Abstract:
An immersion sensor for use with a cushion or mattress for determining the relative immersion of a person within the cushion or mattress comprising a sensor, a ground and a circuit for measuring capacitance. The sensor comprises a sheet of conductive material, and the ground comprises a second sheet of conductive material. The circuit is adapted to send short bursts of electrical current to the sensor and a capacitor. The circuit is further adapted to measure the length of time the burst of current takes to charge the capacitor. Based upon the measured time, the circuit calculates the proximity of the object based upon the time taken to charge the capacitor. A method that may be implemented with the immersion sensor is also disclosed.

Description:
RELATED APPLICATIONS  
       [0001]     The present application claims priority to U.S. Provisional Application Ser. No. 60/725,901 filed Oct. 12, 2005, U.S. Provisional Application Ser. No. 60/725,006 filed Oct. 6, 2005, and also U.S. Provisional Application Ser. No. 60/675,315 filed Apr. 27, 2005. The contents of said applications are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to proximity sensors. More specifically, the invention relates to a sensor for detecting a relative distance of an object to the sensor by detecting changes in charge transfer.  
       BACKGROUND OF THE INVENTION  
       [0003]     Proximity sensors for detecting an actual or relative distance between the sensor and an object are known in the art. For example, U.S. Pat. No. 6,621,278 to Arie Ariav discloses a method of measuring a distance by transmitting a cyclically-repeating wave. The wave is then received at a second location in the medium. The system detects a predetermined point in the cyclically-repeating wave that is received at the second location and continuously changes the frequency of transmission of the cyclically-repeating energy wave in accordance with the detected point of each received cyclically-repeating wave received at the second location such that the number of waves received at the second location is a whole integer. The change in frequency to produce a measurement of the predetermined parameter is used to determine the distance the wave has traveled. However, this system has drawbacks, particularly in that the sensor is unduly complex both in electronic implementation and in sensor construction.  
         [0004]     Other types of detectors, primarily for detecting the presence or absence of an object, use ultrasonic and radio frequency transmitters and detectors that receive reflected energy when an object is present in an area of interest. These detectors however cannot be used practically to detect a relative or actual distance, particularly in very short distances. In certain settings, the amount of RF energy generated by these types of device is unacceptable due to interference. Moreover, some people have concerns about constant exposure to RF energy.  
         [0005]     Many applications require low power consumption and detection of a relative distance within a range of interest. For example, cushions for wheelchairs must be inflated to a pressure that is sufficient to properly immerse the person in the cushion to prevent the formation of decubitus ulcers on the person in the wheelchair. However, often the people bound to the wheelchair do not have the ability to feel when they are properly immersed in the cushion, such as a paraplegic or quadriplegic person. For those people, others must periodically check the person&#39;s immersion within the cushion to ensure the person is not in an overinflated state, such that only a small portion of the person&#39;s body is bearing their weight, or in an underinflated state, such that the person has “bottomed out” and is no longer supported entirely by the cushion. Similarly in a cushion not inflated with air, problems also exist when determining the proper cushion immersion. However, presently, no acceptable means of detecting the immersion of a person in a cushion exists. Only indirect measurement of pressure internally in the cushion is available. This type of measurement is dependant upon the materials of construction and structural conformation all creating significant limitations in the applicability of the measurement.  
