Fluid pressure detection device

A fluid pressure detection device capable of accurately detecting a pressure change of a fluid flowing inside a tube includes a substrate, and piezoelectric elements on the top surface of the substrate. The substrate is substantially rectangular in shape. The piezoelectric elements are arranged along the longitudinal direction of the substrate with long sides of each of the piezoelectric elements substantially perpendicular to the long side of the substrate. The substrate has slits on both sides of each of the piezoelectric elements in the longitudinal direction of the substrate. When detecting the pressure, the tube is deformed with a bottom surface of the substrate.

TECHNICAL FIELD

The present invention relates to a fluid pressure detection device which detects pressure of a fluid flowing inside a tube.

BACKGROUND ART

For example, the heart is a pump in life which circulates blood throughout a whole body when four parts called “right atrium”, “right ventricle”, “left atrium” and “left ventricle” move at the same time. When these parts are moving regularly with a constant rhythm, it can be said that normal “beating” is going on. A word “pulsation” and not “beating” is used for what passes through the inside of a tube such as a blood vessel or piping, and pulsation occurs in the case of a positive displacement reciprocating pump. The pulse waveform is the waveform of artery inner pressure, and there is proposed a method of detecting this waveform of inner pressure from a body surface by using a piezoelectric ceramic or piezoelectric polymer resin.

The Patent Document 1 below relates to a broadband sensor and discloses constitution that the sensor comprises an insulating substrate, a piezoelectric element mounted to the surface of the insulating substrate and a cylindrical member installed to surround the piezoelectric element. By bringing an opening on a side opposite to the insulating substrate of the cylindrical member into contact with a body surface to form an airtight cavity in the inside of the cylindrical member, a pulse wave transmitted from a blood vessel below the body surface is detected as a change in air pressure in the cavity with the piezoelectric element.

PRIOR ART DOCUMENT

Patent Document

SUMMARY OF THE INVENTION

Problem to be Solved by the Invention

When detecting pressure of fluid flowing inside the tube by a fluid pressure detection device using a piezoelectric element, it is important to align the piezoelectric element and the tube, but there is a case where high-precision positioning is difficult. In the case of a blood vessel for example, a mounting position is restricted by surrounding organization, thus it was difficult to accurately detect pressure changes.

It is an object of the present invention which was made in view of this situation to provide a fluid pressure detection device which can accurately detect from outside of a tube a pressure change of a fluid flowing inside the tube.

Means for Solving the Problem

One embodiment of the present invention relates to a fluid pressure detection device for detecting pressure of a fluid flowing inside a tube. The fluid pressure detection device includes:

a substrate; and

a plurality of piezoelectric elements on one surface of the substrate, wherein the tube is deformed with the other surface of the substrate.

The fluid pressure detection device may include a pressing member for pressing the substrate from the one surface side to press the other surface of the substrate against the tube.

The pressing member may apply a pressure of not less than 40 mmHg from the one surface side of the substrate.

The substrate may be substantially a rectangle. The piezoelectric elements may be arranged along the longitudinal direction of the substrate with long sides of each of the piezoelectric elements being substantially vertical to the long side of the substrate.

The long side of the substrate may be substantially parallel to the extending direction of the tube.

The substrate may have slits on both sides of each of the piezoelectric elements in the longitudinal direction of the substrate.

It is to be noted that any arbitrary combination of the above-described structural components as well as the expressions according to the present invention changed among a system and so forth are all effective as and encompassed by the present aspects.

Effect of the Invention

According to the present invention, there can be provided a fluid pressure detection device which can accurately detect from outside of a tube a pressure change of a fluid flowing inside the tube.

EMBODIMENT FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will now be described in detail with reference to the drawings. The same or equivalent constituent parts, members, etc., shown in the drawings are designated by the same reference numerals and will not be repeatedly described as appropriate. The embodiments are not intended to limit the invention but are mere exemplifications, and all features or combinations thereof described in the embodiments do not necessarily represent the intrinsic natures of the invention.

