Ultrasonic flow velocity sensor and method of measuring the velocity of a fluid flow

Abstract: An anemometer comprises an acoustic resonant cavity 1 between two plates 2 and 3 and open in the direction of flow of a fluid, the cavity having at least one pair of electro acoustic transducers 5 which are energised at an Eigenfrequency to produce a standing wave perpendicular to the direction of flow of the fluid and a travelling wave perpendicular to the standing wave. By using the time delays or phase shifts between the signal energising a transducer and the signal generated by the response of the other transducer the velocity of flow of the fluid can be calculated. Compensation is provided for changes in the Eigenfrequency caused by changes in the environment.

Description: This invention relates to a flowmeter of the type used in the measurement of the speed and direction of movement of air, water and other fluids. Flowmeters or anemometers of various types are frequently used in meteorology, but suffer from one or more of many disadvantages including the presence of moving parts in mechanical types, a slow and inaccurate response, susceptability to temperature and pressure variation, a maintenance requirement, a complex construction, the dissipation of relatively large amounts of energy, the production of unduly high disturbances in the fluid flow, low sensitivity at low velocities, and a severely restricted maximum velocity range. Known anemometers include GB-A-2,140,160 which has a pair of spaced parallel plates between which the fluid flows, a standing wave extending in the direction of fluid flow being used to measure the speed of flow. Patent Abstracts of Japan, vol 007, no. 274 and JP 58151564A measures flow in a pipe and utilises a standing wave across the pipe and a travelling wave along the pipe, but the standing wave is not at a resonant Eigenfrequency. EPA 0601475 is similar but uses a transverse travelling wave instead of a standing wave and measures flow in a different way. DE 4,335,394A again measures flow in a pipe and uses two acoustic standing waves inclined to the direction of flow, and in Patent Abstracts of Japan A-60-166821 flow in a pipe is measured using the Doppler effect. An object of this invention is to provide a flowmeter which obviates or mitigates at least some of these disadvantages. Embodiments of the present invention work at very high and very low air velocities, are both fast and accurate, are practically unaffected by changes in air temperature and pressure, are simple in construction and possess no moving parts, require little or no periodic maintenance, dissipate minimal amount of power, cause a negligible disturbance to the fluid being measured, operate under extremely climatic conditions and can be used with other fluids besides air. The present invention provides a flowmeter for measuring the speed of a fluid flow and including an acoustic resonant cavity (1) which is open in the direction of fluid flow and is defined between a pair of reflectors, and at least two electro-acoustic transducers (5) for converting electrical signals into acoustic signals or vice-versa and for establishing in the cavity an acoustic standing wave at an acoustic Eigenfrequency, and an electronic unit which senses the speed of sound to maintain the acoustic resonance by changing the frequency of the acoustic signal and processes the electrical signals to derive the speed of the fluid flow inside the acoustic cavity, characterised in that the acoustic standing wave is perpendicular to the direction of fluid flow in the cavity, that there is a traveling wave perpendicular to the standing wave, and in that signals processed by the electronic unit are derived from the travelling and standing waves, and further characterised by means for measuring the time delay or phase shift between energising one transducer and the response of the other transducer, measuring the time delay or phase shift between energising the other transducer and the response of the one transducer, and comparing the measured time delays or phase shifts to derive the speed of the fluid flow in the acoustic cavity. The unit may process the signals to derive the direction of fluid flow. The transducers may be mounted in a plane parallel to the direction of fluid flow. Preferably the flowmeter includes a housing for the electronic unit and a support pillar situated between the acoustic resonant cavity and the housing of the electronic unit. The present invention also provides a method of measuring the speed of a fluid flow comprising energising one of a pair of electro-acoustic transducers in an acoustic resonant cavity (1), which is open in the direction of fluid flow and is defined between a pair of reflectors (2,3), to establish in the cavity (1) an acoustic standing wave at an acoustic Eigenfrequency, and sensing the speed of sound to maintain the acoustic standing wave by changing the frequency of the acoustic signal, characterised in that the standing wave is perpendicular to the direction of fluid flow in the cavity and that there is a travelling wave perpendicular to the standing wave, and further characterised by