Vibration and Acoustic Wave Detecting Device Employing a Piezoelectric Element

1. Field of the Invention

This invention relates to a vibration and acoustic wave detecting device employing a piezoelectric element. More particularly, this invention relates to an improved construction of a resonant-type vibration and acoustic wave detecting device employing a piezoelectric element which makes it possible to increase its available frequency bandwidth by lowering its quality factor Q.

2. Description of the Prior Art

With respect to a piezoelectric knock sensing device of a type employing a piezoelectric conversion element, a piezoelectric type knock sensor having a construction in which a resistor is connected in parallel with a piezoelectric conversion element so as to broaden its available frequency bandwidth is already disclosed by prior art references, for example, by Japanese Patent Unexamined Pub-ications Nos. 58-142227 and 58-142228.

As mentioned above, it is publicly known to make a resonant-type detector employing a piezoelectric element which has a construction in which a resistor is connected in parallel with the piezoelectric element to decrease its quality factor thereby to have a broad available frequency bandwidth, with a view to removing a defect such that the available frequency bandwidth becomes narrow as the quality factor increases. However, there is no prior art reference which has disclosed any concrete structure for incorporating a resistor in the resonant-type detector employing a piezoelectric element.

It is an object of the present invention to provide an improved construction of a vibration and acoustic wave detecting device employing a piezoelectric element such that, in the manufacture of the detecting device, there occurs no inconvenience in any process of the manufacture thereof and it is made very simple and easy to have a resistor connected in parallel with the piezoelectric element across its two output terminals for the purpose of increasing the available output frequency bandwidth.

According to the detecting device of this invention, due to the fact that, in the manufacturing process thereof, there are arranged a polarization step after a step of mounting a resistor on a piezoelectric element, which has electrodes attached thereto beforehand, and then the required assembly steps including a steps of mounting the piezoelectric element on a conductive member, there are great meritorious effects that the polarization of the piezoelectric element can be effected without causing any inconvenience and that the resistor can be mounted very simply and easily.

With the construction of the detecting device according to this invention, there is another effect that the adjustment of the resistance value of the resistor can be effected after the mounting of the resistor very easily during the manufacture of this detecting device.

With the construction of the detecting device according to this invention, there is still another effect that it is possible to easily obtain a detecting device comprising a plurality of piezoelectric elements connected commonly between two output terminals to operate in parallel with each other using a single resistor in common.

FIG. 1 is a longitudinal sectional view showing a complete overall construction of a detecting device of this invention.

FIG. 2 is an electric circuit diagram showing an equivalent electric circuit of the detecting device shown in FIG. 1.

FIG. 3 is a plan view showing the first surface of the piezoelectric element used in the detecting device of this invention.

FIG. 4 is a bottom view showning the second surface of the piezoelectric element used in the detecting device of this invention.

FIG. 5 is an electric circuit diagram showing an equivalent electric circuit of a detecting device of this invention with its piezoelectric element having electrodes formed thereon having not yet been attached to the surface of the conductive member (diaphragm), thus staying in a state thereof which has not yet reached the complete state of the detecting device of this invention shown in FIG. 1.

FIG. 6 is an overall sectional view of a knock sensing device as an example of practical application of the detecting device of this invention.

FIG. 7 is a longitudinal sectional view showing a longitudinal section of a complete detecting device of another embodiment of this invention using a combination of two or more piezoelectric elements each thereof having electrodes formed thereon.

FIG. 8 is an electric circuit diagram showing an equivalent electric circuit of the detecting device of the embodiment of this invention shown in FIG. 7.

FIG. 9 is a plan view of a piezoelectric element having a number of resistance pieces arranged on its first surface so as to facilitate the trimming of the resistance value of the resistor attached to the piezoelectric element in the detecting device of this invention.

The construction and operation of a vibration and acoustic wave detecting device employing a piezoelectric element (hereinafter simply referred to as a detecting device) will now be described with reference to the accompanying drawings. In the drawings, like reference numerals designate the same or similar constituent elements.

