Abstract:
A method and apparatus is disclosed for sensing when a person is disposed on or arises from a support that flexes or sags when a person is disposed thereon. Such supports include beds, supports for a mattress or a seat, such as a sling-type seat found on wheelchairs. In one embodiment, an assembly is positioned beneath the support and includes a sensor. The sensor indicates when tension on the assembly exceeds a certain amount which is indicative of sagging of the support. The assembly may be adjustable in length to maintain proper tension in the absence of sag. The sensor may be a switch that includes a plunger which operates against the bias of a biasing member which either completes or interrupts an electrical circuit in response to tension placed on the assembly due to sagging of the support. In another embodiment, the assembly may be at least partially elastic. In yet another embodiment, a strain gauge may be employed to detect stretching of the assembly due to sagging of the support. In the method of the invention, a signal received from the sensor may be employed either with or without a processor to activate an alarm or to activate a light or to activate another audible signal or to send a signal over a telephone line. In addition, the signal received may be used to activate or deactivate a brake on a wheelchair.

Description:
BACKGROUND OF INVENTION 
       [0001]    1. Field of Invention 
         [0002]    This invention relates generally to a method and apparatus for detecting the presence or absence of a person on a flexible support, and more particularly, to a mechanism and method to be used with a sling-type seat using an expandable strap. 
         [0003]    2. Discussion of Related Art 
         [0004]    It is desirable to be able to monitor when a person is either present on or is not present on a flexible support, such as a seat or a bed. A typical example of such a seat is a sling-type of seat which typically includes a fabric seat portion which extends between at least two generally parallel, rigid support members. The fabric is flexible and provides a degree of comfort to the person seated, yet must be sufficiently strong so that it can support the weight of such a person. The seat may be made of any suitable fabric, such as canvass, cotton, nylon, Kevlar or the like. Chairs are typically provided with a sling-type of seat to allow for folding of the chair for storage or transport. One typical example of a chair with a sling-type of seat is a wheelchair. 
         [0005]    Because of its non-rigid nature, the seat of a sling-type of seat will sag in the middle. When a person is seated on the seat, the seat, regardless of the material from which it is formed, will stretch slightly, producing a greater sag. The seat will demonstrate some level of sag even when no one is seated therein. With time, the fabric of the sling-type seat will tend to take on a permanent stretch and sag after repeated use. This is commonly termed “sling seat sag”. With respect to a wheelchair, some of this additional sag is due to side supports being pulled toward one another by the weight of the body and part of the sag is due to whatever inherent stretchability is found in the fabric. As this sag increases, the spacing of the bottom of the seat from a horizontal line drawn between the two side supports also increases, and the actual length of the fabric material of the seat is necessarily longer than a straight, generally horizontal line drawn between the two support members. 
         [0006]    When a person arises from such a sling-type of seat, the inherent elasticity in the material of the seat will cause the seat to return ever so slightly toward its pre-stretched length. In addition, the side support rails may return to a wider spacing which is maintained by any support structure and braces. 
         [0007]    It would be desirable to be able to determine when someone becomes seated in a sling-type seat, particularly a wheelchair. It would also be desirable to determine when someone arises from a sling-type seat, such as a wheelchair. This information could be used to monitor movement of a patient for various purposes. 
         [0008]    Currently there exist compression switches that are used in conjunction with wheelchairs and that are activated when a person is seated in a wheelchair. These compression switches, however, are attached to the side rails by an inelastic or rigid member. These types of switches are undesirable because they cause unnecessary pressure from beneath the seat on the person seated therein. This rigid or non-elastic member not only is uncomfortable for a seated person, but causes excessive pressure which could lead to skin breakdown and, after continuous or substantially extended use, a decubitus ulcer. 
       SUMMARY OF INVENTION 
       [0009]    This invention relates generally to a method and apparatus for detecting the presence or absence of a person on a flexible support, including a seat, a bed, a support for a mattress or the like. This invention provides a signal indicative of either the presence or absence of a person on the support to alert a caregiver or some other person. Any sort of response may be provided, such as an audible signal (i.e., an alarm), a visual signal, a recorded telephone message, or a signal over a telephone line. A signal may also be used to either apply or release brakes on a wheelchair. This invention has particular applicability to seats which sag when a person is seated therein, such as the seat of a wheelchair. However, the application of this invention is not limited to wheelchair seats, or even to chairs. It could also be used to detect when a person is lying in a bed or is seated on the edge of a bed. This invention overcomes the problems associated with prior art devices which typically are not flexible, or are not elastic, or both, and which render the support, such as a seat, uncomfortable and could even, in extreme cases, cause injury to the person. 
