Abstract:
A sensing system is disclosed that uses at least one conductive plate and associated electronic circuitry to provide an output that is indicative of an object&#39;s position in relation to the at least one conductive plate. The sensing system is provided with a high impedance drive signal that varies as a result of the location of an object relative to the at least one conductive plate. The electronic circuitry receives a high impedance drive signal value as an input and a processor uses the value to calculate a digital output indicative of the object&#39;s position. The high impedance drive signal value is monitored over time enabling the objects position, displacement, pressure, movement, impact and energy to be determined. This data is output to a display and may also be transmitted to a person located remotely from the object being monitored.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation application of U.S. patent application Ser. No. 11/791,208, filed Nov. 2, 2007, now abandoned, which is a national stage application of and claims priority to PCT/NZ05/00309, filed Nov. 22, 2005, which claims priority to New Zealand application 536,762, filed Nov. 22, 2004, which is incorporated by reference herein. 
    
    
     FIELD OF INVENTION 
     This invention relates to a contactless monitor and particularly to a conductive surface coupling an electric field between conductive bodies that provides a system capable of detecting movement, position and pressure of a body. 
     SUMMARY OF THE PRIOR ART 
     A number of systems are known in the art for providing a contactless monitor or alarm condition responsive to the activity of humans or animals. More commonly known forms of monitor or alarm are for use in a hospital environment or home which responds to respiration and apnea and more particularly its application to preventing apnea in small infants and premature babies. Respiration monitors of this type are used in hospitals providing a visible and/or aural indication of when a patient develops abnormal breathing patterns or has stopped breathing. 
     In U.S. Pat. No. 4,033,332 to Cavitron Corporation, a contactless activity and respiration monitor is disclosed which includes a resilient, capacitive pad (a mattress) or a pad used as a mattress, having a capacitor therein which is responsive to the activity or respiration of a patient lying on the pad. Coupled to the capacitor pad is electronic sensing circuitry and an alarm unit that provides an indication and/or alarm when abnormal respiratory rates change or when apnea occurs. This monitor is capable of responding to and distinguishing between small movements of the patient being monitored due to breathing, including apnea, and larger movements of the patient that would naturally cause the breathing rate of a patient to increase and result in false alarms being indicated. This monitor uses three layers of conductive wire mesh electrodes having foam between each layer. The sandwiched construction is then placed either within or under an infant&#39;s mattress, for example. When an infant is lying on the pad the relative motion between electrodes caused by breathing effectively changes the capacitance between the electrodes. The change in capacitance is sensed by the electronic circuitry and if the detected level falls outside a predetermined threshold level a visual and/or aural alarm is used to alert medical staff. The system uses rudimentary integrator and filtering circuits to detect long term changes of capacitance caused by increased breathing rate or apnea. One of the disadvantages of this type of system is that it cannot measure static parameters such as distance, force or pressure, it is susceptible to electrostatic pick up at mains line frequency, can only measure event changes and not constant values such as would occur when a person sits still. A further disadvantage is that the electronic circuitry responds to electrostatic charges generated by movement of the pad cable, unless non-micro phonic cabling was used and as such the circuit requires a drive conductor and a receive conductor. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a contactless monitoring system which goes some way to overcoming the abovementioned disadvantages in the prior art or which will at least provide the industry with a useful choice. 
     Accordingly, in a first aspect the invention consists in an electric field sensing system used to detect movement, position and pressure of an object comprising: 
     at least one conductive plate, 
     a reference input signal, 
     a high impedance drive signal input generated from said reference input signal and connected to said at least one conductive plate producing an electric field around said at least one conductive plate, 
     a processing circuit that receives as an input said reference input signal and said high impedance drive signal, and generates a digital signal output, and 
     wherein a position of said object in said electric field determines a value of said high impedance drive signal and said processing circuit provides said digital signal output indicative of said object&#39;s position in relation to said at least one conductive plate. 
     Preferably, said high impedance drive signal is also a sense signal. 