         [0006]     Likewise, people bound to hospital beds must avoid decubitus ulcers when confined to the bed for long periods of time. To accomplish this, inflation mattresses are commonly used, and the inflation level of the mattress must be monitored in order to maintain the proper inflation level to prevent overinflation or underinflation of the mattress. Moreover, because the person&#39;s weight is concentrated over their entire back side, multiple locations must be checked for underinflation or overinflation. As a result, a sensor which is divided into zones to check the immersion of the patient within the mattress is needed.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention comprises an immersion sensor for use with a cushion or mattress for measuring the depth of immersion of a person within the cushion or mattress comprising a sensor, a ground and/or shield and a circuit for measuring capacitance. The sensor comprises a sheet of conductive material, and the ground comprises a second sheet of conductive material. The circuit is adapted to send short bursts of electrical current to the sensor and the reference capacitor. The circuit is further adapted to measure the length of time the burst of current takes to charge the capacitor. Based upon the measured time, the circuit calculates the proximity of the object based upon the time taken to charge the capacitor. The present invention also comprises a method that may be implemented with the immersion sensor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is an exploded perspective view of a wheelchair cushion proximity detection device according to an embodiment of the present invention;  
         [0009]      FIG. 2  is a plan view of the conductive and nonconductive layers of the proximity detection device according to an embodiment of the present invention;  
         [0010]      FIG. 3  is a diagram of a circuit according to an embodiment of the present invention;  
         [0011]      FIG. 4  is a diagram of a circuit of a charge transfer device according to an embodiment of the present invention;  
         [0012]      FIGS. 5A-5D  are a flow chart showing the operation of the hardware and software of the circuit of  FIG. 3 ;  
         [0013]      FIG. 6  is an exploded perspective of a proximity detector for a bed air cushion according to an embodiment of the present invention;  
         [0014]      FIG. 7  is a diagram of a circuit according to another embodiment of the present invention;  
         [0015]      FIG. 8  is a diagram of a sensor placement on a bed cushion proximity detector according to an embodiment of the present invention;  
         [0016]      FIG. 9  is a diagram of a circuit according to another embodiment of the present invention;  
         [0017]      FIG. 10  is a diagram of a sensor placement on a bed cushion proximity detector according to yet another embodiment of the present invention;  
         [0018]      FIG. 11  is a diagram of a sensor placement on a bed cushion proximity detector according to yet another embodiment of the present invention;  
         [0019]      FIG. 12  is an exploded perspective view of an automatically adjusting wheelchair cushion according to an embodiment of the present invention;  
         [0020]      FIG. 13  is a perspective view of an embodiment of the device including a first sensor of relatively large area and a second sensor of relatively small surface area according to an embodiment of the present invention;  
         [0021]      FIG. 14  is a perspective view of an embodiment of the device including a first sensor of relatively large area and a second sensor of relatively small surface area with a ground plane according to an embodiment of the present invention;  
         [0022]      FIG. 15  is a diagram of a circuit for operating the embodiment of  FIG. 14 ;  
         [0023]      FIG. 16  is diagram of an embodiment of the present invention including a visual display device; and  
         [0024]      FIG. 17  is a diagram of a sensor placement on a bed cushion proximity detector according to yet another embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]     While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiments illustrated.  
         [0026]     The preferred embodiment of the present invention is a proximity sensor that utilizes charge transfer measuring technology and large-area capacitive sheets to determine the distance of an object from the capacitive sheet. The charge transfer measurement is employed with a short, low duty cycle burst of power. Burst mode permits power consumption in the low microamp range, thereby dramatically reduces radio frequency (RF) emissions, lowers susceptibility to electromagnetic interference (EMI), and yet permits excellent response time. Internally, it is preferred that the signals are digitally processed to generate the required output signals. The charge transfer measurement device switches and charge measurement hardware functions are preferably all internal to the charge transfer measurement device.  
         [0027]     To that end, the invention will be described, by way of example and not by limitation, in reference to a cushion for a wheelchair. Referring to  FIG. 1 , there is shown an inflatable cushion  10 , for example the cushion described in U.S. Pat. No. 4,541,136. Placed below the cushion is a sensor  12  according to the present invention to detect the immersion of a person within the cushion. The sensor  12  comprises two exterior sheets of neoprene rubber  14 . Sandwiched between the sheets of rubber are thin layers of foam  16  and between the foam  16  is a sensor layer  18 .  
