Abstract:
A conductance sensor uses a high impedance feedback oscillator to drive a single coil at the natural resonant frequency of the coil coupled to a measured material. The amplitude of the driving signal is also feedback controlled. The error signal of the amplitude control yields a measurement of resistance of the measured material. The combination of frequency adjustment and amplitude control eliminates non-resistance impedance effects and amplitude-dependent effects allowing high precision, high sensitivity measurement.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. patent application Ser. No. 08/724,388, filed on Oct. 1, 1996. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a sensor for measuring conductance of material and in particular a sensor that may be used, among other applications, for measuring periodic respiratory and cardiac motion by detecting changes the in conductance of patient tissue. 
     BACKGROUND OF THE INVENTION 
     Conductance Measurement 
     The conductance of a material, that is, how well it conducts electricity, may be determined by applying a voltage to the material with a pair of electrodes and measuring the corresponding current flow. 
     This approach, while straightforward, has several disadvantages. First, it is sensitive to localized high resistances at the interface between the electrodes and the material. Second, the electrodes used to apply the voltage must contact the material raising issues of contamination of the materials and corrosion of the electrodes. 
     For this reason it is known to make non-contacting measurement of conductivity using a pair of electrical coils immersed in the material being measured. The premise behind such systems is that electrical coupling of an alternating current signal between the two coils is functionally related to the conductivity of the material between the coils. Such systems have the disadvantage of requiring multiple coils. 
     A single coil resistivity measuring system (resistivity being the inverse of conductivity) is described in U.S. Pat. No. 4,536,713. In this system, eddy currents are induced in a drilling fluid for an oil well or the like by a magnetic field applied via a ferrite core. The measured eddy currents, as reflected on the driving voltage of the coil, provide a coarse indication of drilling liquid resistivity. 
     Respiration Monitoring 
     Electronic monitoring of respiration may be performed for critically ill or comatose patients, small children at risk of sudden infant death (“SID”), or patients with sleep apneas (partial suspension of breathing sometimes associated with insomnia). 
     There are two types of sleep apnea. Obstructive apnea, is caused by blockage of the windpipe or mouth and nose, and is characterized by a change in the phase relationship between respiratory movement of the chest and abdomen. In central apnea, the patient ceases all signs of breathing. 
     Distinguishing central apnea from obstructive apnea requires independent detection of abdominal and thoracic respiratory motion. 
     A number of different types of respiration sensors are known in the art, including mechanical sensors employing air flow measuring turbines or the like, that require the use of a face mask or mouthpiece, and inflatable cuffs which measure chest or abdomen extension manifest as a change in cuff pressure. Such mechanical systems can suffer from problems of reliability. 
     Respiratory motion may be also detected electronically by conductive coils wrapped around the patient. In one prior art device, multiple inductive coils are positioned on the patient to move with respect to each other as the patient breathes. The mutual inductance between the coils changes with their relative motion and is measured to detect respiratory motion. 
     In a somewhat different design, respiratory deformation of a single coil causes a change of inductance in that coil which may be measured by the detuning of an oscillator using the coil&#39;s inductance as a tuning element. The change in frequency of the oscillator is used as a measure of respiratory motion. 
     With these electrical sensors, the sensing coil must be snugly fit to the patient so as to move with the patient and for this reason may create an unpleasant confining sensation or pressure on the patient&#39;s trunk and stomach. Such constraining pressure is particularly problematic with small infants. Such electronic sensors are unsuitable for measuring cardiac motion which does not result in significant distention of the patient&#39;s chest or abdomen. 
     SUMMARY OF THE INVENTION 
     The present invention provides a single-coil non-contacting conductance sensor that provides improved accuracy in conductance measurement. By driving the single coil at its resonant frequency (as coupled to the measured material) capacitive and inductive effects are minimized making the eddy current measurement more robust against the influence of the geometry of the measurement and qualities of the material other than its conductance. 
     Specifically, the sensor includes a conductive coil driven by an auto-tuning oscillator. The auto-tuning oscillator seeks a resonant frequency of the conductance coil as coupled to a measured material and its environment to induce eddy currents in the measured material. An impedance measuring circuit is connected to the conductive coil providing a measure of the effective resistance of the coil at the resonant frequency as reflects the magnitude of the induced eddy currents within the sample. 