     Preferably, said processing circuit uses said values in said high impedance drive signal input to calculate at least one of displacement, position and pressure caused by said object. 
     Preferably, said processing circuit uses said values in said high impedance drive signal input over time to calculate at least one of movement, impact or energy caused by said object. 
     Preferably, said digital signal output is input to a display system incorporating an audible alarm. 
     Alternatively, said digital signal output is input to a display system. 
     Alternatively, said digital signal output is input to an audible alarm system. 
     Preferably, said digital signal output is transmitted from said electric field sensing system via at least one of a radio, mobile communications network and the internet to a person located remotely from a user of said electric field sensing system enabling said remotely located person to receive on an electronic device said digital data in real-time. 
     Preferably, said electronic device includes at least one of a radio, mobile telephone, personal digital assistance, internet connection device and computer. 
     Preferably, said at least one conductive plate is constructed from a solid material. 
     Alternatively, said at least one conductive plate is constructed from at least one of a flexible and stretchable material. 
     Preferably, said solid material is a copper plate. 
     Alternatively, said solid material is a carbon impregnated polyethylene pad. 
     Preferably, said at least one conductive plate is coated with a conductive ink such as silver. 
     Alternatively, said at least one conductive plate is coated with a conductive ink such as carbon. 
     Preferably, said flexible material is a conductive membrane. 
     Alternatively, said flexible material is a plurality of conductive fibres. 
     Preferably, said flexible material is adhered to or sewn to a garment or other section of flexible type of material which, in use, does not inhibit the ability of a user to perform a task. 
     In a second aspect the invention consists in a multilayered electric field sensing system used to detect at least one of movement, position and pressure of an object comprising: 
     a plurality of electrically coupled conductive plates forming a layered construction, 
     a plurality of compressible insulating members interleaved with said conductive plates, 
     a reference input signal, 
     a high impedance drive signal input generated from said reference input signal and connected to at least one of said conductive plates producing an electric field between said layered construction, 
     a processing circuit that receives as an input said reference input signal and said high impedance drive signal, and generates a digital signal output, and 
     wherein a position of said object in said electric field determines a value of said high impedance drive signal and said processing circuit provides said digital signal output indicative of a position of said object in relation to said plurality of electrically coupled conductive plates. 
     Preferably, said high impedance drive signal is also a sense signal. 
     Preferably, said conductive plate layered construction is formed by at least an upper conductive plate, a lower conducive plate and a third conductive plate therebetween. 
     Alternatively, said conductive plate layered construction is formed by a plurality of conductive plates, said plurality of conductive plates being an odd number and there are n odd numbered conductive plates and m even numbered conductive plates. 
     Preferably, said upper and lower conductive plates are electrically connected to ground and said third conductive plate is connected to said high impedance drive signal. 
     Preferably, each of said n odd numbered conductive plates are electrically connected to ground and each of said m even numbered conductive plates are connected to said high impedance drive signal. 
     Preferably, said insulation layers are a compressible medium such as high density foam. 
     Alternatively, said insulation layers are a compressible medium such as an elastomer foam material. 
     Preferably, said processing circuit uses said changes in said high impedance drive signal inputs to calculate at least one of displacement, position, pressure, impact and energy caused by said object. 
     Preferably, said digital signal output is input to a display system incorporating an audible alarm. 
     Alternatively, said digital signal output is input to a display system. 
     Alternatively, said digital signal output is input to an audible alarm system. 
     Preferably, said digital signal output is transmitted from said electric field sensing system via at least one of a radio, mobile communications network and internet to a person located remotely from a user of said electric field sensing system enabling said remotely located person to receive on an electronic device said digital data in real-time. 
     Preferably, said electronic device includes at least one of a radio, mobile telephone, personal digital assistance, internet connected device and computer. 
     Preferably, said at least one conductive plate is constructed from a solid material. 
     Alternatively, said at least one conductive plate is constructed from at least one of a flexible and stretchable material. 
