Abstract:
A capacitive pressure sensor includes first and second of mutually displaceable elastic members each having a respective electrically conductive surface separated by a thin elastic dielectric. Variations in area of pressure-induced contact between the first and second members are used to vary capacitance of the sensor that allows determination of differential pressure between the two elastic members. Both of the elastic members have respective projections, the projections of the first elastic member being disposed in interlocking relationship with the projections of the second elastic member and configured so that as the elastic members are pressed toward each other their respective projections progressively engage and create increasing areas of contact.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Israel Patent Application serial number IL240460, filed Aug. 9, 2015, which is incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The present invention generally relates to systems and methods to convert pressure into capacitance using a flexible sensor and particularly for monitoring body movements and respiration using such a sensor. 
     BACKGROUND OF THE INVENTION 
     The fast development of healthcare technology increases the need for monitoring respiration via the pressure exerted by the human body on appliances, such as a mattress (during sleep), chair and sofa without wearing a sensor. Some applications require a respiration signal to be both DC and relatively noiseless at high-enough quality that enables detection in real time of the onset of breathing phases, e.g. expiration, and breath holding. Such applications may include, for example, breathing pattern modification using guiding tones, as applied by a device called RESPeRATE for inducing relaxation and treating hypertension. This device is described in U.S. Pat. No. 5,800,337. For such applications the sensor should be thin enough not to cause the user any discomfort; flexible, as both the relevant appliances and the human body are usually deformable and soft; highly sensitive, as the pressure modulation elicited by respiration is rather small, and inexpensive, as the main market is for home use. For these reasons, piezo-based sensors that provide AC signals are not optimal. 
     Flexible sensors that convert pressure into capacitance seem to be appropriate for this purpose. In general, two conductive surfaces of area A separated by a dielectric of thickness d generate capacitance C that is proportional to A/d. Noda et at. (U.S. Pat. No. 7,641,618) disclose a capacitance-based pad sensor for heart/respiration monitoring in bed, in which the dielectric is flexible, resulting in the increase of capacitance, C from its unloaded value due to the reduction of d under loading. Brunner et al. (U.S. Pat. No. 4,986,136) disclose a sensitive capacitance-based pressure measurement system, with an upper conductive surface that includes deformable projections that contact, via a thin dielectric, a lower flat conductive surface ( FIG. 3 a   ). The capacitance of this structure increases in response to applied pressure, as the contact area between the upper and the lower conductive surfaces increases due to the deformation of the projections ( FIG. 3 b   ). Alternatively, as shown in  FIG. 8  thereof both upper- and lower conductive surfaces contain mutually opposed parallel strip-like tapered projections that are oriented to each other preferably at right angles. Respective strips of projections in the upper and lower surface intersect thereby forming a plurality of capacitive cells. The capacitance of the sensor may therefore be considered as a matrix of parallel rows of series connected capacitive cells. In an initial displacement of the upper and lower surfaces, the tips of the projections are undistorted and define very low areas of mutual contact pressure. As the surfaces are urged toward each other, the tips of the projections become progressively flattened thus creating progressively increasing areas of pressure contact. Additionally, the distance between the two surface decreases, which further increases the capacitance. It is to be noted, however, that because the projections of the two surfaces are mutually offset, as indeed they must be to create a matrix of capacitive cells, only their respective tips contribute to the increasing areas of pressure contact. Therefore, at no stage during use of the sensor is there any ability for the opposing projections of the two surfaces to interlock or otherwise engage. 
     U.S. Pat. No. 4,437,138 discloses a force sensor comprising capacitor plates formed of metallic cloth bonded to a compressible elastomeric dielectric. The metallic cloth strips are in the form of strips running crosswise on opposite sides of the dielectric to provide a matrix of force sensors. The warp and weft threads of the metallic cloth increase the flexibility of the sensor but the warp and weft threads of one plate do not interlock with or otherwise engage the warp and weft threads of the other plate or affect the capacitance of the sensor, which is determined only by the compression of the intermediate dielectric. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a more sensitive flexible capacitive pressure sensor. 
     It is another object of the invention to use such a sensor in an apparatus for monitoring respiration and other body-generated movements at high sensitivity, very low signal-to-noise ratio, and without restraining the user. 
     These objects are achieved in accordance with a broad aspect of the invention by a capacitive pressure sensor comprising first and second mutually displaceable elastic members each having a respective electrically conductive surface separated by a thin elastic dielectric, wherein: 
     variations in area of pressure-induced contact between the first and second members are used to vary capacitance of the sensor that allows determination of differential pressure between the two elastic members; and 
     both of the elastic members have respective projections, the projections of the first elastic member being disposed in interlocking relationship with the projections of the second elastic member and configured so that as the elastic members are pressed toward each other their respective projections progressively engage and create increasing areas of contact. 
