JPS5911841B2 - electret sensing device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION TECHNICAL FIELD This invention relates generally to sensing devices in general, and more particularly to sensing devices that include multiple values of layers of electret material as sensing elements.

BACKGROUND OF THE INVENTION The common definition of an electret is a dielectric material that has been permanently polarized by heating the material and placing it in a strong electric field as it cools.

Electrets are usually classified based on their pyroelectric or piezoelectric properties, both of which have a dipole moment (di
A change in the voltage (poledmlrrent) produces an electrical signal.

Pyroelectric materials and piezoelectric materials are often considered to be two distinct electret classes, but in reality the primary electrical materials are a generic class within which pyroelectric materials are included as a class. be. Therefore, a large number of materials have both pyroelectric and piezoelectric properties. The use of electrets having both pyroelectric and piezoelectric properties is described in U.S. Pat. No. 3,769,096 to Asyukin et al.
No. 3,733,922 discloses an apparatus for pyroelectrically sensing incident radiation, and Eyer et al., U.S. Pat. No. 3,768,059, describes a strain sensor using piezoelectric principles. As described above, this is done when forming various types of sensing devices.

The types of electrets described in both of the above-mentioned US patents have both pyroelectric and piezoelectric properties. Therefore, the Asyukin et al. sensor may produce erroneous sensing behavior as a result of piezoelectrically generated spurious electrical signals, and the Eyer et al. have similar sensitivities. SUMMARY OF THE INVENTION An object of the present invention is to selectively detect only one variable and nullify the sensing of the other variable when two variables, temperature and stress, are received simultaneously by using a sensing medium that can sense both variables. The object of the present invention is to provide a precise and accurate sensing device free from errors caused by other variables.

To this end, the present invention uses a pair of layers of electret material as the sensing medium. The electret layers are separated by an inner conductive layer in electrical contact with one surface of each layer. The other surface of each layer is covered by an outer conductive layer. An electronic sensing circuit is connected to at least two of the conductive layers to detect electrical signals on the conductive layers. In a first preferred embodiment, the present invention senses pyroelectrically generated electrical signals and cancels piezoelectrically generated spurious signals due to bending.
It is used as an electret sensing device that detects the amount of change in temperature and cancels out the amount of stress change.

Therefore, a precise and accurate sensing operation can be obtained. The two electret layers in this embodiment are polarized in opposite directions, and the sensing circuitry is connected across the inner conductive plate and the outer conductive plate, with the outer conductive plate being grounded. . Thus, by using two electret layers as the sensing medium, the sensing signal is twice as large for a given change in temperature as in a sensor using only one electret layer. By grounding the conductive plate outside V1,
The sensor is shielded from sensing spurious electrostatic changes. In another preferred embodiment, the electret senses the piezoelectrically generated electrical signal due to bending, nullifies the pyroelectrically generated signal, detects the amount of stress change and cancels the amount of temperature change. Used as a sensing device.

Therefore, this embodiment also provides precise and accurate sensing. The present invention uses a sensing medium formed of a polymeric electret material that has both pyroelectric and piezoelectric properties to selectively sense only one variable, temperature or stress change, while detecting other variables. This invention relates to an electret sensing device that eliminates errors due to

The former (biroelectric) property produces an electrical signal due to a change in the ambient temperature of the sensing medium, whereas the latter (piezoelectric) property produces an electrical signal as a result of a change in the axial stress of the sensing medium. The present invention provides an electrical signal generated in the sensing medium by one of the above characteristics, depending on the combination of the pole forming direction of the pair of electret layers and the method of connecting the sensing circuit arrangement and the electret layers. The device is configured to detect and cancel the electrical signals produced by the other characteristic to provide accurate and precise sensing operations. EMBODIMENTS OF THE INVENTION Referring now to FIG. 1, a first preferred embodiment of the invention is shown in the form of a thermal sensor 1, which is adapted to sense pyroelectrically generated electrical signals. and is compensated to substantially cancel out the piezoelectric charge caused by bending.

The sensor 1 preferably comprises two similar non-conducting electret layers 2 and 3, these electret layers being polyvinylidene fluoride, polyvinylidene fluoride, which has both pyroelectric and piezoelectric properties. Can be formed of vinyl, lanthanum modified zirconate, lead titanate or other such amorphous materials. electret layers 2 and 3 act as sensing media in sensor 1;
They are coupled in electrical contact by a conductive layer 4 in surface-to-surface electrical contact with their inner surfaces.