     Preferably, said solid material is a copper plate. 
     Alternatively, said solid material is a carbon impregnated polyethylene pad. 
     Preferably, said at least one conductive plate is coated with a conductive ink such as silver. 
     Alternatively, said at least one conductive plate is coated with a conductive ink such as carbon. 
     Preferably, said flexible material is a conductive membrane. 
     Alternatively, said flexible material is a plurality of conductive fibres. 
     Preferably, said flexible material is adhered to or sewn to a garment or other section of flexible type of material which, in use, does not inhibit the ability of a user to perform a task. 
     In a third aspect the invention consists in a double layer electric field sensing system used to detect movement, position and pressure of an object comprising: 
     an electronic circuit used to generate a multiplexed high impedance signal and a multiplexed low impedance inverted drive signal, 
     a first conductive plate energised by said multiplexed high impedance drive signal, 
     a second conductive plate energised by said multiplexed low impedance inverted drive signal, 
     a compressible insulating layer located between said first and second conductive plates, 
     a processing circuit that obtains as an input said multiplexed high impedance drive signal, calculates variations in said multiplexed high impedance drive signal and generates a digital signal output, and 
     wherein said first conductive plate is electrically orthogonal to said second conductive plate generating a matrix of electrically coupled cells whereby a position of said object determines a number of coupling interactions between said electrically coupled cells and said processing circuit measures said positions and provides said digital signal output indicative of said object position. 
     Preferably, said high impedance drive signal is also a sense signal. 
     Preferably, said insulation layers are a compressible medium such as high density foam. 
     Alternatively, said insulation layers are a compressible medium such as an elastomer foam material. 
     Preferably, said processing circuit uses said changes in said multiplexed high impedance drive signal inputs to calculate at least one of displacement, position, pressure, movement, impact or energy caused by said conductive body. 
     Preferably, said digital signal output is input to a display system incorporating an audible alarm. 
     Alternatively, said digital signal output is input to a display system. 
     Alternatively, said digital signal output is input to an audible alarm system. 
     Preferably, said digital signal output is transmitted from said electric field sensing system via at least one of a radio, mobile communications network and the internet to a person located remotely from a user of said electric field sensing system enabling said remotely located person to receive on an electronic device said digital data in real-time. 
     Preferably, said electronic device includes at least one of a radio, mobile telephone, personal digital assistance, internet connected device and computer. 
     Preferably, said at least one conductive plate is constructed from a solid material. 
     Alternatively, said at least one conductive plate is constructed from at least one of a flexible and stretchable material. 
     Preferably, said solid material is a copper plate. 
     Alternatively, said solid material is a carbon impregnated polyethylene pad. 
     Preferably, said at least one conductive plate is coated with a conductive ink such as silver. 
     Alternatively, said at least one conductive plate is coated with a conductive ink such as carbon. 
     Preferably, said flexible material is a conductive membrane. 
     Alternatively, said flexible material is a plurality of conductive fibres. 
     Preferably, said flexible material is adhered to or sewn to a garment or other section of flexible type of material which, in use, does not inhibit the ability of a user to perform a task. In a fourth aspect the present invention consists in a method of monitoring the performance of a user incorporating any one of the conductive plate arrangements and associated sensing system circuit comprising the steps of: 
     placing said conductive plate arrangement and said associated sensing system in close proximity to said user, 
     measuring changes in electrical characteristics between said conductive plates using said associated sensing system when said user moves in relation to said conductive plates, 
     applying a plurality of algorithms to said measured changes to calculate at least one of movement parameters of displacement, force, shear force and pressure, converting said movement parameters into a digital signal output, and 
     outputting said movement parameters to said user. 
     Preferably, said step of measuring said changes in electrical characteristics includes measuring fluctuations in a high impedance drive signal applied to said conductive plate arrangement as a result of said user&#39;s movement in relation to said conductive plate arrangement. 