First Reference Example

With reference toFIG. 1toFIG. 21, a fluid pressure detection device1according to a first reference example of the present invention will be described hereinunder. The fluid pressure detection device1is used in oil pressure gauges, water gauges and blood pressure gauges. As shown inFIG. 1, the fluid pressure detection device1comprises a substrate10(diaphragm), a piezoelectric element20, a support body30and a lid body40. The substrate10is, for example, a plate-like or sheet-like substrate having a thickness of 10 to 200 μm and made of a metal such as stainless steel. The top surface (one surface) of the substrate10is a surface for mounting the piezoelectric element20. The bottom surface (the other surface) of the substrate10is a surface to be pressed against a tube7shown inFIG. 9. The piezoelectric element20is, for example, a piezoelectric ceramic having a thickness of 10 to 200 μm and formed (mounted) on the top surface (the one surface) of the substrate10. A metal electrode (for example a gold electrode having a thickness of several μm to 20 μm formed by gold sputtering) for taking out signals is formed on both surfaces of the piezoelectric element20but not shown inFIG. 1. The metal electrodes and the piezoelectric element20constitute a piezoelectric unimorph. The substrate10and the piezoelectric element20are substantially circular in the example ofFIG. 1.

The support body30is a circularly annular member having a much larger thickness (height) than the piezoelectric element20and made of a rigid material such as stainless steel. The support body30is provided to surround the piezoelectric element20. As shown inFIG. 1andFIG. 3, the support body30has a stepped part32in the vicinity of the inner edge part of the bottom surface. With the stepped part32, the support body30supports the top surface of the substrate10in the vicinity of the outer edge. As shown inFIG. 2, the support body30has a notch31at the inner edge for leading out wires.

The lid body40is a plate-like or sheet-like body having substantially the same shape as the substrate10in plane view and made of a metal such as stainless steel. The lid body40is mounted to the upper end (one end) of the support body30to close the top opening (one-end opening) of the support body30. As shown inFIG. 7, the lid body40has a through hole41for leading out wires. The through hole41communicates with the notch31of the support body30. On the top surface of the lid body40, a terminal part50is provided. The terminal part50is electrically connected to the piezoelectric element20by wires51. From the terminal part50, cables60extend for connecting to a circuit (FIG. 11orFIG. 13) in an unshown measuring instrument.

The support of the substrate10with the support body30is not limited to support with the stepped part32shown inFIG. 1andFIG. 3. The substrate10may be supported by the bottom surface of the support body30having no stepped part32as shown inFIG. 4. Further, as shown inFIG. 5, the lid body40may not be provided. In a constitution example shown inFIG. 6, the outer edge (outer peripheral surface) of the substrate10is supported by the inner peripheral surface of the support body30. But the support structures shown inFIGS. 3 to 5are preferred from the viewpoint of the reproducibility of pressure detection.

FIG. 9is a schematic diagram showing that the fluid pressure detection device1is directly pressed against the tube7. This schematic diagram shows the cross section of an evaluation device for quantitatively evaluating the fluid pressure detection device1. The tube7is, for example, a silicone tube having flexibility or viscoelasticity. The tube7is held in a tube holder8having a U-shaped cross section. The outer diameter of the tube7is 6 mm. A fluid6supplied from an unshown piston pump flows (transferred with pulsation) inside the tube7. When detecting the pressure of the fluid6, by a force gauge9the fluid pressure detection device1is pressed from the lid body40side (the support body30is pressed from a side opposite to the substrate10) so that the other surface of the substrate10is pressed against the tube7to deform the tube7. Measurement results which will be given hereinafter were obtained when the fluid6was supplied at 96 bpm by the pumping function of the above piston pump. The pressure of the fluid6was also directly detected by an unshown pressure sensor (Model AP-13S of KEYENCE CORPORATION) to evaluate the output of the fluid pressure detection device1.

