Capacitive sensor

A capacitive sensor for measuring pressure comprises a fixed charge plate integral to a printed circuit board, a flexible charge plate that is grounded, a conductive donut-shaped adhesive spacer between the charge plates, a lid, a non-conductive donut-shaped adhesive spacer between the second charge plate and the lid, means of providing a pressure, fixed or variable, to both sides of the flexible charge plate, wherein a microcontroller controls a power supply and provides a voltage to the first charge plate wherein the accumulative voltage may be measured as a means of determining differential pressure.

II. CROSS REFERENCE TO RELATED APPLICATION

U.S. Pat. No. 6,220,244, “Conserving device for use in oxygen delivery and therapy”, McLaughlin, is herein incorporated in its entirety by reference.

III. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

IV. REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX

Not Applicable.

V. BACKGROUND OF THE INVENTION

Refer to U.S. Pat. No. 6,220,244, “Conserving device for use in oxygen delivery and therapy”, McLaughlin.

VI. BRIEF SUMMARY OF THE INVENTION

Note: headings provided herein are for convenience and do not necessarily affect the scope or interpretation of the invention.

VIII. DETAILED DESCRIPTION OF THE INVENTION

FIG. 2adepicts a preferred embodiment of the subject invention—a sensor assembly210including a single capacitor with at least one sensing plate. Sensor assembly210is preferably used as the sensing component of a pressure transducer. Pressure transducers have many applications which are well known in the art and related arts.

FIG. 2aspecifically is an exploded, in part, side view of a single capacitive, or capacitor, sensor assembly210—the invention may include the following fixedly stacked components: a plastic lid201; a first adhesive spacer202; a metalized membrane203; a conductive adhesive spacer204; a copper grounding contact205; a PCB206; a copper shielding plate207; a copper sensing plate208; and a non-conductive mask209.

FIG. 4is an exploded perspective view of plastic lid201; adhesive spacer202, metalized membrane203, and adhesive spacer204and includes the preferred location of ports701aand701bin plastic lid201when the subject invention is used with an oxygen delivery system for general aviation. Ports701aand701bare the ports of the corresponding apertures through lid201which enable the introduction of a first pressure, ambient or other, into chamber219aand thereby to top side of the metalized membrane203when the stacked components are assembled. In this application of the invention the ports are preferably tubularly coupled with ambient pressure and are approximately 0.125 inches in diameter.

FIGS. 2aand2bdo not depict apertures701aor701bnor do they depict means to introduce a second pressure to the bottom side of the metalized membrane203—see chamber219b. Aligned aperture through non-conductive mask209, copper sensing plate208, printed circuit board206, and copper shielding plate207enable the introduction of a second pressure to the bottom side of the metalized membrane203via chamber219b. In this application of the invention the port defined by these aligned apertures in copper shielding plate207(again, not shown) is pneumatically coupled with a cannula or face mask. Apertures are sized and placed so as to evenly and timely introduce pressure changes to chambers219aand219band thereby metalized membranes203and252(see chambers279aand279b) and prevent damage to metalized membranes203and252in the event that the pressure, in either chamber (219aor219b), is so great, or the opposite chamber (219bor219a) negative pressure is so great, so as to deflect the membrane203or252into at least one aperture to damage it sufficiently to effect performance—e.g. plastic deformation.

The single capacitor sensor210inFIG. 2ais preferably used when accurate, precise, and timely pressure measurements are needed when the metalized membrane203deflects toward sensing plate208. Dual capacitor sensor250as depicted inFIG. 2bwould be a preferred alternative embodiment when accuracy, precision, and timeliness are needed when metalized membrane203, or inFIG. 2bmetalized membrane252deflects up or down—a true differential pressure sensor. One means of grounding the components inFIG. 2bis depicted.

