DE102018113671A1 - Method for determining the moisture content of a moisture-absorbing material - Google Patents

Abstract

In a method for determining the moisture content of a moisture-absorbing material (8), in particular a textile, or a moisture-containing material, in particular skin, comprising a moisture sensor (1) comprising a heating element (3) with a control unit (5) and a Evaluating circuit is coupled, by the timing and analysis of the change in temperature of the heating element (3) taking advantage of the finite propagation velocity of the heat pulse in the material (8) and the different temperature effects due to the different heat capacities depending on the moisture content of the distance to the heating element (3), in which the moisture content is measured, influenced.

Description

The invention relates to a method for determining the moisture content of a moisture-absorbing or moisture-containing material, in particular a textile, comprising a moisture sensor comprising a heating element, which is coupled to a control unit, a power supply and an evaluation circuit.

In many technical applications, knowledge of the moisture content of materials is important and is measured, with measurement in relatively thin layers of material often proving problematic.

In the literature, for example in material moisture measurement: Basics - Measurement Methods - Applications - Standards, Contact & Study, Volume 513, Klaus Kupfer, Expert-Verlag, 1997 , a variety of measurement methods are described for different purposes, whereby methods of building and soil moisture measurement are often explained, which have in common that relatively large areas are measured and local delimitation is often not possible. Special requirements, for example measurements close to the body, are only insufficiently met with the known methods.

The gravimetric method enables the water content of a material to be measured by comparing the dry weight of the material with its wet weight. For this purpose, a sample is taken from the material to be examined and, if necessary, dried, the sampling being difficult in some cases, since it is not always possible to separate a sample from an overall system. A continuous measurement of the water content is almost impossible, and measurement results do not indicate the actual water content if the sample cannot be weighed in a timely manner, since part of the moisture absorbed in the material of the sample evaporates. Gravimetric methods are for example in the US 4,316,384 A1 . US 4,666,007 A1 . US 4,750,143 A1 . US 4,798,252 A1 and US 4,889,201 A1 specified.

Furthermore, there are electrical methods, in particular the electrical resistance measurement and the capacitance measurement, for determining the water content in a material, for example from the US 5,402,075 A1 . US 6,647,782 B2 . US 6 222 376 B1 and the US 5,861,758 A1 known. Here, electrodes are arranged in or on the material to be examined and the electrical measurement signal or electrical measurement values are recorded for evaluation. These methods are fundamentally suitable for use in connection with relatively thin layers of material, but the influence of the measured values by ionic impurities or parasitic, often time-varying capacitances, such as those that occur in measurements close to the body, has proven to be disadvantageous.

In thermal measuring methods, the moisture content of the material is determined by the thermal properties, such as thermal conductivity or heat capacity, which change with the water content. In practice, these measurement methods are less susceptible to interference and their main application is in the construction sector.

The EP 1 844 323 B1 discloses a device for determining the moisture in a medium, in particular earth, based on a thermal measurement. The device has at least one measuring sensor, which consists of a temperature sensor and a means for heating the temperature sensor. Furthermore, the device comprises evaluation electronics and a control unit which is used to heat the heatable temperature sensor with a heating pulse and to determine the moisture content of a sensor surrounding the sensor Serve medium. The sensor is surrounded by an intermediate layer made of an absorbent, mechanically deformable and thermally insulating material made of synthetic fibers and / or of felt. A statement regarding the spatial measuring range, which is influenced by varying the shape of the heating pulse and the time of the measurement, cannot be gathered from this disclosure.

From the EP 0 981 737 A1 a sensor for determining the moisture content is known, which is preferably produced as a microsystem using silicon technology and comprises a membrane which can be heated either permanently or by means of a short pulse and has a first temperature sensor and a second temperature sensor which is thermally insulated from the membrane for determining the ambient temperature. An absorbent material, for example wood, paper, cardboard or a porous ceramic, is arranged on the membrane. The heat generated when the membrane is heated is dissipated to the surroundings by the absorbent material. The difference in temperature between the two temperature sensors represents a reference value for the material moisture. Although heating by means of a heating pulse minimizes the energy requirement, it is not possible to limit the measuring range in space.

The US 3,550,439 A1 describes a so-called clothing hygrometer, with which the moisture content of textiles can be determined, using a structure similar to a capacitor and a hygroscopic material. Of the Moisture content is determined without a heat pulse using an electrical capacitance measurement.

The invention has for its object to provide a method and an apparatus of the type mentioned above or make a statement about a moisture content in a moisture-absorbing or moisture-containing material, the spatial measurement range is variable and the moisture content in a thin Material layer or in the immediate vicinity of the moisture sensor is measurable and wherein the spatial measurement range is not affected by a physical barrier, d. H. not influenced by the influence of a surrounding material.

According to the invention the object is solved by the features of the independent claims.

