Calibration of humidity sensor devices

Abstract

The present invention relates to a method for calibrating a humidity sensor device, comprising the following steps: determining a first dew point and a first relative humidity at a first temperature in an environment by the humidity sensor device using a first calibration offset; then heating the humidity sensor device, in particular by self-heating, thereby heating the environment to a second temperature higher than the first temperature; then determining a second dew point at the second temperature by the humidity sensor device; then determining whether the difference between the determined second dew point and the determined first dew point is greater than a predetermined difference; and then, if it is determined that the difference between the determined second dew point and the first dew point is greater than the predetermined difference, changing the first calibration offset of the humidity sensor device by a predetermined value to obtain a second calibration offset.

Description

Technical Field

[0001] The present invention relates to a humidity sensor device, and in particular to calibration of a humidity sensor device configured to measure relative humidity and determine a dew point in an environment. Background Art

[0002] Humidity can be measured by a variety of techniques based on changes in resistance and capacitance of suitable materials. For example, in semiconductor-based systems that determine relative humidity (the ratio of water vapor to water at a given temperature) by measuring capacitance, humidity can be measured based on the reversible water absorption properties of polymer materials used as capacitor dielectrics.

[0003] Typically, a conventional capacitive relative humidity sensor device includes a semiconductor substrate and a pair of electrodes, which are formed on the surface of the semiconductor substrate and face each other at a specific distance. A humidity-sensitive dielectric film is placed between the electrodes and formed on the surface of the semiconductor substrate. The capacitance of the film changes in response to humidity. The sensor detects humidity by detecting the change in capacitance between the pair of electrodes in response to changes in ambient humidity. A humidity sensing element of the capacitive sensing type generally includes a non-conductive structure that is insensitive to moisture, on which a suitable electrode element is mounted or deposited, and a layer or coating of a dielectric and highly moisture-sensitive material covering the electrode, the material being positioned to be able to absorb water from the surrounding atmosphere and reach equilibrium in a short period of time.

[0004] The response offset and slope of the integrated relative humidity sensor device can be set to specific values ​​in order to achieve the desired sensor accuracy value. The accumulated charge of the capacitor element can be read out with the help of an appropriate conditioning circuit, which outputs a DC voltage or a change in the oscillation frequency of an oscillator, which can be detected in response to the accumulation of charge in the dielectric layer and represents a measurement of the capacitance and thus the corresponding humidity. Based on the relative humidity and the detected temperature, the absolute humidity and the dew point can be determined. Thus, for example, the dew point on the windshield of a vehicle can be determined with the help of the humidity sensor device.

[0005] Measuring relative humidity by means of a sensing capacitor requires calibration of the measuring circuit, in particular adjustment of the offset, in order to account for the changing physical properties of the sensing capacitor in terms of relative humidity. Climate chambers are known for precise calibration, which utilize, for example, saturated or unsaturated solutions of salts. However, long stabilization times are required and communication with a test bench is required. The time and therefore money spent on calibration is significant.

[0006] Furthermore, the sensing capacitor and the measuring circuit may be implemented on an integrated circuit chip, for example in the context of CMOS technology, and calibrated at the time of manufacture. A wafer with many IC chips may be manufactured at one time, while the actual electronic devices containing the manufactured IC chips may be assembled and calibrated at a later time and usually at another location. Therefore, it becomes time-consuming and laborious to calibrate each electronic device individually at the time of assembly.