         [0010]    In one aspect, a combination of a support for a person and a sensor is disclosed. In one embodiment of this aspect, this combination includes a flexible support having an upper surface upon which a person may be disposed and a lower surface, and a flexible assembly which is secured adjacent the lower surface of the support, the assembly including a sensor that provides an electrical signal which is indicative of the presence or absence of a person on the upper surface of the support and which is produced by tension on the assembly resulting from downward flexing of the support. This combination may include a strap, and this strap may comprise at least one elastic portion. This elastic portion may include a spring. In another embodiment, the assembly may be fixedly attached to two side rails. 
         [0011]    In another embodiment of this aspect, the sensor comprises a housing, a plunger slidable with respect to the housing, a member biasing the plunger into a first axial position with respect to the housing, a pair of spaced contacts disposed in the housing, and a conductor associated with the plunger for providing an electrical connection between the spaced contacts. In this embodiment, the plunger may be axially slidable from a first axial position to a second axial position upon the application of tension to the assembly resulting from downward flexing of the seat. In another embodiment, the conductor is spaced from the electrical contacts in the first axial position, and is in electrical contact with both of the electrical contacts in the second axial position. In yet another embodiment, the conductor is in electrical contact with both of the electrical contacts in the first axial position, and is spaced from the electrical contacts in a second axial position. 
         [0012]    In yet another embodiment of this aspect, the combination includes a processor configured to receive an electrical signal from the electrical contacts. This processor may be selectively coupled to an alarm, an audible signal device, a light, a telephone jack or a brake associated with a chair. 
         [0013]    In yet another embodiment of this aspect, at least a portion of the assembly is elastic, and the sensor comprises a strain gauge coupled to the elastic portion of the assembly. 
         [0014]    In another aspect of the invention, a system is disclosed for providing an indication of the presence or absence of a person on a support which flexes downwardly when a person is disposed thereon. This system comprises a strap structured to be disposed adjacent a lower surface of the support and is fixed to prevent movement thereof with respect to the support, the strap including an elastic portion. The system also includes a sensor associated with the strap for providing an indication of when downward flexing of the support caused by a person disposed on the support exceeds a predetermined amount or is less than a predetermined amount. In one embodiment, the support is a seat. 
         [0015]    In yet another aspect of the invention, a method is disclosed for determining whether a person is disposed on a support that flexes downwardly when a person is disposed thereon. This method comprises the steps of affixing an assembly with respect to the support beneath the support adjacent a lower surface thereof, adjusting a length of the assembly so that the assembly generally conforms to the shape of the lower surface of the support when no one is disposed on the support, and providing an electrical signal from a sensor associated with the assembly indicative of when tension on the assembly exceeds a predetermined amount resulting from flexing of the support downwardly when a person is disposed thereon. In one embodiment of this aspect, the method includes the further step of activating an alarm in response to the signal. In another embodiment of this aspect, the method includes the step of activating an alarm in the absence of a signal. 