     Preferably, said step of outputting said movement parameters includes inputting said digital signal output from said sensing system to a display system incorporating an audible alarm. 
     Alternatively, said step of outputting said movement parameters includes inputting said digital signal output from said sensing system to a display system. 
     Alternatively, said step of outputting said movement parameters includes inputting said digital signal output from said sensing system to an audible alarm. 
     Preferably, said step of outputting said movement parameters includes transmitting said digital signal output from said sensing system to a remote electronic device. 
     To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting. 
     This invention consists in the foregoing and also envisages constructions of which the following gives examples. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred forms of the present invention will now be described with reference to the accompanying drawings in which; 
         FIG. 1  is a block diagram of the electric field sensing system of the first preferred embodiment of the present invention, 
         FIGS. 2   a  and  2   b  are waveform diagrams illustrating high impedance drive signal variations measured by the electric field sensing system of  FIG. 1 , 
         FIG. 3  is a block diagram of a second preferred embodiment of the present invention using a multilayered electric field sensing system, 
         FIG. 4  is a side view of a layered electrically coupled conductive plate used with the electric field sensing system of the second embodiment of the present invention, 
         FIG. 5  is a block diagram showing a number of conductive plates as applied to the multilayered electric field sensing system of  FIG. 3 , 
         FIGS. 6   a  and  6   b  illustrate a flexible conductive pad applied to the electric field sensing system of  FIG. 1 , 
         FIG. 7  is a block diagram of a third preferred embodiment of the present invention utilising a double layer electric field sensing system, 
         FIG. 8  is a side view of an N×M flexible sensing pad for insertion into a shoe using the electric field sensing system of  FIG. 1 , 
         FIG. 9  is a block diagram of a digital system capable of providing an aural and visual output to a user that can be applied to any of the preferred embodiments of the present invention, 
         FIGS. 10   a  and  10   b  are side views of a layered electrically coupled conductive plate used with the second and third preferred embodiments of the present invention showing the change in distance between the conductors as a result of a force being applied to the conductors, 
         FIGS. 11   a  and  11   b  are side views of a layered electrically coupled conductive plate used with the second and third preferred embodiments of the present invention showing the effects of a shear force being applied to the conductors, 
         FIGS. 12   a  and  12   b  are side views of a layered electrically coupled conductive plate used with the second and third preferred embodiments of the present invention showing the effects on the conductive plates when a pressure is applied to the conductors, 
         FIG. 13  is a block diagram of a circuit attached to any one of the preferred embodiments of the present invention enabling the data output from the sensing system to be transmitted to a person located remotely from a user. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Whilst there are a number of different monitors available for detecting a human, animal or any other forms of non-insulating object movement and/or for detecting changes in respiratory rate or detecting apnea, the present invention is directed to an electric field sensing system  1  which can be utilised in a broad range of applications, such as; infant respiratory monitoring systems, pressure detection systems as a means of preventing bed sores; in wheel chairs to detect the movement of the occupant and provide a means of determining the risk of the occupant developing sores as a result of being in a sedentary position for a prolonged period of time; placing the sensor in a shoe to detect pressure while running in order to determine false leg follow-through based on early foot strike detection or alternatively the system could be used as a proximity sensor to control the sense of touch for robotic limbs. The contactless monitoring system allows static parameters such as force and pressure to be measured. As illustrated by the above examples the number and types of application to which the present invention can be directed is very broad. 
     Referring to  FIG. 1 , a first preferred embodiment of the electric field sensing system of the present invention is shown that uses a single conductive plate  3  that provides improvements over systems currently available in a number of industries and the medical or sports industries in particular. A conductive plate  3  coupled with a microprocessor based circuit is described which provides a means of measuring position and movement variations applied to the conductive plate  3  such that erroneous or false alarm detections are substantially reduced. A system of this type will provide a means of enhancing personnel safety through the provision of visual and/or audible alarms to which medical staff, for example, will react to knowing that there is a minimal risk of the alarm being false. 
