DE2907620A1 - Optical thickness detection circuit - measures intensity of light beam reflected from lubrication film containing fluorescent material additive - Google Patents

Abstract

The appts. uses a flourescent material which is added to the lubrication fluid before it is applied to the lubricated surface. A light source is then used to direct a beam onto the lubrication film (8) at an angle, with the intensity of the stimulated fluorescence measured as an indication of the film thickness. Pref. the beam is provided by a high-press mercury lamp (1), the output (SLe of the shielded lamp (1) passed through a primary filter (3) for selection of one or more Hg lines, e.g. a blue Hg line with a wavelength of 436mm. The output (SA) of the filter falls on the film (8) and the diffuse fluorescence (SF) is detected by a shielded photocell or photomultiplier (5). The latter is preceded by a secondary filter (4), which blocks ambient light.

Description

Method and device for measuring thickness

transparent thin layers, especially lubricant films The invention relates to a method for measuring the thickness of transparent thinner Layers, especially lubricant films, on stationary or moving surfaces as well as facilities for carrying out the procedure.

Such measuring methods and devices come a practical technical one Significance, especially in connection with the investigation of the friction behavior of parts moving against each other, e.g. in bearings, gears or piston-cylinder arrangements, too, since the thickness of the lubricant film plays an important role here.

In the known methods for measuring very thin liquid films a distinction must be made between processes that at least temporarily remove the lubricating film destroy, and those in which the measurement is contactless without destroying the Film takes place.

As a contact measurement method, especially in the painting or. Coating technology,, mechanical processes used, some of which are international are standardized (e.g. ISO 2808 or ASTMD 1212).

The measuring wheels, measuring combs or measuring tips used are very easy to use. The determination of the thickness of almost colorless layers, e.g. of Lubricating oils, however, is difficult. For film thicknesses in the area of surface roughness of the wearer measurement is impossible. Neither can on-line measurements can be carried out on moving, coated parts. aside from that is a perfect recording of the film thickness only on flat or cylindrical Surfaces possible.

Also the known electrical measuring methods for determining the film thickness by means of a measurement of the capacitance or the resistance, the liquid film acts as a dielectric or insulating, generally require a touch of the layer to be measured and therefore require a certain shape of the carrier (just or cylindrical).

The light-section microscope, for example, is used for non-contact film thickness measurement in principle suitable. The evaluation of the measurements is cumbersome and on dormant carriers restricted.

About procedures and facilities in which with the help of radio nuclei Layer thicknesses are determined, e.g. using the beta backscattering method, are in With a view to measuring the thickness of very thin liquid films on metallic Support surfaces so far no experience. In addition, require with radiant Substances working processes in general elaborate safety precautions Avoidance of radiation damage. Although measurements on moving surfaces are possible are, this method can therefore only be used to a limited extent is based on the object of a method for measuring the thickness of transparent thinner Layers, in particular lubricant films, indicate that it allows exact Measurements also on high-speed machines, e.g. inside pneumatic cylinders as well as on bearings or gear parts. It should also be possible gradual differences in thickness at closely spaced measuring points or continuously to be grasped in greater ways.

This object is achieved according to the invention by what is specified in claim 1 Procedure solved. Further training of the process as well as facilities for its implementation are the subject of the subclaims.

The relationship between the strength of the fluorescence 1F caused by a fluorescent liquid layer when illuminated by a fluorescence-stimulating radiation IQ on the concentration of the fluorescent dye C, the extinction coefficient E and the layer thickness d is given by the well-known Lambert-3eersche law. After that is This relationship has so far only been used to determine the concentration and the extinction coefficient, but not to measure the thickness of liquid films. The invention is based on the knowledge that, given constant C, X and Io in a measuring arrangement, the relationship IF t. const.

receives.

The emitted intensity of the fluorescent light is therefore directly proportional to the layer thickness. The sensitivity or

the resolving power of the measuring process can be increased by increasing the Dye concentration, increase in excitation radiation intensity and through Use of a dye with a higher EX, considerably increased ink factor will.

