DE102021201017A1 - Measuring system for determining the condition of a hydraulic fluid - Google Patents

Abstract

The invention relates to a measuring system (20) for monitoring the condition of a hydraulic fluid, a line (30) being provided for the hydraulic fluid, the line (30) being transparent to light at least in sections (32), a first light source (41) being on the outside of the line (30), a first light detector (40) being arranged on the outside of the line (30) opposite the first light source (41) on the line (30), the transparency (32; 32a; 32b) of the line (30) is selected so that light from the first light source (41) can pass through the hydraulic fluid within the line (30) to the first light detector (40). According to the invention, the first light detector (40) comprises an optical system (51) and an image sensor (50), wherein the image sensor (50) comprises a multiplicity of individual sensors (52) arranged in a two-dimensional grid, which are each light-sensitive, the optics (51) being designed and arranged in such a way that they increase the m ith the first light source (41) on the image sensor (50) projected image of the hydraulic fluid causes.

Description

The invention relates to a measuring system according to the preamble of claim 1, a distributed measuring arrangement with at least one such measuring system and a method for operating the measuring system.

From the article "COPS - Combined Oil quality and Particle measurement System" (by T. Bley, E. Pignanelli and A. Schütze, published in "2012 Sixth International Conference on Sensing Technology (ICST)", page 561ff) is a measurement system for Known to determine the quality of oil. In this case, the oil is passed through a measuring cuvette and irradiated with infrared light, for example, with the light emerging from the measuring cuvette being measured using a photodiode. In order to obtain additional information about the particles contained in the oil, the oil is irradiated with laser light, with the laser light emerging from the measuring cuvette being measured using a photodiode. The use of laser beams is complex and expensive. Only comparatively little information can be obtained about the particles floating in the oil.

One advantage of the measuring system according to the invention is that a comparatively large number of key figures which characterize the state of a hydraulic fluid can be obtained in an extremely cost-effective manner. The hydraulic fluid is preferably a liquid and most preferably mineral oil. The many key figures can subsequently be viewed together in order to obtain a statement as to whether the hydraulic fluid can still be used or not. The measuring device is preferably used directly on hydraulic systems or vehicles, for example to decide when the hydraulic fluid needs to be changed. The measuring device according to the invention requires little installation space, so that it can be used in places that were previously inaccessible for monitoring the quality of the hydraulic fluid. Furthermore, the same measuring device can be used to monitor different hydraulic fluids that differ with regard to the aging processes. The particle number can also be determined according to ISO 44060:99. Furthermore, the shape of the particles can be determined and characterized numerically. Gas bubbles can also be detected in the hydraulic fluid. Furthermore, information about precise chemical bonds can be determined, which enables an analysis of the chemical state of the hydraulic fluid.

According to claim 1 it is proposed that the first light detector comprises optics and an image sensor, the image sensor comprising a plurality of individual sensors arranged in a grid, each of which is light-sensitive, the optics being designed and arranged in such a way that they increase the size of the the first light source causes the image of the hydraulic fluid projected onto the image sensor. It is conceivable that the line is completely transparent, being made of glass, for example. One then speaks of a measuring cuvette. The measuring system is preferably used with a hydraulic fluid which is under a pressure which exceeds the ambient pressure. In most cases, this will be the pressure in the tank of the hydraulic system or the low pressure. The pressure can be 30 bar, for example. Then preferably a line made of metal is used, which has transparent sections or small windows made of transparent material. The first light source preferably emits light in the human-visible range of the color spectrum. The image sensor can be a CCD or a CMOS image sensor. The image sensor is preferably designed in such a way that it can generate a digital still image. A photographic shutter can be assigned to the image sensor, with which the exposure time can be controlled. However, it is also conceivable to control the exposure time via the image sensor itself. A photographic aperture can be assigned to the image sensor, with which the depth of field of the optical image on the one hand and the amount of light incident on the image sensor on the other hand can be controlled. The magnification mentioned is to be understood as meaning that a particle which is located in the plane of greatest image sharpness is imaged larger on the image sensor than it actually is. The individual sensors mentioned are preferably designed to be identical to one another.

The grid is preferably an orthogonal grid. The pitch of the grid is preferably constant in both directions. The pitch of the grid is preferably the same in both directions. The image sensor can be sensitive in the visible and/or in the infrared range of the light spectrum.

Advantageous developments and improvements of the invention are specified in the dependent claims.

