WO2016198182A1

WO2016198182A1 - Interferometric device and system for measuring the refractive index of a medium

Info

Publication number: WO2016198182A1
Application number: PCT/EP2016/058064
Authority: WO
Inventors: Ingo Ramsteiner
Original assignee: Robert Bosch Gmbh
Priority date: 2015-06-10
Filing date: 2016-04-13
Publication date: 2016-12-15
Also published as: DE102015210627A1

Abstract

The invention relates to a device (1) for measuring the refractive index n of a medium (2, 2a, 2b), comprising: a white light interferometer (3), which has a light source (11), a beam splitter (14) for splitting the light (21) emitted by the light source (11) into a reference beam (22), and a detection beam (23), which detection beam can be guided through the medium (2, 2a, 2b), a reference path (13, 13a) having a variable length (L), on which reference path the reference beam (22) is guided, and a detector (15) for measuring the interference product (100) formed from the reference beam (22) and the detection beam (23); and an evaluating unit (101), which is designed to determine the refractive index n from the functional dependency of the interference product (100) on the length L. The invention further relates to a system (110) for measuring the concentration of urea (2a) in a solution (2) for the aftertreatment of exhaust gas in a motor vehicle (4), said solution containing at least urea (2a) and water (2b), comprising a device (1) according to the invention, wherein the detection beam (23) of the device (1) is guided through a storage tank (41) for the solution (2) and/or through a line (42) that conducts the solution.

Description

Description Title:

Interferometric device and system for measuring the refractive index of a medium

The invention relates to a simple and insensitive to contamination device for measuring the refractive index of a medium, in particular a working fluid for a motor vehicle.

State of the art

Future stricter emission standards for diesel vehicles will no longer be met by internal engine measures alone, but will also require improved exhaust aftertreatment. To reduce the emissions of nitrogen oxides, a post-treatment with AdBlue, which according to DIN 70070 or ISO 22241 is a solution of urea in water with a urea concentration of 31.8% to 33.2%, has proven very successful. As the liquid ages, the urea content changes

Exhaust aftertreatment system must be considered in the dosage. In addition, as the refilling of AdBlue will not only be done in workshops, but also by the user of the vehicle, it is

desirable to be able to detect a faulty filling of the AdBlue tank with cooling water, oil, fuel or other substances.

AdBlue has a refractive index n in the range of 1.380 to 1.385 at standard urea concentrations. This is significantly higher than the refractive index n = l, 33 of pure water and at the same time lower than the refractive index n> 1, 4 of most fuels. The refractive index n is therefore suitable as a physical measured variable both for the determination of the urea concentration and for the detection of incorrect filling. DE 10 2007 010 805 B3 discloses a method for measuring the

Refractive index n of a liquid in which the critical angle of total reflection between the liquid and a transparent element with a known refractive index is determined. EP 2 543 839 A1 discloses a simpler concept in which the Fresnel coefficient is determined at an interface between the liquid under investigation and another medium. However, these methods appear to be less suitable for a rugged, maintenance-free sensor for the automotive industry because they are devoid of any light scattering

Deposits (particles, deposits, microscopic air bubbles) at the optical interfaces can be disturbed.

To overcome this problem, it is proposed in DE 10 2010 041 136 B4, the refractive index n by refraction on a

to determine prism-shaped measuring cell. Minor deposits at the interfaces then weaken the light intensity, but do not change the direction in which the light is refracted. This is bought with high demands on the manufacturing precision or adjustment of the optical components and is therefore relatively expensive. Furthermore, the refracted light must have a relatively large intensity distance to scattered light, the

Deposits or suspended matter in the medium is generated.

Disclosure of the invention

In the context of the invention, a device for measuring the

Refractive index n of a medium developed. According to the invention, this device comprises a white light interferometer with a light source, a beam splitter for splitting the light emitted by the light source into a reference beam and a detection beam that can be guided through the medium, a reference path with variable length L, on which the reference beam is guided, and a detector for Measurement of the reference beam and the

Detection beam formed interference product. Furthermore, one is

Evaluation unit provided for determining the refractive index n from the functional dependence of the interference product of the length L is formed.

