GB2187552A

GB2187552A - Apparatus for monitoring movement of a fluid

Info

Publication number: GB2187552A
Application number: GB08605360A
Authority: GB
Inventors: Paul Mark Harrison; George Alexander Macdonald
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1986-03-05
Filing date: 1986-03-05
Publication date: 1987-09-09
Anticipated expiration: 2006-03-05
Also published as: GB8605360D0; GB2187552B

Abstract

In apparatus for monitoring movement of a fluid two circular plates 1 and 2 are arranged parallel to one another such that a fluid to be monitored passes between them. Five transducers are located in one of the plates 1, a central transducer 6 acting as a generating transducer and four circumferential transducers 7, 8, 9 and 10 acting as receiving transducers. During operation of the device, a tone burst is generated by the transducer 6 and energy is radiated into the fluid in a non-normal direction to the plate 1 to be received by the receiving transducers after reflection from the plate 2. By comparing the phase difference between pairs of receiving transducers the velocity and direction of the fluid may be determined. <IMAGE>

Description

SPECIFICATION Apparatus for monitoring movement of a fluid This invention relates to appartus for monitoring movement of a fluid and particularly, but not exclusively, to apparatus where the fluid to be monitored is a gas.

In a previously knowrl apparatus for measuring gas flow, a pair of transducers is aligned in a direction at an angle to the flow of the gas to be monitored. Each transducer emits ian ultrasonic pulse which is received by the other. Since one transducer transmits a pulse in a direction against the flow, and the other emits a pulse which travels with the flow, the difference in time taken by the pulses to traverse the flow is directly proportional to the mean velocity of the flow.

However, this technique is limited since pulses require fast rise times and thus wide bandwidth transducers are required to transmit the necessary high frequency components. Dispersion may also prove a problem as high frequency components are more greatly attenuated than lower frequency ones.

The present invention arose in an attempt to provide improved fluid flow monitoring apparatus.

According to a first aspect of this invention there is provided apparatus for monitoring movement of a fluid comprising: first transducer means having an operating surface arranged to generate a burst of ultrasonic energy at substantially a single frequency, part at least of the energy being arranged to be radiated into the fluid in a non-normal direction to the operating surface; second transducer means spaced from the first and arranged to receive the said part at least after transmission through the fluid; and means for monitoring the time taken by the said part at least to travel along a path in the fluid. The time taken by the ultrasonic energy to travel in a direction through the fluid is dependent on the velocity component of the fluid in that direction. Thus by monitoring the time taken, information can be gained regarding the fluid flow.

This is described more fully below, with reference to the accompanying drawings. The operating surface of the transducer means is that surface which is arranged to generate the ultrasonic energy or to receive and detect it. By "ultrasonic" it is meant energy having a frequency above the audible range, a particularly convenient range being from about 20kHz to 80kHz. This frequency range is above that of environmentally generated noise resulting in a high signal to noise ratio being possible. Interference by reflected sound in the region of the apparatus tends not to cause interference as the repeat frequency of the tone burst is comparatively low. The burst of ultrasonic energy at a substantially single frequency may be termed a "tone" burst.

Also, since unlike techniques involving pulses, a range of frequencies is not employed dispersion is not a problem. Since only one frequency is present there is no problem with higher frequency components being more greatly attenuated than lower ones.

In a conventional device, the ultrasonic energy tends to be well collimated and radiated in a normal direction to the operating surface of a transducer. In apparatus in accordance with the invention, the transducer and the substantially single frequency are suitably chosen to give a desired amount of energy radiated into the fluid in a non-normal direction to the operating surface of the transducer.

Since some energy is radiated in a non-normal direction to the operating surface and it is this energy which is received the second transducer means, the operating surfaces may be made flush with the facing surfaces of the plates whilst without operation of the apparatus being impaired. Thus apparatus in accordance with the invention may be made aerodynamically simple and arranged to produce little disturbance in the flow of the fluid. Apparatus in accordance with the invention to be of simple, rugged construction and involve low material and manufacturing costs.

Preferably two plates are included and are arranged to permit fluid to pass in a plurality of directions between their facing surfaces, operating surfaces of the first and second transducer means being positioned such that they lie in substantially the same plane as a facing surface of a plate.

