GB2026165A - Fluid flow measuring apparatus - Google Patents

Abstract

Fluid flow measuring apparatus comprises two ultrasonic transducers (1, 2) between which an ultrasonic energy beam is transmitted through the fluid, the beam being frequency modulated (4, 5) by an audio signal, and the relationship between the frequency or phase of the received modulation signal (6, 7, 8) and that of the transmitted modulation signal gives a measure (10) of the speed of flow of the fluid. Beams of ultrasonic energy are preferably fed in two mutually opposite directions, for example by switching (3) the transducers to act alternately as transmitter and receiver, and the measurement is computed from the two relationships. An anemometer (Figure 7) comprises two such systems arranged to produce mutually perpendicular ultrasonic beams, and a computer (25) to compute wind speed (28) and direction (29) from the frequency or phase relationships. <IMAGE>

Description

SPECIFICATION Fluid flow measuring apparatus This invention relates to apparatus for the measurement of fluid flow. The apparatus may be used for measuring, for example, the velocity of flow of a fluid in a pipe, or the speed and direction of the wind.

In known fluid flow measurement systems, ultrasonic pulses are transmitted into the stream of fluid by a first transducer and are received by a second transducer downstream of the first. The apparent change in frequency or phase of the received sound waves, or the time of flight of the pulses from one transducer to the other, provides an indication of the speed of flow of the fluid.

However, the known systems use very short pulses. In order to obtain a sharp wave front, a high ultra-sonic frequency has to be used, but the frequency is limited by the fact that attenuation of the transmitted pulses increases as the frequency increases. The chosen frequency must, therefore, be a compromise between the need for a sharp wave front and the need to obtain satisfactory reception over the required distance despite attentuation of the pulses in the particular medium in the transmission path.

As regards wind speed, conventional anemometers comprise a turbine or a windmill which is driven round by the wind, and a revolution counter and timer which indicates the speed of rotation of the turbine or windmill. Wind direction is indicated by the position f a pivoted vane. Such rotating devices are liable to give erroneous indications due to variations in the frictional forces acting on the devices.

It is an object of the present invention to provide an improved fluid flow measuring apparatus.

According to the present invention, fluid flow measuring apparatus comprises means to generate an electrical carrier signal; means to generate an audio-frequency modulation signal; means to frequency modulate the carrier signal in accordance with the modulation signal; first and second transducers which are spaced-apart along the fluid flow; means to feed the modulated carrier signal to one of said transducers so that said transducer transmits ultrasonic energy through the fluid to be received by the other transducer; means to monitor the relationshp between the phase or frequency of the audio-frequency modulation at the transmitting transducer and the phase or frequency of the received audio-frequency modulation; and means to indicate the speed of fluid flow from the monitored relationship.

In the present invention, therefore, the ultrasonic energy is used purely as a signal carrier. The ultra-sonic frequency can be chosen to suit the attenuation and/or other characteristics of the path between the transducers. The modulation frequency is not affected by the choice of ultrasonic frequency, and so the optimum modulation frequency can be selected for the required measurement.

Since the ultrasonic signal is frequencymodulated, the amplitude of the ultrasonic signal does not play any part in the measurement. Hence, changes in attentuation in the transmission path can be taken care of by merely using automatic gain controi or by amplifying the signal and limiting it at a constant level. In this way, a good signal-to-noise ratio can also be obtained. Other advantages of frequency modulation, such as immunity to various types of interference, are also obtained.

Preferably, the apparatus includes means to cause the ultrasonic energy to be transmitted in two mutually opposite directions through the flow, and the indication means includes means to compute the flow speed from the monitored relationships for the two transmission directions.

The modulation signal is preferably derived in a phase-locked loop, the difference between the frequencies for the two directions being a measure of the fluid flow, which can be indicated on a digital display. Alternatively, if the phase relationship is monitored, the flow measurement can be displayed as an analogue quantity.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic block diagram of apparatus in accordance with the present invention, Figure 2 is a schematic cross-section of a pipe with the transducers of Figure 1 located therein, Figure 3 is a schematic cross-section of a pipe with a plurality of pairs of transducers located therein, Figure 4 is a circuit diagram of a transmitter forming part of the apparatus of Figure 1, Figure 5 is a circuit diagram of a receiver forming part of the apparatus of Figure 1, Figure 6 is a schematic cross section through a curved pipe with two transducers located therein, and Figure 7 is a schematic block diagram of a wind speed and direction indicating apparatus.

Referring to Figure 1 of the drawings, fluid flow measurement apparatus comprises two ultrasonic transducers 1 and 2 which are to be spaced apart along the direction of the fluid flow. Signals fed to, and received by, the transducers are controlled by a switch unit 3 so that the transducers 1 and 2 act alternately as transmitting and receiving transducers, the ultra-sonic energy thereby being fed in alternately opposite directions through the fluid flow.

A carrier signal with a frequency of, for example, 60kHz is generated by an oscillator and modulator circuit 4, and the output of this circuit is fed to the switching unit 3.

