JP6283812B2 - Load measuring device and load measuring sensor - Google Patents

Description

  The present invention relates to a highly sensitive impedance change type load measuring device using a piezoelectric body. For example, it is suitable for watching a rising, and measuring pressure, load and force such as pulse rate, respiration rate and blood pressure.

  A piezoelectric body generates electric charges when subjected to stress or force. It is used as a piezoelectric load or force sensor using this. However, since charges leak, it is necessary to devise in order to provide static sensitivity.

  The vibration type or resonance type detects a change in the resonance frequency when the piezoelectric body receives stress, and has a static sensitivity. However, since the movement of a specific resonance point is tracked, the detection circuit is complicated. Become.

  In order to solve these problems, for example, in Patent No. 4178600, a current having a constant frequency and a constant amplitude is applied in the vicinity of the resonance point of the piezoelectric body, and a load or force measurement for detecting a change in impedance of the piezoelectric body is performed with a simple circuit. A measuring device to be realized has been proposed.

  However, if the detection sensitivity is low and the resonance sharpness Q is increased, the measurement range becomes narrow, and there is a drawback that sufficient sensitivity cannot be obtained with respect to increase or decrease in load or force. In addition, there is a problem that it is difficult to improve the pyroelectric effect due to the physical properties of the element.

Patent No. 4178600

Satoshi Nagata “Pressure Sensor Using Piezoelectric Ceramics” Automation Technology, 1982

  Hidetoshi Ouchi, Satoshi Nagata, Toshio Kawanishi, “Improvement of Static Characteristics of Force Sensors Using Piezoelectric Elements”, Journal of Precision Engineering, 2001

  The problem to be solved is that the piezo effect includes a piezoelectric effect and a pyroelectric effect. In detecting the load or force, there is a drawback that the impedance of the piezoelectric body itself is small with respect to increase or decrease of the load or force, and sufficient sensitivity cannot be obtained. In addition, there is a problem that it is difficult to improve the pyroelectric effect due to the physical properties of the piezoelectric element.

となり、
電源電圧Ｅ_ｍときの電流Ｉ_ｍは（１）式となる。

する。本発明は、この知見に基づいてなされている。The present invention uses the principle that in a series resonant circuit of a resistor R, a coil L, and a capacitor C, the reactance component changes as the frequency changes. The change in reactance X accompanying the change in frequency is as follows.
In the series resonance circuit, when the resonance frequency f ₀ changes to Δf,

And
Current _{I m} when the power supply voltage _{E m} is (1).

To do. The present invention has been made based on this finding.

ると図２の直列共振回路におけるＥ_ＬｍとＥｓｍは電源電圧Ｅｍより昇圧する。

すなわち、（２）式より、ＰＶＤＦ圧電素子両端電圧Ｅｓｍとコイル両端電圧Ｅ_Ｌｍは電源電圧Ｅ_ｍと選択度Ｑの積となり電源電圧より昇圧するので、検出感度がＱで任意に得られる。

Then, E _L m and Esm in the series resonance circuit of FIG. 2 are boosted from the power supply voltage Em.

That is, equation (2), since the PVDF piezoelectric element voltage across Esm and the coil end voltage E _{L m} is boosted from the power supply voltage is the product of the selectivity Q and the power supply voltage E _m, the detection sensitivity can be obtained arbitrarily Q.

