WO2006040781A2

WO2006040781A2 - Sensor for measuring phisical quantities based on the detection of the variation of an electrical parameter, and method for its fabrication

Info

Publication number: WO2006040781A2
Application number: PCT/IT2004/000566
Authority: WO
Inventors: Davide Gennaretti
Original assignee: Neopress S.R.L.
Priority date: 2004-10-14
Filing date: 2004-10-14
Publication date: 2006-04-20
Also published as: WO2006040781A3

Abstract

Sensor (1) for measuring at least one physical quantity, which comprises at least one capacitive cell (2) which, in use, varies a related electrical parameter as a function of the physical quantity; the capacitive cell (2) comprising at least a first layer (3) of dielectric or insulating compressible elastic material having at opposite sides a first and a second outer contact surface (3a), and at least a first plate (4) made with a film of paint or ink in compressible and electrically conductive elastic material, which is deposited directly onto the first contact surface (3a) of the first layer (3).

Description

SENSOR FOR MEASURING PHYSICAL QUANTITIES BASED ON THE DETECTION OF THE VARIATION OF AN ELECTRICAL PARAMETER, AND METHOD FOR ITS FABRICATION

TECHNICAL FIELD

The present invention relates to a sensor for measuring physical quantities, and to the corresponding method for its fabrication. In particular, the present invention relates to a sensor able to measure a series of physical magnitudes, such as humidity, temperature, or pressure, the latter being the use whereto the description that follows shall explicitly refer, without thereby losing its general nature.

BACKGROUND ART

As is well known, the measurement of the pressure, or more in general the sensing of the distribution of forces on a contact surface is currently performed by means of different types of sensors, some of which are typically indicated with the term "capacitive sensors" comprise one or more capacitors or capacitive cells, which are appropriately distributed on the contact surface of the sensor, and are provided each with a pair of plates made of electrically conductive material, and with a layer of dielectric or insulating material interposed between the armatures.

In use, the measurement of the pressure acting on the contact surface of the sensor is obtained by measuring and appropriately processing the variation in the capacity of the capacitive cells produced by a corresponding variations in the distance between the plates subjected to the external pressure.

The US Patent US-4862743 filed on 16 February 1988 in the name of Peter Seitz describes an example of a sensor provided with a series of capacitive cells, which are distributed on a contact area or surface shaped as a sole for shoes. In use, the capacitive sensor in the shape of a shoe sole is able to be placed in contact with the sole of the foot of a patient (e.g., it can be inserted inside a shoe) to sense the distribution of the pressures on the contact surface of the foot, both in static conditions, in which the foot remains motionless bearing on the contact surface of the capacitive sensor, and in dynamic conditions, in which the foot is in motion.

The aforesaid capacitive sensor has a spatial distribution of the capacitive cells according to a matrix geometric structure, in which each capacitive cell is essentially constituted by three superposed layers, in which a first central layer is made of compressible dielectric elastic material, while a second a third lateral layer are made of insulating plastic material (PVC) .

Each capacitive cell of the sensor further comprises a fourth and a fifth layer, which are obtained by laying an electrically conductive material on the second and respectively on the third insulating plastic layer, in such a way as to constitute the two armatures, facing and opposing each other, of the capacitive cell.

The second and the third layer made of insulating plastic material are fastened to the outer and opposite surfaces of the first central layer, and have characteristics of flexibility which allow the lateral layers to be deformed and to flex in plastic fashion, under the action of the pressure, solely from and towards the first central layer, thereby determining the compression thereof.

Capacitive sensors of the type described above have the great drawback of being poorly flexible and of adapting with difficulty to particularly irregular contact profiles such as the surfaces of the human body having double curvatures or a high de^'formability, variable in the presence of external forces. In other words, the aforesaid capacitive sensor is not able to flex and to be deformed sufficiently in each of its points thereby being able faithfully to follow a corresponding shape variation of the corresponding bearing surface.

In particular, in the capacitive sensors described above, although the second and the third layer of plastic material are partially deformable and plastically flexible in substantially "one-directional" fashion, they are completely rigid and non-extensible, i.e. they cannot be elongated along any. direction parallel to its own superficial plane.

This "planar" rigidity of the second and third layer is a necessary characteristic to assure a correct laying of the electrically conductive material on the outer surfaces of the first central layer; it is well known that any extension of the second or third layer along a "planar" direction could determine an elongation and hence an expansion of the strips and/or of the plates, thereby causing interruptions and discontinuities in the electrical connections.

