阅读说明：本技术 用于测量奶中脂肪的方法及设备 (Method and apparatus for measuring fat in milk ) 是由 斯特凡·普夫吕格尔 沃尔夫冈·德拉赫姆 朱浩 于 2018-12-10 设计创作，主要内容包括：一种用于连续确定具有可变固体分数并且在管线中流动的有可变气体含量的奶的脂肪含量的方法和系统(400),包括：基于布置在管线(410)中的密度计(430)的测量管的至少两个弯曲振动期望模式的本征频率,查明在管线中流动的奶的声速值和平均密度值；借助连接到管线的压力传感器(440)来查明管线中的静压力值；基于声速值、平均密度值和压力值来查明气体体积分数值；基于平均密度值和气体体积分数值来查明在管线中流动的没有气体含量的奶的密度值；借助布置在管线中的微波传感器(420),基于测量奶中微波的传播速度和/或吸收中的至少一种,来查明在管线中流动的奶的介电常数值；以及,基于管线中流动的没有气体含量的奶的密度值和有效介电常数值来计算脂肪分数。(A method and system (400) for continuously determining the fat content of milk having a variable gas content and flowing in a pipeline with a variable solids fraction, comprising: ascertaining a sound velocity value and an average density value of milk flowing in the pipeline based on eigenfrequencies of at least two expected modes of bending vibrations of a measuring tube of a densitometer (430) arranged in the pipeline (410); ascertaining a static pressure value in the pipeline by means of a pressure sensor (440) connected to the pipeline; ascertaining a gas volume fraction value based on the sound velocity value, the average density value, and the pressure value; ascertaining a density value of the milk flowing in the pipeline without the gas content based on the average density value and the gas volume fraction value; ascertaining a value of a dielectric constant of the milk flowing in the pipeline based on measuring at least one of a propagation velocity and/or an absorption of microwaves in the milk by means of a microwave sensor (420) arranged in the pipeline; and calculating the fat fraction based on the density value and the effective dielectric constant value of the milk flowing in the pipeline without the gas content.)

1. A method for continuously determining the fat content of milk having a variable solid fraction and a variable gas content flowing in a line, comprising:

ascertaining a sound velocity value and an average density value of milk flowing in the pipeline based on eigenfrequencies of at least two expected modes of bending vibrations of a measuring tube of a densitometer arranged in the pipeline;

ascertaining a static pressure value in the pipeline with a pressure sensor connected to the pipeline;

ascertaining a gas volume fraction value based on the sound velocity value, the average density value, and the pressure value;

ascertaining a density value of the milk flowing in the pipeline without gas content based on the average density value and based on the gas volume fraction value;

determining, by means of a microwave sensor arranged in the pipeline, a value of the dielectric constant of the milk flowing in the pipeline based on measuring at least one of the propagation velocity and/or the absorption of microwaves in the milk; and

the fat fraction is calculated based on the value of the density and the value of the effective dielectric constant of the milk flowing in the line without gas content.

2. The method of claim 1, wherein to perform the calculation, the milk is modeled as a three-component system, wherein the components include fat, water, and fat-free solids.

3. The method according to claim 2, wherein the solids comprise proteins and carbohydrates, in particular mainly lactose.

4. Method according to one of the preceding claims, wherein the density of the milk flowing in the line without gas content is modelled as a function, for example a linear function, of the concentration of the components contained in the milk with the density values of the pure components as weighting factors;

wherein the effective permittivity of the milk flowing in the line is modeled, taking into account the gas content, as a function of the concentration of the components contained in the milk and the permittivity value of the pure components; and

wherein the concentration of the ingredient is ascertained, which results in the ascertained density value and the effective permittivity of the milk.

5. Method according to one of the preceding claims, wherein determining the dielectric constant takes place in the presence of at least one frequency higher than 1GHz, in particular higher than 2GHz, for example 2.45 GHz.

6. The method according to one of the preceding claims, further comprising:

measuring the temperature of the milk flowing in the line; and

ascertaining a temperature-dependent value of the density and/or dielectric constant of the ingredient contained in the milk.

7. The method of one of the preceding claims, wherein the densitometer comprises a coriolis mass flowmeter, wherein the method further comprises:

ascertaining a mass flow, a volume flow and/or a fat flow and/or a solids flow and/or a fat-free solids flow and/or a water flow in the line.