The essential feature of a white light interferometer in the context of this invention is that the light emitted by the light source is not just one

Wavelength or a few discrete wavelengths, but a continuum of wavelengths within a certain spectral width. If only one wavelength is used, the intensity of the interference product oscillates periodically with the length L of the reference path. If, on the other hand, a continuum of wavelengths is used, there is exactly one interval of lengths L of

Reference path in which an interference product occurs at all. Within this interval, the intensity of the interference product assumes a maximum at exactly one length L. The position of this maximum on the axis of length L is determined by the optical path length change experienced by the detection beam on its way through the medium due to the refractive index n of the medium. Conversely, from the position of the maximum of

Refractive index n of the medium can be uniquely determined. This happens in the device belonging to the evaluation. In the prior art, white light interferometers are used in conjunction with spatially resolved imaging techniques such as microscopy or optical coherence tomography. Such devices are expensive, sensitive laboratory equipment. Prima facie is therefore a white light interferometer for use in a robust, inexpensive, mass production suitable sensor for the automotive sector not suitable. The inventors have recognized that the crucial point of the white light interferometer in the context of the invention is the continuous wavelength spectrum and not the

Spatial resolution. Only one-dimensionally the length L of the reference path is measured. The continuous wavelength spectrum provides a clear link between this length L and the refractive index n; it is therefore the key to determining the refractive index n with a

interferometric method. Here are the requirements of the

Manufacturing precision or adjustment of the optical components low. The decisive factor for the accuracy of the measurement result is the exact knowledge of the length L of the reference path. Means for adjustment or measuring this path with very high precision are generally available on the market. In principle, however, it suffices, in the context of a two-point calibration, to match the variable length L to two reference liquids or test specimens introduced into the detection beam with known refractive indexes which differ from one another.

Deposits on the interfaces with the medium weaken the intensity of the detection beam, but this has no influence on the position of the maximum, which assumes the intensity of the interference product. Although the deposits have a refractive index n ₂ , which fundamentally changes the optical path length of the detection beam. However, these deposits are at most a few microns thick, while the distance that the detection beam travels within the medium is at least three orders of magnitude longer. The optical path length change of the detection beam through the refractive index n ₂ of deposits is therefore negligibly small.

Thus, the present invention as a whole combines the advantages of a

Measuring method based on a volume effect within the medium instead of on an interfacial effect at interfaces of the medium, with a simple, inexpensive and robust apparatus realizability. The

Device is compact enough to build in for sensors in

Automotive sector provided form factor space to find.

In a particularly advantageous embodiment, the detection beam is guided between the medium and the detector via the beam splitter, and / or the

Reference beam is passed between the reference path and the detector via the beam splitter. Optical paths in the device can then be used by one beam bidirectionally or by two parallel beams. This makes the interferometer particularly compact.

Advantageously, an interface is provided at which the detection beam is reflected after a first passage of the medium on a path leading again through the medium to the beam splitter. As a result, the optical path traveled by the detection beam within the medium is doubled.

Accordingly, the detection sensitivity for changes in the optical Path length, which are caused by changes in the refractive index n of the medium, advantageously increased.

In a particularly advantageous embodiment of the invention, the light source, the reference path and the detector are arranged in a common housing.

All three components can then be sealed in one or more cavities against the medium under investigation and against contamination.

Furthermore, this housing, for example, consist of a preferably transparent molded part, which is connected to a base plate and forms together with this sealed against the environment cavities. The

The baseplate may then be, for example, simultaneously an electronic board on which the light source, the detector, means for changing the length L of the reference path, the application specific integrated circuit (ASIC) or the embedded system for controlling the device and other electronic components may be integrated , Furthermore, the

Base plate also accommodate additional optical elements for beam shaping, such as lenses or diaphragms. Advantageously, the light from the light source is guided through a collimator, for example a collimating lens, to the beam splitter. The light then forms a parallel beam. The detection beam and the reference beam are advantageously guided through a common concentrator, for example a focusing lens, to the detector. This maximizes the interaction between the two beams through the interference.

In a particularly advantageous embodiment of the invention is the

Detection beam passed through an area outside the housing.

