JP2000028445A - Load measuring instrument and element for measuring load - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure a static load or force by using a piezoelectric element composed of a high polymer piezoelectric material exhibiting no definite resonance characteristic. SOLUTION: A load measuring instrument 100 is provided with a piezoelectric element 1 which is composed of a high polymer piezoelectric material and receives an external load, a coil 2 which works as a steepening means which steepens the frequency characteristic of the voltage of the element 1, a frequency oscillator 4 which oscillates the element 1 at a constant frequency near the resonance frequency of the element 1, and an amplitude variation detecting circuit which detects the magnitude of an external load applied to the element 1 by detecting the variation of the frequency characteristic of the voltage of the element 1 when the external load is applied as the amplitude variation of the voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a load measuring device for measuring an external load or force by applying an AC voltage to a polymer piezoelectric material. For example, the present invention relates to a load measurement for a human body in a vehicle seat and a body pressure distribution in a bed. It is suitable for use in static load and force measurement, such as measurement of the load.

[0002]

2. Description of the Related Art Conventionally, load measuring devices using a piezoelectric material include a piezoelectric type using a piezo effect and a resonance type for tracking a change in resonance frequency (resonance point) due to stress. But,
In the piezoelectric type, although impedance is interposed in the circuit in order to continue generation of the output voltage, leakage of the generated voltage is inevitable, and as a result, dynamic load and force can be measured, but static It is difficult to measure a proper (steady) load or force. On the other hand, the resonance type has a problem that the circuit configuration is complicated and a long time is required for circuit adjustment.

To solve these problems, for example, Japanese Unexamined Patent Publication No. 52-78483 proposes a load (force) measuring device utilizing an impedance change due to stress near a resonance point of a piezoelectric ceramic. In this technique, the AC voltage applied to the piezoelectric ceramic is fixed at a frequency near the resonance point, and the amount of change in voltage amplitude (output voltage difference) when a static force is applied to the piezoelectric ceramic from the outside is measured. . According to this, since the frequency is fixed,
The circuit configuration can be simpler than that of the resonance type.

[0004]

However, in the above-mentioned conventional publication, the impedance change of the piezoelectric ceramic is used, but it is difficult to construct a detector because the mechanical vibration mode of the piezoelectric ceramic material is complicated. . Further, quartz and piezoelectric ceramics, which are typical piezoelectric ceramic materials, are brittle materials, and are hard and brittle, so that their application range is limited. For example, when used for an occupant detection sensor that measures the load on the human body in a vehicle seat, when a person is seated,
An uncomfortable feeling is caused in the hard sensor portion.

Therefore, the present inventors have proposed that instead of piezoelectric ceramic, a polymer piezoelectric material such as polyvinylidene fluoride (hereinafter referred to as PVDF), which is considered to have a relatively simple vibration mode and has high flexibility, is used as a piezoelectric element. I thought about using it. However, as a result of the examination, the polymer (PVD
F) As shown in the frequency characteristic graph of the voltage of the piezoelectric element and the piezoelectric ceramic element, these polymer voltage materials did not show a clear resonance characteristic, so that a static load could not be measured.

In view of the above, an object of the present invention is to make it possible to measure a static load or force using a piezoelectric element made of a high-molecular piezoelectric material that does not show a clear resonance characteristic. .

[0007]

As described above, in order to measure a static load or force, a piezoelectric element having clear resonance characteristics is required. The present inventors have conducted intensive studies to give a clear resonance characteristic to the polymer piezoelectric material,
By connecting an element (for example, a coil or the like) having an inductance component (L component) to the polymer piezoelectric material in parallel or in series, the frequency characteristic of the voltage of the polymer piezoelectric material is sharpened as shown in FIG. Found that it can be The present invention has been made based on this finding.

That is, according to the first aspect of the present invention, the piezoelectric element (1) is vibrated at a constant frequency near the resonance frequency of the piezoelectric element (1), and an external load or a load is applied to the piezoelectric element (1). In a load measuring device that detects the magnitude of the load or the force by detecting a change in the frequency characteristic of the voltage of the piezoelectric element (1) when a force is applied, the piezoelectric element (1) may be a polymer piezoelectric element. It is characterized by being made of a material, and further provided with a steepening means (2) for steepening the frequency characteristic of the voltage of the piezoelectric element (1).