Advantageously, the operating surfaces of the first and second transducer means lie in substantially the same plane. Thus energy generated by the first transducer means is received by the second transducer means after reflection from the facing surface of the plate in the other plane. According to a second aspect of this invention, there is provided apparatus for monitoring flow of a fluid comprising: two plates arranged to permit fluid to pass in a plurality of directions between their facing surfaces; first transducer means having an operating surface arranged to generate a burst of ultrasonic energy at substantially a single frequency into the fluid; second transducer means spaced from the first and arranged to receive part at least of the energy after it has been transmitted from the fluid and reflected from one of the facing surfaces; and means for monitoring the time taken for the said part at least to travel along a path in the fluid. By sensing reflected energy, an appreciably longer path length is available than if the tone burst were arranged to travel from one facing surface to the other, giving a compact arrangement.

There may be multiple reflections or a single one. Preferably, the operating surfaces of the first and second transducer means lie in substantially the same plane. This aids in reducing manufacturing costs since only one plate need be accurately machined to accommodate the transducer means.

According to a third aspect of the invention, there is provided apparatus for monitoring movement of a fluid comprising: two plates arranged to permit fluid to pass in a plurality of directions between their facing surfaces; first and second transducer means having operating surfaces arranged to lie in substantially the same plane as the facing surface of a plate, the first transducer means being arranged to generate a burst of ultrasonic energy at a substantially single frequency into the fluid, and the second to receive the burst, after it has travelled in the fluid; and means for monitoring the time taken by the burst to travel over a distance in a direction through the fluid whereby a velocity component in that direction can be monitored.

Advantageously, as mentioned above, the operating surfaces of the transducer means lie in substantially the same plane.

Advantageously, where the operating surfaces of the transducer means are positioned in the same plane, the duration of the burst and the arrangement of the transducer means are chosen such that a burst received after reflection and one received directly by the second transducer means do not substantially interfere with one another.

It is preferred that where plates are included, they are arranged substantially parallel to each other, and conveniently each plate has a circular cross-section.

Preferably, the substantially single frequency is a resonant frequency of the transducer means.

Thus efficient operation is achieved and large signal levels may be obtained.

It is preferred that a series of bursts are transmitted, intervals between successive bursts being chosen such that reflections of the energy of one burst have substantially reduced amplitude before the succeeding burst is generated. This again tends to reduce interference and improve signal to noise levels.

Preferably, the means for monitoring the time includes comparison means for comparing the phase of the burst on reception of the second transducer means with that of a reference signal.

It is more accurate to measure phase differences to monitor the time taken than time intervals, since the former are averaged over several cycles. A burst of higher frequency energy will undergo a larger phase shift than one at a lower frequency for a given velocity component. The possibility of ambiguities occurring may tend to limit the upper frequency at which the apparatus may be operated. A reference signal may be generated by arranging a transducer to: receive a burst of ultrasonic energy which has travelled normal to a facing surface. Alternatively, or in addition, a reference signal is advantageously derived from a signal which controls the generation of the tone burst.Alternatively and also advantageously the second transducer means may comprise two transducers and a reference signal is derived from the energy received by one, for comparison with that received by the other.

It is preferred that the second transducer means comprises a plurality of transducers, and advantageously a pair of transducers are spaced apart in a direction orthogonal to another pair of transducers. This then permits that may be termed the x and y velocity components to be obtained.

In a convenient configuration, the first transducer means comprises a transducer located at the centre of a plate and a plurality of transducers of the second transducer means are located circumferentially around one of the plates.

Suitable first and second transducer means may be, for example, a loud speaker and a microphone respectively. However, a particularly rugged construction may be achieved where a transducer has an operating surface to which is attached a piezoelectric member. It is preferred that, when two plates are included which are arranged to permit fluid to pass in a plurality of directions between their facing surfaces, that the operating surface of the transducer is integral with the facing surface of a plate. Thus the transducer may be made completely flush with the facing surface, is secured in one position and is particularly resistant to any damage or shocks.

Advantageously, the or a transducer is surrounded by a vibration absorbing layer.

Apparatus in accordance with the invention is particularly useful in meteorological applications.

Where the operating surfaces of the transducer means are located in the same plane, they may be made integral with or located within a much large member or piece of equipment, a plate bearing no transducers being only exposed above the surface of the member or equipment.

The invention is now further described by way of example with reference to the accompanying drawings in which: Figures 1 and 2 are schematic plan and sectional views of apparatus in accordance with the invention; Figures 3A and 3B are explanatory diagrams relating to the apparatus of Fig. 1; Figure 4 is a schematic block diagram of circuitry associated with. the apparatus of Figs. 1 and 2; and Figure 5 is an explanatory diagram relating to the apparatus of Figs. 1 and 2.

With reference to Figs. 1 and 2, apparatus for monitoring movement of a fluid, which in this case is a gas such as, for example, wind flow, includes first and second circular plates 1 and 2.

