WO1995033998A1

WO1995033998A1 - Laser doppler velocimeter

Info

Publication number: WO1995033998A1
Application number: PCT/FR1995/000729
Authority: WO
Inventors: André Clot; Laurent Romatier; Joseph Hamel
Original assignee: Full Imaging System - Fis
Priority date: 1994-06-07
Filing date: 1995-06-06
Publication date: 1995-12-14
Also published as: AU2741495A; FR2720839A1

Abstract

Portable compact laser Doppler velocimeter which is easy to use and comprises a laser source (1g), a splitter consisting of an opatque objet (6), splitting the beam (1h) from the laser source (1g) into two beams (1l and 1j) focussed by a lense (4f) at the measurement location. The device of the invention is especially suitable for flow measurements of transparent fluids and for teaching applications.

Description

The present Doppler laser velocimeter This patent application relates to a portable Doppler laser velocimeter which is simple to implement, takes up little space and has a low production cost. Doppler laser velocimeters to date are mainly used for laboratory and research applications requiring excellent performance in terms of resolution, dynamic measurement, sampling speed, 3-dimensional measurements, possibility of sizing and quantification of particles. It is mainly for this reason that the equipment available in laser Doppler velodmetry only includes expensive, bulky, fragile and difficult devices to implement. In addition, current flowmeters, whatever the measurement principle - electromagnetism, micro-reel, ultrasound, Vortex effect, Coriolis effect, and pressure-reducing devices - all have limitations, particularly in the area of measurement of weak speeds (less than one decimetre per second). In this area alone, laser Doppler velodmetry can offer an interesting technical solution at a cost comparable to or less than the above measurement means.

The subject of the present application relates to a laser Doppler velodmeter intended inter alia for industrial or educational applications and comprising important simplifications compared to the usual products in Doppler laser velodmetry (VLD).

We will recall in the first part the prindpe of velodmetry la¬ ser Doppler effect in order to clearly highlight the components necessary for the implementation of the principle.

We will then describe a possible realization of the velodmeter reducing and simplifying the necessary components.

Figure 1 provides an understanding of the prindpe used under the term Doppler laser velodmeter. FIG. 2 presents a known simplification of implementation of the veloci-

^FIRE, REPLACEMENT (RULE 26) laser metrics.

Figure 3 illustrates the prindpe.seion the invention. Figures 4 and 5 show other implementations of the invention.

Figures 6, 7, 9, 10 and 11 show several possibilities of applying the prindpe according to the invention.

Figs. _t 12, 13 and 14 present interesting simplifications of laser cyclometry for detecting the direction of flow. The scale of Doppler laser velodmetry, as shown in Figure 1, is based on the offset of the frequency of the light scattered by any moving particle when it crosses a beam of coherent and monochromatic light. .

The frequency shift induced by the Doppler effect is proportional to the speed of the particle and depends on the direction of detection. As this difference is very small, it cannot be measured directly. The heterodyning technique, or beating of two frequencies, is then used. Several configurations allow the implementation of this phenomenon.

Figure 1 shows the configuration using the reference beam mode. One causes the beat of the two frequencies on the photodiode 5a used to capture the Doppler signal: the frequency of the laser source and that containing the shift in frequency due to the Doϋϋler effect.

The beam 1 from the laser source 1c is separated into two beams ia and 1b, by the splitter 4a. A total reflection mirror 4b reoπent the beam 1a, called the reference beam, parallel to the beam 1b said to be of measurement. The 2 beams 1a and 1b are focused by the lens 4c at the measurement point 15. The beam 1b, in the measurement zone at the focus point, during the passage of a micro-particle, will diffuse ia light whose frequency is decaiée compared to that of the beam of ori-rine 1 by the Doppler effect. The photodiode 5a, positioned on the reference beam, captures the beats caused by the rreαuences of the 2 beams, that of the light scattered by the particle and that in the reference beam, and delivers an eiectπαue signal 5b containing the beats DOD- cry.

The so-called interference fringe method constitutes another presentation of the prindpe of Doppler laser velodmetry: the two beams 1a and 1b create, at the focal point, interference fringes 2. Any microparticle 3 in passing light areas will diffuse the light captured by the photodiode 5a.