Provision can be made for the optics to be designed in such a way that an area in a plane of greatest image sharpness is imaged on each individual sensor, the greatest transverse dimension of which is at most 4 μm, preferably at most 1 μm. As a result, all particle numbers defined in ISO 44060:99 can be reliably determined.

It can be provided that the individual sensors are each color-sensitive. For the determination of the number, the size, explained below and the shape of the particles is basically sufficient to use a black-and-white image sensor. However, a color image sensor is common today. Within the scope of the invention, the resulting possibilities can be used, for example, to determine the color of the hydraulic fluid. The color of the particles can also be taken into account, in particular if there is abrasion from colored components in the hydraulic fluid. The individual sensors can each have three separate sub-sensors which are light-sensitive with regard to different areas of the color spectrum, for example with regard to red, green and blue light.

Provision can be made for the first light detector to be assigned a second light source, which is arranged on the same side of the line as the first light detector, the transparency of the line being designed such that light from the second light source, which is emitted by the hydraulic fluid in the Line is reflected, can reach the image sensor. It has been shown that metal particles reflect light much more strongly than particles made of other materials. The method explained below can be used to determine, among other things, how high the proportion of metallic particles in the hydraulic fluid is. Metallic particles, for example, indicate wear in the hydraulic system.

Provision can be made for at least one second light detector to be arranged adjacent to the line, with the second light detector comprising a single light sensor which outputs a single, scalar measured value, with at least one third light source being assigned to the at least one second light detector. The at least one third light source can be located on the side of the line opposite the second light detector and/or on the same side of the line, so that a transmission or a reflection measurement can be carried out as desired. It is conceivable that both types of third light sources, which are used alternately, are assigned to a second light detector. A first type of third light source primarily emits visible light, while a second type of third light source primarily emits infrared light.

It can be provided that at least one optical color filter is assigned to at least one second light detector, which is arranged in the light path between the at least one third light source and the assigned second light detector. When hydraulic fluid ages, certain chemical compounds are formed in the hydraulic fluid. These are set in motion by infrared light of a very specific spectral range, so that this spectral range is absorbed. Other spectral ranges of infrared light are transmitted essentially unhindered. The optical color filters are selected in such a way that the associated second light detector is sensitive to precisely this part of the color spectrum. As a result, it can be determined whether one of the chemical compounds mentioned is present in the hydraulic fluid or not, with a corresponding quantification also being possible. By using optical filters, the part of the color spectrum to be evaluated can be delimited much more precisely than would be possible with digital evaluation of the digital still images of the image sensor. Furthermore, a measurement in the infrared range of the color spectrum is easily possible and is preferably also carried out. The second light detector can be a photodiode, for example.

Provision can be made for at least two second light detectors to be provided, which are sensitive in different areas of the color spectrum, the associated transparent sections of the line having different thicknesses in the direction of the relevant light path. It is preferably considered that at least one second light detector works in the infrared range, with at least one further second light detector working in the visible range or in another section of the infrared spectral range. In the visible range, very thick transparent sections can easily be used in the line, which can be up to 20 mm thick. In the infrared range, on the other hand, thin transparent sections of the line are preferred, which are, for example, 0.05 mm or 0.1 mm thick. The transparent portion is made of glass or a transparent crystal, for example, and CaF ₂ , BaF ₂ , KBr, NaCl, Ge, SiO ₂ or ZnSe is preferably used. It is preferably thought of embedding the second light detector in the glass or a transparent crystal in order to obtain a small optically effective thickness of the transparent section, the transparent section nevertheless withstanding the pressure within the line.

Protection is also claimed for a distributed measurement arrangement with at least one measurement system according to the invention, with a single central data evaluation device being provided which is spatially remote from the at least one measurement system, with the central data evaluation device being connected to all measurement systems via a computer network connection. The data evaluation device preferably includes a programmable digital computer which is programmed according to the method according to the invention. The computer network connection is preferably based on the Internet Protocol. It can be wired, for example, according to the Ethernet standard. However, it can also be wireless, for example according to the WLAN, Bluetooth, GSM, LTE or 5G standards, although mixed forms are also conceivable. The method according to the invention is preferably carried out on the central data evaluation device, so that little computing power has to be installed on the individual measuring system. As a result, for example in a fleet with a large number of vehicles or systems with hydraulic functions, the information obtained about the hydraulic fluid can be brought together at one point in order to coordinate the maintenance of the vehicles or systems. It goes without saying that the present invention is also used in connection with an offline evaluation that is carried out adjacent to the image sensor.