A voltage-controlled oscillator 5 generates an audio-frequency signal which is fed to the circuit 4 to frequency-modulate the carrier signal.

The signal produced by whichever of the transducers 1 and 2 is receiving the ultrasonic energy at any instant is fed via the switching unit 3 to a receiving a amplifier and limiter circuit 6. This circuit ensures that, despite changes in the amplitude of the received frequency-modulated carrier signal due to, for example, changes in the attenuation which is caused by the presence of the fluid between the transducers 1 and 2, the signal presented to a demodulator circuit 7 will always be constant predetermined amplitude.

The demodulated signal from the circuit 7 is fed to a phase comparator 8 which is connected in a phase-locked loop 9 with the oscillator 5. In order to maintain the correct phase relationship between the audio-frequency signal produced by the oscillator 5 and the demodulated signal from the circuit 7, the oscillator 5 changes its output frequency to take into account changes in the time of flight of the ultrasonic signal between the transducers 1 and 2. The frequency shift therefore gives a measure of the speed of flow of the fluid between the transducers. Taking into account any necessary conversion constants, a display unit 10 responds to the audio-frequency signal from the oscillator 5 to give a direct indication of the speed of the fluid flow. This can be an analogue or digital display.

Referring now to Figure 2 of the drawings, a pipe 11 through which fluid is to flow is fitted with transducer probes 12 and 13 which extend into the interior of the pipe through fluid-tight bushes 14 and 15, respectively. The probes are spaced apart longitudinally of the pipe and are also relatively displaced round the periphery of the pipe by 180 . Transducers 1 and 2 at the inner ends of the probes face each other diagonally across the direction of fluid flow.

The time of flight of the transmitted signals will vary according to the velocity of flow of the fluid. If the fluid is flowing towards the right in figure 2 and the times of flight from transducers 1 to 2 and transducers 2 to 1 are T1 and T2, respectively, the fluid flow velocity will be proportional to T2-T1 2 Since the ultrasonic beam is travelling diagonally across the flow, the measured velocity will be the average velocity over the whole cross-section of the flow, assuming that the flow profile remains reason ably constant at all velocities in the range in question.

The transducers can be very small relative to the diameter of the pipe, and so do not need to interfere unduly with the fluid flow. They can be mounted at any desired depth in the flow and so can be used to monitor the flow rate at that particular depth.

Figure 3 shows, schematically, the use of probes 16 and 17 mounted parallel to each other in a pipe 18, with pairs of transducers 1, 2; 1', 2'; etc. located at different depths in the flow. By this arrangement the pairs of transducers monitor paths which are parallel to the directions of fluid flow but which are at respectively different depths in the flow. Each pair of transducers is connected to a respective frequen cy modulation system as shown in Figure 1 or, alternativey, the switching unit 3 of a single system could be designed to switch between the various transducer pairs as well as reversing the direction of the ultrasonic beam. In the latter case, respective display units would have to be switched in synchron ism with the switching between pairs of transducers.

Such an arrangement would measure the flow profile, or could be used to indicate whether or not the pipe is full of fluid.

The sum of the times, T1 ("go") and T2 ("return"), mentioned above, is a function of the speed of sound in the fluid irrespective of the rate of flow of the fluid.

The speed of sound in the flow therefore various only in dependence upon the properties of the fluid, such as its density. If it is required to monitor mass flow instead of volume flow, the temperature of the flowing fluid can be measured, and a correction applied to the indication on the display unit 10.

Alternatively, the mass flow may be computed from the times T1 and T2 and the temperature coefficient of the velocity of sound.

Referring to Figure 6, the transducers 1 and 2 do not necessarily have to be arranged such that there is a straight direct path for the ultrasonic beam from one transducer to the other. In the illustrated arrangement, the transducers are mounted in a U-bend 19 of a pipe with their axes perpendicularto each other. The beam will be reflected at a point 20 by the wall of the pipe if the pipe is full of a homogeneous fluid, but will be reflected at a point 21 if there is an interface 22 between dissimilar fluids, such as a gas and a liquid.

A further arrangement of transducers is shown in Figure 7. This arrangement acts as an anemometer.

Two pairs of piezo-ceramic transducers 1", 2" and 1"', 2"' are so arranged that the paths of the respective ultra-sonic beams are mutually perpendicular. Each pair of transducers is coupled to a respective frequency modulation system as shown in Figure 1. The audio frequency signals generated by the respective oscillator 5 are fed to a computer 25 which computes the wind speed and direction therefrom.

When the air is still and the two paths are the same length, the two signals fed to the computer 25 will be the same, although the actual value will vary with changes in the temperature and humidity. A zero wind speed will therefore be indicated. The transit times will be the same irrespective of the direction of the transmission between the transducers.

However, if a wind is blowing, the effect is to move the air so that the transit times will be different according to the angle of the wind. The audio frequencies for each transmission direction and each pair of transducers provides sufficient information for calculating the wind speed and direction. A micro-processor is easily programmed for this purpose and may have spare capacity for providing the electronic circuitry as shown in Figure 1.

