FLUID FLOW MEASURING APPARATUS

Apparatus (1) for measuring fluid flow is disclosed. The apparatus comprises first, second and third ultrasonic transducers (8), the second and third ultrasonic transducers arranged to receive an ultrasonic wave (13) from the first ultrasonic transducer, wherein the second and third ultrasonic transducers are spaced apart by a given distance. The apparatus further comprises a circuit (6) comprising a phase comparator (21) arranged to compare first and second signals (20) from the second and third ultrasonic transducers.

FIELD

The present invention relates to a fluid flow measuring apparatus.

BACKGROUND

Quantitative airflow monitoring or wind speed and direction measurements are required in a number of applications including the continuous monitoring of airflow in road tunnels to ensure quality of air within the tunnel, or the monitoring of airflow within a building space to ensure adequate ventilation. Each country has its own set of regulations about how airflow should be monitored. Measuring wind speed and direction, using a device as an anemometer has many applications in meteorology, shipping, transportation, use on wind turbines and other civil infrastructure such as buildings. There are also situations where one might want to measure the localised speed and direction of flow in a liquid, such as in a river or other body of water.

Having an accurate fluid flow measuring device with a design that is common to a number of applications can be extremely beneficial, as it can easily be adapted to different measurement applications. A device that also uses low-cost transducer and electrical components, but is still capable of measuring low fluid flow speeds is extremely useful as it facilitates the application of such devices in application fields where previously it would have been prohibitively expensive to do so. A fluid flow measuring device that was also capable of low electrical power consumption would offer further benefits, being capable of battery powered operation for long periods of time.

SUMMARY

According to a first aspect of the present invention there is provided apparatus (or “a system”) comprising first, second and third ultrasonic transducers. The second and third ultrasonic transducers are arranged to receive an ultrasonic wave from the first ultrasonic transducer. The second and third ultrasonic transducers are spaced apart by a known distance (e.g., a predetermined distance or a calibrated (or “measured”) distance). The apparatus further comprises a circuit comprising a phase comparator arranged to compare first and second signals obtained from the second and third ultrasonic transducers respectively. The circuit may include amplifiers and/or signal shapers to condition respective signals generated by the second and third transducers for supply as the first and second signals to the phase comparator. The circuit can be used to determine a component of velocity of a fluid (such as air) moving in a direction along a line between the second and third ultrasonic transducers

Thus, the apparatus can be used to measure speed or velocity of fluid flow. The fluid is preferably air and so the apparatus may be operable as an anemometer.

The apparatus preferably comprises a signal generator, coupled to the first ultrasonic transducer, configured to cause the first ultrasonic transducer to generate a continuous wave (CW) ultrasonic wave.

The circuit may include a bias generator arranged to receive a signal from the phase comparator and generate a dc signal whose level depends on a phase difference between the first and second signals. The bias generator may comprise a low-pass filter. The bias generator may comprise an integrator.

The apparatus may also comprise an analogue-to-digital converter for digitising the dc signal. The analogue-to-digital converter may sample the dc signal at a rate no more than 128 kHz, preferably at a rate between 10 Hz and 128 kHz, more preferably between 100 Hz and 1 kHz.

The apparatus may further comprise a fourth ultrasonic transducer arranged to receive the ultrasonic wave from the first ultrasonic transducer. The fourth ultrasonic transducer may be spaced apart by a given distance from the second ultrasonic. The fourth ultrasonic transducer is not collinear with the second and third ultrasonic transducers. The circuit may comprise further phase comparator arranged to compare the first signal and a third signal from the fourth ultrasonic transducer.

Thus, the apparatus can be used to measure a direction, as well as speed, of the fluid flow.

The second, third and fourth ultrasonic transducers may be arranged in an equilateral triangle.

The apparatus may further comprise a respective amplifier (such as an operational amplifier) for each of the second, third and fourth ultrasonic transducers. Each amplifier may be configured to amplify a signal from a respective ultrasonic transducer having a gain set to saturate the signal.

The apparatus may further comprise a controller. The controller may comprise a signal generator configured to generate a signal for the first ultrasonic transducer. The controller may comprise an analogue to digital converter.