FIG. 10is a graph showing the relationship between the pressure difference of the fluid6flowing inside the tube7and the output of the sensor when the pressing force applied to the fluid pressure detection device1against the tube7was set to 1N, 2N, 3N, 4N and 5N. This graph is a linear approximation graph of the peak-peak value of the output voltage (output voltage Vout1of a circuit shown inFIG. 11which will be described hereinafter) of the fluid pressure detection device1for each pressing force when measurement was made by setting the pressure difference to 40 mmHg (minimum pressure of 50 mmHg to maximum pressure of 90 mmHg), 80 mmHg (minimum pressure of 50 mmHg to maximum pressure of 130 mmHg) and 120 mmHg (minimum pressure of 50 mmHg to maximum pressure of 170 mmHg). It could be confirmed fromFIG. 10that the pressure difference of the fluid6flowing inside the tube7and the output voltage of the fluid pressure detection device1have high correlation with a correlation coefficient of more than 0.98 at any pressing force, and are substantially in proportion to each other. The pressing force was set to 3N in the following measurement.

FIG. 11is a circuit diagram showing an example of an I-V conversion circuit (impedance conversion circuit) which converts the output current of the piezoelectric element20of the fluid pressure detection device1into voltage. This circuit constitutes a closed loop with the piezoelectric element20and a resistor R1, and an output voltage Vout1appears at both ends of the resistor R1. The output voltage Vout1is in proportion to the time differential of a charge generated in the piezoelectric element20, that is, the pressure change rate of the fluid6, where the proportional constant is the resistance value of the resistor R1.FIG. 12is a waveform diagram showing the waveform of the output voltage Vout1of the circuit shown inFIG. 11and the waveform of a direct detection value Vt obtained by directly detecting the pressure of the fluid6with an unshown water pressure sensor. It could be confirmed fromFIG. 12that the output voltage Vout1is linked with the inclination of the direct detection value Vt. The difference in the fluid pressure (maximum pressure−minimum pressure) produced by the beating of a piston pump can be detected by the circuit shown inFIG. 11.

FIG. 13is a circuit diagram showing an example of an integrating circuit which integrates the output current of the piezoelectric element20of the fluid pressure detection device1. This circuit is an integrating circuit utilizing an operation amplifier A1and accumulates the output current of the piezoelectric element20in a capacitor C1provided between the output terminal and the inverted input terminal of the operation amplifier A1. One end of the piezoelectric element20is connected to the ground as a fixed voltage terminal. The other end of the piezoelectric element20is connected to one end of a resistor R2. The other end of the resistor R2is connected to the inverted input terminal of the operation amplifier A1. The non-inverted input terminal of the operation amplifier A1is connected to the ground. The capacitor C1and a resistor R3are connected in parallel to each other between the output terminal and the inverted input terminal of the operation amplifier A1. The resistor R3is provided to prevent the saturation of the output of the operation amplifier A1. The operation amplifier A1is driven by two power sources and connected to a positive side power line (voltage Vcc) and to a negative side power line (voltage −Vcc). Since the inverted input terminal voltage of the operation amplifier A1becomes substantially equal to ground potential by a virtual short, an output voltage Vout2which appears at the output terminal of the operation amplifier A1is voltage between both ends of the capacitor C1. The output voltage Vout2is in proportion to the integral of the output current of the piezoelectric element20where the proportional constant is the reciprocal of the capacitance value of the capacitor C1. The original charge generation output waveform of the piezoelectric element20is obtained by the circuit shown inFIG. 13, thereby making it possible to calculate a pressure change.

FIG. 14is a diagram showing correlation between the output voltage Vout2of the circuit shown inFIG. 13and the direct detection value Vt obtained by directly detecting the pressure of the fluid6with an unshown water pressure sensor when the fluid6flowing inside the tube7is pulsated with a minimum pressure of 50 mmHg to a maximum pressure of 170 mmHg. Results shown inFIG. 14were obtained by measuring for 10 seconds at a sampling frequency of 1 kHz and the number of data pieces was 10,000. These conditions are the same as in the correlation graphs shown inFIG. 15,FIG. 19andFIG. 20. It could be confirmed fromFIG. 14that the output voltage Vout2and the direct detection value Vt have high correlation with a correlation coefficient of more than 0.99. Therefore, the difference between the minimum pressure and the maximum pressure of the fluid6flowing inside the tube7can be detected from the peak-peak value of the output voltage Vout2.