Unfortunately, if on one hand the "planar" rigidity of the second and third layer made of plastic material is still indispensable to assure the electrical continuity of strips and plates made of electrically conductive material, thereby eliminating the risk of possible interruptions during the flexion of the first layer, on the other hand it makes the capacitive sensor unsuitable for use in some specific applications, in which a high degree of deformability and flexibility of its structure is required.

For example, in the field of measuring the distribution of the pressures of a foot on the inner bearing surface of a shoe, the insufficient degree of deformation and flexibility of the capacitive sensor in the shape of a sole (structure in the manner described above) positioned inside the shoe, in addition to being a factor of inconvenience and awkwardness for the user who wears the shoe, can also introduce measuring errors due to the irregular and discontinuous "plastic" deformations of the second and third layer of the capacitive sensor, which occur when the foot bears down.

DISCLOSURE OF INVENTION

The object of the present invention, therefore, is to obtain a sensor for measuring physical quantities such a pressure, which is free from the drawbacks described above.

According to the present invention, a sensor is provided for measuring physical quantities as set out in claim 1 and, preferably, in any one of the subsequent claims depending directly or indirectly from claim 1.

According to the present invention, a method is also provided for fabricating a sensor for measuring physical quantities as set out in claim 12. According to the present invention, lastly, a capacitor as set out in claim 16 is obtained. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention shall now be described with reference to the accompanying drawings, which illustrate a non limiting embodiment thereof, in which: - Figure 1 schematically shows a perspective view of a sensor for measuring physical quantities based on the detection of the variation of an electrical parameter, obtained according to the present invention;

- Figure 2 is a schematic lateral view of a first variant of the sensor shown in Figure 1;

- Figure 3 is a schematic lateral view of a second variant of the sensor shown in Figure 1;

- Figure 4 schematically shows a perspective view and a lateral view of an embodiment of the second variant of the sensor shown in Figure 3;

- Figure 5 schematically shows a perspective view and a lateral view of an embodiment of a third variant of the sensor shown in Figure 1; whilst

- Figure 6 is a schematic lateral view of a fourth variant of the sensor shown in Figure 1.

BEST MODE FOR CARRYING OUT THE INVENTION

It should be stated that hereinafter the term

"sensor" shall explicitly refer to a device able to measure one or more physical quantities such as pressure, and/or humidity, and/or temperature, based on a variation of an electrical parameter such as a capacity, an impedance, or a resistance.

With reference to Figure 1, the reference number 1 globally designates a sensor provided with at least one sensitive cell, i.e. a capacitive cell 2 which, in turn, comprises a central layer of dielectric or insulating compressible elastic material having two outer mutually opposite outer contact surfaces 3a, and at least a pair of plates 4, which are made of conductive electrical material and whereof each is fastened to a respective outer contact surface 3a of the central layer 3 in positions which are opposite to and face each other.

Unlike the plates comprised in known capacitive sensors, each plate 4 of the sensor 1 is obtained by means of a film made of ink or paint of electrically conductive elastic material, which laid dir'ectly onto one of the two outer contact surfaces 3a of the central layer 3.

In particular, each plate 4 obtained with the paint made of electrically conductive elastic material has such elastic properties as to allow compression and simultaneously elastic elongation, i.e. the elastic extension of the plate 4 itself in any direction in wholly similar to the central layer , 3, thus advantageously enabling the sensor 1 to be adapted to the shape of any bearing surface.

In other words, the elastic property of the paint made of electrically conductive elastic material allows the film i.e. the plate 4 to be deformed and to be compressed elastically in each point both along a direction that is substantially perpendicular to the surface 3a of the central layer 3, and along any direction that is substantially parallel to the surface 3a of the layer 3 itself, thereby allowing the plate to elongate and to extend elastically as a result of an elastic extension of the central layer 3, and thus to follow a variation in shape thereof along any direction.

It should be added that the elastic properties of the ink or paint made of electrically conductive elastic material of the plates 4 also allows the latter to be compressed under the action of external pressures in wholly similar fashion to the central elastic layer 3.

According to a first variant illustrated in Figure 2, the sensor 1 lacks one of the two plates 4, whilst the central layer 3 has a free contact surface 3a.

From the above description, it should be stated that, in use, a variation in the capacity or impedance of the capacitive cell 2 occurs when a body, e.g. a human body or any object or element having electrical conductivity characteristics, designated in Figure 2 by the reference number 4e (with dashed lines) is placed to bear on the outer free contact surface 3a of the central layer 3 causing a compression thereof. Obviously, in this case the body and/or the element 4e, having characteristics of electrical conductivity, represent the second plate 4 of the capacitive cell 2.

