JP2003287550A - Acceleration sensor - Google Patents

Abstract

(57) [Problem] To provide an acceleration sensor that is small, lightweight, robust, and capable of reducing the influence of external noise. SOLUTION: In the electrostatic acceleration sensor, a sensor main body 20 is housed in airtight cases 1 and 2, and is supported inside the case away from the inner surface of the airtight case. Since air permeability is provided above and below the sensor body 20, the sensor body 2
By giving the air permeability to itself, a decrease in sensitivity due to viscous resistance of air can be remarkably suppressed. In addition, the influence of external noise on the sensor body 20 due to the air around the sensor body 20 can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an acceleration sensor, and more particularly to an acceleration sensor that can be preferably used as a heart sound microphone.

[0002]

2. Description of the Related Art Conventionally, for example, direct auscultation using a stethoscope has been widely performed in the diagnosis of heart disease, respiratory system and the like. While direct auscultation is useful as a simple diagnostic method, it has the following problems.

1) It lacks objectivity and quantitativeness because it reflects individual differences among doctors. 2) Since heart sounds cannot be recorded, they cannot be compared with past conditions or used for other analyses. 3) Although the main component of heart sound exists in a low frequency band of 30 to 800 Hz, it is difficult to distinguish the low frequency region due to human auditory characteristics, and it is particularly difficult to distinguish the heart sound component in the region of several tens Hz. In order to deal with such a problem, it has been conventionally practiced to detect a heart sound and record it as, for example, a heart sound diagram.

Heart sound meters for detecting and recording heart sounds are mainly
It has a heart sound microphone for detecting a heart sound, an amplifier for amplifying a signal output from the heart sound microphone, a filter for selecting a frequency component to be recorded, a filter for removing a noise component, a recording device for recording a heart sound waveform, and the like.

The heart sound microphone is attached to the chest of a subject and converts the mechanical vibration of the chest wall associated with the heartbeat into an electric signal. As the conversion method, a moving coil type, a piezoelectric type, and an electrostatic type are known. And the output is displacement,
Velocity, acceleration, etc. can be obtained.

Conventionally, as a heart sound microphone, a piezoelectric acceleration sensor having a structure in which a piezoelectric element is used and an acceleration is converted into an electric signal and output is widely used. Specifically, it has a structure in which a vibrating film, which is composed of a piezoelectric element and to which a weight is attached, is supported in a sealed case. When vibration is transmitted to the heart sound microphone having such a configuration, the weight tries to stop at the place according to the law of inertia, so that a force is applied to the piezoelectric element. Since the voltage generated by the piezoelectric element due to this force is proportional to the inertial force of the weight, that is, the magnitude of the input vibration, it is possible to detect the acceleration from the magnitude of the voltage.

The heart sound microphone needs to have high sensitivity because it detects a minute vibration called heart sound that propagates to the chest wall. To increase the sensitivity of the above heart sound microphone,
The weight attached to the vibrating membrane may be increased.

[0008]

However, since the heavy weight has a large size and a large load is applied to the vibrating membrane composed of a piezoelectric element, it is vulnerable to impact and is damaged even if dropped from a height of about 1 m. . Further, since a large weight is used, the heart sound microphone is increased in size. Since the heart sound microphone is mounted on an elastic body such as the chest of the human body and used, if the heart sound microphone is heavy, the treble component of the heart sound is attenuated. Furthermore, the large size makes it difficult to mount the space between the ribs of the subject.

Further, the holder and ring for supporting the vibrating membrane inside the case usually have a shape in close contact with the entire side surface of the case, and not only the vibration in the direction perpendicular to the vibrating membrane that should be detected, but also the side surface. It was easily affected by external noise, etc. input from.

On the other hand, for the acceleration sensor, for example, Japanese Patent Laid-Open No.
As disclosed in Japanese Patent Application Laid-Open No. 232136, a capacitor is formed by a vibrating membrane and an electrode plate which are arranged with a minute gap in a sealed case, and an acceleration is detected from a voltage change due to a capacitance change of the capacitor. Electric type acceleration sensors are known.

Since the electrostatic acceleration sensor does not need to detect the voltage directly from the vibrating membrane, it is not necessary to attach a weight to the vibrating membrane, and is superior to the piezoelectric type acceleration sensor in terms of weight and size.

However, even in the electrostatic acceleration sensor, since the holder for supporting the vibrating membrane and the electrode plate in the case is in close contact with the inner surface of the case almost all over, the vertical direction of the vibrating membrane is In addition to the vibration to be detected, it was easily affected by external noise input from the side surface of the case.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems of the prior art, and its object is to:
An object of the present invention is to provide an acceleration sensor that is small, lightweight, robust, and capable of reducing the influence of external noise.