Regarding the assembly of the single capacitor depicted inFIG. 2a, the first adhesive spacer202is a means for securely fixing the plastic lid201to metalized membrane203wherein the spacer202is preferably square with a round aperture and the first chamber219ais defined therein. Preferably first adhesive spacer202is substantially non-conductive. Preferably the first chamber219ais substantially sealed so the pressure therein may be controlled and accurately measured. The pressure may be a vacuum or preferably (and as described herein) the lid may have an aperture or port, or more than one, which may introduce a pressure—the pressure source may be regulated or controlled, or alternatively may be an unknown and uncontrolled. In the preferred application of the preferred embodiment of the invention two lid ports701aand701bare coupled to ambient pressure as part of a supplemental oxygen conserving delivery system for use in general aviation. The spacer202is preferably substantially non-conductive so as not to affect the charge on the membrane203.

Alternatively the lid may be comprised of a second PCB266. PCB266(or PCB206) may originally include a copper laminate, or copper laminates, which may be etched to form copper sensing plates268and258(or copper sensing plate208), and provide copper shielding plates257and267(or207).

A second copper sensing plate268as illustrated inFIG. 2bwill enable symmetrical sensing which may be a significant improvement for some true differential pressure sensor applications. And an additional copper shielding plate267, which may be integral to the second PCB266, will improve the performance of the dual capacitor sensor as preferably depicted inFIG. 2b.

Shielding plates207,257and267provide electromagnetic shielding so metalized membranes203and252and copper sensing plates208and258and268respectively are electromagnetically isolated so as to improve the performance of the capacitive sensors.

Any of a number of alternative insulating, spacing, and securing means well known in the arts could be employed to achieve the function of spacer202. Alternative means of defining a chamber219aare may include a concave cavity (chamber) on the underside of lid201and alternative means for non-conductively securing the lid201to the membrane203including any of a number of adhesives well known in the art. Alternatively various manufacturing processes could be employed wherein these components and their functions could be combined into different, fewer or even a single part such as a plastic molded top that included the functions of lid201and means to affix to, and insulate from, membrane203.

Preferably, the metalized membrane203is comprised of a flexible aluminized Mylar and is approximately 0.010 inches from the surface of the lid and 0.006 inches from non-conductive mask209). This distance permits the lid201to act as a stop when the membrane experiences a significant pressure (negative pressure from the first pressure source in chamber219aor positive pressure from a second pressure source in chamber219b—see below). The stop prevents the membrane from experiencing excessive excursion which can be damaging, such as plastic deformation or premature fatigue from repeated excessive pressures/loads.

Conductive adhesive spacer204provides a means of grounding the membrane203and securing membrane203to the printed circuit board206and thereby defining chamber219b.

As was the case with adhesive spacer202preferably adhesive spacer204is square with a round aperture therein, but any adequate aperture in the spacers could be equally functional and while it is preferred the spacers have the same dimensions it is not necessary. Alternative means of grounding the membrane203include a separate electrical contact between the membrane203and ground which is independent of the other components inFIG. 2aor wherein membrane203is grounded to copper grounding contact205independent of conductive adhesive spacer204. Preferably membrane203is grounded via spacer204to copper grounding contact205(distinct from substantially insulated from copper sensing plate208) on the PCB206when assembled (or etched there from).

The metalized membrane203is a first charge plate and the copper sensing plate208is a second charge plate of a capacitor. As described herein, printed circuit board206and sensing plate208preferably have apertures which share an axis such that they are coupled to a second pressure source which is introduced to chamber219b. Preferably a non-conductive mask209may be disposed between the copper sensing plate208and the membrane203which will keep the metalized membrane203from shorting in the event it is deflected so as to come in contact with copper sensing plate208.

An alternative embodiment, which does not conceptually depart from the single capacitor sensor depicted inFIG. 2aand described, preferably and alternatively herein, is depicted inFIG. 2b. Preferably this alternative embodiment includes all the components included inFIG. 2bbut it can be appreciated that depending upon the application one skilled in the art could select from the additional components and their functions inFIG. 2bvis-avisFIG. 2aand enable a capacitive sensor. For example, the lid201inFIG. 2amay be replaced with another sensing plate—namely, copper sensing plate268and conductive adhesive spacer251but may not require PCB266or copper shielding plate267or non-conductive mask269.

