FR2551214A1 - Method and device for measuring the local speed within a fluid medium carrying included matter in suspension. - Google Patents

Abstract

Measurement of the local speed of a fluid medium. The device for implementing the method is characterised in that it comprises: - an ultrasound emitter 10; - a wave generator assembly 15 linked to the emitter; - a detection and measurement assembly 16 linked to the emitter and to the generator assembly; and - a calculator assembly 17 linked to the detection and measurement assembly. Application to liquids carrying included matter in suspension.

Description

 The present invention relates to a means for measuring the speed of a fluid medium, for which it is difficult to make speed measurements by the usual techniques, either because of its opacity, its density or its concentration of components of charge either, again, because of its strong gradient of refractive index.

 The invention relates to the technical field of the study, control or determination of the speed, at least local, of a fluid vein and, more particularly, of a fluid vein which can be driven by a movement at low , or even very low speed.

 The invention should be understood as interesting the field of measuring a proper speed of a moving fluid stream or, again, that of the fluctuation of the interface between two immiscible media contributing to the formation of said vein.

 To measure the local speed of a fluid medium, various proposals have already been made.

 These different proposals are based on the measurement of the "Doppler" frequency using either a continuous emission or a pulsed emission of an ultrasonic wave.

We can cite in this regard
- "The in situ measurement of sediment transport by means of
ultra-sound scattering "RHJ JANSEN (Delft Hydraulics
Laboratory 1978),
- "Determination of flow rates by spectral analysis of signals
Ultra sound Doppler "J C. VERA (Thesis - NANCY) 1977,
- "Analysis of blood flow by ultrasonic method"
P. PERONNEAU (These - PARIS-Sud) 1977,
- "Measurement of fluid turbulence based on pulsed ultrasound
techniques "JL GARBINI, FK FORSTER, J. E JORGENSEN
(J. Fluid Mech, May 1982).

 Measurement by continuous emission consists in using two suitably oriented transducers, so that the intersection of their lobes (emission and reception) determines the measurement volume.

 In the case where the measurement range is close to walls or obstacles, the receiving transducer risks receiving waves which, as a result of multiple reflections, do not come from the measurement range. Such a drawback clearly limits the practical interest of such a method.

 Pulsed emission measurement consists in periodically emitting an ultrasonic pulse or wave train and taking into account the echo signal of each pulse reflected by an inclusion.

 This method makes it possible to select the inclusions by taking into account only the echoes occurring with a given delay. This allows the measurement range to be precisely defined. Unfortunately, if the pulses are emitted that are too short to reduce this domain, the echo spectrum widens, which leads to inaccuracy. This disadvantage is all the more important when the speed to be measured is low.

 Furthermore, it is understood that the information collected is supplied at the rate of the transmissions, which imposes certain restrictions for obtaining the Doppler frequency obtained by successive samplings.

 In addition, in one or the other method, the measurement of the Doppler frequency involves an imposed lXx which is the ultrasonic half-length of wave in the fluid medium considered and therefore relates to the measurement of the corresponding At. The Doppler frequency is, in fact, the inverse of the time taken by an inclusion to travel half a wavelength.

 It is understood that the imposed x cannot be modified, unless the transmission frequency and therefore the transmitting transducer are changed. For low speeds to be measured, the ss x is often too large. Such a method therefore becomes practically unusable for low speeds close to zero.

 A method has also been proposed which consists in emitting a light beam by "Laser" and in using the reflected beam for analysis. It is, in fact, a principle analogous to measurement by ultrasonic emission, only the nature of the waves being changed.

 Such a method therefore has the drawbacks already mentioned but, in addition, also a drawback due precisely to the scratching of the waves emitted. It is understood, in fact, that this method cannot be used when the liquid medium to be measured is non-transparent, such as, for example, a highly charged liquid, a flow of liquid stall, a stream of paper pulp, etc.

 The present invention aims to solve the general problem a; if p3se and to eliminate the disadvantages of the methods currently co-sn es) in order to make possible a precise speed measurement within a fluid vein even non-transparent and likely to be driven by a slow, even very low, speed of movement, or even close to zero.