Figure 3 schematically shows a second variant of the sensor 1, which, instead of comprising a single capacitive cell 2, comprises a plurality of capacitive cells 2 (three cells in the illustrated example) , which are provided with a single common central layer 3 made of dielectric or insulating compressible elastic material of appropriate shape and thickness s and a plurality of plates 4, fastened to the two opposite surfaces 3a of the central layer 3 facing each other in pairs and obtained by means of the paint made of electrically conductive elastic material. In particular, the plates 4 can have any shape and thickness and be positioned on the two surfaces 3a according to any geometric distribution.

For example, according to an embodiment shown in Figure 4, the plates 4 are fastened to the two surfaces 3a according to a grid or matrix geometric configuration.

With reference to Figure 4 in particular, N plates 4 obtained with the electrically conductive elastic paint forming a first group of plates are secured by ^■ deposition on one of the two outer contact surfaces 3a of the central layer 3 in such a way as to form a number N of distanced rectilinear segments parallel to a direction H, whilst M plates 4 made of electrically conductive elastic paint forming a second group are instead secured by deposition onto the opposite contact surface 3a of the central layer 3 in such a way as to form a number M of rectilinear segments distanced from each other and parallel to a direction K different from the direction H.

Specifically, in the example illustrated in Figure 4, the rectilinear segments deposited on the two opposite surfaces 3a of the central layer 3 are defined by a plurality of rectilinear and elongated sections positioned on the surface 3a preferably, but not necessarily, equidistant and parallel to each other, while the directions H and K of orientation ^'of the two groups of rectilinear segments are substantially perpendicular to each other and define a grid or matrix having N rows and M columns.

It is evident that according to the aforesaid matrix configuration the sensor 1 has a number N*M of capacitive cells 2, each of which is identified at a point P of crossing between two different rectilinear segments belonging to a row and respectively to a column of the matrix. According to a third variant illustrated in Figure 5, the structure of the sensor 1 differs from that of the sensor 1 shown in Figure 3, in that it i,s provided with a plurality of plates 4a obtained with the ink or paint made of electrically conductive elastic material, which are fastened on a contact surface 3a of the central layer in such a way as to occupy a predetermined area Ai (indicated with a dashed line in Figure 5) , and with an plate 4b obtained with the paint made of electrically conductive elastic material, which is secured on the opposite surface 3a of the layer 3 in a position facing the plates 4a and opposite thereto.

Specifically, the plate 4b extends on the surface 3a in such a way as to cover an area A₂ (indicated with a dashed line in Figure 4) greater or equal to the area Ai occupied by the plates 4a. According to a fourth variant illustrated in Figure 6, the structure of the sensor 1 differs from that of the sensor 1 shown in Figure 5, in that the plates 4a obtained with the ink or paint made of electrically conductive elastic material are interposed between two. layers of elastomer 10 of determined thickness s having each an outer contact surface 10a whereon is deposited an plate 4b also obtained with the film of paint made of electrically conductive elastic material.

In detail, in the example illustrated in Figure 6, the two layers 10 of elastomer have the respective surfaces 10b, opposite to the outer surfaces 10a, secured to each other in such a way as to trap the plates 4a internally, whilst the plates 4b extend on the respective outer contact surfaces 10a in such a way as to be mutually parallel and face each other, thereby covering an area that is substantially equal to or greater than the area Al occupied by the inner plates 4a.

From the above description it should be stressed that the sensor 1 according to the variant described above has a mechanical and electrical structure which, thanks to the presence of a double surface for sensing the physical quantities to be measured, i.e. of the two sensitive layers 10, advantageously has greater sensitivity in the measurement of physical quantities. In addition to the above, the two outer plates 4b constitute an electrical shield able to protect the inner layers of the sensor 1, i.e. the two sensitive layers 10 and the plate 4a from possible external electrical interference which could introduce errors in the measurement of the physical quantities.

According to a fifth variant, not illustrated herein, the sensor 1 may comprise a "multi-layer" structure in which a plurality of layers 10 are superposed parallel to and facing each other in such a way as to trap internally the plates 4a and, alternatively, the plates 4b according to a predetermined configuration. Specifically, the plates 4a and the plates 4b are alternatively positioned between the adjacent faces of two different layers 10.

With reference to Figures 1 through 6, the paint made of electrically conductive elastic material can comprise inks or paints based on water or organic solvents, which can be made electrically conductive by means of carbon and/or metal based molecules trapped in its matrix, in order to maintain appropriate mechanical characteristics of elasticity in any direction, of adhesion in the surface involved by the laying and of tenacity.

