IN-SITU CALIBRATION OF A HUMIDITY SENSOR

Abstract

IN SITU CALIBRATION OF AN INFRARED HUMIDITY SENSOR A calibration method for a humidity sensor based on infrared (IR) radiation measurements, comprising a weighing and surface mass (Gs) calculation step of a dry sample (Scal) using a balance, in which a humidity sensor and a surface mass sensor are used, the method comprising the steps of collecting in parallel surface mass measurement data and infrared radiation measurement data of the sample at different humidity levels, calculating a variable representing the water surface mass of the sample from the surface mass measurement data and the dry sample surface mass, and establishing, for each of the different humidity levels of the sample, a calibration correspondence between the variable representing the water surface mass and the infrared radiation measurement data. Figure to be published: Fig. 2

Description

Title of the invention: IN SITU CALIBRATION OF A HUMIDITY SENSOR Technical field

[0001] The invention relates to a method for calibrating a humidity sensor based on infrared radiation measurements. Prior art

[0002] The field of manufacturing flat materials, or webs, such as paper, fabric or non-woven materials and certain plastics, requires the control of the characteristics of these webs during their formation, and in particular the control of their humidity level.

[0003] For example, a sheet of paper up to several meters wide is formed continuously by passing a paper pulp solution, which is an aqueous solution, over a forming table and then gradually drying it by heated hollow cylinders.

[0004] Controlling the humidity level of the sheet during formation appears essential for good control of the quality of the sheet produced.

[0005] Monitoring the characteristics of sheets such as the sheet of paper in the example above required the development of in situ measuring devices comprising sensors mounted on mechanical frames capable of moving these sensors across the width of sheets being formed, these sensors measuring the humidity level continuously.

[0006] The humidity level can be measured by means of a device for measuring the transmission of infrared rays through the material to be characterized, as disclosed in US patent 9,170,194.

[0007] Such a measuring device requires a calibration establishing a correspondence between the physical quantity directly measured and the quantity that one seeks to characterize, respectively the intensities of infrared rays and the humidity level in the present case.

[0008] Calibration of the measuring device therefore requires the measurement of the transmission of infrared radiation by a sample of known surface, for various humidity levels thereof.

[0009] In practice, the mass of the sample is determined several times for different humidity levels using a precision balance and its infrared transmission is measured each time by the infrared ray measuring device, so as to obtain pairs of surface mass and infrared transmission measurements.

[0010] A measurement of the mass of the dry sample makes it possible to calculate the humidity rate Th of the sample for each of the measurement pairs using formula 1: Th = ^x 100(1) where mh represents the mass of the wet sample and ms the mass of the dry sample.

[0011] Alternatively, the surface mass in water alone Ge of the sample can be calculated by formula 2: Ge = ^(2) where S represents the surface area of ​​the sample.

[0012] A calibration curve establishing a correspondence between the humidity level or the surface water mass of the sample and its infrared transmission is then established.

[0013] Such a calibration procedure is lengthy and prone to errors: the sample is moved manually from the precision balance to the infrared measuring device, during which movement its humidity level can vary significantly, which directly impacts the accuracy of the calibration.

[0014] Furthermore, this procedure requires the continuous presence and repeated manipulations of an operator, with an inherent risk of non-reproducibility of the manipulations and human errors. Statement of the invention

[0015] An objective of the invention is to simplify the calibration procedure of a humidity sensor while increasing its accuracy and reliability.

[0016] For this purpose, the invention relates to a method for calibrating a humidity sensor based on infrared radiation measurements, comprising a step of weighing and calculating the surface mass of a dry sample using a balance, in which a humidity sensor is used which is a first sensor mounted on a mechanical frame supporting a scanning system of infrared radiation detection and emission heads of said humidity sensor and of radiation detection and emission heads of a second sensor which is a surface mass sensor separate from the first sensor, the method further comprising the steps of collecting in parallel, by means of the surface mass sensor and the humidity sensor, surface mass measurement data and infrared radiation measurement data of the sample at different degrees of humidity, calculating,by means of a data processing unit and for each of the different degrees of humidity of the sample, a variable representative of a surface mass of water of the sample from the surface mass measurement data and the surface mass of the dry sample, and establishing, for each of the different degrees of humidity of the sample, a calibration correspondence between the variable representative of the surface mass of water and, infrared radiation measurement data.

[0017] The calibration method according to the invention is thus based on the replacement of the conventional step of measuring the mass of the wet sample with a precision balance by a surface mass measurement.

[0018] According to the method, the sample does not have to be moved repeatedly between the humidity sensor and a precision balance, but can simply be placed in a wet state facing the detection heads, so that a first advantage of the method is that measurements can advantageously be made continuously as the sample dries.