The dependent claims represent advantageous embodiments.

In a method of the type mentioned by the timing and analysis of the change in temperature of the heating element by utilizing the finite propagation velocity of the heat pulse in the material and the different temperature effects due to the different heat capacities depending on the moisture content of the distance to the heating element in which the moisture content is measured , influenced.

The heating element, a resistance heating element, is subjected to heating for a short defined period of time with a defined electric current, wherein the heat is absorbed and dissipated by the material surrounding the moisture sensor. The method makes it possible to determine the moisture content in a material, which may also have a relatively thin cross-section, for example a textile of a garment or a shoe or the like, by means of a thermal method, wherein the finite propagation speed of a heating pulse and the associated limitation of the measuring volume used becomes. The temperature of the heating element as a function of time and the moisture content describes a temperature profile, which is determined to determine the moisture content.

Thus, the temperature change produced by the defined heat pulse or the temporal temperature profile can be used as the measured value for the ambient humidity. Since the heat propagates at finite propagation velocity into the environment, it is possible to define the spatial influence range, by the duration of the heating pulse or the time points of the temperature measurements. By accurately controlling the timing of the temperature measurement, the affected measurement volume of the material to be examined or monitored is effectively limited. It is also possible to measure by skillful choice of the temporal measurement window also targeted areas that border indirectly to the sensor, i. you can focus the measurement on certain discrete layers, which are at a defined distance from the sensor. Thus, it is possible to measure the water content in thin layers, in particular of textile material used in the manufacture of clothing, without the wider environment, such as physical barriers, influencing the measurement result. In particular, the method is suitable for determining the moisture in thin layers whose environment is not defined in more detail. As an example, textile layers are mentioned, which are in more or less well-defined contact with the human body and / or other textile layers and / or the ambient air.

Due to the high thermal conductivity and high heat capacity of water compared to air, the thermal energy dissipated by the environment from the heating element is highly dependent on the water content of its environment.

The heat flow itself may vary during the heat pulse, but the course should be the same for each measurement. It is possible to analyze the entire temperature profile during the heating pulse or at defined times. It is important that the propagation front of the heat remains within the desired measurement volume at the time of the measurement or that the contact with the environment outside the desired measurement volume can not yet have an effect on the measurement at the sensor.

The heating element of the humidity sensor is protected from the environment by a thin electrically non-conductive layer.

The heating element itself can be used as the temperature sensor, in which its temperature-dependent electrical resistance is monitored and evaluated, or a temperature sensor which is attached directly to the heating element and is of course also coupled to the evaluation circuit and the control unit is provided. Of course, in addition to memory elements, interfaces and CPU, the control unit can comprise further electronic components, in particular also be coupled to input and / or output units or be designed for wireless data transmission. The energy supply can be stationary via a power supply unit or also in the form of a battery for, for example, mobile applications or applications close to the body. By choosing the heat pulse length, the intensity or the frequency of a measurement the energy consumption can be adapted to the respective situation.

The measurement volume directly detected by the moisture sensor can consist of the material to be examined for its moisture content. Alternatively, it is possible to embed the moisture sensor in a reference material in close moisture exchange with the material to be examined. The medium to be examined for its moisture content can thus be used e.g. be a textile itself and act directly on the moisture sensor or indirectly via the humidity sensor sensor firmly attached and moisture-absorbing reference material, which is in a moisture exchange with the environment. If the water absorption in the reference material is virtually hysteresis-free and fast enough in comparison to the change in the water content of the environment, then the moisture content in the reference material can be taken as the Aquipotential value, which is often referred to in soil physics as "suction stress", for the ambient humidity.

The moisture sensor suitable for heating and for temperature measurement can be realized in different ways and correspondingly integrated into the measuring environment. When installing the humidity sensor, care should be taken that the heat capacity of its structure is low and the contact with the material to be investigated, which can also be referred to as a measuring body, is sufficiently good for detecting the moisture.

It is possible to have a structure of the humidity sensor on a conventional circuit carrier, e.g. B. a circuit board to build. Alternatively, the sensor structure on a flexible circuit carrier, for. As a film, or a flexible and stretchable circuit carrier, for. As a stretchable film, are applied, so that the moisture sensor can closely conform to the monitored or to be examined material or incorporated into the material, for. B. in a textile.

Preferably, the moisture sensor comprises a gold thin film heating element coated on a thin film-like polyimide substrate. The heat pulse propagates through the polyimide and then reacts to the humidity of the immediate sensor environment.

As a further option, the structure can also be formed as a thread, fiber, wire or as a conductive yarn and can be correspondingly incorporated in e.g. textile fabric, i. immediately in clothing or footwear. As a result, textiles can be constructed with a largely integrated sensor structure, which does not disturb the normal moisture transport of the textile or the garment, whereby substantial advantages in the construction of intelligent clothing can be achieved.

