JP4112883B2 - Acceleration sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an acceleration sensor, and more particularly to an acceleration sensor that can be suitably used as a heart sound microphone.
[0002]
[Prior art]
Conventionally, for example, in a diagnosis of a heart disease or a respiratory system, direct auscultation using a stethoscope or the like has been widely performed. Direct auscultation is useful as a simple diagnostic method, but has the following problems.
[0003]
1) Since individual differences of doctors are reflected, objectivity and quantitativeness are lacking.
2) Since heart sounds cannot be recorded, it cannot be compared with past states 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 of Hz.
In order to cope with such a problem, heart sounds are detected and recorded as, for example, a heart sound diagram.
[0004]
Heart sound detectors that detect and record heart sounds are mainly heart sound microphones for detecting heart sounds, amplifiers that amplify signals output by heart sound microphone microphones, selection of frequency components to be recorded, filters to remove noise components, It has a recording device or the like for recording a heart sound waveform.
[0005]
The heart sound microphone is attached to the subject's chest and converts the mechanical vibration of the chest wall accompanying heartbeat into an electrical signal. Moving coil type, piezoelectric type, electrostatic type are known as conversion methods, and As output, displacement, speed, acceleration, etc. are obtained.
[0006]
Conventionally, as a heart sound microphone, a piezoelectric acceleration sensor having a configuration in which a piezoelectric element is used and acceleration is converted into an electrical signal and output is widely used. Specifically, it has a configuration in which a vibrating membrane, which includes a piezoelectric element and has a weight attached thereto, is supported in a sealed case. When vibration is transmitted to the heart sound microphone having such a configuration, the weight tries to stop in place by the law of inertia, and thus a force is applied to the piezoelectric element. Since the voltage generated by the piezoelectric element by this force is proportional to the inertial force of the weight, that is, the magnitude of the input vibration, the acceleration can be detected from the magnitude of the voltage.
[0007]
The heart sound microphone needs to have high sensitivity in order to detect minute vibrations such as heart sounds propagating to the chest wall. In order to increase the sensitivity of the above-mentioned heart sound microphone, the weight attached to the diaphragm should be made heavy.
[0008]
[Problems to be solved by the invention]
However, the heavy weight is large in size and also has a large load on the vibration film made of a piezoelectric element. Therefore, the heavy weight is vulnerable to impact and is damaged even when dropped from a height of about 1 m. In addition, the use of a large weight has led to an increase in the size of the heart sound microphone. Since the heart sound microphone is used by being mounted on an elastic body called a human chest, if the heart sound microphone is heavy, the treble component of the heart sound is attenuated. Furthermore, the increase in size makes it difficult to wear the subject's claw.
[0009]
In addition, the holder or ring that supports the diaphragm in the case usually has a shape that is in close contact with the entire side of the case, and is input from the side as well as the vibration in the direction perpendicular to the diaphragm that should be detected. It was easily affected by external noise.
[0010]
On the other hand, in an acceleration sensor, as disclosed in, for example, Japanese Patent Laid-Open No. 5-232136, a capacitor is formed by a vibrating membrane and an electrode plate arranged with a minute interval in a sealed case. 2. Description of the Related Art An electrostatic acceleration sensor that detects acceleration from a voltage change caused by a capacitance change is known.
[0011]
Since the electrostatic acceleration sensor does not need to directly detect a voltage from the diaphragm, it is not necessary to attach a weight to the diaphragm, and is superior to the piezoelectric acceleration sensor in terms of weight and size.
[0012]
However, even in the electrostatic acceleration sensor, the holder for supporting the vibrating membrane and the electrode plate in the case still has a structure that is in close contact with the inner side surface of the case, and therefore should be detected in a direction perpendicular to the vibrating membrane. In addition to vibration, it was easily affected by external noise input from the side of the case.
[0013]
[Means for Solving the Problems]
The present invention has been made in view of such problems of the prior art, and an object of the present invention is to provide an acceleration sensor that is small, light and robust and can reduce the influence of external noise.