Based on the findings as described above, the first aspect of the present invention, which has sufficient sensitivity and can also improve the pyroelectric effect due to the physical properties of the piezoelectric element,
A plurality of piezoelectric elements formed by sandwiching a piezoelectric material between two electrodes, and a load for detecting a load applied to the plurality of piezoelectric elements based on a change in impedance between the piezoelectric elements. A detection unit;
Each of the piezoelectric elements includes a plurality of resonance circuits driven at a predetermined resonance frequency by an AC power source,
The resonance circuit has an intersection on the high frequency side of the resonance frequency characteristic of one resonance circuit and a low frequency side of the resonance frequency characteristic of another resonance circuit, and the intersection is a reference point of the load load. It is formed so that it becomes.
According to this aspect, as will be described in detail later with reference to FIG. 4, when a load is applied to the piezoelectric element, the voltage at the reference point (load load = 0) decreases in one resonance frequency characteristic. At the same time, in the other resonance frequency characteristic, the voltage at the reference point (load load = 0) increases. As a result, it is possible to increase the sensitivity of the detection value of the load load detected as a relative difference between the two and to make the detection value of the load load highly accurate.
The second aspect of the present invention is:
In the load measuring device described in the first aspect,
Of the two piezoelectric elements adjacent in the stacking direction, a capacitance formed by sandwiching an insulating layer between one of the electrodes facing each other in the stacking direction is used as a circuit element of the resonance circuit.
According to this aspect, as will be described in detail later with reference to FIG. 3, it is possible to form a capacitance that is a circuit element of the resonance circuit using an insulating layer between the electrodes of the piezoelectric element.
The third aspect of the present invention is:
In the load measuring device described in the second aspect,
The plurality of piezoelectric elements are formed by sequentially laminating one electrode on the upper surface of the insulating layer, the piezoelectric material on the upper surface of the electrode, the other electrode on the upper surface of the piezoelectric material, and the other insulating layer on the upper surface of the other electrode. It is characterized by being formed by laminating a plurality of the same structures formed in the above.
According to this aspect, as will be described in detail later with reference to FIG. 3, an insulating layer, one electrode, a piezoelectric material, the other electrode, and another insulating layer laminated in this order as a unit, The structure described in the second embodiment can be formed only by sequentially laminating.
The fourth aspect of the present invention is:
A combination of two sets of load measuring devices described in the first aspect, the voltage based on the impedance between the electrodes of one of the load measuring devices on which the load is applied, and the piezoelectric element without being affected by the load The temperature correction of the measurement value of the load load is performed based on the difference from the voltage based on the impedance between the electrodes of the other load measuring device based on the ambient temperature.
According to the present embodiment will be described in detail later based on FIG. 8, Ru can cancel the effect of the pyroelectric effect of the piezoelectric material.
In the present invention, as shown in FIG. 3 (a), the load is detected as a pair in which two PVDF piezoelectric elements 1 and PVDF piezoelectric elements 2 are vertically stacked. FIG. 2B schematically shows the configuration of the load measuring sensor. When the insulating film polyester thin film coating surface (8) is Cs1, and the other polyester thin film coating surface (9) is Cs2, a dielectric is formed between the electrodes (4) and (5). As a result, the combined electrostatic quantity Cs1 + Cs2 = Cs, and by forming a pair, the capacitor Cs is added to the PVDF piezoelectric element 1.

Shows when the capacitance C _S to a PVDF piezoelectric element combined resonance characteristic of voltage added to the PVDF piezoelectric element 1, the results of examining the pair of resonance frequency characteristics in FIG. Slightly varying the frequency of the sine wave generation source _{E m,} and the constant amplitude current fed separately to the PVDF piezoelectric element 1 (1) and the PVDF piezoelectric element 2 (2), and the voltage Esm plotted between both terminals. As shown in the figure, the PVDF piezoelectric element 1 moved slightly toward the lower resonance point.

The load measuring sensor is a load measuring sensor constituted by a pair of the PVDF piezoelectric element 1 (1) and the PVDF piezoelectric element 2 (2), and each output voltage is an inter-electrode voltage _{ESm of the} PVDF piezoelectric element 1 (1). 1 and the interelectrode voltage E _Sm 2 of the VDF piezoelectric element 2 (2).

The point indicated by f _S0 in FIG. 4 is set by fixing the frequency of the sine wave generating power supply (13) so that the voltage between the electrodes (E _Sm 1 = E _Sm 2) of the pair of PVDF piezoelectric elements becomes the same voltage. did.

From the drawing, when an external force is applied to the pair of PVDF piezoelectric elements 1 (1) and PVDF piezoelectric elements 2 (2), the frequency of the voltage of the elements slightly moves to fso + Δf. The voltage Esm1 between terminals of the PVDF piezoelectric element 1 (1) is lower than when the frequency is fso. Similarly, the voltage between the terminals Esm2 of PVDF piezoelectric element 2 (2), Ru high as will the differential output voltage than when the frequency fs0 is obtained.

  The load measuring sensor of the present invention utilizes the principle of a vibration type sensor that moves to a higher voltage frequency when a load or force is applied. The detection sensitivity can be arbitrarily set by using the selectivity Q of the series resonance circuit of the piezoelectric element.

