JPH05172679A - Pressure detector - Google Patents

Abstract

PURPOSE:To obtain a pressure detector constituted so that the piezoelectric element 7 provided in the main metal fitting 1 of a pressure sensor is formed from a material having good heat resistance and durability and showing no pyroelectric properties and capable of obtaining a stable combustion waveform from which the drift of the combustion waveform caused by the output of low frequency especially generated by a temp. change is removed. CONSTITUTION:A pressure sensor S using a single crystal piezoelectric material of LiNbO3 having a Curie point higher than the combustion temp. in a cylinder is used as the piezoelectric element provided in a main metal fitting. A charge amplifier A for converting the generated charge of the pressure sensor S to voltage and a high-pass filter F for removing the low frequency component of output voltage are connected to the pressure sensor S.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressure detecting device equipped with a pressure sensor for detecting a change in cylinder internal pressure of an internal combustion engine by a piezoelectric element.

[0002]

2. Description of the Related Art A detection element such as a piezoelectric element and a pressure transmission rod is housed inside a metal shell mounted in a sensor mounting screw hole provided in a cylinder block of an internal combustion engine, and a lower end surface located inside the cylinder. A pressure sensor is known in which a diaphragm is sealed to the cylinder, the internal pressure of the cylinder is propagated to the piezoelectric element through the diaphragm and the pressure transmission rod, and an electric signal proportional to the magnitude of the internal pressure of the cylinder is extracted from the element. is there. A piezoelectric element made of a ceramic material such as lead zirconate titanate or lead titanate is generally used as the piezoelectric element.

[0003]

By the way, since such a pressure sensor is exposed to a high combustion temperature (1000 ° C.) together with the cylinder internal pressure like a spark plug, a ceramic type piezoelectric element is The Curie point is about 250 ° C for lead zirconate titanate system and about 350 ° C for lead titanate system.
℃ and lower than the combustion temperature, both of them,
The element part becomes extremely hot (around 400 ° C), and as a result, the piezoelectric characteristics deteriorate due to depolarization, etc., and it cannot be used. Therefore, it is usually used after adding a cooling means for maintaining the piezoelectric element at an appropriate temperature. As a result, the structure is complicated, the size is increased, and the price is also increased. Further, there is a problem in that the pressure in the cylinder cannot be accurately detected due to the temperature dependence of the piezoelectric element, in which the generated charge increases as the temperature rises, and the pyroelectric characteristics.

On the other hand, in order to solve such a drawback, instead of the ceramic-based piezoelectric material, a SiO ₂ single crystal having no temperature dependence and pyroelectric characteristics or a single crystal piezoelectric material having the same effect as these single crystals is used. It is conceivable that these single-crystal materials have a smaller amount of electric charge generated than ceramic-based piezoelectric materials, so they are easily affected by noise in the spark plug, noise from radio waves, etc., and the S / N ratio deteriorates. There is a problem.

Therefore, in Japanese Patent Application No. 3-125414,
It has been proposed to use a circular or rectangular plate-shaped piezoelectric element made of lithium niobate (LiNbO ₃ ) or lithium tantalate (LiTaO ₃ ) or a single crystal piezoelectric material having the same effect as these single crystal materials. The Curie point of this lithium niobate is as high as about 1200 ° C., and it has excellent heat resistance and low temperature dependence. In addition, LiTaO ₃ , S
Compared to iO ₂ , the generated charge is large and the S / N ratio is excellent, which is an excellent characteristic. However, although the element characteristics are excellent, when a structure is used as a pressure sensor, the component size changes due to the temperature difference during combustion (temperature difference between each member), and an output similar to pyroelectric characteristics is obtained. Sometimes occurred. It is an object of the present invention to eliminate the problems of the conventional structure.

[0006]

According to the present invention, a metal shell mounted on a cylinder of an internal combustion engine is provided with a shaft hole in which a detection opening is located in the cylinder, and a piezoelectric element is provided inside the shaft hole.
A pressure sensor that houses a detection element such as a pressure transmission rod, and further has an opening end of the shaft hole sealed with a metal diaphragm, and uses a single crystal piezoelectric material of lithium niobate as the piezoelectric element, and the pressure sensor. And a filter having a function of removing low-frequency components of the output voltage, the pressure detecting device. As the single crystal piezoelectric material of lithium niobate, it is desirable to use one having a Z-axis component in the polarization direction of 20 ° or less. Further, the threshold value for removing the low frequency of the filter is preferably in the range of 0.01HZ to 1.0HZ.