         [0016]    In yet another embodiment of this aspect, the method further includes the step of sending a signal to a processor. In another embodiment, the method includes the step of activating a light in response to the signal or in the absence of the signal. In another embodiment, the method includes manipulating a brake associated with a seat in response to the signal. In yet another embodiment, the method further includes sending a signal over a telephone line in response to the signal received from the sensor. In yet another embodiment, the method includes the step of providing an elastic portion on the assembly. In yet another embodiment, the method includes providing another electrical signal from the sensor indicative of tension on the assembly below a certain amount. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]    The objects, advantages and features of this invention will be more clearly appreciated from the following detailed description, when taken in conjunction with the accompanying drawings, in which: 
           [0018]      FIG. 1  is a side schematic view illustrating the sensor of the present invention installed on an empty support, such as a seat; 
           [0019]      FIG. 2  is a side schematic view of the support and sensor of  FIG. 1  illustrating the effect of a person being seated thereon; 
           [0020]      FIG. 3  is a partial, cross-sectional side view of one embodiment of the sensor assembly of this invention; 
           [0021]      FIG. 4  is a partial, cross-sectional side view of the sensor assembly of  FIG. 3  in a switched condition; 
           [0022]      FIG. 5  is a partial, cross-sectional side view of the sensor assembly of  FIG. 4  illustrating stretching of the strap; 
           [0023]      FIG. 6  is a partial, cross-sectional side view of another embodiment of the sensor of this invention having a switch in a closed position; 
           [0024]      FIG. 7  is a partial, cross-sectional side view of the sensor of  FIG. 6  showing a switch in an open position; 
           [0025]      FIG. 8  is a partial side view of another embodiment of the invention; and 
           [0026]      FIG. 9  is a schematic diagram illustrating a processor and associated systems for use with this invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0027]    This invention relates to a sensor assembly for use with a flexible support. Examples of a support with which this invention may be used include a seat, a bed or a support for a mattress or the like. This invention is particularly suited for use with a sling-type seat, to provide an indication of when a person is seated in that seat, and/or when a person has left that seat. A typical sling-type seat is found on a wheelchair. In one aspect, the sensor assembly preferably is disposed beneath but closely spaced from the support, and is activated as a result of the stretching and resultant sagging of the material of the support when a person is disposed thereon, or by the absence of stretching and sagging of the material when the person leaves the support. The sensor assembly typically is capable of stretching and/or expanding to accommodate stretching and/or sagging of the support material caused by the presence of a person thereon. 
         [0028]    The sensor assembly typically includes a strap and a sensor. The sensor used in conjunction with this invention may be any type of device which senses flexing or stretching of the support as a result of a person being present. In one embodiment, the sensor is a switch which is biased into an open position and is closed when a person is disposed on the support. In another embodiment, the switch is biased into a closed position, and is opened by a person being on the support. In either embodiment the switch may include a plunger which is biased into one portion and slides against the bias into another position in response to stretching and/or sagging of the support to accommodate such stretching and/or sagging. In yet another embodiment, the sensor is a strain gauge calibrated for the degree of stretching of the support material expected to occur when a person is present thereon. 
         [0029]    In any embodiment, a resulting signal may be sent to a processor which provides an indication that a person is present on the support, or that a person has left the support. The signal may be used in a variety of ways, such as sending an alarm, activating a visual alarm, placing a call for assistance, or activating or releasing a braking mechanism for a wheelchair. 
         [0030]    In another aspect of the invention, the sensor assembly may include an elastic component which accommodates stretching or flexing or sagging of the support after the switch is opened or closed to prevent damage to the switch and to minimize upward forces on the support and the user of the support. 
         [0031]    With reference now to the drawings, and more particularly to  FIGS. 1 and 2  thereof, one aspect of this invention will be described. Sensor assembly  20  will now be described with respect to one embodiment of the support of this invention, a chair  10  with a seat  12 . However, it is to be understood that the operation and structure of sensor assembly  20  when used with another embodiment of the support of this invention, such as a bed or a support for a mattress, are substantially the same as that described with respect to chair  10  and seat  12 . It is to be understood, of course, that seat  12  could represent a bed or a support layer for a mattress upon which a person could sit or lie. 
         [0032]      FIG. 1  is a schematic illustration of a chair  10  with a flexible seat  12 , such as a sling-type seat. Seat  12  may be suspended between two side rails  14 . Rails  14  are typically, but not necessarily, parallel and are mounted to a support structure (not shown) of chair  10 . This support structure of chair  10  typically includes side supports  16 . One example of a typical chair  10  is a wheelchair. In a wheelchair, wheels are typically mounted on the lower end of side supports  16 . These wheels may include a braking system or the like  130  (see  FIG. 9 ). Since seat  12  is flexible, side support  16  and rails  14  may be collapsed toward one another to fold chair  10  into a condition which allows it to be either stored or transported. Seat  12  may be attached to rails  14  in any manner well-known to those skilled in the art, such as by wrapping a portion of seat  12  about rails  14  and affixing seat  12  to rails  14  such as by screws, rivets or the like  15 , as shown in  FIG. 2 . 
         [0033]    Seat  12  may be formed of any material commonly known to be used with respect to flexible seats, such as sling-type seats. Typical materials for seat  12  include canvass, cotton, nylon, foam or the like. Regardless of the material used, because it is flexible and unsupported at its center, seat  12  is subject to a certain amount of deformation and stretching when a person sits on seat  12 . 