FIG. 15is a diagram showing correlation between the output voltage Vout2of the circuit shown inFIG. 13and the direct detection value Vt obtained by directly detecting the pressure of the fluid6with an unshown water pressure sensor in cases where the fluid6flowing inside the tube7is pulsated with a minimum pressure of 50 mmHg to a maximum pressure of 130 mmHg.FIG. 16is a diagram showing the waveform of the output voltage Vout2of the circuit shown inFIG. 13and the waveform of the direct detection value Vt obtained by directly detecting the pressure of the fluid6with an unshown water pressure sensor in the above case. It could be confirmed fromFIG. 15that the output voltage Vout2and the direct detection value Vt have high correlation as inFIG. 14. Also, it could be confirmed fromFIG. 16that the waveform of the output voltage Vout2and the waveform of the direct detection value Vt are almost the same.

FIG. 17is a schematic diagram showing that the fluid pressure detection device1is pressed against the tube7through human skin gel7a. This schematic diagram shows the cross section of an evaluation device for quantitatively evaluates the fluid pressure detection device1.FIG. 17differs fromFIG. 9in that the human skin gel7ahaving flexibility or viscoelasticity is added, but is the same in other points. By adding the human skin gel7a, a state is made close to the pressure measurement (blood pressure measurement) of a blood flowing inside a blood vessel of a human body.

FIG. 18is a waveform diagram of the output voltage Vout2of the circuit shown inFIG. 13when the fluid6flowing inside the tube7is pulsated with a minimum pressure of 50 mmHg to a maximum pressure of 150 mmHg in a case where the fluid pressure detection device1is pressed against the tube7without the human skin gel7a(FIG. 9) and in a case where the fluid pressure detection device1is pressed against the tube7through the human skin gel7a(FIG. 17). It could be confirmed fromFIG. 18that though sensitivity (amplitude) drops, almost the same waveform is obtained between when the fluid pressure detection device1is pressed through the human skin gel7aagainst the tube7and when it is pressed not through the human skin gel7aagainst the tube7.

FIG. 19is a diagram showing correlation between the output voltage Vout2of the circuit shown inFIG. 13and the direct detection value Vt obtained by directly detecting the pressure of the fluid6with an unshown water pressure sensor when the fluid pressure detection device1is pressed against the tube7without the human skin gel7ain a case where the fluid6flowing inside the tube7is pulsated with a minimum pressure of 50 mmHg to a maximum pressure of 150 mmHg.FIG. 20is a diagram showing correlation between the output voltage Vout2of the circuit shown inFIG. 13and the direct detection value Vt obtained by directly detecting the pressure of the fluid6with an unshown water pressure sensor when the fluid pressure detection device1is pressed against the tube7through the human skin gel7ain the above case. In the measurement which is the basis of the results ofFIG. 20, the human skin gel (Model H0-2) of EXSEAL CORPORATION was used as the human skin gel7a. It could be confirmed from comparison betweenFIG. 19andFIG. 20that when the fluid pressure detection device1is pressed against the tube7through the human skin gel7a, the output voltage Vout2and the direct detection value Vt have high correlation with a correlation coefficient of more than 0.97, though the correlation coefficient slightly drops compared with the case where the fluid pressure detection device1is pressed against the tube7without the human skin gel7a.