[0007] In view of the above, the problem to be solved by the present invention is to provide an effective and reliable procedure for calibrating a relative humidity sensor device. Summary of the invention

[0008] The present invention overcomes the above mentioned problems by providing a method for calibrating a (relative) humidity sensor device as claimed in claim 1. The method comprises (in the order of the indices) performing the following steps in sequence:

[0009] a) determining, by the humidity sensor device, a first dew point and a first relative humidity at a first temperature in the environment using a first calibration offset;

[0010] b) heating the humidity sensor device, in particular by self-heating, thereby heating the environment to a second temperature higher than the first temperature;

[0011] c) determining, by the humidity sensor device, a second dew point at the second temperature;

[0012] d) determining whether the difference between the determined second dew point and the determined first dew point is greater than a predetermined difference (tolerance); and

[0013] e) If it is determined that the difference between the determined second dew point and the determined first dew point is greater than a predetermined difference (eg a predetermined function of the first calibration offset), changing the first calibration offset of the humidity sensor device by a predetermined value to obtain a second calibration offset.

[0014] The method uses heating of the humidity sensor device, in particular self-heating of the humidity sensor device. In the prior art, self-heating (heat caused by operation) is considered a serious problem that must be overcome by appropriate calibration. In contrast, according to the present invention, the self-heating phenomenon can be used for the calibration process. Self-heating can be caused by the processing means (e.g., a microcontroller) of the humidity sensor device. However, it is noted that the disclosed process also (additionally or alternatively) utilizes some external working of the humidity sensor device.

[0015] The disclosed procedure makes use of the fact that in a stable environment / enclosed volume of air, an increase in temperature leads to a decrease in relative humidity, so that the dew point (temperature) remains constant. Any deviation from the constant value of the dew point is due to some de-calibration or calibration error. Therefore, by adjusting the calibration offset based on the determined difference between the dew point determined after controlled heating of the sensor and the dew point initially determined before this heating, a self-calibration (automatic calibration) can be achieved in a very reliable and cost- and time-saving manner. The calibration offset is related to the relative humidity detected by the humidity sensor device (see also the detailed description below). The correct offset allows, for example, a measured voltage to be correctly converted into the actual relative humidity present in the environment.

[0016] In particular, according to an embodiment, if the second dew point is determined to be lower than the determined first dew point, the first calibration offset is increased by a predetermined value; and if the second dew point is determined to be higher than the determined first dew point, the first calibration offset is decreased by a predetermined value.

[0017] The method of calibrating a humidity sensor device of the present invention may include the following steps performed in sequence:

[0018] f) determining a new value for the first dew point based on the changed calibration offset;

[0019] g) then determining, by the humidity sensor device, a third dew point at a third temperature;

[0020] h) then determining whether the difference between the determined third dew point and the determined new value of the first dew point is greater than a predetermined difference; and

[0021] i) Then, if it is determined that the difference between the determined third dew point and the determined new value of the first dew point is greater than the predetermined difference, the calibration offset of the humidity sensor device is changed by the predetermined value.

[0022] Steps f) to i) may be repeated until the predetermined difference is no longer reached. Steps f) to i) may be repeated until the respectively determined new (third, fourth, fifth, etc.) dew point (at the respective third, fourth, fifth, etc. temperature) does not differ from the respectively determined new dew point value of the first dew point by more than the predetermined difference. Thus, an accurate calibration may be achieved by an iterative procedure.

[0023] To achieve automatic recalibration of the humidity sensor device during use, steps a) to e) or steps a) to i) (including repetition until a predetermined difference is no longer reached) may be performed periodically, for example in time periods of several hours or minutes.

[0024] According to a specific embodiment, the difference between the third temperature and the second temperature is less than 10% of the second temperature, and / or the predetermined difference between the predetermined second dew point and the predetermined first dew point, and the predetermined difference between the predetermined third dew point and the new value of the predetermined first dew point are in the range of 0.05°C to 0.5°C.

[0025] At least steps a) to e) of the method of the invention can be automatically performed at a predetermined time period, in particular between 10 and 2 times per hour. Steps a) to i) can be automatically performed at a predetermined time period and can be repeated until the determined dew point difference no longer exceeds or matches the predetermined difference.

[0026] According to a further embodiment, the first dew point, the determined first relative humidity and the first temperature can in particular be stored in a memory of the humidity sensor device in order to be used for a calibration process.