     In some embodiments, the projections are realized by a concave cavity and in others by a convex protrusion. It will be appreciated that both concave and convex structures may be regarded as projections since they are formed not in the surface of the elastic members but rather by these surfaces. Consequently, whether a projection appears convex or concave is solely dependent on the direction in which the elastic members are viewed: from one side the projections will appear as convex protrusions and from the opposite side they will appear as concave cavities. 
     The term “interlocking” as used in the description and the appended claims means that the respective projections in the opposing surfaces engage each other in such a manner that they cannot be completely separated by lateral displacement along two mutually orthogonal axes. To qualify and clarify this, there is no requirement that the interlocking be tight and consequently, there is no requirement that lateral displacement to any extent must be impeded. The projections may interlock loosely such that some degree of lateral displacement is possible, but not to the extent that the two surfaces can be completely separated along both axes. 
     In accordance with another aspect of the invention, there is provided a capacitive pressure sensor comprising first and second mutually displaced elastic members each having a respective electrically conductive surface separated by a thin elastic dielectric, each of the elastic members having at least one respective contact area, the respective contact areas being configured to effect pressure-induced contact as compression is applied to the displaced elastic members, wherein: 
     variations in the areas of contact pressure between the two members are used to vary capacitance that allows measurement of the compression; 
     the respective contact areas of the first and second elastic members each have at least one finite radius of curvature; and 
     at each point of pressure-induced contact between the two elastic members, all of the respective radii of curvature of both elastic members are oriented in an identical direction. 
     A major benefit of the present invention over known structures is in the enhanced sensitivity of a capacitive-based pressure sensor due principally to the fact that interlocking projections offer increased rate of change in areas of contact per unit of pressure as the surfaces are pressed together. Specifically, areas on the side surfaces of the projections effect mutual contact and the projections may be shaped to ensure that significantly larger areas of the projections mutually engage even in response to a relatively small increase in pressure. This greatly enhances the sensitivity of the sensor. 
     In some embodiments, the elastic members and include projections with one-dimensional symmetry, e.g. parallel projections or tubes located on the X-Y plane, or two-dimensional symmetry, e.g. the projections have the form of half ellipsoids arranged as an array on the X-Y plane, or three-dimensional symmetry, e.g. plain weave structure, or any combination of the above-mentioned embodiments. 
     In some embodiments, there is provide a sleep monitor using a flexible sensor that has at least one sensing unit that is subject to pressure exerted by the user body on a support that are both, in general, deformable and not planar, in an attempt to monitor desired variables that include at least respiratory movements. Some embodiments include a pad-like flexible sensor used on a mattress, or on the back of a chair or sofa; a flexible sensor integrated into a neck supporting pillow sensor, and a belt-type respiration sensor. 
     In embodiments where the flexible sensor includes more than one sensing unit the capacitance of the individual sensing units may be connected to a capacitance-to-data converter via a multiplexer that is operative to connect the sensing units in a selected order and timing controlled by the system. 
     In some embodiments, multiple sensing units are provided to measure variation in pressure distribution over time, possibly as a measure for body movements. 
     In some embodiments, raw or analyzed sensor data are transmitted to a host device, which performs at least one of the following functions:
         generating stimulus input to user that may be used in affecting respiration and other body movements in a closed or open loop;   generating stimulus input to appliances; providing indication inputs to user; data storage and communicating with a remote site;   receiving user commands; and   controlling the operation of an electronic sensor circuit via bi-directional communication.       

     In embodiments where the raw data are transmitted to the host device, the latter also performs the data analysis to obtain the desired variables. 
     In some embodiments, the host device is a hand-held device such as a mobile phone, iPad, iPod, laptop, etc. which may effect bi-directional communication with the electronic circuit using Bluetooth™ low energy (BLE) technology or another suitable short-range protocol. 
     In some embodiments, the host device generates in real time audiovisual or tactile stimuli in response to monitored body movements. The user synchronizes with these stimuli body movements, preferably respiration, in a way that leads gradually to modification of these movements in a desired direction known to be beneficial to the user. For example, slowing respiration and relatively prolonging exhalation induces relaxation and sleep. It will be appreciated that the generating stimuli in response to single- or multiple respiration patterns, or to non-respiratory movements, may be useful for detracting the user attention from wondering thoughts and external stimuli that interfere with falling asleep or relax. 
     In some embodiments, an electronic circuit switches its mode of operation between inactive and active states, depending on whether the flexible sensor is unloaded or loaded, respectively. 
     In some embodiments, real-time or nearly real-time analysis of sensed data from multiple sensors is applied in attempt to select the sensor that provides the best quality desired variable. The selection process may be repeated, if necessary and the user may be notified if selection fails. In some embodiments, the desired variable is indicative of respiration. 