The outer surfaces of layers 2 and 3 are in surface-to-surface electrical contact with equally sized conductive layers 7 and 8, respectively, which conductive layers are coated on the outer surfaces of electret layers 2 and 3. It's fine. Layers 7 and 8 are grounded together by a conductor 9. A sensing circuit 10, such as a voltmeter, ammeter or other suitable sensing circuit device, is electrically connected between the conductive layer 4 and ground. For clarity purposes, layer 2
, 3, 4, 7 and 8 are exaggerated. The sensor has layers 2 and 3, typically about 50 microns, and typically about 2
It has layers 4, 7 and 8 of 0 micron. but,
Layers 2 and 3 may be as thin as 6 microns and layers 4, 7 and 8 may be as thin as 1 micron.
The electret layers 2 and 3 are preferably uniformly polarized in terms of size, but polarized in opposite directions, as indicated by arrows 11 and 12 for layers 2 and 3, respectively.

Pole formation of the electret layers 2 and 3 prepares dipoles at right angles to the planar surfaces of the layers when those layers are heated above a special temperature called the pole formation temperature. At this pole formation temperature, the dipoles of the electret layers 2 and 3 are oriented in accordance with the applied electric field. The degree of bipolar orientation is a function of the heating temperature of the layers 2, 3, the applied electric field strength and the duration of the applied electric field. For example, in polyvinylidene fluoride, substantial pole formation begins when an electric field of at least 4000 volts per thickness 111 is applied and heated to a temperature above 90° C. for about 15 minutes. By increasing the consistency and/or the applied electric field, the degree of pole formation achieved will progressively increase up to a saturation maximum. Once the electret layers 2 and 3 are formed into poles,
Subsequently, after cooling below its pole formation temperature, the applied electric field is removed and the dipoles of layers 2 and 3 are held in the orientation determined by the applied electric field. In order to prevent the dipoles of layers 2 and 3 from returning in a haphazard direction, care must be taken to ensure that layers 2 and 3 are not heated above the pole forming temperature over an extended period of time. After pole formation is complete, layers 2 and 3, due to their pyroelectric properties, will develop electrostatic charges of opposite polarity on their planar surfaces as the ambient temperature changes.
Also, when layers 2 and 3 are subjected to axial stress, the piezoelectric properties of layers 2 and 3 will create electrostatic charges of opposite polarity on the surfaces of layers 2 and 3. The conductive layers 7 and 8 as well as the electret layers 2 and 3 are of the same size as each other, and when the layers 2 and 3 are heated or stressed, an equal level of electrostatic charge is applied to the conductive layers 7 and 8. It is preferred to have polar formations of equal size, as seen above.

However, such a condition is not essential to the invention, since the layers 2, 3 or 7, 8 may differ in size. However, the same amount of charge will be stored on conductive layers 7 and 8 when using layers 2 and 3 that are polarized to different degrees. In order to better explain the invention, the operation of sensor 1 will now be described.

When the temperature of the sensor 1 is changed in a uniform manner so that the electret layers 2 and 3 change temperature similarly, the second
As shown in the figure, a uniform distribution of pyroelectrically induced electrostatic charges of opposite polarity accumulates on the surfaces of layers 2 and 3 in direct proportion to the polar formation magnitude and surface area of layers 2 and 3. Will. Since the pole formation directions of layers 2 and 3 are different,
Only one polarity of electrostatic charges will appear on the outer surface of each of layers 2 and 3, and only electrostatic charges of the opposite polarity will appear on the inner surfaces of layers 2 and 3. Thus, there is an appreciable electrical potential between the outer and inner surfaces of layers 2 and 3. Since layers 2 and 3 are substantially non-conductive, the electrically connected outer conductive layers 7 and 8 and inner conductive layer 4 transfer charges from the surfaces of layers 2 and 3 to sensing circuit 10. Acts as a device to conduct electricity. The sensor 1 thus serves to provide an electrical sensing signal when exposed to temperature changes.