The laying of the paint made of electrically conductive elastic material onto the outer contact surface 3a or 10a of the layer 3 or of the layers 10 can be effected through any printing process (automatic or manual) such as a serigraphy process or any other similar known printing method able to deposit a film of paint or ink on the face of a layer. Processes for printing a paint onto a surface are known and therefore they shall not be described any further.

In regard instead to the central layers 3 and 10, they can have any shape and thickness s. In particular, the layers 3 and 10 can be obtained with any dielectric or insulating elastomeric material able in use to be compressed to vary its thickness s as a function of the external pressure applied on the surface 3a or on the external contact surfaces 10a of the layers 3 and respectively 10.

Specifically, the layer 3 or the layers 10 can be obtained, for example, with any synthetic or natural polymer, expanded or compact, which is able to vary one or more . mechanical characteristics, such as its own thickness s, or electrical such as the dielectric constant and/or the loss factor of a corresponding capacitive cell 2, as a function of one or more external physical quantities such as pressure, temperature, or humidity.

More in detail, if the external quantity to be measured is pressure, the central layer 3 or the layers 10 of the sensor 1 can be made of an elastomeric polymer constituted by neoprene and/or natural rubber or any other type of similar compressible elastic material.

If instead the central layer 3 or the layers 10 of the sensor 1 are obtained with an insulating elastic polymer able to vary the loss factor of the capacitive cell 2 as a function of temperature, the sensor 1 serves as a temperature sensor; in this case, measuring the variation of the loss factor it is possible to determine the superficial distribution of temperature on a defined contact surface, according to the embodiment shown in Figure 2, by the free surface 3a. Lastly, the central layer 3 of the capacitive sensor can be obtained by means of an elastic polymer able to vary its own dielectric constant as a function of absorbed external humidity; in this case, -the sensor 1 is able to serve the function of humidity sensor. Specifically, the central layer 3 can be obtained with polymides, such as Kapton, or with any other type of similar elastic polymer able to vary its dielectric constant as a function of humidity. From the above description, it should be stated that the capacitive cell or cells 2, i.e. the sensitive cells that define the sensor 1 can be electrically connected through external connectors (not shown) to a processing module (not shown) , for example a microprocessor and/or analogue circuits, which is able to acquire electrical parameters to be measured, in particular the variation of capacity or impedance of each cell 2, and to process them to output an indication about the value of the measured physical quantities. For example, the processing module (not shown) can be able to determine the pressure and/or humidity and or temperature present in one or more points of a surface, as a function of the variation in capacity, impedance, or resistance which takes place in the capacitive cells 2 positioned in correspondence with said points.

It is readily apparent that the variation in the electrical parameter (impedance, capacity or resistance) obtainable from the processing module (not shown) can be caused by a corresponding variation in thickness s of the layer 3 and 10 of elastomeric material due to the compression thereof caused by external pressure, and/or by a variation in the dielectric constant as a result of the absorption of humidity by the layer 3 and^' 10, or by the variation in the loss factor which occurs in the presence of a temperature change. Each capacitive cell 2 of the sensor 1 is connected to the connector (not shown) through a series of conductive electrical strips (not shown) which are preferably, but not necessarily, obtained with the paint made of electrically conductive elastic material appropriately deposited on the outer surfaces, 3a or 10a of the elastomeric material layer 3 or 10.

The operation of the sensor described and illustrated above is readily apparent from the above description and therefore requires no further explanation.

The advantages of the sensor 1 are evident: the elastic properties of the paint made of electrically conductive elastic material deposited directly onto the central layer of elastomeric material provide the sensor 1 with a high degree of flexibility in any direction, assuring, on one hand, the electrical continuity of strips and plates when any type of deformation occurs, and on the other hand a good compactness and a high ability to adapt to bearing surfaces characterised by double and irregular curvatures, such as those of the human body.

Lastly, it is readily apparent that the sensor 1 described and illustrated herein can be subject to modifications and variants, without thereby departing from the scope of the present invention.

Claims

C L A I M S

1. A sensor (1) for measuring at least one physical quantity, which comprises at least one sensitive cell (2) which, in use, varies a related electrical parameter as a function of a physical quantity; said sensor (1) being characterised in that said at least one sensitive cell (2) comprises at least a first layer (3) of dielectric or insulating compressible elastic material having at opposite sides a first and a second outer contact surface (3a), and at least a first plate (4) made with a paint of compressible and electrically conductive elastic material, which is deposited directly onto the first contact surface (3a) of said at least a first layer (3) .