Protection is also claimed for a method in which a measurement system according to the invention is used, with at least one digital still image being recorded with the image sensor, with the at least one digital still image being subjected to digital filtering, with the filtering being selected in such a way that a filtered still image is obtained results in which each pixel is assigned a single color value with two possible color states. Digital filtering can involve filtering out certain regions of the color spectrum, the result being a still digital image in which each pixel has a single scalar color value. This is also referred to as a grayscale image. In the original color image, on the other hand, a pixel has three color values. The color filtering mentioned can be followed by a contrast enhancement, in which a first color state is assigned to the scalar color values that are below a predetermined threshold value, with a second color state being assigned to all other scalar color values. The named pixels (picture elements) preferably correspond to the measured value of an individual sensor or the measured values of the corresponding three sub-sensors. All filters that are available, for example, in the image editing software Gimp (https://github.com/GNOME/gimp) can be used as digital filters.

It can be provided that contiguous areas of the filtered still image are determined, which uniformly have a second color value. In a first approximation, the contiguous areas each correspond to a particle that is carried along by the hydraulic fluid.

It can be provided that the number of contiguous areas in a predetermined evaluation area of the filtered digital image is determined. The number mentioned is a key figure that characterizes the contamination of the hydraulic fluid. The filtered still image is typically rectangular, with the evaluation area being able to be circular, for example. This means that the numerical value obtained does not depend on the rotational position of the image sensor relative to the line. The evaluation area can also include the entire filtered still image.

It can be provided that a largest and/or a smallest transverse dimension of at least one contiguous area is determined. The corresponding key figures are an indication of the origin of the particle in question. All contiguous areas in the evaluation area are preferably evaluated, with a statistical frequency distribution of the transverse dimensions mentioned being determined. In this case, a plurality of dimension classes are preferably formed, each of which extends over a sub-range of the possible transverse dimensions, with the number of measured transverse dimensions falling into a dimension class being determined.

Provision can be made for a quotient to be determined from the largest and smallest transverse dimension. Alternatively, a quotient of the largest transverse dimension and a transverse dimension orthogonal to the largest transverse dimension can be determined. The quotients mentioned each form a key figure which characterizes the shape of the particles. This in turn is an indication of the origin of the particles. If the quotient is close to one, there could be gas bubbles in the hydraulic fluid, with the corresponding connected area being excluded from the evaluation if desired, since gas bubbles are not an indication of the aging of the hydraulic fluid. It is also conceivable that a separate indicator is created for these cases.

Provision can be made for a first digital still image to be recorded with the image sensor using the first light source, with a second digital still image being recorded using the second light source at a short time interval from the first digital still image, with a comparison of the first with the second digital still image it is determined whether a particle consists of metal. For this purpose, use is made of the fact that metal particles reflect the light comparatively strongly, so that these particles are comparatively bright in the second digital still image. Said short time interval can be between 0.1 ms and 10 ms. As part of the data evaluation, it is preferably taken into account that the flowing hydraulic fluid has moved somewhat further in the short time interval. The order in which the first and the second digital still image to be recorded can be chosen arbitrarily.

It can be provided that a color of the hydraulic fluid is determined by recording a third digital still image with the image sensor, with the first or the second light source being operated with a reduced brightness compared to the determination of individual particles. An average color over an evaluation area is preferably determined from the third digital still image, this color characterizing the color of the hydraulic fluid.

• - gesamte Partikelanzahl innerhalb des Auswertebereichs
• - Partikelanzahlen in verschiedenen Partikelgrößenklassen innerhalb des Auswertebereichs
• - Partikelanzahlen in verschiedenen Formklassen innerhalb des Auswertebereichs
• - Mittelwert der Farbe über den gesamten Auswertebereich
• - Differenz des ersten und des zweiten Standbilds, wobei vorzugsweise eine maximale Differenz als Kennzahl verwendet wird.

It can be provided that at least one characteristic number obtained by means of the image sensor and at least one characteristic number obtained by means of the at least one second light detector are used to characterize the state of the hydraulic fluid. In the simplest case, the state of the hydraulic fluid is characterized by the two states “hydraulic fluid OK” and “hydraulic fluid not OK”. For example, the following key figures can be obtained with the image sensor:

• - total number of particles within the evaluation range
• - Particle numbers in different particle size classes within the evaluation range
• - Number of particles in different shape classes within the evaluation range
• - Mean value of the color over the entire evaluation range
• - Difference between the first and the second still image, a maximum difference preferably being used as a key figure.

