WO1980001953A1 - Device for measuring in a compressible medium the local density and the time variation thereof - Google Patents

Abstract

The device for measuring the local density and the time variation thereof is based on the measure of the local refraction index and the variations thereof. The device is particularly used for measuring the local density of compressible mediums when such mediums, for example gas liquid flows, must not be disturbed. The measuring device is applicable to monitoring flows having a speed close to or higher than the sound speed for monitoring the flow of mediums having temperature gradients and it is also applicable to the flow acoustic field. To this effect, the problem to be solved is to provide a device which is readily transportable and easy to manipulate, with which it is possible to measure, by means of the measurement of the corresponding refraction index, the local density of the medium at the measurement location or within the measurement volume and to follow the evolution thereof in the time. To solve this problem, two laser beams (1, 1") coherent between each other are directed so as to intersect within the measurement volume (2) so as to cause interference fringes (3) spaced between each other by a distance (a) to appear at this location. By means of powerful optical means (5) sufficiently remote from the measuring volume (2), an intermediary optical image of the interference fringes (3") is generated and, by means of a micro-lens (6), a considerably enlarged image of a transversal cross-section of this measuring volume is projected in a recording region in the form of a system of parallel interference fringes (3""). At this location, intensity sensitive detectors, for example opto-electronic detectors (8, 8", 8"", 8"""), are arranged in a predetermined configuration. By zero-adjusting the differential signal provided by the detectors (8, and 8"), the particular extreme value of the intensity of the interference fringes (3""") appearing between them is fixed in the space and such value is used as a reference point for the enlargement of the fringe system when the interval (a"") between the fringes varies. At the recording detectors (8"" and 8""") a differentiel electric signal appears corresponding to the spacing of the fringes. This differential signal which is proportional to the spacing (a) of the fringes and, subsequently, inversely proportional to the refraction index, therefore to the density of the medium contained within the measuring volume (2), is converted into a measuring signal (U(t)) by means of an electronic circuit (9").

Description

 -. 1 -

Arrangement for measuring the local density and its changes over time in compressible media

The invention relates to an optically and optoelectronically acting device with which it is possible to locally measure the refractive index of an optically transparent medium and its rapid changes over time and thereby to the corresponding local density or its temporal change changes, for example within the flow of a compressible fluid. Insofar as coherent electromagnetic radiation is also available outside the optical wavelength range, the invention also relates to this radiation and to media which it penetrates largely without scattering.

In many areas of research and technology, in particular in the field of flow technology and research on compressible media, there is often a need for a measurement method with which the measurement of the local density of the examined medium or its often faster changes in time is possible . Such questions are typical, for example for problems of sound-related and supersonic flows, the flow of media with temperature gradients and for the field of flow acoustics.

In these areas, density measurements have so far predominantly been made using shadow-optical, streak-optical, interferometric

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and, more recently, holographic methods are also possible. These methods are used in many special modifications, of which the differential interferometry method is particularly worth mentioning here. They represent all integral measurement methods, which means that their effect used for the measurement is an integral effect along the optical transmission paths used. They therefore do not allow a local density measurement.

A method that has recently been used frequently for the purpose of local density measurement is methods which use the Raman solution of narrowband light. Such methods require highly sensitive equipment and experienced scientific staff and will therefore mainly be reserved for use in the laboratory.

The invention described here permits the local measurement of the refractive index and its rapid changes over time with the aid of a device which, if necessary, can be combined in a compact measuring unit and transported to the location of the measurement as a closed unit. Since the refractive index of a medium is in many cases directly related to its density, a corresponding refractive index measurement makes it possible to measure and track the associated local density of the medium over time.