Thus, the steepening means (2) sharpens the voltage frequency characteristics of the piezoelectric element (1) made of a polymer piezoelectric material which does not exhibit clear resonance characteristics, ie, clarifies the resonance characteristics (Q value). ), It is possible to clearly detect a change in the frequency characteristic of the voltage due to a change in the impedance of the piezoelectric element (1) when an external load or the like is applied.
Therefore, according to the present invention, a static load or force can be measured.

The invention according to claims 2 and 3 provides concrete steepening means, and the steepening means is provided with a coil unit (parallel or series connected to the piezoelectric element (1)). 2). Here, a resonance circuit is constituted by the piezoelectric element (1) and the coil section (2), and if the inductance value of the coil is selected, the resonance frequency and the Q value can be arbitrarily set, and therefore, the measurement sensitivity can also be arbitrarily selected. .

According to a fourth aspect of the present invention, a plurality of piezoelectric elements (1) are provided, and a coil section (2) is connected to each of the plurality of piezoelectric elements (1), and an inductance value of the coil (2) is provided. Is selected in accordance with the frequency characteristics of the voltage of each piezoelectric element (1). If a plurality of piezoelectric elements (1) and coil portions (2) are arranged at a plurality of locations, It can measure static loads and forces.

Further, in the frequency oscillating means (4) according to the first to fourth aspects, in order to vibrate the piezoelectric element (1),
A constant amplitude sine wave voltage is applied, or a constant amplitude sine wave current is applied. That is, a sine wave signal in which either the voltage or the current is constant is applied. According to the study of the present inventors, it is necessary to obtain an accurate output signal (output voltage difference) according to an external load or the like. It is preferable to apply a sinusoidal current having a constant amplitude, as in the fifth aspect of the present invention (see FIG. 3B).

Further, the material described in claim 6 can be used as the polymer piezoelectric material. According to a seventh aspect of the present invention, there is provided a load measuring device comprising a piezoelectric element (1) made of a polymer piezoelectric material and a steepening means (2) for steepening a frequency characteristic of a voltage of the piezoelectric element (1). To provide an element for use. According to the present invention, it is possible to provide a load measuring element with a clear resonance characteristic, vibrate the load measuring element at a constant frequency near the resonance frequency, and detect a change in the frequency characteristic of the voltage when an external load or the like is applied. Then, static load and force can be measured.

Note that the reference numerals in parentheses above are examples showing the correspondence with specific means described in the embodiments described later.

[0015]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a first embodiment of the present invention. The load measuring device according to the present embodiment is applied as a load sensor, and has a steep resonance characteristic by connecting a coil in parallel or in series to a piezoelectric element made of a polymer piezoelectric material. FIG. 1 shows a circuit configuration of the load sensor 100 according to the present embodiment.

Reference numeral 1 denotes a piezoelectric element as a pressure receiving portion which is made of a polymer piezoelectric material and receives an external load or force, and has a structure in which electrodes are provided on a polymer piezoelectric material formed in a film shape. Examples of the polymer piezoelectric material include polyvinylidene fluoride (PVDF), polyvinyl fluoride, a copolymer containing polyvinylidene fluoride and / or polyvinyl fluoride as a main component, odd-numbered nylon, polyurea, and vinylidene cyanide as a main component. Materials selected from copolymers, polyurethanes, acrylonitrile copolymers, and polyvinyl chloride can be employed.

Reference numeral 2 denotes a coil (coil portion) as steepening means for steepening the frequency characteristic of the voltage of the piezoelectric element 1. In the present embodiment, the piezoelectric element 1 and the coil 2 correspond to a load measuring element, and both 1 and 2 are connected in parallel to form a resonance circuit. A frequency oscillator (frequency oscillating means) 4 is connected to the resonance circuit in series via a resistor (impedance) 3.