Because the sensor 1 has this property, it has a wide variety of applications. However, many of the possible applications involve intentional or unplanned bending of the sensor 1. However, sensor 1 would be completely undesirable for use in bending applications without piezoelectric compensation, since layers 2 and 3 would develop spurious electrostatic charges when subjected to bending moments. This is because such charges will lead to inaccurate sensing unless the sensor 1 is provided with a compensation device for such spurious charges. The structure of the sensor 1 is adapted to provide an inherent piezoelectric compensation for bending to ensure precise and accurate operation. When sensor 1 is stressed as a result of bending, layers 2 and 3
An electrostatic charge is created on the surface of the layer, the magnitude of which is directly proportional to the amount of tensile or compressive stress in layers 2 and 3.

The polarity of this electrostatic charge depends on the polar formation direction of layers 2 and 3 and on whether the stress is tensile or compressive. Taking these two factors into account, sensor 1 is
Although the electrostatic charges caused by the bending cancel each other out, temperature changes in layers 2 and 3 are made to produce an appreciable sensing voltage. Such charge cancellation is achieved by making the toughness of the inner conductive layer 4 even greater than the composite toughness of layers 2 and 3 and their respective conductive coatings 7 and 8. Therefore, when sensor 1 is bent, conductive layer 4 acts as a point of zero compression or zero tension, layer 3 is stretched and layer 2 is stretched, as shown in the exaggerated view of FIG. , compressed. However, sensor 1
It should be understood that flexing causes compression of layer 3 and stretching of layer 2. By tensioning layer 3,
Positive and negative charges are generated on its outer and inner surfaces respectively, while compression of layer 2 generates negative charges and positive electrolytes on its outer and inner surfaces.

A conductive layer 7 connects together the oppositely charged inner surfaces of layers 2 and 3 and a conductor 9 electrically contacts the oppositely charged outer surfaces of layers 2 and 3. and 8 are connected, the piezoelectrically generated charge of sensor 1 is
There is a substantially zero net charge across the sensing circuit 10.
Besides offering the advantage of compensating the piezoelectrically generated charge for a given temperature change, the sensor 1 also offers the advantage of increasing sensitivity.

This is because the pyroelectrically generated sense charge created by the two electret layer configuration is twice the sense charge generated by a sensor using a single layer of electrets 2 or 3. It is. The earth, grounded, conductive outer layer prevents the sensor 1 from reacting to spurious electrostatic charges that would result in inaccurate operation of the sensor 1.
to shield. Referring now to FIG. 4, a second embodiment of the present invention is represented by thermal sensor 18. As shown in FIG. Similar to sensor 1, sensor 18 includes two electret layers 1
9 and 20, the inner surfaces of both electrets are joined with a conductive layer 21, and the outer surfaces thereof are connected with a conductive layer 2, respectively.
2 and 23. This sensor 18 is
As shown in 4 and 25, it has electret layers 19 and 20 with poles formed in the same direction, and outer conductive layers 22 and 23.
This sensor differs from the sensor 1 in that a sensing circuit 26 is electrically connected across the sensor. With this arrangement of the present invention, uniform temperature changes in layers 19 and 20 produce electrostatic sensing potentials of opposite polarity at the surfaces of electret layers 19 and 20, as shown in FIG. However, as previously discussed regarding the operation of sensor 1, flexing sensor 18 produces only one polarity of charge on the outer surface of electret layers 19, 20, and only an opposite polarity of charge on its inner surface. will occur. Therefore, the sensing circuit 26
experiences essentially zero net charge. Thus, as demonstrated by the embodiments described above, the invention provides a simple device for compensating piezoelectric voltage potentials caused by bending in thermal sensors.

However, the present invention is not limited to such uses. This is because it can also be used to compensate for pyroelectrically generated voltage potentials in sensors that rely on the sensing of piezoelectrically generated voltage potentials. Referring to FIG. 5, sensor 28 is shown compensating for pyroelectrically generated voltage potentials.

Sensor 28 consists of two electret layers 29 and 30.
, both layers being bonded at their inner surfaces by a relatively tough conductive layer 31. The outer surfaces of layers 29 and 30 are coated with conductive layers 32 and 33, respectively, and are electrically grounded by conductor 34. A sensing circuit 36 connected across the conductive layer 31 and the conductor 34 is for sensing the potential difference between the inner conductive layer 31 and the outer conductive layers 32 and 33. Although the arrangement of parts forming sensor 28 is the same as that of sensor 1, these two sensors differ from each other in the following points.