The circuit may be configured to convert ultrasonic wave signals from the second, third and, optionally, fourth ultrasonic transducers into square wave signals, and to generate a series of pulses by comparing two of the square wave signals so as to compare the ultrasonic wave signals received at the second, third and, optionally, fourth ultrasonic transducers.

The circuit may be further configured to generate a dc voltage output based on the width of pulses from the output of a logic gate used to measure the phase difference between two received signals.

The apparatus may further comprise a plurality of sets of ultrasonic transducers, each set comprising first, second, and third ultrasonic transducers.

According to a second aspect of the present invention there is provided apparatus comprising first, second and third ultrasonic transducers arranged along a surface and having respective transducer faces flush with the surface, a first reflector supported by the surface arranged so as to reflect a pressure wave emitted by the first ultrasonic transducer towards the second ultrasonic transducer and a second reflector supported by the surface arranged so as to reflect a pressure wave emitted by the first ultrasonic transducer towards the third ultrasonic transducer.

The circuit preferably comprises a phase comparator for comparing signals from the second and third ultrasonic transducers. The first, second and third ultrasonic transducers may be arranged collinearly. The first, second and third ultrasonic transducers may be arranged collinearly along a line which is parallel with a longitudinal axis of a tunnel in which the apparatus is installed.

The apparatus may further comprise a controller operatively connected to the second ultrasonic transducer and first and second amplifiers connected to the first and third ultrasonic transducers respectively. The controller may further comprise first and second signal shapers. The XOR logic gate may be operatively connected to the first and second signal shapers.

The apparatus may further comprise at least one diffuser (or “diffusing structure”). A diffuser may be an absorber, for example, in the form of a region of energy-absorbing material. A diffuser may take the form of scatterer, such as reflector or patterned surface, for directing ultrasonic waves away from the second and third transducers.

According to a third aspect of the present invention there is provide a method of determining speed and/or direction of a fluid flow. The method comprises transmitting an ultrasonic wave from a first ultrasonic transducer towards a second, third and, optionally, fourth ultrasonic transducers, and obtaining speed or velocity by comparing phase difference between first and second ultrasonic waves received by a first pair of the second, third and/or fourth ultrasonic transducers and, optionally, comparing phase difference between first and second ultrasonic waves received by a second pair of the second, third and/or fourth ultrasonic transducers.

Obtaining the speed and/or direction of the fluid flow may further comprise, for the first and second ultrasonic waves, converting ultrasonic wave signals received at the second, third and, optionally, fourth ultrasonic transducers into first and second square wave signals, and generating a first series of pulses by comparing the first and second square wave signals of the first pair of ultrasonic transducers and, optionally, generating a second series of pulses by comparing the first and second square wave signals of the second pair of ultrasonic transducers.

The first or second series of pulses may be generated by applying exclusive OR logic on the first and second square wave signals of the first pair of ultrasonic transducers and, optionally, on the first and second square wave signals of the second pair of ultrasonic transducers. Determining the speed or velocity of fluid moving past the second, third and, optionally, fourth ultrasonic transducers may be based on the widths of the pulses.

The method may further comprise generating a dc voltage output based on the rate of pulses in the first or second series of pulses.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

Introduction

Referring toFIG.1, a system1(or “apparatus”) for measuring fluid flow2, in particular air flow, in a space3is shown. The space3may be partially enclosed or fully enclosed by one or structures4, such as walls (which may include windows and doors), floors and ceilings. For example, the space3may be partially enclosed by a tunnel, pipe or conduit, or may be fully enclosed in a room (or in another enclosed space in a building), a vehicle or other closed space.

The system1comprises a sensor5(or “fluid flow measuring device”) and a circuit6(or “measurement circuit”) for providing drive signals to the sensor5and processing measurements from the sensor5, and a controller7.

The sensor5comprises plurality of ultrasonic transducers8, one or more optional reflectors9and one or more optional diffusers10disposed in or supported by a structure11which may be an open structure, such as a frame, or closed structure, such as a housing or enclosure. The transducers8take the form of flexural ultrasonic transducers (also referred to as “unimorph ultrasonic transducers”) operating at, for example, 40 kHz.