[0014]
In order to achieve this object, an acceleration sensor according to the present invention includes a vibrating membrane, a back pole disposed opposite to the vibrating membrane at a predetermined interval, and a weight attached to a surface of the vibrating membrane that does not face the back pole. A capacitor is formed by the gas existing between the diaphragm and the back electrode, and the diaphragm and back electrode are held in an acceleration sensor that detects the acceleration against the diaphragm based on the capacitance change of the capacitor due to vibration of the diaphragm A substantially cylindrical inner case having a hole on the outermost surface on the diaphragm side, an inner case for holding a rear plate having a hole on the outermost surface on the back pole side, and an airtightness for housing the inner case of and a resin outer case, the outer case has a support means for supporting the inner case by remembering outer case inner surface and spacing, the support means, integral to the inner bottom surface and inner surface of the outer case A made ribs or protrusions, the cross-sectional shape of ribs or protrusions, have a narrowing shape increases toward the tip from the inner surface of the outer case, with holes are provided on the vibrating membrane and the backplate, the weight is vibration film has a hole corresponding to the hole provided in the cover a hole provided holes and the rear plate to the weight, characterized in that it have the acoustic resistance member having air permeability.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail based on preferred embodiments with reference to the drawings. FIG. 1 is an exploded perspective view showing a configuration example of an acceleration sensor according to the present invention, and FIG. 2 is a vertical sectional view taken along line AA in FIG.
[0016]
As can be seen from FIG. 2, the acceleration sensor 100 according to the present embodiment is held in a unit case 2 as an internal case in an airtight outer case composed of a resin lower case 1 and an upper case 17. The sensor main body 20 is supported. As will be described later, by supporting the sensor body 20 apart from the inner surface of the housing, it is possible to ventilate the upper and lower sides of the sealed sensor, reducing the sensitivity loss of the sensor, and detecting unnecessary vibrations. It can be reduced.
[0017]
With reference to FIG. 1 and FIG. 3 which is a vertical sectional view of the sensor body 20, the configuration of the acceleration sensor according to the present embodiment will be described in more detail.
[0018]
The sensor body has a configuration in which the components 3 to 16 are stacked in a metal unit case 2 having a substantially cylindrical shape, and the components are held in the case by the flange portions 21 and 22 constituting the upper and lower surfaces of the unit case 2. ing. A front ring 3 is provided on the uppermost surface in the unit case 2 (which is the lowermost surface when used, but will be described with reference to the drawings. The same applies to other components below), and an adjustment weight 5 and a unit case described later. It plays the role of a spacer with the inner surface of the two jaws 21.
[0019]
The vibration film 7 is made of, for example, a resin thin film deposited with metal, and has a predetermined tension by being fixed to the vibration film ring 8 around the film. A ventilation hole 71 is provided in the center of the vibration film 7. A ring-shaped weight 6 having a common center is attached to the surface of the vibration film 7, and a ring-shaped adjustment weight 5 is also provided on the weight 6 as necessary. The adjustment weight 5 is an adjustment weight for sensor sensitivity and resonance point, and is sufficiently lighter than the weight 6. The adjustment weight 5 may be unnecessary or a plurality of adjustment weights may be used.
[0020]
As will be described later, the shape of the weight 5 has a great significance in maintaining a constant distance between the vibrating membrane 7 and the back electrode 10 and preventing the vibration film 7 and the back electrode 10 from being adsorbed.
[0021]
The adjustment acoustic resistor 4 is a breathable film attached so as to cover the hole of the weight 6 (and the adjustment weight 5), and is the other adjustment acoustic resistor provided in the hole 16a of the rear plate 16. Together with (not visible in FIG. 1 but shown in FIGS. 2 and 3), it functions as a fluid resistance to the air passing through the sensor.
[0022]
The adjustment acoustic resistor 4 can finely adjust the sensitivity of the sensor, the resonance point of the vibrating membrane, the sharpness of resonance, and the like. The acoustic resistance 4 for adjustment can be comprised from a nonwoven fabric, for example.
[0023]
Annular spacer 9, and the monitor by a predetermined distance separates the back plate 10 below the diaphragm 7 by its thickness, provided these to insulate. Therefore, the vibrating membrane 7 and the back electrode 10 form a capacitor using a layer of air having a thickness equal to the distance by the spacer 9 as a dielectric. The thickness of the spacer 9 is about several tens of μm.