The interfrange x is linked to the wavelength L of the laser source and to the geometry of the beams defined by the angle 2β, by the relation x = L / 2sin β A particle moving at a speed v perpendicular at the interference fringes will therefore generate a signal of frequency f such that

The speed of the particles is thus linked to the Doppler frequency by the relation: v = f * L / 2 sin_3 The processing, that is to say the extraction of the Doppler beat frequency carrying the speed information, can be performed using different methods described later.

Many works have been written (Durst / Melling / Whitelaw - 1987 - Theory and practice of laser velodmetry with Doppler effect) which concern only the choice of particles and more precisely the diameter of these to obtain a good quality of signal. We will not develop this aspect of technique; the present innovation is mainly concerned with applications which do not require seeding but which can operate with the only particles present in the fluid to be instrumented. In the main applications targeted, on fluids frequently encountered (water, steam, other transparent fluids,) there are statistically sufficient particles allowing speed measurement under good conditions, without the need for seeding. . Generally, these applications are more particularly interested in the measurement of flow rates or volumes circulating in the measurement of instantaneous speed. If we refer to the diagram in Figure 1 explaining the measuring prindpe, the essential functionalities for the implementation of a VLD sensor are as follows:

- monochromatic and single-mode light source, - separation into 2 beams, with the possibility of balancing the separation of these beams,

- focusing of these 2 beams on the measurement point,

- signal reception.

The example shown in FIG. 1 uses a separating blade 4a making it possible to obtain two coherent beams la and lb from the same source la¬ ser le, a mirror 4b reorienting the beam parallel to the beam lb and a lens 4c focusing the two beams at the measurement point. The quality of the signal depends in particular on the precision of the opti¬ path of the beams, which itself depends on the quantity of the components and their positioning. Thus, FIG. 2 illustrates a known and interesting possibility of producing a sensor comprising only a limited number of components: laser source ld, separating blade 4d, of which a 4th part has been treated to allow total reflection of the beam. the; the two beams le and lf at the outlet of the separating blade are always, without any other precaution, parallel.

Likewise, a good Doppler signal requires having coherent beams, that is to say that the difference in optical path of the beams is less than the coherence length of the laser. Any difference in optical path thus leads to a risk of non-coherence of the beams, particularly if the coherence length of the laser source used is small.

The patents FR 2654215, registered under No. 90 13540 and published on 05/10/91, and FR 2637085, registered under N ° 89 08632 and published on 03/30/90, relate in particular to beam splitters allowing the production of compact Doppler laser anemometers. Despite undeniable advantages in compactness, the means allowing the separation of the laser beam into two beams in both patents, however, include several optical elements requiring careful positioning and adjustments. Patent 89 08632 for example, requires a divider cube or a symmetrical divider for the division of the laser beam and of mirrors with total reflection, one per partial beam, for deflecting the beams and making their crossing in the measurement volume.

Patent 90 13540 also uses two mirrors with total reflection, the separation being carried out by a semi-transparent mirror. In both patents, at least three components are necessary for the division of the beams, which implies, apart from any price element, special precautions as to the positioning of each of them and, by way of consequently, possibilities of adjustment. To avoid using expensive optical means making it possible to overcome this kind of problem, the present invention proposes a separation method which is simple to implement, reliable and which makes it possible to obtain a very low interference order, the difference optical path at the central fringe being zero.