The reflector(s)9may be used to re-direct ultrasonic waves from one transducer8towards another transducer8, and a reflector9may take the form of a flat surface. A diffuser10may be absorber, for example, in the form of a region of energy-absorbing material. Alternatively, a diffuser10may take the form of scatterer, such as reflector or a patterned surface which is used to direct ultrasonic waves away from transducer(s)8and so discourage multiple reflections. For example, one transducer8may be mounted at a distal end of a frustum or other angled support (such as an arm) projecting from an inner wall of a passage such that it faces other transducers arranged along an opposite wall. Thus, any ultrasonic wave reflected back from the opposite wall or other transducers8is deflected off the angled wide wall(s) of the frustum generally away from the other transducers (rather than back towards them). Additionally or alternatively, an inner wall of a passage may be convoluted or suitably patterned and/or be formed from a material, for absorbing direct or reflected ultrasonic waves.

In a first arrangement, the transducers8are orientated generally in the same direction, and at least one reflector9is used to redirect ultrasonic waves from one transducer8(the “transmitter”) towards two other transducers8(the “receivers”) so each path from the transmitter to a respective receiver8includes reflection. In the first arrangement, the transmitter8is typically interposed between the receivers8, for instance, collinearly or in an arc.

In a second arrangement, a transducer8(the “transmitter”) is orientated in the opposite direction to the other transducers8(the “receivers”) so it faces the receivers8. Thus, each path from the transmitter8to a respective receiver8is direct. Diffuser(s)10may be provided to help reduce unwanted reflections.

Referring also toFIG.2, in both arrangements, a signal generator11is used to generate a time-varying signal12which is fed to the transmitter8which transmits a continuous-wave (CW) ultrasonic wave13which is received, via a direct path14or a reflected path15, by the receivers8which generate respective signals16which are fed to the circuit6. The signal generator11may be provided by the controller7.

The time-varying signal12may be repeatedly switched on and off. The time-varying signal12is ON for sufficiently long, for example, at least 30 cycles, that the transducers8reach stable operating conditions. For an operating frequency of 40 kHz, this corresponds to a minimum ON time of 750 μs. The time-varying signal12is ON for between 100 and 10,000 cycles. Thus, a CW wave13an intermittently transmitted. 13.48.

The circuit6includes amplifiers17for generating amplified signals18, optional signal shapers19for shaping the amplified signals18and generating shaped or conditioned signals20(“square-wave signals”). A pair of square-wave signals20are provided to a phase comparator21(or “phase detector”) for generating a phase difference signal22. If there are more than one pairs of signals20, then a phase comparator21may be provided for each pair of signals20. The or each phase comparator21may take the form of an exclusive OR gate.

As will be explained in more detail later, the gains of the amplifiers17are set to be sufficiently high to saturate the outputs and so produce saturated or clipped signals18. The signal shapers19, which take the form of comparators which compares the saturated signals18to a threshold value, can be used to sharpen the rising and falling edges and thus produce square-wave signals20.

The phase difference signal22comprises a train of pulses23(FIG.9). The width of the pulses varies according to phase difference which depends on fluid speed or velocity.

The phase difference signal22is provided to a bias generator24which includes an integrator25that produces a dc signal26whose amplitude corresponds to fluid speed or velocity. As fluid speed changes, the de signal26also changes and so the signal26can vary over time. The dc signal26is sampled by an analogue-to-digital converter27at a rate less than or equal to 128 kHz, preferably at a rate less than 1 kHz, to produce a digital signal28which is processed by a processor29in the controller7which may take the form of a microcontroller. The bias generator24and/or the analogue-to-digital converter27may be provided by the controller7.

By using CW signals and sampling a slow varying dc signal26instead of sampling high-frequency ultrasonic wave signals16, the circuit6and controller7can be implemented using simpler and cheaper integrated circuits.

Systems1and devices5which use the first and second arrangements will now be described in more detail.

First Fluid Flow Measuring Device

Referring toFIG.3, a first fluid flow measuring device51has a body10which generally takes the form of box-like or cylinder-like structure having at least first and second sides33,34. The body first and second sides33,34may be opposite each other, the first and second sides33,34may be parallel to each other. Alternatively, the first and second sides33,34may be adjacent to each other forming an angle. The angle may be less than 180°. The first and second sides33,34each have a passage opening,35,36and are connected by a passage38which passes through the body10to allow air (or other fluid) to flow through the body10of the first fluid flow measuring device21. The first and second side openings35,36may be any suitable shape, for example, square, circular or oval. The passage38may take the form of a pipe-like structure and may have any suitable profile, for example, square, circular, oval or hexagonal. The shape of the first and second openings35,36and the profile of the passage38may be adapted to the location and requirements of the fluid flow measuring device.