The plates 1 and 2 have a diameter of about 175mm and are arranged substantially parallel to one another, their facing surfaces 3 and 4 being separated by a distance d of approximately 32mm by thin spacers 5. Five transducers 6, 7, 8, 9 and 10 are included in one of the plates 1, one transducer being arranged at its centre and the other four being circumferentially distri buted equidistant from one another. Each transducer is arranged such that its operating surface lies in substantially the same plane as that of the facing surface 3 of the plate 1, thus presenting a substantially smooth overall surface area to gas flowing between the two plates 1 and 2.Each transducer is surrounded by a rubber jacket 11 to isolate it from vibration and comprises an operating surface which is a thin aluminium diaphragm 12, having a diameter of about 20mm and 0.5mm thickness, to which a piezoelectric disk 13 of PZT 5A of 8mm diameter and 0.2mm thick, is bonded using a conductive silver epoxy.

During operation of the apparatus, an ultrasonic tone burst is generated at the central trans ducer 6 by applying a signal to its piezoelectric disc 13 at a resonant frequency of the diaphragm 12.

Figs. 3A and B illustrate the directional characteristics of the transducer 6, Fig. 3A illustrating operation at the fundamental frequency, which in this case is 10kHz, and Fig. 3B illustrating the second harmonic resonant frequency which is about 49kHz. Although it would be possible to operate the transducer 6 at its fundamental frequency, since there is a substantial component energy transmitted in a non-normal direction, in this case it is preferred to operate it in its second harmonic since more pronounced and directional side lobes are available during this mode of operation.The directionality of the beam may be determined with reasonable accuracy by using the following expression

where A is the wavelength of operation, the total angular width of the main beam alpng the central axis of the diaphragm 12 is a, w is the diameter of the diaphragm 12, and A is smaller than w. The tone burst generated by the operating surface of the transducer 6 follows the path illustrated by the broken lines in Fig. 2, being reflected at the plate 2 and impinging on, the other transducers 7, 8, 9 and 10 where it is received. The receiving transducers 7, 8, 9 and 10, have outputs which are proportional to the pressure variations caused by the impinging tone burst, the output signals then being processed to obtain information regarding the fluid flow.

With reference to Fig. 4, a square wave generator 14 is initially applied to the transmitting transducer 6 via a timing circuit 15 which gates the output of the generator 14 into a series of tone bursts, as is shown at A of Fig. 5. The outputs of the receiving transducers 7, 8, 9 and 10 are applied to respective amplifying and filtering circuits 16 and, via squaring circuits 17, to respective phase comparators 18. The signals received by transducers 10 and 8 are shown after squaring at B and C respectively of Fig. 5. The phase comparators 18 compare the phase of the received signals with that of the output of the square wave generator 14 which acts as a reference signal to obtain a phase difference which is indicative of the gas flow.The outputs of the phase comparators 18 are then applied to a data processor 19 which computes the information received and displays it at a display device 20.

As previously mentioned the outputs of the phase comparators 18 may be used to derive the required fluid flow information, as is now further described.

The separation between the transmitting transducer 6 and each of the receiving transducers 7, 8, 9 and 10 is Q, the separation between the plates is d and 0 is the angle the direction of propagation of the tone burst makes with plate 2. The time of travel in which the energy travels from the transmitting to a receiving transducer, in the absence of any fluid flow, is t where Q t= c cos 6 and c is the speed of sound in the fluid.

The transducers 8 and 10 may be considered to lie along an x axis, and transducers 7 and 9 along a y axis. When the gas is flowing in a direction generally shown by an arrow in Figs. 1 and 2, the time taken tl for the tone burst to reach the downstream transducer 10 on the x axis is then

where Vx is the component of gas flow velocity in the x axis direction. It can then be shown that if the time interval required for the tone burst to reach the upstream transducer 8 on the x axis is t2 then

and

This gives measurement of c, the speed of sound in the gas flow, which may be useful for calculations in respect of other equipment, for example, where the apparatus is used in conjunction with a gun aiming system.

It can be further shown that

where K is a constant which is dependent on the physical dimensions of the apparatus.

Similarly, the component of gas flow in the direction of the y axis, Vy, and the times t3 and t4 required for the tone burst to reach the downstream and upstream y axis transducers 7 and 9 respectively are related by the following expression:

Thus it can be seen that the components Vx and Vy of the gas flow velocity may be determined and that also the direction of the gas flow may be calculated, this being represented by the angle fi which it makes relative to the x axis, since

As mentioned previously, it is more accurate to measure phase differences rather than time intervals, because the determination of the phase difference is averaged over several cycles.

