Relative humidity and condensation measurement with a capacitive humidity sensor

A method and apparatus for measuring relative humidity including condensing environmental conditions using a circuit with a capacitive humidity sensor and a reference resistor each connected to an input of a switch device and thence a quadrature sampling circuit. A sinusoidal source is first connected to the reference resistor and secondly to the capacitive humidity while a signal ground is first connected to the capacitive humidity sensor and secondly to the reference resistor. This produces a first voltage and a second voltage that are each sampled in quadrature. A complex ratio of the sampled voltages is calculated and converted into a representation of relative humidity.

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BACKGROUND

Prior Art

Capacitive humidity sensors are widely used for the determination of relative humidity as their capacitance changes nearly linearly with changes in relative humidity, they exhibit small hysteresis, have low temperature coefficients, and are highly reliable. Capacitive humidity sensors such as the JLC International HC103/HC104, GE Panametrics MiniCap-2, Rotronic CS 30, and Humirel 2030 are excellent examples of capacitive humidity sensors that are readily available in the market at the present time. Typically, the capacitances of these sensors range from 140 pF (picofarads) to 380 pF. Typical sensitivities (slope of capacitance versus relative humidity) range from 0.3 pF per % RH (percent relative humidity) to 0.5 pF per % RH. The temperature coefficients of these sensors are generally small, with values of up to 0.01 pF change per degree Celsius.

Capacitive humidity sensors generally require electronic circuitry, components, and devices to translate their capacitances to a convenient, readable, or usable form. Such electronic circuitry, components, and devices translate sensor capacitance to voltages, currents, frequency, and/or time. By means well known in the art, said voltages, currents, frequency, or time are then usually converted to a more convenient human readable form such as digital data or a number on a display which is representative of a relative humidity measured by the sensor. Within the scope of the present invention, “measurement circuit” and “measurement circuitry” shall be used to represent the electronic circuitry, components, and devices which provide a means for the measurement of voltages, currents, frequency, or time, and conversion of said voltages, currents, frequency, or time to a more convenient human readable form such as digital data or a number on a display. In addition, the terms “humidity sensor” and “humidity sensors” shall be used to represent one or a plurality of capacitive type humidity sensors.

Some applications of humidity sensors require accuracy approaching 0.1% RH. Resolving 0.1% RH with a typical humidity sensor, requires measuring its capacitance to an accuracy of 30 fF (femptofarads) to 50 fF, or approximately 1 part in 10,000. At this level of performance, not only must measurement circuitry be capable of an accuracy of 1 part in 10,000 in capacitance, but parasitic capacitance and parasitic conductance arising from a sensor's measurement circuitry needs to be known and stable to 1 part in 10,000 or better. In addition, this level of performance must be maintained over the full range of operating temperatures and relative humidity's encountered.

A practical alternating current circuit model for a humidity sensor, operated at a frequency f, consists of two components: an ideal capacitor, with capacitance Cxand susceptance Bx=2πfCx, connected in parallel with a frequency dependent resistance Rxand its conductance Gx=1/Rx. The parallel conductance Gxaccounts for both the capacitive dielectric loss and the current leakage of the humidity sensor. In complex notation, the admittance Yxfor the humidity sensor is:
Yx=Gx+jBx

The measurement circuitry of a humidity sensor may add a parasitic admittance. This parasitic admittance Yp, consists of a parasitic susceptance Bpand a parasitic conductance Gpboth of which act in parallel with the humidity sensor. In complex notation, the parasitic admittance Ypis:
Yp=Gp+jBp

The parasitic susceptance Bpadds to the humidity sensor's apparent susceptance for a total susceptance of Bx+Bp. The parasitic conductance Gpadds to the humidity sensor's apparent conductance for a total conductance of Gx+Gp. Parasitic susceptance and parasitic conductance can thereby lead to measurement errors in determining relative humidity. Not only does parasitic admittance create a fixed error in measurement of relative humidity, but it often varies undesirably and unpredictably with changes in environmental conditions both during manufacturing and during use, making the accurate determination of relative humidity difficult.