The invention takes advantage of a long-known but little-used fact. As is customary today in laser Doppler anemometry, for example, a locally localized real interference fringe system is used as a sectional image through an interference layer system in the intersection area of two to one another - 3 - ^» -

generated coherent laser light bundle, the distance a _{0 of} the interference fringes depends on the convergence angle ___ _{0 of} both light bundles according to the relationship

λ is the output wavelength of the laser light that is still unaffected by changes in the refractive index, n is the refractive index valid for this initial or reference state, which therefore prevails before irradiation into the measuring medium. The non-indicated state is the state in the region of interest of the measurement volume, which is identical to the cutting volume of the light beam, the wavelength, the refractive index n and the cutting angle prevailing there - the light beam. With the law of refraction of optics then applies:

bzw.

respectively.

The changed interference fringe spacing is then

^«■ λ n sin ^β / 2

which results in no ^a o

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/ ,. WIPO,. *. The distance between the interference fringes changes inversely proportional to the refractive index.

It is irrelevant whether the refractive index change occurs suddenly at a phase interface or as a result of a finite refractive index gradient.

The invention now solves the problem in that the cutting volume of the beam bundles as a measurement volume with the aid of a sufficiently long focal length image is reproduced one or more times and is finally projected through a strongly magnifying micro-optics into the istrian area, in which there is sufficient integrity-sensitive optoelectronic Detectors.

The focal length of the first imaging optics should be chosen at least so large that the optics themselves do not mechanically influence the measurement object, for example a flow. The intermediate image enables the micro-objective to be brought up for sufficient magnification right up to the image of the measurement volume itself, without having to be introduced into the flow, which would disturb it in an intolerable form.

Further intermediate images can be used, for example, for introducing such an image into an electro-optical cell, which, as will be explained later, can be used for refractive index compensation in the form of a zero method.

In the registration area, which can contain a suitable projection surface, for example, the system exists strongly

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vergrössterter, in good alignment stripes parallel Interferenz¬, in which the fringe spacing determined by absolute or ^'relative acting methods or is exploited, thus to generate a the fringe spacing, and thus the refractive index ent speaking, the density of the electrical signal proportional to the measurement volume .

For this purpose, optoelectronic detectors are used, which are provided with a pinhole, the diameter of which is small enough to detect only a small part of the flank of an intensity strip. To increase the signal strength, the pinhole can be replaced by a narrow slit, which must then be aligned parallel to the strip system.

Two such detectors, which are expediently of approximately the same sensitivity, are aligned in such a way that their distance perpendicular to the strip direction actuates approximately half of the projected strip width or an odd multiple thereof. The difference between the electrical signals of these two fixing detectors becomes zero when their diaphragms select areas of the same flank height of the intensity profile of the strips. If this is not the case, the differential voltage is used to generate a lateral displacement of the projected strip system in such a way that it occupies this excellent position and the differential signal thus becomes zero. In this way, the position of a reference point of a fixed phase position in the projected strip system is fixed locally, which is a prerequisite for the further measurement.

The interference strip offset to be compensated for in the manner described perpendicular to the strip direction can be

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fluctuations in the index along the path of the laser light bundles to the measurement volume are generated, but also by slight mechanical vibrations of the measurement arrangement itself.

The lateral offset is compensated for by one or more electro-optical cells, for example Eckels or Kerr cells, which are inserted into one or both of the incident laser light bundles in front of the measuring location and controlled by the electrical difference signal of the two fixing detectors mentioned above via an electronic control system stem can be activated electrically. The refractive index changes which can thus be generated in the cell produce corresponding phase shifts of the light waves of the two coherent laser light bundles with respect to one another, as a result of which the interference fringe system in the measurement volume undergoes the necessary position correction in the measurement volume due to lateral migration, which correction occurs both in the intermediate images and in the projected strip system ¬ occurs.

A further route can be followed by displacing the micro-objective used for the enlargement of the strip perpendicular to the direction of the strip to the extent that a correction of the position of the projected strip system is necessary. This can be achieved by mounting the micro-objective on, for example, a piezo body, which in turn is controlled by the differential voltage of the fixing detectors.

It is essential that this position correction receives the minimum time constant necessary for the measurement problem in order to be able to follow the greatest occurring speed of change of position, which is possible with the help of modern electronic control technology without difficulty.