FIG. 21is a schematic diagram showing that the fluid pressure detection device1is pressed against the surface (skin) of a human body5to deform the tube7which is an artery in the human body5. A pressing member70is used to press the bottom surface of the substrate10against the tube7by pressing the support body30from a side opposite to the substrate10. The pressing member70may be just a belt or an adhesive tape such as an adhesive bandage having a viscous surface. By pressing the bottom surface of the substrate10against the tube7through the support body30to deform the tube7as shown inFIG. 21, the difference between the minimum pressure (minimum blood pressure) and the maximum pressure (maximum blood pressure) of the fluid6(blood) flowing inside the tube7can be detected from outside the human body5more accurately than before. The existence of the support body30makes it possible to carry out stable detection with excellent reproducibility.

First Measurement

FIG. 22AandFIG. 22Bare schematic sectional views of a measuring instrument used in first to third measurements.FIG. 22Ais of before measurement.FIG. 22Bis of during measurement. This measuring instrument was made by combining a frame body81, a round wire coil spring82and a cap83with the fluid pressure detection device1ofFIG. 7whose support body30having an outer diameter of 12 mm. The frame body81is provided to guide the pressing of the cap83and hold the pressed cap83. The round wire coil spring82has an outer diameter of 8 mm, a total length of 15 mm and a spring constant of 0.1 N/mm. The cap83is made of polyurethane and has an outer diameter of 18 mm and an inner diameter of 14 mm. The spot facing depth “a” of the cap83, the thickness “b” of the fluid pressure detection device1and the total length (natural length) of the round wire coil spring82were set to ensure that a pressure of 40 mmHg should be applied to the top surface of the fluid pressure detection device1by the round wire coil spring82when the cap83was pressed to the lower limit position as shown inFIG. 22B.

The above measuring instrument was fixed to the skin (position at which a pulse can be felt) above the superficial temporal artery of each of eight subjects by using the surgical tape of3M Company while the round wire coil spring82was compressed as shown inFIG. 22B, and the output voltage Vout1of the I-V conversion circuit (impedance conversion circuit) ofFIG. 11was measured. As a result, stable output was obtained in all the subjects. Therefore, it was found that, when a pressure of 40 mmHg is applied to the top surface of the fluid pressure detection device1, normal blood pressure measurement is possible above the superficial temporal artery.

Second Measurement

The same measurement as in the first measurement was made on the common carotid artery of each of eight subjects. That is, the above measuring instrument was fixed to the skin (position at which a pulse can be felt) above the common carotid artery in the same manner as in the first measurement to measure the output voltage Vout1. As a result, stable output was obtained in all the subjects. Therefore, it was found that when a pressure of 40 mmHg is applied to the top surface of the fluid pressure detection device1, normal blood pressure measurement is possible above the common carotid artery.

Third Measurement

The same measurement as in the first measurement was made on the radial artery of each of eight subjects. That is, the above measuring instrument was fixed to the skin (position at which a pulse can be felt) above the radial artery in the same manner as in the first measurement to measure the output voltage Vout1. In this measurement, three types of round wire coil springs82were used. That is, in addition to a type used in Example 1, a round wire coil spring82having an outer diameter of 8 mm, a total length of 20 mm and a spring constant of 0.1 N/mm, and a round wire coil spring82having an outer diameter of 10 mm, a total length of 15 mm and a spring constant of 0.2 N/mm were used. And then by adjusting the spot facing depth of the cap83, a pressure of 40 mmHg, 60 mmHg, 80 mmHg, 100 mmHg or 120 mmHg was applied to the top surface of the fluid pressure detection device1to carry out the above measurement.

FIG. 23is a table showing measurement results (peak-peak values Vpp of output voltages Vout1) in the third measurement.FIG. 24is a bar graph obtained from the table ofFIG. 23. The numerical values of the measurement results are average values of the peak-peak values Vpp (maximum−minimum) of last 10 beats in a measurement time of 30 seconds. InFIG. 23andFIG. 24, the expression “fixing with tape” means that the fluid pressure detection device1is fixed with a surgical tape without using the frame body81, the round wire coil spring82and the cap83to prevent its displacement, without being conscious of pressure. In the case of “fixing with a tape”, output could not be obtained in two subjects No. 6 and No. 8 as shown inFIG. 23andFIG. 24. Meanwhile, when a pressure of 40 mmHg, 60 mmHg, 80 mmHg, 100 mmHg or 120 mmHg was applied to the top surface of the fluid pressure detection device1, output could be obtained in all the subjects. Therefore, it was found that when a pressure of 40 mmHg is applied to the top surface of the fluid pressure detection device1, normal blood pressure measurement is possible above the radial artery.