[0019] A second advantage of the method is that the infrared radiation measurements and the surface mass measurements are made in parallel, i.e. simultaneously and not sequentially, eliminating errors arising from the variation in the humidity level of the sample between the weight measurement and the infrared measurement of conventional methods and therefore increasing the accuracy of the calibration.

[0020] A third advantage also allowing to increase the accuracy of the calibration is that the number of measurement points can be considerably increased, limited only by the response times of the sensors used, and not by the time taken by the manipulations of the sample in conventional methods, which allows to increase the accuracy of the calibration.

[0021] A fourth advantage is based on the simplification of operations: handling of the sample between two measurements is made unnecessary, which reduces the sources of handling error, the process can also be very simply automated.

[0022] Finally, a fifth advantage lies in the fixed nature of the sample with respect to the sensors, which leads to excellent reproducibility of the measurements.

[0023] The calibration method according to the invention can have the following particularities:

[0024] - the variable representing the surface mass of water can be chosen from the surface mass of water in the sample, the moisture content of the sample and the water carryover rate of the sample;

[0025] - the dry sample can be weighed before the parallel data collection step;

[0026] - the dry sample can be weighed after the parallel data collection step;

[0027] - the detection heads can be fixed relative to the sample during the step parallel data collection;

[0028] - during the parallel data collection step, the heads can be moved detection relative to the sample substantially parallel to an alignment axis of the detection heads, so as to probe a portion of the sample common to surfaces probed respectively by the humidity sensor and by the surface mass sensor;

[0029] - during the parallel data collection step, the heads can be moved detection relative to the sample substantially perpendicular to an axis (Al) of alignment of the detection heads;

[0030] - a motorized sample holder can move the sample; and

[0031] - the scanning system can move the detection heads. Brief description of the drawings

[0032] The present invention will be better understood and other advantages will appear on reading the detailed description of the embodiment taken as a non-limiting example and illustrated by the appended drawings, in which:

[0033] [Fig.l]- [Fig.l] illustrates a diagram of the calibration method according to the invention;

[0034] [Fig.2]- [Fig.2] illustrates a humidity sensor head scanning system and of surface mass, viewed horizontally, with the detection heads of the sensors aligned parallel to the scanning direction;

[0035] [Fig.3]- [Fig.3] illustrates in top view the characterization of a sheet N by sensor scanning, with the sensor detection heads aligned perpendicular to the scanning direction;

[0036] [Fig.4A]- [Fig.4A] illustrates the irradiation of a sample by in-radiation infrared and radiation;

[0037] [Fig.4B]- [Fig.4B] illustrates a first geometric calibration configuration;

[0038] [Fig.4C]- [Fig.4C] illustrates a second geometric calibration configuration;

[0039] [Fig.4D]- [Fig.4D] illustrates a third geometric calibration configuration;

[0040] [Fig.4E]- [Fig.4E] illustrates a fourth geometric calibration configuration;

[0041] [Fig.5]- [Fig.5] illustrates a table in which data are collected measured or calculated for calibration; and

[0042] [Fig.6]- [Fig.6] illustrates curves deduced from the data measured during the calibration method according to the invention.

[0043] Description of an embodiment of the method according to the invention

[0044] In order to characterize during manufacture the humidity level and the surface mass of webs such as paper, plastics, fabric or non-woven materials, one solution is to use mobile humidity and surface mass sensors arranged so as to scan the web across its width.

[0045] The humidity sensor is, in this embodiment, based on the variations in the transmission of one or more wavelengths of infrared radiation by a sheet sample according to the humidity level of this sample, at least one of the wavelengths being absorbed by the water molecules and, optionally, one or more others being absorbed by the material forming the sheet to be characterized and / or serving as a reference in order to take into account, for example, refraction or impurities. present in the water table.

[0046] Thus, the humidity sensor comprises an H1R.E head for emitting infrared radiation and an H1R D head for detecting infrared radiation passing through the sheet to be characterized.

[0047] The surface mass sensor is based on the transmission of X-rays through the sheet to be characterized, transmission varying with the surface mass of the sheet, and comprises an HXE head for emitting X-rays and an Hx D head for detecting the X-rays passing through the sheet to be characterized.

[0048] Alternatively, the surface mass sensor could be based, for example, on the transmission or backscattering of beta radiation and could comprise a beta radiation emission head and a head for detecting beta radiation transmitted or backscattered by the sheet to be characterized.

[0049] Generally speaking, measurements can be made by transmission or backscattering of electromagnetic radiation, ultrasound or particles.