Figure 3 shows schematically a possibility of achieving this separation. The present invention achieves separation by interposing, as shown in this figure 3, simply on the beam lh at the output of the laser source lg, an opaque body or object 6, the constant thickness of which in its useful part and opposing when the beam lh passes, is less than the width of the initial beam, coming from the diode or laser source lg, of elongated shape, at least in the direction perpendicular to the plane formed by the direction of the speed to be measured and that of the beam lh of the laser source, of length in this direction greater than the width of the beam lh. This opaque object 6 separates the initial beam lh into two sub-beams li and lj which are then focused by the lens 4f on the measurement point 15a, creating at this point interference fringes. The signals resulting from the passage of a particle through these interference fringes are detected by the detection means 5c. In an advantageous implementation of the invention, the opaque object 6 constituting the separator is a cylinder with a diameter less than the width of the beam lh, as shown in FIG. 3. The advantages of such an implementation are numerous, the main ones being the following: extreme simplicity of implementation - negligible cost of the components necessary for this separation - simplified development. The drawbacks (waste of energy, fixed geometry of the separation of the beams) can certainly limit its applications, particularly in the field of laboratory measurement. Its simplicity, on the other hand, opens the way to many applications in the industrial field, difficult to envisage with conventional products in laser velocimetry with Doppler effect. This separation can be achieved using a variety of means, the list of those described below is by no means limiting. A simple way of making this separation, as shown diagrammatically in FIG. 4, consists in simply sticking a strip of adhesive tape 7, at the outlet of the laser source. Another way offering a little more adjustment possibilities is shown diagrammatically in FIG. 5. It implements a cylinder 8 which separates the initial beam in two. This cylinder, in an interesting embodiment of the invention, is mounted eccentrically on a support 9 rotating around an axis and located outside the beam field and perpendicular to the plane passing through the direction of the beam and that of the speed to be measured. The rotation of this support 9 makes it possible to move the cylinder 8 from one edge to the other of the width of the beam, to position the separation strip at will over this width, and to adjust the balance in intensity and geometry of each beam. Figures 6-top view and 7-side view show an application involving the invention and illustrating its simplicity of implementation. In this application, the invention is used as an educational bench for initiation to laser velocimetry with Doppler effect. The sensor 10 according to the invention comprises the laser source 11, the separator 13 and the focusing lens 26 and is associated with a glass tube 18 in which water circulates. The laser source 11 after positioning the beam in the measurement direction is blocked by the screw 12. The sensor 10 and the receiver 14 are fixed on a plate 15 movable relative to the bench 16. The measurement point 17 can thus be moved to any point of the inside diameter of the tube 18 in which water is circulated. The assembly makes it possible to produce the profile of the flow velocities inside the tube.

In an interesting embodiment of the invention as shown in Figures 6, bottom view, and 7 ₁ side view, the receiver is simply constituted by a support mounted on the plate 15 and supporting an optical fiber 19 receiving the Doppler signal. This optical fiber 19 transmits the signal to a photodiode 20 which converts the optical signal into electric current; this current is transformed into voltage and amplified by the electronic card 21. This signal can then be directly viewed on an oscilloscope 22. A memory osdlloscope makes it possible to select the interesting measurements and to calculate the frequency of the Doppler signal thus visualized . Knowing the coefficient of the probe, taking into account the wavelength L of the laser source and the geometry of the beams, makes it possible to calculate the flow speed at the point of measurement according to the relationship mentioned above: v = f * L / 2 sin β

Such a system is perfectly suited to educational applications, by presenting in a simple manner the different possible configurations for implementing a sensor based on laser velodmetry with effect.

Doppler; it also allows interesting applications in the industrial field.

In another interesting embodiment of the invention, the sensor includes a processing of the signal 41 allowing at the output of the current-voltage converter to digitize the signal and to process it to extract the Doppier frequency therefrom. Most Frequently Doppler Frequency Extraction Methods encountered are:

. counting: the time corresponding to "n" zero crossings of the periodic signal containing the Doppler frequency is counted; it is a technique which is simple to implement and which gives good results whatever the application; validation constraints making it possible to ascertain the reality of the signal, however, lead to the elimination of interesting data; in fact, to validate the calculated frequency, one can for example compare the time corresponding to "n" zero crossings, double the time for "n / 2" passages by zero; values outside of previously established tolerance thresholds are eliminated; ced leads to the rejection of interesting values, particularly in the case of turbulent flows; the measurement time can become abnormally long;

. phase-locked sulk system: the frequency of the Doppler signal is compared to that of a voltage-controlled oscillator; this method can only be used in the case where the signal is permanent and of frequency varying only slightly;

^' . rapid FURNING transforms; without going into the limitations linked to the calculation method well known to specialists, in the presence of non-permanent signals, the window, corresponding to the signal obtained during the passage of a particle in the measurement volume, is too narrow to allow satisfactory processing by FOURLER transforms. Another interesting possibility is signal processing by mathematical parameterization.

Let be a sinusoidal signal of frequency f sampled at a period T. If we denote S (n) = (cos Ω * nT) the value of the signal sampled at the n ^th sampling period, we can write for the (n + l ) th sample:

S (n + 1) = 2cos Ω TS (n) - S (n-1) second order recursive function. Our problem is to determine the value of cos ΩT and therefore the value of the frequency f = Ω / 2π which verifies the relation: S (n + 1) + S (nl) = a ^ n) where a _! = 2 cos Ω T Generally this type of treatment includes:

- signal digitization, after current / voltage conversion and signal amplification,

- a recursive linear processing in which an algorithm calculates the pulsation Ω so as to minimize the difference between the estimate and the real value of the signal.

