JPH0663725B2 - Piezoelectric sensor - Google Patents

Description

The present invention relates to a piezoelectric sensor capable of exhibiting a highly accurate piezoelectric conversion function without being affected by temperature changes.

(Prior Art) As a tactile sensor for a robot hand, conduction or non-conduction between electric conductors such as a micro switch, a conductive rubber, and a carbon-containing sponge has been conventionally used. (System and Control, VOL 22, No.
7, PP409-416, 1978).

(Problems to be Solved by the Invention) Each of the above-mentioned sensors is a contact sensation that identifies whether or not it has contacted a work. Considering a case where a fragile object is gripped without damage, the fingers are It is necessary to detect the pressure exerted on the finger and control the opening and closing of the finger.

Conventionally, strain gauges are often used as sensors of this type, and these are attached by mounting elastic beams with large bending displacement on part of the finger, so in order to increase sensitivity, the elastic beams must be thin. Although it has to be done, if it is thin, the fingers are likely to break during gripping.

Therefore, it is necessary to use a thick elastic beam to some extent, but this has a disadvantage in that the sensitivity is lowered.

Further, since the characteristics vary greatly depending on the temperature, it is necessary to combine a dummy strain gauge to form a bridge circuit as a means for improving the characteristics, and thus the detection unit has a large shape,
The required space is considerably large.

As a means for solving such a problem, a piezoelectric polymer material is attached to the fingertip portion of a robot like artificial skin, and distortion of the piezoelectric polymer material caused by gripping an object such as a work is corrected by an electric voltage such as a voltage. A method of capturing as a signal is known.

However, when actually using this polymeric piezoelectric material, the material has piezoelectricity and pyroelectricity at the same time, that is, it has the property of emitting a voltage in response to a change in temperature. The actual situation is that the electric signal cannot be used due to a remarkable change in the electric signal when it is gripped.

The present invention has been made to provide a piezoelectric sensor that can solve the above problems and solve the conventional problems.
In particular, by using a basic structure in which a dummy gauge is attached as well as a strain gauge using a polymeric piezoelectric material, it is possible to correct variations in piezoelectric conversion characteristics due to changes in temperature,
The purpose is to make it possible to sufficiently exhibit the stabilization of the characteristics without being affected by the temperature fluctuation.

(Means for Solving Problems) However, in the present invention, as is apparent from the drawings showing the embodiments, the piezoelectric thin film (2) made of a polymer piezoelectric material is formed into a conductive film (3) as a ground electrode. The two piezoelectric thin films (2, 2) are adhered to both surfaces, and further, the conductive films (4) as detection electrodes for extracting piezoelectric conversion signals are adhered to the outer surfaces of the piezoelectric thin films (2, 2). The sheet-like piezoelectric element (1) is formed by a sheet having a bimorph structure obtained by polarization in the same direction, and is applied to the output system of one conductive film (4) as the detection electrode, and to the output voltage of the output system. On the other hand, a correction means (5) for performing conversion with a predetermined conversion coefficient determined by the voltage characteristics of both detection electrodes with respect to temperature changes is provided, and the output of the other electric membrane (4) as a detection electrode and the correction means (5). ) Output when the planar piezoelectric element (1) is not deformed. It is characterized by the configuration of a piezoelectric sensor provided with an addition means (7) for destructively adding the different polarities.

(Operation) In the present invention, since the piezoelectric thin film (2) as the dummy gauge for temperature correction and the piezoelectric thin film (2) as the detection sensor are two thin layers, the temperature of the object is directly measured. Since the dummy gauge is subjected to, and the charges generated by the temperature are destructively added as different polarities so as to cancel each other, temperature correction can be appropriately performed.

Moreover, when a bending displacement is applied, the charges generated in both piezoelectric thin films (2) are added with the same polarity, so that the desired piezoelectric conversion characteristics are exhibited.

Thus, accurate temperature correction is possible, and since the bimorph structure is employed, the function as a piezoelectric sensor is satisfied without any deterioration in flexibility.

(Example) Hereinafter, an example of the present invention will be described with reference to the accompanying drawings.

The structure of the piezoelectric conversion element portion of the sensor of the present invention is as shown in FIG. 1, in which the piezoelectric thin films (2) and (2) made of a polymeric piezoelectric material are replaced with a conductive film (3), for example, an aluminum thin film (3). Are bonded to each other by thermocompression bonding or using an adhesive to form one body, and further, a conductive film (4), for example, an aluminum vapor deposition film (4) is closely adhered to both the front and back surfaces of the planar piezoelectric element (1). ) Is formed.

Such a sandwitch structure is called a bimorph structure.

The thermocompression bonding is a hot press.
Polymer piezoelectric material / aluminum thin film / polymer piezoelectric material is laminated in a sandwich shape to raise the temperature up to the melting point of the polymer piezoelectric material to melt the polymer piezoelectric material,
In addition, it is a method of applying pressure in the vertical direction to perform pressure bonding, which is a general method of joining polymer resins.