     Thus, it is another object of the invention to provide a simplified conductance measuring sensor that is resistant to inductive and capacitive effects. By driving the coil at its natural resonant frequency, the resistive effects to be measured dominate. 
     The auto-tuning oscillator may include a gain controlled amplifier having a gain control input signal reflecting the amplitude of the signal driving the coil and the impedance measuring circuit may derive resistance from the gain control signal. 
     Thus it is another object of the invention to provide a conductance sensor that is resistant to amplitude dependent effects in the conductance measuring process. 
     The gain control signal may be provided by a synchronous rectifier, rectifying an oscillator signal from the oscillating current synchronously with the oscillating current. The synchronous rectifier may be a multiplier multiplying the oscillator signal by itself. 
     Thus it is another object of the invention to provide a conductance sensor that is resistant to asynchronous noise sources. The synchronous rectifier is not switched by larger amplitude out of phase or different frequency signals which will then be averaged out in a subsequent filtering stage. 
     One application of the conductance sensor is electronic monitoring of respiratory (or cardiac) motion which relies on the measurement of changes in conductivity in the patient&#39;s tissue within a limited cross-sectional area. Because the measurement does not rely on changes in the coil&#39;s dimensions, the sensor may be less constraining to the patient and more robust against deformation of the coil in use. 
     Specifically, the conductive coil is sized to fit about the body of a person. The auto-tuning oscillator, connected to the conductive coil, drives the coil with an oscillating current at a resonant frequency to induce eddy currents in the person. Attached to the conductive coil is the impedance measuring circuit which provides a measure of the effective resistance of the coil at the first frequency as reflects the magnitude of the induced eddy currents within the person. An electronic filter receiving this impedance signal extracts a periodic plethysmographic signal. 
     Thus, it is one object of the invention to provide a method of measuring periodic physiological motion, such as cardiac or respiratory motion, by detecting changes in areal conductance of patient tissue within a coil without the need to deform the coil in time with the physiological motion. Because the invention does not require outward distention of the patient, it is equally effective to measure periodic internal conductance changes, for example those caused by the heartbeat, as with those conductance changes which are reflected in outward movement of the patient, such as breathing. 
     The patient monitor may include two coils sized to fit about the body of the person, one in the abdominal area of the person and one in the thoracic area of the patient. Here, the impedance measuring circuit provides measures of the effective resistance of the two coils separately. An alarm may compare these separate resistance measurements to determine when distress of the person is indicated. 
     Thus, it is another object of the invention to provide an electronic respiratory detector that can provide separate measures of two different regions of the body in proximity to each other. Because only a single coil is needed to measure conductivity, multiple measurements may be made by adjacent multiple coils. 
     The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims herein for interpreting the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a belt incorporating the present invention and shown in position on an infant; 
     FIG. 2 is a perspective view of the belt of FIG. 1 when open showing the localization of the magnetic fields from two coils in the abdominal and thoracic region; 
     FIG. 3 is a plot of magnetic field strength versus distance along line  3 — 3  of FIG. 2 when the belt is closed; 
     FIG. 4 is a schematic diagram of a circuit used to drive the coils of FIG.  2  and to provide a conductance measurement of the region of the magnetic fields shown in FIG. 2; 
     FIG. 5 is a block diagram showing subsequent processing of the conductance measurement; and 
     FIG. 6 is a perspective view of a general-purpose conductance sensor employing the principle of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Belt Construction and Loop Placement 
     Referring to FIG. 1, the present invention may be incorporated into a belt  10  for wrapping about the trunk of a patient  12 , such as an infant, with the upper edge of the belt being positioned about the patient&#39;s thoracic region and the lower edge of the belt being positioned about the patient&#39;s abdomen. 
     Referring also to FIG. 2, the belt  10  may include an inner fabric sleeve  12  providing electrical insulation and cushioning to the patient  12  against various rigid portions of the belt  10  as will be described. 
     Attached to the outside of the sleeve  12  near the superior and inferior edges of the belt  10  are first and second flexible electrical conductors  14  and  16  which open into a C-shaped form when the belt  10  is opened. Connectors  18 , attached to one end of each of conductors  14  and  16 , physically join together the ends of conductors  14  and  16 , to form two loops, one about the thoracic and one about the abdominal regions of the patient  12  when the belt  10  is closed, as shown in FIG.  1 . The opposing ends of each of the conductors  14  and  16  remain electrically isolated so that an oscillating electrical signal may be applied across the ends of conductor  14  and conductor  16 . 