[0027] Further, a computer program product is provided, comprising one or more computer readable media having computer executable instructions for performing the method of one of the above embodiments. In addition, a (relative) humidity sensor device is provided, comprising a memory for storing the computer program product and a processor device configured to run the computer program product. The humidity sensor device can be suitable for use in automotive applications, for example for detecting relative humidity and / or dew point on a windshield of an automobile.

[0028] Additional features and advantages of the present invention will be described with reference to the accompanying drawings. In the specification, reference is made to the accompanying drawings which are intended to illustrate preferred embodiments of the present invention. It should be understood that such embodiments do not represent the full scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The dependence of the dew point on the temperature in a closed environment is shown.

[0030] Figure 2 A flow chart illustrating a calibration process according to an embodiment of the present invention is shown.

[0031] Figure 3 Another flow chart illustrating a calibration process according to a specific embodiment of the present invention is shown.

[0032] Figure 4 Another flow chart is shown that illustrates an automated recalibration process for an initially calibrated and aged relative humidity sensor device.

[0033] Figure 5 A schematic diagram illustrating basic components of a humidity sensor device according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0034] The invention provides a method for (self)calibrating a (relative) humidity sensor device based on heating of the humidity sensor device. The heating may be a self-heating of the humidity sensor device occurring during operation of the humidity sensor device, for example due to self-heating of at least one of the circuitry, the processing means, the microcontroller, etc. In a closed environment (e.g. considered as a canonical system), the heating of the humidity sensor device (at least locally at the measurement location) results in an increase in the ambient temperature. Data processing is performed by the processing means of the humidity sensor device.

[0035] Figure 1 The dependence of the dew point on temperature and relative humidity in a closed environment is shown. Straight lines (with constant slope) are plotted for different values ​​of relative humidity. The dew point τ is

[0036]

[0037] in:

[0038] T and φ represent temperature and relative humidity, i.e., the ratio of vapor partial pressure to saturated vapor pressure, and K2 and K3 are constants of the Magnus formula, i.e., for a temperature range of -45°C to 60°C, K3 = 243.12°C, K2 = 17.62°C.

[0039] In a closed environment, an increase in temperature causes a decrease in relative humidity, thereby keeping the dew point (at least approximately) constant. Therefore, if some heating of the humidity sensor device causes a change in the dew point determined based on the measured relative humidity, the humidity sensor device cannot be accurately calibrated. In other words, if the relative humidity calibration offset (e.g., relating the value of the measured voltage to the relative humidity value) is incorrectly adjusted, the determined dew point will not be constant at changing temperatures (which results in correspondingly changing relative humidity levels).

[0040] After the humidity sensor device is manufactured, the conditioning circuit must be calibrated. In the case of a resistive or capacitive humidity sensor device, the resistivity or capacitance changes with the humidity in the environment in which the humidity sensor device is to be operated. For example, in a capacitive humidity sensor device, some voltage or charge time is output and must be converted to the correct relative humidity by an appropriate offset. Next, we will refer to Figure 2 A calibration process for determining an appropriate offset according to an embodiment of the present invention is described.

[0041] During a startup phase, the dew point τ1 and the relative humidity value φ1 are determined (10) by the humidity sensor device based on an initial calibration offset for the measured relative humidity. The humidity sensor device is operated in a closed air volume at a given temperature T1 with a certain first calibration offset. All determined values ​​(including the temperature T1 which may also be detected by the humidity sensor device) may be stored, for example, in a memory of the humidity sensor device.

[0042] Next, the humidity sensor device is heated (11) by some external device, by a heater provided in the humidity sensor device, or by self-heating caused by the operation of the humidity sensor device. For example, the operation of certain circuits of the humidity sensor device (particularly certain data processing means) may cause self-heating.