[0050] In the case of backscattering, the emission heads and the detection heads are located on the same side of the sample, possibly integrated into the same housing.

[0051] In this embodiment, the humidity sensor based on infrared radiation and the surface mass sensor based on X-ray radiation are used simultaneously in transmission, the radiation emission heads being mounted contiguous to one another on the same side of the sheet to be characterized, on a first mobile part MPI of a scanning system MP mounted on a mechanical frame M, and the detection heads are mounted contiguous to one another on the other side of the sheet to be characterized, on a second mobile part MP2 of the scanning system.

[0052] The detection heads of the two sensors can be aligned in a direction DI of scanning of the sheet by the heads of the detectors or in a machine direction D2 perpendicular to the scanning direction D1, as illustrated respectively by figures 2 and 3.

[0053] The moving parts and the emission and detection heads are positioned in such a way that the detection heads can respectively measure the infrared and X radiation emitted by the emission heads and transmitted through a sheet N passing through the mechanical frame M, as illustrated by [Fig.2] where these radiations are indicated respectively by IR and X.

[0054] During its formation, this sheet N is continuously moved in the direction of its length through the mechanical frame, in the direction D2, and the scanning system is used to move the heads of the sensors in the direction D1 so as to characterize the sheet over its entire width.

[0055] More specifically, the scanning system is designed to move the first moving part MPI and the second moving part MP2 of the scanning system while maintaining the alignment of the heads and so that they scan the sheet over its entire width in the scanning direction DI, perpendicular to the machine direction D2.

[0056] The scanning system comprises a control / command unit C / C controlling the movement of the heads of the sensors and the activation of the latter, as well as a data processing unit CPU from the sensors and a computer memory Mem storing the raw data from the sensors as well as the processed data and data calculated from the raw data by the calculation unit.

[0057] In order to carry out characterization measurements on a sheet in formation, the infrared radiation sensor and the X-ray radiation sensor must be calibrated.

[0058] The X-ray sensor follows a standard calibration process.

[0059] According to the invention, the infrared radiation sensor follows a calibration method 100 illustrated by [Fig.l].

[0060] A first step S10 of the calibration method 100 consists of placing a wet Seal calibration sample between the infrared radiation emission heads H1R_E and X-ray emission heads HxEd on the one hand and the detection heads H1R D and Hx D on the other hand, the sample drying naturally in a progressive manner.

[0061] Following step S10, a second step S20 consists of emitting infrared radiation and X-ray radiation through the calibration sample by means of the emission heads of the sensors kept fixed, then collecting in parallel by means of the corresponding detection heads surface mass measurement data, here X-ray radiation data transmitted through the sample, as well as infrared radiation measurement data transmitted through the sample.

[0062] The parallel acquisition of surface mass measurement data and infrared radiation measurement data means obtaining sets of these data at the same time or over a short time interval for each set, i.e. for the same degree of humidity of the sample.

[0063] A short time interval corresponds to a time interval such that the degree of humidity does not vary significantly during this interval, which may depend on the nature of the sample and the temperature and humidity conditions during the measurement, or even on the level of precision required by the user.

[0064] The measurements are acquired cyclically in this way during the drying of the sample and stored in the memory Mem under control of the control / command unit C / C and the data processing unit CPU, which makes it possible to collect a number n of sets of data averaged at different degrees of humidity, each set comprising, for each degree of humidity of the sample, an averaged measurement of surface mass and at least one measurement by infrared radiation averaged at at least one wavelength.

[0065] The averages are preferably taken over a sufficiently short time interval so that the humidity level does not change significantly and / or over a defined surface area of ​​the sample.

[0066] Table 500 of [Fig.5] is an illustration of the manner in which the n data sets are arranged, by means of the data processing unit, respectively in rows 1 to n of the table, the averages of surface mass measurements by X-ray transmission of indices 1 to n, G_1 to G_n, occupying column Col.l and the averages of humidity measurements by infrared radiation transmission of indices 1 to n for four different IR wavelengths IR1_1 to IR1_n, IR2_1 to IR2_n, IR3_1 to IR3_n and IR4_1 to IR4_n occupying columns Col.2 to Col.5 respectively.

[0067] Note that this is only an example, the wavelengths and their number may differ depending on the applications and the wishes of the operators.

[0068] Columns Col.l to Col.5 of a row i of the table correspond to a set E_i of data collected for the sample for a humidity degree i.

[0069] A third step S30 consists of measuring the mass of the dry sample to obtain an absolute reference which will be used to calculate the humidity rate or the surface mass of water of the sample for each of the measurement sets.