As the polymer piezoelectric material in this case, for example, polyvinylidene fluoride (PVDF) is used, and the polymer piezoelectric material that is polarized as described later is used.

By the way, a polymeric piezoelectric material first exhibits piezoelectricity by applying a high DC electric field between its surfaces, and this is called polarization treatment.

In the case of the bimorph structure, there are two types of polarization directions, as shown in FIGS. 2 (a) and 2 (b). The conductive film (3), which is the central layer against bending deformation of the polymeric piezoelectric material, is used as the neutral layer. The first layer, for example the upper layer of FIG. 2, becomes stretched and the second layer becomes contracted.

Accordingly, the piezoelectric conversion signals are taken out, and the conductive films (4) as front and back detection electrodes are connected to the external wiring as equipotential electrodes or counter potential electrodes.

Therefore, the present invention uses the same-direction polarization method shown in FIG.

When heat is applied from the outside to the planar piezoelectric element (1) which has been subjected to such polarization treatment, electric charges are generated as shown in FIG. 3, but this phenomenon is called pyroelectricity.

If the characteristics of both layers of the piezoelectric thin film (2) are exactly the same, the generated electric charge is the same, so the signal extraction (wiring) shown in FIGS. 2A and 3 is performed. Then, the charges should cancel each other out.

However, in reality, the two layers of the piezoelectric thin film (2) have slightly different thicknesses, or there is a difference in the amount of generated charges.

Therefore, as shown in FIG. 4, the buffer circuit (6) including high impedance input operational amplifiers (OP ₁ ) and (OP ₂ ) having FETs (field effect transistors) at the input terminals receives piezoelectric charges. An external circuit is formed, and the output corresponding to the generated charges is output from the terminals (CT-1) and (CT-2) while changing the temperature of the thermostat while the planar piezoelectric element (1) is housed in the thermostat. Let it be taken out and measure.

To further explain this in detail, the electric charge generated by pyroelectricity is obtained by multiplying the temperature change by the pyroelectric rate. On the other hand, the pyroelectric rate is physically a characteristic peculiar to a material. The pyroelectric sensitivity varies depending on the thickness, the polarization treatment conditions, and the like.

Therefore, in the planar piezoelectric element having the bimorph structure shown in FIG. 4, the upper piezoelectric thin film (2) and the lower piezoelectric thin film (2) have different thicknesses in manufacturing. Therefore, the amount of pyroelectric charge generated for the same environmental temperature change is different between the upper piezoelectric thin film and the lower piezoelectric thin film because the pyroelectric sensitivity is different.

Electric charges are physically generated in the piezoelectric thin film due to the pyroelectric (or piezoelectric) effect, and the high impedance input operational amplifiers (OP ₁ and OP ₂ ) having the FET (Field Effect Transistor) at the input end of the electric charges are charged. It can be converted into a voltage by being received by the buffer circuit as an element.

Therefore, in the observation terminal CT-1 portion shown in FIG. 4, the output voltage due to (pyroelectric) charges obtained from the upper piezoelectric thin film is obtained from the lower piezoelectric thin film in the observation terminal CT-2 portion ( It is possible to take out the output voltage due to the pyroelectric charge.

Then, as shown in Fig. 4, the bimorph type planar piezoelectric element composed of the upper and lower two piezoelectric thin films is placed under the same environmental temperature change, and the observation terminals CT-1 part and CT-2 part are placed. By calculating the output ratio Av with respect to temperature change from the output from
That is the pyroelectric sensitivity ratio with respect to the temperature change of the two piezoelectric thin films.

From this measurement result, output ratio (voltage characteristic) against temperature change
Av = | Δv ₂ | / ΔT ÷ | Δv ₁ | / ΔT is calculated.

However, Δv ₁ : change in output of operational amplifier (OP ₁ ) Δv ₂ : change in output of operational amplifier (OP ₂ ) ΔT: change in temperature of thermostatic chamber The output ratio Av thus obtained is used for one conductive film (4 )
It is given as an amplification factor in a correction circuit (5) as a correction means that is connected in series to an operational amplifier (OP ₁ ) connected to the output terminal of the correction circuit (5) and the other conductive film (4). By directly connecting the output terminal of the operational amplifier (OP ₂ ) to the adder circuit (7) as an adder means, the charge due to the temperature fluctuation generated in one piezoelectric thin film (2) and the other piezoelectric thin film ( The inherent difference with the similar charges in 2) will be canceled out with certainty.