 Another advantage of the object of the invention is to provide, in addition, by the same measurement operation, the speed of movement and the direction of this movement.

 An additional advantage of the object of the invention lies in the fact that it becomes possible to locally measure the speed of a fluid vein by adopting a measurement method following the inclusion responsible for the echo signal taken into account. We can thus measure the Lagrangian speed of inclusion. This advantage thus makes it possible to have precise information concerning the possible variations in speed affecting all or part of a vein and thus to assess the conditions of movement as a function of various parameters, such as pressure, temperature, flow, obstacle, path , section variations, etc ...

 Another advantage of the subject of the invention lies in the fact that the means of the invention make it possible to obtain useful measurement results, even at low speeds, for an attractive cost compared to that involved in known techniques.

To achieve the above aims, the method according to the invention consists in
- emit in the environment, at a reference frequency
determined tition, an ultrasonic pulse pat
a transducer whose beam is directed according to
the desired component of the speed to be measured,
- collect on the same transducer (or on a
separate transducer) the echoes of each im
drive for inclusions,
- measure the phase difference at each pulse
expressed as a fraction of emission period, between
the signal emitted and the echo signal occurring at
after a certain time,
- differentiate between two corresponding measurements
laying on two successive impulses,
- and multiply this difference by the product
of the half-speed of ultrasound in the medium
considered by the repetition frequency for
get the unit travel speed of
time.

 Various other characteristics will emerge from the description given below with reference to the appended drawings which show, by way of nonlimiting example, an embodiment of the subject of the invention.

 Fig. 1 is a diagram showing, diagrammatically, certain phases of the process.

 Fig. 2 is a diagram illustrating the implementation of the method.

 Fig. 3 is a diagram illustrating a characteristic of the process.

 Fig. 4 is a diagram similar to that of FIG. 1 but illustrating another arrangement of the method.

 Fig. 5 is a diagram showing the functional composition of a device for implementing the method of the invention.

 The method according to the invention is intended to determine the local speed of a liquid or gaseous fluid vein, incorporating inclusions which can be of any kind. Thus, it may be solid particles of any material, but also gas bubbles having a lifetime sufficient to represent reflective elements. The implementation of the method consists in using an ultrasonic emitter applying pulsed energy to an emitting transducer responsible for emitting in the fluid medium an ultrasonic pulse at a determined adjustable frequency, directed according to the desired component of the speed to be measured. In this sense, any material offered for this purpose on the market can be used or adapted by those skilled in the art. For example, one can use a material working at frequencies of a few E-ZHZ.

 Fig. 1 shows that the first phase therefore consists in generating, in a regular and continuous manner, a signal in form 1 by a precise clock.

 A pulse 2 of at least one wave is taken from this signal, this pulse, illustrated in I, being applied by the emitting transducer, not shown, to the fluid medium.

 From the origin O of emission of the pulse 2, a delay t is determined to involve, in II, a fixed time reference Rt from which we await the reception of an echo reflected by an inclusion. This reference Rt is placed in phase with the transmission signal and, for example, in coincidence with a falling edge of the signal 1.

 Echo 3 (III) corresponding to the transmitted signal is received and analyzed by a characteristic point detector. In the present case, the detector is designed so as to emit a signal 4 (IV) each time the echo passes through the value zero following a downward variation.

 The first signal 4 emitted after the time reference Rt is applied to a flip-flop 5 (V) whose change of state 1 or 2 is placed simultaneously under the dependence of the reception of a toggle or hold signal generated by the Falling edge of each wave of the transmission signal.

The measurement period Tg is thus between the reference Rt and the return to the original being of the flip-flop and defines the measurement domain. In this period, the time interval
O, comprised between the reference Rt and the change of state signal of the flip-flop 5 corresponds, expressed as a fraction of the transmission period, to the measurement of phase difference between the transmitted signal 1 and the perceived echo 3.

 One proceeds, periodically, as said above, by transmitting another signal 2 after reception of the echo 3 of the previous signal 2.

 Fig. 2 shows, overall, the emission of a first signal 21 reaching an inclusion or particle P located at x1 in the fluid medium. This particle P is responsible for an echo 31 which is taken into account, after a delay r ,, from the reference Rt. The value e1 measured corresponds to the phase difference between the transmitted signal 2 and the echo signal 3 and determines the half-distance between the origin of emission and the point x1, taking into account the half-speed of the ultrasound in the medium considered.