     It will be appreciated that the electric field sensing system  1  as described in the first preferred embodiment of the present invention can be used in a broad range of applications including the medical and sports industries generally but will now be described below with reference to use in the neonatal care environment to monitor the respiratory rate of neonates and in particular for the early detection of apnea (cessation of breathing). It will be appreciated that the present invention can be applied to various applications within the hospital environment, including but not limited to a contactless electric field pad. 
     Electric Field Sensing Circuit 
     The conductive plate  3  is constructed from a conductive material such as copper or carbon impregnated polyethylene and may also be coated with conductive inks such as silver or carbon. Alternatively, the plate  3  may be constructed from a flexible material using one or a number of sandwiched membranes. The circuit which provides the signal input to drive the electric field sensing system  1  is illustrated at  FIG. 1 . In the first preferred embodiment, an alternating current (AC) source  4  generates a sine wave signal (reference signal)  5  of a preselected frequency. To generate a high impedance signal  7  to drive the electric field sensing system  1 , the reference signal  5  is input through a high impedance resistor  6  before being applied to conductive plate  3 . The reference signal  5  is also used as an input clocking signal to an analogue-to-digital converter (ADC)  10 . Hence, as the reference signal  5  and ADC clock signal are in phase, synchronisation of signal peaks and troughs can be measured using the ADC  10 . 
     An electric field  26  is generated around the conductive plate  3  being driven with the high impedance drive signal  7 . When a conductive body or object  2  moves over or near the conductive plate  3 , the electric field  26  between the conductive body  2  and the conductive plate  3  is altered. The movement alters the electric field coupling of the high impedance drive signal  7 . Moving a conductive body or object  2  closer to the conductive plate  3  increases the coupling between the high impedance drive signal  7  and the object  2  thereby attenuating the high impedance drive signal  7 . Hence, the greater the common area between the conductive plate  3  and object  2 , the higher the attenuation of the high impedance drive signal  7  and the subsequent changes in electric field coupling are measured as a voltage by the processing circuitry as shown in  FIG. 1 . An example of changes in voltage as a result of changes in electric field coupling due to the movement of the object  2  in relation to the conductive plate  3  is shown in  FIG. 2 . Measuring the change in high impedance drive signal strength provides a means of measuring the distance between the conductive plate  3  and the object  2  or common area between the conductors  2 ,  3  to be measured. Hence, the high impedance drive signal  7  is also used as the sense signal. 
     With reference to  FIGS. 2   a  and  2   b , the waveform illustrates the changes in the high impedance drive signal due to an object or conductive body  2  moving towards the conductive plate  3 .  FIG. 2   a  shows a sine wave of voltage V c , having a peak-to-peak amplitude V p-p  that corresponds to the high impedance drive signal  7  applied to the conductive plate  3  with no movement detected by the electric field sensing system  1 . When the electric field sensing system  1  has an external force applied or conductive body  2  changes position, the high impedance drive signal  7  amplitude V p-p  is reduced as a result of the electrical coupling between the conductive plate  3  in relation to the conductive body  2 . The change in amplitude V p-p  is shown in  FIG. 2   b . The reduction in high impedance drive signal  7  amplitude V p-p  is measured by the processing circuitry which applies a number of algorithms to convert the signal variations into a meaningful data output to a display system and/or audible alarm system (not shown). Whilst a symmetrical sine wave alternating current is applied to the system as shown in  FIG. 1 , the processing circuitry is capable of performing the necessary calculations and provide a meaningful output when other symmetrical or unsymmetrical waveforms are applied. 
     To increase the sensitivity of the electric field sensing system  1 , the reference signal  5  may be input to a difference amplifier (not shown) along with the high impedance drive signal  7 . The difference between the reference signal  5  and high impedance drive signal  7  can be measured when the separation distance between conductors  2 ,  3  is varied thereby causing the high impedance drive signal voltage to vary as a result of the change in electric field coupling. These voltage variations are buffered  9  and input to the ADC  10  thereby converting the resultant analogue voltage into a digital data output which is input to the microprocessor  11  for further processing. The microprocessor  11  implements a number of algorithms in order to measure sensed voltage signal variations representative of movement and force or pressure variations which correspond to changes in breathing patterns, for example. 