     In some embodiments, there is provided a sleep monitor comprising a pad-like flexible sensor with multiple sensing units. The sensor contains an electronic circuit that communicates bi-directionally with a host device, preferably a mobile phone for monitoring body pressure distribution on the pad, respiratory movements using automatically selected sensing units and body movements unrelated to respiration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a prior art monitor suitable for use with an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view and graphical nomenclature for the structural elements of a sensing unit employed in the system of  FIG. 1 ; 
         FIG. 3  includes a cross-sectional view of a prior art sensing unit and graphs that depict its performance under variable pressure; 
         FIG. 4  includes a cross-sectional view of the sensing unit employed in the system of  FIG. 1  and graphs that depict its performance under variable pressure, in accordance with one embodiment of the present invention; 
         FIG. 5  is a cross-sectional view of the sensing unit employed in the system of  FIG. 1 , in accordance with another embodiment of the present invention; 
         FIG. 6  includes a cross-sectional view of the sensing unit employed in the system of  FIG. 1  and graphs that depict its performance under variable pressure, in accordance with another embodiment of the present invention; 
         FIG. 7  includes a sectional view of the sensing unit employed in the system of  FIG. 1  and graphs that depict its performance under variable pressure, in accordance with another embodiment of the present invention; 
         FIG. 8  is a perspective view of the sensing unit employed in the system of  FIG. 1 , in accordance with another embodiment of the present invention; 
         FIG. 9  includes a perspective- and top view of the sensing unit employed in the system of  FIG. 1 , in accordance with another embodiment of the present invention; 
         FIG. 10  includes a cross-sectional- and top view of the sensing unit employed in the system of  FIG. 1 , in accordance with another embodiment of the present invention; 
         FIG. 11  includes a sectional- and exploded perspective view of the sensing unit employed in the system of  FIG. 1 , as described in  FIG. 4 , in accordance with another embodiment of the present invention; 
         FIG. 12  includes perspective view of the sensing unit employed in the system of  FIG. 1  attached to a user or appliance in a number of embodiments of practical interest, in accordance with embodiments of the present invention; 
         FIG. 13  includes schematic top view and block diagram of the flexible sensor employed in the system of  FIG. 1  and its outcomes emerging via a host device, in accordance with embodiments of the present invention; 
         FIG. 14  is a block diagram showing the functional units employed in the system of  FIG. 1 , in accordance with one preferred embodiment of the present invention; 
         FIG. 15  is a block diagram showing an example of the processes involved in getting desired variables from the sensing unit data with reference to the functional units employed in  FIG. 14 , in accordance with one preferred embodiment of the present invention; and 
         FIG. 16  is a graph showing a typical respiration signal expressed as variation over time in the frequency obtained from the capacitance of the sensing unit, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following description of some embodiments, identical components that appear in more than one figure or that share similar functionality will be referenced by identical reference symbols. 
       FIG. 1  illustrates a sleep monitor comprising a flexible capacitive pressure sensor  1  placed between and compressed by the human body and a support that are both, in general, deformable and not planar. The sensor  1  includes at least one sensing unit  2 , each of which responds to the local compression by increasing its capacitance. The sensing units  2  are connected to a suitable transducer  3  that converts a change in capacitance to a corresponding signal. The signal is typically a variable frequency signal whose frequency is a function of capacitance and is processed by a data processing circuit  4  to provide a measure of capacitance or a function thereof Typically, the data processing circuit  4  samples and averages the signals and provides desired variables relating to the time variation of the local pressure applied to the sensing units  2 , e.g. respiration, heart beats and variables derived from the pressure distribution as body movements. 
     Reference is now made  FIG. 2  that provides graphical nomenclature for the structural elements of the sensing unit  2  employed in the system of  FIG. 1  in order to simplify the description of the various embodiments.  FIG. 2 a    illustrates schematically the structure of the sensing unit  2  that comprises two elastic members  100  and  150 , each having a deformable and elastic bulk  110  and  130 , and surfaces  120  and  140 , respectively. The surfaces  120  and  140  are electrically conductive, but isolated from each other by a thin elastic dielectric layer  170 . The dielectric layer may be laminated on at least one of these surfaces to form an integral part of the surface, or it may be a physical barrier that separates between the two layers without reducing their elasticity. The surfaces  120  and  140  are found in contact with one another at one or more contact regions, where the total area of the contact regions increases for greater compression of the sensing unit  2 . It is to be noted that surfaces  120  and  140  are conductive at least at the maximum contact region generated in the desired pressure range (marked by a thick line) and that all conductive regions in each surface are interconnected. The surfaces  120  and  140  or the bulks  110  and  130 , if conductive, are connected electrically via conductors  180  and  190 , respectively, to the transducer  3  that converts capacitance into data. It is worthwhile to note that at a given pressure the sensor sensitivity in detecting pressure changes is determined by the relative rate of change of the capacitance C of the sensing unit  2  with the change in pressure ΔP, i.e. (ΔC/C(P))/ΔP. Since most of the capacitance of sensing unit  2  is generated at the contact regions, this sensitivity expresses the relative rate of change of the total contact area with compression. 