For the implementation of the method according to the invention, it is essential that that a standard is available as a comparison object for the calibration of the facility stands, which is a layer of known thickness from the same liquid and with the same Has fluorescent agent concentration like the liquid film to be measured. For The invention makes suggestions for training safe standards as well as for one Device for producing an open film of lubricant that can be precisely determined and uniform thickness. Finally, it is also stated how the method according to the invention to control the lubricant supply to moving parts in the sense of an optimal Lubrication can be used.

Exemplary embodiments of devices according to the invention are shown in Drawings shown. They show schematically: FIG. 1 with a measuring device Representation of the corridor, Figures 2 to 4 several versions of standards for calibrating the measuring device, Figure 5 shows a device for producing a thin, open lubricant film of a certain thickness, Figure 6 shows a device for Measurement and control of the lubricant film thickness on fast moving machine parts, FIG. 7 shows the dependence of the frictional torque on the thickness of the lubricating film (Tribeck curve).

Figure 1 shows an apparatus for measuring the thickness of thin films of liquid on flat surfaces, with the path of the rays of the excitation and fluorescent light is shown simplified. A radiation source 1, e.g. a high pressure mercury vapor lamp is arranged in a lamp housing 2.

With the help of a primary filter, the consists of commercially available filters or filter combinations, one or more Hg lines selectively selected. These filters only have a certain optical permeability, e.g. for a blue Hg line with a wavelength of 436 nm. All other areas of the spectrum. the Hg lamp are absorbed by this filter.

With this transmitted excitation radiation SA is on a Base plate 7 with a flat surface located layer 8 one with a fluorescent dye offset liquid illuminated at point 9 and excited to fluorescence. The diffuse fluorescence radiation SF generated in the process coincides with the reflected one Part of the SAR of the excitation radiation on a secondary filter 4. This secondary filter 4 is designed in such a way that it retains the excitation radiation SAR and any extraneous light and only allows the fluorescence radiation SF to pass through which is directed to a measuring housing 6 Light intensity measuring device 5, e.g. a photocell or a photomultiplier, got.

The current generated by the light intensity meter 5 is known in the art Way, e.g. measured by means of a sensitive ammeter 10 and can after appropriate The device is measured as a measure of the thickness of the liquid layer 8.

Since the light intensities involved are very low, in particular with small film thicknesses below 1 µm, currents in the 3 range from about 10-6 to 10-9 amps.

The measurement of such small currents often causes difficulties. It is therefore advisable to use the voltage drop across a resistor as the output signal to use. The measurement result can be displayed e.g. with a rY writer. as possible fluorescent dyes are particularly dyes that are derived from the Mineral oil industry can be used to color mineral oil products. Suitable For example, the color solvent listed under No. 45550 in the Color Index is green 4th

To measure the facility, standards are used in which a layer the same liquid with the same fluorescent concentration as the one to be measured Layer can be produced which has a certain measurable thickness and its Fluorescence radiation can be measured while maintaining the same measurement conditions can.

The standard according to FIG. 2 has one against a cuvette housing 16 by an O-ring 14 sealed base plate 11, the one in the cover 13 of the housing 16 inset plane-parallel glass plate 12 with little, everywhere facing the same distance. The gap between the base plate 11 and the glass plate 12 is filled with the liquid to be compared via an inlet Z, wherein any excess can escape through an outlet A. The base plate 11 is Movable in the housing 16 and via a so-called piezo-bar 15 in the housing supported. This piezo-bar 15 consists of individual layers stacked on top of one another Piezo-ceramic discs Material. He's using a high voltage source tied together. The piezo-bar 15 expands proportionally to the applied voltage UP out or contract. This causes a corresponding change in the gap between the base plate 11 and the glass plate 12 and thus the thickness of the liquid layer F between the plates. Depending on the applied voltage UP, reproducible reference layer thicknesses can be generated in this way. With lighting a reference layer of a certain thickness of the standard due to the excitation radiation SA a certain fluorescence radiation SF is obtained.