 With a repetition period TR greater than r ", a new signal 22 is emitted which reaches the particle in the position x2. The echo signal 32 is received and taken into account as before to supply a value # 2 corresponding to the new resulting phase difference of the displacement of the particle P from x1 to x2 during the time separating the emissions 21 and 22.

The phase difference 02 e, oI therefore corresponds to the displacement of P from x1 to x2. Thus, it becomes possible to know the local average speed imposed on P by applying the equation
v = c / 2 FR. (# 2 - # 1) in which cc is the speed of ultrasound in the medium
considered,
and FR the frequency of repetition.

 From the above and from the examination of fig. 2, it can be seen that the method of the invention aims to measure the value lix proportional to the value ae resulting from two successive measurements defined by the predetermined repetition period TR. Thus, it becomes possible to vary TR at will and to adjust or adjust this parameter according to the speed to be measured. Unlike current methods, the method of the invention therefore offers the possibility of measuring low, or even very low, travel speeds close to zero.

In addition, the difference 92 - 01 is directly affected
2 I of a sign which provides, consequently, and without additional measurement, the direction of movement of the fluid stream relative to the origin of emission.

 In order to effectively implement the inversion method, it is necessary to be certain of the identity of the terms taken into account for two successive measurements. If the first term Rt is) brvu of ambiguity, on the other hand, the second, which is the first drawing by zero after Rt of a falling slope, can correspond to any one of the waves of the echo signal 3. The method of the inven too, in order to remove this ambiguity, consists in choosing a repetition period TR, in such a way that, taking into account the speed of the particle P, the displacement of this particle between two successive emissions is less than one half wavelength of the emitted ultrasound, so as shown in the diagram in fig. 3, the successive measurements of 0 will be presented in successive stages, the maximum value of 6 being practically equal to a period T of the echo. This figure makes it possible to note that the value of B experiences a sudden variation corresponding in fact to a phase shift resulting from a change of a zero point consecutive to the displacement of the characteristic wave initially taken into account outside the measurement period Tg and its replacement by the following wave.

 The method of the invention eliminates such maximum variations be which are not significant of the displacement of the particle in the fluid medium, by providing, for example, to calculate the difference ## 2 - ## 1 between two successive measurements, which amounts to calculating the acceleration of P.

 The inention process is completed by validation means so as to eliminate any parasitic echo collected and likely to be taken into account for the calculation of 0.

 One of these means, illustrated in FIG. 4, consists in defining a measurement window W opening in time, slightly in advance of the reception of the echo signal 3. Upon reception of the signal, each passage through zero is detected in downward slope, for example of successive waves. We count a determined number N of successive zero crossings, for example, four, five or six and we then trigger, only, the taking into account of the time reference Rt after which we will then retain the first subsequent zero crossing to measure the difference 8 corresponding to the phase difference.

 This makes it possible to be certain that the signal 3 received is indeed an echo signal corresponding to a first state of reflection from a particle P.

 The method of the invention also comprises, in the same direction, means for eliminating too brief and / or non-successive echoes.

 Fig. 5 shows a functional diagram of a device for implementing the above method.

 This device comprises an ultrasonic transmitter 10 comprising a power amplifier 11 supplying a transmitting transducer 12 as well as possibly a receiver. In the latter case, the transducer 12 is connected to a receiver 13.

The device comprises a set 15 of signal generator in the form of wave trains, a set 16 of detection and measurement, a set of computer 17, as well as two validation chains 18 and 19
The assembly 15 includes a clock 20, the output of which is connected to an adjustable divider 21 capable of supplying, from the original frequency Fg of the clock, an adjustable repetition frequency FR. The divider 21 is connected to a selector 22 of the number of half-waves of the frequency Fg transmitted for each transmission at the frequency FR. The output of the selector 22 is connected to the input of the amplifier 11.

