GB2140160A - Apparatus for sensing the movement of a fluid - Google Patents

Abstract

A resonance cavity is formed by two parallel spaced discs (1, 2, Fig. 1). A transducer 7 sets up a standing wave in the cavity through which a gas, the flow of which is to be measured, may pass in all directions. The flow of the gas causes a phase shift in the standing wave, as measured by microphones 11 and 13, and 10 and 12. The phase difference of signals produced by microphones 11 and 13 gives an indication of the velocity component of the gas in the direction in which they are spaced, and similarly for the other pair of microphones 10 and 12. The velocity components are determined by multiplying the phase differences by the resonance frequency of phase-locked oscillator 18, which controls the transducer 7, in multiplier circuits 25 and 30. The resulting signals are applied to a display 31 which gives an indication of the speed and direction of the gas flow. In further embodiments a microprocessor is fed the outputs of the multiplier circuits so as to give separate fluid speed and direction read-outs, and the apparatus is extended to measure three-dimensional flows. The apparatus is useful in projectile aiming systems. <IMAGE>

Description

SPECIFICATION Apparatusforsensing movement of a fluid This invention relates to apparatus for sensing the movement of a fluid.

Oneconventionaltechniquefor measuring flow of gas employs an acoustic pulse transmitted in a diagonal direction across the flow direction ofthe gas from a transmitter to a detector. The time taken bythe pulse in passing from the transmitter to the detector is dependent on the velocity component of the gas in the direction travelled by the acoustic pulse. However,accuracy of such a technique is limited by acoustic dispersion within the gas and bythe limited bandwidth of detectors. Also high frequency waves in a gas at approximately room temperature and pressure are greatly attenuated. Usually such atechnique must be carried outundera pressure of at leasttwo atmospheres, making it unsuitable for measuring, for example, wind speed and direction.

A method which may be used for monitoring wind velocity utilises the variation in temperature of electrically heated wires which occurs when the wind flows over them. However, this method can be somewhat unsatisfactory.

According to an aspect ofthe invention there is provided an apparatusforsensing movement of a fluid comprising: means for setting up an acoustic standing wave in thefluid; and means for measuring a first phase difference between two locations spaced along the standing wave in a first direction and a second phase difference between two locations spaced along the standing wave in a second direction different from the first, thefirst and second phase differences being indicative of velocity components ofthefluid in thefirstand second directions respectively.

Theterm "acoustic" is notrestricted to audible sound frequencies, but also includes subsonic and ultrasonic frequencies. By employing the invention the speed andfordirection ofthefluid may be determined.

A standing wave can be taken to comprise first and second sinusoidal travelling waves propagating in opposite directions. When a standing wave is imposed on. afluid which is moving, velocity compo- nents ofthe travelling waves in the direction offlow of the fluid are increased or decreased bythe velocity of the fiwel, depending on whetherthe component is in the same or opposite sense respectively to the direction of flow. Hence the phase difference between two points along the standing wave is modified by an amountwhich depends on the component of the velocity of the fluid in the direction ofthe straight line oiningthetwo points.Therefore, by observing the relative phase between the two locations spaced in a direction along the standing wave, an indication of the velocity component in that direction is obtained.

This is shown by the following one dimensional analysis.

The first sinusoidal travelling wave is given by the following expression:

and the second sinusoidal travellingwave bythe expression:

where A1 and A2 are the respective amplitudes of the first and second travelling waves at a position x and time to) is the angularfrequency, vx is the component offlowvelocity ofthe fluid in the x-direction, c is the speed of sound in the fluid and Ao isthe peak amplitude.

The addition of expressions (1) and (2) gives the expression for a standing wave, which has an amplitudeAgiven bythefollowing expression (3):-

At a point x1 on the standing wave, the phase relative tothat att = o is given by: cox1 v, C2 - vx2 and at a point x2 by: (dX2 Vx c2 - v2 A phase difference A between the points x1 and x2 is given by: ## = #.Vx . (x1 - x2) C2 ~ vx2 =ov, . d c vx (4) where d is equal to xl - x2.

The velocity of the fluid may be determined by solving the quadratic equation of expression (4) or, if The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

c2; v2, by using a linear approximation to give: ## = #v x d ( ) #v,d (5) c which gives an error of less than 1% if v < c/10 If c is the normal atmospheric sound velocity, this means thatv must be less than about 30 ms-1.

When the fluid flows in a resonator similar to an organ pipe, characteristic resonances occur, with resonance frequencies fN given approximately by: fN= N.c (s) 21 where N is the mode number and I is the length of the resonator.

These resonances occurwherethere are an integral number of half wavelengths along the length ofthe resonator so that reflections from either end ofthe resonator interfere constructively with each other.

Therefore, substituting for c in expression (5) gives: ## = wind . N2 fN2 . 22 . 12 = ##x d . N2 4 fN2 12 = 2 iffN . Vx d. N2 4. fN2 12 = = TvX d. N2 2 fN 12 (7) and, since N, d and I are constant, ## a Vx (8) fN Vx &alpha;fN .## (9) Thus the veiocity V, is proportional to the resonance frequency multiplied by the relative phase ##.

lf the fluid flows in a resonator comprising two coaxial circular discs parallel to each other and spaced apart by a small gap, a number of resonances occurwhen there are an integral number of half wavelengths across the diameter ofthe the discs. In this case, resonance frequencies are given approximately by: fun = N c (10) 4r where r is the radius of the discs.

Substituting for c in the expression (5) gives: = = 2 #fN Vx d N fN2 16 r2 = #Vx d N2 8fN r2 N, d and r are constant, therefore:- Am a Vx fN and again Vx &alpha; fN . ## By measuring the said first and second phase differences in the said first and second directions to find the velocity components ofthe fluid in those directions, the actual velocity ofthefluid may be obtained.

Preferably,thefirst and second directions are at right angles two each other, in which case the speed of the actual flow velocity V is given by: Vo:fN(A(Px2+Ay2)1,2 (11) where ##x and Ay are the phase differences of the standing wave between two locations spaces in x andy-directions respectively, the x- andy- directions being at right angles to each other.

The direction of the flow of the fluid is then given by: e = tan-1 A < py (12) ##x where 0 is the angle measured from the x- direction to the direction of flow.

it is also preferable that the acoustic standing wave is set up in the fluid in a cavitythrough whichthefluid can pass in a plurality of different directions.

Preferably, the standing wave is radial, having wavefrontstravelling along in all directions in one plane, and having pressure nodes and antinodes concentric about one point.

An acoustic standing wave.may be considered in two ways. Firstly, in terms of pressure variations and secondly in terms of "displacement velocity" of molecules. A node in terms of pressure corresponds to an antinode in terms of molecule displacement velocity, and vice-versa. At an open end of a resonatorthere will be a pressure node and at a closed end a pressure antinode.

Preferably, the cavity comprises two coaxial discs arranged separated from and substantially parallel to each other. The two discs could be held in position by spacers which are thin enough notto affect fluid flow through the cavity to any great extend. Such an arrangementcould be used where the fluid is a liquid, but is more suited to use with a gas, since there is better reflection of a travelling acoustic wave at the boundary between the gas contained within the cavity and that surrounding itthan is the case for a liquid, and it is more accurate. A cavity otherthan one comprising two such discs could be employed, for example a wide but short cylindrical pipe may be used, but phase differences in a direction other than its longitudinal axiswould bedifficultto detectand the fluid would not be able to pass through it in a wide range of directions, resulting in a less accurate indication ofthe fluid velocity. Another possible arrangement comprisestwo square plates but a standing wave set up within a cavity so formed would be more complicatedthan that obtainable with circular discs.

It is preferablethatthe standing wave is maintained at a fixed resonance frequency. This can be achieved by employing a phase lock loop. This keeps the standing wave at a selected resonance frequency if the characteristics ofthe resonator alter, for instance as a result of a change in the temperature ofthefluid, or the use of a differentfluid. The mode number ofthe standing wave is not fundamentally important, but a greater phaseshiftfor a given flow rate, and hence greater sensitivity, will be obtained for a standing wave having a higher mode number than another.

According to another aspect ofthe invention there is provided apparatus for sensing movement of a fluid comprising: means for setting up at least two acoustic standing waves in the fluid; and means for measuring for each standing wave a phase difference between two locations spaced along the standing wave in respective different directions for different standing waves, the phase difference between two locations spaced in a direction being indicative of a velocity component of the fluid in that direction.

The standing waves may be set up in separate cavities,forexample in aseries of pipe arranged in different directions. Preferably, cavities are included through which the fluid is able to pass in a plurality of directions and in which the standing waves are set up.

A series of discs could be used to form several cavities.

According to afeature ofthe invention, a projectile aiming system includes apparatus as described above. This enables compensation to be made to the trajectory of a projectile so as to compensate forthe effectofwind driftandthe like.

The apparatus described above may be used for high or low pressure fluids, and is suitable for use in sensing wind speed and direction, unlike the pre viouslymentioned ultra-sonic pulse techniques. Also dispersion ofacousticsignals in the fluid is nota problem because a single frequency is employed, or, if a number of standing waves are used, each one is at a single frequency.

The invention is now further described by way of example, with reference to the accompanying drawings, in which: Figure lisa schematic view of apparatus according to the invention, the section being taken on line 11 of Figure 2; Figure 2 is a schematic exploded perspective view ofthe apparatus of Figure 1; Figures 3 and 4 illustrate a standing wave produced in a resonance cavity in the apparatus of Figure 1; and Figure5 isa schematic block diagram of circuitry associated with the apparatus of Figures 1 and 2.

Referring to Figure 1 and 2, a first circular disc 1 is arranged parallel to and coaxial with a second circular disc 2 of the same diameter with a small gap between them. The first and second discs 1 and 2 are held in position by four spacers 3,4,5 and 6, which are small columns arranged between facing surfaces ofthe discs 1 and 2 at equal intervals around their circumference and near to their edges. Free movementofgaswhoseflowisto be measured is permitted in any direction through the gap.

Atransducer 7 is fixed to the second disc 2 and is located at its centre. It comprises a thin aluminium diaphragm with a slab of piezoelectric material bonded to it. An alternating voltage can be applied across the piezoelectric material, causing the di aphragm to flexurallyvibrate and generate acoustic waves into the gap which acts as resonator cavity 8. A rubber gasket 9 serves to insulate the disc 2 from any mechanical vibration caused bythetransducer 7.

Four microphones 10, 11, 12 and 13 are located on the first disc 1. They are equally spaced around its circumference, each at the same distance from the centre ofthe disc 1 as the others. They are insulated byrubbergaskets 14,15, 15,16and 17 from any mechanical vibration of the disc 1. The microphones 10,11,12 and 13 are arranged to detect pressure vibrations ofthe gas in the resonator cavity 8.

In operation the diaphragm of the transducer 7 vibrates, causing pressure variations to the gas in the vicinity ofthe diaphragm. These pressure vibrations propagatethrough the gas in the form of pressure waves. The waves radiate outwards from the centre of the discs 1 and 2 and are reflected attheiredges by the gas outsidethe cavity8 because of the change in acoustic impedance. Thus two travelling sets of waves are produced travelling in opposite directions.

At certain frequencies characteristic of the resonance cavity 8, the waves interfere constructively and result in a radial standing wave. Aseries of pressure nodes and antinodes are set up radially as illustrated by side and plan views ofthe cavity 8 in Figures 3 and 4 respectively, with pressure nodes present at the edges ofthe discs. Nodes are denoted by 'N' and antinodes by 'A'.

With reference to figure 5, the frequency at which the diaphragm of the transducer 7 vibrates is controlled by an oscillator 18 and is arranged to be equal to a chosen frequency ofthe resonance cavity 8 determined by the physical dimensions ofthe gap and bythespeed of sound in the gas.

Two ofthe microphones 11 and 13 lie on the same diameter of the discs 1 and 2. They give sinusoidal output signals which are proportional to acoustic pressures of the standing wave at these points where they are located. The output of microphone 11 is applied to a phase sensitive detector 19. The output of the oscillator 18 is also applied to the detector 19. The detector 19 generates an output control signal representative ofthe difference in phase of its two input signals, The control signal from the detector 19 is applied to a low pass filter 20 which cuts out any highfrequencycomponents and thus gives a d.c.

output which is proportional to the mean value ofthe phase difference. The d.c. output of the low pass filter is then applied to the oscillator 18 to constitute a negative feedback phase locked loop. Under the action ofthis loop, the oscillatorfrequency is altered until resonance is established. Under thins condition the level ofthe outputsignal from the detector 19 is equal to the magnitude ofthe control signal required by the oscillator 18 to maintain that resonance frequency. Any change in the resonance frequency arising,forexample,from a change in temperature of the gas causes a compensating shift in the frequency of the oscillator 18so as to maintain it at a selected resonance frequency.

A phase shift in the standing wave due to the gas flowing through the cavity 8 cuases the frequency to move slightly off resonance, but this effect is slight and does not unduly affect operation ofthe apparatus.

Instead of using only the signal from microphone 11 ,the outputs of microphones 11 and 13 could be added and the resultant signal applied to the detector 19to help maintain the frequency at resonance.

The output ofthe microphone 11 is also applied to a squaring circuit 21 which acts to produce a first pulse train correspondingtothe sinusoidal outputand having the samefrequency. Similarly, the sinusoidal output of microphone 13 and is applied to another squaring circuit 22 which generates a corresponding second pulse train.

The first and second pulse trains are applied to a first exclusive OR gate 23 which gives an output representative ofthe phase difference between them.

The output ofthe first OR gate 23 is passed through a second lowfilter24to produce a d.c. signal proportional to the phase difference, which arises because of the gas flow through the cavity 8, between the signals from microphones 11 and 13.

The d.c. signal is applied to a first multiplier circuit 25, as is signal derived from the oscillator 18 via a frequency-to-voltage converter 26. These two signals are combined to give an outputfrom the first multipliercircuit 25 which is proportional to the product of the resonance frequency and the phase difference between thefirst and second pulse trains.

The output ofthe first multiplier circuit25 is directly representative ofthe velocity component ofthe gas in the direction ofthe diameter on which microphones 11 and 13 lie.

The othertwo microphones 10 and 12 lie on a diameter which is at right angles to that on which microphones 11 and 13 lie. The output of microphone 10 is appliedto a squaring circuit 26 and that of microphone 12to another squaring circuit 27, giving third and fourth pulse trains which are then applied to a second exclusive OR gate 28. The output of the second OR gate 28 is representative ofthe phase difference between the two pulse trains and is passed via a low pass filter 29to a second multiplier circuit 30 where it is combined with a signal from the oscillator 18 and frequency-to-voltage converter 26. The output ofthe second multiplier circuit 30 is representative of the velocity component ofthe gas in the direction in which the microphones 10 and 12 are separated.

The outputs of the first and second miltiplier circuits 25 and 30 are applied toan X-Y display 31. The display 31 displays a vector,the length of which is proportional to the flow velocity and whose orientation shows the direction of the gas flow.

In an alternative embodiment (not shown) the outputs of the multiplier circuits 25 and 30 are applied to a microprocessorto give separate read-outs of the flow velocity and flow direction. They may also be applied to a projectile aiming system to give an indication ofwind speed and direction.This indication can then be used to adjustthe pointing direction of the boresight of the system to compensate for the effects of wind drift on the trajectory ofthe projectile and enable greater accuracyto be achieved.

The display may give an indication ofthe actual gas velocity or may operate only when a certain predetermined condition of gas flow is reached.

Although the embodiments described above determine the velocity ofthe gas in two dimensions it would, of course, be possible to extend the informa tion to three dimensions by using a second apparatus similarto that described above, but orientated in a different plane.

Also, it is possible to set up a separate standing wave for each direction in which the component of velocityisto befound. For example,two sets of discs may be used each having two microphones, or a number of pipes may be used, in each of which a standing wave is set up, and arranged in different directions, a phase difference being measured along the longitudinal axis of each one.

Claims (12)

1. Apparatus for sensing movement of a fluid comprising: means for setting up an acoustic stand ing wave in the fluid; and means for measuring a first phase difference between two locations spaced along the standing wave in a first direction and a second phase difference between two locations spaced along the standing wave in a second direction different from the first, the first and second phase differences being indicative velocity components ofthefluid in thefirstand second directions respectively.

2. Apparatusasclaimed in claim 1 andwherein the acoustic standing wave is set up in the fluid in a cavitythrough which the fluid can pass in a plurality of different directions.

3. Apparatus as claimed in claim 1 or2 and wherein the standing wave is radial, having wavefronts travelling in all directions in one plane and having pressure nodes and antinodes concentric about one point.

4. Apparatus as claimed in claim 2 or 3 and wherein the cavity comprises two coaxial discs arranged separated from and substantially parallel to each other.

5. Apparatus as claimed in any preceding claim and wherein the first and second directions are at rightanglesto each other.

6. Apparatus as claimed in any preceding claim and wherein signals representing the first and second phase differences are applied to a displayto give a visual indication oftheflowofthefluid.

7. Apparatus as claimed in any preceding claim and wherein the standing wave is mounted at a fixed resonance frequency.

8. Apparatus as claimed in any preceding claim and including means for multiplying the resonance frequency with one ofthe said phase differencesto givean indicationofthevelocityofthefluid.

9 Apparatus for sensing the movement of a fluid comprising: meansforsetting up at least two acoustic standing waves in the fluid; and means for measuring for each standing wave a phase difference between two locations spaced along the standing wave in respective different directions for different standing waves, the phase difference between two locations spaced in a direction being indicative of a velocity component of the fluid in that direction.

10. Apparatus as claimed in claim 9 and including cavities through which the fluid is able to pass in a plurality of different directions and in which the standing waves are set up.

11. A projectile aiming system including apparatus as claimed in any preceding claim.

12. Apparatus substantially as illustrated and described with reference to the accompanying drawings.