Referring also toFIG.4, the passage38has first and second opposite walls40,41running between the first and second passage openings35,36. First, second and third ultrasonic transducers81,82,83are arranged along the first passage wall40. The first ultrasonic transducer81may also be referred to as “a transmitting ultrasonic transducer”, “a transmitter”, “a generation transducer”, or “a transmit transducer”. The second and third ultrasonic transducers may also be referred to as “first and second receive transducers”, “receive transducers” or “receivers”. Each of the ultrasonic transducers8has a respective face45. The face45of each of the ultrasonic transducers8may be flush with the surface of the first passage wall40. In some cases, the reflector9may be integrally-formed with the wall40as a single piece. One or more reflectors9are arranged on, along or alongside the second passage wall41to face the ultrasonic transducers8to reflect a pressure wave13(FIG.2) emitted from the first ultrasonic transducer81to towards the second and third ultrasonic transducers82,83. The reflector9may be provided on, by or along the second passage wall41. The reflector9may be flush with the second passage wall41. Air (or other fluid) flows through the passage38from the first opening35towards the second opening36. The first fluid flow measuring device51may have more than three ultrasonic transducers8.

The ultrasonic transducers8may be arranged collinearly along the first passage wall40along a first axis47. The portion of the first passage wall40where the ultrasonic transducers8are arranged may be planar around any one of the transducers. The first passage wall40may be planar along its whole length, from the first opening35to the second opening36. The portion of the second passage wall41where the reflector9is arranged may be planar. The second passage wall41may also be planar along its whole length, from the first opening35to the second opening36.

As will be explained hereinafter, the device51may be used in a tunnel101(FIG.28) having a wall103(FIG.28) and a longitudinal axis104runs parallel to the tunnel wall103(FIG.28). The first passage axis47may be parallel with the longitudinal axis104of a tunnel101(FIG.28), or parallel with the most dominant vector of airflow through the tunnel101(FIG.28).

Referring also toFIG.5, each of the ultrasonic transducers8comprises a transducer element48, for example a piezoelectric transducer and a suitably-shaped transmission element49for directing ultrasonic pressure waves13from the first transducer81to the reflector(s)8. The ultrasonic pressure waves13may be emitted from the second transducer82at first and second angles θ1, θ2with respect to the inner surface of the first wall40towards the reflector(s)9. The first and second angles θ1, θ2may be between 20° and 60° and preferably between 30° and 45°. The first and second angles θ1, θ2may be the same. The ultrasonic pressure waves131,132are reflected by the reflector9as first and second reflected waves131′,132′ at first and second reflected angles α1, α2with respect to a line perpendicular to the second fluid flow measuring device wall41. The first and second reflected angles α1, α2may be the same.

The first and second reflected waves131′,132′ travel to the faces of the second and third transducers82,83respectively. The first and second ultrasonic transducers81,82may be separated or spaced apart by a first distance TD1, and the first and third ultrasonic transducers81,83may be separated or spaced apart by a second distance TD2. The first and second distances TD1, TD2may be less than 200 mm and preferably between 50 mm to 100 mm. The first fluid flow measuring device51may have additional ultrasonic transducers (not shown) located on the same wall and spaced apart by the first and second distances TD1, TD2.

Air may flow in a first direction42from the opening35to the second opening36or in a second direction43from the second opening36to the first opening35. The first and second walls40,41are separated by given distance, for example by a known distance or a calibrated distance WD, thus, at least the relative positions between each of the first, second and third transducers are known.

Referring toFIG.6, the reflector9may take the form of first and second reflectors91,92supported by the first wall40, or surface, arranged to reflect an ultrasonic pressure wave13(FIG.5) towards the second and third ultrasonic transducers82,83. The first and second reflectors91,92may take the form of a metal bracket secured to the first wall40. This can help reduce the number and/or area of reflective surfaces which can reduce other possible strong reflective paths between the generation transducer81and detection transducers82,83.