In order to achieve this object, the acceleration sensor according to the present invention uses a vibrating membrane, a back pole facing the vibrating membrane at a predetermined distance, and a gas present between the vibrating membrane and the back pole. Is an acceleration sensor that detects acceleration to the vibrating film based on a change in the capacitance of the capacitor due to the vibration of the vibrating film, and includes a substantially cylindrical inner case that holds the vibrating film and the back pole, and the inner case. It has an airtight outer case, and the outer case has a supporting means for supporting the inner case with a space from the inner surface of the outer case.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below in detail based on its preferred embodiments with reference to the drawings. Figure 1
An exploded perspective view showing a configuration example of an acceleration sensor according to the present invention,
FIG. 2 is a vertical sectional view taken along line AA of FIG.

As can be seen from FIG. 2, the acceleration sensor 100 according to the present embodiment has an airtight outer case composed of a resin lower case 1 and an upper case 17, and a unit case 2 as an inner case. The sensor main body 20 held is supported. As will be described later, by supporting the sensor body 20 away from the inner surface of the housing, it is possible to ventilate the sensor above and below the sensor, reduce the sensitivity loss of the sensor, and detect unnecessary vibration. It can be reduced.

The configuration of the acceleration sensor according to this embodiment will be described in more detail with reference to FIG. 1 and FIG. 3 which is a vertical sectional view of the sensor body 20.

The sensor main body has a structure in which the respective components 3 to 16 are laminated in a metal unit case 2 having a substantially cylindrical shape, and the components are held in the case by flanges 21 and 22 constituting the upper and lower surfaces of the unit case 2. have. The uppermost surface in the unit case 2 (the lowermost surface when used, but will be described with reference to the drawings. The same applies to other components below)
The front ring 3 is provided in the
And plays the role of a spacer between the inner surface of the jaw 21 of the unit case 2.

The vibrating membrane 7 is made of, for example, a metal thin film made of metal vapor deposited, and has a predetermined tension by being fixed to the vibrating membrane ring 8 at the periphery thereof. Further, a ventilation hole 71 is provided in the center of the vibrating membrane 7. A ring-shaped weight 6 having a common center is attached to the surface of the vibrating membrane 7, and a ring-shaped adjustment weight 5 is also provided on the weight 6 if necessary. The adjustment weight 5 is an adjustment weight for adjusting the sensor sensitivity and the resonance point, and is sufficiently lighter than the weight 6. In some cases, the adjusting weight 5 is unnecessary, and in some cases, a plurality of adjusting weights 5 are used.

As will be described later, the weight 5 has a vibrating film 7 shape.
Has a great meaning in maintaining a constant distance between the back electrode 10 and the back electrode 10 and preventing adsorption between the vibrating membrane 7 and the back electrode 10.

The adjustment acoustic resistance 4 is a breathable film attached so as to cover the hole of the weight 6 (and the adjustment weight 5), and the other adjustment is provided in the hole 16a of the rear plate 16. Acoustic resistance (not visible in Figure 1, but not in Figure 2
And (shown in FIG. 3) together, acts as a fluid resistance to the air passing through the sensor.

The sensitivity of the sensor is adjusted by the adjusting acoustic resistance 4.
It is possible to finely adjust the resonance point of the vibrating film, the sharpness of the resonance, and the like. The adjustment acoustic resistance 4 can be made of, for example, a nonwoven fabric.

The ring-shaped spacer 9 is provided to separate the vibrating membrane 7 and the back electrode 10 described later by a predetermined distance due to the thickness thereof and to insulate them. Therefore, the vibrating membrane 7 and the back electrode 10 form a capacitor having a dielectric layer of air having a thickness equal to the distance by the spacer 9. The spacer 9 has a thickness of about several tens of μm.

The back electrode 10 is composed of a metal plate to which a piezoelectric film is pressure-bonded, and ventilation holes 10a are provided at four positions in this embodiment.

The back electrode 10 and the contact plate 11 are held by the insulator 12 around the periphery. The contact plate 11 is various circuits mounted on the printed wiring board (PC board) 13, especially FETs for impedance adjustment.
14 is a conductor for electrically connecting the back electrode 10 and the back electrode 10.

The PC board 13 has an F for impedance adjustment.
Besides ET14, an electric circuit element (not shown) for generating a sensor output is mounted. The PC board 13 is also provided with ventilation holes 13a.