  When the generated electrostatic capacity is used by a paired configuration in which two piezoelectric elements are superposed, a slight difference in resonance point occurs between them. When the external force applied to the pair of piezoelectric elements is increased or decreased, the frequency shifts accordingly, and accordingly, the voltage between both elements increases or decreases. Therefore, the voltage between the terminals is increased or decreased by the detecting means (14). A load measuring sensor capable of detecting with high sensitivity can be provided.

FIG. 1 is an explanatory diagram showing the relationship between the frequency change of the series resonant circuit and the current. FIG. 2 is an explanatory diagram showing an equivalent circuit of the piezoelectric element. FIG. 3 is an explanatory diagram showing the configuration of the PVDF piezoelectric element. FIG. 4 is an explanatory diagram showing the measurement principle. FIG. 5 is an explanatory diagram showing the configuration of the load measuring sensor. FIG. 6 is an explanatory diagram showing an equivalent circuit of the load measuring sensor. FIG. 7 shows the output-load characteristics. Example 1 FIG. 8 shows a block diagram of the temperature correction circuit. (Example 2)

  The load measuring sensor is a PVDF piezoelectric element made of a polymer piezoelectric material and receiving an external force. Polymer piezoelectric materials include polyvinylidene fluoride (PVDF), polyvinyl fluoride, polyvinylidene fluoride and / or copolymers based on polyvinyl fluoride, odd-number nylon, polyurea, and vinylidene cycyanide as main components. A material selected from a copolymer, polyurethane, acrylonitrile copolymer, and polyvinyl chloride can be used.

The configuration of the load measuring sensor according to the embodiment of the present invention is shown in FIG. The circuit is a series resonant circuit using a resistor (11), a coil (12), and a polymer piezoelectric material as a load measuring sensor.
Two piezoelectric elements having the same resonance frequency were stacked one above the other and fixed to form a pair of load sensors. The magnitude of the load was detected by the detection means (14) for the output voltage between the piezoelectric elements.
Since the electrode surface of the element is insulated by the polyester thin films (8) and (9), the electrodes of the two elements are not in contact with each other.

When the polyester thin film coating surface (8) of two PVDF piezoelectric elements having the resonance frequency of the piezoelectric element voltage is C _S1 and the other polyester thin film coating surface (9) is C _S2 , the electrode (4) and the electrode ( Between 5), the polyester thin film coating surface becomes a dielectric, and a capacitance C _S1 + C _S2 = C _S is generated. The capacitance C _S is added to the PVDF piezoelectric element 1, and the resonance frequency of the two piezoelectric elements is slightly different. This is a load measuring sensor that causes

From the above, the load measuring sensor in which two piezoelectric elements are paired receives a load and a force from the outside at the same time. When an external force is applied, the inter-element voltage Esm2 shown in FIG. 6 increases and the other inter-element voltage Esm1 decreases due to the resonance characteristics, so that a differential output voltage load measuring sensor can be provided. The voltage amplitude at both ends of the coil may be taken out.

As shown in FIG. 4, a sinusoidal wave so that the voltage between terminals Esm1 at the time of Δf ₁ of the PVDF piezoelectric element 1 (1) and the voltage between the terminals Esm2 at the Δf ₂ of the PVD piezoelectric element 2 (2) are the same potential. The frequency of the generated power source (13) was varied and fixed at the _fSO point in the figure.

Due to the steepness of the frequency of the output voltage of the piezoelectric element, when an external force is applied to the PVDF piezoelectric element 1 (1) and the PVDF piezoelectric element 2 (2), the frequency slightly shifts (f _S0 + Δf (see FIG. 4)). The inter-terminal voltage Esm of the piezoelectric element 1 (1) has a lower value when the frequency is f _S0 Similarly, the inter-terminal voltage Esm of the PVDF piezoelectric element 2 (2) is the same as that when the frequency is f _S0 . Since the output voltage of the load measuring sensor is a differential output voltage, the load and force can be measured with high sensitivity according to the present invention.

FIG. 6 is an equivalent circuit diagram of an embodiment of the present invention, in which two PVDF piezoelectric elements (thickness: 28 μm, electrode dimensions: 10 × 15 mm) are stacked one above the other to form a resistor (50Ω) 11 A constant frequency f _S0 (about 164 kHz, 2 Vp-p) near the resonance point of the PVDF piezoelectric element and a constant amplitude voltage were applied to a series resonance circuit in which a coil (1.5 mH) 12 was connected in series by a sine wave generation power source 13. .