[0024]
The back electrode 10 is composed of a metal plate to which a piezoelectric film is pressure-bonded, and vent holes 10a are provided at four locations in this embodiment.
[0025]
The back electrode 10 is held around the insulator 12 together with the contact plate 11. The contact plate 11 is a conductor for electrically connecting various circuits mounted on the printed wiring board (PC board) 13, in particular, the impedance adjusting FET 14 and the back electrode 10.
[0026]
An electric circuit element (not shown) for generating a sensor output is mounted on the PC board 13 in addition to the FET 14 for impedance adjustment. The PC board 13 is also provided with a vent hole 13a.
[0027]
The fixing ring 15 is a spacer provided so that 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. Further, as described above, the adjustment acoustic resistance 4 is provided in the rear plate 16 so as to cover the ventilation hole 16a.
[0028]
The sensor body 20 having such a configuration is further housed and sealed in a case constituted by the upper case 17 and the lower case 1 to constitute an acceleration sensor. In the acceleration sensor according to the present embodiment, the sensor body 20 is supported by the protrusions or the apexes of the ribs provided on the inner surface of the case, so that the contact area between the sensor body 20 and the inner surface of the case is reduced. The influence on the main body can be greatly reduced.
[0029]
For example, in the present embodiment, the sensor body 20 is supported in the case by a ring-shaped rib 1 a provided on the inner bottom surface of the lower case 1 and a ring-shaped rib 1 b provided on the inner side surface of the lower case 1. These ribs 1a and 1b may be integrally formed when the case is made of resin, or the ribs 1a and 1b may be fitted into grooves provided 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 body 20, a shape that becomes narrower toward the tip as shown in FIG. 4 is preferable.
[0030]
Further, it is not always necessary to support the entire circumference, and the sensor body may be supported by a plurality of protrusions as long as the sensor body 20 can be supported without rattling. In this case, it is preferable to support three or more points on the sensor side surface and two or more points on the jaw portion 21 of the unit case 2.
[0031]
Further, the position of the rib or protrusion for supporting the sensor main body 20 is not necessarily the position shown in FIG. 2 or FIG. 4, the sensor main body 20 can be supported away from the inner surface of the case, and the operation of the sensor is hindered. Any position that is not given can be supported.
[0032]
The height of the ribs or projections supporting the sensor body 20 can be arbitrarily set. However, if the height is too high, the overall size of the sensor is increased, and if it is too low, the external noise is buffered by the air layer around the sensor body 20. Since the effect is reduced, it is preferable to provide ribs or protrusions that are as high as possible within a range where the sensor satisfies a desired size.
[0033]
In the acceleration sensor according to the present embodiment, the rear plate 16, the PC plate 13, the back electrode 10, the vibrating membrane 7, the weight 6, and the adjustment weight 5 that constitute the sensor body 20 are provided with holes, and these components are further provided. Among them, the adjustment weight 5 located on the outermost side and the adjustment acoustic resistance 4 attached so as to cover the hole of the rear plate 16 also have air permeability, so that air can pass through the sensor body 20. Furthermore, since the sensor main body 20 is supported away from the inner surface of the case, air can also travel between the upper and lower surfaces of the sensor main body 20 while being supported inside the case. Accordingly, it is possible to suppress a decrease in sensitivity due to the viscous resistance of air.
[0034]
In the acceleration sensor having such a configuration, when a predetermined voltage is applied to the vibrating membrane 7 and the back electrode 10 via the unit case 2 and the FET 14, charges are accumulated on the opposing surfaces of the vibrating membrane 7 and the back electrode 10, and the capacitor is connected. Form. 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 vibration film 7 tends to stay in the original position due to inertia. As a result, the vibrating membrane 7 vibrates and the distance from the back electrode 10 changes, whereby the capacitance of the capacitor formed by the vibrating membrane 7, the back electrode 10 and the air layer therebetween changes. A 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.
[0035]
Next, the shape of the weight 5 attached to the vibration film 7 will be described. As described above, the distance between the vibrating membrane 7 and the back electrode 10 is as narrow as several tens of μm. In addition, during operation, charges having opposite polarities appear on the facing surface of the vibrating membrane 7 and the back electrode 10, so that a force that attracts the vibrating membrane 7 and the back electrode 10 is generated.