[0043] After a heating period and / or after a certain temperature of the humidity sensor device (and thus the temperature of its environment) has been reached, the dew point τ2 is determined a second time by the humidity sensor device at a second temperature T2 (12). The second dew point τ2 determined at temperature T2 is compared (13) with the first dew point τ1 previously determined at temperature T1.

[0044] If the comparison (13) shows that the determined second dew point τ2 exceeds the determined first dew point τ1 and the difference is greater than a predetermined (tolerance) range Δτ, τ2>τ1+Δτ, it is determined that the offset value is too high, i.e. the relative humidity is overestimated, and therefore the first calibration offset used for the measured relative humidity is reduced (14a) by a predetermined value ΔO, for example, a certain percentage of the first calibration offset, for example, 0.05% to 0.2% of the first calibration offset.

[0045] On the contrary, if the comparison (13) shows that the determined second dew point τ2 is lower than the determined first dew point τ1 and the difference is greater than the predetermined range Δτ, τ2>τ1-Δτ, it is determined that the offset value is too low, that is, the relative humidity is underestimated, and therefore the first calibration offset is increased (14b) by a predetermined value ΔO, for example, a certain percentage of the first calibration offset, for example, 0.05% to 0.2% of the first calibration offset.

[0046] Then, a new value for the first dew point is calculated using a new calibration offset for the measured relative humidity that differs by ΔO from the previously used value. Figure 2 The process shown in FIG. 1 is continued until a constant value of the dew point within the predetermined range Δτ is determined. After the constant value of the dew point within the predetermined range Δτ is determined, the humidity sensor is well calibrated.

[0047] Figure 3Another specific example of the calibration process of the present invention is shown. At time t0, the dew point τ0 is determined (20) by the humidity sensor device, which is to be calibrated in a closed environment at temperature T0. The determination of the dew point τ0 is based on an initial calibration offset O0 of the measured relative humidity. The temperature T0 can be, for example, T0 = 24.7°C. The dew point τ0 is determined by the measured relative humidity φ0. For example, τ0 = 11.9°C, φ0 = 44.8%.

[0048] After a period of time t1, the dew point τ1 is determined from the measured relative humidity φ1. During the time period t1-t0, the self-heating of the humidity sensor device causes the temperature of the local environment to rise to a temperature T1>T0. For example, T1=27.7°C, τ1=12.8°C and φ1=39.6%. In this example, the determined dew point τ1 has increased by more than a certain tolerance Δτ, for example Δτ=0.1°C, relative to τ0. Therefore, the relative humidity is measured too high, that is, the calibration offset O0 is too high. Therefore, if the predetermined amount ΔO is selected as 0.1%, the calibration offset is reduced by the predetermined amount ΔO to obtain a new calibration offset O1 (22) for the measured relative humidity, for example, O1=O0-0.1%.

[0049] Using the new calibration offset O1, the dew point initially determined by (23) is re-determined to obtain the corrected value of the initial dew point: τ0 c =f(O1). Subsequently, at time t2, a new value τ2 of the dew point is determined (24) using the new calibration offset O1. During the time period t2-t1, self-heating of the humidity sensor device occurs, causing the temperature of the local environment to rise to T2>T1>T0, for example, T2=28.1°C.

[0050] The new dew point value τ2 is equal to the initial dew point correction value τ0 c The comparison is made and if these values ​​are identical within a predetermined tolerance Δτ, it is determined that the new calibration offset O1 for the measured relative humidity is the expected correct value (26) and the calibration process is terminated (27). On the other hand, if τ2>τ0 c +Δτ, the calibration offset is further corrected to O2 = O1-0.1%. The initially determined dew point is re-determined again using the corrected offset O2 to obtain the corrected value of the initial dew point: τ0 c =f(O2). Once again, a new value of the dew point is determined after time t3>t2, and the above process is repeated until the corresponding new value determined for the dew point does not differ from the corresponding corrected new value of the initial dew point by more than a predetermined tolerance Δτ. At the end of the iterative process, the humidity sensor has been well calibrated.