Since in this method the glass plate 12 lies in the radiation path, the measured value must be corrected for the reflection and transmission losses. With the help of the value calculated in this way, the film thickness measuring device can be measured point by point calibrate, the relationship between the measured current intensity and the associated Film thickness is approximately linear for small film thicknesses of up to about 30 fun.

A structurally particularly simple implementation of a standard is shown by Figure 3. On a base plate 21 three gauge blocks are blown, of which the outer gauge blocks 23 and 24 have the same thickness h1, while the middle Gauge 25 has a smaller thickness h2. A glass plate is attached to the gauge blocks 23 and 24 22 placed or stretched with plane-parallel surfaces. In the gap between the comparison liquid is introduced into the middle gauge block 25 and the glass plate 22, which spreads evenly there by capillary action. The thickness h5 of the reference layer F of the comparison liquid is h5 = h1 - h2- By changing the gauge blocks Any desired reference layer thickness can be generated.

In the standard according to FIG. 4, there is only one base plate 31 a gauge block 33 is placed at a distance tO parallel to a reference edge K of the base plate runs. On the reference edge K and the end piece 33 there is a thick one Plate 32 from optical Glass supported with plane-parallel surfaces Glass plate 32 is pressed by forces P, for example by a not shown Housing cover can be generated. The comparison liquid is in the wedge-shaped Gap inserted between the glass plate 32 and the base plate 31 and fills this by capillary action. As a result, a reference layer F is formed, the thickness of which continuously from the value 0 at the reference edge to the value h0 in the distance 1, increases. Depending on the distance Ii, which can be read off on a scale, for example any reference layer thicknesses between these limit values can be used Calibration can be used. The following applies here: ho / 1; 0 = h1 / l1.

The creation of an exposed lubricant film with low, precisely determinable layer thickness on a hollow cylindrical surface, where because of the lack of a glass cover plate means that no corrections to the calibration curve are required, is possible with the device shown in Figure 5. This facility exists from a prismatic housing with a lower part 41 and an upper part 42, the are firmly connected to one another via screws 45 and dowel pins 46. The case 41, 42 contains a bore 40, the axis X-X of which coincides with the axis of the housing. At the ends of the housing 41, 42, the cylindrical bore 40 is through cover 43, 44 locked. The covers 43, 44 are attached to the housing by screws and dowel pins 41, 42 connected and have centering bores 48 which are exactly on the central axis the boron 40 lie.

The device assembled in this way is placed in a rotating device, E.g. in a lathe, clamped centrically to the drive axis.

Through openings 47 in the lids 43, 44 a certain Amount of lubricating oil dissolved in a volatile solvent into the cylindrical Hole 40 entered. When rotating the device around the horizontally lying one Bore axis X-X, the liquid is thrown against the boron wall and forms there an evenly thick layer. The evaporating can through the openings 47 Solvents, e.g. ethyl acetate or Petroleum benzine will escape. After the solvent has evaporated, a uniform, thin film of lubricating oil F, the thickness of which is determined by the amount of lubricating oil entered and can be calculated precisely from the wall area of the hole.

With the arrangement shown in Figure 6, the thickness of the Lubricant film measured on rotating or periodically moving machine parts and can be regulated by the appropriate supply of lubricant during operation. Included it is assumed that there is a known relationship between the lubricant film thickness in the area of the frictional contact of the machine parts and after passing through the contact present film thickness at a point accessible for observation.