Referring to FIG. 1 illustrating the complete overall construction of the detecting device of this invention, numeral 1-1 designates a piezoelectric element made of a typical material such as lead titanate zirconate ceramics. Numeral 1-2 designates a first electrode deposited on a first surface of the piezoelectric element 1-1, 1-3 a second electrode deposited on a second surface of the piezoelectric element 1-1, and 1-4 a side electrode deposited across the first and second surfaces of the piezoelectric element 1-1 to surround its side surface. These electrodes 1-2, 1-3 and 1-4 are discretely arranged so as to be electrically isolated from each other on the surfaces of the piezoelectric element 1-1. It is to be noted that these electrodes are formed by depositing a material such as gold, silver, a silver-palladium alloy on the surfaces of the piezoelectric element 1-1 by using a printing technique or the like. Numeral 1-5 designates a conductive member made of a metallic material such as stainless steel (SUS) or Kovar which serves the purpose of supporting the piezoelectric element 1-1 attached thereonto by an adhesive or the like and functions as a diaphragm for transmitting the vibrations imparted thereto from an object to be tested to the piezoelectric element 1-1.

Numeral 2 designates a resistor formed by depositing through a printing technique or the like a pasty electric resistance material containing RuO.sub.2, etc., on the first surface of the piezoelectric element 1-1 so that it extends across the respective ends of the first electrode 1-2 and the side electrode 1-4. Numeral 1-6 designates a hole which is formed through the complete detecting device shown in FIG. 1 and around a longitudinal center line thereof and which is used to fix the detecting device to a support of a detecting device supporting member when incorporating the detecting device in a detecting device assembly.

FIG. 2 shows an equivalent electric circuit of the detecting device shown in FIG. 1. FIG. 3 is a plan view showing the first surface of the piezoelectric element unit used in the above-mentioned detecting device of this invention, and FIG. 4 is a bottom view showing the second surface of the piezoelectric element unit.

Next, a description will be made of the features of the construction and manufacturing process steps of the detecting device of this invention as shown in FIG. 1. It will be seen that this invention as shown in FIGS 1-4 is different from those of the prior art device.

The printing process for depositing the resistor 2 on the first surface of the piezoelectric element 1-1 including a baking step. The resistor is printed so that it extends across the first surface electrode 1-2 and the side electrode 1-4. However, due to the baking step, the surface of the piezoelectric element 1-1 is usually exposed to a high temperature, of over 500.degree. C., in the baking step. If the piezoelectric element 1-1 is polarized in advance of the baking step, the polarization characteristics of the piezoelectric element 1-1 are deteriorated by the application of heat in the baking step. Therefore, the polarizing step of the piezoelectric element 1-1 must be performed after the step of baking the resistor 2. Here, however, in the polarizing step of the piezoelectric element 1-1, a high dc field of several kilovolts/mm is applied to the piezoelectric element 1-1 for more than several minutes. Therefore, in this case, if the construction of the detecting device is represented by the equivalent circuit shown in FIG. 2, a heavy current flows through the resistor 2 during the polarizing step, so that a resistor 2 having a small current capacity will be burnt. Therefore, if a low current capacity resistor is used, the equivalent circuit of FIG. 5 is desirable during polarization. FIG. 5 shows the state where the first surface electrode 1-2, the second surface electrode 1-3 and the side electrode 1-4 have been deposited on the surface of the piezoelectric element 1-1. The the resistor 2 is deposited on the first surface of the piezoelectric element 1-1 so as to extend across the first surface electrode 1-2 and the side electrode 1-4 as shown in FIGS. 3 and 4. However, conductive member 1-5 has not been attached to the piezoelectric element 1-1. By not attaching conductive member 1-5, the equivalent electric circuit shown in FIG. 5 is yielded. Thus, in this state, even if a high dc voltage is applied across the first surface electrode 1-2 and the second surface electrode 1-3 to polarize the piezoelectric element 1-1, no current flows through the resistor 2, and therefore the polarization of the piezoelectric element 1-1 can be effected without any problem.