     The microprocessor  11  uses a crystal clock (not shown) to clock the circuit internal digital signals and is also used as a source to drive the conductive plate or sensor plate  3 . This provides an advantage over other systems as the microprocessor  11  inherently knows the frequency of the drive  5  and sense signals  7 . Hence, using software programmes, the microprocessor  11  can phase lock with the sense signal  7 . This allows the ADC  10  to be triggered in phase and in a frequency dependent way thereby allowing synchronous detection of changing events without the use of external devices. 
     Therefore, the sensing system  1  of the first preferred embodiment of the present invention is inherently frequency locked and the ADC  10  synchronously detects the time varying signal, allowing the time varying signal to be referenced to a zero frequency reference. Hence, timing variations due to changes in the timing source over time and changes in environmental conditions are naturally tracked and cancelled. 
     Multilayered Electric Field Sensing System 
     With reference to  FIGS. 3 and 4 , a second preferred embodiment of the present invention is shown which uses at least three conductive plates  15 ,  16 ,  17 , having an insulating layer  18  placed between each conducting plate  15 ,  16 ,  17 , forming a sandwich. The sandwiched conducting plate system coupled with a microprocessor based circuit as described in this embodiment provides a means of measuring position, movement and pressure variations applied to the sandwiched conductive plates such that erroneous or false alarm detections are substantially reduced. The use of a multilayered electric field sensing system  14  provides a means of calculating pressure variations as well as movement and position variations. 
     As illustrated in  FIG. 5 , the multilayered sandwiched conductor plate sensing system  30  need not be limited to three conductive plates.  FIG. 5  shows a number of conductors N 1 , M 1 , N 2 , M 2  . . . M z , N z , having a compressible medium  18  between each conductor of the type provided in  FIGS. 3 and 4 . The conductors N 1  to N z  may be either solid conductors or alternatively, each of the “plates” can be a flexible conductive surface such as a membrane adhered to or sewn into material. As can be seen in  FIG. 5 , each alternative conductive surface (N 1 , N 2  . . . N z ) has been grounded while the interleaved conductive surfaces (M 1 , M 2  . . . M z ) are driven by the high impedance drive signal  7  enabling a sensed signal to be detected as previously detailed in the first preferred embodiments of the present invention. 
     Conductive Plates 
       FIG. 4  shows the multilayered construction of the sensing system  14  having at least three conductive plates  15 ,  16 ,  17  with insulated layers  18  between each plate forming a sandwich type construction. The insulated layers  18  are formed of a compressible material such as high impact foam or other elastomer foam. Each of the conductive plates  15 ,  16 ,  17  are of the same construction as those as described with reference to the first preferred embodiment as shown in  FIG. 1 . The upper and lower conductive plates  15 ,  16  are electrically connected to ground in order to provide a shield to isolate the sensor plate (middle conductive plate)  17  from electromagnetic interference whilst acting as coupling plates, such that movement of these plates  15 ,  16  causes the high impedance drive signal  7  supplied to the sensor plate  17  to vary. 
       FIGS. 6   a  and  6   b  show an alternative type of conductor  40  whereby the conductor  40  is made from a stretchable medium such as a membrane sewn or adhered to material. The conductor  40  has metallic fingers  41 , conductive ink or conductive fibres which are electrically connected to the drive and sensing circuitry of any of the preferred embodiments of the present invention. Hence, when the flexible conductive material  40  is stretched, as shown in  FIG. 6   b , the coupling changes between the metallic fingers  41  thereby causing the distance between adjacent metallic fingers  41  to vary and hence the high impedance drive signal  7  supplied to alternate fingers  41  to vary. 