       FIG. 2 b    illustrates a number of optional structures that may be appropriate for the bulk and the surface of the sensing unit  2 , depending on its preferred geometry and manufacturability. This may include for the bulk  110  or  130  filled or hollow structures made, for example, of injected or extruded thermoplastic elastomers foam, and open structures made, for example, from  10  malleable material, which is embossed into elastic dome-like protrusions. In case the bulk is made of electrically conductive material, e.g. conductive polyurethane foam, its surface directed towards the other bulk may be coated by a thin elastic dielectric film  160 , e.g. a thermally laminated 5-30 microns elastomer film. In case the bulk is an insulator, it must be coated with an elastic and electrically-conductive layer  170 , e.g. elastic and conductive fabric. It will be appreciated that only one elastic member has to include the dielectric layer  160 . It is worthwhile to note that for the purpose of the present invention good electrical conductivity of the elastic members is not required, which may reduce considerably the cost of the sensor materials. Since according to the present invention the surfaces of the elastic members  110  and  130  rub against one another, there may be an advantage of adding a thin external layer  165  for reducing friction in at least the regions of potential contact between the elastic members, e.g. by adding a sprayed layer or film of Teflon. 
       FIG. 2 c    illustrates an example of a two-dimensional array of elastic members  100  and  150 , wherein any neighboring pairs of elastic members  100  and  150  operates as described above with reference to  FIG. 2 a    with different contact areas I and II along the Y and X directions, respectively, where contact I is the first to be established. Such an array may generate the flexible sensor  1  or the sensor unit  2 , as shown in  FIG. 1 . It is self-evident that the structural concept depicted in  FIG. 2 c    can be extended to three-dimensional arrays. 
     With this understanding, the embodiments disclosed hereafter can be simplified, if desired, by illustrating only surface lines; marking contact regions and omitting the electrical conductors  180  and  190 . It should be noted that although  FIGS. 1 and 2  show features that are known per se, to the extent that similar features are also used in different embodiments of the present invention, the same properties or alternatives are applicable also to the invention. For example, the dielectric may be deposited on one of the elastic members or it may be realized by an ultra-thin elastic barrier that separates the plates but does not reduce their ability to engage and for their respective projections to interlock. 
     It will also be understood that the following principles are demonstrated in a number of embodiments, which serve only for clarifying these principles, but can be implemented in other different embodiments including also combinations of embodiments. 
     Reference is now made to  FIG. 3 a    that illustrates schematically the principle of operation of a known capacitive sensor  2  as shown in U.S. Pat. No. 4,986,136. The sensor comprises an electrically conductive first elastic member  100 , includes a series of rounded or tapered projections  200  that may represent the cross-section of parallel structures. The projections  200  make pressure contact with a second electrically conductive flexible elastic member  150  via a dielectric layer. Under minimal pressure (denoted by an arrow), a contact region I (marked by a dotted rectangle) is established between projections  200  and the elastic member  150 .  FIG. 3 b    illustrates the deformation of the projections  200  under greater pressure. The resulting compression increases the area of the contact region I.  FIG. 3 c    shows the variation of the capacitance C in picofarads per unit of area and the sensitivity (ΔC/C(P))/ΔP in percent per unit of area generated by testing such a sensor  2  at the pressure range characterizing a human body on a mattress. These curves will subsequently be referred to as ‘performance curves’. 
       FIG. 4 a    illustrates schematically the structure of the sensor  2  constructed and operating in accordance with an embodiment of the invention. Both elastic members  100  and  150  include respective rounded interlocking projections  200  and  250  that may, or may not have the same size and form, but are presented here as identical for the sake of simplicity. Under minimal pressure, the contact regions I are diagonal on both sides of the projections  200  and  250 .  FIG. 4 b    and  FIG. 4 c    show that upon increasing compression, regions I increase while at some point an additional contact region II is generated. The resulting performance curves are shown in  FIG. 4 d   . The sensitivity of this embodiment is considerably greater than that of the prior-art sensor taken from 
       FIG. 3 c    and shown by dotted curve especially at low pressures, which are preferable for detecting respiration in a human lying on a mattress. It is noteworthy that the sensitivity curve can be modified by changing the gap between the projections  200  and  250 , as a larger gap makes the contact region II appear earlier and vice versa. 
