GB1585549A

GB1585549A - Temperature and stress compensated sensing apparatus

Info

Publication number: GB1585549A
Application number: GB1541877A
Authority: GB
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1976-04-14
Filing date: 1977-04-13
Publication date: 1981-03-04
Also published as: DE2716723A1; FR2348481A1; FR2348481B1; JPS5950084B2; JPS52149399A

Description

(54) TEMPERATURE AND STRESS COMPENSATED SENSING APPARATUS (71) We, MINNESOTA MINING AND MANUFACTURING COMPANY, a corporation organised and existing under the laws of the State of Delaware, United States of America, of 3M Center, Saint Paul, Minnesota 55101, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: Background of the Invention (1) Filed of the Invention The present invention relates in general to sensing apparatus; and more specifically to such apparatus that employ electret material having both pyroelectric and piezoelectric properties for sensing.

(2) Description of the Prior Art In this application the term electret means a dielectric material that is permanently or semi-permanently polarized and has pyroelectric and piezoelectric properties.

Such materials produce electric charge due to a change in what is commonly referred to as the dipole moment of the polarized material. Although pyroelectric and piezoelectric materials are often thought of as two distinct classes of materials, actually piezoelectric materials are a generic class in which pyroelectric materials are included as a species.

Sensing apparatus using pyroelectric and/or piezoelectric materials have recently become well known. Attention is drawn. for example. to the specification and claims of British patent 1.312.879 which is for a pyroelectric device formed from the polymeric material known as polyvinylidene fluoride. Materials having pyroelectric properties characteristically develop electric charges on their surfaces and voltage potentials between such surfaces when subjected to a change in temperature; and materials having piezoelectric properties characteristically develop electric charges on their surfaces and voltage potentials between such surfaces when subject to a change in stress.

Accordingly, pyroelectric materials are useful in sensing apparatus that detect temperature changes. However, one major problem inherent in a pyroelectric temperature sensing apparatus is that it must be temperature compensated in order to minimize erroneous readings due to ambient temperature changes in the vicinity of the apparatus.

Another major problem inherent in a pyroelectric temperature sensing apparatus is that it must be stress compensated in order to minimize erroneous readings due to stresses placed on the apparatus by the ambient environment.

The present invention provides an electret sensing apparatus having temperature and stress compensation, which comprises a flexible element and sensing means, said element comprising a continuous layer of flexible electret material that has both pyroelectric and piezoelectric properties, said layer having first and second areas, said first area being poled so that it produces electric charges on its broad surfaces when at least one of its temperature and stress is varied, the charges on one broad surface of said first area being opposite in polarity to the charges on the other broad surface of said first area. said second area being poled so that it produces electric charges on its broad surfaces when at least one of its temperature and stress is varied, the charges on one broad surface of said second area being opposite in polarity to the charges on the other broad surface of said second area. and said first and second areas being poled in opposite directions from one another. and a pair of con tinuous conductive plates in surface-tosurface contact with said layer of electret material, one of said continuous conductive plates being on one broad surface of said material and coextensive with at least one portion of each of said first and second areas, and the other of said continuous conductive plates being on the other broad surface of said material and coextensive with at least said one portion of each of said first and second areas, and said sensing means being operatively connected between said pair of continuous conductive plates for detecting charges on said pair of plates.

When the ambient temperature of the electret layer is varied, each of the first and second areas produces (pyro) electric charges on its surfaces. The charges on opposite surfaces of each of the first and second areas are of opposite polarity and the charges on the same broad surface of the electret layer are of opposite polarity in the first and second areas. When the pyroelectric coefficient and size of each area interconnected by the plates are in proper proportion, the sensing means detects no significant change in electric charges on the pair of plates in response to a uniform temperature variation.

When the stress on the electret layer is varied, each of the first and second areas produces (piezo) electric charges on its surfaces. Again. the charges on opposite surfaces of each of the first and second areas are of opposite polarity and the charges on the same broad surface of the electret layer are of opposite polarity in the first and second areas. When the piezoelectric coefficient and size of each area interconnected by its plates are in proper proportion, the sensing means detects no significant change in electric charges on the pair of plates in response to a uniform stress variation.

When the temperature of the electret layer is selectively varied from ambient, such as by heating the first areas more than the second areas. a net charge develops on each plate and a potential exists between the plates. The sensing means provides an appropriate indication and thus. the device operates as a thermal sensing apparatus. In like manner, the present invention may be employed as a stress sensor for detecting selective variations in the stress on the electret layer.

The present invention, however. is not limited to only first and second oppositely poled areas. Preferably an apparatus according to the present invention has first and second groups of oppositely poled areas with each such group having at least three areas.