For example, an area may be provided on the path of the detection beam, in which a first wall of the housing is initially through the

Detection beam is guided on a second wall of this housing and then runs parallel to this second wall of the housing. A physical or chemical change of the medium then propagates particularly quickly into the area in which the detection beam is passed through the medium. The device then reacts very quickly to such changes. At the same time, all components of the device, in particular the electronic and optical components, are enclosed in airtight, dry cavities. The beam splitter can be equipped, for example, as a separate optical element on the base plate. In a particularly advantageous embodiment, however, it is part of the housing. As a result, the production of the device is further simplified.

Advantageously, the housing extends from a center in which, for example, the beam splitter can be located, in two mutually perpendicular spatial directions. Advantageously, the housing extends in both directions equally far and / or in at least one of the two

Spatial directions symmetrical to the center. Advantageously, the part of the

Housing through which the detection beam is guided away from the beam splitter and back to him, the same outer dimensions as the part of the housing through which the reference beam is guided away from the beam splitter and back to him. Thermal length changes in both parts of the housing, the

For example, be caused by the temperature of the medium under investigation, then compensate and remain without influence on the measurement result for the refractive index n. Advantageously, the molding is transparent. It may for example be made of a transparent plastic. If the molded part is not transparent, for example because ambient light is to be shielded, transparency must be produced at all points at which light is to pass. This relates in particular to an outer wall to an area outside the housing, through which the detection beam is guided, as well as the beam splitter, if it is part of the housing. Transparency can be produced, for example, by installing corresponding windows in the molded part. However, it is also possible, for example, conversely, for an initially transparent molded part to be selectively colored where it should no longer be transparent.

Advantageously, the emission spectrum of the light source is at least partially in the wavelength range from 400 nm to 1100 nm, preferably from 500 nm to 900 nm. In the wavelength range below 1100 nm advantageously inexpensive silicon detectors, such as a photodiode for the

Interference product can be used. In the range between 500 nm and 900 nm is an aqueous urea solution for use in one

Exhaust after-treatment system particularly transparent.

The spectral width of the light determines its coherence length. The

Coherence length is inversely proportional to the spectral width. The smaller this coherence length, the sharper the unique maximum of

Intensity of the interference product defined. The sharper this maximum is defined, the steeper depends on the determined value for the opposite

Refractive index n of the length L of the reference path from. Accordingly, the accuracy requirements for the measurement or increase

Setting this length L. Advantageously, the light source emits light having a spectral width of at least 10 nm, preferably from 20 nm to 50 nm.

The light source can be, for example, a superluminescent diode or a light-emitting diode, which can optionally be combined with a bandpass filter. The light source can also be, for example, an incandescent lamp. The light of a superluminescent diode is per se most suitable for carrying out the measurement, but a light-emitting diode is significantly cheaper and therefore an economic compromise for many applications.

To vary the length L of the reference path, the reference path advantageously includes a mirror that is movable by an actuator along the reference path. The spatial resolution of this actuator then determines in conjunction with the spectral width of the light, the accuracy with which changes in the refractive index n can be detected. The measuring range, ie the detectable interval of the refractive index n, is determined by the maximum displacement of the actuator or the mirror in interaction with the length on which the detection beam is guided through the medium. With the same maximum adjustment path of the actuator an extension of the path on which the detection beam is passed through the medium reduces the measuring range.

Conversely, extending the path on which the detection beam passes through the medium increases the sensitivity of the device. This means that the minimum detectable change in the refractive index n becomes smaller. The longer the path on which the detection beam is passed through the medium, the more larger displacement paths of the mirror corresponds to a given change in the refractive index n.

For example, if not yet guided by the medium

Detection beam and the reference beam aligned so that the

Interference product between the detection beam and reference beam has a maximum intensity, and then the detection beam is passed over a length of 10 mm through a medium with refractive index n = l, 4, the length L of the reference path must be extended by 4 mm to this maximum restore.

The actuator can be realized in different ways. It may be, for example, a stepper motor or a piezoelectric actuator. A piezoelectric actuator is particularly reproducible and wear-free, but offers a lower displacement compared to a stepper motor. The lifetime of the calibration between the set value for the length L at maximum interference on the one hand and the determined refractive index n on the other hand is given primarily by the long-term stability and reproducibility of the actuator. In a further particularly advantageous embodiment of the invention is the

Evaluation unit designed to determine the concentration C of at least one substance in the medium of the refractive index n.