     Reference is now made to  FIG. 5 a    that illustrates schematically the structure of the sensing unit  2  constructed and operating in accordance with another embodiment. Both elastic members  100  and  150  include rounded interlocking projections  200  and  250 , respectively that have substantially the same height and nearly parallel sides. Under minimal pressure, the projections  200  and  250  touch the opposite members  150  and  100 , respectively, generating the contact regions I but they do not touch each other, as shown.  FIG. 5 b    and  FIG. 5 c    illustrate the progressive increase in size of the contact regions I in response to increased pressure showing that at sufficiently high compression new contact regions II are formed in the vertical direction, when the projections  200  and  250  touch each other. This increases the sensitivity of the sensor unit  2  over hitherto-proposed sensors similarly to that shown in  FIG. 4 . It is worthy of note that for deformations of the form shown here hollow projections  200  and  250  have been found to be particularly effective. 
     Reference is now made to  FIG. 6 a    that illustrates schematically the structure of the sensing unit  2  constructed and operating in accordance with another embodiment, which is different from that illustrated in  FIG. 5 a    in one aspect: The height of the projections  200  is smaller than that of the projections  250 . As a result, under minimal pressure, contact region I is generated on the elastic member  100  only.  FIG. 6 b    shows that at elevated pressure, contact region II is generated on the elastic member  150 .  FIG. 6 c    shows that under further compression additional contact regions III are generated, of the type illustrated in  FIG. 5 c   .  FIG. 6 d    shows the pressure dependency of the capacitance per cm 2  and the sensitivity of the sensor unit  2 , which is far greater that prior art sensors (dotted line) and of the embodiment shown in  FIG. 4   d.    
     Reference is now made to  FIG. 7 a    that illustrates schematically the structure of the sensing unit  2  constructed and operating in accordance with another embodiment. Here also, there are provided a pair of mutually interlocking elements but they differ from those illustrated in  FIG. 3 a    (prior art) in the following respects: The elastic body  100  is not flat but includes concave structures  200  that enclose complementary convex structures  250  provided in the elastic member  150 . As explained previously, both the concave and the convex structures  200 ,  250  may be regarded as projections. The contact region I is generated under minimal pressure.  FIG. 7 b    illustrates the mutual deformation of elements  200  and  250  under greater pressure that increases the contact region I.  FIG. 7 c    illustrates a unique property of this embodiment—detecting forces applied at different angles, a case relevant to pressures exerted by the edge of the human body on a deformable support, such as a mattress. Such sensor location appears to be most sensitive for detecting respiration.  FIG. 6 d    shows the performance curves of this embodiment. The sensitivity is greater than prior art sensors at low pressures, but similar and even lower at higher pressures. It is noteworthy that the sensitivity of this sensing unit at low pressures is controlled by the difference between the curvatures and elasticity of elements  200  and  250  at region I. By making the curvatures of units  200  and  250  close, even a small pressure will increase considerably the area of the contact region I, which means large low-pressure sensitivity. 
     Reference is now made to  FIG. 8 a    that illustrates schematically the structure of the sensor  1  constructed and operating in accordance with another embodiment, which shares some similarity with the embodiment illustrated in  FIG. 7 . It comprises a cylindrical elastic member  150  with a circular cross-section enclosed by an elastic member  200  in the form of a hollow cylinder with a circular or elliptic-like cross-section of larger dimensions than those of the member  150 , such that under minimal pressure elastic members  100  and  150  form contact regions I. The electrical conductors  180  and  190  are marked schematically. The increase of the contact regions I at greater pressures and the rationale for enhanced low-pressure sensitivity, are the same as disclosed in the previous embodiment shown in  FIG. 7 . This sensor unit is self-contained and can be used in applications that favor pressure detection over a relatively large distance along a line. Both members  100  and  150  can be preferably manufactured in the form of cylinders by extrusion. While member  150  has to be hollow, member  100  may be filled to assure mechanical stability of the sensor under bending. The unloaded sensor  1  has minimal capacitance generated by the random contacts between the members  100  and  150 . This is an important issue when the pressure is applied only in part of the sensor  1 . This embodiment responds to forces applied in all directions that are not parallel to its axis. 
     Reference is now made to  FIG. 9  that illustrates schematically perspective and top views of the sensing unit  2  structure constructed and operating in accordance with another embodiment. This embodiment shares some similarity with the embodiments shown in  FIG. 7  and  FIG. 8 , but differs by the following feature: Two elastic members  100  and  100 ′ each having mutually aligned outwardly-directed projections  200  and  200 ′ are juxtaposed back to back so that the respective projections of the two members form ‘tunnels’ that enclose tubular elastic members  150 . The elastic members  100  and  100 ′ can be manufactured according to the options detailed in  FIG. 2 b   . This embodiment may be appropriate for a relatively large sized sensing unit  2 . The figure also illustrates schematically the location of the electrical conductors  180  and  190 , where the conductor  180  is common to both elastic members  100  and  100 ′, while each of the elastic members  150  has its own conductors  190 . In this way, a weighted average of pressure applied along a line can be detected. 