[0051] In principle, the above reference Figure 2 and 3The described process can be used to overcome initial calibration problems or aging problems of manufactured humidity sensor devices. Thus, during aging of the humidity sensor device, automatic recalibration can be performed to maintain the correct operation of the already initially calibrated humidity sensor device during the service time. Field calibration is useful for fine-tuning after a first calibration under laboratory conditions. For example, automatic recalibration is performed during any start-up operation of a humidity sensor device installed in the field, such as in a car. Automatic recalibration can also be initiated periodically. For example, during a ten-minute period, ten measurements can be performed in one second using the self-heating function while increasing the processor load.

[0052] Figure 4 An automatic recalibration process according to an example of the present invention is shown. Operation of the humidity sensor device is started at step 30. Operation can be started periodically, for example, within a period of several hours or about 10 minutes. Immediately after the start of operation, the dew point τ0 is determined by the humidity sensor device based on the measured value φ0 of the relative humidity with an initial calibration offset O0 of the relative humidity and stored (31) in a memory of the humidity sensor device.

[0053] After a predetermined period of self-heating of the humidity sensor device, a new value τ1 of the dew point is determined (32) and a difference between the new value τ1 and the initial value τ0 is determined (33) by a processing device of the humidity sensor device. It is determined (34) whether the absolute value of the difference exceeds a predetermined tolerance Δτ of the calibration error. If the predetermined tolerance Δτ is not exceeded, no calibration adjustment is required and the process is terminated (35). If the predetermined tolerance Δτ is exceeded, an initial calibration offset O0 of the relative humidity (e.g. set by an initial calibration process, e.g. performed after manufacturing the humidity sensor device and before first operation in the field) must be adjusted (36). In this case, if τ1>τ0, the initial offset O0 is reduced by a predetermined amount ΔO and if τ1<τ0, the initial offset O0 is increased by a predetermined amount ΔO to obtain a corrected offset O1.

[0054] Next, the initial value of the dew point is re-determined (37) using the adjusted offset O1 to obtain the corrected initial dew point τ0 c In a further measuring / determining step (38), the dew point τ2 is determined after a predetermined period of time and the newly determined dew point τ2 is compared with the corrected initial dew point τ0. c Again, if the absolute value of the difference exceeds the predetermined tolerance Δτ, the actual calibration offset O1 is adjusted (increased or decreased depending on the sign of the difference) by a predetermined amount ΔO, and the corrected initial dew point τ0 is re-determined using the new corrected offset O2. cThis process can be repeated (e.g., measuring every 100 milliseconds) until the offset is adjusted so that the newly determined dew point does not differ from the actual corrected initial dew point by more than a predetermined tolerance. After the iterative process is complete, the humidity sensor is well calibrated.

[0055] Note that the calibration slope can also be adjusted to account for aging issues. For example, a relative humidity sensor may have been calibrated well for 50% relative humidity, but incorrectly calibrated at 30% relative humidity. Instead of calibrating for the offset at 30% relative humidity, the operator can alternatively calculate a slope that results in an accurate calibration at 30% relative humidity that is just as good as at 50% relative humidity. The adjustment of the slope can be based on some linear or non-linear function. Adjusting the slope over the entire range of 0% to 100% allows for accurate recalibration.

[0056] Refer to above Figures 2 to 4 The described procedure allows for a very efficient and reliable and time-saving way of calibrating the humidity sensor device both in the initial state before the first operation of the sensor and during service time. Figure 5 The block diagram of shows the configuration of a humidity sensor device 100, in which the above-described process is implemented. The humidity sensor device 100 comprises a humidity sensing device 110, which comprises a sensing capacitor 112 and is configured to measure relative humidity.

