JPH0746069B2 - Electrostatic Dissolution Type Pressure Sensor - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitance type pressure sensor in which a capacitance formed by two parallel flat plates changes according to an applied pressure.

2. Description of the Related Art In a capacitance type pressure sensor that utilizes the fact that the capacitance formed by two parallel flat plates changes depending on the applied pressure, as a prior art for selecting the electrode diameter to a specific value, , JP-A-58-198739, there is a capacitance type pressure sensor. In this method, a pressure-sensitive capacitance electrode is formed in the center of the diaphragm, and a reference capacitance electrode is formed in the periphery, and the ratio of both capacitances is measured to eliminate temperature dependence.
By setting the distance between the reference capacitance electrode and the insulating layer (glass layer) within a certain range (4 to 15%), the stray capacitance between the reference electrode and the insulating layer is suppressed.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, in such a conventional configuration, the temperature dependence can be removed, but the sensitivity of the pressure-sensitive capacitance (load-capacitance characteristic) cannot be improved. That is, the conventional example shown in the above publication is to be mounted on an automobile, and is struggling to improve only the requirement of severe temperature characteristics, and no consideration is given to the sensitivity itself of the sensor.

The present invention aims to increase the sensitivity of the sensor with reference to the idea of limiting the electrode design shown in the conventional example.

Means for Solving the Problems In order to solve the above problems, the present invention provides a ratio r / R of a diameter r of a pressure-sensitive capacitance electrode and a diameter R of an insulating layer between 33% and 75%. It is something to choose.

The capacitance-type pressure sensor of the present invention has a diameter r of the pressure-sensitive capacitance electrode.
By selecting the ratio r / R to the insulating layer diameter R between 33% and 75%, the peripheral area of the electrode that does not contribute to the pressure-sensitive capacitance is scraped off, reducing the initial capacitance when no load is applied. By doing so, the sensitivity of pressure-sensitive capacitance (load-capacitance characteristic) can be improved.

Embodiment Hereinafter, a capacitance type pressure sensor according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is an exploded perspective view of a capacitance type pressure sensor showing one embodiment of the present invention, and FIG. 2 is a schematic sectional view of the same.

In both figures, the diaphragm 1 is provided with an electrode 2 on its inner surface, and faces the diaphragm 4 with a certain distance through the insulating layer 3. On the other hand, the diaphragm 4 is also provided with an electrode 5, and a capacitance is formed between the electrode 2 and the electrode 5 facing each other with this slight distance.

The diaphragms 1 and 4 are made of an alumina sintered body, and the electrodes 2 and 5 on the inner surface are formed by printing a gold paste on the alumina sintered body and then firing it. The electrodes 2 and 5 are connected to the soldering electrodes 8 and 9 via the lead portions 6 and 7.

The insulating layer 3 is made of glass containing beads, and the gap is determined by the particle size of the beads. The bead diameter is selected to be, for example, about 45 microns, and after the electrode is baked,
Printed over the electrodes. The two diaphragms are then stacked and fired with a load to complete.

The solder electrodes 8 and 9 are formed on the lead portions 6 and 7 of the electrodes 2 and 5, and are formed by applying silver palladium. The change in electrostatic capacitance is input to the detection circuit through a lead wire or the like through the solder electrodes 8 and 9.

Now, FIG. 3 is a diagram showing the result of computer simulation of how the diaphragm 1 deflects when a load P is applied to the center of the diaphragm 1.

As shown in FIG. 2, A, B, C, and D in the figure are fixed points where the insulating layer 3 and the diaphragms 1 and 4 are in contact with each other, and the condition of the sample for which this data was calculated is that the gap (particle size) is 45
μm, the thickness of the diaphragm 1 made of alumina sintered body is
It is 0.5 mm.