Alternatively, lid201may simply be replaced with printed circuit board266if the device needs another board—the PCB266could easily provide all the functions as lid201. The non-conductive mask269and copper shielding267are preferred if this alternative is a dual capacitor sensor which requires a second sensing plate to enable the second capacitor—in this case copper sensing plate268. The second sensing plate will provide for two capacitors which is preferred if the application is for a symmetrical differential pressure sensor. Obviously, and consistent with the embodiments described herein apertures in the conductive mask269, copper sensing plate268, copper shielding plate267and printed circuit board266would be necessary to maintain a port so as to introduce a pressure to chamber279a. Introduction of a pressure to chamber279bwould be akin to the chambers219aand219bdepicted inFIG. 2a.

Capacitive sensors depicted inFIGS. 2aand2bare driven by the circuits depicted inFIGS. 1aand1brespectively.FIG. 1adepicts a simple RC circuit101which includes control means preferably a microcontroller102—any of a number of adequate off the shelf controllers are well known in the art including Microchip PIC12C672 or PIC16F676. While the circuit can be driven any number of ways, for example the rise or fall times may vary or the voltage may vary, preferably a digital output102aof microcontroller102is a pulse of 5 volts103(preferably the rise and fall times are 1 microsecond or less), which is applied through resistor104of a known value—preferably 1 M ohm. The resistor limits the current of the applied voltage and may vary based upon principles well-known in the electronic arts. An impedance buffer, preferably an operational amplifier105, tracks the voltage and applies it to the analog-to-digital converter input102bon the microcontroller102wherein the means for measuring the accumulated voltage takes place. The voltage source103and resistance104are of known values. The accumulated voltage across the capacitor for a given amount of time will therefore represent the distance between the charge plates in the single capacitive sensor210. The components are calibrated and the microcontroller is programmed so the value of the capacitor varies with the air pressure placed upon it—thereby rendering a transducer. Preferably, the device is calibrated such that metalized membrane203is an initial distance from fixed charge plate (copper sensing plate)208when the pressures in chambers219aand219bare equal and deflects based upon the differential pressure in said chambers. So the pressure put upon the flexible charge plate (metalized membrane203) can be calculated (by software or firmware or a functional equivalent preferably in or downloaded to the microcontroller102)—from a single pressure source for absolute pressure or two pressure sources for differential pressure.

As depicted inFIG. 1andFIG. 2a, the preferred embodiment of the invention is not a true differential pressure sensor but a sensor for use in an oxygen delivery system wherein precise, accurate and timely data on exhalation is not necessary for desirable oxygen conservation. Deflection of membrane203toward sensing plate208preferably occurs during inspiration or inhalation and deflection toward lid201occurs during expiration or exhalation. Accurate, precise and timely data enables the timely delivery of a bolus of oxygen. As depicted inFIG. 6, the microcontroller output line602represents the conditioned signal from the sensor210for external use—in this case signaling valve assembly605to open valve608which enables a bolus of oxygen to be delivered to the user.

To illustrate a function of the RC circuit101, refer toFIG. 3a. One measurement cycle starts with raising the voltage at SD_R301from zero to a known value—preferably 5 volts. This is followed by measuring the accumulated voltage across the capacitor at three points A/Ds_1, A/Ds_2and A/Ds_3(302a-c)—a single measurement cycle. Sigma305represents the addition of these three voltages and may be used to approximate the area under curve303. Multiple measurements help zero out noise. This is followed with lowering the voltage to zero—see306aand306bfor a time period that allows the capacitive sensor210to discharge to or near zero. Another measurement cycle cannot begin until sufficient time has elapsed for the capacitor to discharge to near zero. The zero point can be calibrated by the microcontroller102to a baseline if fast repetition rates are necessary. In the most preferred embodiment of the subject invention16measurement cycles are made and the values summed and conditioned (including averaging to reduce noise and improve the accuracy of the correlation between accumulated voltage and the pressure on metalized membrane203) to create a value that closely approximates the area under the asymptotic curve303. Other means of measuring the accumulated voltage may be employed—as long as these values are properly calibrated to represent a distance between the metalized membrane203and sensing plate208(which is preferably copper) and therefore a pressure. It should be noted that curve303may not be asymptotic—depending upon the circuit characteristics and the pulse characteristics the accumulated voltage may be linear or some other shape.

