JPS6255612B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a piezoelectric humidity sensor in which a hygroscopic substance is coated on the main vibration surface of a piezoelectric vibrator that provides a stable vibration frequency.
In other words, the relationship between the thickness of the coating layer of the hygroscopic material and the contour dimensions of the piezoelectric diaphragm is such that the main vibration mode of the piezoelectric vibrator is combined with the unnecessary vibration mode, and the humidity is such that oscillation is stopped at a predetermined humidity. It is intended to be configured to operate as a switch.

In a rectangular thickness-shear vibrating piezoelectric vibrator, there are many high-order parasitic vibrations near the main vibration frequency whose frequencies depend on the length l and width W of the rectangular diaphragm, and depending on the temperature etc. This higher-order parasitic vibration may be coupled to the main vibration. With this combination,
Depending on the type of parasitic vibration, an extremely steep entrainment phenomenon may occur, resulting in a peculiar phenomenon called an activity dip, in which the equivalent resistance of the main vibration increases significantly, and eventually oscillation stops.

This is an abnormal phenomenon for a piezoelectric vibrator, and conventional methods have been designed to prevent this kind of behavior from occurring, that is, to avoid the possibility of coupling with unnecessary vibration modes near the main vibration frequency, and to maintain stable vibration at a predetermined frequency. Various considerations were made to ensure that this was done.

The present invention uses this phenomenon to the contrary, and the piezoelectric vibrator stops oscillating when the main vibration mode and unnecessary vibration mode (hereinafter abbreviated as main mode or unnecessary mode) are combined at a specific humidity, and acts as a humidity sensor. The details will be explained below.

In general, a transducer that provides a stable frequency
It is required that there is no coupling with unnecessary modes over a wide temperature range, and it is known that even in filters made of vibrators, these unnecessary modes have an adverse effect on the characteristics.

The main vibration frequency f ₀ of the rectangular thickness-shear oscillator is
In the case of a crystal resonator, it is inversely proportional to the thickness t, and has the following relationship.

f ₀ = K1/t... (1) In other words, in the case of an AT cut plate, the constant K = 1.67, and if f ₀ = 10MHz, the thickness of the diaphragm is 0.167 mm.
becomes.

Here, the unnecessary mode is a contour where the length l of the rectangular plate depends on the width W, width slip,
It occurs in each vibration mode such as vertical vibration, and higher-order vibrations of each mode appear near the main vibration frequency of thickness shear.
This is one of the causes of the activity dip.

In a rectangular thickness-shear oscillator, if the long side direction of the diaphragm is taken parallel to the crystal axis x, in the case of an AT cut plate, there are many parasitic vibrations, that is, higher-order contour vibration frequencies f _l as shown in the following equation. .

Here, l is the long side dimension of the rectangular diaphragm, n is the Overton order in the x-axis direction, and t is the plate thickness. This parasitic vibration depends on the long side dimension (x-axis direction) of the rectangular diaphragm and is unrelated to the width (short side) dimension of the diaphragm.

Next, regarding the width (short side) direction (Z' axis) of the rectangular diaphragm, there are sliding and vertical vibration frequencies f _W according to the following equations.

Here, W is the width (short side) dimension, ρ is the density, and C _ij
indicates the elastic constant of the crystal.

As is clear from the above relational expression, there are countless high-order unnecessary modes near the main vibration frequency.

On the other hand, in a vibrator designed so that the unnecessary mode is intentionally combined with the main mode, if the electrode thickness is changed, the main vibration frequency decreases in proportion to the electrode thickness, but the unnecessary mode frequency decreases. It doesn't change much. Therefore, if a suitable hygroscopic material (for example, polyamide) is applied to the electrode after the electrode is formed, the moisture absorbed by the hygroscopic material will act as if the thickness of the electrode itself has changed, and the humidity will be reduced. Therefore, the main vibration frequency changes.

According to actual measurement results, at a main vibration frequency of 10 MHz, the humidity changes by nearly 200 Hz per 1% humidity.
Therefore, when the humidity changes, a combination of the main mode and the unnecessary mode occurs at a certain humidity, and the equivalent resistance of the vibrator suddenly increases at this humidity, eventually stopping oscillation. Note that when the humidity changes further, oscillation starts again. Therefore, by changing the humidity at which the main mode and unnecessary mode are combined, the vibrator can be made to stop oscillating at an arbitrary humidity, and can be operated as a temperature sensor.

Hereinafter, the present invention will be explained with reference to Examples.

Figure 1 shows a state in which a hygroscopic substance is applied to the entire surface of the electrode of a piezoelectric vibrator, where 1 is a rectangular piezoelectric diaphragm;
Reference numeral 2 denotes electrodes provided on the front and back sides of the electrode, which are connected to the electrode leg 3 through a lead-out portion 2a. Further, a film 4 formed by applying a hygroscopic substance is provided on the surface of the electrode 2, and 5 is a holder.

When the thickness of the hygroscopic material film 4 is changed while keeping the humidity and temperature constant, the main vibration frequency changes at a rate of -1.0Hz/Å, as explained below, but the frequency change of the unnecessary mode is much smaller than this, about -0.1Hz/Å.