Environmental conditions can often affect the operation of measurement circuitry causing changes in: device admittances, parasitic admittances, frequencies, currents, offset currents, leakage currents, voltages, threshold voltages, offset voltages, component values, temperature coefficients and overall measurement circuit gains. These environmental conditions can include dust, chemical vapor(s), water vapor, water condensation, air currents, and temperature, or combinations thereof. For example, under condensing environmental conditions, liquid water on surfaces of a humidity sensor and its measurement circuitry can mix with surface dust and other surface compounds. This then can lead to a large increase in total conductance and susceptance, thereby causing large measurement errors, and even stopping measurement circuit operation altogether. Such undesirable and unpredictable changes present demanding measurement challenges that are not taught in the prior art.

Early exemplary prior art is found in U.S. Pat. Nos. 4,295,090 and 4,295,091 wherein is taught the use of an integrator comprising a humidity sensor as an ideal capacitance connected as a feedback element between an amplifier's input and output, and a resistor connected to the amplifier's input. With the humidity sensor as the feedback capacitance, the amplifier is less sensitive to parasitic capacitances at its input and output. In operation, the integrator repeatedly charges and discharges the humidity sensor. The output of the integrator connects to an input of a comparator having a threshold voltage. When the output of the integrator alternately crosses the threshold voltage of the comparator, the comparator alternately changes its output between high and low voltage. The output of the comparator is then fed back to drive the integrator charge and discharge cycles. The frequency of the signal at the output of the comparator is thereby dependent on the capacitance of the humidity sensor. U.S. Pat. Nos. 4,295,090 and 4,295,091, however, do not account for changes in comparator threshold voltage, offset voltages, offset currents, or for changes in high and low output voltages of the comparator. Changes in these parameters can cause undesired changes in the charge and discharge of the capacitive humidity sensor, undesired changes in the oscillation frequency, and thereby erroneous indications of changes in relative humidity. In addition, under condensing conditions, values of capacitance and conductance for a humidity sensor can increase by orders of magnitude, causing failure of the charge or discharge of the integrator to reach threshold voltage, halting oscillation.

Additional exemplary prior art is found in U.S. Pat. Nos. 4,636,714, and 6,647,782, and 6,888,358, and 7,032,448 wherein humidity sensors are taught as ideal capacitors incorporated into a switched capacitor circuit. The circuits comprise an amplifier with a reference capacitor CRand a parallel connected switch as feedback elements. Not taught or anticipated by these patents are offset voltage errors that arise from operation of the feedback switch, which exhibits different offsets between its closed and open states. When the feedback switch is closed a voltage appears at the amplifier's output equal to its input offset voltage, Vosclosed. When the feedback switch is open, another offset voltage Vosopenarises due to the feedback capacitor CRand due to all capacitances connected to the inverting input of the amplifier. These include a capacitive humidity sensor with capacitance Cxand conductance Gx, other parasitic capacitances Cp, and parasitic conductances Gp. Vosopenis given by:

Vosopen=Vosclosed+Vosclosed⁡(Cx+CpCR+Gx+GpCR⁢t).
An elapsed time t is determined upon opening of the feedback switch. In addition, an input bias current Ibto the amplifier causes an additional offset voltage VosIbthat can be approximated as follows:

VosIb≈Ib⁢tCR.
In U.S. Pat. Nos. 6,647,782 and 7,032,448, no offset correction is taught or anticipated for Vosclosedor for Vosopen. In U.S. Pat. Nos. 4,636,714, and 6,647,782, 6,888,358, and 7,032,448 an offset correction is taught for Vosclosedwithout anticipation of additional offset errors, namely:

Vosclosed⁡(Cx+CpCR+Gx+GpCR⁢t)+Ib⁢tCR
In addition, none of these patents anticipate condensing conditions, where values of humidity sensor capacitance and conductance increase by orders of magnitude, thereby causing offset changes during switching that can lead to erroneous, even meaningless humidity indications.