     The conductors  14  and  16  may be flat metal springs of a conductive material such as brass so as to open and close without permanent deformation. 
     An outer insulating fabric sleeve  22  covers the inner sleeve  12 , sandwiching the conductors  14  and  16  between the inner sleeve  12  and the outer sleeve  22 . Medial edges  25  of the outer sleeve  22  support opposite halves of a strip fastener  24 , such as Velcro, so that the medial edges  25  may be removably attached to one another to close the belt  10  about the patient  12  and to resist outward forces which might serve to disconnect conductors  14  and  16  from connectors  18 . 
     Additional circuitry  20  which includes two oscillators, to be described below, and a low powered radio transmitter to transmit signals from the patient  12 , and a battery is connected to each of the connectors  18  and thus to both ends of each conductors  14  and  16  by wires  26  running along the inner surface of the outer sleeve  22  and passing through the outer sleeve  22  to the circuitry  20 . 
     Conductivity Measurement with Eddy Currents 
     Referring now also to FIG. 3, when the conductors  14  and  16  are formed into a loop with a closing of the belt about the patient  12 , alternating current passing through the conductors  14  and  16  generates oscillating magnetic fields  26  within the patient  12 . FIG. 3 shows the amplitude of magnetic fields along a transverse plane through the patient caused by current flow through conductors  14  and  16 . 
     As shown in FIG. 3, the amplitude of the magnetic field A is concentrated within the patient  12  and of relatively low magnitude outside of the patient, the boundaries of which are indicated by vertical lines  30 . The strength of the magnetic field falls off rapidly in a direction perpendicular to the plane of the coils of the conductors  14  and  16  and thus interference in the measurements obtained by the coils  14  and  16  is low. 
     The oscillating magnetic fields  26  will induce eddy currents within the patient  12 , the magnitude of which are proportional to the conductivity of the tissue. For example, if the tissue of the patient  12  were perfectly conductive, the eddy currents would be sufficient to generate a magnetic field (back EMF) opposing the magnetic field  26  to perfectly cancel that magnetic field  26 . On the other hand, to the extent that the tissue of the patient  12  is not perfectly conductive, the eddy fields will be lower, and the magnetic field  26  will be reduced but not canceled. Thus, a measurement of the back EMF created by the eddy currents within the patient provides an indication of the conductivity of the patient tissue. 
     Referring now to FIG. 4, the effective circuit of a conductor  14  or  16  in proximity to the patient  12  may be modeled as a first inductor L 1  (comprised of either conductor  14  or conductor  16 ), inductively coupled (as if in a transformer) to a second inductor L 2 , connected in series with a resistance R 2 . The inductor L 2  and resistance R 2  are not discrete elements, but are the equivalent lumped values incorporating the distributed inductance and resistance of many looping current paths of the eddy currents through tissue of the patient  12 . Generally, however, R 2  will reflect a total conductivity (1/R) in the region within a given loop  14  and  16 . 
     The Relationship Between Conductivity And Physiological Motion 
     This resistance R 2  in turn will depend generally on the real resistance, (W/cm 2 ), of the tissue in the area enclosed by the loop  14  or  16  times the area enclosed by the loop  14  or  16 . For this reason, changes in the cross-sectional area of the patient  12  within either loop  14  or  16  will produce changes in the value of R 2  measured with that loop  14  or  16 . 
     Further, changes in the cross-sectional area of an organ within the patient  12  and within the plane of a loop  14  or  16 , and having a noticeably different areal resistance than the surrounding tissue will also be reflected in a change in the value of R 2 . For this reason, the value of R 2  will show both respiratory motion in which the patient&#39;s overall cross-sectional area changes, and cardiac motion in which a relative area of highly conductive blood changes within the patient without significant overall change in the overall patient cross-sectional area. 
     Resistance Measuring Circuitry 
     The value of R 2  is detectable as a decrease in the quality factor of an oscillator employing conductor  14  or  16  as an inductive component of an oscillator tank circuit. 