The differential output voltage of the output voltage E _Sm 1 and the output voltage E _Sm 2 of the detection unit was measured for the magnitude of the load by the detection means 14.

FIG. 7 shows the relationship between the output voltage and the output voltage-load characteristic when a load is applied by the detecting means. The output voltage changed linearly according to the applied load, and good results were obtained as a load measuring device.

In the pyroelectric property resulting from the physical properties of the piezoelectric element, a voltage is generated between the elements even in a no-load state due to spontaneous polarization without applying an external force when the temperature is changed. In particular, the influence of the pyroelectric effect cannot be ignored in measuring weak loads and forces that require high sensitivity.

FIG. 8 is a block diagram of temperature correction according to the second embodiment. An embodiment in which two pairs of sensors are used and a load measuring sensor is used for temperature detection will be described. Two PVDF piezoelectric elements (thickness: 28 μm, electrode dimensions: 10 × 15 mm) 1 and PVDF piezoelectric elements (thickness: 28 μm, electrode dimensions: 10 × 15 mm) 2 were stacked one above the other to form a pair. A piezoelectric element is connected to a series resonant circuit of a resistor (50Ω) 11 and a coil (1.5 mH) 12 with a sine wave generation power supply 13 having a constant frequency f _S0 (about 164 kHz) and a constant amplitude voltage near the resonance point of the PVDF piezoelectric element. Excited.

  As shown in the figure, the load measurement sensor output voltage (A) and the temperature detection sensor output voltage (C) are inputs to the temperature correction amplifier (E), and the load measurement sensor output voltage (B) and the temperature detection sensor output voltage (D) ) As an input to the temperature correction amplifier (F), the temperature is corrected by calculation and the output voltage is an embodiment in which the magnitude of the load is detected by the amplifier circuit (G). The temperature detection sensor was used as a load or force dummy. Although not shown in the figure, it was confirmed that the change in the output voltage of the amplifier circuit (G) within the range of about 20 degrees to 40 degrees was within several mV.

  This load measuring apparatus is suitable for use in nursing and medical monitoring, and for fetal heart rate monitor, apnea monitor, pulse monitor, body pressure distribution sensor, blood pressure monitor, body temperature monitor and the like.

1 PVDF piezoelectric element 1
2 PVDF piezoelectric element 2
DESCRIPTION OF SYMBOLS 3 Electrode of PVDF piezoelectric element 2 Electrode of PVDF piezoelectric element 2 Electrode of PVDF piezoelectric element 1 Electrode of PVDF piezoelectric element 1 Acrylic coating surface 8 of PVDF piezoelectric element 2 Polyester thin film surface 9 of PVDF piezoelectric element 2 PVDF piezoelectric element 1 Polyester thin film surface 10 Acrylic coating surface of PVDF piezoelectric element 1 Resistance 12 Coil 13 Sine wave generation constant current power supply 14 Detection means

Claims (4)

A plurality of piezoelectric elements formed by sandwiching a piezoelectric material between two electrodes, and a load for detecting a load applied to the plurality of piezoelectric elements based on a change in impedance between the piezoelectric elements. A detection unit;
Each of the piezoelectric elements includes a plurality of resonance circuits driven at a predetermined resonance frequency by an AC power source,
The resonance circuit has an intersection on the high frequency side of the resonance frequency characteristic of one resonance circuit and a low frequency side of the resonance frequency characteristic of another resonance circuit, and the intersection is a reference point of the load load. A load measuring device formed so as to become. In the load measuring device according to claim 1,
Of the two piezoelectric elements adjacent in the stacking direction, a load formed by sandwiching an insulating layer between one of the electrodes facing each other in the stacking direction is a circuit element of the resonance circuit measuring device. In the load measuring device according to claim 2,
The plurality of piezoelectric elements are formed by sequentially laminating one electrode on the upper surface of the insulating layer, the piezoelectric material on the upper surface of the electrode, the other electrode on the upper surface of the piezoelectric material, and the other insulating layer on the upper surface of the other electrode. A load measuring device formed by laminating a plurality of components having the same structure. A combination of two sets of the load measuring device according to claim 1, a voltage based on an impedance between electrodes of one load measuring device on which a load load acts, and a surrounding of the piezoelectric element without being affected by the load load load measurement equipment being characterized in that to perform the temperature compensation of the measured value of the applied load based on the difference between the voltage based on the impedance between the electrode of the other of the load measuring device based on the temperature.

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