     Multilayered Electric Sensing Circuit 
     A circuit that provides the signal input to drive the multilayered electric field sensing system  14  is illustrated at  FIG. 3 . This sensing circuit can also be applied to the multilayered electric field sensing system  30  as illustrated in  FIG. 5 . The sensor plate  17  is connected to an alternating current (AC) source  50  which generates a sine wave signal (reference signal)  51  from which a high impedance signal  53  to drive the sensing system  14  is generated when the reference signal  51  is input through a high impedance resistor  52  before being input to sensor plate  17 . The reference signal  51  is also used as an input clocking signal to the analogue-to-digital converter (ADC)  55 . Hence, as the reference signal  51  and ADC clock signal are in phase, synchronisation of signal peaks and troughs can be measured using the ADC  55 . 
     As the sense plate  17  is sandwiched between an electrically grounded upper and lower conductive plate  15 ,  16 , and separated by a compressive medium  18 , changes in the position of the conductive plates  15 ,  16  either horizontally or vertically, alters the electric field coupling of the high impedance drive signal  53 . Moving one or both of the conductors  15 ,  16  closer to the sense plate  17  increases the coupling between the high impedance drive signal  53  and the conductor  17  thereby attenuating the high impedance drive signal  53 . Alternatively, moving the conductors  15 ,  16 ,  17  horizontally in relation to each other changes the common area between the conductors  15 ,  16 ,  17 . This also results in a change in electric field coupling thereby changing the high impedance drive signal  53 . Hence, the greater the common area between conductors  15 ,  16 ,  17 , the higher the attenuation of the high impedance drive signal  53 . The subsequent changes in electric field coupling are measured as a voltage by the processing circuitry  56 . Measuring the change in high impedance drive signal strength provides a means of measuring the distance between two conductors or common area between conductors to be measured. 
     The sensitivity of the multilayered electric field sensing system  14  can be increased in the manner already described with reference to  FIG. 3  using a difference amplifier (not shown) along with the high impedance drive signal  53 . 
     Matrix System Circuit 
     A third preferred embodiment of the present invention is shown in  FIG. 7 . A multiplexed electric field sensing system  19  is provided using two conductive plates  20 ,  21  separated by a compressive medium  18  of the type described with reference to  FIGS. 3 and 5 . A matrix of cells is generated using orthogonal drive signals in conjunction with multiplexed or switched electric fields. The lower conductor (rows)  21  is driven with a multiplexed high impedance drive signal  23  and the upper conductor (columns)  20 , an electrically orthogonal conductive plate, is electrically grounded or driven with a multiplexed low impedance inverted drive signal  22 . Hence, where the upper and lower conductors  20 ,  21  electrically overlap, changes in distance between the two conductors  20 ,  21  causes a change in the amplitude and phase of the high impedance drive signal  24  between the conductive plates  20 ,  21 . This change in high impedance drive signal  24  is converted from an analogue signal to digital data via the ADC  10  and provided as an input to the microprocessor  11 . By selectively driving different orthogonal conductors in the matrix, each cell in the matrix may be measured and position and movement within the matrix can be determined. Furthermore, multiplexing several drive signals in time with multiple orthogonal receiving signals enables an X-Y resolution of force on the conductive plates  20  and  21  to be determined. 
     Pressure and force measurements can be used to determine a person&#39;s gait. The measured data can be used as a feedback mechanism for control loops. These loops can control various pressure plates within a person&#39;s shoe, the follow-through of an artificial leg or control the dispensing of medicine.  FIG. 8  shows an n×m matrix sensing pad whereby a sensing pad is inserted into a user&#39;s shoe which is made up using a number of conductive plates or flexible membranes, (na, nb, nc, . . . nz)×(ma, mb, mc, . . . mz), adhered to the shoe inner. This plate system operates in the same manner as the matrix sensing system as described above and is capable of sensing events in three dimensions. 