Various problems can limit this type of treatment. We are able to provide an estimate of al even if the signal is not sinusoidal. The algorithm used in the present invention comprises a first phase in which the nature of the signal is verified. For this, we calculate the coefficients of the recursion relation: S (n + 1) = a S (n) + a ₂ S (nl) For a 100% sinusoidal signal, we must have a ₂ = -1. Since the signal is modulated randomly in amplitude, a margin (around 1% to 2%) is tolerated around -1; if a ₂ is not located in the corresponding interval, the same calculation is carried out on the following sample. Many industrial applications of the Doppler laser velocimeter sensor according to the present invention can be envisaged; the examples which follow are presented by way of simple illustration and are in no way limiting. FIG. 9 shows a possibility of implementing the sensor according to the invention for measuring the speed of water circulation or the flow rate of fluid circulating in a pipeline. As in the examples of FIGS. 6 and 7, it is the microparticles present in the fluid which are at the origin of the Doppler signals detected by the receiver 14. Portholes 31 and 32 arranged along the axis of the laser beam in the wall of the pipe, allow optical access to the flowing fluid. The sensor 10 including the laser source, its splitter and its focusing lens, and the receiver 14 are positioned on either side of the pipeline as in the example of FIGS. 6 and 7 of the educational bench. The signal processing is then carried out in an electronic unit 33 and delivers an electrical signal proportional to the parameter to be measured: point speed, average speed, flow taking into account the diameter of the pipe or quantity of fluid delivered after integration of the flows over time. . The sensor of figure 9 can also be used for any fluid with sufficient qualities of transparency, ad hoc adaptations, as to the nature of the components, in particular portholes, which may be necessary depending on the fluid transported: chemical aggressiveness or temperature in the case of vapor transport by example- Another example of embodiment is presented in FIG. 11 where the transmitter 35 and the receiver 36 are mounted on the same mechanical support 37. A single tap 40 in the pipe 43 is necessary to implement the sensor according to the invention. An opening 39 in the body of the mechanical support 37 of the sensor allows the passage of the fluid without disturbing the flow in the measurement zone between the transmitter and the receiver. Likewise, the laser velocimeter with Doppler effect according to the invention can be used for non-contact scrolling measurement problems such as production of paper pulp, production of laminates or speed measurement of a mobile, using, not detectors located in the axis of the beams, which is possible with a transparent medium, but arranged on the same side as the laser sources, using the reflection of the surface of the mobile. This last example, which does not limit the possibilities of application, is illustrated in FIG. 10 where a mobile 41 equipped with a sensor 42 and traveling on rails 34, measures the running speed of the rail 34. This application can be applied to any mobile moving on a sufficiently flat surface. The simplicity and the compactness of such a sensor according to the invention open the way to numerous applications and in particular the possibility of producing a sensor giving the direction of flow without requiring the use, for example, of a Braggs cell, allowing the frequency shift of one of the two beams, bulky and expensive.

FIG. 12 represents a velocimeter with two laser sources 44 and 45, one of which 44 may be that lg represented in FIG. 3, and the second 45, with its separator 57 positioned parallel to the first separated from it by a distance d. Their focusing lens 46 is common to the two sensors and consists of a Ticket lens, such that the measurement zones 50 and 51 of the detection means, are separated by a distance defined by construction and dependent on the thickness of the wafer 48 removed from said Ticket lens 46. Indeed, Ticket lenses are lenses that have been modified in accordance with FIG. 13. On lens 47, it has been removed by machining wafer 48; the two remaining elements are then joined together forming the assembly 49. With such a focusing lens, on the assembly according to FIG. 12, the sensor 44 has its focal point at 50, the sensor 45 its focal point at 51. A microparticle moving perpendicular to the measurement axes and passing through the measurement volumes containing the interference fringes, will first cross that of the sensor 45 then that of the sensor 44, if the flow direction is that of arrow 52 The comparison of the moments of appearance of detection of the signal from each sensor gives the direction of flow: - direction of the arrow 52 in the case of a detection on 45 then 44, as shown diagrammatically in FIG. 8,

- opposite direction to that of arrow 52 in the case of detection on 44 then 45. The same result can be obtained by using an ordinary conventional converging lens 55 and by tilting the beam coming from one of the two sensors made up of the laser sources 53 and 54 and separators 59 and 60, with respect to the axis of the lens 55, in the plane formed by the axis of the lens and the beams of the sensors, so as to offset the corresponding measurement volume in said plane and create two measurement zones slightly offset along an inclined plane 58, in accordance with FIG. 14.