measuring the time delay or phase shift between energising the one transducer and the response of the other transducer, energising the other transducer and measuring the time delay or phase shift between energising the other transducer and the response of the one transducer, and comparing the measured time delays or phase shifts, to derive the speed of fluid flow in the acoustic cavity. An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a vertical section of an anemometer according to the present invention for measuring the velocity of a flow of air; Fig. 2 is a horizontal section through the anemometer of Fig. 1 showing the location of transducers in an acoustic resonant cavity; Fig. 3 is a horizontal section similar to Fig. 2 but of a modified embodiment using three transducers in a triangular formation; and Fig. 4 is a vertical section showing a modified acoustic resonant cavity with restricted air passage for use at high air velocity. As shown in Figure 1, the anemometer comprises an acoustic resonant cavity 1 formed between an upper reflector 2 and a lower reflector 3. The pair of reflectors are shown in Figs. 1 and 2 as circular plates of equal size but other shapes and unequal sizes are permissible. The pair of reflectors 2 and 3 are held together by spacers 4, in this embodiment four rods, but again other configurations are possible. The acoustic resonant cavity 1 operates to support a standing acoustic wave in a direction perpendicular to the direction of air flow, i.e. vertically as shown in Fig. 1, and to support travelling waves in the direction(s) along the air flow, i.e. horizontally as shown in Fig. 1. It should be noted that the resonant frequency to support the standing wave is dependent on the frequency of the acoustic signal, the speed of sound, and the seperation between the reflectors 2 and 3 which must be an integer multiple of half wavelengths. The size of the reflectors is not critical, and beyond some minimum size required to contain the acoustic signal the resonant frequency is independent of the size of the reflectors which allows for a compact design of the anemometer while retaining aerodynamic efficiency. Inside the acoustic resonant cavity 1, acoustic waves are generated and received by a number, in this embodiment four, of electro-acoustic transducers 5. The transducers 5 can of any known type such as piezoelectric, inductive, or electrostatic and operate to convert electrical signals into acoustic signals and vice-versa. In operation any one of the transducers 5 receives an electrical signal generated by the electronic unit (not shown) and excites a proportional acoustic signal; by action of the acoustic resonant cavity 1, the acoustic signal is impressed on the other transducers 5 which accept the acoustic signal and generate proportional electrical signals which are, in turn, fed back to the electronic unit for processing. In propagating from one transducer A to another transducer B the acoustic signal and its derivative electric signal experience a time delay (and equivalent phase shift) which is related to the speed of sound in the medium and the net distance between the transmitting A and receiving B transducers. In still air, an identical time delay is measured when the direction of signal transmission is reversed i.e. from transducer B to A since (a) the distance travelled by the signal and (b) the sound velocity remain the same whether the signal is transmitted from A to B or vice versa. If the air moves, on the other hand, say from A to B by some speed v, it proportionally increases the net speed of sound in the direction A to B; conversely it proportionally reduces the speed of sound in the direction B to A. Consequently the electronic unit which measures the time delay, or equivalently the phase shift, detects a difference in time delay (and phase shift) when reversing the direction of the signal. The magnitude of the time delay, and resulting phase shift, is related to the speed of the air along the adjoining line between the pair of transducers. Using a second pair whose adjoining line lies in some other direction the velocity in a second direction can be found. It is usually sufficient to know the air speed in just two directions, as speed in any other direction can be derived by mathematical formulae, but more than two pairs may be employed to improve accuracy. For example, in Fig. 3 just three transducers are employed in a triangular formation, and by grouping two transducers at one time, three pairs can be formed, which can sense the speed of air along the directions of the sides of the triangle. In the direction perpendicular to the air travel, (shown in figure 1 as the vertical direction), the resonant acoustic cavity 1 supports an acoustic standing wave. By action of the resonant acoustic cavity 1, this standing wave can exist to any significant magnitude if and only if the frequency of the acoustic signal (and proportional electric signal) belongs to a set of distinct frequency values known as Eigenfrequencies. The Eigenfrequencies values are dependent on a) the physical dimensions, construction and composition of the resonant acoustic cavity 1 