         [0028]     The sensor layer  18  of  FIG. 2  comprises a conductive sheet  20  adhered to a nonconductive sheet  22 . The conductive sheet  20  is preferably made from copper, and the nonconductive sheet  22  is preferably made from a polyester film. The sensor layer  18  may also be made from any other conductive material, such as a conductive polymer. The conductive sheet  20  when made from copper preferably has a thickness of about 0.0005 of an inch. The conductive sheet  20  is interrupted, preferably by etching or die cutting, along an area  24  to form a sensor area  26  and a grounding plane area  28 . While the sensor layer  18  is described as copper and polyester sheets, the nonconductive sheet is not required and may be omitted and the conductive sheet may be made from any conductive material, such as a conductive braid, mesh or screen printing a conductive material onto a nonconductive base. Additionally, while the sensor area  24  is shown as rectangular, the sensor area  26  may be appropriately shaped and located in order to provide the optimum geometry to the object to be sensed. In the example of  FIG. 2 , the sensor area is confined to a rear portion of the sensor where a person&#39;s buttocks would be located when seated in the wheelchair. Since most of a person weight is distributed in this location while seated, this located is at the greatest danger of bottoming out. However, it is within the scope of the present invention to provide a sensor at any location or multiple locations of the seating area.  
         [0029]     The problem solved by the ground layer with using charge transfer or capacitive technology with wheel chair cushions is that there is no good ground to use as reference. The grounding plane area  39  being in the area around the sensor area  26  allows a capacitance measurement to be made relative to the distance between the person and the sensor and ground areas  26  and  28 . The present invention is attached to a circuit  30  as shown in  FIG. 3 . The circuit generally comprises a microcontroller  32 , such as a 16LF818 available from Microchip Technology, Inc. of Chandler, Ariz. The microcontroller  32  is powered by a 3.5 volt battery  34 . Attached to the enable line  36  of the microcontroller  32  is a voltage regulator  39  for regulating the input voltage to the microcontroller  32 . Attached to the clock line  38  and the data line  40  is a charge transfer sensor  42 . The data line  40  transmits data from the charge transfer sensor  42  to the microcontroller indicting the distance of an object, in this case a person&#39;s buttocks, from the sensor area  26 . The data is preferably in the form of a hexadecimal number representative of the relative distance of the person from the sensor area. In the preferred embodiment, the charge transfer sensor  42  is a QProx QT117 available from Quantum Research of Hamble, Southampton, United Kingdom. A ground line  44  is also connected to the charge transfer sensor  42 , as well as to the grounding plane  28 . The capacitive sensor  42  also requires a capacitor  46 , having a capacitance C s , attached to two lines of the sensor  42 . The capacitance of the capacitor  46  is preferably 0.022 μF and a temperature stable dielectric such as COG, but such value will change based upon the size and the application of the sensor.  
         [0030]     Also attached to the microcontroller  32  are various outputs to alarms and indicators  48 , inputs from an on/off switch  50  and an operator input switch  52 , and inputs from other controls  54 , such as if the circuit  30  is used as a feedback loop to automatically control the inflation of the cushion, as described below.  
         [0031]     Referring to  FIG. 4 , the charge transfer sensor  42  employs a short, low duty cycle burst of charge-transfer cycles with a burst controller  58  and amplifier  62  to acquire its signal. Internally the signals are digitally processed with an analog to digital converter (ADC)  60  to generate the required output signals. The charge transfer sensor  42  switches and charge measurement hardware functions are all internal to the sensor  42 . The ADC  60  is 14-bit single-slope switched capacitor ADC including both the required sensor  42  charge and transfer switches in a configuration that provides direct ADC conversion. The burst length is inversely proportional to the rate of charge buildup on the capacitor  46  (C s ), which in turn depends on the values of C s , C x  (the load capacitance of the sensor) and V cc . V cc  is used as the charge reference voltage. Larger values of C x  cause the charge transferred into C s  to accumulate more rapidly. As a result, the values of C s , C x  and V cc  should be fairly stable over the expected operating temperature range.  
         [0032]     The internal ADC  60  treats C s  as a floating transfer capacitor. As a direct result, the sensor  26  can be connected to either SNS1 or SNS2 with no performance difference. The polarity of the charge buildup across C s  during a burst is the same in either case. C s  must be of within a certain range for proper operation. It is important to limit the amount of stray capacitance on both terminals, especially if the load C x  is already large, for example by minimizing trace and wire lengths and widths so as not to exceed the C x  load specification and to allow for a larger sensing electrode size if so desired. The circuit board traces, wiring, and any components associated with or in contact with SNS1 and SNS2 will become proximity sensitive and should be treated with caution.  