8. Measuring device for determining the fat content of milk in a pipeline, in particular with a method according to one of the preceding claims, comprising:

a densitometer having at least one oscillatable measurement tube for ascertaining density and sonic velocity measurements of a medium contained in the measurement tube based on at least desired mode eigenfrequencies of at least two desired modes of flexural vibration;

a pressure sensor for measuring an absolute pressure of a medium;

a microwave sensor for ascertaining the absorption and/or propagation speed of a microwave signal in a medium; and

a computer unit for calculating fat content based on the measurements of the densitometer, the pressure sensor and the microwave sensor.

9. The measurement device for determining the fat content of milk in a line according to claim 8, wherein the densitometer, the pressure sensor and the microwave sensor are installed in the line.

10. The measurement device of claim 8 or 9, wherein the densitometer comprises a coriolis mass flowmeter.

Technical Field

The invention relates to a method and apparatus for cream measurement:

background

Milk and intermediates obtained therefrom can be described as a mixture of various ingredients, mainly water, milk fat and solids, wherein the solids essentially comprise protein, carbohydrates (among them lactose in particular) and small amounts of minerals.

In the processing chain from raw milk and extending to the manufacture of milk products, the percentages of these components are important parameters for controlling the process, performing process and quality checks and balancing the product flow. Common practice is to determine the percentage using laboratory samples and standard methods. This means that only a small number of samples can be evaluated and that only after sampling an analytical result with a significant delay is obtained.

Indeed, it is possible to perform process-related spectral analysis by automated sampling in the infrared region, which is, however, firstly very expensive and, secondly, based only on the collection of a small number of samples over a relatively large time interval. Therefore, this analysis is only conditionally applicable for process control.

Density measurements, for example by means of coriolis flow meters, are indeed suitable for on-line measurements for continuous process monitoring and can also be used for determining the fat content or the percentage of solids of milk when the assumption of a given ratio of the percentages of solids of fat, carbohydrates (in particular lactose), proteins, etc. is reasonable. As the validity of this assumption decreases, the measurement results become correspondingly inaccurate.

Another difficulty in determining fat content with coriolis flow meters arises from the distribution of air microbubbles distributed in the milk, which on the one hand reduces the effective density and on the other hand leads to a change in the relationship between the eigenfrequency of the oscillating measuring tube of the flow meter and the density of the measured substance in the measuring tube due to the oscillation of the now compressible milk compared to the measuring tube. The limited applicability of coriolis mass flowmeters for determining the composition of a gas-containing liquid is described, for example, in US7,363,800B2. This patent teaches an apparatus having firstly a microwave sensor for ascertaining dielectric parameters of a medium, secondly a coriolis mass flowmeter, thirdly a separate sensor for determining the gas composition of the medium, and fourthly a signal processing unit for processing the different sensors. However, this is a complex and expensive device.

Disclosure of Invention

It is therefore an object of the present invention to provide a method and a corresponding measuring device for reliably and continuously measuring the fat fraction of milk even with variable solid fraction and variable gas content.

According to the invention, this object is achieved by a method as defined in independent claim 1 and a measuring device as defined in independent claim 8.

The method of the invention for continuously determining the fat content of milk with variable gas content flowing in a line with variable solid fraction, comprises:

ascertaining a sound velocity value and an average density value of the milk flowing in the pipeline on the basis of eigenfrequencies of at least two expected modes of bending vibrations of a measuring tube of a densitometer arranged in the pipeline;

ascertaining a static pressure value in the pipeline by means of a pressure sensor connected to the pipeline;

ascertaining a gas volume fraction value based on the sound velocity value, the average density value, and the pressure value;

ascertaining a density value of the milk flowing in the pipeline without gas content based on the average density value and based on the gas volume fraction value;

determining, by means of a microwave sensor arranged in a pipeline, a value of the dielectric constant of the milk flowing in the pipeline based on measuring at least one of the propagation velocity and/or the absorption of microwaves in the milk; and

the fat fraction is calculated based on the density value and the effective dielectric constant value of the milk flowing in the pipeline without gas content.

In a further development of the invention, milk is modeled as a three-component system, where the components include fat, water and fat-free solids.

In a further development of the invention, the solids comprise proteins and carbohydrates, among which especially lactose.

In a further development of the invention, the density of the milk flowing in the pipeline without gas content is modeled as a function, for example a linear function, of the concentration of the components contained in the milk with the density values of the pure components as weighting factors; wherein the effective permittivity of the milk flowing in the line is modelled as a function of the concentration of the components contained in the milk and the permittivity values of the pure components, taking into account the gas content; and wherein the concentration of the ingredient is ascertained, which yields the ascertained density value and effective permittivity of the milk.