The thermal moisture measurement method is robust against many interferences such as e.g. ionic impurities, parasitic capacitances or electrical interference fields, which is why they can also be used in a harsh environment, ie in particular close to reality and under particular stress by a wearer of the clothing as well as environmental influences. In addition, the thermal moisture measurement method is a nondestructive method so that the moisture sensor is directly applicable to the material without having to separate a sample from the material. Due to the short heat pulse or heat pulse, it is possible to limit the measuring range to a small range. It is therefore possible to measure thin layers without having to accept a greater influence on the wider environment.

Unlike many other measurement methods, such as. As the capacitive humidity measurement, the thermal moisture measurement method is very sensitive even in areas with very high moisture content. If the moisture sensor is incorporated into a textile in the form of a thread-like structure, various measurement tasks in the field of sports, health, ambient assisted living (AAL) can be solved relatively easily, without influencing the normal moisture transport. It is also possible to build sensors based on perforated films, so that the normal moisture transport is not hindered.

Due to the robust operation of the measuring principle, the moisture sensor can be used in systems worn close to the body, for example in portable electronic devices (wearables) or directly in textiles or clothing, in particular functional or protective clothing, in order to quantify the humidity climate or else to actively control ventilation systems. Due to the very short heating pulses, energy-saving small systems are possible. The ventilation systems themselves may be active or passive in nature, e.g. Flaps or fans.

Essential is the possibility of controlling the distance from the heating element in which the measurement of the moisture content takes place. This is done via the timing of the heating pulse and taking advantage of the finite propagation speed of the heat pulse in the material and the different temperature effects due to the different heat capacities depending on the moisture content.

In the simplest case, the spatial measuring range can be limited so that the measurement can be limited to a thin layer around the moisture sensor. It is also possible to analyze layers defined over the duration of the heat pulse at a certain distance from the heating element.

The sensor can be structured differently. Practical embodiments are e.g. thin gold tracks on a very thin polyimide film or as a semiconductor device or thin conductive threads. It is essential that the heat capacity be reduced by a miniaturized design, e.g. in thin film technology or microtechnology, can be kept very small, which improves the sensor sensitivity.

It goes without saying that the features mentioned above and those yet to be explained below can be used not only in the respectively specified combination but also in other combinations. The scope of the invention is defined only by the claims.

The invention will be explained in more detail below with reference to an embodiment with reference to the accompanying drawings.

• 1 eine schematische Darstellung einer Anordnung zur Durchführung des Verfahrens nach der Erfindung,
• 2 eine schematische Darstellung einer Wärmeverteilung,
• 3 Diagramme zur Darstellung der Zusammenhänge zwischen der Heizleistung und der Temperatur über die Zeit in trockenem und feuchtem Material,
• 4 ein Diagramm zur Darstellung der Eindringtiefe der Wärme in das Messmaterial und die Umgebung über der Zeit,
• 5 ein Diagramm einer Feuchtigkeitsmessung,
• 6 ein Diagramm einer Feuchtemessung, in dem die Widerstands- sowie die berechnete Temperaturänderung des Sensors über der Zeit aufgetragen sind, wobei unterschiedliche Feuchtezustände gezeigt sind (W=Wet, feucht; D=Dry, trocken).

It shows:

• 1 1 shows a schematic representation of an arrangement for carrying out the method according to the invention,
• 2 a schematic representation of a heat distribution,
• 3 Diagrams to show the relationships between the heating power and the temperature over time in dry and moist material,
• 4 a diagram showing the depth of penetration of the heat into the measuring material and the environment over time,
• 5 a diagram of a moisture measurement,
• 6 a diagram of a moisture measurement, in which the resistance as well as the calculated temperature change of the sensor are plotted over time, different moisture states being shown (W = wet, moist; D = dry, dry).

The moisture sensor 1 includes a structure 2 on which a heating element 3 is arranged via electrical connections 4 with an electronic control unit 5 which is an evaluation circuit 6 and of course includes a CPU in addition to memory elements, and an energy source 7 , for example in the form of a battery or an accumulator is coupled to the heating element 3 to apply a defined heating current for a predetermined period of time and thus generate a heating pulse or heat pulse and the temperature increase to determine the moisture content of the material that the moisture sensor 1 is assigned to measure.

The heating element 3 is designed as a thin-film heating element based on the thin polyimide structure 2 evaporated gold and encapsulated with a further polyimide layer. Other materials and manufacturing processes are possible.

2 it can be seen that by the arrows 10 indicated heat distribution based on the moisture sensor 1 into the material to be examined or monitored 8th , which can be, for example, a textile of a piece of clothing or a diaper, does not go into the environment within the measuring time 9 ,

After 3 is illustrated how the abrupt course of a heat flow Q over time t according to the line 11 to a core temperature T over time t affects the curve 12 of dry material 8th and the curve 13 on damp / wet material 8th is determined. Due to the high thermal effusiveness of water, heat is generated by the material 8th with higher water content better from the moisture sensor 1 dissipated. Other courses of the heat flow, such as a very short pulse, are also possible.