Then, after the completion of the polarizing step, the polarized piezoelectric element 1-1 is attached to the conductive member 1-5, whereupon the second surface electrode 1-3 and the side electrode 1-4 are electrically connected with each other through the conductive member 1-5. Thus, an equivalent circuit of the detecting device in the complete state becomes as shown in FIG. 2, and the polarized piezoelectric element 1-1 and the resistor 2 are electrically connected in parallel between a first output terminal 4 and a second output terminal 8 of the detecting device.

The detecting device of this invention operates in the following manner. Mechanical vibrations of an object under test are transmitted to the piezoelectric element 1-1 through the detecting device supporting member and the conductive member 1-5 acting as a diaphragm.

Thus, the piezoelectric element 1-1 is subjected to bending vibrations together with the conductive member 1-5, and the vibrations of the object are detected by an output voltage which is produced in the piezoelectric element 1-1, appears across the first surface electrode 1-2 and the second surface electrode 1-3 and is delivered from the output terminals 4 and 8.

Since the detecting device of this invention is of a resonant type, it has a disadvantage such that its available frequency bandwidth becomes narrow, although a high output voltage is obtained in a frequency range around the resonant frequency, as described before. This disadvantage can be removed by connecting the additional resistor 2 in parallel with the piezoelectric element 1-1 as shown in the equivalent circuit of FIG. 2 and by effecting the trimming (fine adjusting) of its value. With this arrangement, it is possible to easily obtain a detecting device which is capable of reducing variations in the sensitivity and possessing a suitable extended available frequency bandwidth, yet having a required output voltage level, even though the magnitude of the output voltage level is reduced by the provision of the resistor 2. Here, it is known from the disclosures of the aforementioned prior art references that, if R represents the resistance of the resitor 2, C the capacitance of the piezoelectric element 1-1 and f the resonant frequency, a maximum available frequency bandwidth is obtained when the resistance of the resistor 2 satisfies a relation R=1/(2.pi.f.multidot.C).

Next, an example of practical application of the detecting device of this invention as a knock sensing device will be described with reference to FIG. 6. FIG. 6 is an overall sectional view illustrating a construction of an example of practical application of the detecting device of the present invention in which it is incorporated in a knock sensing device assembly. In FIG. 6, numeral 6 designates a connector case made of an electrically insulating material, and 4 a connection pin for external connection of the output of the detecting device which is made of a conductive material and is molded in the connector case 6. Numeral 3 designates a lead wire one end of which is connected to the first surface electrode 1-2 of the piezoelectric element 1-1 and the other end of which is connected to an inner end of the pin 4. Numeral 5 designates a metal base. Fastened by welding or the like to the metal base 5 is the conductive member 1-5 acting as a diaphragm which is attached onto the second surface of the piezoelectric element 1-1 having the second surface electrode 1-3 deposited thereon. Numeral 7 designates a housing which has a threaded portion for the mounting thereof on the engine block and which encloses the base 5 and a base portion of the connector case 6 and holds them in a pressed state. Thus, the second surface electrode 1-3 of the piezoelectric element 1-1 is electrically connected to the housing 7 through the conductive member 1-5 and the base 5, so that the housing 7 functions as the second output terminal 8 in FIG. 2.

The detecting device of this invention may also have such a construction as that of the embodiment shown in FIG. 7, in which a piezoelectric elment having the first surface electrode, second surface electrode and side electrode and a resistor deposited thereon and one or more other piezoelectric element having only a first surface electrode and a second surface electrode deposited thereon are attached onto a single conductive member. In this case, the piezoelectric elements which are attached to the opposite surfaces of the conductive member are polarized with polarities opposite to each other before being attached to the conductive member. Referring to FIG. 7 numerals 1-7 and 1-8 designate the first surface and second surface electrodes of the other piezoelectric element, respectively. FIG. 8 shows an equivalent circuit of the embodiment shown in FIG. 7.

As described hereinabove, in the detecting device of this invention, the trimming of the resistance of the resistor 2 is performed by using a laser beam or the like to have an optimum value thereof. Here, in order to facilitate the trimming operation, it is possible to deposit a large number of small resistors (resistance pieces) 2, each thereof having a suitable width, on the first surface of the piezoelectric element as shown in FIG. 9, and then these small resistors may be cut one by one as occasion demands, thereby attaining the trimming more easily.