[0007]

When the pressure sensor is mounted on the engine block to detect the internal pressure of the engine block, the temperature changes drastically such that the temperature changes between idling, high rotation and high load. When the pressure sensor is attached to a portion where the temperature changes drastically as described above, the output having the pyroelectric characteristic is likely to cause the output variation (drift) as shown in FIG. This output fluctuation is
The output waveform has a low frequency.

Therefore, a predetermined low frequency is used as a threshold by a filter, and only a pressure waveform having a frequency equal to or higher than the threshold is transmitted, and a low frequency below the frequency is removed to remove a waveform corresponding to the drift of the output. can do. It is desirable that this threshold value is in the range of 0.01HZ to 1.0HZ. If it is less than 0.01 HZ, the drift cannot be removed well, and 1.0 HZ
This is because if it exceeds, the original pressure waveform will be deformed, and it will not be possible to detect an appropriate pressure, and the accuracy may decrease.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of a pressure sensor S applied to the present invention, in which 1 is a male screw 2 screwed into a cylinder block on the outer periphery of a lower end 1a, and a tightening tool such as a wrench on the outer periphery of an upper end 1b. A metal shell 1 having a bolt structure in which a hexagonal portion 3 that conforms to the above is formed, and a shaft hole 4 is formed therein. The shaft hole 4 has different diameter holes 4a, 4b, 4c penetrating the lower end 1a side and a large diameter hole 4 penetrating the upper end 1b side.
and d are continuously formed. The different diameter holes 4a, 4b, 4
In c, the hole 4b on the inner diameter side is provided with the metal plate 5, the first terminal member 6, and the piezoelectric element 7 provided with electrodes 7a, 7b on the front and back surfaces from the side closer to the hole 4a on the smaller diameter side. The second terminal member 8 and the electrical insulating ring 9 are sequentially arranged in layers, and are insulated from the metal shell 1 by the insulating material 10 that covers the periphery of the laminate. Further, a connecting hand 8a for connecting to a signal carrying cable 16 which will be described later is projected from the upper surface of the second terminal member 8 by inserting the insulating ring 9 therethrough.

Further, the pressure transmission rod 1 is provided in the hole 4a on the smaller diameter side.
1 is inserted, one end thereof is pressed against the metal plate 5, and the other end thereof is bonded to a metal diaphragm 12 joined so as to hermetically seal the end face of the lower end portion 1a of the metal shell 1. This metal diaphragm 12 is required to have heat resistance, and for example, a material such as Inconel or a heat resistant metal material corresponding to SUS630 is preferable. Further, a spring ring 13 is provided between the pressure transfer rod 11 and the stepped portion of the holes 4a and 4b of the metal shell 1, and the spring ring 13 causes strain due to the difference in thermal expansion and the difference in heat transfer between the components, and the male screw portion. It absorbs the stress caused by the elongation due to the tightening torque. Further, a cap screw 18 is screwed into a female screw engraved in the large diameter hole 4c, and a predetermined preload is applied to the piezoelectric element 7 by the screw 18 so as to correspond to a negative pressure state at the time of introducing intake air during combustion. There is.

On the inner surface of the large diameter hole 4d on the hexagonal portion 3 side,
A stepped seat 4d 'is formed, and a metal sleeve 14 having a collar 14a for seating is formed on the stepped seat 4d'.
The annular thin piece 3a protruding from the upper end of the hexagonal portion 3 together with a and the electric contact member 15b is bent by being bent inward.

One end of a signal carrying cable 16 is inserted and fixed inside the metal sleeve 14, and the cable 16
Relay conductor 17 in which the bare core wire 16a is insulated
Is connected to the second terminal member 8 via. Therefore, one electrode of the piezoelectric element 7 is connected to the core wire 16a of the cable 16 through the second terminal member 8 and the relay lead wire 17, and the other electrode is connected to the first terminal member 6, the metal plate 5, and the transmission rod pressure transmission. It is grounded to the metal shell 1 through the rod 11.

The configuration up to this point is conventionally known, but the feature of the pressure sensor S that constitutes the present invention is that the piezoelectric element 7 disposed between the first terminal member 6 and the second terminal member 8 is provided. To
A circle made of lithium niobate (LiNbO ₃ ) (Fig. 2
(See FIG. 3) or a rectangular (see FIG. 3) plate-shaped piezoelectric element.