Although a pressure at which the highest peak-peak value Vpp was obtained differed according to each subject, in all the subjects, a higher peak-peak value Vpp was obtained when a 60 mmHg pressure was applied than when a 40 mmHg pressure was applied, and a higher peak-peak value Vpp was obtained when a 80 mmHg pressure was applied than when a 60 mmHg pressure was applied. Further, in all the subjects except subject No. 2, a higher peak-peak value Vpp was obtained when a 100 mmHg pressure was applied than when a 80 mmHg pressure was applied. Meanwhile, in all the subjects except subjects No. 5 and No. 6, a lower peak-peak value Vpp was obtained when a 120 mmHg pressure was applied than when a 100 mmHg pressure was applied. From the viewpoint of a burden on each subject, a lower pressure is more preferred. Therefore, in consideration of balance between sensitivity and a burden on subject, it was found that a pressure of not more than 100 mmHg or not more than 80 mmHg is preferred.

For the measurement of blood pressure with the fluid pressure detection device1, it is not necessary to use the frame body81, the round wire coil spring82and the cap83as in the above measurement instrument. To obtain a required pressure, an elastic body such as rubber may be placed on the fluid pressure detection device1, and the fluid pressure detection device1may be fixed on a measurement point with a surgical tape from above the elastic body. Alternatively, the fluid pressure detection device1may be fixed on a measurement point with an elastic tape stretched by a predetermined length. At this point, a spacer (such as a pad) may be placed on the fluid pressure detection device1to obtain a required thickness. The fluid pressure detection device1and members for fixing it on the measurement point may be as a whole considered as a fluid pressure detection device.

FIG. 25is a diagram showing the waveform of the output voltage Vout1of the I-V conversion circuit (impedance conversion circuit) ofFIG. 11and the waveform of the output voltage Vout2of the integrating circuit ofFIG. 13both of which were obtained from measurement (40 mmHg) on a subject No. 8 in the third measurement. It could be confirmed fromFIG. 25that waveforms having little noise were obtained.

Second Reference Example

FIG. 26AandFIG. 26Bare plane views of the substrate10and the piezoelectric element20of a fluid pressure detection device according to a second reference example of the present invention.FIG. 26Ashows a first constitution example andFIG. 26Bshows a second constitution example. The fluid pressure detection device of this reference example differs from the fluid pressure detection device of the first reference example in that the substrate10and the piezoelectric element20are substantially rectangular and the support body30is rectangularly annular like the outer shape of the substrate10, but is the same in other points. For the detection of the pressure of the fluid6, the longitudinal direction of the substrate10and the extending direction of the tube7are made parallel to each other. In the first constitution example shown inFIG. 26A, the long sides of the piezoelectric element20are substantially vertical to the long sides of the substrate10(substantially parallel to the short sides). In the second constitution example shown inFIG. 26B, the long sides of the piezoelectric element20are substantially parallel to the long sides of the substrate10. When the support body30is of an all-side support type shown inFIG. 27A, it supports all the sides (four sides) and therearound of the substrate10. When the support body30is of a long-side support type (having a recessed part33in a lower part of a short side) shown inFIG. 27B, it supports two long sides and therearound of the substrate10(no contact with a short side). When the support body30is of a short-side support type (having a recessed part34in a lower part of a long side) shown inFIG. 27C, it supports two short sides and therearound of the substrate10(no contact with a long side). In the following description, the support body30is of the all-side support type shown inFIG. 27A.