Figure 6 shows as an example a measuring and control device for Monitoring of the lubrication conditions on two gear wheels 51 meshing with one another and 52. A point 59 on a tooth flank of the pinion 51 is through a focused light source controlled intermittently by a chopper 57 50, e.g. a laser device or a strobe light, so illuminated that the pinion 51 appears to stand still and always the same point 59 through which the primary filter 53 passing excitation radiation SA is illuminated. The one on the tooth flank As already described for FIG. 1, the lubricant film contains a fluorescent dye, which is excited to fluorescence by the radiation SA. The resulting fluorescence radiation SF passes through the reflected part of the excitation radiation SA and any Secondary filter 54, which retains external light, switches to a filter which is sensitive to short light pulses Light intensity meter 55s e.g. a photomultiplier. The output current of the light intensity meter 55 is entered as the actual value in a target / actual comparator (Komparatõr) - 56, the a z <B. Setpoint by the machine control that depends on the load on the tooth flanks 5uge £ ti6t $ i-rd If the comparator 56 determines that at the point 59 a too low There is a lubricating film and thus also the lubricating film that is about twice as thick on the Frictional contact point 60 between the gears 51 and 52 too thin is, a lubricating device 61 is actuated, e.g. by means of a metering pump, gives a small amount of lubricant to the friction points of the tooth surfaces.

If dangerous underlubrication is detected, a direct connection from the comparator 56 to the machine control 58, the machine is switched off.

The relationship between frictional force and lubricant film thickness is shown in FIG. 7. In this diagram, known as the Stribeck curve, the frictional force is above the Applied friction speed.

At low friction speeds there is between the friction partners Mixed friction, which causes a high frictional force. With increasing friction speed the frictional force decreases and reaches a minimum at point I (notch point). Beyond fluid friction occurs at the notch point I. With increasing friction speed the losses due to fluid friction, flexing work, heating of the lubricating oil larger, so that the frictional force also increases. This rise according to curve II forms sish but only fully if there is plenty of lubricant available at the friction point (e.g. with flooding lubrication). If the amount of lubricant is lower, the result is smaller thicknesses of the lubricant layer and a smaller increase in the frictional force, e.g. after curve III. This shows that the friction speed-dependent fully elasto-hydrodynamic lubricating film only up to a certain friction speed forms, above which the frictional force remains approximately constant. With further reduction of the amount of lubricant and thus the thickness of the lubricant film, curve IV is obtained, which corresponds to the smallest possible frictional force. When the lubrication decreases in addition the frictional force increases sharply due to metallic contact between the friction partners on (curve V).

From the diagram of the Stribeck curves (Figure 7) it can be seen that it it is beneficial to keep the lubricant film thickness in a range that creates a friction force curve results slightly above curve IV.

Then it can be assumed with certainty that a sufficient Separation of the metallic surfaces is present and the frictional force is close their minimum. This area shown in dotted lines in FIG. 7 can can be reliably controlled with the arrangement according to FIG.

Only with this arrangement according to the invention has it become possible that due to the small amount of lubricant added, there are also very high quality lubricants can be applied at a reasonable cost. Due to the small amount of lubricant the flexing work and thus the friction and heating is reduced, so that this results in a higher efficiency of the system lubricated in this way.

Above, the invention is thinner in connection with the thickness measurement Liquid - especially lubricant films described because it is opposite the known methods and devices achieve particularly clear advantages. For some applications, however, the invention can also be made thinner for measuring the thickness Films or foils made of transparent, solid materials, e.g. made of plastics, are useful if a fluorescent agent is added to them in a uniform distribution can.