The piezoelectric element 7 made of a single crystal may be a circular plate, but a rectangular plate as shown in FIG. 3 is preferable from the viewpoint of economy and mass productivity. In this case, as shown in FIG. A large number of piezoelectric elements 7, 7, ... Can be manufactured at one time without waste by making a grid-shaped cutting line t on one large single crystal substrate s. Further, it is desirable that the thickness thereof has an outer dimension of 0.3 mm to 1.5 mm. That is, these single crystal materials have poor mechanical strength and may be damaged by a normal cutting method such as a diamond cutter, an ultrasonic cutter, or a laser beam, but the thickness thereof is 0.3 mm to 1.
Limiting to 5mm solves this problem. If it exceeds 1.5 mm, cracking may occur due to thermal shock during cutting. Therefore, it is preferably 1.0 mm or less. However, if it is less than 0.3 mm, there is a problem in mechanical impact during cutting. Further, in such a pressure sensor, accuracy and parallelism of the pressure receiving surface are required, and usually surface polishing is often performed. From this point, 0.3 mm or more is desirable.

Further, the electrodes 7a and 7b adhered to the front and back surfaces of the piezoelectric element 7 can be formed by plating, vapor deposition or the like, but a metal such as nickel or silver is used in order to improve the adhesion with the piezoelectric element. It is preferable that an organometallic ink dispersed in an organic solvent is used, and this is formed and printed by printing. In this case, after printing the organometallic ink, the first,
By stacking the second terminal members 6 and 8 and baking them integrally, the adhesion between the piezoelectric element and the electrode and between the electrode and the terminal member is increased, and contact failure during use is reliably prevented. There is an advantage to

Next, for the piezoelectric element 7 using a lithium niobate single crystal, its pyroelectric characteristics were confirmed by the following means. Pyroelectric properties were tested by alternately immersing a lithium crystal of lithium niobate single crystal in various angles as a sample, and immersing it in a tank containing silicone oil at 0 ° C and a tank containing silicone oil at 150 ° C. did. The result was as shown in FIG. Where x is the element size φ6
The outer diameter dimension of x 0.5t, y is φ4 x 0.7
The outer diameter of t is shown. The vertical axis shows the electric charge generated by the temperature change, and the horizontal axis shows the angle θ of the Z-axis component.
Indicates. Here, the angle θ of the Z-axis component is defined as shown in FIG. 6, and refers to the deviation angle of the polarization axis from the plane direction (X, Y direction) in the thickness direction (Z direction). That is, θ = 90 ° means polarized in the same direction as the thickness direction, and θ = 90 °
= 0 ° means that it is polarized in the plane direction.

From these results, it was found that the smaller the Z-axis component, the better. Further, in order to reduce the pressure sensitivity to 10 atm or less, it is preferable that the angle θ of the Z-axis component is 20 ° or less, and 10 ° or less, which is 5 atm or less, is even better, and the filtering on the circuit is easy. .. Moreover, in these cuts, the piezoelectric characteristics were sufficient. No Z direction (θ
= 0), the sample was very good, with almost no pyroelectricity observed. Then, in the sample without Z direction (θ = 0), when X-cut and Y-cut products were prepared and examined, the Y-cut product had better piezoelectric characteristics than the X-cut product, Almost the same as other cuts,
Further, good results were obtained in mechanical strength.

From these results, a Y-cut product of lithium niobate was used as the pressure sensor S, and the above-mentioned assembly was carried out by ultrasonic cutting to obtain an outer diameter of φ5 × 0.5t. The inner and outer diameters of the diaphragm were φ7 / φ9, and the thickness of the diaphragm was 0.15 mm. Also,
The pressure applied by the holding screw 14 was set to 10 MPa.

The pressure sensor S having such a structure is shown in FIG.
As described above, the charge amplifier A for converting the electric charge generated by the pressure sensor S into a voltage and the high-pass filter F having the function of removing the low-frequency component of the output voltage of the charge amplifier A are sequentially connected to each other to realize the present invention. Configure the pressure detection device,
The pressure sensor S was attached to an engine with a total displacement of 2000 cc, and the relationship between the low frequency threshold of the high pass filter F and the combustion pressure waveform passing through the high pass filter F was investigated.

FIG. 7 shows various thresholds for cutting the low frequency of the high pass filter F. The engine rotation is idling (1200 rpm) and 5000 rpm.
Shows the combustion pressure waveform when it is constant. Here, the one-dot chain line in the center is a reference line indicating atmospheric pressure. From this waveform diagram, at 0.01HZ to 0.5HZ, the raw waveform without a filter (shown by the broken line) at any rotation speed
It can be understood that the above means that the waveforms do not greatly differ, and that proper accuracy can be maintained. Also, in the case of 1HZ, it can be understood that the predetermined accuracy can be managed somehow although the deviation from the raw waveform is slightly generated at the time of idling. However, it has been found that when a high-pass filter having a threshold value of 2HZ and 3HZ is provided, a drift occurs at a low level, a large difference from the raw waveform occurs, and an error of 1 atmospheric pressure or more occurs.