Figure 2 shows how the main oscillation frequency decreases when the thickness of the polyamide coating film used as the hygroscopic substance increases, where the horizontal axis is the film thickness (Å) and the vertical axis is the amount of frequency decrease (KHz). So, the main vibration frequency is -1
Changes are made in Hz/Å.

Further, FIG. 3 shows the resonance characteristics of the thickness shear vibration measured by the Yarman circuit of FIG. 4, using the thickness of the coating film of the hygroscopic substance as a parameter. In the characteristic curves h ₁ to _{h 5} in FIG. 3, the film thickness of the hygroscopic substance on the electrode increases sequentially from h ₁ to h ₅ . Also the fourth
In the figure, the frequency applied to the piezoelectric vibrator 7 from the signal generator 6 is sequentially changed while keeping the output constant, and the resonance characteristics are measured from the indications from the voltmeter 8 for each frequency.

In this case, as is clear from Fig. 3, as the thickness of the hygroscopic material film on the diaphragm electrode increases, the resonance characteristics of the main oscillation mode change from h ₁ to h ₅ , and the resonance frequency increases. From f ₁ to _{f 5} , as described above, changes greatly at a rate of −1 Hz/Å with respect to changes in film thickness. On the other hand, the resonance frequency of the unwanted mode is
As mentioned above, the actual measurement results from f ₁ ′ to f ₅ ′ are -0.1
The change is only a small amount of Hz/Å. Therefore, in the case of the same figure (not shown for simplicity), the resonance frequencies of the main oscillation mode and the unwanted oscillation mode match between resonance curves _h3 and _h4 , that is, coupling of both modes occurs. .

Next, the humidity characteristics of such a vibrator, that is, when the thickness of the hygroscopic material film is kept constant and the temperature is kept constant and the humidity is changed, the amount of moisture absorbed by the hygroscopic material film acts as an increase in the mass of the electrode. FIG. 5 shows a state in which the resonant frequency decreases. In the same figure, the horizontal axis indicates relative humidity (%), and the solid line curve represents the case where the thickness of the hygroscopic material film on the diaphragm electrode is set to a size that does not cause coupling with unnecessary modes, and the curve shows the change in humidity. A shows the change in the resonant frequency with respect to the current, and B shows the change in the equivalent impedance. In the same figure, the dashed curve shows the case where the thickness of the hygroscopic substance on the diaphragm electrode is adjusted appropriately so that the main mode is combined with the unnecessary mode at relative humidity P (82%), and a is the resonance The frequency b is a characteristic curve of equivalent impedance for each humidity.

In this way, the thickness of the hygroscopic material film is adjusted so that the main mode and unnecessary mode are combined at a predetermined humidity, and the equivalent impedance increases rapidly at that humidity, controlling the oscillation to stop. be able to. It should be noted that the characteristic change in response to the humidity change was extremely rapid, with almost no delay observed.

In the above explanation, in order to provide a hygroscopic substance film, an alcoholic solution of polyamide used as a hygroscopic substance is applied to the electrode surface, dried, and this is repeated to gradually increase the film thickness until a predetermined film thickness is obtained. And so.

In addition, the above-mentioned hygroscopic substance is not necessarily applied to the entire surface of the electrode, but only to a portion of the electrode or the diaphragm.
Alternatively, almost the same results were obtained by applying the solution over the entire surface of the diaphragm and further immersing the diaphragm in the solution.

Next, we will consider the influence of temperature on this effect.

Generally, the temperature-related frequency change exhibited by the unwanted mode is more than an order of magnitude larger than the main mode, AT cut thickness shear vibration, and has a large dependence on temperature. Therefore, by selecting an unnecessary mode with a relatively small temperature coefficient and by increasing the rate of change of humidity versus frequency as much as possible, the influence of temperature on operational accuracy can be minimized.

As described above, the piezoelectric humidity sensor according to the present invention is characterized in that it is a digital type highly sensitive humidity sensing element that converts humidity into a frequency, and at the same time, it intentionally stops oscillation at an arbitrary humidity. It performs the function of a humidity switch, which is to control humidity, and is extremely practical and has great effects.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing a crystal resonator according to an embodiment of the present invention, and FIG. 2 is a characteristic diagram showing the relationship between film thickness and main vibration frequency when a hygroscopic material film is provided on the electrode surface of the resonator. , Fig. 3 is a resonance characteristic diagram of thickness shear vibration using the film thickness of the hygroscopic material on the electrode surface as a parameter, Fig. 4 is a circuit diagram showing the Yarman circuit used to measure the above resonance characteristics, and Fig. 5 is a diagram FIG. 3 is a characteristic diagram showing changes in resonance frequency and equivalent resistance due to relative humidity of a vibrator with a constant film thickness of a hygroscopic substance. 1... Rectangular piezoelectric diaphragm, 2... Electrode, 4...
Hygroscopic substance film.

Claims (1)

[Claims] 1. A moisture-sensitive pressure vibrator in which a hygroscopic substance is coated on the vibration surface of a piezoelectric diaphragm, characterized in that the film thickness of the hygroscopic substance is determined so that the main vibration mode and the unnecessary vibration mode are combined at a predetermined humidity. Piezoelectric humidity sensor.