Referring toFIG.7, a first ultrasonic flow-measuring system11includes the first fluid flow measuring device51, the driving and measurement circuit6and the controller7.

When enabled, the signal generator11generates a signal12which is supplied to the first ultrasonic transducer8,81. The first ultrasonic transducer8,81transmits a CW ultrasonic pressure wave13towards the reflectors91,92. The ultrasonic pressure wave13is reflected by the reflectors91,92as first and second reflected pressures waves131′,132′ which are directed towards the second and third ultrasonic transducers82,83respectively.

The second and third ultrasonic transducers82,83receive the first and second reflected ultrasonic pressure waves131′,132′ and generate first and second ultrasonic transducer signals161,162respectively. The first and second ultrasonic transducer signals161,162are amplified by first and second amplifiers171,172respectively to produce first and second amplified signals181,182. The transducer signals161,162are preferably over-amplified, i.e., saturated or “clipped”. The first and second amplified signals181,182are shaped (or “conditioned”) to reduce the signal rise and fall times thereby producing first and second square wave signals201,202.

The first and second square wave signals201,202are fed into a phase comparator21(or “phase detector”), for example, in the form of an XOR logic gate. The phase comparator21generates a signal22comprising a series of pulses23(FIG.9). The width w of the pulses23are a function of the airflow velocity and so the airflow velocity is determined using the width w of the pulses23.

Using two receive transducers82,83, it is possible to achieve a high accuracy airflow measurement (for example, down to velocities of 1 mms−1) without using high-frequency (>128 kHz) analogue-to-digital converters (ADCs) and digital signal processors (DSPs) (which are expensive and tend to have a higher power consumption than ADCs having lower sampling rates), or a multiplexer to switch the ultrasonic transducers between sending and receiving ultrasonic signals.

The first fluid flow measuring device51may be calibrated at installation to correct for errors caused by the transducers81,82,83. For example, the manner in which transducers81,82,83are fixed to or embedded in the first passage wall40, slight variations in construction, and/or temperature variations may affect the resonant frequency of any one of the transducers, and could vary by up to 2%, or between 1 and 2%. The slight differences in the mechanical response may cause the electronic voltage signal from the second and third transducers82,83to vary slightly in phase so that even at zero flow there might appear to be a flow that was not real. This zero-flow offset can also be trimmed at installation.

The location of the first, second and third ultrasonic transducers81,82,83on the first wall40can allow simplify manufacture and maintenance, and/or servicing of the device51.

Referring also toFIG.8, the signals16output by the second and third ultrasonic transducers82,83have sinusoidal waveforms. The amplified or over-amplified signals18have generally steep, straight rising and falling edges and are clipped (i.e., saturated at a maximum and minimum values). The shaped signals20take the form of square-wave signals having vertical rising and falling edges.

Referring toFIG.9, first and second shaped signals201,202are compared using the phase comparator21(FIG.7) which in this case takes the form of an XOR gate. When there is no flow (i.e., flowrate is zero), the first and second signals201,202exhibit a constant phase offset which may be non-zero. The phase relationship between the first and second processed signals201,202may be obtained using the phase comparator21(FIG.7).

The phase comparator21(FIG.7) generates a signal22which includes a train of pulses23. The degree of phase offset is reflected by the pulse width wo. The bias generator24converts the pulsed, phase-dependent signal22into a bias signal26which has a dc offset which depends on the pulse width w. At zero flow rate, w=wo.

The bias generator24can be a low-pass filter, a loop filter or other suitable form of passive or active circuit which generates a signal whose bias depends on phase offset.

Referring toFIG.10, as flowrate increases in magnitude a first direction42(FIG.5), the first output ultrasonic pressure wave131and first reflected ultrasonic pressure wave131′ take longer to travel from the first transducer81(i.e., the generation transducer) to the second transducer82upstream of the first transducer81than the second output ultrasonic pressure wave132and reflected ultrasonic pressure wave132′ will take to travel from the first transducer81to the third transducer83downstream of the first transducer81. Thus, with increasing flowrate in the first direction42, the phase difference between the first and second signals201,202decreases and the pulse width w of the phase comparator21(FIG.7) decreases. Thus, at a positive flow rate, w+<wo.