FIG. 28is a bar graph showing comparison between the standardized sensitivity (mV/mmHg) of each of the fluid pressure detection devices of the second reference example and the standardized sensitivity of each of the fluid pressure detection devices of comparative examples. The term “standardized sensitivity” is obtained by dividing the peak-peak value of the output voltage Vout1of the circuit shown inFIG. 11by a difference between the minimum pressure and the maximum pressure and based on an actual measurement value. InFIG. 28, #1 represents a comparative example in which the substrate10is circular with a diameter of 9 mm, the support body30is circularly annular and the piezoelectric element20has the same shape as that of the first constitution example of this reference example. #2 represents the first constitution example of this reference example in which the size of substrate10is 8 mm×18 mm. #3 represents a comparative example in which the substrate10is circular with a diameter of 9 mm, the support body30is circularly annular and the piezoelectric element20has the same shape as that of the second constitution example of this reference example. #4 represents the second constitution example of this reference example in which the size of the substrate10is 8 mm×18 mm. In all of #1 to #4, the size of the piezoelectric element is 2 mm×4 mm.

It was found from comparison between #1 and #2 that higher sensitivity is obtained with the rectangular substrate10than with the circular substrate10when the longitudinal direction of the piezoelectric element20is parallel to the short-length direction of the substrate10even if the short sides of the rectangle are shorter than the diameter of the circle. It was also found from comparison between #3 and #4 that there is not so much difference in sensitivity between the circular substrate10and the rectangular substrate10when the longitudinal direction of the piezoelectric element20is parallel to the longitudinal direction of the substrate10.

FIG. 29is a graph showing the relationship between the displacement against the tube7and sensitivity change rate in the fluid pressure detection devices of the second reference example and the fluid pressure detection devices of the comparative examples. The displacement on the abscissa axis inFIG. 29is the relative amount of displacement of the tube7in the short-length direction of the substrate10while the extending direction of the tube7remains parallel to the longitudinal direction of the substrate10. “0 mm” on the abscissa axis means that the tube7passes the center of the substrate10in the short-length direction. It was found fromFIG. 29that when the amount of displacement is not more than 2 mm, the reduction of sensitivity caused by displacement is suppressed more in #2 and #4 in which the substrate10is rectangular than #1 and #3 in which the substrate10is circular.

According to this reference example, the reduction of sensitivity caused by displacement can be suppressed as compared with a case where the substrate10is circular and the support body30is circularly annular. Therefore, even when the mounting position of the fluid pressure detection device is limited, the reduction of sensitivity can be suppressed. When the longitudinal direction of the piezoelectric element20is parallel to the short-length direction of the substrate10as in the first constitution example, sensitivity can be further improved as compared with a case where the substrate10is circular.

FIG. 30is a graph showing the relationship between pressing force and standardized sensitivity according to the shape of the piezoelectric element20in the fluid pressure detection devices of the second reference example. #5 represents an example in which the size of the piezoelectric element20is 1 mm×8 mm in the second constitution example of this reference example. It was found fromFIG. 30that the standardized sensitivity of the constitution of #2 was the highest. Therefore, in an embodiment which will be described hereinafter, further improvement will be made based on the constitution of #2.

Embodiment

FIG. 31AandFIG. 31Bare plane views of the substrate10and the piezoelectric elements20of a fluid pressure detection device according to an embodiment of the present invention.FIG. 31Ashows a first constitution example andFIG. 31Bshows a second constitution example. The first constitution example of this embodiment is the same as the first constitution example (FIG. 26A) of the second reference example except that three piezoelectric elements20are arranged (arrayed) in the longitudinal direction of the substrate10. The second constitution example of this embodiment is the same as the first constitution example except that the substrate10has slits11on both sides in the short-length direction of each piezoelectric element20. The slits11are parallel to the longitudinal directions of the piezoelectric elements20. To distinguish the three piezoelectric elements20from one another, they are represented by E1, E2and E3from top in the figure. The I-V conversion circuit shown inFIG. 11and the integrating circuit shown inFIG. 13are provided for each piezoelectric element20.