The index, which is obtained with a second light detector, is preferably its measured value or its measured value averaged over a predetermined period of time.

It can be provided that the specified key figures are evaluated by means of a discriminant analysis and/or a logarithmic regression and/or a linear regression in order to characterize the state of the hydraulic fluid. For example, reference is made to the website https://de.wikipedia.org/wiki/Diskriminanzanalyse for discriminant analysis. For logarithmic regression, reference is made to the website https://de.wikipedia.org/wiki/Logistic Regression, for example. Both methods open up the possibility of using a particularly large number of key figures in the context of data evaluation. As a result, the state of the hydraulic fluid can be reliably inferred even if the key figures used only imprecisely reflect the actual conditions.

Within the scope of the present patent application, a still image is mentioned several times. It goes without saying that videos or moving images can also be recorded with the image sensor, with the method according to the invention being carried out with an individual image of the video which corresponds to a still image.

It goes without saying that the features mentioned above and those still to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

• 1 eine grobschematische Ansicht einer verteilten Messanordnung, welche ein erfindungsgemäßes Messsystem umfasst;
• 2 ein digitales Standbild, welches mit dem Bildsensor aufgenommen wurde; und
• 3 ein gefiltertes Standbild.

The invention is explained in more detail below with reference to the accompanying drawings. It shows:

• 1 a rough schematic view of a distributed measurement arrangement, which includes a measurement system according to the invention;
• 2 a digital still image captured by the image sensor; and
• 3 a filtered still image.

1 shows a rough schematic view of a distributed measurement arrangement 10, which includes a measurement system 20 according to the invention. The measuring system 20 comprises a line 30 inside which the hydraulic fluid to be examined flows along the direction of flow 23 . The hydraulic fluid is preferably a liquid and most preferably is oil. In the present case, the line 30 is formed by a metal tube which is provided with a plurality of transparent sections 32; 32a - 32f each formed in the manner of a window of glass or transparent crystal. A metal block with a bore can also be used instead of a tube. However, it is also conceivable to produce the line entirely from a transparent material, in particular from glass.

A first light source 41, which emits visible light, for example, is arranged outside the line. The corresponding light enters the line 30 through the transparent section 32a, shines through the hydraulic fluid and exits the line 30 again through the transparent section 32b on the opposite side. A first light detector 40 is provided there, which includes an optical system 51 and an image sensor 50 . The optic 51 is shown in simplified form as an optical lens, which is actually magnifying optics such as those found on a microscope. The image sensor 50 comprises a multiplicity of ones which are identical to one another Individual sensors 52, which are distributed in a grid over a rectangular or square area. A single individual sensor 52 is within 1 shown enlarged. This includes three sub-sensors 53, each of which is sensitive to light with regard to different areas of the color spectrum, for example with regard to the colors red, green and blue. The image sensor 50 can be implemented in CCD or in CMOS design.

An inexpensive image sensor such as that found in digital still cameras can be used within the scope of the invention.

The ISO 4406:99 standard stipulates that particles down to a minimum size of 4 µm are determined in order to determine the cleanliness class of oil. It is therefore preferred within the scope of the invention that such small particles can be imaged sufficiently clearly with the image sensor 50 . It follows from this that the optics preferably have a high magnification factor. As a result, the depth of focus of the optical image is comparatively small. Essentially only the particles floating in the plane of greatest image sharpness 54 are sharply imaged, with all particles floating in the beam path in front of or behind being imaged out of focus, with a comparatively small distance from the plane 54 already leading to significant blurring. The optics 51 are preferably designed in such a way that particles with a largest transverse dimension of 1 μm just completely fill an individual sensor 52 . It goes without saying that the optics 51 can include a device with which the position of the plane of greatest image sharpness 54 can be adjusted.

The digital still image obtained with the image sensor 50 is preferably fed to a data collection device 21, on which all the light detectors 40; 60 and all light sources 41; 42; 61 in 1 are connected. The data collection device 21 transmits the measured data to a central data evaluation device 11 via a computer network connection 12, in particular via the public Internet. It is preferably intended to connect a plurality of similarly constructed measurement systems 20 to a single, shared data evaluation device 11 . The method according to the invention is preferably carried out exclusively by the data evaluation device 11 . If necessary, the data collection device 21 preprocesses the measurement data so that they are suitable for transmission via the computer network 12 .