         [0033]     The microcontroller  32  operates according to the flow chart of  FIG. 5 . In a first step, the device is powered on  100  and enters a continuously monitoring state  102 . From this state, the microcontroller  32  monitors whether an input operator input switch  52  has been depressed in decision step  104 . If it is has not, the microcontroller  32  returns to the monitoring state  102 . If the switch  52  has been depressed, the next step is to determine whether the depression was for three seconds or less in decision step  106 . If for three seconds or less, the battery health is checked in step  108  and a present reading of the distance of the person from the sensor area  26  is determined in step  110 .  
         [0034]     If a button  52  is determined to have been pressed greater than three seconds in step  106 , then in step  112 , the microcontroller  32  causes an alarm  48  to beep momentarily and proceeds to step  114  where the circuit again determines of the button  52  has been depressed for more than three more seconds. If so, the microcontroller  32  cycles through a series of five sensitivity settings as indicated to the user by a rapid succession of beeps of the alarm  48  in step  116 . The sensitivity setting is then stored in step  118  and the circuit continues to step  110  to read the present distance.  
         [0035]     If in step  114  it is determined that the button  52  has not been depressed for an additional three seconds, a value indicating the present distance is stored as the preferred set point in step  120 , and the circuit sounds an alarm and continues to step  110  to read the present distance.  
         [0036]     If in step  110 , the present value of the distance of the person from the sensor area  26  is not readable, the circuit continues to step  122  and flashes yellow and red LEDs alternatively. If the value is readable, the microcontroller  32  continues to step  124  and sets a tolerance above and below the current setpoint which will be considered within acceptable range from the setpoint. Next, in step  126 , the microcontroller  32  decides whether the present reading is within range or above or below range.  
         [0037]     If the reading is above range, in step  128 , the microcontroller  32  determines whether the current reading is greater than or equal to two counts over the previously chosen and stored sensitivity plus the setpoint. If the condition is true, the microcontroller  32  proceeds to step  130  where the microcontroller  32  determines it is not presently being used and goes to sleep until a reading is in the normal range. If the condition is not true, the microcontroller  32  flashes a yellow LED  48  to indicate that the cushion is overinflated. In either event, the microcontroller  32  next optionally proceeds to step  134 , where it logs the current condition date and time. If the embodiment is not one in which the data indicating inflation status is logged, the microcontroller will proceed to step  136 .  
         [0038]     In step  136 , if the current reading is below the acceptable range, the microcontroller will flash the red LED  48  and sound an audible alarm  48  to indicate underinflation if the current reading is the second consecutive reading to determine underinflation and proceeds to step  134 .  
         [0039]     After step  134 , the microcontroller  32  determines whether a user has pushed the button  52  to silence the audible alarm  48  in step  138 . If yes, the microcontroller  32  proceeds to step  140  and disables the audible alarm  48  until a second button push or a current sensor reading shows a reading with the acceptable range. After steps  138  and  140 , the microcontroller  32  proceeds to step  102 .  
         [0040]     If it is determined in step  126  that the setpoint is within the acceptable range, the microcontroller  32  continues to step  142  where the microcontroller  32  determines if the present reading was initiated by a button  52  press. If yes, in step  144  the green LED  48  is flashed and the microcontroller  32  returns to the monitoring state in step  102 . If no, in step  146 , the microcontroller  32  reinstates the timer and return to step  102 .  
         [0041]     Returning back to step  102 , if in the monitoring state ten minutes have elapsed, the microcontroller  32  will initiate a current reading automatically by proceeding to step  148  by performing a battery check and proceeding to step  110 .  