Next, FIG. 8 shows the state when the engine speed is changed. Even though the pressure sensor S using the piezoelectric element 7 having no pyroelectric characteristic or temperature characteristic is applied, without the high-pass filter F, the entire waveform fluctuates up and down, and engine drift occurs. I understand. Even when the high-pass filter F having a threshold of 0.01 HZ is connected, the drift appears on the waveform. However, in the case where the high-pass filter F having the thresholds of 0.05HZ, 0.1HZ, and 0.3HZ was connected, the drift could be removed on the waveform. The drift can also be dealt with by the high-pass filter F having a threshold value of 1 HZ or less.

Therefore, the threshold value is 0.01HZ to 1.0HZ.
It can be said that the high-pass filter F in the range is capable of removing the drift, is not significantly different from the raw waveform, and can have the accuracy thereof. In addition, it is preferable to obtain a better result by using the high-pass filter F having a threshold value of 0.05 to 0.5 HZ.

[0023]

The pressure detecting device of the present invention is as described above, and the piezoelectric element mounted inside the metallic shell has a LiNb having a Curie point higher than the combustion temperature in the cylinder.
Since the pressure sensor using the single crystal piezoelectric material of O ₃ is used, it is not necessary to cool the piezoelectric element by the cooling means as in the conventional case. Further, the Z-axis component in the polarization direction of the piezoelectric element is set to 20 ° or less, so that the piezoelectric element does not have pyroelectric characteristics. Further, the pressure sensor S further includes a device (charge amplifier A) for converting electric charges generated by the pressure sensor into a voltage, and a filter F having a function of removing a low frequency component of the output voltage.
Since the and are connected, the drift of the combustion waveform due to the low frequency output caused by the temperature change or the like is removed, and a stable combustion waveform can be obtained. Therefore, the present invention has an excellent effect that the pressure detecting device having no pyroelectricity, small size and low cost, good output waveform, and high accuracy can be put on the market.

[Brief description of drawings]

FIG. 1 is a vertical sectional side view of a pressure sensor showing an embodiment of the present invention.

FIG. 2 is a perspective view of a piezoelectric element 7 formed in a circular shape.

FIG. 3 is a perspective view of a piezoelectric element having a rectangular shape.

FIG. 4 is a perspective view showing a cutout unit of the piezoelectric element 7.

FIG. 5 is a graph showing the influence of a pyroelectric effect on a lithium niobate single crystal cut at various angles.

FIG. 6 is a coordinate showing the concept of an angle θ of a Z-axis component.

FIG. 7 is a waveform diagram of 1200 rpm and 5000 rpm, showing a relationship between a high-pass filter F having different thresholds and its output waveform.

FIG. 8 is a waveform diagram showing a drift when the rotation speed is changed.

FIG. 9 is a conceptual diagram showing a basic configuration of the present invention.

[Explanation of symbols]

 1 Metal shell 7 Piezoelectric element 11 Pressure transmission rod S Pressure sensor A Charge amplifier F High pass filter

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hirofumi Hayako, 14-18 Takatsuji-cho, Mizuho-ku, Nagoya City Nippon Special Ceramics Co., Ltd.

Claims (3)

[Claims] 1. A metal shell mounted on a cylinder of an internal combustion engine, comprising a shaft hole in which a detection opening is located in the cylinder,
A detection element such as a piezoelectric element and a pressure transmission rod is housed inside the shaft hole, and the opening end of the shaft hole is sealed with a metal diaphragm, and a single crystal piezoelectric material of lithium niobate is used as the piezoelectric element. A pressure detecting device comprising: a pressure sensor used; a device for converting charges generated by the pressure sensor into a voltage; and a filter having a function of removing low-frequency components of the output voltage. 2. The pressure detecting device according to claim 1, wherein the plate-shaped piezoelectric element of the pressure sensor is made of a single crystal piezoelectric material of lithium niobate having a Z-axis component in the polarization direction of 20 ° or less. 3. The pressure detecting device according to claim 1, wherein a threshold value for removing low frequencies of the filter is in a range of 0.01 HZ to 1.0 HZ.