FIG. 2 is a characteristic diagram showing the frequency characteristics of the voltage of the piezoelectric element 1. Since the coil 2 is added in parallel to the piezoelectric element 1, the frequency characteristic (frequency-voltage characteristic) of the voltage is reduced as shown by the black square plot in FIG. It is steeper than in the case (comparative example, shown by a white square plot in FIG. 2).

The characteristic example is 40 mm × 40 m
m, a piezoelectric element 1 using a PVDF film having a thickness of 40 μm, a coil 2 having an inductor of 36 mH, and an impedance 3 of 10 kΩ. From the frequency oscillator 4, the difference between the maximum value and the minimum value is 5.6 V. _This was performed by applying an AC voltage with a constant amplitude of _PP and changing the frequency.
The other polymer piezoelectric materials described above also show the same steepening tendency.

5 is an amplitude change detection circuit (detection means)
This is a configuration including an appropriate storage circuit and a differential amplifier. Then, the voltage amplitude at both ends of the resonance circuit (load measuring element) is taken out as an output signal. Note that the voltage amplitude at both ends of the resonance circuit may be extracted as the detected voltage amplitude, but the voltage amplitude at both ends of the impedance 3 may be used.

The operation of the load measuring device 100 having the above configuration will be described. The frequency oscillator 4 causes the resonance circuit to vibrate the piezoelectric element 1 at a constant frequency near the resonance frequency of the piezoelectric element 1. At this time, as shown in FIG. 2, the frequency characteristic of the voltage of the piezoelectric element 1 is sharpened by the addition of the coil 2, and a clear resonance characteristic is exhibited.

When a load is applied to the piezoelectric element 1 from the outside, the frequency characteristic of the voltage changes. FIG. 3A schematically shows how this frequency characteristic changes. FIG.
In (a), a waveform A1 indicated by a solid line is a frequency characteristic when no external load is applied, and R1 indicates a resonance point. Here, when an external load is applied, the frequency characteristic changes from the waveform A1 to a waveform A2 indicated by a broken line, and the resonance point moves from R1 to R2.

Therefore, if the piezoelectric element 1 is vibrated at a constant frequency near the resonance frequency, for example, the frequency f1, the output voltage difference V1 can be detected by the amplitude change detection circuit 5 when an external load is applied. On the other hand, if the piezoelectric element 1 is vibrated at a frequency f2 near the resonance frequency, the output voltage difference V2 can be detected when an external load is applied. These output voltage differences V1, V2 and the external load have a relationship as shown in FIG. 3B, for example, and the magnitude of the load can be detected using the output-load characteristics. The sign of the output-load characteristic changes depending on which of the output voltage differences V1 and V2 is output.

In order to vibrate the piezoelectric element 1 by the frequency oscillator 4, a sine wave voltage having a constant amplitude or a sine wave current having a constant amplitude is applied.
According to the study of the present inventors, it is preferable to apply a sinusoidal current having a constant amplitude. This is because the use of a sinusoidal current having a constant amplitude can reduce the hysteresis H of the output voltage difference change that occurs in the increasing and decreasing directions of the load, as shown in FIG.

As described above, according to the present embodiment, the addition of the coil 2 sharpens the frequency characteristic of the voltage of the piezoelectric element 1 made of a polymer piezoelectric material that does not show a clear resonance characteristic, that is, makes the resonance characteristic clear. Therefore, a change in the frequency characteristic of the voltage due to a change in the impedance of the piezoelectric element 1 when an external load or the like is applied can be clearly detected, and a static load or force can be measured.

Here, in FIG. 1, the coil 2 is connected in parallel to the piezoelectric element 1, but as shown in FIG.
The same operation and effect can be obtained even when the coil 2 is connected in series. The steepening means is not limited to a coil, but may be any element having an inductance component (L component). Next, the present embodiment will be further described based on examples shown below.

[0027]

(Example 1) A coil (L = 36.5 mH) 2 is connected in parallel to a piezoelectric element 1 in which Al is deposited on both sides of PVDF (thickness: 40 μm, dimensions: 40 × 40 mm). Then, a sine having a constant frequency (12.5 kHz) and a constant amplitude (voltage: 5.6 V) near the resonance point from the oscillator 4 is provided by the oscillator 4 via a resistor (10 kΩ) 3 in series with a resonance circuit including the piezoelectric element 1 and the coil 2. Wave alternating current was applied.