Additional exemplary prior art is found in a paper published in Measurement Science Technology, Vol. 9, 1998, pages 510-517, by Kolle and O'Leary entitled “Low-cost, high-precision measurement system for capacitive sensors”. Kolle and O'Leary teach a circuit and method for measuring humidity using a humidity sensor and a current-to-voltage converter with a reference resistor, whereby the humidity sensor's capacitance and its conductance are measured using quadrature detection methods. These measurements are less sensitive to variations in offsets, parasitic admittance, reference signal source amplitude, and circuit gain.

In particular, Kolle and O'Leary teach a two part quadrature modulation to obtain two quadrature signals whereby one quadrature signal is subtracted from the other to remove offset voltage. In addition, the signal input of a current-to-voltage converter is periodically switched between a reference resistor and a humidity sensor as a means for an auto-calibration. A ratio is calculated between the response when the humidity sensor is connected and the response when the reference resistor is connected. This ratio ideally cancels out circuit gain and its variations, and signal source amplitude and its variations from the determination of humidity sensor capacitance. However, Kolle and O'Leary do not account for the loop gain difference between when the humidity sensor or when the reference resistor is connected to the input of the current-to-voltage converter. Loop gain, well known in the prior art of feedback control, includes a feedback factor β which depends on the ratio of feedback admittance to the sum of feedback admittance and current-to-voltage converter input admittance.

With the humidity sensor connected, the total input admittance is the sum of the admittance of the humidity sensor Yx, the parasitic admittance of the circuit Yp, the admittance of the current-to-voltage converter input Ya, and the total feedback admittance Yf. In this case, the feedback factor is given by:

On the other hand, with the reference resistor connected, the total input admittance is the sum of the reference resistor's conductance Gr, the parasitic admittance of the circuit Yp, the admittance of the current-to-voltage converter input Ya, and the total feedback admittance Yf. In this case, the feedback factor is given by:

As the loop gain of the circuit depends on the feedback factor β, the loop gain differs depending on whether the humidity sensor or the reference resistor is connected to the current-to-voltage converter. The ratio computed by Kolle et al, therefore, does not lead to complete correction for a) the parasitic admittance of the circuit, b) the reference signal source amplitude, c) the overall circuit gain, or d) for their variations. In addition, under condensing environmental conditions, the conductance of the humidity sensor can increase dramatically, causing the gain of the current-to-voltage converter coupled to the humidity sensor to increase by many orders of magnitude. This results in distortion or severe limiting of the current-to-voltage converter's output signal, or in unwanted oscillation of the current-to-voltage converter, thereby leading to grossly erroneous indications of relative humidity.

SUMMARY

An objective of the present invention is to overcome the disadvantages of prior art to assure accurate measurement of relative humidity when using a capacitive humidity sensor.

A second objective of the present invention is to overcome the disadvantages of prior art to assure continued, sensible and reproducible indications under condensing conditions when using a capacitive humidity sensor.

A third objective of the present invention is to overcome the disadvantages of prior art to assure sensible and continuous indications during the transition from non-condensing to condensing conditions when using a capacitive humidity sensor.

These objectives are advantageously attained by an embodiment comprising:a) A quadrature sampling circuit having it's input connected to an output of a humidity sensor and an output of a reference resistor.Quadrature sampling circuits are well known in the prior art for determining the real and imaginary components of a sinusoidal signal. In the context of an embodiment of the present invention, a quadrature sampling circuit is a circuit that samples a sinusoidal signal synchronously with said signal. Sample times within a given period of the sinusoidal signal occur at

tsample=m⁡(T4)where T=the period of the sinusoidal signalm=0, 1, 2, 3 representing four samples taken in a given periodb) An input of the humidity sensor and an input of the reference resistor are alternately connected to a signal ground or a sinusoidal source by a double-pole double-throw (DPDT) switch having two switch states. The two states of the DPDT switch result in a first voltage and a second voltage.)c) The first voltage is generated when the DPDT switch is configured to connect the input of the humidity sensor to signal ground and the input of the reference resistor to the sinusoidal source.d) The second voltage is generated when the DPDT switch is configured to connect the input of the humidity sensor to the sinusoidal source and the input of the reference resistor to signal ground.e) The first and second voltages are both sinusoidal. The quadrature sampling circuit creates a first and second set of data samples from the first and second voltages, respectively.f) A computer determines a first peak-to-peak complex voltage from the first set of data samples, and determines a second peak-to-peak complex voltage from the second set of data samples.In the context of an embodiment of the present invention, the peak-to-peak complex voltage is derived from the difference of real components separated by T/2 and the difference of imaginary components separated by T/2.g) The computer then takes a complex ratio of the second complex peak-to-peak voltage to the first complex peak-to-peak voltage. Said complex ratio cancels out parasitic admittance, voltage offsets, the sinusoidal source amplitude, and measurement circuitry gain.h) From the complex ratio, the computer calculates a value representative of relative humidity.