[0036]
Since the periphery of the vibration film 7 is fixed, the movable amount increases toward the center. Moreover, when a weight is attached to increase the sensitivity, there is a tendency to be centered to obtain a large effect with a small number of weights. Therefore, the difference in the amount of movement between the central portion and the peripheral portion further increases, and the central portion of the vibrating membrane 7 comes closer to the back electrode 10 during vibration. As a result, in some cases, the vicinity of the center portion of the vibration film 7 and the back electrode 10 may be adsorbed, and the function as a sensor may not be performed.
[0037]
In view of such a situation, the present invention has improved the shape of the weight 6 attached to the diaphragm 7 to prevent the diaphragm 7 from adsorbing to the back electrode 10 and to obtain a stable sensor output. .
[0038]
That is, in the acceleration sensor according to the present invention, the weight 6 attached to the vibrating membrane 7 is configured so that the attachment area thereof is as large as possible. In other words, if the weight has the same weight, it is configured to be thin and have a large outer diameter. By attaching the weight having such a shape to the diaphragm, the planarity of the diaphragm can be maintained in a wider range as compared with the case of using a weight with a small mounting area (FIG. 5A) ( FIG. 5B).
[0039]
As a result, the distance between the vibrating membrane and the back pole is uniform over a wide range, so that it is possible to prevent the center portion from being attracted and the variation in the distance between the vibrating membrane and the back pole due to vibration is also wide. Therefore, it is possible to obtain a change in capacitance due to vibration with higher accuracy.
[0040]
In the present embodiment, in order to increase the outer diameter of the weight 6, a ring shape having a hollow center portion is formed. Further, by providing a ventilation hole in the central portion of the vibration film 7 to which the weight 6 cannot be attached, the central portion is prevented from being adsorbed even on the shape surface.
[0041]
As will be apparent to those skilled in the art, in determining the shape of the weight 6, it is not necessary to simply increase the outer diameter. In other words, the weight becomes too thin and the planarity of the diaphragm cannot be maintained due to insufficient rigidity. In the case of a ring shape, even if the outer diameter is large, if the ring is too thin, the planarity at the center of the diaphragm cannot be obtained. Further, 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 is difficult to move. Therefore, (1) the shape of the weight is determined so as to increase the mounting area with the vibrating membrane within a range that does not hinder the vibration of the vibrating membrane and (2) the flatness of the central portion of the vibrating membrane can be maintained. Become. In the present specification, the “attachment area” means an area where the planarity of the diaphragm is substantially maintained by the weight, and the entire area does not need to be fixed to the diaphragm.
[0042]
The change in characteristics due to the shape of the weight will be further described. The acceleration sensor having the configuration of FIG. 2 is a vibration membrane (thickness) in which a hole having a diameter of 1 mm is provided in the center in a stainless steel vibration membrane ring having an outer diameter of 14.5 mm and an inner diameter of 12.5 mm, as shown in FIG. 4μm, made of PPS, gold deposited on the surface) attached with adhesive, using brass back electrode with FEP electret film crimped and acceleration sensor arranged at 50μm interval, shape with the same weight (0.7g) I / O characteristics were measured by attaching different weights.
[0043]
Three types of weights having a diameter of 6 mm, 8 mm, and 10 mm were used in the shape of a disk with a hole having a diameter of 1 mm in the center. The weight and the vibration film were attached to the vibration film with no holes so that the positions of the holes correspond to each other with an adhesive, and then a hole with a diameter of 1 mm penetrating the vibration film and the weight was provided. FIG. 7 shows input / output characteristics at a frequency of 20 to 1 kHz when a constant acceleration (0.98 m / S ² ) is applied. As can be seen from FIG. 7, even when the weights have the same weight, a larger output is obtained by using the weight having a large outer diameter. This is considered to be due to the fact that the planarity is maintained over a wide range of the vibration film, and the capacitance change due to vibration can be detected by the configuration. In any case, adsorption between the vibrating membrane and the back electrode was not observed.
[0044]
As described above, according to this embodiment, the acceleration sensor is suitable for detecting weak vibrations such as heart sounds, which is lightweight, robust and excellent in durability, and realizes highly sensitive acceleration detection while suppressing the influence of external noise. Is realized.