         [0034]    Sensor assembly  20  is disposed directly below seat  12  typically closely adjacent a lower surface  13  of seat  12 . Sensor assembly typically conforms to the shape of lower surface  13  of seat  12 . Sensor assembly  20  senses the sagging and/or flexing and/or deformation and/or stretching of seat  12  that occurs when someone sits on seat  12 . Sensor assembly  20  preferably is flexible so as to allow it to be folded with seat  12  when chair  10  is collapsed. Sensor mechanism  20  typically also is sufficiently stretchable or expandable to permit it to accommodate the sagging and/or deformation and/or flexing and/or stretching of seat  12  in a manner so as to not exert any substantial upward force on seat  12  and a person seated therein. As a result, no discomfort is produced with respect to the person, and there is no concern that the person will in some way be injured by sensor assembly  20 . 
         [0035]    In one embodiment, sensor assembly  20  includes a sensor  22  and a flexible strap  24 . Assembly  20  may also include an affixation device  26  disposed at or near each end of strap  24  to affix strap  24  to opposed rails  14 , or other portions of the support structure for chair  10 . 
         [0036]    Affixation device  26  may be any type of connector which fixedly attaches strap  24  to rails  14  or to some other portion of the support structure for chair  10 . Affixation device  26  typically is a connector which will not stretch, bend or otherwise deform under the weight of a person sitting on seat  12 . In one embodiment, affixation device  26  may be a metal hook which is secured to rails  14 . In another embodiment, affixation device  26  may be a screw, rivet or the like which secures strap  24  to rails  14  directly beneath seat  12 . In this embodiment, devices  26  may be the same fastening mechanisms  15  used to secure the edges of seat  12  to rails  14 . 
         [0037]    As shown in  FIG. 2 , when a person sits on seat  12 , it tends to flex under the weight of the person. A sag may be produced which is accompanied by a slight stretching of seat  12 . This flexing, deformation and/or stretching results in a downward force on sensor assembly  20 . Since each end of strap  24  is anchored, the downward force applied by the body of a person necessarily applies a tension or longitudinal force on assembly  20  which is directed along the length of assembly  20 . It is this tension that is sensed by sensor  22 . 
         [0038]    Sensor  22  may be any type of device which will respond to the longitudinal tension or force applied parallel to the length of sensor assembly  20  which results from sagging, flexing, deformation and/or stretching of seat  12  caused by a downward force on seat  12  produced by a person sitting on seat  12 . 
         [0039]    One example of a suitable sensor  22  is a switch. One embodiment of a suitable switch  40  is illustrated in  FIG. 3 . Switch  40  may include an outer housing  42 , a plunger  44  having a shaft  56 , a biasing member  46 , contacts  48  and a conductor  50 . 
         [0040]    Shaft  56  of plunger  44  extends past lips  52  which typically surround shaft  56  on the interior of housing  42 . End  60  of shaft  56  extends through opening  58  in one end of housing  42 . Plunger  44  includes a head  54  on an end opposite end  60 . Disposed on shaft  56  generally between head  54  and end  60  is an enlarged portion  64  which together with lips  52 , limits movement of shaft  56  to the left as shown in  FIG. 3 , or toward head  54  and away from contacts  48 , thereby limiting the extent to which plunger  44  can move away from contacts  48 . Conductor  50  is mounted on shaft  56  on a side of enlarged portion  64  facing contacts  48 . Conductor  50  may be a ring encircling shaft  56  and may be formed of any electrically conductive material. Contacts  48  extend into the interior housing  42  such that they face and may be contacted by conductor  50 , but such that conductor  50  is spaced from contacts  48  in the normal, open position of switch  40 , as shown in  FIG. 3 . Wires  68  extend externally of housing  42  from contacts  48 . 
         [0041]    Plunger  44  is slidable in a longitudinal or axial direction parallel to shaft  56  within and with respect to housing  42 , typically in a direction generally parallel to the tension or longitudinal force applied to sensor assembly  20 . Biasing member  46  typically is a compression spring, although member  46  could be any elastomeric member which is compressible, and which is sufficiently elastic to substantially return to its normal, uncompressed condition upon the release of a force. Biasing member  46  is positioned between and bears against lips  52  formed on the interior of housing  42 , and enlarged head  54  formed on shaft  56 . In this way, as illustrated in  FIG. 3 , plunger  44  is normally urged by biasing member  46  in a leftward direction as shown in  FIG. 3 , or in a direction in which conductor  50  is spaced from contacts  48  in the longitudinal direction. 