Some unique advantages of using the previously described embodiment include:a) elimination of gain changes due to switch operation by using a voltage follower that has a gain unaffected by switch operation;b) elimination of off-set errors by the measurement of peak-to-peak complex voltages;c) elimination of undesired parasitic admittance by using the complex ratio of peak-to-peak complex voltages;d) elimination of undesired variations in sinusoidal source amplitude by using a complex ratio of peak-to-peak complex voltages;e) elimination of variations in measurement circuit gain by using a complex ratio of peak-to-peak complex voltages; andf) elimination of distortion or severe limiting during condensing conditions by providing for peak-to-peak complex voltages that are equal to or smaller than the sinusoidal source even when humidity sensor admittance increases by many orders of magnitude thereby assuring continued, sensible and reproducible indications under condensing conditions and during the transition between non-condensing and condensing conditions.

DETAILED DESCRIPTION

Shown inFIG. 1is an embodiment for accurate measurement of relative humidity comprising the following components: a humidity sensor17, a reference resistor10, an electronically actuated double-pole double-throw (DPDT) switch5, a sinusoidal source1capable of producing a sinusoidal signal, an analog-to-digital converter (ADC)31, a computer35containing a memory36, and a display44. Memory36contains a set of instructions for execution by computer35that include controlling sinusoidal source1, controlling DPDT switch5, controlling ADCC31, controlling display44, and for performing mathematical calculations. In addition, memory36contains a set of calibration data which relate a plurality of complex ratios to a corresponding plurality of values of relative humidity. Also illustrated is a parasitic admittance50.

DPDT switch5includes a first input7, a second input8, and a first output6. First output6is connected either to first input7or second input8dependent on a control input9. DPDT switch5also includes a third input23, a fourth input24, and a second output20. Second output20is connected either to third input23or fourth input24dependent on control input9.

Sinusoidal source1includes a digital port2and an output3. Reference resistor10includes an input11and an output12. Humidity sensor17includes an input18and output16. ADC31includes an input32and a digital port33. Computer35includes a first digital port37, a second digital port38, a third digital port40, and a fourth digital port41. Display44includes a digital port45.

Digital port2of sinusoidal source1is connected to first digital port37of computer35via a connection42. Output3of sinusoidal source1is connected to first input7and to fourth input24of DPDT switch5via a connection4. A signal ground26is connected to second input8and to third input23of DPDT switch5via a connection25. First output6of DPDT switch5is connected to input11of reference resistor10via a connection14. Second output20of DPDT switch5is connected to input18of humidity sensor17via a connection19. Output12of reference resistor10and output16of humidity sensor17are both connected to input32of ADC31via a connection13and a connection15respectively. Control input9of DPDT switch5is connected to second digital port38of computer35via a connection43. Parasitic admittance50is connected between input32of ADC31and signal ground26.

Third digital port40of computer35is connected to digital port33of ADC31via a connection34. Fourth digital port41of computer35is connected to digital port45of display44via a connection46.