         [0042]     As another example shown in  FIG. 6 , the sensor can be used in a hospital bed to determine whether a patient has bottomed out when using an inflatable air mattress. In this instance, the bed comprises a bed frame  200  comprising a spring support  202 . Placed upon the spring support are a shield plane  204  and a sensor plane  206 . Upon the sensor plane  206  is placed an air mattress  208 . The shield plane  204  acts to isolate the metallic items of the bed  200 , particularly the spring support  202 , from the sensor plane  206 . The sensor plane  206  in its simplest application comprises a single sheet of conductive material, as with the previously discussed embodiment. The driven shield isolates the metal items of a bed and chair below the sensor plane  206 . In a device without a driven shield the effect of surrounding metal is subtracted by the user creating a setpoint based on the desired immersion level and the relative reading observed at that immersion.  
         [0043]     Just as with the wheelchair cushion proximity detector, the circuitry  30  operates in the same manner except that the shield plane  204  is driven to provide isolation from the metallic structure of the bed. The distance between the sensor plane  206  and the shield plane  204  is preferably about ⅛″ to about ⅜″. A problem posed by the hospital bed situation is the amount of metal in the bed and mattress support structure. The driven shield under the sensor or sensor area in the case of multiplexed units (described below) shields the sensor plane  206  in that direction of the location of the shield plane  204  giving increased sensitivity in the desired direction and ignoring changes in conductive materials and noise generating devices with position changes of the relative position of the device with the bed or other devices.  
         [0044]     In this regard and referring to  FIG. 7 , the original circuit  30  is modified to form circuit  30 ′. The numerals of circuit  30 ′ that correspond to circuit  30  are unchanged. However, the circuit  30  further comprises an amplifier  302  which is driven from an output of the charge transfer sensor  42  and serves to drive the shield plane  204  to isolate the sensor plane  206  from the metal portions of the bed  200 .  
         [0045]     In another embodiment shown in  FIG. 8 , the bed  200  may be equipped with multiple sensors  400 - 414  in the sensor plane  206 . For example, the first sensor  400  would be placed in the area of the patient&#39;s head, two more sensors  402  and  404  in the area of a patient&#39;s shoulders, yet another sensor  406  in the area of the patient&#39;s buttocks, and finally two more sensors  408  and  410  in the area of the patient&#39;s feet. Entrapment sensors  412  and  414  are also located near the bed rails to provide an indication that the patient has rolled to one side of the bed and has possibly become entrapped in the railing.  
         [0046]     The sensors  400 - 414  are all conductively attached to a charge transfer sensor to form a single sensor plane  206 . The shield plane  204  is similarly divided into portions that correspond to the size and the shape of the sensors  400 - 414 . The result is that one charge transfer sensor  42  is required for each sensor  400 - 414 .  
         [0047]     To provide the ability to monitor an even greater number of sensors, a circuit  30 ″ as shown  FIG. 9  can be implemented. The circuit is identical to the circuit  30 ′ except that a multiplexer  500  is inserted between the output of the charger transfer sensor  204  and a plurality of sensors  206 ,  206 ′ and  206 ″. The multiplexer  500  switches from sensor  206  to sensor  206 ′ to sensor  206 ″, in turn, in order to determine the distance of the relevant portion of the lying person from the sensors  206 ,  206 ′,  206 ″. In this manner, only one circuit  30 ″ is required to poll a multiplicity of sensors  400 - 414 . Because of timing limitations of available charge transfer sensors, a limited number of sensors can be daisy chained. Also, due to stray capacitance issues the number of sensors that can be reasonably multiplexed, a combination of multiplexed and daisy chained sensors may be implemented in order to maximize the number of sensors. Thus, for example, sixty-four sensors may be implemented by arranging the sensors as eight daisy chains of sensors multiplexed to the circuits  30 ″ with each chain having eight sensors  420 , as shown in  FIG. 17 .  
         [0048]     In that regard and referring to  FIG. 10 , an embodiment is shown wherein thirteen sensors  500 - 524  are provided which determine the patient&#39;s immersion within the air cushion and two more sensors  526  and  528  are provided that determine whether the patient has become entrapped in the bed rails. These sensors  500 - 528  may be either daisy chained, attached to their own circuits or multiplexed. Moreover, a combination of daisy chaining sensors and multiplexing sensors may be performed.  