         [0042]    In another aspect of the invention, sensor assembly  20  may also include an elastic component which will accommodate additional stretching of seat  12  beyond that which is necessary to activate sensor  22 . In one embodiment, as shown in  FIG. 3 , this elastic component may be an elastic portion  30  of strap  24 . Elastic portion  30  may comprise all or part of strap  24 , or may compromise a plurality of separated segments of strap  24 . Elastic portion  30  may be an elastic webbing, an extension spring or the like. If it is an elastic webbing, portion  30  may be any commonly available knitted or woven elastic which is sufficiently strong to withstand the forces applied, and to provide the desired level of elasticity to accommodate any additional stretching of seat  12  beyond that accommodated by switch  40 . The purpose of elastic portion  30  includes prevention of damage to switch  40 , and prevention of sensor assembly  20  from becoming undesirably tight beneath seat  12  to avoid discomfort and injury to the person seated in the seat caused by an upward force on the seat. In this embodiment, the spring constant of biasing member  46  of switch  40  typically is lower than the spring constant of portion  30 , so that biasing member  46  will be compressed to the point where conductor  50  touches contacts  48  before the elastic portion  30  stretches significantly. In this way, the switch will always be closed before substantial stretching of elastic portion  30  occurs, and stretching of portion  30  will not override or thwart the operation of switch  40 . 
         [0043]    As shown in  FIG. 3 , in one embodiment, elastic portion  30  is affixed to end  60  of plunger  44  by a non-elastic connector  62 . Elastic portion  30  may be affixed to connector  62  in any manner known to one of ordinary skill in the art. In one example, as illustrated in  FIG. 3 , an end piece of elastic portion  30  is folded over connector  62  and onto itself and is sewn to itself by stitch  31 . Portion  25  of strap  24  may be affixed to end  70  of housing  42  by another connector  62  in the same manner. Portion  25  typically is non-elastic. However, portion  25  may also be elastic in another embodiment of the invention. Of course, the positions of portions  30  and  25  may be reversed so that portion  30  is affixed to end  70  and portion  25  is affixed to end  60 . Connector  62  may be any commonly used device, such as a metal bracket or the like, having any shape desired, such as triangular, square, rectangular or the like. In addition, instead of using a connector  62 , portion  25  and elastic portion  30  may be affixed directly to respective end  70  and end  60  such as by screws, rivets, adhesives or the like. 
         [0044]    The operation of the embodiment of sensor assembly  20  illustrated in  FIG. 3  will now be described with particular reference to  FIGS. 3 ,  4  and  5 .  FIG. 3  illustrates the condition of switch  40  when seat  12  is in the condition shown in  FIG. 1 , in which no one is seated. In  FIG. 3 , biasing member  46  urges plunger  44  to the left, as shown in  FIG. 3 , or in a direction such that conductor  50  is spaced from contacts  48 . Typically, although not necessarily, enlarged portion  64  may be urged into engagement with lips  52 . The switch is in an open condition as there is no contact between conductor  50  and contacts  48 . Elastic portion  30  is in a substantially unstretched or relaxed condition. 
         [0045]      FIG. 4  illustrates the condition of switch  40  when a person first begins to sit on seat  12 , as illustrated in  FIG. 2 . As shown in  FIG. 4 , plunger  44  is urged to the right, or in a direction which moves conductor  50  toward contacts  48 . This movement results from the initial downward force exerted on seat  12  by the weight of the occupant of seat  12 . This downward force flexes, deforms and/or stretches seat  12 , thereby causing a sag in seat  12 . This sag in seat  12  produces a longitudinal force or tension on sensor assembly  20  and thus strap  24 , and switch  40 , since strap  24  is fixedly secured on either side of seat  12 , such as to rails  14 . This longitudinal tension overcomes the bias of biasing member  46 , compressing biasing member  46  and urging plunger  44  in a direction that pushes conductor  50  toward contacts  48 , until conductor  50  is in electrical contact with contacts  48 , thus closing the switch. As shown in  FIG. 4 , plunger  44  moves a maximum distance of ΔX, which is the distance necessary to place conductor  50  in contact with contacts  48 . 