, IPO On the interference strip system which is locally fixed in the manner described, a differential voltage is then generated with two detectors, which is related to the strip spacing. For this purpose, these two measuring detectors, one of which can also be identical to one of the fixing detectors, are arranged at a distance of approximately half a strip spacing or an odd multiple thereof. The electrical difference signal generated by the measuring detectors is then proportional to the strip spacing a.

Aligning the detectors to about half the height of the intensity edges is not absolutely necessary, but maximizes the sensitivity of the measuring device and the approximation to a linearity between the difference signal and the strip spacing. In addition, the case of half the strip spacing of the measuring detectors or the odd multiple thereof is a way of maximizing the difference signal compared to all other cases. The use of positions for the measuring detectors, which are danced across as many strip widths as possible compared to those of the fixing detectors, increase the sensitivity of the measuring arrangement in proportion to this distance and should therefore be optimized from this point of view.

In addition, the greater the number of interference strips between the fixed point of the strip system and the measurement detectors, the smaller the influence of small changes in position of the strip system in the course of the fixing control. One of the measuring detectors can be arranged on one side of the pair of fixing detectors, taking into account the criteria mentioned. - B -

Since the relationship between the difference signal of the measuring detectors and the strip spacing is generally not exactly linearly proportional, the actual relationship for the direct conclusion from the difference signal to the refractive index must be taken into account. This necessity can be avoided by using the measurement signal as a control signal for a zero method.

For this purpose, a further intermediate image of the measurement volume is generated within an electro-optical cell (e.g. Pockels or Kerr cell). The refractive index of the cell, which can be changed by the voltage applied to this cell, is controlled by the measuring voltage via an electronic control circuit so that it adjusts itself again to zero or to a fixed reference value. Then the stripe spacing of the intermediate image within the cell is equal to that of the value predetermined by the reference value of the difference signal.

In order to maintain this strip spacing, a sufficiently rapid readjustment of the voltage applied to the electro-optical cell is necessary due to the deviation of the electrical difference signal from the desired value. It must be fast enough to follow the changes in the refractive index in the actual measurement volume. If this is the case, the activation voltage for the cell is a measure of the current refractive index or the current density of the medium in the measurement volume.

Since the physical relationship between the refractive index of an electro-optical cell and its activation voltage is known, the refraction is also based on a reference value - 9 -

index and thus the density in the measurement volume within the measurement object is known.

Estimates show that, for example, the relative change in the stripe width of the interference fringe system within the medium of air and in the vicinity of the normal state has the following orders of magnitude:

Dependence on pressure;

_Q ___ | da / dp / _ =. 3rd 10 (mWs)

Dependence on the temperature:

da / dT r = έ> 1. 1θ ^"6 (grd) ^{" 1}

It is taken into account that an increase in the sensitivity of the measurement compared to these values by the arrangement of the measurement detectors on two adjacent flanks of the strip system by a factor of 4, in addition when measuring over 100 strip widths, for example, by a factor of 100, that is, a sensitivity overall ¬ increase by a factor of 400 is quite realistic, with the opto-electronic detectors available today the resolution of a change in density due to a 1 degree temperature difference or due to small pressure fluctuations using the example of air is possible.

The essence of the invention will be explained with reference to FIGS. 1 to 5.

Fig. 1 shows two at the angle "** ^ * overlapping mutually coherent laser light beams 1 and 1 ^• which form the measurement volume 2 in its sectional area. - 10- -

FIG. 2 shows the common cutting area of the light beams 1 and 1 ', the measuring volume 2, in a greatly enlarged form, with the stationary interference surfaces which are parallel to one another and which appear through the vertical cut with the plane of the drawing as interference strips 3 with the distance a from one another nen. The refractive index n is present in the area of the measuring volume.