Referring toFIG.11, when the flowrate increases in magnitude in a second, opposite direction43, the second output ultrasonic pressure wave132and second reflected ultrasonic pressure wave132′ take longer to travel from the first transducer81to the third transducer83downstream of the first transducer8, than the first output ultrasonic pressure wave131and first reflected ultrasonic pressure wave131′take to travel from the first transducer81to the second transducer82upstream of the first transducer81. Thus, the phase difference between the first and second signals201,202increases and the pulse width w of the phase comparator21(FIG.7) increases. As explained earlier, the bias generator24(FIG.7) generates a bias signal26according to the width w of the pulses23.

A digital counter (not shown) may be used to control the number of pulses supplied to an op-amp integrator60(FIG.12), after which the output of the integrator may be read electronically and then reset. Alternatively, if a continuous or very long pulse of cycles is generated, the output of the XOR can be input to an op-amp integrator60(FIG.12) with a time constant such that it delivers a continuous time averaged value of the integrated input.

Referring toFIG.12, a first integrator circuit60may sum the widths of all the input pulses input to it. The integrator circuit60may be reset after reading the summed value for a given number of input pulses.

Referring toFIG.13, a second integrator circuit61may sum the time averaged widths of all the input pulses input to it. In the second integrator circuit61, the averaging time or time constant is determined by the capacitor and resistor in the feedback loop.

When the output22of the phase comparator21(FIG.7) for pulses of a given fixed amplitude is integrated, the value of the integrator circuit output26is related to the sum of the widths w of the pulses23. The voltage of the integrator output26of the integrator circuit60,61can be converted to a digital value, for example, by a low-speed (i.e., with a sampling rate less than 128 kHz) analogue to digital converter27(FIG.2).

To increase the dynamic range of a flowmeter, the operating frequency of the ultrasonic transducers81,82,83can be changed, such that the time period of the first and second signals201,202supplied to the phase comparator21is increased.

Alternatively, the dynamic range of a flowmeter can be increased by amplitude modulating a drive signal to the generation transducer. For example, the drive signal of the first transducer81may be amplitude modulated, so that the ultrasound output161(FIG.7) of the first transducer81is a modulated ultrasound output. The second and third transducers82,83may then receive the first and second modulated reflected ultrasound waves131′,132′ and generate first and second modulated voltage output signals. The first and second modulated voltage output signals may then be filtered, for example, electronically filtered or filtered using suitable software, to generate two different frequency signals: a modulation frequency and an ultrasonic sound frequency. Next, two output signals with different frequencies from the second transducer82and two output signals with different frequencies from the third transducer83are generated. Signals with the same frequency will be input into an XOR gate to measure the flowrate in the same way as described earlier. If the flowrate is sufficiently high that the phase difference between the high frequency signals leads to an output signal that exceeds either the lower limit of output pulse width w (thereby causing the output pulse width w to increase beyond that point) or the higher limit of output pulse width w (thereby causing output pulse width w to decrease beyond that point), the lower frequency output signal will not have reached its limit and the high frequency output signal for the XOR gate can be used to give more accurate readings in combination with the lower frequency output signal from the other XOR gate.

Referring toFIGS.14to17, a drive signal65may be modulated using a modulation signal66, for example a lower frequency signal to produce a modulated drive signal67. When the modulated drive signal67is emitted from a transducer, a modulated wave68is generated. For illustration, the drive signal65has a frequency of 4 kHz (although a frequency 40 kHz would typically be used) and the modulation signal66has a frequency of 625 Hz.

Using a filter and amplifier to detect the low frequency modulation from the first and second signals201,202output from second and third receive transducers82,83it is possible to recover a signal similar to the modulation signal66. This signal may then be amplified into a square wave of the same frequency. This process may be performed for signals travelling both upstream and downstream and the square waves from each may be fed into the phase comparator21(FIG.7). It is possible to perform the same method for a higher frequency signal, for example, between 40 kHz and 68 kHz (with a time period of between 25 μs and 14.7 μs). A combination of the low-frequency integrator pulses may then be used to give the coarse measurement of flowrate over a wider range than could be achieved by using the 40 kHz signal alone. The 40 kHz signal may be used in addition to provide a more accurate measurement of flowrate using the information from the lower frequency integrated signal.