[0070] For this purpose, following data collection, the sample is removed from the measuring device and weighed dry on a precision balance, for example after being dried as completely as possible in an oven, or using a heating balance.

[0071] Alternatively, the sample may be dried and weighed prior to collecting the moisture and surface area measurement data, as a first step in the calibration process, and then moistened prior to being placed at the sensors in step S10.

[0072] The surface area of ​​the dry sample being known (measured or defined by an operator during sample preparation) and its mass measured, its dry surface mass Gs is determined.

[0073] Following steps S20 and S30, a fourth step S40 consists of calculating a variable representative of the surface mass of water Ge_i contained in the sample for each set of index i, such that the surface mass of water Ge_i obtained by subtracting the dry surface mass Gs from the surface mass G_i for each set of index i.

[0074] For the calibration method, it is also appropriate to calculate, instead of the surface mass of water Ge_i, any variable that can be deduced unequivocally, such as the humidity rate Th_i or the water carryover rate Tem_i of the sample.

[0075] For example, the humidity rate Th_i of the sample for each of the different humidity levels is expressed by the formula 3: Ti = x 100 (3) where Ge_i represents the surface mass of water and G_i the surface mass of the sample at the humidity index i.

[0076] Similarly, the carrying rate Tem_i is expressed by formula 4: Tem_i = x 100 <4) where Ge_i represents the surface mass of water in the sample at humidity index i and Gs represents the surface mass of the dry sample.

[0077] Any variable that can be deduced unequivocally from the surface mass of water and the value of the surface mass of water itself are variables representative of this surface mass of water, which can be used to establish a calibration curve for the humidity sensor linking the data obtained from the humidity sensor to the data obtained from the surface mass sensor.

[0078] A fifth step S50 consists of establishing in the data processing unit a calibration correspondence between a variable representative of the surface mass of water (Ge_i, Th_i, Tem_i) calculated in step S40 and the infrared radiation measurement data (IRl_i, IR2_i, IR3_i, IR4_i) collected in step S20, the data processing unit adding to the table 500 this variable representative of the surface mass of water in column 6.

[0079] Column 6 of table 500 taken as an example consists of the values ​​of the surface mass of au Ge_i at the different humidity degrees 1 to n of the sample calculated in step 40 from the X-ray data, for the n values ​​of the index i.

[0080] Of course, the method operator can order the data differently, as long as correspondences between the surface mass measurement data and the infrared radiation measurement data are established.

[0081] [Fig.6] illustrates curves representing the temporal evolution of the surface mass of the sample (Ge curve) and of the absorbance of the infrared radiation of the latter for 4 wavelengths of the infrared domain (curves IR1 to IR4), plotted from the data sets E_i and Ge_i for i increasing from 1 to n.

[0082] In this example, the curves IR1 and IR2 correspond to data obtained for wavelengths absorbed by water molecules and the curves IR3 and IR4 correspond to data obtained for wavelengths not absorbed by water, serving as reference.

[0083] From these measured and calculated data stored in the table, a conventional calibration method such as that described in US patent 9,170,194 can be applied, with the advantages of a large number of data collected and a excellent precision of these for the reasons explained above.

[0084] In step S20 of the method described above, the heads of the detectors are kept fixed relative to the sample, but it is possible to further improve the quality of the calibration by increasing the reliability of the measurements by using the variants set out below.

[0085] Indeed, at step S20, the IR and X rays from the detectors do not pass through the same area of ​​the sheet.

[0086] The infrared radiation and the X-ray radiation are emitted in the form of beams which irradiate two distinct zones of the sample, respectively Z1R and Zx, which do not have the same extent or the same location, as illustrated by [Fig.4A], so that an uncertainty in the measurements due to a possible inhomogeneity of the sample is introduced, as is conventionally the case.

[0087] In a step S20' alternative to step S20, during the data collection, the sensor heads scan the sample during each measurement in a direction perpendicular to the alignment axis of the detection heads, and therefore carry out averaged measurements on the surfaces Sir and Sx of the sample scanned respectively by the IR and X-ray radiation during each measurement, the surfaces Sir and Sx being entirely distinct as illustrated by FIGS. 4B and 4C.

[0088] Step S20' represents an improvement over step S20 in that a possible effect of sample inhomogeneity is reduced.

[0089] To implement step S20' in the geometric configuration of [Fig.3], in which the detection heads are aligned along the machine direction D2, the scanning of a sample by the sensor heads can be obtained by using the scanning system on which the heads are mounted with the sample held fixed, as illustrated by [Fig.4B], the arrows Depl indicating the movement of the heads.