As the output signal, the voltage amplitude at both ends of the resonance circuit was taken out, and the output voltage difference was detected by the amplitude change detection circuit 5 using an operational amplifier. FIG. 5 shows an output-load characteristic when a load is applied to the piezoelectric element 1 constituted by the measurement circuit. In this embodiment, the output linearly changes in accordance with the applied load, and a good result as a load sensor was obtained.

Example 2 PVDF (thickness: 40 μm,
A coil (L = 36.5 mH) 2 is connected in parallel to a piezoelectric element 1 in which Al is deposited as electrodes on both surfaces of dimensions (40 × 40 mm), and a resistance is serially connected to a resonance circuit composed of the piezoelectric element 1 and the coil 2. Through a (10 kΩ) 3, a sine wave alternating current having a constant frequency (12.79 kHz) and a constant amplitude (voltage: 5.6 V) near the resonance point was applied from the oscillator 4.

As for the output signal, the voltage amplitude at both ends of the resonance circuit was taken out, and the output voltage difference was detected by an amplitude change detection circuit 5 using an operational amplifier. FIG. 6 shows an output-load characteristic when a load is applied to the piezoelectric element constituted by the measurement circuit.
Shown in In this embodiment, the output linearly changes in accordance with the applied load, and a good result as a load sensor was obtained.

Example 3 PVDF (thickness: 80 μm,
A coil (L = 3) which gives resonance characteristics to the piezoelectric element 1 in which Al is vapor-deposited as electrodes on both surfaces of dimensions: 40 × 40 mm
6.5 mH) 2 connected in parallel, the piezoelectric element 1 and the coil 2
A constant frequency (16.5 kΩ) near the resonance point from the oscillator 4 via a resistor (10 kΩ) 3 in series with a resonance circuit composed of
Hz) and a sine wave alternating current having a constant amplitude (voltage: 5.6 V) was applied.

As the output signal, the voltage amplitude at both ends of the resonance circuit was taken out, and the output voltage difference was detected by the amplitude change detection circuit 5 using an operational amplifier. FIG. 7 shows an output-load characteristic when a load is applied to the piezoelectric element 1 constituted by the measurement circuit. In this embodiment, the thickness of the polyvinylidene fluoride was increased compared to the first and second embodiments, so that the sensitivity was lowered. However, a good result was obtained as a load sensor whose output linearly changed in accordance with the applied load. Obtained.

(Comparative Example) This comparative example uses a piezoelectric element without a coil. PVDF (thickness: 40
μm, dimensions: 40 × 40 mm)
A constant frequency (12.5 kHz) near the resonance point from the oscillator via a resistor (10 kΩ) in series with the piezoelectric element on which
z), a sinusoidal alternating current having a constant amplitude (voltage: 5.6 V) was applied. Although the voltage amplitude was detected in the same manner as in the first embodiment,
As shown in FIG. 8, no change in the output voltage difference was observed even when a load was applied.

Although the above embodiment has been described including the above embodiments, the load sensor 100 uses a thin film and highly flexible polymer piezoelectric material for the piezoelectric element 1 as the pressure receiving portion. So, for seats and beds of vehicles, etc.,
By incorporating the piezoelectric element 1 serving as the pressure receiving portion, the load and the load distribution on the human body can be measured without giving a sense of strangeness to a person.

Accordingly, the present load sensor 100 includes an occupant detection sensor for arranging the piezoelectric element 1 on the seat of an automobile to detect the weight of the occupant and control the opening condition of the airbag, or the piezo-electric element 1 is used for a patient. It is suitable for use in a body pressure distribution sensor for detecting body pressure distribution, beating, pulse, etc. of a caregiver or a floor-slip prevention sensor, etc., which is arranged on a nursing bed. Further, the present load sensor 100 can be used as a hydraulic sensor or an air pressure sensor that wraps a flexible piezoelectric element 1 around a hose through which oil, air, or the like flows, to detect a hydraulic pressure or an air pressure, and a weight detection sensor or the like. .