[0057] The sensing capacitor 112 includes two parallel metal plates, each having a covered film having a polyimide dielectric layer serving as the capacitor dielectric. The polyimide dielectric layer absorbs moisture, changing the dielectric constant of the capacitor. Thus, since a constant reference capacitance is known (e.g., established by a reference capacitor), the relative humidity can be calculated by measuring the change in capacitance of the sensing capacitor 112, as the capacitance changes with the humidity of the ambient condition.

[0058] The humidity sensing device 110 is connected to an analog-to-digital converter 120 for providing a digital data stream to a control logic 130. The control logic 130 includes a processing device 132, such as a microcontroller, and a memory 134. Calibration data and data for the calibration process may be stored in the memory 134. In addition, the memory 134 stores a program product for executing the calibration process according to the above-described embodiments.

[0059] The control logic 130 is connected to an interface 140 for outputting data via data lines 151 and 152. The interface 140 may be, for example, an I2C interface having signal lines SDA and SCL. 2 C interface. Figure 5 At least some of the components shown may be formed on a single microchip or printed circuit board.

[0060] All previously discussed embodiments are not intended to be limiting but rather serve as examples to illustrate the features and advantages of the present invention. It should be understood that some or all of the above features may also be combined in different ways.

Claims

1. A method for calibrating a humidity sensor device, comprising the following steps: a) determining, by a humidity sensor device, a first dew point and a first relative humidity at a first temperature in an environment using a first calibration offset related to a first relative humidity detected by the humidity sensor device; b) then heating the humidity sensor device, in particular by self-heating, thereby heating the environment to a second temperature higher than the first temperature; c) then determining, by the humidity sensor device, a second dew point at the second temperature; d) then determining whether a difference between the determined second dew point and the determined first dew point is greater than a predetermined difference; as well as e) then, if it is determined that the difference between the determined second dew point and the determined first dew point is greater than a predetermined difference, changing the first calibration offset of the humidity sensor device by a predetermined value to obtain a second calibration offset; f) determining a new value for the first dew point based on the changed calibration offset; g) then determining, by the humidity sensor device, a third dew point at a third temperature; h) then determining whether the difference between the determined third dew point and the determined new value of the first dew point is greater than a predetermined difference; as well as i) then, if it is determined that the difference between the determined third dew point and the determined new value of the first dew point is greater than the predetermined difference, changing the calibration offset of the humidity sensor device by the predetermined value; Steps f) to i) are repeated until the difference between the determined respective dew point determined in step g) and the determined respective new value of the first dew point determined in step f) is smaller than the predetermined difference.

2. The method according to claim 1, wherein: The first and second calibration offsets are related to relative humidity determined by the humidity sensor device; and i) if it is determined that the second dew point is lower than the determined first dew point, increasing the first calibration offset by the predetermined value; and ii) if it is determined that the second dew point is higher than the determined first dew point, reducing the first calibration offset by said predetermined value.

3. The method according to claim 1 or 2, wherein: The predetermined value is some predetermined function of the first calibration offset.

4. The method according to claim 1 or 2, wherein: The difference between the third temperature and the second temperature is less than 10% of the second temperature.

5. The method according to claim 1 or 2, wherein: The predetermined difference is in the range of 0.05°C to 0.5°C.

6. The method according to claim 1 or 2, wherein: At least steps a) to e) are automatically performed at a predetermined time period, in particular between 10 and 2 times per hour.

7. The method of claim 1 or 2, further comprising storing the first dew point, the determined first relative humidity, and the first temperature.

8. The method according to claim 1 or 2, wherein: The heating of the humidity sensor device is a self-heating by means of the processing device of the humidity sensor device.

9. The method according to claim 1 or 2, wherein: Steps a) to e) or a) to i) are performed periodically.

10. A computer program product comprising one or more computer readable media having computer executable instructions for executing the steps of the method of any one of claims 1 to 9.

11. A humidity sensor device comprising a memory for storing the computer program product of claim 10 and configured to run the computer program product of claim 10.

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