3 shows the measurement volume 2 with the interference fringes 3 generated by the laser light bundles 1 and 1 'within the measurement object 4 with the variable refractive index n, its intermediate image 2' with 3 'generated with the aid of the optics 5 located outside the measurement object is projected onto the registration surface 7 by means of the micro-optics 6 and there represents the sine-like intensity curve of the intensity stripes indicated by 3 "in a strongly expanded form. Two opto-electronic detectors 8 and 8 'are positioned at a distance of about half a stripe width from each other, in 9, their electrical differential signal is formed, which controls the electronic control circuit 10. This activates the electro-optical cell 11, which, by shifting the phase of the laser light beam passing through it, compensates for any lateral offset of the strip system 3 that may have arisen perpendicularly to the direction of the interference strips so that which produces in 9 e differential voltage becomes zero. As a result, the interference strip system 3 "is fixed locally in the registration area 7, which is a prerequisite for the further measurement.

The piezzoelectric or magnetostrictive body 12 represents a possible alternative to the electro-optical cell 11 11 ^*

can, likewise activated by the control circuit 10, shift the micro-optics 6 perpendicular to the strip system in such a way that undesired lateral displacements of the strip system in the registration area 7 are corrected by making the difference signal from 9 zero.

Another pair of detectors 8 ″ and 8 ′ ″ serves to register the desired measurement signal U (t). Both detectors can also be arranged separately on each side of the fixing detectors 8 and 8 ¹ . Alternatively, one of the registration detectors 8 "and 8 '" can be replaced by one of the fixing detectors 8 or 8'.

An electronic circuit 9 'forms the electrical difference signal of the two measuring detectors 8 "and 8"', which is proportional to the strip spacing a and thus inversely proportional to the refractive index or the density of the medium in the area of the measuring volume 2. It is converted into the measurement signal U (t) by the electronics 13.

FIG. 4 again shows schematically the projected interference fringe system 3 "with the sinusoidal intensity curve and the arrangement of the detectors 8, 8", 8 "and 8 '" which is in principle optimal for this purpose, of which here the parallel to the slit diaphragms 17, 17 ', 17 "and 17'" aligned with the interference fringes between the maxima 15 and the mini a 16 of the intensity curve are indicated.

By zero-regulating the difference signal between 8 and 8 ', the special extreme of extremity lying between them is spatially fixed and thus serves as a reference point for the expansion of the

OMPΓ Λ. WIPO - 1 2 "

Strip system when changing the strip spacing a ". The difference in intensity between the detector positions 8" and 8 "'or the corresponding electrical difference signal is then clearly related to the strip spacing. Because of the generally small change in the strip spacing, the uniqueness of the display is normally not clear at risk.

By varying the actual strip width a using the

Choice of the angle 06 o as well as by the variation of the

The distance between the detector pairs 8 and 8 'or 8 "and 8"' covered stripes can be used to adapt the measurement conditions to the respective measurement problem within wide limits and to optimally design the sensitivity of the measurement arrangement.

FIG. 5 essentially shows the arrangement according to FIG. 3, but with an additional intermediate image 2 ″ of the measurement volume with the intensity stripe system 3 ″, which is generated within an electro-optical cell 19. The optics 18 then generate the intermediate image 2 'for the subsequent projection of the strip system by the micro objective 6. An additional electronic control circuit 14 is now controlled by the differential signal of the measuring detectors from the circuit 9 ¹ and in turn activates the cell 1. Via the refractive index which is variable by the activation voltage in the cell 19, the strip spacing in the strip system 3 '' can be influenced in such a way that it is always kept constant, which is achieved by adjusting the difference signal from 9 'to zero or a predetermined reference value.

The voltage V {t) taken from FIG. 14 and used to activate the cell 19 is then a measure of the deviation of the refraction ^' 1 3 "

index in the actual measurement volume from a reference value defined at the start of the measurement. Known physical relationships between the refractive index of the cell 19 and the activation voltage allow the conclusion to be drawn about the refractive index and its change in the actual measurement volume.

The zero method acting in this way enables a large temporal resolution of the refractive index changes to be measured and thus the density changes in the measurement volume, is distinguished by a high sensitivity and is independent of the non-linearities of the intensity curve on the flanks of the interference fringes used.