Such a sensor can become, under certain conditions, a vector sensor. Indeed, we then have two components of the speed in this plane 58, along the perpendiculars to the interfringes of each sensor whose orientations are known by construction, making it possible to determine the direction of the speed; comparing the moments of detection of the signal on one and the other of the sensors 53 or 54, makes it possible to define the direction of the speed. This supposes that the same particle crosses the measurement zones corresponding to each sensor 53 and 54. A simple remedy to this situation, made possible by the use of inexpensive sensors according to the invention, then consists in associating other sensors according to a geometry allowing the detection of microparticles by at least two sensors, whatever the direction of the speed.

Claims

1. Doppler laser velocimeter using at least one laser source (lg), a focusing lens (4f) and detection means (5c), characterized in that it comprises a separator interposed on the beam (lh) in output of the laser source (lg), consisting of a body (6) having a constant thickness in its useful part, less than the width of the initial beam (lh) coming from the laser diode (lg), of at least elongated shape in the direction perpendicular to the plane formed by the direction of the speed to be measured and that of the beam (lh) of the laser source, of length in this direction greater than the width of the beam (lh) and separating it into two beams ( li) and (lj) crossing at the focal point (15a).

2. Doppler laser velocimeter according to claim 1, characterized in that the separator consists of an adhesive tape (7), of width less than that of the initial beam of the laser diode (lg) glued at the output of the diode laser (lg).

3. Laser Doppler velocimeter according to claim 1, characterized in that the separator consists of a cylinder (6) of diameter less than the width of the beam.

4. Doppler effect laser velocimeter according to claim 3, characterized in that said cylinder (8) is mounted eccentrically on a support rotating about an axis.

5. Laser Doppler velocimeter according to claim 1, characterized in that it comprises a second laser source (45) positioned parallel to the first, with its separator (57), and spaced from the first source by a distance d , the focusing lens being common (46) and constituted by a Billet lens, such that the measurement zones (50) and (51) of the detection means are separated by a distance defined by construction and dependent on the thickness of the wafer (48) removed from said ticket lens (46).

6. Doppler laser velocimeter according to claim 1, characterized in that it comprises a second laser source (45) positioned parallel to the first with its separator (57) is spaced from the first source by a distance d, the focusing lens (55) being common and consisting of a conventional converging lens and in that 13

one of the two laser sources (53) and (54) has its beams slightly inclined relative to the lens axis (55), so as to create two measurement zones slightly offset along an inclined plane (58).

7. Doppler effect laser velocimeter according to claim 1, suitable for educational use, characterized in that it comprises a sensor

(10), including said laser source, its splitter and its focusing lens, and means for receiving its signal (14), associated and positioned on either side of a glass tube (18) in which circulates of water and in that the sensor (10) and the receiver (14) are supported by a plate (15) movable relative to the bench (16) supporting the tube (18) allowing the profile of the speeds of l flow inside the tube (18).

8. Doppler effect laser velocimeter according to claim 1 for measuring the flow rate of transparent fluids circulating in a pipe, characterized in that it comprises a sensor (10), including said laser source, its separator and its focusing lens, and a receiver (14) positioned on either side of the pipeline, comprising portholes (31, 32) along the axis of the laser beam.

9. Doppler effect laser velocimeter according to claim 1, for measuring the flow rate of transparent fluids circulating in a pipe (43), characterized in that the sensor, including said laser source, its separator and its focusing lens, and the receiver (36) are mounted on a mechanical support (37) on either side of an opening (39) in said support (37), said opening (39) allowing the passage of the flowing fluid in the pipes (43 ), which support (37) being introduced into the pipeline (43) by means of a nozzle (40).

10. Doppler effect laser velocimeter according to any one of claims 1 to 9, characterized in that the processing of the signal picked up by the reception means and making it possible to extract the Doppler frequency, uses a mathematical algorithm with recursive computation which comprises a check of the sinusoidal nature of the signal.