The output of the computer 25 can be two continuous signals representing the speed and direction, and these signals can be fed to indicators 28 and 29 o4 can be fed to a line in pulse or frequency form suitable for transmission to other locations.

The energy level required to operate the system is very small, typically 1/4 watt, and the system has no inherent time lags and no warm up time so that the connection of a power supply to the system gives immediate speed and direction readings.

The system may conveniently be powered by a battery 26 via a switch 27 which is closed manually or automatically when a ready is required and which then returns to the open position. The battery 26 can be a battery of rechargeable secondary cells which can be trickle-charged by power from a solar cell.

The system can therefore be self-contained and portable.

The anemometer could be mounted on a boat or other vehicle to measure the "apparent wind".

Suitable parameters for the anomometer system are: 1 metre transducer spacing, 100kHz carrier frequency, 3,310Hz modulation frequency when air is still, 3,320Hz modulation frequency when wind speed is 1 metre/sec. along the direction of the relevant ultra-sonic beam. The working range can be very wide, but for meterological use a range of O - 30 metres/sec. wind speed is typical, and this may conveniently be divided into two ranges, viz. 0 - 3 metres/sec. and 0 - 30 metres/sec. The instrument may alternatively be used in respect of other fluids, for example water. In the latter case, a carrier frequency of 500kHz and a modulation frequency of 3kHz would be suitable.

Itwill be appreciated that, whilst in any of the above embodiments each transducer can act alternately as both transmitter and receiver as shown, separate transducers can be provided for transmit and receive at each position, with separate sets of associated circuitry, to avoid the need for switching the direction of the beam. The transmit transducers can be supplied with frequency-modulated signals of respectively different ultrasonic carrier frequencies to avoid interference between the "go" and "return" signals. The carrier frequencies may be, for exaple, 25kHz and 38kHz.

Figure 4 illustrates one circuit configuration which may be used for the phase-locked loop 9 and the carrier generator and modulator circuit 4 of Figure 1.

The phase-locked loop comprises an integrated circuit 30 with associated components. The demod ulated signal is fed to the circuit 30 via a line 31 and the nominal frequency is set by means of an input to a line 32. the carrier generator and modulator circuit comprises an integrated circuit 33 and the carrier frequency is set by a variable resistor 34. The integrated circuits 30 and 33 are supplied with power from a stabilised 12 volt d.c. supply line 36.

The modulated carrier signal is fed to an amplifier comprising transsistors 37 and 38 and the amplified signal is fed via a line 39 to the switch unit 3 of Figure 1.

Figure 5 illustrates one suitable configuration for the circuits 6 and 7 of Figure 1. The amplifier circuit 6 comprises transistors 40, 41 and 42 and associated components, and the demodulator circuit 7 compris es an integrated circuit 43. The demodulated audio frequency output is amplified by a transistor 44 and is then fed to the line 31 of Figure 4.

Variations in the above-described apparatus are possible without departing from the scope of the present invention.

Claims (9)

1. Fluid flow measuring apparatus, comprising means to generate an eiectrical carrier signal; means to generate an audio-frequency modulation signal; means to frequency modulate the carrier signal in accordance with the modulation signal; first and second transducers which are spaced-apart along the fluid flow; means to feed the modulated carrier signal to one of said tranducers so that said transducer transmits ultra-sonic energy through the fluid to be received by the other transducer; means to monitor the relationship between the phase or frequency of the audio-frequency modulation at the transmitting transducer and the phase or frequency of the received audio-frequency modulation; and means to indicate the speed of fluid flow from the monitored relationship.

2. Apparatus as claimed in Claim 1, including means to cause the ultrasonic energy to be transmitted in two mutually opposite directions through the flow, and wherein the indicating means includes means to compute the flow speed from the monitored relationships for the two transmission directions.

3. Apparatus as claimed in Claim 2, wherein the carrier signals for the two transmission directions are of respectively different frequencies.

4. Apparatus as claimed in Claim 2, wherein the means to cause the transmission in two mutually opposite directions comprises switching means to feed the modulated carrier signal to the first and second transducers alternately.

5. Apparatus as claimed in any preceding claim, wherein the transducers are mounted on opposite sides of a pipe in which the fluid flows, whereby the ultrasonic energy traverses a cross-section of the flow.

6. Apparatus as claimed in any one of Claims 1 4, including a plurality of pairs of said first and second transducers, each pair being arranged to transmit and receive ultrasonic energy along mutually parallel paths at different regions of the cross-section of the flow, whereby the indicator means indicate the profile of the flow.

7. Apparatus as claimed in any one of Claims 1 4, including two pairs of said first and second transducers arranged to transmit the ultrasonic energy along respective paths which are mutually perpendicular; and wherein the indicator means includes means to compute wind speed and direction from the monitored relationships for the two paths.

8. Apparatus as claimed in any preceding claim, wherein the transmitted and received audio frequency modulation signals are controlled by a phaselocked loop.

9. Apparatus as claimed in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.