The electric layer and the conductive plates of the present invention are flexible.

The electret layer of the present invention is preferably a polymeric material. such as polyvinylidene fluoride.

In a first embodiment. the electret layer has six side-by-side rectangular areas that have equal size and uniform magnitudes of poling. Each area is oppositely poled with respect to its adjacent areas. Each broad surface of the electret layer is entirely covered by a thin continuous conductive plate and a sensing means is connected between the plates.

A second embodiment is similar to the first except the side-by-side rectangular areas poled in one direction have twice the size and half the magnitude of poling of the rectangular areas poled in the opposite direction.

The present invention provides a lowprofile, inexpensive, reliable and highly accurate temperature and stress compensated sensing apparatus. Because it is made of a continuous layer of electret material and a pair of continuous conductive plates, a sensor according to the present invention can have pluralities of temperature and stress compensating areas and yet need only two external connections for connection to the sensing means.

Brief Description of the Drawing Figure 1 is an end view of a temperature and stress compensated sensing apparatus representing a first embodiment of the present invention; Figure 2 is a sectional view taken along the line 2-2 of Figure 1 that illustrates with dotted lines a plurality of poled areas having the same size; Figure 3 is an end view of a temperature and stress compensated sensing apparatus representing a second embodiment of the present invention; and Figure 4 is a sectional view taken along the line 4-4 of Figure 3 that illustrates with dotted lines a plurality of poled areas having differing sizes.

Detailed Description of the Invention Referring to the drawings and specifically to Figures 1 and 2. an electret sensing apparatus 10 having temperature and stress compensation is shown. The apparatus 10 includes an electret layer 12 that has both pyroelectric and piezoelectric properties.

The layer 12 has two opposite surfaces which are broad relative to the thickness of the layer. Continuous conductive plates 14 and 16 are disposed on the opposite broad surfaces of the layer 12. A sensing means 18 is connected between plates 14 and 16 by leads 20 and 22 and detects electric charges thereon.

The electret layer 12 can be a thin continuous film of polyvinylidene fluoride and has first and second groups of areas, which areas are shown separated by dotted lines only for illustration purposes in Figures 1 and 2. Each of the areas 24 through 29 is rectangular and has the same size and magnitude of poling. The first group includes areas 24, 26 and 28 which are poled in one direction and the second group mcludes areas 25, 27 and 29 which are poled in the opposite direction, as represented by the arrows in the drawing. to provide a pattern of alternating poled areas from the first and second groups. The poling magnitudes will tapet off where the areas approach each other.

The pyroelectric and piezoelectric proper ties of materials are commonly referred to as alignment of the dipoles of the material.

While a few electret materials have dipoles that are naturally aligned in a poled relation ship, normally the dipoles of electret materi al's are essentially arranged in random fash- ion. These dipoles can be rearranged in poled orientation when an electret material is heated above a particular temperature known as the poling temperature and an electric field is applied.

The dipoles of an electret material will orient themselves in accordance with the applied electric field. The degree of dipole orientation is a function of the temperature to which the electret material is heated. the applied field strength and length of time the field applied. For example. substantial pol ing begins in polyvinylidene fluoride when it is heated to a temperature greater than 90 C. with an electric field of at least 4.000 volts per millimeter of thickness applied for approximately 15 minutes. Increasin the temperature and/or the applied electric field will progressively increase the degree of pol ing achieved up two a maximum of saturation.

Once an electret material is poled and then cooled below its poling temperature, the applied field may be removed and the dipoles will remain as oriented by the applied field.

The pyroelectric property of the electret material will thereafter produce opposite electric charges on its broad surfaces when heated or cooled from its ambient tempera ture. The polarity of such charges will there- after be determined by the direction of the poling field and the direction of the tempera ture change. The piezoelectric property of the - poled electret material will produce opposite electric charges on its broad sur faces when stressed by tension or compres- sion from its ambient condition. The polarity of such charges will thereafter be determined by the direction of the poling field and the type of change in the stress. Care should be taken tn ensure that the poled material is not heated above its poling temperature for extended periods in order that the dipoles are not permitted to return to a random orientation.

The electret layer 2 of Figures 1 and 2 can be'poled by first spray coating a first broad surface of the layer 12 with the conductive plate 14 that is coextensive with the areas 24 through 29. Then a plurality of conductive electrodes that do not contact one another are placed against the other broad surface of layer 12 with each such electrode overlaying one of the areas 24 through 29. One polarity of voltage is applied between the electrodes overlaying areas 24, 26 and 28, and layer 14. and the opposite polarity of voltage is applied between the electrodes overlaying areas 25. 27 and 29, and layer 14 to pole the areas with opposite directions of poling as previously described. Care should be taken to adequately insulate the opposite polarity electrodes overlaying the areas 24 through 29 from each other.