         [0046]    If the sag on seat  12  is to be great, longitudinal tension on sensor assembly  20  exceeds the force necessary to produce contact between conductor  50  and contacts  48 , and no additional movement of plunger  44  is permitted, as conductor  50  abuts contacts  48 . If further expansion of sensor assembly  20  is not permitted, this tension could damage switch  40 , and/or cause discomfort for or injury to the occupant of seat  12 . As illustrated in  FIG. 5 , any additional tension is accommodated by stretching of elastic portion  30 . Elastic portion  30  is permitted to stretch a distance ΔY to accommodate this additional longitudinal tension, to prevent injury or discomfort to the occupant of seat  12  and/or damage to switch  40 . 
         [0047]    When a person arises from seat  12 , and the sag is removed from seat  12 , the force applied by biasing member  46  acts to return plunger  44  to the position shown in  FIG. 3  in which conductor  50  is spaced from contacts  48 , thereby producing an opening of switch  40 , and elastic portion  30  retracts elastically to return strap  24  to its original length. 
         [0048]    In another embodiment, an alternative switch  80  for use with sensor assembly  20  will now be described with particular reference to  FIG. 6 , in which like numbers are used for like parts in  FIGS. 1-5 , where applicable. Like switch  40 , in switch  80 , elastic portion  30  is connected to end  60  of plunger  44  and portion  25  is connected to end  70  of housing  42  (or vice versa), such as by connectors  62 . The primary difference between switch  40  and switch  80  is that switch  80  is in a normally closed condition in the absence of a person seated in seat  12 . Biasing member  46  urges plunger  44  to the left as shown in  FIG. 6 , or into a condition in which conductor  82  is in electrical contact with contacts  84  to provide a closed circuit. Contacts  84  may be positioned on lips  52  facing enlarged portion  64 , or contacts  84  may be spaced axially away from lips  52  toward enlarged portion  64 , but on a side of lips  52  facing end  60 . In this embodiment, conductor  82  typically is disposed on the opposite side of enlarged portion  64  from conductor  50  and facing contacts  84  and lips  52  so that conductor  82  is urged into electrical contact with contacts  84  when biasing member  46  is in an extended position as shown in  FIG. 6 , or in which plunger  44  is disposed to the left with respect to housing  42  in  FIG. 6 . Conductor  82  may be similar to conductor  50 , such as a ring disposed on and encircling the shaft of plunger  44 . Contacts  84  are coupled to an external circuit (not shown) by wires  86 . 
         [0049]    The operation of the embodiment of  FIG. 6  will now be described with particular reference to  FIGS. 6 and 7 . As previously discussed with respect to  FIGS. 1-5 , when a person sits on seat  12 , a longitudinal tension or force is applied to assembly  20  which urges plunger  44  axially toward the right with respect to housing  42 , by compressing biasing member  46 , as shown in  FIG. 7 . Such axial movement of plunger  44  separates conductor  82  from contacts  84  and opens the switch. Movement of plunger  44  in an axial direction with respect to housing  42 , to the right, as shown in  FIG. 6 , is permitted to continue until enlarged portion  64  strikes shoulders  88  of housing  42 . At this point, further axial movement of plunger  44  with respect to housing  42  is prevented. As discussed with respect to  FIGS. 1-5 , any further longitudinal tension applied to assembly  20  is accommodated by stretching of elastic portion  30 . As a person arises from seat  12 , biasing member  46  returns plunger  44  to the position shown in  FIG. 6  in which contacts  84  are in electrical contact with conductor  82  to close the switch, and portion  30  contracts in the same manner as discussed with respect to switch  40 . 
         [0050]    Typically, the spring constant of biasing member  46  is lower than that of portion  30  to permit opening of switch  80  prior to any significant stretching of portion  30 . Moreover, biasing member  46  of switch  80  may have a somewhat higher spring constant than that of biasing member  46  of switch  40  to prevent inadvertent opening of switch  80  in response to random forces applied to seat  12  that are not caused by a person sitting in the seat. 