Claims

1 4 arrangement for measuring the local density and its changes over time in compressible media claims 1. Arrangement for measuring the local density and its temporal changes, in particular of compressible media by measuring the local refractive index, characterized in that the distance between the interference fringes of an interference stripe system, which in the intersection volume of two mutually coherent laser Beams of light are created as a measure of the refractive index present in the area of this cutting volume or used by producing an optical intermediate image of the measuring volume by means of bright optics sufficiently distant from the measuring location and then using a micro-objective to produce a greatly enlarged image of a Cross-section of this measurement volume in the form of a system of parallel interference fringes is projected into the registration area, in which the intensity-sensitive sensors used for the measurements, for example electrophotographic receivers, are positioned. 2. Arrangement according to claim 1, characterized in that the greatly enlarged image of the interference fringe system in • ^ JREX OMPI 5 " the registration area can also take place via more than just an intermediate image. 3. Arrangement according to claim 1 and 2, characterized in that opto-electronic detectors of high intensity sensitivity and high time resolution, such as, for example, photo diodes or photo electron multipliers, are used for the registration of the interference fringe width in the registration area. 4. Arrangement according to claim 1 to 3, characterized in that a sufficient spatial resolution of the intensity measurement on the flanks of the interference fringes is achieved by pinhole diaphragms of small diameter or, to increase the signal strength, by narrow slit diaphragms which are in front of the optoelectronic detectors the slit diaphragms are aligned parallel to the interference fringes. 5. Arrangement according to claim 1 to 4, characterized in that the electrical difference signal of two detectors, which are positioned at a mutual distance of about half an interference stripe width or an odd multiple thereof, serves as a control variable for the electronic regulation, by means of which the offset of the interference fringe system perpendicular to the direction of the fringe is compensated for, which can be caused by changes in the refractive index along the optical paths over time; the fringe system is thereby spatially fixed, in particular in the registration area. 6. Arrangement according to claim 5, characterized in that the lateral fixation of the strip system by an electronically controlled and controlled by the differential voltage electrical - 1 6- One or more optoelectronic cells are acted upon, for example Pockels or Kerr cells, which are inserted into one or both beam paths of the laser light bundles incident on the measurement volume. 7. Arrangement according to claim 5, characterized in that the lateral fixation of the strip system is carried out by an electronically controlled and controlled by the differential voltage electrical application to a piezo body, which slightly shifts the micro-objective perpendicular to the direction of the interference fringes and thereby the lateral Corrected the position of the projected strip system in the registration area accordingly. 8. Arrangement according to claims 1 to 7, characterized in that two further optoelectronic detectors, one of which can also be identical to one of the fixing detectors according to claim 5, are in turn spaced apart from one another as measuring detectors and at about half a strip spacing or an odd multiple thereof adjusted approximately to the middle area of the flanks of the intensity strips in the registration area, deliver an electrical differential signal which is proportional to the change in the interference strip width and the greater the number of those between the fixing detectors and is the number of strips lying on the measuring detectors. 9. Arrangement according to claim 1 to 8, characterized in that a further intermediate image of the measuring volume is placed in an electro-optical reference cell which, controlled and regulated by the differential voltage of the measuring detectors, is acted upon electrically so that its own refractive index changes Distance of the interference fringes in your own and all OMPI - 17 - the subsequent intermediate images keep constant that they thus compensate for the refractive index changes present at the measuring location in the manner of a zero method and that their activation voltage represents a measure of the instantaneous refractive index in the actual measuring volume. 10. The arrangement as claimed in claim 1 to 9, characterized in that the coherent light beams radiated onto the measurement volume are brought as close as possible to the measurement location in tubes with the smallest possible diameter or in glass rods in order to protect the light beams from optical distortions caused by refraction. To protect index fluctuations of the medium surrounding them along their path up to the measurement volume and thus to reduce the cross fluctuations of the strip system in the measurement volume.