[0045]
[Other Embodiments]
In the above-described embodiment, the case where the atmosphere in the case is air has been described. However, other gases such as an inert gas (both single component gas and mixed gas are possible) can be filled.
[0046]
In the above-described embodiment, only the case where the shape of the weight attached to the diaphragm is a ring shape or a disk shape has been described, but it is not necessarily a perfect circle. Further, the number of weights is not limited to one, and a plurality of weights may be arranged on the vibration film as long as the flatness of the vibration film is maintained in a wide range and does not hinder vibration. .
[0047]
Further, the adsorption prevention effect and the sensitivity increase effect by improving the shape of the weight are not necessarily obtained only for the acceleration sensor having the vibration film provided with the vent hole, and other than the electrostatic type, For example, it is also effective for a piezoelectric acceleration sensor.
[0048]
In order to further prevent the vibration film and the back electrode from being attracted, it is also possible to provide minute ribs or protrusions on the attachment surface side of the weight 6 to the vibration film 7 as shown in FIG. With such a configuration, even when there is contact between the vibrating membrane 7 and the back electrode 10, the contact area is reduced, and the vibrating membrane 7 and the back electrode 10 are easily separated. In this case, it is preferable that the ribs and protrusions provided on the weight 6 have a round shape because the vibration film may be damaged if the apex is too sharp. Also, the height is set so as not to hinder vibration.
[0049]
【The invention's effect】
As described above, according to the acceleration sensor of the present invention, by holding the acceleration sensor body with a gap from the inner surface of the case, ventilation can be performed on the upper and lower surfaces of the acceleration sensor. In addition to this, it is possible to mitigate the influence of unnecessary external noise on the acceleration sensor due to the air present in the gap between the acceleration sensor main body and the case inner surface.
[Brief description of the 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 cross-sectional view taken along the line AA of FIG.
FIG. 3 is a vertical sectional view of the sensor main body 20 of FIG.
4 is a partial perspective view showing an example of a rib structure of the lower case 1 that supports the sensor main body 20. FIG.
FIG. 5 is a diagram for explaining a difference in the movement of the diaphragm depending on the shape of the weight.
FIG. 6 is a diagram for explaining parameters of a vibrating membrane and a weight used in the experiment.
FIG. 7 is a diagram showing output characteristics when the shape of a weight is changed.
FIG. 8 is a diagram showing the shape of a weight of an acceleration sensor according to another embodiment of the present invention.

Claims (4)

A vibration membrane; a back pole disposed opposite to the vibration membrane at a predetermined interval; and a weight attached to a surface of the vibration membrane that does not face the back pole, between the vibration membrane and the back pole. In an acceleration sensor that forms a capacitor with the gas present in and detects an acceleration with respect to the diaphragm based on a change in capacitance of the capacitor due to vibration of the diaphragm,
A substantially cylindrical inner case for holding the vibrating membrane and the back electrode, having a hole in the outermost surface on the vibrating membrane side and holding a rear plate having a hole in the outermost surface on the back pole side An inner case ,
An airtight resin outer case for housing the inner case;
The outer case has support means for supporting the inner case with a space from the inner surface of the outer case;
The support means is a rib or a protrusion integrally formed on the inner bottom surface and the inner side surface of the outer case , and the cross-sectional shape of the rib or the protrusion has a shape that becomes narrower from the inner surface to the tip of the outer case. ,
The vibration membrane and the back electrode are provided with holes, and the weight has a hole corresponding to the hole provided in the vibration membrane,
Covering the hole in the provided hole and said rear plate to said mass, an acceleration sensor, characterized by chromatic acoustic resistance member having air permeability. The acceleration sensor according to claim 1, wherein a rib or protrusion of the weight, on the mounting surface side to the vibrating film. Wherein the hole provided in the vibration film provided in the center of the vibrating membrane, the acceleration sensor according to claim 1 or claim 2, wherein said weight has a ring shape. The acceleration sensor as claimed in any one of claims 1 to 3 between the vibration film and the rear plate and having a gas permeability by the distance between the outer casing inner surface and the inner case.

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