After poling, the second broad surface of the layer 12 is spray coated with the conduc tive plate 16. The sensing means 18, which is operatively connected to the plates via leads 20 and 22. can be for example, an ammeter, a voltmeter, a coulombmeter, or other device .which indicates current.- voltage or charge, The temperature. compensation, of -the apparatus 10 operates for example when the ambient environment uniformly changes the temperature of the electret layer 12, such as by increasing the ambient temperature. In response to such temperature change, each of the areas 24, 26 and 28 in the first group of poled areas will produce pyroelectric charges on the broad surfaces of the layer 12 for example the charges on the surface adjacent plate 16. being positive and those oil. the sur- face adjacent plate 14 being negative@ and each of the areas 25, 27 and 29 in the second group of poled areas will produce pyroelec tric charges on the broad surfaces of the layer 12, for example, the charges on the surface adjacent plate 16 being negative and those on the surface adjacent plate 14. being posi- tive. Because each of the poled areas is the same size. has the; same magnitude of poling; and experiences the same temperature change. and because the poled areas in the first and second groups are oppositely-poled as previously described. the net amount of pyroelectric charge developed on each sur face of.the layer 12 is zero and the sensing means 18 detects no significant change in charge on or voltage between the plates 14 and 16 in response to such uniform tempera ture change.

.The stress compensation of the apparatus 10 operates. for example. when the ambient environment uniformly changes the stress on the electret layer 12. Such change in stress could be provided by a change in the mag nitude of air pressure on the exposed broad surfaces of the plates 14 and 16 such as caused by acoustical vibrations. which in turn could result in a uniform compression of the layer 12. In response to such stress change. each of the areas 24. 26 and 28 in the first group of poled areas will produce piezoelec tric charges on the broad surfaces of the layer 12. for example. the charges on the surface adjacent plate 16 being positive and those on the surface adjacent plate 14 being negative; and each of the areas 25, 27 and 29 in the second group of poled areas will produce piezoelectric charges on the broad surfaces of the layer 12. for example. the charges on the surface adjacent plate 16 being negative and those on the surface adjacent plate 14 being positive. Because each of the poled areas is the same size. has the same magnitude of poling. and experiences the same stress change. and because the poled areas in the first and second groups are oppositely poled as previously described. the net amount of charge developed on each surface of the layer 12 is zero and the sensing means 18 detects no significant change in charge on or voltage between the plates 14 and 16 in response to such uniform stress change.

The apparatus 10 can be operated as a thermal detector by selectively heating or cooling part of the electret layer. Such selective heating or cooling may be accomplished in a variety of ways. depending on the condition that the apparatus 10 is adapted to sense. For example. a radiant source that provides heat energy may be sensed by the use of a slotted housing that permits only the portion of layer 16 overlaying the poled areas 24. 26. 28 in the first group to be exposed to the radiation. Thus. the areas 24 26 and 28 in the first group will increase in temperature relative to the areas 25. 27. 29 and in the second group to provide. for example. a net amount of positive pyroelectric charge on the surface of the layer 12 adjacent the plate 16 and a net amount of negative pyroelectric charge on the surface of the layer 12 adjacent the plate 14. In response thereto. the sensing means provides an indication representative of the magnitude of change in the charge on or the voltage between the plates 14 and 16. It should be understood by those skilled in the art that the apparatus 10 can be employed in like fashion as a stress sensor for detecting selective variations in the stress on the electret layer.

While the embodiment shown in Fi-gures I and 2 has continuous conductive plates 14 and 16 that are coextensive with the poled areas in each of the first and second groups. such coextensive feature is not essential to the present invention. Instead. it would be considered swithin the scope of the present invention if the continuous conductive plates 14 and 16 are coextensive with a portion of each poled area 24 through 29.

While the embodiment shown in Figures 1 and 7 has a uniform magnitude of poling throughout the areas. it is possible that due to practical poling problems all portions of the areas will not be poled with exactly the same magnitude. Thus, a variation in the charges developed on the surfaces of the layer 12 will result when at least one of the temperature and stress of the layer 12 is altered. To compensate for such variation in developed charges. the portions of the areas covered by the plates 14 and 16 are selected in size to provide electric charges on each plate which are equal in magnitude and opposite in polarity in response to uniform variation in at least one of the temperature and stress of the layer 12 so that the sensing means detects no significant change in charge on or voltage between the pair of plates.