Associated with the first light detector 40 is a second light source 42 located on the same side of the conduit 30 as the first light detector 40. The first light detector 40 detects the light emitted by the second light source 42 reflecting off hydraulic fluid becomes. With this approach, metal particles floating in the oil can be easily detected, as they reflect the light comparatively strongly. The second light source 42 preferably emits visible light. A light shield 22 is preferably provided between the second light source 42 and the first light detector 40, so that only light waves from the second light source 42, which propagate via the hydraulic fluid, reach the image sensor 50.

The measuring device 20 also comprises a plurality of second light detectors 60; 60a; 60b; 60c, each designed as a photodiode, for example. In contrast to the image sensor 50, these do not supply a digital image, but rather a single scalar measured value which describes the luminous flux incident on the relevant second light detector 60. So that the second light detector 60 is only sensitive to a specific part of the color spectrum, optical color filters 64 can be provided with which the desired spectral range is filtered out purely optically before it reaches the associated second light detector 60a. In 1 a switchable color filter device 63 is provided with which, for example, four color filters 64 can be selected by sliding them in front of the second light detector 60a. It goes without saying that, alternatively, four second light detectors 60, each with an assigned third light source 61, can be provided, with each of which a single color filter 64 is permanently assigned. the inside 1 The left second light detector 60a works preferably in the infrared range of the light spectrum. The associated transparent sections 32c; 32d in the line are therefore made thin, being 0.1 mm thick, for example. A parallel bypass line can be provided in this area, which is designed in such a way that a larger fluid flow flows via the bypass line, with a comparatively small fluid flow at the transparent sections 32c; 32d flows past. The transparent sections 32a; 32b; 32e; 32f, which are provided for light in the visible range of the light spectrum, can be made significantly thicker, being up to 20 mm thick. The light spectrum of the at least one third light source 61 is preferably adapted to the light spectrum to which the associated second light detector 60 is sensitive.

on the inside 1 right side are two more second light detectors 60b; 60c provided, which are designed, for example, as a photodiode. These preferably work in the visible range of the light spectrum. They are arranged on the outside of the line 30 that they share a common associated third light source 61b work. The second light detector 60b measures the light that shines through the hydraulic fluid. The second light detector 60c measures the light reflected from the hydraulic fluid. These second light detectors 60b; 60c can be used, for example, to adapt the entire data evaluation to the basic color of the oil under consideration, so that different evaluation parameters are used for light-colored, clean oil than for dark oil, which is already heavily contaminated.

2 shows a digital still image captured by the image sensor (No. 50 in 1 ) has been recorded. This is an example of a color image in which many different types of particles can be seen, namely metal particles, fibers and organic particles.

3 shows a filtered still image 71. 3 has no direct relation to 2 and is only used to illustrate the evaluation process. First, a circular evaluation area 74 is drawn in. As part of the evaluation, preferably only the particles within the evaluation area 74 are taken into account in order to obtain more comparable results. Various connected areas with a uniform color value are also drawn in, each of which corresponds to a potential particle in the hydraulic fluid. As explained above, the contiguous areas were determined by digital filtering from the digital still image, which was output directly from the image sensor. These areas 73 can be recognized automatically and their transverse dimensions can be measured. Under transverse dimension, the width of a contiguous area 73 in the image plane of the 3 be understood.

Different transverse dimensions result from different directions in which said width is measured.

Contrary to what is shown in 3 it is preferred that a comparatively large number of particles or connected areas 73 are located within the evaluation area 74 . This allows more reliable statements to be made about the hydraulic fluid. To achieve this goal, for example, the enlargement of the optics (No. 51 in 1 ) can be adjusted. It goes without saying that with a brand-new hydraulic fluid, only a few particles or areas 73 will be present in the filtered digital still image 71 .

Reference List

10

distributed measurement arrangement

11

central data evaluation device

12

computer network connection

20

measuring system

21

decentralized data collection device

22

light shielding

23

Flow direction of the hydraulic fluid

30

Management

31

metal pipe

32

transparent section

32a

transparent section

32b

transparent section

32c

transparent section

32d

transparent section

32e

transparent section

32f

transparent section

40

first light detector

41

first light source

42

second light source

50

image sensor

51

optics

52

single sensor

53

sub-sensor

54

level of greatest sharpness

60

second light detector

60a

second light detector

60b

second light detector

60c

second light detector

61

third light source

61a

third light source

61b

third light source

63

color filter device

64

color filter

70

digital still image

71

filtered still image

73

contiguous area with a uniform color value

74

evaluation area

Claims (17)