A measurement responsive to relative humidity is performed as follows:

A first operation executed by computer35includes:a) Computer35, via connection43, actuates DPDT switch5, connecting first output6to first input7of DPDT switch5, and connecting second output20to third input23of DPDT switch5. This, thereby, connects output3of sinusoidal source1to input11of reference resistor10and connects input18of humidity sensor17to signal ground26. This, thereby, produces a first voltage at input32of ADC31.b) Computer35, via connection34, then causes ADC31to quadrature sample and convert the first voltage synchronously with sinusoidal source1, creating a first set of data samples.FIG. 2Ais an illustration of the timing for quadrature sampling the first voltage. A first voltage100is substantially sinusoidal with a horizontal time axis120and a vertical voltage axis110. First voltage100has substantially the same frequency as sinusoidal source1(whereas their relative phase and amplitude may be different). The frequency f of first voltage100determines a time period T where:

T=1f.Sample times for ADC31are denoted sequentially by150,151,152and153. Sample time150occurs at a fixed time with respect to sinusoidal source1. Subsequent sample times151,152and153are spaced by one-fourth of period T. At these sample times, first voltage100has voltage values I150, Q151, I152, and Q153corresponding to sample times150,151,152, and153respectively.c) Computer35then causes ADC31to transfer the first set of data samples to computer35. Computer35then stores the first set of data samples in memory36.d) With the first set of data samples, Computer35then subtracts the voltage value taken at sample time152from the voltage value taken at sample time150, giving a peak-to-peak real component I1of the first voltage where:
I1=I150−I152Computer35also subtracts the voltage value taken at sample time153from the voltage value taken at sample time151, giving a peak-to-peak imaginary component Q1of the first voltage where:
Q1=Q151−Q153A first complex peak-to-peak voltage, comprising the peak-to-peak real and peak-to-peak imaginary components of the first voltage, is then stored in memory36as a first result R1where:
R1=I1+jQ1

A second operation executed by computer35includesa) Computer35, via connection43, actuates DPDT switch5, connecting first output6to second input8of DPDT switch5, and connecting second output20to fourth input24of DPDT switch5. This, thereby, connects output3of sinusoidal source1to input18of humidity sensor17and input11of reference resistor10to signal ground26. This, thereby, produces a second voltage at input32of ADC31.b) Computer35, via connection34, then causes ADC31to quadrature sample and convert the second voltage synchronous with sinusoidal source1, creating a second set of data samples.FIG. 2Bis an illustration of the timing for quadrature sampling the second voltage. A second voltage200is substantially sinusoidal with a horizontal time axis220and a vertical voltage axis210. Second voltage200has substantially the same frequency as sinusoidal source1(whereas their relative phase and amplitude may be different). The frequency f of second voltage200determines a time period T where:

T=1fSample times for ADC31are denoted sequentially by250,251,252and253. Sample time250occurs at a fixed time with respect to the sinusoidal source1. Subsequent sample times251,252and253are spaced by one-fourth of period T. At these sample times, second voltage200has voltage values I250, Q251, I252, and Q253corresponding to sample times250,251,252, and253respectively.c) Computer35then causes ADC31to transfer the second set of data samples to computer35. Computer35then stores the second set of data samples in memory36.d) With the second set of data samples, computer35then subtracts the voltage value taken at sample time252from the voltage value taken at sample time250, giving a peak-to-peak real component I2of the second voltage where:
I2=I250−I252Computer35also subtracts the voltage value taken at sample time253from the voltage value taken at sample time251, giving a peak-to-peak imaginary component Q2of the second voltage where:
Q2=Q251−Q253A second complex peak-to-peak voltage comprising the peak-to-peak real and peak-to-peak imaginary component of the second voltage, is then stored in memory36as a second result R2where:
R2=I2+jQ2

An electronic circuit analysis of the embodiment illustrated inFIG. 1, gives the following values for R1and R2:

R1=KVs⁢GrGr+Yx+YpR2=KVs⁢YxGr+Yx+Yp
where K is an overall circuit gain, Vsis a peak-to-peak amplitude voltage of sinusoidal source1at output3, Gris the conductance value of reference resistor10, Yxis the admittance value of humidity sensor17, and Ypis the admittance value of parasitic admittance50.

Computer35then computes R2divided by R1as a complex ratio R3as a third result, and stores the third result in memory36. The third result R3can be written as follows:

Substituting in the electronic circuit analysis values from above gives:

The third result R3, as shown above, consists of components I3and Q3. Component I3is directly proportional to sensor conductance Gx. Component Q3is directly proportional to sensor susceptance Bx, whereby the relative humidity seen by humidity sensor17is a function of sensor susceptance Bx.