     Reference is now made to  FIG. 10 a    that illustrates schematically a top view of a flexible sensor  1  (or sensing unit  2 ) with a plain weave structure constructed and operating in accordance with another embodiment. All the elastic members  150  are illustrated as black stripes in the X direction, while the elastic members  100  are in the Y direction.  FIG. 10 b    illustrates the crossing point of the elastic members  100  and  150  in cross-sectional view (top) and top view (bottom). This crossing point is the basic sensing element of the sensor, under zero or minimal pressure, where the curvatures of the members  100  and  150  in the X-Z or Y-Z planes are conceptually similar to the ones illustrated in  FIG. 7 .  FIG. 10 c    shows the same structure under elevated pressure that increases the area of the contact region I. The contact region has a three-dimensional form of a saddle. In this embodiment, the sensor is preferably made of flexible fibers having a circular cross-section to assure mechanical stability and can be manufactured by extrusion following the options presented in  FIG. 2 b   . A textile-like sensor of this kind is extremely flexible and responds to both compression applied at some angle around the Z direction and to tension applied along the X or Y axes. It is noteworthy that the sensor sensitivity at low pressures increases with increasing density of weave structure, as member  150  becomes more wrapped around member  100  and vice versa ( FIG. 10 b    top). 
     It will be appreciated that other types of weave structures can be constructed along the principles described here by those who are skilled in the art. 
     Reference is now made to  FIG. 11  that illustrates an exploded- and a cross-sectional view of one example of the sensing unit  2  as described in  FIG. 4 , embedded in the flexible sensor  1 , constructed and operating in accordance with an embodiment of the invention. The flexible sensor  1  includes a nonconductive elastic sheet  300 , to which the sensing unit  2  is attached in a mechanically stable way, and which contains the electrical conductors  180  and  190 . The elastic member  150  includes a flat base attached to the elastic sheet  300  and hollow projections  250 . The elastic member  100  has a flat top and the hollow projections  200 , diagonal sides  210  and a base  220  attached to the elastic sheet  300 . Elastic members  100  and  150  are electrically connected to the conductors  180  and  190 , respectively, e.g. via rivets at points  230  and  260 , without forming an electrical contact with each other. The appropriate options for the surface structure of the elastic members are as described above with reference to  FIG. 2 b   . The structure of the elastic member  100 , as shown, enables the capacitance of an unloaded sensing unit  2  to be minimized by keeping an initial gap  255  between projections  200  and  250  as shown. In order to distribute the applied pressure homogeneously over the flexible sensor  1 , the elastic sheet  300  is covered with a sensor cushion  400 , which may be foam or any elastic structure, preferably having thickness versus pressure dependency similar to that of the sensing unit  2  and being embedded in appropriately sized bores in the sensor cushion  400 . Preferably, the unloaded thickness of the cushion  400  is somewhat larger than that of sensing unit  2 , which leaves a gap  410  that determines together with gap  255  the minimal pressure required for generating the contact region. 
     It will be appreciated that the embodiment illustrated by  FIG. 11  can be used also with the structure of the sensor unit illustrated in  FIG. 5  and  FIG. 6 . In another implementation similar to the one described in FIG. lithe projections  250  can have the form of half ellipsoids that can be embossed in an elastic sheet  300  made of thermoplastic elastomer previously covered by elastic and conductive fabric and a dielectric layer at selected functional domains according to the principles described in  FIG. 2 . 
     Reference is now made to  FIG. 12  that illustrates four embodiments, in which the user&#39;s body subjects the flexible sensor  1  to pressure, a small part of which is caused by pressure variations generated by respiratory movements. The sensor output (marked by an arrow) may serve as an input to products that require high-quality monitoring of the respiration pattern. 
       FIG. 12 a    illustrates the flexible sensor  1  in the form of a pad placed on mattress  500  over or under the sheet and under the user&#39;s torso when lying in a dorsal position, prone position or lateral position,. The sensor may include single or multiple sensing units  2  (not shown). It will be appreciated that such a pad sensor can have the form of a sheet that covers the whole mattress. Furthermore, the sensor pad can be placed below or under pillows having a structure that causes respiratory-related head movements. 