[0090] Alternatively, to implement step S20' in the geometric configuration of [Fig.2], in which the detection heads are aligned along the scanning direction D1, the scanning of the sample by the sensor heads can be obtained by means of a motorized sample holder PEM moving the sample along the machine direction D2 with the detection heads held fixed, as illustrated by [Fig.4C], the arrow Depl indicating the movement of the sample.

[0091] On the other hand, even if in step S20' the measurements are averaged over the scanned surfaces, the surfaces S1R and Sx of the sample probed respectively by the infrared radiation sensor and by the surface mass sensor are entirely distinct, which can introduce a bias into the calibration.

[0092] This bias can be reduced by carrying out a step S20' ' alternative to steps S20 and S20', in which, during measurement, the sample is moved relative to the sensors in a direction parallel to the alignment axis of the two detection heads of the two sensors, so that these heads scan a portion of the sample common to the surfaces probed respectively by the humidity sensor and by the surface mass sensor, the surfaces S1R and Sx probed respectively by the IR and X radiation being at least partially superimposed, as illustrated by [Fig.4D] and 4E.

[0093] To implement step S20” in the geometric configuration of [Fig.2], the scanning of a sample by the sensor heads can be obtained by using the scanning system on which they are mounted with the sample held fixed, as illustrated by [Fig.4D], the arrow Depl indicating the movement of the heads.

[0094] Alternatively, to implement step S20” in the geometric configuration of [Fig.3], the scanning of the sample by the sensor heads can be obtained by means of the motorized sample holder PEM moving the sample in the machine direction D2 with the detection heads held fixed, as illustrated by [Fig.4E], the arrows Depl indicating the movement of the sample.

[0095] This further improves the accuracy of the calibration by minimizing the sources of errors and biases during data collection.

[0096] It goes without saying that the present invention cannot be limited to the embodiment set out above, which may be subject to modifications without departing from the scope of the invention.

Claims

Claims

1. Method for calibrating a humidity sensor based on infrared radiation (IR) measurements comprising a step of weighing and calculating the surface mass (Gs) of a dry sample (Seal) using a balance (S30), characterized in that a humidity sensor is used which is a first sensor mounted on a mechanical frame (M) supporting a scanning system (MPI, MP2) of detection heads (H1R D) and emission (H1R E,) of infrared radiation of said humidity sensor and of detection heads (HXD) and emission (HXE) of radiation of a second sensor which is a surface mass sensor distinct from the first sensor, said mechanical frame (M) being designed to be crossed by a sheet in formation, the method further comprising the steps of: - collecting in parallel (S20, S20', S20”), by means of the surface mass sensor and the humidity sensor, measurement data (G_i) of surface mass and measurement data (IRl_i, IR2_i, IR3_i,IR4_i) by infrared radiation of the sample (Seal) at different humidity levels; - calculating (S40), by means of a data processing unit (CPU) and for each of the different humidity levels of the sample, a variable representative of a surface mass of water (Ge_i, Th_i, Tem_i) of the sample from the surface mass measurement data (G_i) and the surface mass (Gs) of the dry sample; and - establishing (S50), for each of the different humidity levels of the sample, a calibration correspondence between the variable representative of the surface mass of water (Ge_i, Th_i, Tem_i) and the infrared radiation measurement data (IRl_i, IR2_i, IR3_i, IR4_i).,

2. The calibration method according to claim 1, characterized in that the variable representative of the surface mass of water is chosen from the surface mass of water (Ge_i) of the sample, the humidity rate (Th _i) of the sample and the water carryover rate (Tem_i) of the sample.

3. The calibration method according to claim 1 or 2, characterized in that the dry sample (Seal) is weighed before the step of parallel data collection.

4. The calibration method according to claim 1 or 2, characterized in that the dry sample (Seal) is weighed after the step of parallel data collection.

5. The calibration method according to any one of claims 1 to 4, characterized in that the detection heads are fixed relative to the sample during the step of parallel data collection.

6. The calibration method according to any one of claims 1 to 4, characterized in that, during the step of collecting the data in parallel, the detection heads are moved relative to the sample substantially parallel to an axis (Al) of alignment of the detection heads, so as to probe a portion of the sample common to surfaces (Sk, Sx) probed respectively by the humidity sensor and by the surface mass sensor.

7. The calibration method according to any one of claims 1 to 4, characterized in that, during the step of collecting the data in parallel, the detection heads are moved relative to the sample substantially perpendicular to an axis (Al) of alignment of the detection heads.

8. The calibration method according to claim 6 or claim 7, characterized in that a motorized sample holder moves the sample.

9. The calibration method according to claim 6 or claim 7, characterized in that the scanning system moves the detection heads.

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