A representation of the depth of penetration x the heat or the heat flow of the heating pulse into the material 8th depending on the time t , is 4 can be seen, the depth of penetration also increases with increasing time. The depth of penetration is a root function of the time and the thermal conductivity of the environment and can be described using the "mean quadratic shift" of statistical physics. The heat continues to spread even after the heating current has been switched off. It is therefore also possible, for example, to use only a short heating pulse and then the cooling behavior of the moisture sensor 1 consider.

5 represents how the temperature T of the moisture sensor 1 with a sudden heat flow Q behaves. Depending on the time t is the temperature profile through the polyimide structure 2 of the moisture sensor 1 , then through the immediate sensor environment, namely the material 8th and its moisture content, and when measuring for a long time due to the material 8th subsequent environment 9 affected. By contemplation a certain time interval can influence the environment 9 be discriminated against from the moisture measurement. This means that the moisture measurement must be limited to the immediate sensor environment.

6 shows a diagram with the typical signal curve of a simulated measurement on a textile material 8th and six different moisture levels from air dry (D) to completely soaked (W). It shows the change in resistance of the heating element (ΔR / R) and its change in temperature in arbitrary units, both of which are directly related (PTC resistance properties). The time range from 100ms to 300ms (Mx) is suitable for this measurement to estimate the moisture in the textile.

LIST OF REFERENCE NUMBERS

1.

humidity sensor

Second

structure

Third

heating element

4th

connection

5th

control unit

6th

evaluation

7th

energy

8th.

material

9th

Surroundings

10th

arrow

11th

curve

12th

Curve

13th

Curve

Q

heat flow

t

Time

T

temperature

x

penetration depth

Mx

measuring range

D

dry

W

wet

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 4316384 A1 [0004]
• US 4666007 A1 [0004]
• US 4750143 A1 [0004]
• US 4798252 A1 [0004]
• US 4889201 A1 [0004]
• US 5402075 A1 [0005]
• US 6647782 B2 [0005]
• US 6222376 B1 [0005]
• US 5861758 A1 [0005]
• EP 1844323 B1 [0007]
• EP 0981737 A1 [0008]
• US 3550439 A1 [0009]

Cited non-patent literature

• Basics - Measurement Methods - Applications - Standards, Contact & Study, Volume 513, Klaus Kupfer, Expert-Verlag, 1997 [0003]

Claims (12)

Method for determining the moisture content of a moisture-absorbing material (8), in particular a textile, or a moisture-containing material, in particular skin, with a moisture sensor (1) comprising a heating element (3), which is coupled to a control unit (5) and an evaluation circuit is characterized in that the time control and analysis of the temperature change of the heating element (3) using the finite rate of propagation of the heat pulse in the material (8) and the different temperature effects due to the different heat capacities, depending on the moisture content, the distance to the heating element (3) in which the moisture content is measured is influenced. Procedure according to Claim 1 , characterized in that the control unit (5) applies a defined heating current to the heating element (3) for a predetermined period of time and the temperature profile, that is to say the time-dependent temperature change, is measured in order to determine the moisture content. Procedure according to Claim 1 or 2 , characterized in that the evaluation circuit (6) detects the temperature change during and / or after the heating current is applied to the heating element (3) by detecting the temperature resistance of the heating element (3) or a signal from a temperature sensor arranged on the heating element (3). Procedure according to one of the Claims 1 to 3 , characterized in that the control unit (5) varies the level of the heating current and the duration of the heating current application, the course of the heating current being the same for individual measurements of the temperature. Moisture sensor with a heating element (3) and a temperature sensor, characterized in that the heating element (3) is surrounded by an electrically non-conductive layer. Humidity sensor after Claim 5 , characterized in that the structure (2) of the moisture sensor comprises a flexible circuit carrier. Humidity sensor after Claim 5 , characterized in that the structure (2) of the moisture sensor (1) is designed as a film made of a polyimide. Humidity sensor after Claim 6 or 7 , characterized in that the structure (2) is perforated or perforated. Humidity sensor after Claim 5 or 6 , characterized in that the structure (2) of the moisture sensor (1) is thread-like or fiber-shaped. Humidity sensor after Claim 5 , characterized in that the heating element (3) is made of a gold foil. Arrangement for performing the method according to Claim 1 , characterized in that the moisture sensor (1) is integrated directly into the material to be monitored (8) or is arranged adjacent to the material to be monitored. Arrangement for performing the method according to Claim 1 , characterized in that the moisture sensor (1) is embedded in a reference material which is connected to the material to be examined (8).

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