In the above embodiment, the piezoelectric element 1 and the coil 2 connected in parallel or in series with the piezoelectric element 1 constitute a load measuring element, and the load measuring element has a clear resonance characteristic. Provides devices. If a plurality of the load measuring elements are provided and a plurality of the elements are arranged at appropriate places on a seat or a bed, static loads and forces at a plurality of places can be measured. In this case, the inductance value of the coil 2 may be selected in accordance with the frequency characteristics of the voltage of each piezoelectric element 1.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating an example of a load measuring device according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a frequency characteristic of a voltage of a piezoelectric element in the embodiment.

3A is an explanatory diagram illustrating a change in frequency characteristic of a voltage when an external load is applied, and FIG. 3B is an explanatory diagram illustrating an output-load characteristic.

FIG. 4 is a circuit diagram showing another example of the load measuring device according to the embodiment.

FIG. 5 is a characteristic diagram illustrating a relationship between a load and an output voltage in the first embodiment.

FIG. 6 is a characteristic diagram illustrating a relationship between a load and an output voltage according to the second embodiment.

FIG. 7 is a characteristic diagram showing a relationship between a load and an output voltage in the third embodiment.

FIG. 8 is a characteristic diagram illustrating a relationship between a load and an output voltage in a comparative example.

FIG. 9 is a graph showing frequency characteristics of a polymer (PVDF) piezoelectric element and a piezoelectric ceramic element.

[Explanation of symbols]

1: piezoelectric element, 2: coil, 3: impedance, 4:
Frequency oscillator, 5 ... amplitude change detection circuit.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kin Kono 1-1-1, Showa-cho, Kariya-shi, Aichi Pref. (72) Inventor Hidetoshi Ouchi 3008-1 Kofunaka-cho, Kofu City, Yamanashi Prefecture (72) Inventor Sasa 1-1-8 Asagi, Kofu City, Yamanashi Prefecture

Claims (7)

[Claims] 1. A piezoelectric element (1) made of a polymer piezoelectric material and subjected to an external load or force; a steepening means (2) for steepening a frequency characteristic of a voltage of the piezoelectric element (1); Frequency oscillating means (4) for vibrating the piezoelectric element (1) at a constant frequency near the resonance frequency of the piezoelectric element (1)
And detecting means (5) for detecting the magnitude of the load or force by detecting a change in the frequency characteristic of the voltage when the load or force is applied to the piezoelectric element (1). Characteristic load measuring device. 2. The piezoelectric device according to claim 1, wherein the steepening means includes a piezoelectric element.
The load measuring device according to claim 1, wherein the load measuring device is a coil portion (2) connected in parallel to the load measuring device. 3. The steepening means includes the piezoelectric element (1).
The load measuring device according to claim 1, wherein the load measuring device is a coil portion (2) connected in series to the load measuring device. 4. A plurality of the piezoelectric elements (1) are provided, the coil section (2) is connected to each of the plurality of piezoelectric elements (1), and an inductance value of the coil (2) is 4. The load measuring device according to claim 2, wherein the load measuring device is selected in accordance with a frequency characteristic of a voltage of each of the piezoelectric elements (1). 5. The frequency oscillating means (4) vibrates the piezoelectric element (1) by applying a sinusoidal current having a constant amplitude at a constant frequency near a resonance frequency of the piezoelectric element (1). The load measuring device according to any one of claims 1 to 4, characterized in that: 6. The polymer piezoelectric material is polyvinylidene fluoride, polyvinyl fluoride, a copolymer containing polyvinylidene fluoride and / or polyvinyl fluoride as a main component,
6. A material selected from the group consisting of an odd-numbered nylon, a polyurea, a copolymer containing vinylidene cyanide as a main component, a polyurethane, an acrylonitrile-based copolymer, and polyvinyl chloride. The load measuring device according to any one of the above. 7. A load measuring element that changes the frequency characteristics of a voltage when a load or force is applied from the outside under a vibration at a constant frequency near a resonance frequency, and generates a signal corresponding to the load or the force. A load measuring element comprising: a piezoelectric element (1) made of a polymer piezoelectric material; and steepening means (2) for steepening the frequency characteristic of the voltage of the piezoelectric element (1). .