     The oscillator is formed from the parallel combination of inductance :L 1  being one of conductor  16  or  14 , and a capacitor C 1  contained within the electronics circuitry  20 . In a preferred embodiment, C 1  is selected to tune the combination of L 1  and C 1  into parallel resonance at approximately 4.5 MHz. For conductors  14  and  16  formed into a loop having a diameter of approximately 100 millimeters, C 1  will be a capacitor of 330 pF. 
     As is known in the art, the quality factor of an oscillator provides a measure generally indicating how long an oscillator would continue to oscillate without the input of additional energy (free oscillation). Without eddy currents, the oscillator formed of conductors  14  or  16  would be expected to oscillate for a time limited only by the intrinsic resistance of the conductors  14  and  16  and the other components forming the oscillating resonant circuit. With eddy currents, the resulting back EMF adds an effective power dissipating resistance to the oscillator, shortening the time of free oscillation. Thus, a measure of the quality factor of oscillators formed from conductors  14  and  16  provides an indication of the conductivity of the patient  12 . 
     The measurement of quality factor of a resonant circuit is well known in the art. However, in the present invention an extremely precise measurement is required and the measurement must be robust in the environment of a moving patient and in a situation where the value of L 1  may change from measurement-to-measurement based on some inadvertent deformation of the conductors  14  and  16 , and in variations of attaching the belt  10  to the patient  12 . 
     In order to improve the quality factor measurement, the measurement is made at the resonant frequency of the parallel combination of C 1  and L 1 . In fact, the oscillator driving the loops  14  and  16  effectively tracks to the resonant frequency of the parallel combination of C 1  and L 1  as will be described. Thus the effect of variations in the resonant frequency of L 1  and C 1  with respect to the oscillator frequency, such as might effect the quality factor measurement, is reduced. It will be further understood that my driving the coil at resonant frequency, undesired capacitive and inductive influences on the measurement are reduced because the inductive components of the detected signal will cancel the capacitive components of that signal. 
     In addition, the amplitude of the oscillator driving the parallel combination of L 1  and C 1  is precisely controlled to a constant value. Thus, the effect of amplitude on the quality factor measurement is eliminated and apparent changes in quality factors such as might be caused by a slight detuning of the oscillator with respect to the inductive parallel combination of L 1  and C 1 , are reduced. 
     In a preferred embodiment, an oscillator providing the desired feature of tracking the resonance of C 1  and L 1  and providing a constant amplitude oscillation is provided through the use of an operational transconductance amplifier (OTA)  32  connected at its output  38  to one junction between capacitor C 1  and inductor L 1 . The remaining junction between C 1  and L 1  is connected to ground. 
     As is understood in the art, an operational transconductance amplifier provides a current at its output  38  that may be modeled as a gain factor G m  times the voltage across inverting and non-inverting inputs  36  (indicated by a minus and plus sign, respectively) of the operational transconductance amplifier  32 . The value G m  is determined by an amplifier bias current I abc . 
     The output  38  of the OTA  32  is also connected to the non-inverting input  36  of the OTA  32 . In this positive feedback configuration, the output current of the OTA  32  will naturally oscillate at the resonant frequency of the parallel combination of C 1  and L 1  to produce an oscillator signal  41 . 
     This oscillator signal  41  on output  38  is also received by an amplitude detector  40 , which is a precision synchronous rectifier and low-pass filter providing a DC voltage proportional to the amplitude of the oscillator signal  41  at output  38 . The synchronous rectifier is realized in the preferred embodiment by a multiplier that accepts at its two factor inputs the output  38 . Any noise signal on output  38  that is asynchronous with the oscillator signal  41  will average to zero in the low pass filter. The output of the amplitude detector  42  is received by an inverting input of a standard high-gain operational amplifier  42 , whose positive input receives a precision reference voltage  44  designated as V r . 
     The amplifier  42  operates open-loop, and hence it will be understood that if the voltage on the inverting input of the amplifier  42  is greater than V r , the output of amplifier  42  will be a negative value. On the other hand, if the voltage on the inverting input of the amplifier  42  is negative with respect to V r , the output of amplifier  42  will be positive. The output of the amplifier  42 , termed V out , is applied to limiting resistor  43  to become amplifier bias current I abc . 