As shown in FIG. 3, it is only the central portion of the diaphragm that is actually largely deflected, and the peripheral portion is hardly moved. That is, the peripheral portion of the electrode hardly contributes to the sensitivity. However, the electrostatic capacitance Co under no load is Co = εS / d where ε: Dielectric constant S: Electrode area d: Gap. Naturally, the peripheral electrodes also have a great influence on the initial capacitance Co under no load. .

That is, the electrodes in the peripheral portion increase the initial capacitance, but the contribution to the pressure-sensitive capacitance is low. Therefore, if the peripheral electrodes are scraped off, the sensitivity can be improved.

FIG. 4 shows a state in which a capacitance type pressure sensor is attached to an oscillation circuit as an example of the detection circuit. The oscillating circuit is a triangular wave / square wave oscillator using two circuits of the operational amplifier 10, the pressure sensor Cs is connected to the triangular wave circuit, and the output terminal Vout
Outputs a square wave with a frequency corresponding to Cs.

FIG. 5 shows the relationship between the initial capacitance of the pressure sensor and the circuit sensitivity in such a circuit. From the figure, it can be seen that the circuit sensitivity is improved by reducing the initial capacitance. That is, not only does the peripheral part of the electrode not contribute as a pressure-sensitive capacitance, but it also has a bad function of reducing the sensitivity by increasing the initial capacitance and relatively reducing the amount of capacitance change. it is obvious.

Now, as a conventional capacitance type sensor, a gap of 45 μm,
An electrode with a diameter of 18 mm and an insulating layer of 24 mm has been put into practical use.
It is installed mainly in the microwave oven.

This has an initial capacitance of 40 to 50 pF and a circuit sensitivity of 1.5 Hz / g, which is far from satisfactory in terms of sensitivity as shown in FIG.

In the present invention, the sensitivity is improved by making the electrode diameter r relatively smaller than the conventional example r / R = 75%.

On the other hand, when the r / R value is made smaller and smaller, the output is not determined to be fluttering and the circuit sensitivity becomes apparently unstable, as shown by the shaded area in FIG. This is because the electrode diameter becomes too small, the stray capacitance of the circuit cannot be ignored, and the output is affected and fluctuates. Since such a stray capacitance is usually considered to be about several pF to several tens of pF, it is necessary to set the initial capacitance larger than this value. In the present invention, the lower limit of the r / R ratio is set to 33% in order to avoid the instability of sensitivity due to the stray capacitance.

Now, with the above configuration, the sensitivity of the pressure sensitive capacitance can be made high and stable.

EFFECTS OF THE INVENTION As described above, in the capacitance type pressure sensor of the present invention, the diameter r of the pressure sensitive capacitance electrode is selected to be a specific value with respect to the diameter R of the insulating layer. The capacity can be stably reduced, and the amount of change in the pressure-sensitive capacity under load can be relatively increased.

Further, since such an electrode is formed by printing and firing a gold paste, the cost can be significantly reduced by making the electrode diameter small. Further, it is possible to reduce the number of process problems such as uneven coating when printing gold paste.

[Brief description of drawings]

FIG. 1 is an exploded perspective view of a capacitance type pressure sensor showing an embodiment of the present invention, FIG. 2 is a schematic sectional view of the same, and FIG. 3 is a diagram showing a result of simulating displacement of the diaphragm. FIG. 4 is an oscillator circuit showing an embodiment of the detection circuit, and FIG. 5 is a characteristic diagram showing the relationship between the initial capacitance and the circuit sensitivity of the capacitance type pressure sensor by the circuit. 1 ... diaphragm, 2 ... electrode, 3 ... insulating layer, 4 ...
... diaphragm, 5 ... electrode, r ... electrode diameter, R ... insulating layer diameter.

Claims (1)

[Claims] 1. A pair of diaphragms, electrodes provided on the inner surfaces of the diaphragms, and an insulating layer connecting the two at the outer peripheral portion of the diaphragms, the diameter r of the electrodes and the diameter R of the insulating layer. The capacitance type pressure sensor is characterized in that the ratio r / R is selected between 33% and 75%.