 80 ° C. was used, and the resonance frequency fr and G-max were measured in the temperature range of room temperature to about 90 ° C. Prior to the experiment, it was previously confirmed that the influence of the temperature characteristic of the quartz substrate itself was negligibly small compared to the temperature change of the gas oil viscosity, that is, the change in fr and G-max accompanying the change in viscosity.

The measured resonance frequency fr, G of the quartz oscillator
-max is shown in FIG. The experimental value is

(Equation 3) This is consistent with previous research reports and theories. The sensitivity to each viscosity can be evaluated by the slopes dfr / dη and dG-max / dη of the regression line, but the actual detection resolution is Δf
r / (dfr / dη), ΔG-max / (dG-max / dη)
Was evaluated. Here, Δfr and ΔG-max are measurement variations, and 4σ (= Xavg. ± 2) using the standard deviation σ.
σ). The measurement variation is clearly smaller in the G-max measurement from FIG. Table 1 shows the detection resolution.

[0019]

[Table 1]

Next, using an AT cut quartz substrate 1 having a thickness of 415 μm, as shown in FIGS. 4A and 4B, a circular shape having a diameter of 10 mm and a thickness of 0.2 μm is opposed to both surfaces thereof.
A 0.4 μm Au electrode 2 was formed by sputtering film formation and photolithography, and four vibrators (circuit elements) were provided on the same quartz substrate to form an impedance bridge circuit as shown in FIG. The input lead 3 and the output lead 4 of this bridge circuit are provided on both sides of the substrate 1 in a form separated from input and output, as shown in FIG. At the time of measurement, a sine wave of Vp-p = 2 V is applied from the input lead 3 by the function generator, and
The frequency was swept in the vicinity of MHz, and the AC voltage generated at the output lead 4 was observed with an oscilloscope, and the maximum voltage when the amplitude reached the maximum was observed. In addition, as a test liquid, a light oil whose density and viscosity were known in advance was prepared and measured at room temperature.

FIG. 5 shows the output voltage when the sensor was immersed in various types of light oil whose viscosity and density were previously determined as described above and measured. From the equivalent circuit analysis of the impedance bridge circuit, the output voltage is

(Equation 4) It is expected that the density of light oil used as the test liquid is constant at approximately 0.82 g / cm ³ , and therefore, the output voltage is approximately

(Equation 5) Is proportional to That is, for a liquid whose density is known in advance, the viscosity can be measured using the output voltage of the impedance bridge element.

The viscosity measurement sensitivity is very high for a low-viscosity liquid, and as shown in FIG. 5, it is possible to distinguish various light oils very clearly. In addition,
Although the output voltage is obtained as an AC voltage, if this is converted into a DC voltage through a detection and rectification circuit, it can be used as an output signal for viscosity detection. For example, if the DC voltage is proportional to the viscosity value in the arithmetic circuit, the viscosity can be displayed using a voltmeter, and the output signal (information) relating to the liquid viscosity obtained in the in-line measurement is obtained in the preceding stage. Alternatively, the information can be transmitted to a subsequent control system and used as information for system control.

[0023]

According to the present invention, since the liquid viscosity sensor is constituted by an impedance bridge circuit using a crystal unit and a capacitance element as circuit elements, it is small and highly sensitive, and is particularly effective for detecting the viscosity of a low viscosity liquid. is there. Further, since the circuit elements of the sensor are integrated on the same substrate, variations in the characteristics of each circuit element are small, the stability of the output characteristics is good, and the apparatus is suitable for use in in-line measurement.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an impedance bridge circuit.

FIG. 2 is an equivalent circuit of a quartz thickness slip resonator.

FIG. 3 is a graph showing the relationship between the resonance frequency fr and G-max of a quartz oscillator and the viscosity of light oil.

FIG. 4 is a schematic diagram of an impedance bridge circuit element formed on the same substrate.

FIG. 5 is a diagram illustrating output characteristics of an impedance bridge circuit including circuit elements on the same substrate.

[Explanation of symbols]

1 substrate 2 Au electrode 3 input lead 4 output lead X ₁ , X ₂ crystal oscillator C ₁ , C ₂ capacitor

Claims (6)

[Claims] 1. A liquid viscosity sensor having an impedance bridge circuit using a crystal unit and a capacitor as circuit elements. 2. The liquid viscosity sensor according to claim 1, further comprising an impedance bridge circuit for enabling measurement of a Q value of the quartz oscillator in the test liquid. 3. The liquid viscosity sensor according to claim 1, wherein the circuit elements are formed side by side on the same quartz substrate. 4. The liquid viscosity sensor according to claim 3, wherein the admittance and the resonance frequency of each of the circuit elements are made different by making the magnitude of the mass load of the electrode part constituting the circuit element different. 5. The liquid viscosity sensor according to claim 1, wherein the capacitance of the capacitance element is equal to the parallel capacitance of the crystal unit. 6. An impedance bridge circuit comprising a crystal unit and a capacitance element, wherein the bridge circuit is configured by arranging four units on the same crystal substrate, and the Q value of the unit is Detected by a bridge circuit, the admittance and resonance frequency of each vibrator are adjusted according to the magnitude of the mass load on the electrode section. A liquid viscosity sensor characterized by being adjusted by changing the film thickness of the electrode material, laminating various materials on the electrode, or any combination thereof.