In a further development of the invention, the determination of the dielectric constant takes place in the presence of at least one frequency above 1GHz, in particular above 2GHz, for example 2.45 GHz.

In a further development of the invention, the method further comprises measuring the temperature of the milk flowing in the line; and ascertaining temperature-dependent values of the density and/or dielectric constant of the ingredients contained in the milk.

In a further development of the invention, the density meter comprises a coriolis mass flowmeter, wherein the method further comprises:

the mass flow, the volume flow and/or the fat flow in the line are ascertained.

Measuring device of the invention for determining the fat content of milk in a pipeline, in particular with a method according to one of the preceding claims, comprising:

a densitometer having at least one vibratable measurement tube for ascertaining a density measurement and a sound velocity measurement of a medium contained in the measurement tube based on at least desired mode eigenfrequencies of at least two desired modes of bending vibration;

a pressure sensor for measuring an absolute pressure of the medium;

a microwave sensor for ascertaining the absorption and/or propagation speed of a microwave signal in a medium; and

a computer unit for calculating the fat content based on the measurements of the densitometer, the pressure sensor and the microwave sensor.

In a further development of the invention, the densitometer, the pressure sensor and the microwave sensor are installed in a pipeline.

In a further development of the invention, the densitometer comprises a coriolis mass flowmeter.

Drawings

The invention will now be explained in more detail on the basis of an example of embodiment shown in the drawings. The drawings are as follows:

FIG. 1: a flow chart of an example of an embodiment of the method of the present invention;

FIG. 2: a more detailed flow chart of an example of an embodiment of a first sub-process of the method of the present invention;

FIG. 3: a more detailed flow chart of an example of an embodiment of a second sub-process of the method of the present invention; and

FIG. 4: schematic illustration of an example of an embodiment of the device of the invention.

Detailed Description

The components of milk can be essentially summarized in four groups, namely water, fat, protein and carbohydrates, wherein the carbohydrates comprise for example more than 95% lactose and a small fraction of glucose and galactose. Furthermore, depending on the physical process conditions, in particular on the existing flow conditions, the entrained air may be present in the form of micro-bubbles, which need to be taken into account in the analysis. The following table presents the physical properties of the ingredients and air by way of example:

with the aid of these variables, in each case according to the fraction a of the component_iThe effective density and dielectric constant of the mixture are given as follows:

ρ_{milk with air}＝f((1-a_{Air (a)})a_{Water (W)}，(1-a_{Air (a)})a_Fat，(1-a_{Air (a)})a_SNF，a_{Air (a)})

′_eff＝f((1-a_{Air (a)})a_{Water (W)}，(1-a_{Air (a)})a_Fat，(1-a_{Air (a)})a_SNF，α)

Air fraction a_{Air (a)}Can be ascertained by means of a densitometer and an auxiliary variable pressure and taken into account in the equation as a known parameter. These relationships naturally hold true also in the case of processes in which the air fraction is process-dependent and absent (a)_{Air (a)}＝0)。

Since the density and dielectric constant of carbohydrates and proteins are almost identical, they can be essentially unquestionably combined as one ingredient, fat-free Solids (i.e. Solids-no cat (snf)), and considered by calculating the average density and dielectric constant, which is obtained using the typical mixing ratio of the two ingredients in milk. In the case of milk, this will be about 58% carbohydrate (96% of which is lactose) and 42% protein, according to wikipedia.

The average density can basically be calculated as a weighted average of the individual densities.

To determine of'_effA typical mixing equation is the brugman equation:

in this case:

_MG：’_eff

_h: dielectric constant of matrix phase (water)

_n: dielectric constant of additive (fat, fat-free solid, air)

f_n: volume fraction of each component

Markel-Introduction to the Maxwell Garnettap approximation guide, Journal of the Optical Society of American A

As a result:

ρ_milk＝f(a_{Water (W)}，a_Fat，a_SNF) (equation 1)

’_eff＝f((1-a_{Air (a)})a_{Water (W)}，(1-a_{Air (a)})a_Fat，(1-a_{Air (a)})a_SNF，a_{Air (a)}) (equation 2)

Wherein, a_{Air (a)}Is the volume fraction of the gas content.

The third equation is derived from the sum of the volume fractions:

a_{water (W)}+a_Fat+a_SNF1 (equation 3)

Three equations with three unknowns are obtained, from which the fractions of water, fat and fat-free solids can be determined without other assumptions.