and its boundaries, and on b) the speed of sound in air. Because of the physical boundaries of the acoustic resonant cavity 1, the air velocity in the direction of the standing wave is practically zero and the behaviour of standing wave is unaffected by air movement. However as the speed of sound in air is affected by environmental changes, (primarily temperature) the Eigenfrequencies, or permissible operating frequencies, are altered accordingly. In operation, the electronic unit senses any change in the speed of sound due to environmental factors, which change manifests itself in a loss of signal amplitude and phase, and takes corrective action to ensure that the acoustic resonant cavity 1 stays in tune. The tuning action can be conveniently carried out by changing the operating frequency. This last action of the electronic circuit is equivalent to maintaining a fixed operating wavelength in the vertical direction (the direction of the standing wave pattern) which is also the direction of zero air velocity. Under these conditions the indication of the anemometer can be made insensitive to environmental changes. Air entering the cavity from any direction in the horizontal plane, does not experience any obstacle beside the thin wall of the upper reflector 2 and lower reflector 1. Similarly air travelling inside the cavity does not experience any significant obstacle and hence the indication of the anemometer is truly representative of the air speed and direction. Moreover by spacing the upper reflector 2 or the lower reflector 3 or both, the passage of air can be restricted in a symmetrical manner which does not perturb the indication of direction but it reduces the sensitivity in speed terms thereby enabling the anemometer to cope with extremely large air velocities. Fig. 4 shows a modified acoustic resonant cavity having a shaped upper reflector 2 which restricts the passage of air symmetrically in all horizontal directions and which is suitable for operation at high air velocities. The opposite is also true, and by opening up the entrance of the cavity the sensitivity can be increased. The basic anemometer of the embodiments previously described measures air flow in the horizontal or xy directions, but by simple modifications it is possible to make a three axis anemometer which is capable of measuring air flow in the vertical or z direction as well. If one or more secondary reflectors is or are inserted in the resonant cavity and spaced from the primary reflectors 2 and 3, then the cavity can be made to resonate at more than one frequency. The new resonances are associated with different acoustic field distributions or eigenfunctions. As shown in Fig. 1, a secondary reflector 8 in the form of a small circular plate is suspended from the upper reflector 2 by means of a pin 9, but other configurations are possible. This arrangement gives rise to a pair of distinct resonant frequencies. One frequency is primarily associated with the space between the lower reflector 3 and the upper reflector 2, while the second frequency is primarily associated with the space between the lower reflector 3 and the secondary reflector 8. In operation the electronic unit excites an acoustic wave at a first resonant frequency F1 and measures the average air flow in the cavity space associated with that frequency. The electronic unit also excites an acoustic wave at-the second resonant frequency F2 and measures the the average air flow in the cavity space associated with the second resonant frequency. In practice the pair of measurements can be conducted simultaneously by exciting the two resonances together. When the air flow is parallel to the plane of the reflectors, near uniform air flow is obtained throughout the cavity volume and the measurements at F1 and F2 agree. If the direction of air flow is inclined at some angle to the plane of the reflectors a non-uniform distribution arises because of the dynamics of air flow in the presence of the body of the anemometer and the measurements at F1 and F2 become unequal by some amount which depends, among other factors, on the angle of inclination. The electronic unit can extract the desired inclination angle from knowledge of the two measurements at F1 and F2. An anemometer of this type can measure simultaneously air speed and azimuth direction, as explained previously, and by means of the secondary reflector(s) can measure the inclination of the air flow relative to the reflectors and at the same time monitor the cavity response and compensate the readings against environmental changes. In another application of the invention, the acoustic field distribution of a travelling wave in the direction of fluid flow and a standing wave perpendicular to the direction of flow can be applied to fluids flowing inside pipes to measure the amount of flow. Normally two transducers will be employed and they will be fixed to the wall of the pipe at some distance along the length of the pipe. The wall of the pipe acts in the same way as the reflector arrangement of Fig. 1 but otherwise the measurement of fluid velocity and compensation will be essentially the same as previously described.