The fixing ring 15 is a spacer provided so that the circuit components on the PC board 13 do not contact the rear plate 16. The rear plate 16 constitutes the bottom surface of the sensor body 20. The rear plate 16 is provided with a hole for drawing out a cable (not shown) for taking out an output signal from a circuit on the PC board 13 and a pair of ventilation holes 16a. In addition, as described above, the rear plate 16
An acoustic resistance 4 for adjustment is provided so as to cover the hole 16a for ventilation.

The sensor body 20 having such a structure
Is further housed and sealed in a case composed of an upper case 17 and a lower case 1 to form an acceleration sensor. In the acceleration sensor according to the present embodiment, the sensor main body 20 is supported by the apexes of the protrusions or ribs provided on the inner surface of the case, so that the contact area between the sensor main body 20 and the inner surface of the case is reduced, and external noise added to the case is directly detected by the sensor. It is possible to greatly reduce the influence on the main body.

For example, in the present embodiment, the sensor body 20 is supported in the case by the ring-shaped rib 1a provided on the inner bottom surface of the lower case 1 and the ring-shaped rib 1b provided on the inner side surface of the lower case 1. To be done. These ribs 1a,
When the case is made of resin, the case 1b may be integrally formed, or the ribs 1a and 1b may be fitted in grooves formed on the inner surface of the case. The cross-sectional shape of the rib is arbitrary, but in order to reduce the contact area with the sensor main body 20, it is preferable that the rib becomes narrower toward the tip as shown in FIG.

Further, it is not always necessary to support the sensor body over the entire circumference, and if the sensor body 20 can be supported without rattling, the sensor body may be supported by a plurality of protrusions. In this case, it is preferable to support three or more points on the side surface of the sensor and two or more points on the jaw 21 of the unit case 2.

The positions of the ribs or protrusions that support the sensor body 20 do not necessarily have to be the positions shown in FIGS. 2 and 4, and the sensor body 20 can be supported separately from the inner surface of the case and the operation of the sensor is possible. It can be supported at any position as long as it does not hinder the operation.

The height of the ribs or protrusions that support the sensor body 20 can be arbitrarily set, but if it is too high, the size of the sensor as a whole is increased, and if it is too low, foreign matter due to the air layer existing around the sensor body 20 is introduced. It is preferable to provide ribs or protrusions that are as high as possible within the range in which the sensor satisfies the desired size, because the noise buffering effect decreases.

In the acceleration sensor according to this embodiment, holes are provided in each of the rear plate 16, the PC plate 13, the back electrode 10, the vibrating membrane 7, the weight 6, and the adjusting weight 5 which form the sensor body 20, Further, since the adjustment weight 5 located at the outermost side of these parts and the adjustment acoustic resistance 4 attached so as to cover the holes of the rear plate 16 also have air permeability,
Air can pass through the inside of the sensor body 20. further,
Since the sensor main body 20 is supported away from the inner surface of the case, air can move between the upper and lower surfaces of the sensor main body 20 in the state instructed inside the case. Therefore, it is possible to suppress a decrease in sensitivity due to the viscous resistance of air.

In the acceleration sensor having such a structure,
When a predetermined voltage is applied to the vibrating film 7 and the back electrode 10 through the unit case 2 and the FET 14, the vibrating film 7 and the back electrode 10 are applied.
A charge is accumulated on the opposite surface of the capacitor to form a capacitor. When the case vibrates in this state, the vibration is transmitted to the sensor body 20. Although the sensor body 20 vibrates, the weight 6 (and the adjustment weight 5) attached to the vibrating membrane 7 tries to stay in the original position due to inertia. As a result, the vibrating membrane 7 vibrates,
As the distance to the back electrode 10 changes, the capacitance of the capacitor composed of the vibrating membrane 7, the back electrode 10 and the air layer therebetween changes. The change in the capacitance of the capacitor is observed as a change in the voltage value between the vibrating membrane 7 and the back electrode 10, and the acceleration applied to the sensor body 20 can be detected from this voltage value.

Next, the shape of the weight 5 attached to the vibrating membrane 7 will be described. As described above, the distance between the vibrating membrane 7 and the back electrode 10 is very small, several tens of μm. Moreover, during operation, electric charges having opposite polarities appear on the opposing surfaces of the vibrating membrane 7 and the back electrode 10, so that a force attracting the vibrating membrane 7 and the back electrode 10 is generated.