         [0051]    Another embodiment of this invention will now be described with specific reference to  FIG. 8 . In  FIG. 8 , like numbers are used for like parts in  FIGS. 1-5  where applicable. In  FIG. 8 , a strap  100  extends beneath seat  12  and is affixed at or near its ends, such as to rails  14 . Strap  100  includes somewhere along its length a strain gauge  102 , which measures the amount of stretch of strap  100 . Typically, strap  100  is elastic, or includes an elastic portion, so that as a person sits on seat  12 , strap  100 , or a portion thereof, will stretch. Conversely, strap  100  contracts once a seated person leaves seat  12  to return strap  100  to its original length. The amount of stretch of strap  100  depends on the amount of flexibility and stretch of seat  12  and upon the sensitivity of strain gauge  102 . As a person sits on seat  12  and applies a longitudinal stretching force or tension on strap  100 , causing stretching of strap  100 , or a portion thereof, strain gauge  102  measures the amount of stretch. Gauge  102  then sends a signal to a remote location via wires  104 . Once the amount of stretch exceeds an amount indicative of a person seated in seat  12 , a signal may be generated. Conversely, when the measured strain drops below that amount, a signal may be generated. Strain gauge  102  may be any commercially available gauge which senses elongation of a material. 
         [0052]    In another aspect of each of the embodiments of  FIGS. 1-8 , an adjustment device  110  may be employed with sensor assembly  20  and/or strap  100  to adjust the length of assembly  20  and/or strap  100 . The length of assembly  20  and/or strap  100  preferably should be adjusted so that the assembly  20  and/or strap  100  are relatively taut and are positioned adjacent the lower surface  13  of seat  12  so that the desired amount of stretching of seat  12  caused by a person sitting on seat  12  will be detected. However, assembly  20  and strap  100  should be not so taut as to cause a signal to be sent from switch  40  or  80  and strain gauge  102  caused by inadvertent forces applied to seat  12  and not by someone sitting on seat  12 . This adjustment device  110  can also provide an adjustment of the tension on straps  24  and/or  100  to conform to the amount of stretch, flex and/or deformation permitted by seat  12  when one is seated in seat  12  which could vary with time and use of seat  12 . Of course assembly  20  and strap  100  should not be so loose that when one sits on seat  12 , no signal is provided by switch  40  or  80  and strain gauge  102 . A typical example of an adjustment device  110  is a buckle. Other types of adjustment devices would include mating hook and loop fasteners, snaps, buttons or the like. 
         [0053]    For each of switch  40 , switch  80  and strain gauge  102 , the signal output therefrom (or lack thereof) over respective wires  68 ,  86  and  104  may be provided to a processor  120 , as shown in  FIG. 9 . Processor  120  may be programmable, or may be hard wired and may be conventional. Processor  120  typically is coupled to one or more devices which could, for example, include an alarm  122 , another audible device  124 , such as a recorded call for help, an inaudible device, such as a light  126 , a telephone jack  128  for sending a recorded message or a braking system  130  on a chair  10 . It should be understood that any single one or more of these devices may be coupled to processor  120 . Also, these devices may be directly coupled to the output wires  68 ,  86  and  104  of respective switches  40  and  80  and gauge  102 , and not to processor  120 . It is to be understood that with regard to the braking system  130 , the action of being seated in the chair could cause either a release or activation of the brake, while the action of arising from the chair could cause either activation or release of the brake depending upon the desired result. 
         [0054]    Alarm  122 , device  124 , light  126  and telephone jack  128  are commonly known in the art, and need not be described further. The braking systems also would be conventional, and could include a solenoid or mechanically actuated brake which either applies the brake or releases the brake. It is to be understood that any or all of these devices could be triggered either by someone being seated in the chair or by someone arising from chair  12 . 
         [0055]    The method of the present invention will now be described with reference to  FIGS. 1-9 . As noted, this invention is to be used with a support that is flexible and preferably stretches or sags. The support may be a seat such as one that is known as a sling-type seat. This invention involves detecting when a person is seated on, lying on, or arises from a flexible support, such as a seat, bed or support for a mattress. A sensor assembly  20  or strap  100  is positioned directly beneath a lower surface  13  of the support, such as seat  12 . It is preferred that sensor assembly  20  or strap  100  be disposed closely adjacent lower surface  13  so that any flexing, deformation, sag and/or stretch will produce a corresponding tension in assembly  20  or strap  100 . Assembly  20  and strap  100  are typically tightly attached to the frame on either side of the support. In the examples shown in  FIG. 1 , and in  FIG. 8 , respective straps  24  and  100  are affixed to rails  14 . However, it is to be understood that straps  24  and  100  could be affixed to other portions of a frame and need not be affixed to rails  14 . Once strap  24  or  100  has been affixed, typically at its ends (although not necessarily), and typically by an affixation device  26 , typically assembly  20  or strap  100  is tensioned so as to conform relatively closely to the shape of lower surface  13  when no one is on the support. Also, assembly  20  or strap  100  may be approximately equally spaced from lower surface  13  generally along its length. This tension and conformity and spacing may be adjusted utilizing adjustment device  110 . 