FIG. 3billustrates a range of rates of accumulated voltage based upon the processes described inFIG. 3a—see SD_A2inFIG. 3b. Each curve represents a different pressure put on membrane203which will be described in more detail. These values may be compared to other calculated values derived from the accumulated voltage to either determine a differential pressure in a true differential pressure sensor or alternatively if the capacitive sensor210is part of an electronic oxygen conserving delivery system (SeeFIGS. 5a,5band6) as a means for tracking respiration to determine the optimal bolus of oxygen and the timing thereof.

FIG. 7depicts time-voltage curves for a single measurement cycle representative of various points in a respiratory cycle—exhalation801, no breathing802, a small rate of inhalation803, a moderate rate of inhalation804and large rate of inhalation805. The x axis is time in seconds (note exponent)—accordingly16measurement cycles may be made in a fraction of a second.

FIG. 5ais an event timing diagram of a waveform of 2 respiratory cycles with a bolus of oxygen505delivered during the second inspiration event. Other embodiments of this application of the subject invention may deliver gases other than oxygen.

FIG. 5bis time-voltage curve of a single respiratory cycle derived from the amplitudes calculated from multiple measurement cycles.FIG. 5billustrates how the data derived from the accumulated voltage in single capacitor sensor210and described inFIGS. 3a,3b, and7is manipulated to construct the waveform representative of respiration or breathing. The accumulated measurements of voltage inFIG. 7andFIGS. 3aand3b, measured in seconds (note x axis exponent) are averaged and added to construct the wave form inFIG. 5bwherein511represents a state of no breathing,512represents the beginning of an inspiration event (510btrip threshold for breath detection),513represents when an inspiration event may be confirmed and a bolus of oxygen (preferably) is delivered, the area below the baseline (for no breathing)510aand above respiration curve516estimates the total volume of inspiration,514represents expiration and515represents no breathing. Baseline510amay represent zero pressure per calibration of the sensor210, and may change based upon accumulated data from prior respiration events). It should be appreciated that other means of mathematical manipulation of the data derived from the accumulated voltage across sensor210, or alternatively250, may yield the same results if the system or device is properly calibrated.

To elaborate, inFIG. 5a501a-cindicate zero pressure, that is, no inspiration or expiration which means membrane203is neither trending up or down for which it is calibrated.502a-bindicate a negative pressure or inspiration.503a-bindicate positive pressure or expiration.504indicates a triggering event wherein the microcontroller opens the valve608in the valve assembly605for a calculated time interval to provide a bolus505to the cannula or face mask.502b(dotted line) indicates the inspiration superimposed by the bolus505and508indicates the follow-through of that inspiration event.

The bolus delivered to the inspiration tube606may be delivered to a delivery device such as a cannula or face mask. The bolus will vary depending upon the physical characteristics of the delivery device used by the patient or pilot. It should be appreciated that while the subject invention has been described for use in an oxygen delivery system there are many other applications, non-medical and medical for which it could be utilized. In particular the subject invention could be utilized in a respiratory monitoring system to detect, measure, and report respiratory characteristics based on calculated differential air pressures put upon sensor210or alternatively250.

FIG. 1bdepicts two simple RC circuits which drive the dual capacitor sensor250. Microcontroller112serves the same functions as microcontroller102but drives an additional circuit, see digital outputs112band112cand processes additional data, see analog inputs112aand112d. Other devices are depicted inFIG. 1bwhich may enhance the performance of the device such as barometer117, which may be used to determine when a pilot may need supplemental oxygen among other uses. It is well known in the art of aviation that barometers are used to measure pressure altitude. Temperature sensor118may also provide data on ambient temperatures which may be useful in optimizing the performance of the device. The interface transceiver119, LCD120, keypad121, and alert device123may facilitate the use and enhance the performance of the device. The memory device112may store respiration and system data to provide a record for later retrieval which may be used to monitor system performance.