Thus, where 0 is the phase, in terms of phase differences fi, Vx, and Vy are

where f is the frequency of the ultrasonic tone burst, K1 is a constant and s0, is related to t1, and similarly for 02, 03, and 94.

Claims

1. Apparatus for monitoring flow of a fluid comprising: first transducer means having an operating surface arranged to generate a burst of ultrasonic energy at a substantially single frequency, part at least of the energy being arranged to be radiated into the fluid in a nonnormal direction to the operating surface; second transducer means spaced from the first, and arranged to receive the said part at least after transmission through the fluid; and means for monitoring the time taken by the said part at least to travel along a path in the fluid.

2. Apparatus as claimed in claim 1 and including two plates arranged to permit fluid to pass in a plurality of directions between their facing surfaces, operating surfaces of the first and second transducer means being positioned such that they lie in substantially the same plane as a facing surface of a plate.

3. Apparatus as claimed in claim 2 and wherein the operating surfaces of the first and second transducer means lie in substantially the same plane.

4. Apparatus for monitoring flow of a fluid comprising: two plates arranged to permit fluid to pass in a plurality of directions between their facing surfaces; first transducer means having an operating surface arranged to generate a burst of ultrasonic energy at substantially a single frequency into the fluid; second transducer means spaced from the first and arranged to receive part at least of the energy after it has been transmitted through the fluid and reflected from one of the facing surfaces; and means for monitoring the time taken for the said part at least to travel along a path in the fluid.

5. Apparatus as claimed in claim 4 and wherein operating surfaces of the first and second transducer means are arranged to lie in substantially the same plane.

6. Apparatus as claimed in claim 4 or 5 and wherein part at least of the ultrasonic energy generated at the operating surface is arranged to be radiated into the fluid in a non-normal direction to the operating surface.

7. Apparatus for monitoring movement of a fluid comprising: two plates arranged to permit fluid to pass in a plurality of directions between their facing surfaces; first and second transducer means having operating surfaces arranged to lie in substantially the same plane as the facing surface of a plate, the first transducer means being arranged to generate a burst of ultrasonic energy at a substantially single frequency into the fluid, and the second to receive the burst, after it has travelled in the fluid; and means for monitoring the time taken by the burst to travel over a distance in a direction through the fluid whereby a velocity component in that direction can be monitored.

8. Apparatus as claimed in claim 7 and wherein the operating surfaces of the transducer means lie in substantially the same plane.

9. Apparatus as claimed in any of claims 3 to 6 or claim 8 and wherein the duration of the burst and arrangement of the transducer means are chosen such that a burst received after reflection and one received directly by the second transducer means do not substantially interfere.

10. Apparatus as claimed in any of claims 2 to 9 and wherein the plates are arranged substantially parallel to each other.

11. Apparatus as claimed in any of claims 2 to 10 and wherein each plate has a circular cross-section.

12. Apparatus as claimed in any preceding claim and wherein the substantially single frequency is a resonant frequency of the transducer means.

13. Apparatus as claimed in any preceding claim and wherein a series of bursts are transmitted, the intervals between successive bursts being chosen such that reflections of the energy of one burst have substantially reduced amplitude before the succeeding burst is generated.

14. Apparatus as claimed in any preceding claim and wherein the means for monitoring the time includes comparison means for comparing the phase of the burst on reception at the second transducer means with that of a reference signal.

15. Apparatus as claimed in claim 14 and wherein the second transducer means comprises two transducers and a reference signal is derived from the energy received by one transducer for comparison with that received by the other.

16. Apparatus as claimed in any preceding claim and wherein the second transducer means comprises a plurality of transducers.

17. Apparatus as claimed in claim 16 and wherein a pair of transducers are spaced apart in a direction orthogonal to another pair of transducers.

18. Apparatus as claimed in claim 16 or 17 when dependent on claim 11 and wherein the first transducer means comprises a transducer located at the centre of a plate and the plurality of transducers of the second transducer means are located circumferentially around one of the plates.

19. Apparatus as claimed in any preceding claim and wherein the first or second transducer means comprises a transducer having an operating surface to which is attached a piezoelectric member.

20. Apparatus as claimed in claim 19 and, where two plates are included and arranged to permit fluid to pass in a plurality of directions between their facing surfaces, wherein the operating surface of the transducer is integral with the facing surface of a plate.

21. Apparatus as claimed in any of claims 16 to 20 and wherein the or a transducer is surrounded by a vibration absorbing layer.

22. Apparatus for monitoring the flow of a fluid substantially as illustrated in and described with reference to the accompanying drawings.