     The connection of the output of the amplifier  40  to the OTA  32  provides feedback control of the amplitude of the oscillator signal  41  to the value of V r . As connected in this manner, the value of V out  being the amplitude error signal indicates generally how much additional energy must be input into the oscillator of C 1  and L 1  to maintain oscillation at the desired amplitude Of V r  and thus provides a measure of the quality factor of the oscillator of C 1  and L 1  and hence a measure of R 2 . Generally, the higher the value of V out , the less the value of R 2 . 
     Referring now to FIG. 5, the circuit of FIG. 4 is repeated once for each of the conductors  14  and  16  and hence two values of V out  will be obtained, each indicating the conductivity of the patient within the area encircled by respective loops  14  and  16 . These two signals distinguished as V out2  (referring to the signal from coil  16  and from the abdomen) and V out1  (referring to the signal from coil  14  and from the thoracic region), are each received by separate low-pass filters  50  and  52 , respectively, which are tuned to reject the fundamental frequency of the oscillator signal  42  driving the loops  14  and  16 , but to pass lower frequency signals caused by a slowly varying change in the value of R 2  with respiration or cardiac motion. In the following discussion, it will be assumed that the frequency of the low-pass filters  50  and  52  is adjusted to extract respiratory motion such as may be needed to diagnose sleep apnea. 
     The signals from the low-pass filters  50  and  52  are then provided to a low-powered radio transmitter  54  contained within the circuitry  20  shown with respect to FIG. 2. A nearby receiver  56  receiving this signal extracts two respiratory signals. The transmitter and receiver circuits  54  and  56  are constructed according to techniques wellknown in the art, and may be a simple low-power FM link with a different frequency assigned to each of the different respiratory signals. 
     The receiver  54  provides the respiratory signals to a detector and alarm circuit  58 , which is programmed to detect patient distress. The detector and alarm circuit  58  may provide an alarm indicating a loss of respiratory signal extending for more than a certain period of time. Alternatively, or in addition, the alarm and detector circuit  58  may detect a loss of phase coherence between the signals V out1  and V out2  such as may indicate obstructive apnea as described before. The detector and alarm  58 , in a preferred embodiment, are a microprocessor programmed to receive digitized samples of the respiratory signals and to monitor them according to the criteria described above or other criteria that may be selected by a physician. 
     It will be recognized that this approach of detecting cardiac and respiratory signals through a measurement of changes in conductivity does not require that the loops  14  and  16  be deformed or flexed with motion of the patient  12 , and in fact, the circuit would work if loops  14  and  16  were perfectly rigid and unyielding. As a result, the patient  12  is not unduly compressed about the torso by pressure by the belt  10  other than that minimally necessary to hold the belt  10  in position. 
     Referring now to FIG. 6, the above-described circuit may also find use in measuring the conductivity of an arbitrary material, such as a liquid  58  passing through a pipe  60 . Here a single conductor  14  is used passing in one or more loops about the circumference of the tube  60  so as to again generate an internal oscillating magnetic field (not shown) inside the tube  60 . The back EMF of the eddy currents so induced is detected by measuring the quality factor of an oscillator formed of the conductor and a parallel capacitor (not shown) as described above. 
     Unlike other conductance sensors in the prior art, no direct contact is required between the liquid  58  and one or more electrodes of the circuit, nor is it necessary that the loop  14  be immersed in the liquid  58  such as may produce clogging or unnecessary turbulence, or that the loop  14  be used in conjunction with a second loop  16 . 
     The measurement of conductivity in this manner may provide an indication of the composition of the liquid  58  and may detect inclusions such as air bubbles or the like. 
     The liquid  58  need not fill the tube  60 , however. To the extent that the liquid  58  occupies only a portion of the cross-sectional area excited by the coil  14 , the conductivity will look proportionately lower. 
     Many other modifications and variations of the preferred embodiment which will still be within the spirit and scope of the invention will be apparent to those with ordinary skill in the art. For example, the conductance probe may be used over a large volume and thus as a means of detecting the presence of people in a room. On a smaller scale the conductance sensor may be used to detect nearby high or low conductance materials including metal and thus function as a proximity switch. In order to apprise the public of the various embodiments that may fall within the scope of the invention, the following claims are made.