       FIG. 12 b    illustrates a similar sensor pad placed freely on the back of a sofa  510 , in a way that the sensing units are located against the back of the user.  FIG. 12 c    illustrates a neck support pillow for travel  520  that is compressed by the user&#39;s neck against the back of a head/back support chair  530 . The sensing unit  2  is attached to or integrated into the neck support pillow  520 .  FIG. 12 d    illustrates an elastic belt sensor  540  worn typically around the torso that converts the torso circumferential changes induced by the breathing movements into stretch variations in the belt. A sensing unit  2  integrated into the belt is stretched and compressed simultaneously. It is appreciated that the selection of the specific sensor unit  2  for a specific application depends on the type, magnitude, directionality and spatial spread of forces involved and the desired variables. 
     Reference is now made to  FIG. 13  that illustrates schematically the general structure of a flexible sensor  1  containing at least one sensing unit  2  constructed and operative in accordance with an embodiment of the invention. The sensing unit  2  includes a sensor that communicates bi-directionally and preferably wirelessly with a host device  700 , preferably a mobile phone, via a specialized Application (‘App’) or with any other device having similar functionality, e.g. iPad, iPod, laptop etc. It is appreciated that wired communication via a cable is also possible. 
     Each of the sensing units  2  serves as an input to the sensor circuit  450 . In practice, one of the electrical conductors  180  or  190  may be common to all sensing units  2 , as shown. The electronic circuit  450  performs at least the function of capacitance conversion into data for selected sensing units  2  and may also handle some of the data processing and bi-directional communication with the mobile phone  700 . 
     The electronic circuit  450  is energized preferably by disposable or rechargeable batteries. A button  460  may be provided for manual activation or deactivation of the system, whose state can be indicated to the user by an arrangement of dynamic LEDs, if desired. The flexible sensor  1  and the electronic circuit  450  are preferably enclosed by a cover  470  that protects the various sensor parts and makes the sensor comfortable and aesthetic according to the application, e.g. it is preferably machine washable for the application shown in  FIGS. 12 a    and  12   b.    
     The host device  700 , preferably a mobile phone, may further process the sensor data for the following purposes: i) calculating desired variables including, for example, single- and multiple respiration pattern characteristics and body movements that may be useful, for example, in determining the sleep structure and quality and markers for abnormal breathing; ii) controlling the operation of the electronic circuit  450  in response to the analyzed data and user commands, and providing indication inputs and feedback to the user, e.g. the status of sensor connectivity with the App, status of the battery charge and status of respiration detection and the level of variation of respiration characteristics over time; iii) generating from the processed data stimuli inputs to the user in order to modify respiration pattern by generating guiding to breathing movements via tones or other stimulations, as disclosed by the present inventor in U.S. Pat. Nos. 5,076,281 and 5,800,337 to induce, for example, relaxation, or stimulations intending to elicit alertness, e.g. waking up a user upon detecting a prolonged apnea; iv) storing data and communicating with remote site including, for example, uploading to the iCloud both raw and analyzed data, sharing data with other users and communicating with technical support over the internet, etc.; and v) providing stimuli to appliances in response to measures generated by the data analysis, which is possible in the so-called ‘smart home’. For example, the App may turn on/off lights and radio accordance to the detected sleep phase, or turn on minimal light when a user leaves bed at night and thus unloads the sensor. 
     Reference is now made to  FIG. 14 , which is a block diagram showing the functional units of the electronic circuit of an embodiment illustrating the operation of a monitor employed in the system of  FIG. 13  in case of wireless communication with a mobile phone  700 . A multiplexer  600  connects the different sensing units to the capacitance-to-frequency converter  610  following a selected order and timing. The conversion can be made by standard methods, e.g. Schmidt trigger inverter oscillator, where the output frequency is reciprocally related to the sensing unit capacitance, and thus related functionally to the pressure exerted on the sensing unit, as illustrated, for example, by  FIG. 6   d.    
     The sampling unit  620  samples this frequency in a given time window and may average several consecutive samples to obtain the average frequency per sensing unit at this time point. 
     Units  600 ,  610  and  620  establish the capacitance-conversion-into-data circuit  3  shown in  FIG. 3 . These data are analyzed by a pattern-detecting unit  630 , which derives the desired variables. Unit  630  is equivalent to the data processing circuit  4  shown in  FIG. 3 . The data, including desired variables and various status indicators, are transmitted wirelessly to the mobile phone  700  preferably via a Bluetooth™ low energy chip (BLE)  640 . It is appreciated that the function of the pattern-detecting unit  630 , all of it or in part, may also be performed by the mobile phone  700 , as described above. The BLE receives from the mobile phone  700  commands including parameters that control the operation of the other units comprising the electronic circuit  450 . The START command may be provided to the BLE manually, for example, by pressing the button  460  or by loading the flexible sensor  1 , provided that the electronic circuit  450  is in a ‘sleep’ state. It is appreciated that in case of a single sensing unit, e.g. in the case of a belt sensor illustrated in  FIG. 12 d   , no multiplexer is needed. 