The ultrasonic transducers81,82,83may have a wider frequency range than mentioned above, for example, between 1 kHz and 1 MHz.

The dynamic range of the first fluid flow measuring device51can also be increased by adding additional, more closely-spaced receive transducers. Shortening the propagation distance reduces the phase shift between the signals from waves that have travelled upstream or downstream, but in doing so measurement accuracy can be reduced for lower phase shifts at lower flowrates.

Referring toFIG.18, a second fluid flow measuring device52is shown.

Referring also toFIGS.19and20, the second fluid flow measuring device52is similar to the first fluid flow measuring device51(FIG.3) except that the device52does not have a reflector9and the first ultrasonic transducer8, is provided in its position on the second fluid flow measuring device wall41. The first, second and third ultrasonic transducers81,82,83are arranged so the second and third ultrasonic transducers82,83can receive a transmitted ultrasonic wave13directly (i.e., without reflection) from the first ultrasonic transducer81.

Referring in particular toFIG.20, the face45of the first ultrasonic transducer81may be flush or coplanar with the inner surface of the second wall41. The first ultrasonic transducer81transmits an ultrasonic wave13towards the second and third ultrasonic transducers82,83at first and second transmission angles α1, α2with respect to a line perpendicular to the inner surface of the wall41. The transmitted ultrasonic wave13is received at the faces45of the second and third ultrasonic transducers82,83at first and second angles θ1, θ2respectively with respect to the inner surface of the first wall40. The first and second angles θ1, θ2may be between 20° and 60° and preferably between 30° and 45°. First and second angles θ1, α2may be the same. The first and second walls40,41are separated by a known or calibrated distance WD, thus, at least the relative positions between each of the first, second and third transducers are known.

The second and third ultrasonic transducers82,83may be closer to each other in the second fluid flow measuring device52than in the first fluid flow measuring device51(FIG.3), allowing for a more compact device.

The second fluid flow measuring device52may be able to measure air flow speeds of up to 20 ms−1and even 30 ms−1. At a speed of 20 ms−1, the phase difference between the first and second ultrasonic waves131,132, may be π/2, to allow direction detection with the phase comparator. A frequency of 20 kHz yields a transducer separation of 18.4 mm.

Referring toFIG.21, the first ultrasonic transducer81can be supported by a bracket69(for example, a metal bracket) secured to the first wall40. This can reduce the number of reflective surfaces which can reduce other possible strong reflective paths between the generation transducer81and detection transducers82,83.

Referring toFIG.22, the measurement circuit6for the second ultrasonic flow measuring-system12is the same as that used in the first ultrasonic flow-measuring system11(FIG.7).

Referring toFIG.23, a third fluid flow measuring device53includes first, second, third and fourth ultrasonic transducers81,82,83,84. The third fluid flow measuring device53is generally cylindrical in shape having first and second opposite end walls80,81supported by connecting members82for example in the form of columns or struts. The third fluid flow measuring device53generally has open sides and so air83can flow into the space84between the first and second end walls80,81from any direction (i.e., air is not constrained to flow in a passage).

The first ultrasonic transducer81is arranged on a first end wall80(or “plate”). The second, third and fourth ultrasonic transducers82,83,84, are arranged on a second end wall81. The first ultrasonic transducer81is arranged to transmit an ultrasonic wave towards the second, third, fourth ultrasonic transducers82,83,84, which are in turn arranged to receive the transmitted ultrasonic wave13.

Referring toFIG.24, the arrangement of the transducers81,82,83,84in the third fluid flow measuring device53is similar to that in the second fluid flow measuring device52(FIG.18). The fourth ultrasonic transducer84also has a transducer element48and a transmission element49. The fourth ultrasonic transducer84is generally co-planar with the second and third ultrasonic transducers. The second, third and fourth transducers82,83,84are not, however, collinear. The faces45of the second, third and fourth ultrasonic transducers82,83,84may be coplanar, and may be coplanar with the surface of the end wall81. The ultrasonic wave13transmitted by the first ultrasonic transducer81is received by the second, third and fourth ultrasonic transducers at first, second and third angles α1, α2, α3, with respect to the first fluid flow measuring device wall.

Air83can flow over the first to fourth ultrasonic transducers81,82,83,84in any direction and can be measured in a plane parallel to the end walls80,81.