         [0049]     In  FIG. 11 , yet another embodiment is shown wherein the coverage area of the bed is higher, but with fewer sensors  600 - 610 . This arrangement may be more appropriate for monitoring not whether a person is properly immersed, but rather if they are present or absent from their bed. Such an application would be useful in hospitals and nursing homes. Again, these sensors  600 - 610  may be either attached to their own circuits or multiplexed.  
         [0050]     Another application for the present invention defined in the claims is for use as a feedback loop in the auto-inflation or auto-deflation of a cushion for wheelchair. Referring to  FIG. 12 , such an embodiment is shown. Specifically, an output of the microcontroller  32  notifies a valve  700  to change positions to add air, release air or remain closed based upon the inflation status of the cushion  10 . The valve  700  is attached to a source of compressed air  702 , which supplies compressed air when an underinflation status is detected. Likewise, when an overinflation status is detected the valve  700  slowly releases air from the cushion  10  until the proper inflation level is achieved. Similarly, in a low air loss cushion for a hospital bed the circuit may similarly serve as a feedback loop to control mattress inflation, such as by providing feedback to a bed blower control.  
         [0051]     In the embodiments shown above, it is necessary to manually “teach” the microprocessor the extents of the travel by indicating the microprocessor the extents of proximity of the detected object. In that manner, the microprocessor can determine a relative proximity of the detected object within the known range. In the embodiment of  FIG. 13 , the device may comprise a sensor within a sensor.  
         [0052]     In this embodiment, there is provided a first sensor  800  comprising a large area with respect to a second, smaller sensor  802 . In the embodiment of  FIG. 13 , the second, smaller sensor  802  is surrounded by the first, larger sensor  800 . Below the first and second sensors  800  and  802 , and electrically isolated therefrom, is a ground plane  804  and a driven shield The first sensor  800  is made fairly large to anticipate contact points over a surface of interest (for example, the area under a person&#39;s buttocks in a wheelchair cushion application). The large sensor  800  gives a reading of charge transfer that is highly dependant on the size of the individual above the sensor. As a result, without manually setting the range of extents of travel of the person in the wheelchair cushion example, it is difficult to determine the precise proximity of a person of unknown size.  
         [0053]     Merely by way of example, a large person may range between a value of 76 and 120 at the extents of travel of that person&#39;s proximity to the sensor  800 . A small person may range between values of 100 and 150 at their extents of proximity. Therefore, at the closest extent of travel, a large person may show a reading of 76 and the small person may show a reading of 100 making it difficult to determine the proximity of a person of unknown size.  
         [0054]     However, the charge transfer of only the small sensor  802  is not as dependent on the size of the person above of the sensor. This is because the area of the sensor is small in relation to the person above the sensor. Unfortunately, however, the small sensor  802  cannot monitor a large area of interest.  
         [0055]     In the embodiment of  FIG. 13 , the multiplexer or switch  806  ( FIG. 15 ), for example a single pole double throw analog switch such as the FSA3157 available from Fairchild Semiconductor of South Portland, Me., is used to alternately electrically connect the charge transfer sensor  42  to either the small sensor  802  or to both the large sensor  800  and the small sensor  802 . The microcontroller  32  may then read the proximity value of the small sensor  802  and determine, over the small area, the relative proximity of the object above. Next, the large sensor  800  and the small sensor  802  are electrically connected to the charge transfer sensor  42  and the proximity value of the object of interest will be determined. By correlating this value to the value determined by the small sensor  802 , the range of values of proximity for the large sensor  800  and small sensor  802  together can be determined based upon the present value for the small sensor  802 . Alternatively, rather than using the value of the small sensor  802  to correlate with the value of the large sensor  800  and small sensor  802  together, the value of the large sensor  800  alone could be detected and correlated with the value small sensor  802  to obtain a proximity value over only the large sensor&#39;s  800  area.  