In particular, the density and the dielectric constant depend on the temperature, and the dielectric constant depends on the measurement frequency. Temperature measurements that take into account the temperature dependence of the material properties in the solution of the above set of equations can achieve the required accuracy.

As shown in FIG. 1, the method 100 begins by determining a density ρ of milk without an air fraction_MilkThe first step 110. This is done by finding the average density and thus the density ρ of the milk containing air_{Milk with air}Finding out the air component a_{Air (a)}And is performed by removing air components to correct the density.

In a second step 120, the effective dielectric constant 'is then determined'_effThis is achieved by measuring the propagation characteristics of electromagnetic waves in the milk.

In a third step 130, the set of equations equation 1, equation 2, equation 3 is solved in order to determine the fat fraction and, in the given case, the fractions of the other components.

As shown in fig. 2a, step 110 is subdivided into the following steps:

in step 111, f occurs to determine the coriolis mass flow measurement sensor₁-flexural vibration mode and f₃The eigenfrequency of the flexural vibration mode, which is also used here for density measurements. For this purpose, f can be excited simultaneously₁-flexural vibration mode and f₃-a bending vibration mode. The eigenfrequency sought can be ascertained by varying the excitation frequency to maximize the ratio of the vibration amplitude to the mode-specific excitation power.

Based on the ascertained eigenfrequency f_iIn step 112, the initial density value ρ₁And ρ₃Is determined as:

wherein c is_0i、c_1iAnd c_2iIs the mode correlation coefficient.

In step 113, determining the speed of sound of the gas-containing liquid and, in given cases, determining a correction term for the density measurement occurs.

Then, in step 114, the gas volume fraction a is calculated from the sound velocity and pressure measurements_{Air (a)}And the density of the milk minus air is calculated, as explained in more detail below.

As shown in FIG. 2b, step 113 includes determining the correction term, first, in step 1131, a ratio V of the initial density values is calculated, thus, for example, the initial density values ρ₁And ρ₃Divided to form V: rho ═ p₁/ρ₃。

Then, in step 1132, a value of the speed of sound c is determined, which is related to the eigenfrequency f of the measured bending vibration mode₁And f₃The ratio V of the observed initial density values is given in the following equation:

where r is, for example, 0.84, b is 1 and g is a scaling factor between the sound velocity and the resonance frequency associated with the measuring tube, which may assume, for example, a value of 10/m. The sound velocity value satisfying the above equation is a sought value of the sound velocity of the gas-containing liquid.

Based on the ascertained speed of sound values, then in step 1133 of the method in fig. 2b, a mode-specific correction term K for the resonator effect can be calculated_i：

Finally, in step 1134, the density value ρ of the milk containing air may be calculated_{Milk with air}The following were used:

the determination of the air fraction and calculation of the density of the milk without air in step 114 is shown in more detail in fig. 2C and is based on the following relationship between the sound velocity of the gas-containing liquid and additional parameters:

in this case, a_{Air (a)}Is the volume fraction of air, c_{Air (a)}Is the speed of sound in air, c_MilkIs the speed of sound in the air-free milk, gamma is the air adiabatic coefficient, p is the current pressure of the air-containing milk, and ρ_MilkIs the density of milk without air.

The density of the air-containing milk is the weighted sum of the individual densities. The density of air at standard pressure is for example three orders of magnitude lower than that of pure milk, and the volume fraction of air is in the order of a few percent, then the density of air-containing milk can be estimated as follows:

ρ_{milk with air}＝ρ_Milk(1-a_{Air (a)})+ρ_gα

ρ_{Milk with air}≈ρ_Milk(1-a_{Air (a)}) (M2)

Thus, the equation of sound speed C1 can be written as:

by ignoring a_{Air (a)}The square term in (a) yields:

solving for a_{Air (a)}Values for air volume fraction are given

In fact, in the pressure range associated with milk processing, the denominator is substantially dominated by the third term, resulting in the following approximate result:

here, the reference value can be used for the speed of sound c in pure milk without air_Milk。

As shown in FIG. 2c, to determine the air fraction in step 1141, the pressure value of the gas-containing liquid is ascertained, which is at the measurement eigenfrequency f₁And f₃Is located in the milk so that with the equation M1, the density p can be ascertained_{Milk with air}And using equation C1, the speed of sound C of the milk containing air can be ascertained_{Milk with air}。

For the adiabatic coefficient γ, it can be considered that:

γ＝c_p/c_vwhere f is the number of molecular degrees of freedom of the gas, which is equal to 1.4 at room temperature, for example for nitrogen and dry air.