Since the vibrating membrane 7 has its periphery fixed, the moving amount increases toward the center. Moreover, when a weight is attached to increase the sensitivity, there is a large tendency to attach the weight to the center in order to obtain a large effect with a small weight. Therefore, the difference in the moving amount between the central portion and the peripheral portion further increases, and the central portion of the vibrating membrane 7 comes closer to the back pole 10 during vibration. As a result, the vicinity of the central portion of the vibrating membrane 7 and the back electrode 10 may be adsorbed in some cases, and the function as a sensor may not be fulfilled.

In view of such a situation, the present invention takes the vibrating membrane 7 into consideration.
By improving the shape of the weight 6 attached to, the adsorption of the vibrating membrane 7 and the back electrode 10 was prevented, and stable acquisition of the sensor output was made possible.

That is, in the acceleration sensor according to the present invention, the weight 6 attached to the vibrating membrane 7 is constructed so that the attachment area is as large as possible. In other words, if the weights have the same weight, they are thin and have a large outer diameter. By attaching the weight having such a shape to the vibrating membrane, it becomes possible to maintain the planarity of the vibrating membrane in a wider range as compared with the case where the weight having a small mounting area is used (FIG. 5A) ( Figure 5
(B)).

As a result, since the distance between the vibrating membrane and the back electrode is uniform over a wide range, it is possible to prevent the central portion from being adsorbed, and also the distance variation between the vibrating membrane and the back electrode due to vibration. Since it also occurs in a wide range, it is possible to obtain the change in capacitance due to vibration with higher accuracy.

In the present embodiment, in order to increase the outer diameter of the weight 6, it has a ring shape with a hollowed central portion. Further, by providing a ventilation hole in the center portion of the vibrating membrane 7 to which the weight 6 is not attached, adsorption of the center portion is prevented even in terms of shape.

As will be apparent to those skilled in the art, the outer diameter of the weight 6 does not have to be simply large when determining the shape of the weight 6. That is, the weight becomes too thin and the rigidity is insufficient to maintain the flatness of the vibrating membrane. In the case of a ring shape, if the ring is too thin even if the outer diameter is large, the flatness of the central portion of the vibrating membrane cannot be obtained. If the difference between the inner diameter of the diaphragm ring to which the diaphragm is attached and the outer diameter of the weight 6 is too small, the diaphragm becomes difficult to move. Therefore, the shape of the weight is determined so that (1) the vibration of the vibrating membrane is not hindered, and (2) the flatness of the central portion of the vibrating membrane can be maintained, and the mounting area with the vibrating membrane is large within a range. Become. In the present specification, the "mounting area" means an area where the flatness of the vibrating membrane is substantially maintained by the weight, and the entire area does not need to be fixed to the vibrating membrane.

The change in characteristics due to the shape of the weight will be further described. An acceleration sensor having the configuration of FIG.
As shown in FIG. 6, outer diameter 14.5 mm, inner diameter 12.5 m
A brass back with a vibrating membrane (thickness: 4 μm, made of PPS, gold deposited on the surface) with a hole with a diameter of 1 mm in the center attached to the stainless vibrating membrane ring of m with an adhesive and crimping an FEP electret film. Input / output characteristics were measured by using acceleration sensors arranged at 50 μm intervals from the poles and attaching weights having the same weight (0.7 g) and different shapes.

Three types of weights having a diameter of 6 mm, 8 mm, and 10 mm and having a disk shape with a hole having a diameter of 1 mm in the center were used. The weight and the vibrating membrane were attached to the non-perforated vibrating membrane with an adhesive so that the positions of the holes corresponded to each other, and then a hole having a diameter of 1 mm was provided through the vibrating membrane and the weight. In the case of adding a predetermined acceleration (0.98m / S ^2), frequency 20
The input / output characteristic at 1 kHz is shown in FIG. As is clear from FIG. 7, even if the weights have the same weight, a larger output can be obtained by using a weight having a larger outer diameter. It is considered that this is because the flatness is maintained in a wide range of the vibrating membrane and the capacitance change due to vibration can be detected by the configuration. Further, in any case, adsorption between the vibrating membrane and the back electrode was not seen.

As described above, according to this embodiment, it is suitable for the detection of weak vibrations such as heart sounds, which is lightweight and robust, has excellent durability, and realizes highly sensitive acceleration detection while suppressing the influence of external noise. The acceleration sensor is realized.

[0045]

[Other Embodiments] In the above embodiments, the case where the atmosphere in the case is air has been described, but other gas such as an inert gas (both single component gas and mixed gas are also possible) may be filled. It is possible.