         [0056]     Additionally, when sensing the proximity value of the small sensor  802 , it is desirable to electrically connect the large sensor  800  to the ground plane  804 . This is accomplished by using a control line from the microcontroller that controls the switch  806  and connects the peripheral sensor area either ground or part of the sensor. Alternatively, this may also be accomplished by utilizing the frame output of the charge transfer device to make a logic switch after the first reading each time the device is powered up.  
         [0057]     While the embodiment of  FIGS. 13 and 14  is shown having a driven shield and a ground plane, it will be appreciated by one of ordinary skill in the art that an embodiment not having the driven shield may also be implemented without departing from the scope of the present invention.  
         [0058]     Referring to  FIG. 15 , another embodiment of the present invention provides a visual display for graphically representing a relative proximity value for a sensor or group of sensors. In this embodiment, the sensor array and its associated microcontroller  32  of  FIG. 14  (shown in  FIG. 15  as reference numeral  900 ) is electrically connected to a reader device  902 , which comprises a circuit board that provides an interface between the sensors and microcontroller  32  and a display device  904 , which in the preferred embodiment is a computer. The reader device  902  preferably connects to the display device  904  via a USB cable  906 . The display device  904  runs a program which continuously reads the digital value of each sensor in the array, and represents those values graphically. The reader device  902  is not required to be a separate unit. Its functionality could be incorporated into either the sensor circuit or the display device  904 .  
         [0059]     Because the sensors are not calibrated, and because the actual digital value for a particular proximity level is influenced by a number of factors (such as sensor size, shape, and material, and mattress or cushion density and thickness), the display device  904  should provide a method of correlating the actual digital values with proximity levels for each sensor, for each particular system. For example, it can provide a table of maximum and minimum values for each sensor. The maximum value is set to the actual digital value that results from a proximity level of infinity (a body in farthest proximity), and the minimum value is set to the actual value that results from a proximity level of zero (a body in nearest proximity). Then, the digital values within the maximum and minimum range are translated and displayed more meaningfully as proximity values. These values are determined and entered manually, or by way of an auto-range mode in the display device. In this mode, it would monitor the digital values for each sensor, and automatically adjust the table entries as it observes new maximum and minimum values, and as a technician provides appropriate near and far stimulus to each sensor.  
         [0060]     While the invention is described above as separate devices used in conjunction with a hospital bed or wheelchair cover, the devices may be integrally formed with the wheelchair cushion or hospital mattress or with the wheelchair or hospital bed without departing from the scope of the present invention.  
         [0061]     Other applications for the proximity sensor would be as a bed/chair occupancy detector to notify hospital or nursing home attendants as to the presence or absence of the patients from a bed or chair. Similarly, it could serve as a toilet seat occupancy device for notifying when a disabled patient has been left on a toilet seat for too long. Moreover, it may be used for car seat occupancy detection to control air bag deployment in a crash. Another application would be for seat occupancy detection on an airplane.  
         [0062]     There are several veterinary applications for the invention as well. For example, before giving birth horses will lay down in their stall. Horse breeders will typically keep a close eye on a horse about to give birth. In order to ease the burden of checking on the horse, a sensor can be placed in the floor of the stall. When the animal lies down, the breeder would be notified by the circuit to attend to the horse. Additionally, it could be used in horse trailers to monitor the horse.  
         [0063]     It could similarly be used on a person as a geriatric fall monitor. The sensor would be placed on the person&#39;s body and when proximity with the floor was detected, an alarm for help automatically sounded. Possible locations would be on the person&#39;s hip or shoulder.  
         [0064]     Finally, if the conductive layer were placed in close proximity contact with the torso, it could be used to monitor patient vital signs, such as respiration and heartbeat.  
         [0065]     The above examples show that the invention, as defined by the claims, has far ranging application and should not be limited merely to the embodiments shown and described in detail. Instead the invention should be limited only to the explicit words of the claims, and the claims should not be arbitrarily limited to embodiments shown in the specification. The scope of protection is only limited by the scope of the accompanying claims, and the Examiner should examine the claims on that basis.