That is, the present invention is a bimorph type planar piezoelectric element,
Malfunction caused by pyroelectric effect when subjected to temperature change (Originally, this piezoelectric element is for obtaining a piezoelectric signal corresponding to strain of polymer piezoelectric material due to contact force. A temperature change ΔT (this temperature change is simultaneously received by the two piezoelectric thin films forming the bimorph-type planar piezoelectric element).
, The pyroelectric voltage generated by the upper piezoelectric thin film (the voltage observed at the observation terminal CT-2 part) is Δv ₁ , and the pyroelectric voltage generated by the lower piezoelectric thin film (at the observation terminal CT-2 part) Assuming that the observed voltage) is −Δv ₂ , the amplification factor of the correction circuit (5)
By multiplying Av by the pyroelectric voltage Δv ₁ from the upper piezoelectric thin film and adding the pyroelectric voltage −Δv ₂ from the lower piezoelectric thin film in the adding circuit (7), Δv ₁ × Av + (− Δv ₂ ) = Δv ₁ × Δv ₂ / ΔT ÷ Δ
Since v ₁ / ΔT + (− Δv ₂ ) = 0, it can be seen that the pyroelectric voltage can be canceled.

Needless to say, the charges generated with respect to the bending displacement are added without any cancellation.

In the piezoelectric sensor having the above structure, when the planar piezoelectric element (1) having a bimorph structure has a temperature change, as described above, it is caused by the temperature change in both conductive films (4) and (4). The charges cancel each other out in the external circuits connected to the conductive films (4) and (4) (see FIG. 3).
As is apparent from FIG. 2A, the electric charges due to the bending deformation are added, so that the piezoelectric conversion signal can be accurately extracted while the characteristics with respect to temperature are corrected.

Although the embodiment explained above shows the one using the buffer circuit (6) as the first stage input circuit, the present invention can be modified structure in each of the embodiments of FIGS. 5 to 7. .

In the example shown in each of FIGS. 5A and 5B, a circuit is provided between the conductive films (4) and (4) and the correction circuit (5), and a current / voltage conversion circuit (8) and an integration circuit (9) are provided in two stages before and after. The configuration shown in FIG. 6 is a single-stage configuration of the charge amplifier circuit (10).

Further, in the example shown in FIGS. 7A and 7B, in contrast to the example shown in FIG. 4 in which the correction circuit (5) is formed as a non-inverting amplifier circuit,
In the configuration using the subtraction circuit (11) instead of the inverting correction circuit (5) ′ and further adding the addition circuit (7) in the subsequent stage,
The function is the same as the example shown in FIG.

In addition, based on the same principle as each example described above,
As shown in the figure, the output of the buffer circuit (6) in FIG. 4 is converted into a digital signal by the analog-to-digital conversion circuit (12) and taken into the microcomputer (13), and the output ratio Av = ΔV ₂ / ΔT ÷ ΔV
The calculation of ₁ / ΔT, the amplification operation similar to that of the correction circuit (5) and the addition operation similar to that of the addition circuit (7) may be performed by software.

The piezoelectric sensor having the above configuration can be directly attached to the work holding portion of the robot hand as artificial skin and used, but it is also applicable to other general piezoelectric sensors for force-electric conversion.

(Effects of the Invention) As described above, the present invention has a bimorph structure in which two piezoelectric thin films (2) made of a polymeric piezoelectric material are stacked and combined with a correction circuit (5) for temperature correction. Due to the configuration, the output and sensitivity are higher than when one sheet of polymer piezoelectric material is used, and there is no change in the piezoelectric conversion characteristics with respect to temperature fluctuations, enabling stable and accurate detection. is there.

[Brief description of drawings]

FIG. 1 is a principle structure diagram of a planar piezoelectric element according to an embodiment of the present invention, FIGS. 2 (a) and 2 (b) are explanatory views of polarization and connection modes in the planar piezoelectric element, and FIG. Similarly, FIG. 4 is an explanatory view of the influence of heat, and FIGS. 4 to 8 are schematic structural views according to each embodiment of the present invention. (1) ... planar piezoelectric element, (2) ... piezoelectric thin film, (3) ... conductive film, (4) ... conductive film, (5) ... correction means, (7) ... addition means,

Claims (1)

[Claims] 1. A piezoelectric thin film (2) made of a piezoelectric polymer material.
To the both surfaces of the conductive film (3) as the ground electrode, and further to the outer surfaces of the piezoelectric thin films (2, 2) with the conductive films (4) as the detection electrodes for extracting the piezoelectric conversion signals. A sheet-like piezoelectric element (1) is formed from a sheet having a bimorph structure obtained by polarizing two piezoelectric thin films (2, 2) in the same direction, and one conductive film (4) as the detection electrode is formed. The output system is provided with a correction means (5) for converting the output voltage of the output system with a predetermined conversion coefficient determined by the voltage characteristics of the detection electrodes with respect to the temperature change, and at the same time, the other conductive member serving as the detection electrode. Adder means (7) for cancelingly adding the output of the film (4) and the output of the correction means (5) as different polarities when the planar piezoelectric element (1) is not deformed.
A piezoelectric sensor characterized by being provided with.