FIG. 32AandFIG. 32Bare graphs showing the relationship between the displacement against the tube7and standardized sensitivity in the fluid pressure detection device of the embodiment.FIG. 32Ashows the first constitution example andFIG. 32Bshows the second constitution example.FIG. 33AandFIG. 33Bare graphs showing the relationship between the displacement against the tube7and sensitivity change rate in the fluid pressure detection device of the embodiment.FIG. 33Ashows the first constitution example andFIG. 33Bshows the second constitution example. It was found from comparison betweenFIG. 32AandFIG. 32Band comparison betweenFIG. 33AandFIG. 33Bthat the reduction of sensitivity caused by a displacement amount of not more than 2 mm is suppressed when the substrate10has the slits11.

FIG. 34is a diagram for explaining the angle deviation between the substrate10and the tube7. The angle deviation to be studied hereinafter is an angle at which the extending direction of the tube7turns relative to the longitudinal direction of the substrate10while the tube7passes the center of the substrate10as shown inFIG. 34. 0° means that the extending direction of the tube7is parallel to the longitudinal direction of the substrate10.

FIG. 35AandFIG. 35Bare graphs showing the relationship between the angle deviation against the tube7and standardized sensitivity in the fluid pressure detection device of the embodiment.FIG. 35Ashows the first constitution example andFIG. 35Bshows the second constitution example.FIG. 36AandFIG. 36Bare graphs showing the relationship between the angle deviation against the tube7and sensitivity change rate in the fluid pressure detection device of the embodiment.FIG. 36Ashows the first constitution example andFIG. 36Bshows the second constitution example. As shown in the graphs, it was found that, even when an angle deviation occurs, the central piezoelectric element20can maintain a high sensitivity of not less than 80% as compared with a case where there is no angle deviation. When a certain amount of displacement is added to an angle deviation as shown inFIG. 37AandFIG. 37B, one of the piezoelectric elements20except the central piezoelectric element20can maintain high sensitivity. Stated more specifically, in the case ofFIG. 37A, a lower piezoelectric element20in the figure maintains high sensitivity. In the case ofFIG. 37B, an upper piezoelectric element20in the figure maintains high sensitivity.

According to this embodiment, the reduction of sensitivity caused by the angle deviation (and a combination of angle deviation and displacement) of the tube7can be suppressed by arraying the piezoelectric elements20. Also, the reduction of sensitivity caused by the displacement of the tube7can be suppressed by forming the slits11as in the second constitution example. The number of the piezoelectric elements20is not limited to three and may be 2, or 4 or more. The piezoelectric elements20may be arranged in a matrix in the longitudinal direction and short-length direction of the substrate10or in two arbitrary directions of the substrate10. The presence of the support body30provides an advantage in the reproducibility of pressure detection. However, even without the support body30, the reduction of sensitivity caused by the angle deviation (and a combination of angle deviation and displacement) of the tube7can be suppressed as compared with the case where there is only one piezoelectric element20. And, even without the support body30, the reduction of sensitivity caused by the displacement of the tube7can be suppressed as compared with the case where there is no slit11. If a configuration wherein an external pressing member presses only a part, which is supported by the support body30, of the base material10, the reproducibility of pressure detection of the fluid pressure detection device can be improved even without the support body30as same as the support body30is provided.

While the invention has been described in its preferred embodiments, it is to be understood by a person having ordinary skill in the art that variations may be made on each constituent element and process of the embodiments without departing from the scope of the following claims. Variations of the invention will be described hereinafter.

The substrate10, the support body30and the lid body40may be insulators made of a resin or the like. The support body30is not limited to a single annular member. A plurality of support bodies30may be provided on both sides of or around the piezoelectric elements20. Specific numerical values (sizes of the substrate10and the piezoelectric elements20, pressing force, etc.) shown in above embodiments are just examples and may be suitably changed according to required specifications.

EXPLANATIONS OF LETTERS OF NUMERALS