 The assembly 16 comprises an adjustable divider 23 connected to the output of the clock upstream of the divider 21, as well as to the latter. The divider 23 is responsible for defining the position in time of the time reference Rt in phase with the transmission of the clock 20. The divider 23 is connected to a logic control block 24 and to a module 25 for developing the window signal W. This module is connected to one of the inputs of an "AND" gate 26, the other input of which is connected to a detector 27 of zero crossing of the half-waves of the echo signal 3 supplied by the receiver 13. The output of gate 26 is connected to one of the inputs of a flip-flop 28 whose other input is connected to the output of clock 20, in order to receive the frequency Fg. The output of the flip-flop 28 is connected to an integrator 29 triggered by the logic block 24. The integrator 29, responsible for measuring the value z resulting from two changes of state of the flip-flop 28, is connected to a memory 30 which supplies by requesting block 24 a computer 31 providing a value ae resulting from two successive measurements and directly usable.

 The memory 30 and the computer 31 form the assembly 17.

 The first validation chain 18 comprises, at the output of door 27, a counter 32 for zero crossings included in the window W, as well as a detector 33 for lack of signal. The counter 32 and the detector 33 are connected to a logic block 34 controlling a switch 35 controlling the connection between the integrator and the calculation unit 17.

 The second validation chain 19 comprises, following the computer 31, a memory 36 controlled by the block 24 to supply a computer 37 for the acceleration from the speed measurement provided by the assembly 17. The computer 37 controls a switch 38 placed on the output of the assembly 17.

 The invention is not limited to the example described and shown, since various modifications can be made thereto without departing from its scope.

Claims (11)

 1 - Method for measuring the local speed in a fluid medium carrying suspended inclusions, characterized in that:  - emits in the middle, at a repetition frequency  tion determined, an ultrasonic pulse by  a transducer whose beam is directed according to  the desired component of the speed to be measured,  - collects on the same transducer (or on a  separate transducer) the echoes of each im  drive for inclusions,  - measure at each pulse the phase difference ex  awarded in fraction of emission period, between  the signal emitted and the echo signal occurring at  after a certain time,  - makes the difference between two corresponding measurements  laying on two successive impulses,  - multiply this difference by the product of  the half speed of ultrasound in the middle  considered by the repetition frequency for  get the unit travel speed of  time.  2 - Measuring method according to claim 1, characterized in that the repetition frequency is chosen, according to the speed to be measured, so that the difference between two successive phase differences corresponds to a displacement of the inclusions less than half a length of the ultrasonic signal emitted.  3 - Measuring method according to claims 1 and 2, characterized in that eliminates the results of the speed measurements corresponding either to too short echoes, or to non-successive echoes, or to an acceleration of inclusions greater than one fixed and adjustable threshold.  4 - Measuring method according to claim 1, charac terized in that the phase difference is determined by measuring the time elapsing between, on the one hand, a time reference in phase with the emission signal, defined by a delay determined relative to the start of the transmission and, on the other hand, the passage through zero of the first half-wave of the echo signal, received after detection of the reference.  5 - Measuring method according to claim 1, charac terized in that one takes into account the sign of the difference between two successive deviations to determine the direction of movement of the medium.  6 - A measurement method according to claims 1 and 3, characterized in that the acceleration is determined as one of the validation criteria by making, with each pulse, the difference between the result of the speed measurement and the previous result.  7 - Measuring method according to claims 1 and 4, characterized in that the delay is adjustable and determines the position of the measuring range.  8 - Measuring method according to claim 1, character r; é in that we take into account several successive results or  o i to obtain an average speed.  9 - Device for implementing the method according to one of claims 1 to 8, characterized in that it comprises  - an ultrasonic transmitter (10),  - a wave generator assembly (15) connected to  the transmitter,  - a set (16) of detection and measurement connected  to the transmitter and to the generator set,  - and a computer assembly (17) connected to the set  detection and measurement.  10 - Device according to claim 9, characterized in that it comprises a chain (18) for validation and recognition of the echo signals connected to the detection and measurement assembly (16) and controlling a switch (35) controlling the circuit between said assembly and that of calculation (17).  11 - Device according to claim 9, characterized in that it comprises a validation chain (19) placed in series at the output of the computer assembly (17) and appreciating the acceleration of the speed measurement delivered by the latter .