2. Sensor as claimed in claim 1, characterised in that said at least one sensitive cell (2) comprises at least a second plate (4) of paint made of compressible and electrically conductive elastic material, which is deposited directly on the second contact surface (3a) of said at least a first layer (3) ; said first and second plates (4) of said sensitive cell (2) being positioned on said first and second contact surfaces (3a) , mutually opposite and facing each other.

3. Sensor as claimed in claim 1 or 2, characterised in that it comprises a plurality of sensitive cells (2) .

4. Sensor as claimed in claim 3, characterised in that said first and second plates (4) of said sensitive cells (2) are defined by respective rectilinear segments positioned on said first, and respectively, second contact surface (3a) of said at least one first layer (3) according to a matrix or grid shaped geometric configuration.

5. Sensor as claimed in claim 1, characterised in that it comprises a plurality of first plates (4a) of paint made of compressible and electrically conductive elastic material, separately from each other, which are deposited on said first contact surface (3a) according to a predetermined geometric distribution in such a way as to occupy a first area (Ai) of pre-set dimensions; said sensor comprising a second plate (4b) of paint made of compressible and electrically conducive elastic material, which is deposited on the second contact surface (3a) of said at least a first layer (3) and faces said first plates (4a), in such a way as to cover an area (A₂) that is greater than or equal to said first area (Ai) .

6. Sensor as claimed in claim 1, characterised in that said at least one sensitive cell (2) comprises at least one pair of first layers (10) made of dielectric compressible elastic material, which are secured to each other in such a way that said at least first plate (4) is interposed between said first layers (10) .

7. Sensor as claimed in claim 6, characterised in that it comprises a plurality of first plates (4a) interposed between said first layers (10) ; said sensor comprising at least a second and at least a third plate (4b) made of compressible and electrically conductive elastic material deposited on respective mutually opposite outer contact surfaces (10a) of said first layers (10) .

8. Sensor as claimed in claim 7, characterised in that it has a multi-layer structure c'omprising a plurality of first layers (10) made of dielectric or insulating compressible elastic material, mutually superposed, which are able to trap between the inner contact surfaces said first plates and, alternatively, said second or third plates.

9. Sensor as claimed in any of the previous claims, characterised in that said first layers (3,10) are made of a compressible elastomeric material.

10. Sensor as claimed in any of the previous claims from 1 through 9, characterised in that said first layers (3,10) are made of a dielectric material able to vary their dielectric constant as a function of temperature.

11. Sensor as claimed in any of the previous claims from 1 through 9, characterised in that said sensitive cell (2) is a capacitive cell, and in that said first layers (3,10) are made of a dielectric material able to vary the loss factor of the sensitive cell (2) as a function of humidity.

12. A method for fabricating a sensor (1) for measuring at least one physical quantity comprising at least one sensitive cell (2) which, in use, varies a related electrical parameter as a function of the physical quantity to be measured; said method being characterised in that it comprises the step of depositing on at least a first contact surface (3a) of a first layer (3) of dielectric or insulating compressible elastic material a film of paint of compressible and electrically conductive elastic material, in such a way as to define a first plate (4) of said sensitive cell (2) .

13. Method as claimed in claim 12, characterised in that it comprises the step of depositing on at least a second outer contact surface (3a) of said first layer

(3) made of dielectric or insulating compressible elastic material opposite to said first surface (3a) , a film of paint made of compressible and electrically conductive elastic material in such a way as to define a second plate (4) of said sensitive cell (2) .

14. Method as claimed in claim 12 or 13, characterised in that said step of depositing said film of paint made of elastic compressible and electrically conductive material comprises the step of printing the film of paint made of elastic compressible and electrically conductive material on said outer contact surface (3a) of said first layer (3) .

15. Method as claimed in claim 13 or 14, characterised in that said step of depositing said film of paint comprises the step of obtaining rectilinear segments of paint made of elastic compressible and electrically conductive material on said first, and respectively, second outer contact surface (3a) of said first layer (3) according to a matrix ' geometric configuration.

16. A capacitor comprising at least a first central layer (3) of dielectric or insulating material having at opposite sides a pair of outer surfaces (3a) whereon are positioned two respective plates made of electrically conductive material; said capacitor being characterised in that at least one of said plates is obtained with a film of paint made of compressible and electrically conductive elastic material, which is deposited on the related outer surface (3a) of said at least one central layer (3) .