     Data Display 
     The calculated data can be output via an RS232 port (not shown) or other known connection ports, to a display system  70  providing a map illustrating the pressure difference, for example, being applied to the electric field sensing system  1 ,  14 ,  19 ,  30  as shown in  FIG. 9 . Alternatively, the display system  70  may provide a user, suchc as medical personnel, with a numerical readout of pressure, force or movement variations as a result of changes in the electric field sensing system  1 ,  14 ,  19 ,  30 . An audible alarm  71  can also be generated providing an aural warning that a patient has been sedentary for an extended duration, or that breathing has ceased for example. 
     Calculations 
     As mentioned previously, the microprocessor  11  applies a number of software algorithms to determine pressure, force, displacement and types of forces applied to and sensed by the conductive plates for each of the embodiments of the present invention.  FIGS. 10   a  to  12   b  illustrate the types of forces on conductor plates  16  and  17  and compressible material  18  that can be detected and measured.  FIGS. 10   a  and  10   b  shows a force being applied to the upper conductive plate  17 . Hence, by way of example, with the multilayered electric field sensing system  14  as shown in  FIG. 3 , as the drive signal frequency and amplitude remain constant, then knowing the force constant and density of the compressible medium  18 , the derived force can be determined by multiplying the displacement of conductive plates  15 ,  16 ,  7  by the compressible medium force constant.  FIGS. 11   a  and  11   b  show the effects of a shear force being applied to the electric field sensing system  14 . The electrical coupling between the plates  16  and  17  can be proportional to the shear forces as well as the orthogonal forces applied to the plates  16 ,  17  enabling shear force to be determined. Also, knowing the area of the sensor plate  17 , the pressure can also be calculated by dividing the calculated force by the sensor plate area. The effects on the conductive plates when a pressure is applied to the conductors are illustrated in  FIGS. 12   a  and  12   b . Other events that may be measured are listed below: 
     a. Energy which is calculated based on force applied over time. 
     b. Impact can be calculated based on a determination of the energy applied over time and the speed of changes of force applied to the system. 
     c. Breathing, diaphragm expansion and heart rate can be calculated based on the expected frequency content of each of these events and the displacement caused by the sensor system. 
     d. Activity based events such as seating position, walking or running gait, kicking a ball or swing a golf club for example, can be determined by adding several of the sensed events over time. 
     It would also be possible to determine for example, a mode of transport or type of physical activity by combining the electric field sensing system  14  with tilt and vibration sensors (not shown). Whilst other more complex calculations are undertaken by the microprocessor  11 , details of these algorithms go beyond the scope of the object of the present invention and are therefore not disclosed. 
     Communications 
     The sensing and processing circuit of each of the preferred embodiments of the present invention can be modified to incorporate the output of sense data to a communications device as shown in  FIG. 13 . The incorporation of a communications device will provide a means of remotely sensing a physical state or method of activity in a sports event, for example. Hence, data can be output to a coach, medical practitioner or other person monitoring the sensed data via a radio, mobile telephone network  61  or alternatively via the Internet. The data is sent over the radio or mobile phone link  62  to the remote person&#39;s radio, mobile phone, personal digital assistant (PDA), Internet connected device, computer or other electronic device  68  capable of receiving radio, Internet and/or mobile phone type signals. Hence, real-time feedback can be obtained by an athlete&#39;s coach, a doctor or other person monitoring the sensing system  60 . Hence, medical compliance and the characteristics of potential medical problems can be logged and transmitted enabling medical personnel, for example, to monitor a person and enable prognosis and/or diagnosis to be undertaken as soon as a problem arises. 
     Furthermore, a user&#39;s position along with sensed data  60  may also be monitored remotely by adding GPS circuitry  63  at the output of the sensing circuit  60 . This type of data would be of great benefit to personnel involved in a search and rescue type of situation particularly if the sensing system user  64  is in difficulty and is known to be a diabetic for example, and as such, the risk of the user going into a coma is greatly reduced.