Further, in the above-mentioned embodiment, only the case where the weight attached to the vibrating membrane has a ring shape or a disk shape has been described, but the weight need not necessarily be a perfect circle.
Also, the number of weights is not limited to one,
A plurality of weights may be arranged on the vibrating membrane as long as the flatness of the vibrating membrane is maintained in a wide range and the vibration is not hindered.

Further, the adsorption preventing effect and the sensitivity increasing effect by improving the shape of the weight are not necessarily obtained only in the acceleration sensor having the vibrating membrane provided with the ventilation hole, and the electrostatic type Other than this, for example, it is effective for a piezoelectric type acceleration sensor.

Further, in order to prevent the vibration film and the back electrode from being attracted to each other, as shown in FIG. 8, it is possible to provide minute ribs or protrusions on the mounting surface side of the weight 6 to the vibration film 7. . With such a configuration, the vibrating membrane 7 and the back pole 10
Even when there is a contact with the contact area, the contact area is reduced, and the vibrating membrane 7 and the back electrode 10 are easily separated. In this case, the weight 6
The ribs and the protrusions provided on the base plate may have a rounded shape because the vibrating membrane may be damaged if the apex is too sharp. Also, set the height so that it does not interfere with vibration.

[0049]

As described above, according to the acceleration sensor of the present invention, by holding the acceleration sensor main body with a gap between the acceleration sensor main body and the inner surface of the case, it is possible to ventilate the upper and lower surfaces of the acceleration sensor, thereby increasing the sensitivity. In addition to realizing a good acceleration sensor, it is possible to mitigate the effect of unnecessary external noise on the acceleration sensor due to the air present in the gap between the acceleration sensor body and the inner surface of the case.

Furthermore, by using a weight having a shape such that the mounting area to the vibrating membrane is as large as possible, it is possible to maintain the planarity of the vibrating membrane in a wide range, prevent adsorption to the back electrode, and achieve high accuracy. It is possible to detect various capacitance changes.

[Brief description of drawings]

FIG. 1 is an exploded perspective view showing a configuration example of an acceleration sensor according to an embodiment of the present invention.

FIG. 2 is a sectional view taken along line AA of FIG.

3 is a vertical cross-sectional view of the sensor body 20 of FIG.

FIG. 4 is a partial perspective view showing an example of a rib structure for supporting the sensor body 20 of the lower case 1.

FIG. 5 is a diagram illustrating a difference in movement of a vibrating film depending on a shape of a weight.

FIG. 6 is a diagram illustrating parameters of a vibrating membrane and a weight used in an experiment.

FIG. 7 is a diagram showing output characteristics when the shape of the weight is changed.

FIG. 8 is a diagram showing a shape of a weight of an acceleration sensor according to another embodiment of the present invention.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Naoki Nishimura             3-39-4 Hongo, Bunkyo-ku, Tokyo Fukuda Electronics             Within the corporation

Claims (9)

[Claims] 1. A vibrating membrane, a back electrode facing the vibrating membrane at a predetermined distance, a gas present between the vibrating membrane and the back pole to form a capacitor, and An acceleration sensor for detecting an acceleration with respect to the vibrating film based on a change in the capacitance of the capacitor, comprising: a substantially cylindrical inner case that holds the vibrating film and the back electrode; and an airtight outer case that houses the inner case. The acceleration sensor is characterized in that the outer case has a supporting means for supporting the inner case with a space from the inner surface of the outer case. 2. The acceleration sensor according to claim 1, wherein the vibrating membrane and the back electrode are provided with holes to have air permeability inside the inner case. 3. The support means is a rib or a protrusion provided on the inner surface of the outer case.
Alternatively, the acceleration sensor according to claim 2. 4. The acceleration sensor according to claim 1, further comprising a weight attached to a surface of the vibration film that does not face the back pole. 5. The acceleration sensor according to claim 4, wherein the shape of the weight is determined such that the mounting area of the weight with respect to the vibration film is maximized within a range that does not hinder the vibration of the vibration film. 6. The acceleration sensor according to claim 4, wherein the weight has a hole corresponding to a hole provided in the vibrating film. 7. The acceleration sensor according to claim 6, wherein a hole provided in the vibrating film is provided at the center of the vibrating film, and the weight has a ring shape. 8. A breathable acoustic resistor, wherein the inner case holds a rear plate having a hole on the outermost surface on the back electrode side, and covers the hole provided in the weight and the hole of the rear plate. The acceleration sensor according to claim 6 or 7, further comprising a member. 9. The gap between the vibrating membrane and the rear plate is breathable due to the gap.
The acceleration sensor described.