For example, for a medium that is a mixture of two components, for the concentration C of one of the components, at least at an interval of this concentration C, an unambiguous association between the one determined

Refractive index n and the concentration C. This applies, for example, to AdBlue, whose refractive index varies from 31.8% to 33.2% in the range from n = 1.380 to n = 1.385 at standard urea concentrations. Advantageously, a temperature sensor for measuring the temperature T of

Medium provided, and the evaluation unit is adapted to additionally use this temperature T for determining the concentration C of the substance. If the refractive index n of the medium or a

Component herebefore is temperature dependent, can in this way between changes in the refractive index n due to the temperature T and changes of refractive index n can be distinguished by changing the composition of the medium.

In the light of the above, the invention also relates to a system for measuring the concentration of urea in a solution containing at least urea and water for exhaust aftertreatment in a motor vehicle. This system comprises the device according to the invention in the

Embodiment in which the evaluation unit is designed to, the

To determine concentration of urea in the solution of the refractive index n. The detection beam of the device is passed through a storage tank for the solution and / or through a line carrying the solution. The

Use of the device according to the invention combines a simple apparatus design and an acceptable price for mass production with good accuracy and a significantly improved tolerance to deposits on optical surfaces. This makes the system robust and maintenance-free for a long time.

Further measures improving the invention will be described in more detail below together with the description of the preferred embodiments of the invention with reference to figures.

embodiments

It shows:

Figure 1 embodiment of the system according to the invention in supervision.

2 shows a section through the embodiment shown in Figure 1 along the

Line A-A.

3 shows a section through the embodiment shown in Figure 1 along the

Line B-B.

According to FIG. 1, a light source 11 emits light 21, which is passed through a collimator 16 to a beam splitter 14. By this beam splitter 14, the light 21 in half in a detection beam 23 and in a reference beam 22nd

split. The reference beam 22 is guided on a reference path 13, which runs over an adjustable mirror 13a and is thus passed bidirectionally. The detection beam 23 is guided in a region 33 through the medium 2. This region 33 ends at an interface 34, which is reflective for the detection beam 23. As a result, the region 33 is traversed twice by the detection beam 23, which doubles the optical path length. All that is required for this is that the material to which the medium 2 adjoins via the interface 34 has a refractive index n ₃ sufficiently different from the medium 2. The interface 34 may be blank or diffusely reflective. But it can also be advantageous mirrored.

The reference beam 22 is passed through the beam splitter 14 and the concentrator 17 to the detector 15. The detection beam 23, which has been reflected by the interface 34 back to the beam splitter 14, by the same

Concentrator 17 also passed to the detector 15. At the detector 15, the reference beam 22 and the detection beam 23 are brought into interference; the measurement signal 100 of the detector 15 is proportional to the intensity of the interference product formed from both beams 22 and 23. As explained above, it comes only to interference when the reference beam 22 and the

Detection beam 23 have undergone comparable total optical path lengths. Since the wavelength spectrum of the light source 11 is continuous, there is exactly one length L of the reference path 13, for which a maximum of

Intensity of the interference product 100 sets. The evaluation unit 101 determines the refractive index n of the medium 2 from the position of this maximum.

The medium 2 is a solution containing at least urea 2a and water 2b, which is used for exhaust gas aftertreatment in a motor vehicle 4. It is contained in a storage tank 41, from which a line 42 leads away. The

White light interferometer 3 is completely immersed in the solution 2 in the storage tank 41. Its housing 31, 32 comprises a transparent molding 31, which is designed so that the region 33 in which the detection beam 23 is guided through the medium 2, outside of the molding 31 and thus outside of the housing 31, 32 is arranged. In this area 33 is an additional

Temperature sensor 35 is arranged, which reports the temperature T of the medium 2 to the evaluation unit 101. From the with the help of the white light interferometer 3 determined refractive index n of the medium 2 and the temperature T of the

Medium 2 determines the evaluation unit 101, the concentration C of urea 2a in the solution. 2