Regarding microcontroller112(or102) any of a number of adequate off the shelf controllers are well known in the art would suffice including Microchip PIC12C672 or PIC16F676. While the circuits depicted inFIGS. 1aand1bmay be driven any number of ways that are well known to those skilled in the art, the preferable means of driving the circuits in the dual capacitor sensor250, see digital outputs112cand112b, is a 5 volt pulse113aand113brespectively, which is alternately applied through resistors114aand114brespectively, which are of a known value—preferably 1 M ohm. The resistor limits the current of the applied voltage and may vary based upon principles well-known in the electronic arts. Impedance buffers, preferably an operational amplifier115aand115b, tracks the voltage and applies it to the analog-to-digital converter inputs112aand112don the microcontroller112wherein the means for measuring the accumulated voltages takes place. The voltage sources and resistances are of known values. The accumulated voltage across the capacitor for a given amount of time will therefore represent the position of metalized membrane252in dual capacitive sensor250. The components are calibrated so the value of the capacitor varies with the net air pressure (see chambers279aand279b) placed upon metalized membrane252, so the pressure put thereon can be calculated (by software or firmware or a functional equivalent preferably in or downloaded to the microcontroller112)—the accuracy and precision of the dual capacitor sensor250is preferably symmetric.

FIG. 6is a schematic of an oxygen delivery system601which conserves oxygen—an implementation of the subject invention. The inspiration sensor210resides on the PCB206. The microcontroller102controls the power source603to provide a voltage103to a charge plate (either the flexible metalized membrane203or the fixed copper sensing plate208but preferably the sensing plate208) in inspiration sensor210. When an inspiration event is detected the microcontroller102sends an output signal604to the valve assembly605which opens valve608and a bolus505is delivered to inspiration tube606. The power source603may simply be at least one off the shelf battery for a lightweight and/or portable oxygen delivery systems preferably operating at 4.2 volts. Alternatively, the power source may be external to the oxygen delivery system such as the typical 12 volt power available in general aviation aircraft. An adapter may be internal or external to the oxygen delivery system.

For an oxygen delivery system, or a respiratory monitoring system, preferably the first pressure source introduced to chamber219ais ambient air and the second pressure source introduced to chamber219bby the user via a respiratory tube606.

The metalized membrane203or252is preferably a metalized Mylar. Due its properties it may be heated to predictably or controllably shrink, which increases the tension in the membrane, which controls the calibration point and may provide a robust and reliable sensor that is easy to make and easy to calibrate and which provides precise measurements in the capacitor210.

An earlier version of the oxygen delivery system601is described in detail in the referenced U.S. Pat. No. 6,220,244 issued to the applicant. Many of the embodiments therein can be implemented into the subject oxygen delivery system including: a plurality of status indicators both visual and audio; power conservation methods and devices; means of measuring altitude to improve sensor performance and oxygen delivery performance—including changing the bolus; compilation of sensor data to more accurately detect the optimal time to deliver the bolus and duration of the bolus; and means of rejecting spurious data.

In regards to the means of detecting barometric pressure to detect changes in altitude, the barometric sensing device107or117may provide an input signal to the microcontroller when a sufficient change is altitude warrants a modification in oxygen delivery to the pilot or patient or indicates that supplemental oxygen must be used per laws and/or regulations.

While the '244 patent had a start drive line and sustain drive line in recognition that the solenoid valve in the valve assembly needed less power to be held open than to initially open, the subject invention saves power by going into pulse width modulation to not only use the least power possible to sustain an open valve but to change the duty cycle depending upon the power available—for example the battery voltage. This provides improved energy conservations.

The disclosed invention has been set forth in the forms of its preferred and alternative embodiments, and described for use in specific applications, but numerous modifications, which do not require independent invention, may be made to the disclosed devices, systems and methods without departing from inventive concepts embodied in the single capacitor sensor210which is disclosed and/or claimed herein.

Specifically, while an application of the subject invention discloses use in an oxygen conserving delivery system and certain embodiments have been directed to a system for pilots it should be assumed aspects of the subject invention and the embodiments thereof are equally applicable to general medicine wherein patients are in need of supplemental oxygen or medical treatment requires careful, accurate and timely respiratory monitoring. Moreover, the improved capacitive sensor may have myriad applications outside of general aviation or medicine.