That is, the electret layers 29 and 3 of the sensor 28
0 is polarized in the same direction as shown by arrows 37 and 38, whereas the electret layer of sensor 1 is polarized in the opposite direction. Since the layers 29 and 30 have poles in the same direction, uniform temperature changes can be achieved by
As shown in FIG. 5, there is a distribution of static charges on the surfaces of layers 29 and 30, which cancel each other out to produce a zero net potential for sensing circuitry 36. In contrast to the static charge cancellation that occurs as a result of temperature changes on the sensor 28,
The bending of sensor 28 creates a sense potential across circuit 36 .

The relatively strong conductive layer 31 is bent so that the electret 29 is compressed and the electret layer 30 is pulled, as shown in FIG. 6, so that the sensor 28 can respond to the bending force. Ru. Therefore, layers 29 and 30
An electrostatic charge builds up on the surface of the sensor 28, creating a sensed potential that is proportional to the degree of flexion of the sensor 28. The fourth embodiment of the present invention is
A sensor 39 as shown in FIG. 7 includes electret layers 40 and 41, an inner conductive layer 42, and outer conductive layers 43 and 44. The sensor 39 has a sensing circuit 45 connected across outer conductive layers 43 and 44,
The electret layer differs from sensor 28 of FIG. 5 by forming poles in opposite directions, as indicated by arrows 46 and 47. Although uniform temperature changes in sensor 39, like sensor 28, do not affect the net voltage potential across circuit 45 as shown in FIG. Generates a potential. As explained above, according to the present invention, by arbitrarily selecting the combination of the electrode formation directions of the electret layer and the connection direction of the electronic sensing circuit and the electret layer, temperature and stress can be controlled by one sensing device. Accurate sensing operation is possible by detecting one of the variables and canceling out errors caused by other variables.

[Brief explanation of the drawing]

FIG. 1 shows a first embodiment of the present invention used as a thermal sensing device.
1 is a schematic diagram of an example. FIG. 2 is the sensor of FIG. 1 showing the electrostatic charge that occurs when the temperature changes. FIG. 3 is the sensor of FIG. 1 showing the electrostatic charge caused by bending. FIG. 4 is a schematic illustration of a second embodiment of the invention used as a thermal sensing device, showing the electrostatic charge generated when changing temperature. FIG. 5 is a third embodiment of the present invention used as a stress sensing device, illustrating the electrostatic charge generated by uniformly changing temperature.
FIG. 6 is the sensor of FIG. 5 showing the electrostatic charge caused by bending. FIG. 7 is used as a stress sensing device,
Figure 3 is a schematic diagram of a fourth embodiment of the present invention showing electrostatic charges caused by uniform temperature changes; 1...sensor, 2,3...
Electret layer, 4, 7, 8... conductive layer, 10...
- Sensing circuit, 18... Sensor, 19, 20... Electret layer, 21, 22, 23... Conductive layer, 26.
... Sensing circuit, 28... Sensor, 29, 30... Electret layer, 31, 32, 33... Conductive layer, 34
...Conductor, 36...Sensing circuit, 39...Sensor,
40, 41... electret layer, 42, 43, 44
... Conductive layer, 45... Sensing circuit.

Claims (1)

[Claims] 1. Electret sensing device 1, 18, 28, which receives both temperature change and stress change, senses one of the changes, and nullifies the other of the changes.
39, the first electret layer 2, 19, 29 of a pole-forming material having both piezoelectric and pyroelectric properties and generating electrostatic charges of opposite polarity on the surface in response to changes in temperature and stress; , 40 and a first conductive layer 7 in surface-to-surface contact with one side of the first electret layer.
22, 32, 43, which are located almost parallel to the first electret layer, have both piezoelectric and pyroelectric properties, and have opposite polarity statics on the surface in response to temperature and stress changes. a second electret layer 3, 20, 30, 41 of a charge-producing pole-forming material; a second conductive layer 8, 23, 33, 44 in surface-to-surface contact with one side of said second electret layer; a third conductive layer 4, 21 interposed between the first electret layer and the other side of the second electret layer and in surface-to-surface electrical contact therewith;
31, 42, an electronic sensing element connected to at least two of the conductive layers to detect electrostatic charges generated by temperature or stress changes on the electret layer of the device. Circuit devices 10, 26
. An electrostatic charge generated on the electret layer due to one of the changes in temperature and stress is detected depending on whether the polar formation directions of the layers are opposite to each other or in the same direction. Electrostatic charges generated on the electret layer are nullified by the other of the temperature and stress changes, so that only one of the temperature and stress changes can be selectively detected. An electret sensing device characterized by: