WO1992001208A1

WO1992001208A1 - Vortex flowmeter with interferometrical vibration sensor

Info

Publication number: WO1992001208A1
Application number: PCT/GB1991/001060
Authority: WO
Inventors: David Alfred Jackson; Beatrice C. B. Chu; Trevor Paul Newson
Original assignee: Sira Limited
Priority date: 1990-07-03
Filing date: 1991-06-28
Publication date: 1992-01-23
Also published as: GB9014708D0; AU8211791A

Abstract

Apparatus for measuring the flow of fluid comprising: vortex producing means for producing vortices in the fluid flow, a vortex detector member mounted to be moved by said vortices, movement detector for detecting movement of said vortex detector member, said movement detector comprising: an interferometer having two reflective surfaces, a first one connected to move with the vortex detector member as it is moved by said vortices and the second one mounted stationarily and, means to detect interferometrically relative movement between said two reflective surfaces.

Description

Vortex Fl owmeter with interferometrical vibration sensor

The present invention relates to a flowmeter for detecting or measuring the flow of fluid. Apparatus for detecting the flow of liquids and gases will be described.

There are many types of flowmeter. However, it is preferable to provide a flowmeter which has the electrical and electronic components remote from the fluid so that the meter may operate with flammable fluids or in otherwise hazardous situations, with the capability of operating at high temperature.

US Specification 4 706 502 shows a flow meter including means to measure the vortex shedding frequency. In this case, an optical fibre 1 extends across the fluid flow and there is an interferometer which includes means for comparing a reference signal with an optical signal derived from a reflective remote end 15 of the fibre. A major disadvantage of such an arrangement is that the optical fibre, which is relatively fragile, passes through the fluid flow and this restricts the use of the apparatus.

The present invention provides apparatus for measuring the flow of fluid comprising:

vortex producing means for producing vortices in the fluid flow,

a vortex detector member mounted to be moved by said vortices,

movement detector for detecting movement of said vortex detector member, said movement detector comprising; an interferometer having two reflective surfaces, a first one connected to move with the vortex detector member as it is moved by said vortices and the second one mounted stationarily and, means to detect interferometrically relative movement between said two reflective surfaces.

With such an arrangement, the vortex producing means produces ("sheds") vortices at a rate which is proportional to the fluid flow in a known manner, and the vortices are detected by means for detecting movement of the vortex detector member, the movement of the vortex detector member being detected by an interferometric method which can simply use radiation whereby the electrical and electronic components can be mounted remotely from the movement detector means.

Furthermore, only the vortex detector member extends into the fluid flow and this can be made of robust construction so as to be able to cope with a variety of fluids without damage.

There may be provided a radiation source and optical means whereby to pass radiation to both said reflective surfaces.

Through this specification we will refer to radiation which may comprise, for example, optical wavelengths, ultraviolet or infrared, and we will use the adjective "optical" which may be read as including means which are operational in respect of said radiation.

One of said reflective surfaces may comprise a part-spherical mirror, and the other of said reflective surfaces may comprise an end of an optical fibre opposite said part-spherical mirror, radiation from said radiation source, in use, passing down said optical fibre from a remote end.

In another arrangement, one of said reflective surfaces comprises one end of an optical fibre, and the other of said reflective surfaces comprises a reflective discontinuity in said fibre, relative movement between the two reflective surfaces being caused by change of length of the optical fibre between the two reflective surfaces caused by stretching of said optical fibre, radiation from said radiation source, in use, passing down said optical fibre from an end remote from said first reflective surface.

In either of the preceding two paragraphs, said means to detect relative movement between said two reflective surfaces may include first means to detect radiation passing from said source and second means to detect radiation passing from said reflective surfaces and. comparator means to compare them.

The comparator means may include means to change the frequency of. the radiation from the radiation source in such a manner as to maintain the relative value of the radiation detected from said source and radiation detected from said reflective surfaces identical or in some predetermined relationship. Such an arrangement will be referred to as an "active wavelength tuning homodyne system".

Said comparator means may include a feedback circuit, preferably including a differential amplifier between the two radiation detectors and the radiation source.

In an alternative arrangement, the comparator means may include a phase-lock loop circuit and the frequency of said radiation source may be varied, in a reglar pattern (e.g. saw tooth waveform) about a particular frequency. This arrangement will be referred to as a pseudoheterodyne system.

Preferred arrangements of this invention will now be described by way of example only with reference to the accompanying drawings in which:

Fig. 1 is a diagrammatic plan view of part of a vortex shedding type flowmeter,

Fig. 2 is an end view of the flowmeter of Fig. 1 showing one arrangement of movement detector,

Fig. 3 is an enlarged detail of part of an optical system of Fig. 2, Fig. 4 is a view corresponding to part of Fig. 2 showing an alternative flowmeter of the invention incorporating an alternative movement detector,

Fig. 5 is an enlarged view of part of an optical system of Fig. 4,

Fig. 6 is a diagram of a first signal processing circuit, Fig. 7 is a diagram of a second, alternative, signal processing circuit,

Fig. 8 is a diagram showing the phase of an interferometric fringe over successive ramp periods, Fig. 9 shows a phase-lock loop, and,

Fig. 10 shows a block diagram of a phase-lock loop signal recovery scheme.

Referring to Fig. 1 there is shown a diagrammatic plan view of a vortex shedding type flowmeter mounted within a pipe 11, through which a fluid, for example, water or gas passes.

The direction of flow is shown by the arrow 13. Mounted upstream and (normally) extending completely across the pipe 11 is a bluff body 14. The bluff body should be rigidly fixed to pipe 11 and may comprise, for example, a generally flat strip, the width of the upstream face 16 being wider than the downstream face 17 so as to provide acute angles between the upstream face 16 and each of the side edges 18, 19 and obtuse angles between the downstream face 17 and side edges 18, 19. The shape is designed so as to cause vortices 15 to be shed from each edge 18, 19, as is well known. Downstream of the bluff body 14 is provided a probe 21 which comprises a cylindrical body extending parallel to the length of the bluff body 14. The diameter of the probe 21 is substantially the same as the width of the bluff body 14.

In general terms, as is clear from Fig. 1, the side edges 18, 19 shed vortices (and it is found that they shed vortices alternately); the vortices travel parallel to the axis of the pipe 11 and strike the probe 21. As vortices arrive from opposite side edges 18, 19 successively, the probe 21 is deflected back and forth transverse the axis of the pipe 11 at a frequency which is governed by the frequency of shedding of the vortices.

The probe 21 should be mounted in the pipe 11 in such a manner that it's lower end (as can be seen from Fig. 2), is spaced from the wall of the pipe 11. The probe 21 is mounted in the pipe 11 in such a manner as to allow it to move freely transversely of the axis of the pipe 11, but other movements, such as parallel to the axis of the pipe (caused by the pressure of the flow of fluid through the pipe), are restrained. It is well known that the frequency of shedding of vortices is directly proportional to the rate of fluid flow. The frequency of vortex shedding may be calculated as follows:

y

f = S * d

Where, F - Frequency of vortex shedding

v - mean velocity of flow

d - width of bluff body

S - Strouhal number

The Strouhal number is given by:

19.7

S = 0.198 (1 - Re ) for 250 < Re < 2 x 10⁵

Re is the Reynolds number given by:

where, v - velocity of flow

p - fluid density

l - linear dimension (e.g. diameter of pipe) η - fluid viscosity

Fig. 2 shows an end view of the apparatus shown in Fig. 1 with the addition of a movement detector 31 for detecting movement of the probe 21 transverse the axis of the pipe 11. The specification will describe two arrangements of movement detector. We will initially describe the first arrangement shown in Fig. 2. The movement detector 31 comprises a support 32 mounted to the end of the probe 21 and extending outside the pipe 11. The support 32 mounts a con-focal mirror 33. This con-focal mirror 33 may be a semi-spherical mirror and may be formed, for example, from a block of aluminium into which is pressed a polished spherical member such as a ball bearing. It will be understood that the mirror 33 will vibrate back and forth with the support 32 and probe 21.

Mounted rigidly, in this case to the outer diameter of the pipe 11, is an optical fibre 34, the cut end 36 of which is arranged on an axis passing through the radius of the semi-spherical mirror 33. The uncoated fibre end 36 and the semi-spherical mirror 33 provide two reflective surfaces of an interferometer. In general terms, therefore, the fibre end 36 which is stationarily mounted and the semi-spherical mirror 33 which is movable with the probe 21 cause movement of interference fringes.

As is clear from Fig. 2 the monomode optical fibre 34 is actually mounted to the outer surface of the pipe 11 by means of a bracket 38 which in turn mounts a translation stage 39. The bracket 38 mounts by means of three screws 41 a clamp 42 which rigidly clamps the fibre 34, adjustment of the screws 41 allowing limited movement of the fibre end 36 relative to the semi-spherical mirror 33.

Fig. 3 shows the optical fibre 34 and semi-spherical mirror 33 to a larger scale although diagrammatically. The optical fibre itself comprises a central transparent quartz fibre core 43 surrounded by a higher refractive index cladding sheath 44, surrounded by a protective jacket 45 which in turn is surrounded by a capillary tube 46. The capillary tube 46 is only present adjacent to the clamp 42 and the form of the clamp itself can be seen from Fig. 3.

It will be clear from Fig. 3 that movement of the semi- spherical mirror 33 by the probe 21 will cause the distance between the spherical-mirror 33 and fibre 36 to vary.

As already referred to above, an alternative movement detector 31 is illustrated in Figs. 4 and 5. In this arrangement, the free end of an optical fibre 51 is rigidly attached by, for example, adhesive to a support 32 mounted to the end of the probe 21. At a distance remote from the free end of the fibre 51, the fibre is clamped to a plate 52 rigidly attached to the pipe 11. A V clamp arrangement will be suitable in this case, the V clamp 53 clamped to the optical via a protective jacket 54.

On the side of fibre 51 remote from the free end 56 there is provided a reflective discontinuity in the fibre. As shown in Fig. 5, this is provided by cleaving the fibre. Once the break has been formed in the fibre itself, then the complete fibre including cladding 43 and protective jacket 45 are inserted in a close fitting capillary tube and several either side of the reflective discontinuity using UV curing cement, this maintaining an accurate coaxial relationship.

The break or reflective discontinuity 58 in the optical fibre 51, and the reflective free end 56 form two reflective surfaces between which there may be optical interference. In general terms, vibration of the probe 21 will strain the fibre 51 which will cause movement of the free end 56 relative to the V clamp 53; i.e. the length of the part of the fibre between the clamp 53 and free end 56 and hence the distance between the free end 56 and reflective discontinuity 58 changes. Changes in interference effects will therefore be provided.

It will be understood, therefore, that radiation, for example, from a laser 61 (see Figs. 6 and 7) is passed to an end 62 of optical fibre 34 or 51, the radiation passing through the optical fibre and interference effects being provided by the two reflective surfaces provided either by the reflective discontinuity 58 and free end 56 of the optical fibre 51 (Figure 4) or between the semi-spherical mirror 33 and fibre end 36 of optical fibre 34 (Figure 2). The diode may be 3mW Mitsubishi diode laser of wavelength 789nm. Two different means for detecting those interference effects will now be described, the first with reference to Fig. 6 and the second with reference to Figs. 7 and 8. In both cases it is necessary to detect radiation passing directly from the laser passing from the laser, and radiation passing from the reflective surfaces.

Referring to Fig. 6 there is shown a directional coupler 66 connected to the optical fibre 34/51 remote from the probe 21. The directional coupler can be at a considerable distance from the probe 21, for example, several metres if desired. As .is well known, the directional coupler includes two output optical fibres 67, 68, in this case output optical fibre 67 detecting radiation received from the reflective surfaces, and output optical fibre 68 receiving radiation direct from the laser 61. Such directional couplers are well known and comprise two optical fibres laid side by side, their outer protective sheaths being removed or reduced between the two fibres so that there is some spillage of radiation from one optical fibre to the other, one of the optical fibres forming the two output optical fibres 67, 68.

An optical isolater is provided in the length of the optical fibre 34/51 between the directional coupler 66 and laser 61 to prevent radiation reflected by the reflective surfaces passing back to the laser.

Radiation from the output optical fibres 67, 68 are passed to radiation detectors 71, 72 respectively. The output signals from the radiation detectors 71, 72 form the two inputs to a differential amplifier 73, the output of which is passed through a feedback circuit the output of a feedback circuit 74 being used to control the frequency of the laser 61. The arrangement shown in Fig. 6 is generally referred to as an "Active Wavelength Tuning Homodyne". The mode of operation of the arrangement of Fig. 6 is as follows. Radiation from laser 61 is launched into the end 62 of the optical fibre 34/51 and passes to the directional coupler 66. Most of the radiation will continue along the optical fibre 34/51 but some will pass through to the output optical fibre 68.

Radiation passing along the optical fibre 34/51 will pass to one or other of the systems shown in Fig. 2 or Fig. 4. As the probe 21 vibrates, the distance between the two reflective surfaces will vary and thus the reflected beam passing back along the optical fibre 34/51 will vary in intensity. The returning beam passes through the directional coupler 66 and some of the returning beam passes down the output optical fibre 67.

The radiation from the output optical fibres 67, 68 is detected by respective radiation detectors 71, 72 and compared in the differential amplifier 66. If there is a difference in the intensity of the two radiation signals detected by the radiation detectors 71, 72, then the differential amplifier 66 provides an output signal which is fed back to the laser. The arrangement of the feedback and control of the laser is such that the frequency of the laser is shifted. Shifting the frequency of the laser of course changes the interferometric effect in the interferometer and the intention is such that the frequency is changed so that the intensity of the radiation received by the radiation detector 71, 72 is maintained in a predetermined relationship, (normally, identical).

It will be understood, therefore, that by this homodyne process, as the probe 21 vibrates and moves, instead of the interferometric effect, that is the intensity of the reflected radiation substantially changing, because of the feedback arrangement, the frequency of the output of the laser compensates for the change in distance between the two reflective surfaces to maintain the same interferometric effect, that is the same intensity of reflected radiation. In this way, it is possible to measure the movement of the probe by means of the change of frequency of the laser.

In more detail, signal recovery by this Active Tuning Homodyne technique requires the interferometer to be unbalanced in order that the feedback servo 74 can be used to control the laser emission frequency such that the output of the interferometer is always maintained at quadrature (i.e. the point at which the change of reflected radiation level with respect to movement of the probe 21 is at a maximum). From Fig. 6 feedback voltage v is applied to the diode laser source in order to change its injection current and hence its frequency. The signal detected by detector 71 varies as the interferometer is driven away from the quadrature point, detector 72 providing the quadrature reference signal. For solid state lasers, a change in injection current, Δi , leads to a change, Δv, in the emission frequency given by:

Δυ = K_d (ω) Δi

where K_c (ω) is the frequency dependent current to emission frequency conversion factor of the laser.

For an unbalanced interferometer, a change, Δv, m the laser emission frequency results in a change in the phase of the optical output given by:

where L is of optical path length of the interferometer. Thus a change in the phase in the interferometer caused by a movement of the prove 21 may be cancelled by changing Δv in the opposite sense.

The signal for the signal processing may be low gain bandwidth signal at output 76 or high gain bandwidth signal at output 77. In the low gain bandwidth mode, the cut off frequency is set such that the output remains at quadrature for very slowly varying signals (e.g. thermal draft), hence signals produced by the probe 51 above the cut off frequency are recovered from the output of detector 76. In the high gain bandwidth mode the cut off frequency of the servo is above the highest vortex shedding frequency and hence the output of the interferometer is always at quadrature. A signal directly proportional to the vibration frequency of probe 51 may now obtained directly from the signal output 77.

We now refer to the alternative means for detecting the interference shown with reference to Figs. 7 and 8. Many of the components shown in Fig. 7 are similar to those in Fig. 6 and will not described further. The outputs of the radiation detectors 71, 72 are fed in this case to a phase-lock loop circuit 81. The laser 61 is driven by a ramp generator 82 which provides a saw-tooth wave form which sweeps the frequency of the output of the laser 61 in the manner shown in the upper half of Fig. 8. The amplitude of the signal produced by the ramp generator 82 is such that the interferometer is driven over approximately one fringe, that is, during one cycle of the saw-tooth wave form produced by the ramp generator, the interference fringes move over one complete interfringe distance and thus the reflected radiation signal moves over a complete cycle of change of intensity (see the lower half of Fig. 8 which shows the change of reflective radiation signal corresponding to the ramp signal above.)

Referring to Fig. 8, it will be seen therefore, that if the probe 51 was stationary, the sweeping of the frequency of the laser by means of the ramp generator 82 will, so far as the reflective radiation signal is concerned, produce the output signal as shown in the lower half of Fig. 8, at a particular frequency, being the frequency of the saw-tooth wave form from the ramp generator 82.

It will further be understood that if the probe 51 moves so that the second reflecting surface is in a different position, the wave form shown in the lower half of Fig. 8 will be displaced from which it will be understood that movement of the probe 21 modulates the frequency of the return signal opposite the reflected radiation signal or the frequency of change of the reflective radiation signal in such a way as to effectively produce a heterodyne signal, that is a signal having a main central frequency value and two side bands (or at least two side bands) of a frequency shifted from the main signal by a frequency corresponding to the displacement of the second reflective surface. By analysis the signal produced it is therefore possible to produce a measure of the movement of the probe 21.

This Pseudo-Heterodyne Demodulation technique will now be described in more detail.

The emission frequency of the (diode) laser 61 is linearly ramped by injecting the current saw-tooth waveform onto the d . c . bi as current of the l aser . For an unbalanced interferometer, of path length imbalance L, a change in laser emission frequency, Δv , results in a change, ΔΦ, in the phase of the output fringe pattern given by

The rate of change of the output phase of the

interferometer is then

where is the slope of the current ramp. The general

form of output using this scheme is shown in Fig. 8, where the interferometer phase is driven over approximately 1 fringe during each ramp period.

The electronics required to generate the pseudoheterodyne carrier are indicated in Fig. 7. The required phase change in the interferometer is only 2τr radians. However when generated in a relatively short cavity interferometer (i.e. having a small distance between reflective surfaces) this frequency change is always accompanied by an intensity change. In principle these intensity effects can be reduced by using a larger path imbalance, however the sensor then becomes more susceptible to unwanted vibrations and the effects of noise signals produced by frequency instabilities in the source are greater.

The motion of the probe 21 causes the instantaneous frequency of change of the reflected radiation (the carrier signal) to deviate at the vortex shedding frequency. The spectrum of this signal closely resembles that of a frequency modulated F.M. carrier hence the carrier signal can be demodulated by generally conventional FM techniques. In this case a phase locked loop is used. The phase lock loop may be a NE565 chip supplied by the RS component Ltd. which contains (see Fig. 9) a phase detector 86, band pass filter 87, amplifier, and voltage controlled oscillator (VCO) 88. The phase detector 86 is used to compare the phase of the input signal f_IN with that generated by the VCO 88 and generates an output proportional to their phase difference. If the phase of f_IN does not equal that of f_VCO constant. This error signal is proportional to the derivative of the motion of probe 21 and requires integration if the amplitude of this motion is required. As we are only interested in frequency of shedding of the vortices it is unnecessary to integrate the signal. Amplitude modulation will also occur when the laser frequency is modulated over a finite range. This amplitude modulation has deleterious effects on the operation of the phase-lock loop. This problem may be solved by using a divider chip, AD 534, to divide the photodetector signal from the interferometer by a second photodetector signal used to monitoring the intensity of the laser source. Fig. 10 shows the block diagram of the circuit in this signal recovery scheme.

The arrangement so far described may be used to measure the flow rate of liquids or gases.

Claims

1. Apparatus for measuring the flow of fluid comprising: vortex producing means for producing vortices in the fluid flow,

a vortex detector member mounted to be moved by said vortices,

movement detector for detecting movement of said vortex detector member, said movement detector comprising; an interferometer having two reflective surfaces, a first one connected to move with the vortex detector member as i t i s moved by sai d vorti ces and the second one mounted stationari l y and , means to detect i nterferometri cal l y rel ative movement between said two reflective surfaces.

2. Apparatus as claimed in claim 1 characterised in that vortex detector member comprises a probe,

3. Apparatus as claimed in claims 1 or 2 characterised in that there is provided a radiation source and optical means whereby to pass radiation to both said reflective surfaces.

4. Apparatus as claimed in claim 3 characterised in that said optical means includes an optical fibre, and one of said reflective surfaces comprises a part-spherical mirror, and the other of said reflective surfaces comprises an end of an optical fibre opposite said part-spherical mirror, radiation from said radiation source, in use, passing down said optical fibre from a remote end, some of said radiation being reflected back from said end of the optical fibre, and some of said radiation being transmitted to said part-spherical mirror, reflected thereat, and the reflected radiation is received back at said end of the optical fibre.

5. Apparatus as claimed in claim 3 characterised in that said optical means includes an optical fibre, and one of said reflective surfaces comprises one end of said optical fibre, and the other of said reflective surfaces comprises a reflective discontinuity in said fibre, relative movement between the two reflective surfaces being caused by change of length of the optical fibre between the two reflective surfaces caused by stretching of said optical fibre, radiation from said radiation source, in use, passing down said optical fibre from a remote end.

6. Apparatus as claimed in claims 4 or 5 characterised in that said means to detect relative movement between said two reflective surfaces includes first means to detect radiation passing from said source and second means to detect radiation passing from said reflective surfaces and comparator means to compare them.

7. Apparatus as claimed in claim 6 characterised in that the comparator means includes means to change the frequency of the radiation from the radiation source in such a manner as to maintain the relative value of the radiation detected from said source and radiation detected from said reflective surfaces identical or in some predetermined relationship.

8. Apparatus as claimed in claim 7 characterised in that said comparator means includes a feedback circuit, including a differential amplifier between the two radiation detectors and the radiation source.

9. Apparatus as claimed in claim 8 characterised in that the comparator means includes a phase-lock loop circuit and the frequency of said radiation source is variable, in a regular pattern about a predetermined frequency.