In step 1142, the ascertained density ρ of the air-containing milk is then based on the pressure measurement_{Milk with air}And the found sound velocity c of the air-containing milk_{Milk with air}The air volume fraction a is calculated by the equation A1_{Air (a)}。

In step 1143, the density ρ sought for the air-free milk is then calculated_Milk：

This provides the first measured variable to solve the set of equations equation 1, equation 2, equation 3.

A second step 120 will now be described, in which a second measured variable, i.e. the relative permittivity, is ascertained.

This is based on a measurement 121 of the propagation characteristics of the electromagnetic wave (amplitude and phase of the received signal relative to the transmitted signal) within the medium in the pipeline between the transmitting antenna and the receiving antenna separated by a distance d. This measurement 121 can be performed with electromagnetic waves of different frequencies f, so that at 122 the transfer function in the frequency domain s (f) within a frequency band of, for example, 2GHz-4GHz is ascertained.

In practical measurement systems, the measured spectrum s (f) contains not only the (medium-dependent) propagation characteristics in the distance between the transmitting and receiving antennas, but indeed also the attenuation and phase rotation of the antennas, the connecting cables and the transition positions. In the given case, the effect of multiple reflections in the area of the connecting cable is added to this. These effects can be characterized largely by appropriate reference measurements, and as a result, the measurements are compensated at 123 so that only the relevant part of the transfer function between the transmit and receive antennas is preserved.

An impulse response in the time domain may be calculated at 124 according to a transfer function s (f) using an inverse fourier transform. Due to the measurement of the restricted band region, there is then also an impulse response of the system to the excitation with the band-limited pulse, in the form of a window function applied to the inverse fourier transform. From the aboveThe position of the maximum of the delayed pulse with respect to the time axis, the group propagation time τ in the measured band-limited region can be ascertained at 125_g. Thus, the propagation velocity of the signal can be estimated in a simple manner at 126:

in many polar media, dispersion occurs (the dielectric constant and the propagation speed depend on the frequency of the electromagnetic wave). The above-described estimated mean propagation times are therefore only suitable to a limited extent in the sense of group propagation times for directly determining the medium properties. To enable accurate measurements, the phase response can be exploited by calculating the phase propagation time as a function of frequency in the measured frequency band at 127:

by selecting n so that τ is_phAnd τ_grWith minimum deviation therebetween to removeThe integer n can describe the ambiguity of the phase response. In this manner, at 128, the phase now responds, and passes

The exact response of the propagation velocity to frequency can be determined. The behavior of the decay d is directly known from the magnitude response of s (f).

These can be directly converted to physical variables of the complex-valued dielectric constant of the medium at 129 according to the now known behavior of C and α expressed by the following two equations ∈^*＝∈′+j∈″

Wherein:

ω: angular frequency (ω ═ 2 π f)

μ: permeability, μ ═ μ₀μ_r

μ₀: the constant of the magnetic field is constant,

μ_r: relative magnetic permeability

₀: the constant of the electric field is constant,

now ∈ determined from the measurement by using the value of the measurement frequency defined earlier or by processing the total measurement data vector in equation 2^*And ∈' may be used in equation 2.

Based on the above, everything is ready to solve the set of equations equation 1, equation 2, equation 3 and thus determine the fraction of the component in milk, in particular the fat fraction.

Finally, fig. 4 shows a measuring device 400 of the invention for determining the milk fat fraction, in particular by means of the method of the invention. The measuring device 400 includes: a measuring device installed in the pipeline 400, i.e., a microwave sensor 420; a coriolis mass flow meter 430 for recording the density and mass flow of the medium flowing in the line 410, in particular a coriolis mass flow meter with two curved measuring tubes; and an absolute pressure sensor 440 having a measurement output connected to the secondary signal input of the coriolis mass flowmeter. The measuring device 400 further comprises a computer unit 450, the computer unit 450 being connected to the microwave transmitterThe signals of the sensor 420 and the coriolis mass flowmeter 430. The microwave sensor 420 is adapted to record the value of the dielectric constant and/or the absorption of the medium flowing in the pipeline based on the signal propagation time and to output it to the computer unit 450. Coriolis mass flowmeter 430 is adapted to ascertain in addition to mass flow

Density outside, air fraction a_{Air (a)}And the medium temperature T, and outputs these to the computer unit 450. The computer unit 450 is adapted to ascertaining the composition of the medium flowing in the line on the basis of these input variables and to outputting this composition assuming that the medium is milk.