L e r s e i t e

Claims (14)

CLAIMS 1. Method of measuring the thickness of clearer thin layers, especially lubricant films, on stationary or moving surfaces, characterized in that the liquid forming the film prior to application A fluorescence-causing substance is added to the surface, the film through a illuminates the fluorescent radiation and illuminates the intensity of the film generated fluorescence radiation is determined as a measure of the film thickness. 2. The method according to claim 1, d a d u r c h g e k e n n z e i c h n e t da9 as a substance causing fluorescence is a substance that is soluble or in the liquid suspendable fluorescent dye or a naturally fluorescent liquid is added. D. The method according to claim 1 or 2, d u r c h g e n n z e The fluorescent radiation is measured by means of a light intensity measuring device, e.g. a photomultiplier is measured. 4. The method according to claim 3, d a d u r c h g e k e n n z e i c h n e t that to measure the light intensity meter while maintaining the other measurement conditions, a standard is used that is accessible to the measurement Layer of known or measurably variable thickness from the same liquid with the same concentration of the substance causing the fluorescence as that to be measured Has liquid film. 5. Device for performing the method according to the claims 1 to 4, d u r g e k e n n n z e i c h n e t by a on the to be measured Layer directional radiation source (1) with only the excitation radiation (SA) for the Fluorescence permeable primary filter (3) and through a fluorescence radiation (SF) detecting light intensity measuring device (5) with upstream extraneous light and the reflected portion (sir) of the excitation radiation (SA) sifting out secondary filter (4). 6. Device according to claim 5, characterized in that for alignment of the light intensity measuring device (5) on certain measuring points an optical known per se Target device is provided. 7. Device according to claim 5 or 6, characterized in that for irradiating a lubricant layer and for absorbing fluorescent radiation An endoscope is provided inside machine housings, e.g. in compressed air cylinders its connections via light guides, e.g. fiber optic bundles out of the housing are led to the radiation source or to the light intensity meter. 8. Device according to claim 5 or 7, characterized in that for measuring the thickness of the lubricating film on periodically moving machine parts, e.g. teeth wheels, the radiation source (50) as in time with the machine period, e.g. by a Chopper (57), controllable stroboscope and the light intensity meter (55) is responsive to short light pulses, e.g. designed as a photomultiplier. 9. Device according to claim 8, characterized in that the light intensity measuring device (55) a set-actual value comparator (comparator 56) is connected downstream, which supplies the lubricant to the machine parts (51,52) influenced so that the lubricant film in the area the contact surfaces (60) has a predetermined optimal thickness or maintains it. 10. Standard for performing the method according to claim 4, consisting from a glass plate (12) fixed in a housing (13, 16) with plane-parallel surfaces and from one of the glass plate (12) at a distance, fixed or movable Base plate (11) which, together with the glass plate (12), has one with the fluorescent Includes liquid-fillable gap of measurable thickness (h). 11. Standard according to claim 10, characterized in that the im Housing (16) movable base plate (11) sealed by an O-ring (14) in the housing (16) on a column of stacked piezoceramic Disks. (Piezo-bar 15) is supported, the length of which depends on a the column applied variable voltage (UP) is changeable (Figure 2). 12. Standard according to claim 10, characterized eiclinet that between the base plate (21) and the plane-parallel glass plate (22) as spacers two Gauge blocks (23,24) are arranged rmd that limit the gap for the Liquid layer (F) on the base plate (21) between these gauge blocks (23,24) another gauge block (25) - correspondingly small thickness is located (Figure 5). 13. Standard according to claim 10, d a d u r c h g e k e n n z e i c h n e t that on the base plate (31) at a distance (I,) from its reference edge (K) End dimension (39) is arranged and that over the reference edge (K) and the parallel to it Edge of the gauge block (33) the glass plate () 2) is stretched, creating a wedge-shaped Gap for the liquid layer (F). Is formed. 14. Process for the production of a lubricant film with certain Uniform thickness, in particular as a standard according to claim 4, d a d u r c h g e k e n n n e i n e t that a certain amount of the possibly with a Fluorescent dye mixed lubricant in a rapidly evaporating solvent, E.g. ethyl acetate, is dissolved, this solution into a longitudinally divisible hollow cylinder introduced and evenly by rotating the hollow cylinder around its horizontal axis is distributed on its inner wall, the solvent during the rotation evaporates, so that a lubricant film with a computationally precisely determinable more uniform Thickness remains on the wall surface.