Measuring system (20) for monitoring the condition of a hydraulic fluid, a line (30) being provided for the hydraulic fluid, the line (30) being at least partially (32) transparent to is light, a first light source (41) being arranged on the outside of the duct (30), a first light detector (40) being arranged on the outside of the duct (30) opposite the first light source (41) on the duct (30), wherein the transparency (32; 32a; 32b) of the line (30) is selected such that light from the first light source (41) can reach the first light detector (40) through the hydraulic fluid within the line (30), characterized in that the first light detector (40) comprises optics (51) and an image sensor (50), the image sensor (50) comprising a multiplicity of individual sensors (52) which are arranged in a two-dimensional grid and are each light-sensitive, the optics (51) is designed and arranged in such a way that it causes an enlargement of the image of the hydraulic fluid projected onto the image sensor (50) by the first light source (41). Measuring system (20) according to claim 1 , the optics (51) being designed in such a way that on each individual sensor (52) an area is imaged in a plane of greatest image sharpness (54), the greatest transverse dimension of which is at most 4 μm, preferably at most 1 μm. Measuring system (20) according to claim 1 or 2 , wherein the individual sensors (52) are each color-sensitive. Measuring system (20) according to one of the preceding claims, wherein the first light detector (40) is assigned a second light source (42) which is arranged on the same side of the line (30) as the first light detector (40), the transparency ( 32; 32b) of the line (30) is designed such that light from the second light source (42), which is reflected by the hydraulic fluid in the line (30), can reach the image sensor (50). Measuring system (20) according to one of the preceding claims, wherein at least one second light detector (60) is arranged adjacent to the line (30), wherein the second light detector (60) comprises a single light sensor which outputs a single scalar measured value, the at least one at least one third light source (61) is assigned to the second light detector (60). Measuring system (20) according to claim 5 wherein at least one second light detector (60) is assigned at least one optical color filter (64) which is arranged in the light path between the at least one third light source (61) and the assigned second light detector (60). Measuring system (20) according to claim 5 or 6 , wherein at least two second light detectors (60; 60a; 60b; 60c) are provided, which are sensitive in different areas of the color spectrum, the associated transparent sections (32c; 32d; 32e; 32f) of the line (30) pointing in the direction of the relevant Light paths are of different thicknesses. Distributed measuring arrangement (10) with at least one measuring system (20) according to one of the preceding claims, wherein a single central data evaluation device (11) is provided, which is arranged spatially remote from the at least one measuring system (20), wherein the central data evaluation device (11) is connected to all measuring systems (20) via a computer network connection (12). Method, wherein a measuring system (20) according to one of the preceding claims is used, wherein at least one digital still image (70) is recorded with the image sensor (50), the at least one digital still image being subjected to digital filtering, the filtering being selected in this way is that a filtered still image (71) results in which each pixel is assigned a single color value with two possible color states. procedure after claim 9 , coherent areas (73) of the filtered still image (71) being determined which uniformly have a second color value. procedure after claim 10 , the number of contiguous areas (73) in a predetermined evaluation area (74) of the filtered still image (71) being determined. procedure after claim 10 or 11 , wherein a largest and/or a smallest transverse dimension of at least one contiguous area (73) is determined. procedure after claim 12 , whereby a quotient is determined from the largest and the smallest transverse dimension. Procedure according to one of claims 9 until 13 , wherein a measuring system according to one of Claims 4 until 8th is used, with a first digital still image being recorded with the image sensor (50) using the first light source (41), with a second digital still image being recorded using the second light source (42) at a short time interval from the first digital still image, wherein by comparing the first with the second digital still image, it is determined whether a particle consists of metal. Procedure according to one of claims 9 until 14 , wherein a color of the hydraulic fluid is determined by using the image sensor (50) a third digital still image is recorded, the first or the second light source (41; 42) is operated with a reduced brightness compared to the determination of individual particles. Procedure according to one of claims 9 until 15 , where a measurement system according to claim 5 is used, at least one characteristic number obtained by means of the image sensor (50) and at least one characteristic number obtained by means of the at least one second light detector (60) being used to characterize the state of the hydraulic fluid. procedure after Claim 16 , wherein the parameters mentioned are evaluated by means of a discriminant analysis and/or a logarithmic regression and/or a linear regression in order to characterize the state of the hydraulic fluid.

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