While the embodiment shown in Figures 1 and 2 has areas which are closely adjacent -one another. the poled areas within the continuous layer 12 could be spaced separately from one another. Also. the present invention would work with only one area in each of the first and second groups. However. in general. it is preferable to have at least three areas in each of the first and second groups.

Figures 3 and 4 show an apparatus 40 representing a second embodiment according to the present invention. The apparatus 40 includes a poled electret layer 42 having continuous conductive plates 44 and 46. similar to the plates 14 and 16. on the opposite broad surfaces thereof. A sensing means 48. similar to the sensing means 18. is connected between the plates 44 and 46 by leads 50 and 52 and detects charges on the pair of plates in a manner similar to that of the apparatus 10.

The poled electret layer 42 of the apparatus 40 also has first and second groups of areas. The first group includes areas 54.56 and 58. and the second group includes areas i5.57 and 59. The areas 54.56 and 58 of the first group have one-half the poling magnitude of the areas 55. 57 and 59 of the second group. and the areas in the first group are twice the size of the areas in the second group. The plates 44 and 46 are coextensive with the areas 54 through 59 and as previously described the sizes of the areas within each group have been selected to compensate for the nonuniformity of poling magnitude. Thus. in response to uniform variation in at least one of the temperature and stress of the electret layer. each surface of the first and second groups will develop equal magnitude and opposite polarity charges such that the sensing means detects no significant change in charge on or voltage between the plates. Other ratios of magnitudes of poling and sizes of areas can be used to provide similar results.

Example A sheet of biaxially oriented polyvinylidene fluoride film (obtained from Kureha Chemical Co.. Japan) 25 micrometers thick is spray coated entirely over a first broad surface to provide a conductive silver coating thereon approximately 2 micrometers thick. Then, six equal-sized rectangular conductive silver plates approximately 2 micrometers thick are formed on the second broad surface by spray coating through a patterned mask to provide an interdigitated set of plates sufficiently spaced from each other to avoid arcing. Alternate rectangular plates are electrically connected to form first and second groups. The film is poled by placing an electric potential of +2000 volts on the first group of plates and an electric potential of -2000 volts on the second group of plates, both of which potentials being with respect to the coating on the first broad surface of the film, while the film is heated to 1200C. The electric potentials are maintained under these conditions for 20 minutes and the film is then cooled to room temperature prior to removing the electric potentials. Then, a thin conductive silver coating is sprayed over the entire surface having the rectangular plates.

A voltmeter is connected to the silver coatings and acts as the sensing means.

Claims

WHAT WE CLAIM IS:

1. An electret sensing apparatus having temperature and stress compensation, which comprises a flexible element and sensing means, said element comprising a continuous layer of flexible electret material that has both pyroelectric and piezoelectric properties. said layer having first and second areas, said first area being poled so that it produces electric charges on its broad surfaces when at least one of its temperature and stress is varied. the charges on one broad surface of said first area being opposite in polarity to the charges on the other broad surface of said first area. said second area being poled so that it produces electric charges on its broad surfaces when at least one of its temperature and stress is varied. the charges on one broad surface of said second area being opposite in polarity to the charges on the other broad surface of said second area. and said first and second areas being poled in opposite directions from one another. and a pair of continuous conductive plates in surface-tosurface contact with said layer of electret material. one of said continuous conductive plates being on one broad surface of said material and coextensive with at least one portion of each of said first and second areas. and the other of said continuous conductive plates being on the other broad surface of said material and coextensive with at least said one portion of each of said first and second areas and said sensing means being operatively connected between said pair of continuous conductive plates for detecting charges on said pair of plates.

2. A sensing apparatus according to claim l. wherein said plates are coextensive with said first and second areas and said areas are the same size and have the same magnitude of poling such that in response to uniform variation in at least one of the temperature and stress of said electret layer, said sensing means detects no significant change in charge on said pair of plates.

3. A sensing apparatus according to claim 1, wherein said plates are coextensive with said first and second areas, said areas have different mangitudes of poling, and the sizes of said areas are selected to provide charges on each surface of said areas which are equal in magnitude and opposite in polarity in response to uniform variation in at least one of the temperature and stress of said electret layer whereby said sensing means detects no significant change in charge on said pair of plates.

4. A sensing apparatus according to claim 1, wherein the portions of said first and second areas covered by said plates are selected to provide charges on each covered surface of said areas which are equal in magnitude and opposite in polarity in response to uniform variation in at least one of the temperature and stress of said electret layer whereby said sensing means detects no significant change in charge on said pair of plates.

5. A sensing apparatus according to any preceding claim, wherein said electret layer comprises a film of polyvinylidene fluoride.

6. An electret sensing apparatus substantially as herein described with reference to the accompanying drawings.