Claims: A flowmeter for measuring the speed of a fluid flow and including an acoustic resonant cavity (1) which is open in the direction of fluid flow and is defined between a pair of reflectors, and at least two electro-acoustic transducers (5) for converting electrical signals into acoustic signals or vice-versa and for establishing in the cavity an acoustic standing wave at an acoustic Eigenfrequency, and an electronic unit which senses the speed of sound to maintain the acoustic resonance by changing the frequency of the acoustic signal and processes the electrical signals to derive the speed of the fluid flow inside the acoustic cavity, characterised in that the acoustic standing wave is perpendicular to the direction of fluid flow in the cavity, that there is a traveling wave perpendicular to the standing wave, and in that signals processed by the electronic unit are derived from the travelling and standing waves, and further characterised by means for measuring the time delay or phase shift between energising one transducer and the response of the other transducer, measuring the time delay or phase shift between energising the other transducer and the response of the one transducer, and comparing the measured time delays or phase shifts to derive the speed of the fluid flow in the acoustic cavity. A flowmeter as claimed in claim 1, characterised in that there are at least three transducers (5) and the electronic unit processes the signals to derive the direction of fluid flow. A flowmeter as claimed in claim 1 or claim 2, characterised in that the transducers (5) are mounted on one of the reflectors (3). A flowmeter as claimed in any preceding claim, in which the resonant cavity is a pipe. A flowmeter as claimed in any of claims 1 to 3, characterised in that the reflectors (2,3) are parallel plates. A flowmeter as claimed in any of claims 1 to 3, characterised in that one reflector (2)is shaped to restrict the passage of air through the cavity symmetrically in all directions. A flowmeter as claimed in any of claims 1 to 3, characterised in that one reflector (2) is shaped to facilitate the passage of air through the cavity symmetrically in all directions. A flowmeter as claimed in any preceding claim when independent of claim 4, characterised in that the transducers (5) are mounted in a plane parallel to the direction of fluid flow. A flowmeter as claimed in any preceding claim, characterised by including a secondary reflector for establishing with one of said reflectors secondary standing and travelling waves at a second frequency. A method of measuring the speed of a fluid flow comprising energising one of a pair of electro-acoustic transducers in an acoustic resonant cavity (1), which is open in the direction of fluid flow and is defined between a pair of reflectors (2,3), to establish in the cavity (1) an acoustic standing wave at an acoustic Eigenfrequency, and sensing the speed of sound to maintain the acoustic standing wave by changing the frequency of the acoustic signal, characterised in that the standing wave is perpendicular to the direction of fluid flow in the cavity and that there is a travelling wave perpendicular to the standing wave, and further characterised by measuring the time delay or phase shift between energising the one transducer and the response of the other transducer, energising the other transducer and measuring the time delay or phase shift between energising the other transducer and the response of the one transducer, and comparing the measured time delays or phase shifts, to derive the speed of fluid flow in the acoustic cavity. A method as claimed in claim 9, characterised by repeating the measurements with a second pair of transducers having an orientation that is different to that of the first pair, and using the measurements to derive the direction of flow of the fluid. A method as claimed in claim 10 or claim 11, characterised by maintaining a fixed operating wavelength perpendicular to the direction of fluid flow thereby making the anemometer less sensitive to environmental changes. A method as claimed in any of claims 10 to 12, characterised in that the standing and travelling waves are primary waves and including establishing in said cavity a secondary standing wave and associated secondary travelling wave at a second resonant frequency, measuring the velocity of flow using the secondary waves, and comparing the velocities measured using the primary and secondary waves to measure the inclination of the flow to the plane of the travelling wave.

Inventors: KAPARTIS SAVVAS, KAPARTIS, SAVVAS
Applicants: F T TECHNOLOGIES LTD, F T TECHNOLOGIES LIMITED
Application year: 1997
Main classification: G01P5
IPC codes: G01P13, G01F1, G01P5