         [0056]    In the embodiment of  FIGS. 3-5 , when a person is disposed on the support, the tension, or longitudinal force applied to assembly  20  initially produces longitudinal movement of plunger  44  with respect to housing  42  to the right as shown in  FIG. 4  against the bias of biasing member  46 . Because the spring constant of biasing member  46  is lower than that of elastic portion  30 , movement of plunger  44  will occur prior to any substantial stretching of elastic portion  30 . However, it is to be understood, that movement of plunger  44  may also be accompanied by some stretching of elastic portion  30 . Once plunger  44  has moved with respect to housing  42  to the position shown in  FIG. 5 , conductor  50  is in electrical contact with contacts  48 , thereby closing the switch. This closed switch causes a signal to be sent through wires  68 , such as to processor  120 . Processor  120  may do nothing or it may activate alarm  122 , audible device  124 , light  126 , a connection to a telephone jack  120 , or brake system  130  associated with a chair  10 . Processor  120  may also activate more than one of the foregoing, depending on how it is wired or programmed. For example, processor  120  could release brake system  130  as well as activate alarm  122  and phone jack  128 . 
         [0057]    Further tension or longitudinal force applied to assembly  20  after engagement of conductor  50  by contacts  48  is accommodated by stretching of elastic portion  30 . In this manner, the person disposed on the support typically is not caused any discomfort or injury. 
         [0058]    When a person on the support arises, plunger  44  returns to the position shown in  FIG. 3  and elastic portion  30  retracts to its original length. At this point, the switch is opened. This opening of the switch could also result in a signal sent such as to processor  120  and/or cause an alarm and/or an audible signal and/or an inaudible signal such as a light and/or connection to a telephone jack and/or release or activation of a brake on the chair. 
         [0059]    With respect to the embodiment of  FIGS. 6 and 7 , when a person is disposed on the support, plunger  44  axially moves with respect to housing  42  from the position shown in  FIG. 6  to that shown in  FIG. 7 . When this movement of plunger  44  occurs, the switch is opened. This opening of a switch may result in a signal sent, such as to processor  120  and may cause an alarm and/or an audible signal and/or an inaudible signal such as a light and/or connection to a telephone jack and/or activation or deactivation of a brake on a chair  10 . 
         [0060]    Movement of plunger  44  in  FIG. 6  continues until it reaches the position shown in  FIG. 7  in which enlarged portion  64  is in contact with shoulders  88 . Thereafter, any additional longitudinal tension or force applied to assembly  20  is accommodated by stretching of elastic portion  30  as previously discussed. 
         [0061]    When a person arises, plunger  44  returns to the position shown in  FIG. 6  at which time the switch is closed, again sending a signal such as to processor  120  which could do nothing, or the signal could cause activation of an alarm and/or some other audible signal and/or an inaudible signal, such as a light and/or a connection to a telephone jack and/or a brake on the chair. In addition, as before, elastic portion  30  returns to its original, unstretched condition. 
         [0062]    With respect to the embodiment of  FIG. 8 , when a person is disposed on the support, strap  100 , at least a portion of which is elastic, stretches. After a certain predetermined amount of stretch, as measured by gauge  102 , a signal is sent such as to processor  120  which can do nothing or can activate an alarm and/or some other audible signal and/or an inaudible signal, such as a light and/or a connection to a telephone jack and/or a brake on the chair. When a person arises in  FIG. 8 , the elastic strap  100  returns to its original, unstretched condition. Gauge  102  can either send a signal to processor  120 , or interrupt the signal previously sent to processor  120 . In either event, processor  120  can do nothing in response, or it may activate an alarm and/or some other audible signal and/or an inaudible signal such as a light and/or a connection to a telephone jack and/or a brake on a chair. 
         [0063]    This invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
         [0064]    Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.