The first end wall80which supports the first ultrasonic transducer81preferably has minimal reflective surfaces, and may only consist of support structures, e.g., a concentric annular rings and spokes, sufficient to hold the first ultrasonic transducer81in position.

Referring toFIG.25, the second, third and fourth ultrasonic transducers81,82,83,84may be arranged in an equilateral triangle, although any known triangle is suitable. Angle k may therefore by 60°, and angle j may be 30°. Each of the second, third and fourth ultrasonic transducers82,83,84are separated by a known or calibrated distance TD1, TD2, TD3. The first ultrasonic transducer81may be arranged at the centre of the equilateral triangle formed by the second, third and fourth ultrasonic transducers82,83,84. The distances TD1, TD2, TD3between the second, third and fourth ultrasonic transducers82,83,84are such that they are close enough to cover dynamic range of flow velocity for the fluid being measured, and the expected speed of flow.

Referring toFIG.26, a third ultrasonic flow-measuring system13includes a third fluid flow measuring device53, the driving and measurement circuit6′ and the controller7.

The driving and measurement circuit6′ is similar to the driving and measurement circuit6(FIG.6) hereinbefore described except that there is an additional amplifier173, an additional signal shaper193, two phase comparators241,242and two bias generators241,242.

The first and second square-wave signals191,192are supplied to a first phase comparator211and the second and third square-wave signals192,193are supplied to a second phase comparator212. First and second phase-dependent signals221,222are provided to respective bias generators241,242. Thus, the controller7can compute not only the speed, but also the direction of the air using flow equations and trigonometry.

Applications

Referring toFIGS.27and28, the fluid flow measuring devices5may be used as an anemometer in a variety of different applications. The measurement circuit6(FIGS.6,22,27) may be provided in the same housing as the fluid flow measuring device5, or in a separate housing and connected by a wired or wireless connection. For clarity, only the fluid flow measuring devices5are shown.

Referring toFIG.27, a tunnel101is shown in which one or more fluid flow measuring devices5, such as the first flow measuring device51(FIG.2) or the second flow measuring device52(FIG.18), is/are installed.

The tunnel101includes a tunnel wall103on which the first fluid flow measuring device5may be mounted. Each section of the tunnel101has a longitudinal axis104running parallel to the tunnel wall103. Depending on the length of the tunnel, there may be more than one fluid flow measuring device5mounted to the wall of the tunnel, separated or spaced apart by a distance D. The tunnel has first and second openings105,106located at the first and second ends107,108of the tunnel101. The distance D may be less than 200 mm and preferably between 50 mm and 100 mm.

The tunnel101contains air which may enter and leave the tunnel101through the first and second openings105,106. The tunnel101may be, for example, a road tunnel for road traffic, or a rail tunnel for rail traffic. The tunnel may have a surface10below the first fluid flow measuring device5. The surface110may be suitable for road or rail traffic.

Referring toFIG.28, a room201is shown in which one or more fluid flow measuring devices5, such as the third flow measuring device53(FIG.23), is/are installed.

The fluid flow measuring device5may be placed, for example, on a table202, shelf (not shown) or stand (not shown), or on the floor203, or mounted to a wall204or ceiling (not shown), and may be used to identify whether there is sufficient ventilation in the room201(or other similar space).

The device5may be used to provide information about effectiveness of measures to limit airborne transmission of pathogens, such novel coronavirus (COVID-19), and/or to provide sufficient ventilation.

The device5may be linked to a warning device (not shown) which may trigger a warning message if air flow speed drops below a threshold value. The warning device may, for example, may emit an audible alarm and/or transmit a warning message to a phone, which can inform the room occupants or a facilities manager to take appropriate measures, for example, to open a window205or door206or increase speed of an air conditioning unit (not shown).

The flow measuring device5may be used in a conduit of an air conditioning system (not shown) for measuring air flow.

Modifications

It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design and use of fluid flow measuring devices, flow measuring systems and component parts thereof and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment.

Other circuits for measuring phase difference can be used. For example, other logic gate configurations (other than used XOR gates) can be used. Further ultrasonic transducers may be provided. Further amplifiers may be provided. Further signal shapers may be provided. Further phase comparators and corresponding dc bias generators may be provided.