Computer35then converts the susceptance Bxinto a display value of relative humidity and sends the display value to display44via connection46for viewing.

Alternative Embodiments

Alternatively R3could be a ratio of R1to R2. This then results in an equivalent series impedance Zsxfor humidity sensor17consisting of a series resistance Rsxand series reactance Xsxwritten as:

A value of relative humidity may then be derived from Xsx.

Alternatively, more than 1 set of 4 data samples per period may be taken allowing for computing a plurality of peak-to-peak real and imaginary components for the first set of data samples and a plurality of peak-to-peak real and imaginary components for the second set of data samples. This then gives a plurality of complex ratios and a corresponding plurality of display values of relative humidity. This would allow for the display of relative humidity values as a function of time.

This would also allow an averaging of relative humidity values over time to provide an improvement in signal to noise ratio. Furthermore, a plurality of peak-to-peak real and peak-to-peak imaginary samples for the first complex peak-to-peak voltage could be averaged, and a plurality of peak-to-peak real and imaginary samples for the second complex peak-to-peak voltage could be averaged to improve signal-to-noise of the complex ratio. This could thereby the improve the signal-to-noise ration of the indicated relative humidity.

Alternatively, quadrature samples, as shown in the above embodiment, need not occur all in one period of the sinusoidal source. They may also be spaced by integral multiples of the voltage period T such that the time between samples is then

tsample=n⁢⁢T+m⁡(T4)
where n is an integer, and m=0,1,2,3 for each of the four quadrature samples of the first and second voltages.

Alternatively, DPDT Switch5could be replaced by any plurality of switches wired in such a way as to be substantially equivalent to the function of the DPDT switch described herein.

Alternatively, sinusoidal source1could comprise a digital to analog converter, having an output3, that converts digital data fed to an input2from computer35, or could comprise an analog oscillator, having an output3, synchronized with computer35via an input2, or could comprise a direct digital synthesis circuit with an output3that is controlled by data fed to an input2from computer35.

Alternatively, a voltage follower having a gain one or less with an input connected to output12of reference resistor10and output16of capacitive humidity sensor17and with an output connected to input32of ADC31could be placed in the circuit. This would ensure that input voltages to ADC31are limited regardless of the impedance of the humidity sensor, such as in condensing conditions.

In addition, integrated circuits could comprise a plurality of the components and connections of the embodiment ofFIG. 1without departing from the spirit and scope of the present invention.

Advantages

From the description above, a number of advantages of some of the embodiments of the present invention become evident:a) The complex ratio used to compute third result R3cancels out the parasitic admittance Yp, the overall circuit gain K, and the sinusoidal source amplitude Vs. Third result R3is thereby independent of parasitic admittance and its variation, independent of sinusoidal source amplitude and its variation, and independent of overall circuit gain and its variation.b) The use of peak-to-peak complex values removes any DC offsets created by the switch or the ADC.c) The electronic circuit arrangement removes loop gain differences and responds without fail for all values of the sensor admittance Yx, thereby ensuring accurate measurement of relative humidity and continued, sensible and reproducible indications under condensing conditions.d) Consequently, sensor susceptance Bx(the imaginary part of third result R3) and thereby measured relative humidity, is independent of undesired variations due to many environmental influences on measurement circuitry.

Conclusion, Ramifications, And Scope

Accordingly, the advantageous circuit arrangements and calculations described above overcome the disadvantages of prior arta) by eliminating undesirable off-set errors;b) by eliminating undesirable differences in loop gain;c) by canceling undesirable parasitic admittance;d) by canceling undesirable gain dependence;e) by eliminating undesirable humidity and temperature influences on circuitry, components and devices;f) and by eliminating undesirable gross errors or failure of measurement circuitry in condensing conditions and in transitions between non-condensing and condensing conditions.

Various changes in the form and details of this invention by those skilled in the art may be made without departing from the spirit and scope of the present invention.