Figure 2 shows the embodiment shown in Figure 1 in one

Sectional drawing along the line A-A. In this sectional drawing, the optical path to the detector 15 and the reference path 13 are visible. It turns out that the mirror 13a forming the end of the reference path 13 is slidable along the reference path 13 by an actuator 12. This actuator 12 is driven by the evaluation unit 101, so that the evaluation unit 101, the length L of the reference path 13 is always known. The reference path 13 and the

Beam path to the detector are encapsulated by the medium 2. For this purpose, the transparent molded part 31 is a base plate 32 sealed and forms together with this base plate 32, the housing 31, 32nd

FIG. 3 shows the exemplary embodiment shown in FIG. 1 in a further sectional drawing along the line B-B. This sectional view shows the light source 11 with associated collimator 16 as well as the beam path of the detection beam 23. The detection beam 23 emerges from the beam splitter 14 through a least specular, ideally non-reflective,

Interface 36 on the molded part 31 in the region 33 via, which is arranged outside of the housing 31, 32. This area 33 is filled with the medium 2. The detection beam 23 is reflected back at an interface 34, which is arranged on the shaped part 31 at the end of the region 33, through the region 33 in the direction of the beam splitter 14. In the region 33, the wall of the molded part 31 is guided directly onto the base plate 32. When passing through the region 33, the refractive index n of the medium 2 causes the speed of light c to be reduced to c / n. This corresponds to a geometric extension of the region 33 by the factor n.

Claims

claims

1. Device (1) for measuring the refractive index n of a medium (2, 2a, 2b), comprising a white light interferometer (3) with a light source (11), a beam splitter (14) for splitting the light emitted by the light source (11) (21) into a reference beam (22) and through the medium (2, 2a, 2b) feasible detection beam (23), a reference path (13, 13a) with variable length L, on which the reference beam (22) is guided, and a detector (15) for measuring the interference product (100) formed from the reference beam (22) and the detection beam (23), further comprising an evaluation unit (101) for determining the refractive index n from the functional

Dependency of the interference product (100) of the length L is formed.

2. Device (1) according to claim 1, characterized in that the

Detection beam (23) between the medium (2, 2a, 2b) and the detector (15) via the beam splitter (14) is guided and / or the reference beam (22) between the reference path (13, 13a) and the detector (15) is guided over the beam splitter (14).

3. Device (1) according to claim 2, characterized in that an interface (34) is provided, on which the detection beam (23) after a first passage of the medium (2, 2a, 2b) on a again through the medium (2 , 2a, 2b) leading to the beam splitter (14) is reflected.

4. Device (1) according to one of claims 1 to 3, characterized

in that the light source (11), the reference path (13, 13a) and the detector (15) are arranged in a common housing (31, 32).

5. Device (1) according to claim 4, characterized in that the

Detection beam (23) through a region (33) outside this housing (31, 32) is guided.

6. Device (1) according to one of claims 4 to 5, characterized

in that the beam splitter (14) is part of the housing (31, 32).

7. Device (1) according to one of claims 1 to 6, characterized

characterized in that the emission spectrum of the light source (11) is at least partially in the wavelength range from 400 nm to 1100 nm, preferably from 500 nm to 900 nm.

8. Device (1) according to one of claims 1 to 7, characterized by a light source (11) which emits light (21) having a spectral width of at least 10 nm, preferably from 20 nm to 50 nm.

9. Device (1) according to one of claims 1 to 8, characterized

in that the evaluation unit (101) is designed to determine the concentration C of at least one substance (2a) in the medium (2, 2a, 2b) from the refractive index n.

10. Device (1) according to claim 9, characterized in that a

Temperature sensor (35) for measuring the temperature T of the medium (2, 2a, 2b) is provided and that the evaluation unit (101) is adapted to additionally use this temperature T for determining the concentration C of the substance (2a).

11. System (110) for measuring the concentration of urea (2a) in a solution (2) containing at least urea (2a) and water (2b) for exhaust gas aftertreatment in a motor vehicle (4), characterized in that it comprises a device (1 ) according to one of claims 9-10, wherein the detection beam (23) of the device (1) through a storage tank (41) for the solution (2) and / or through a solution leading line (42) is guided.