     Reference is now made to  FIG. 15 , which is a block diagram showing an example of the processes involved in getting desired variables from the frequency signal corresponding to the capacitance measured by the sensing unit  2 , as generated by sampling unit  620  and will be called hereafter ‘channel’ (CH). This diagram is constructed in accordance with the embodiments of the present invention illustrated in  FIG. 15 . Upon START all channels are scanned cyclically by unit  621  following a command given to the multiplexer. If the frequency of all channels is below a given threshold, it means that no load was placed on the flexible sensor  1 . This may happen, for example, if the user turns on the system shown in  FIG. 12 a    prior to entering bed, or leaves the support ( FIGS. 12 a  and 12 b   ) during use of the system, or when the belt shown in  FIG. 12 d      20  becomes loose. In any of these cases unit  622  will provide an indication input # 1  to the user for clarifying the situation. In case unit  622  finds that at least one CH frequency is greater than the threshold, i.e. at least one sensing unit  2  is loaded, then three processes start simultaneously: i) Unit  625  analyses all CHs representing loaded sensing units in an attempt to detect CHs that may represent respiration activity. Such analysis is disclosed, for example, in U.S. Pat. No. 5,800,337; ii) providing a time limit to this analysis by activating a timer unit  623 . In case the user places the flexible sensor  1  in a location at which respiratory movements cannot be detected, the time will exceed a predetermined threshold T 1  (unit  624 ) and the user will be notified by indication input # 2 ; iii) providing the value of all channels in predetermined time intervals. These values refer to time-dependent pressure distribution generated by the user body on the flexible sensor  1 . The variation of the distribution over time may be used for monitoring the body movements that are known to be associated with sleep structure and quality. This task is handled by unit  634 . In case unit  625  detects respiration activity in a number of channels, unit  626  selects the most appropriate CH for respiration monitoring, e.g. selecting the CH with the largest stability or signal-to-noise ratio. In case the time variation of the pressure distribution determined by unit  634  is much slower than that required from respiration, or is of no interest, the unit that selects the respiration monitoring CH may provide a command to the multiplexer to connect only the selected CH. It is to be noted that the present flowchart is applied continuously, so any loss of detected respiration restarts the relevant part of the process required for detecting respiration. The data provided by units  632  and  634  (both raw and analyzed) serve as inputs to the BLE. It is appreciated, that all or some parts of the flow chart can be handled by the host device  700 , in general or mobile phone, in particular, and the BLE role is just transmitting the channel&#39;s frequency generated by unit  620 . It is also appreciated that in the case where the flexible sensor  1  includes a single sensing unit  2  no multiplexer is needed and there are other obvious simplifications in the schemes presented in  FIG. 14  and  FIG. 15 . 
     Reference is now made to  FIG. 16  that illustrates a typical respiration signal of a user under spontaneous breathing expressed as variation over time in the frequency obtained from the capacitance of the sensing unit  2 , in accordance with an embodiment of the invention. It is noteworthy that the respiration amplitude is about 3% of the average frequency that corresponds to pressure of about 15 gm/cm 2  this being the low-pressure range at which the sensing units constructed according to the invention are especially sensitive. Furthermore, the respiration signal as shown represents the raw data to which no filtration or other processing has been applied. 
     It will be appreciated that the flexible sensor  1  may frequently detect heartbeats in a way that enables to determine heart rate. However, this may not be an optimal way of detecting heartbeats. Therefore, it will also be appreciated that additional sensors can be integrated into the flexible sensor  1 . For example, piezo films that monitor effectively heart rate and body movements. Such combinations may be important for some healthcare applications. 
     It will be further appreciated that the flexible sensor  1 , if large enough, e.g. in the form of a sheet on a large mattress, may detect desired variable in more than one user, e.g. a mother and her child. Differentiation between users monitored simultaneously can be made easily using heart rate monitors integrated into the flexible sensor  1  at different locations. 
     It is to be appreciated that additional desired variables generated by the analysis of the flexible sensor  1  may be temporal correlations between dynamic variations in respiration structure (single and multiple pattern) and pressure distribution pattern. For example, coughing, vomiting and suffocation in babies are likely to be manifested by such correlations, suggesting that the invention has a potential in early detection of high-risk states that may result in death, e.g. sudden infant death syndrome (SIDS). 
     It is also to be noted that while various distinctions of the invention over the prior art have been mentioned, these distinctions are not to be construed as the only distinctions over the prior art. 
     It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described above. Rather the scope of the present invention is defined only by the following claims.