GB2205645A

GB2205645A - Fluid flow measurement

Info

Publication number: GB2205645A
Application number: GB08713761A
Authority: GB
Inventors: Milan Herman Miessler
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-06-12
Filing date: 1987-06-12
Publication date: 1988-12-14
Also published as: GB8713761D0

Abstract

A phase-locked-loop (PLL) circuit includes a phase comparator 10, a low-pass filter 12 and a voltage controlled oscillator 14 providing an ultrasonic oscillating signal f1 to be transmitted from one transducer 18 through a flowing fluid to another transducer 20. The phase comparator 10 is arranged to compare the phase of the received signal from the transducer 20, in order to improve the performance of the measuring circuit, the frequencies of both the loop signal and the received signal are divided in circuits 16 and 22 by a factor n before being applied to the phase comparator 10. The circuit is then arranged and operated such that n full standing waves are maintained in the delay gap between the transducers 18 and 20. Fig. 3 (not shown) discloses an arrangement of transducers recessed into curved portions of a U-shape conduit to avoid resonant effects. <IMAGE>

Description

FLUID FLOW MEASUREMENT This invention relates to apparatus for measurement of fluid flow and to methods of measuring fluid flow, and in particular to such apparatus and methods using the principle of continuous wave flow measurement.

It is known to transmit ultrasonic pulses into a stream of flowing fluid and to receive the pulses downstream of the transmitting location. The received ultrasonic pulses have a different frequency or phase to those transmitted and this difference in frequency or phase, or the time taken for the pulses to travel through the fluid flow, can be used to derive an indication of the fluid flow speed.

In, for example, US Patent Nos. 3,420,102 and 3,751,979, it has been proposed to use a phase-locked-loop (PLL) to track and detect phase differences between the transmitted and received pulses. However, a difficulty with these techniques is that a high ultrasonic frequency has to be used in order to obtain a steep wave front, otherwise detection of the timing of the wave front is rendered uncertain. On the other hand, attenuation of ultrasonic waves in a fluid medium increases with frequency. Thus, when using these techniques, it is necessary to select a frequency of transmission which is a compromise between the ability to obtain a steep wave front and the requirement for sufficiently low levels of attenuation to be achieved so as to ensure satifactory reception of the ultrasonic signal.A solution to this problem is proposed in US Patent No 4,320,666 in which a continuous wave ultrasonic beam is frequency modulated by an audio signal, and the relationship between the frequencies or phases of the transmitted and received modulation signals is used to provide an indication of the fluid flow speed. Since the ultrasonic signal is used as a signal carrier, the ultrasonic frequency can be optimised to suit the attenuation characteristics of the path through the fluid. The modulation frequency can be selected independently of the chosen ultrasonic frequency and therefore it can be optimised for the required measurement. The modulation signal is utilised in the PLL, the output of the PLL being connected to a modulator for modulation by the ultrasonic signal, and the received signal being demodulated before being applied to the input of the PLL.However, the disclosed technique, in common with the other methods described previously, has the significant disadvantage that, whereas relatively slow changes in fluid flow speed can be followed by the apparatus and measured adequately, the PLL may not be able to track fast changes and thus will lose acquisition and become unstable under these conditions.

In continuous wave flow measuring systems, there are two conditions which affect the performance of the systems.

Firstly, in order to achieve high resolution and high sensitivity, it is desirable to maintain more than one full standing wave in the flow path.

Secondly, in order to achieve a high precision meter capable of following fast changes in the speed of the flow, it is desirable that the PLL should have a fast response.

However, these two requirements are contradictory and cause a fundamental difficulty in keeping the operation of the PLL stable.

According to a first aspect of the invention there is provided apparatus for measurement of fluid flow, the apparatus comprising: first and second transducers arranged along the path of the fluid flow such that an ultrasonic beam between the transducers can pass through the fluid flow; and a phase-locked-loop circuit including a phase comparator and a voltage controlled oscillator providing an oscillating signal to one of the transducers for transmission through the fluid flow, the phase-locked-loop circuit being responsive to the oscillating signal received at the other of the transducers after passing through the fluid flow;; wherein the phase-locked-loop circuit includes a first dividing means in the loop path from the voltage controlled oscillator to the phase comparator for dividing the frequency of the oscillating signal from the voltage controlled oscillator by a predeterminded factor and supplying the resulting divided signal to the phase comparator, and a second dividing means is arranged to divide the frequency of the oscillating signal received at the other of the transducers by the predetermined factor and to supply the resulting divided signal to the phase comparator, whereby the operating characteristics of the phase-locked-loop circuit are those associated with a frequency equal to the frequency of the voltage controlled oscillator divided by the predetermined factor.

According to a second aspect of the invention there is provided a method of measuring fluid flow in which a phase-locked-loop circuit having a phase comparator provides an oscillating signal transmitted as an ultrasonic beam from one transducer to another transducer through the fluid flow, the phase comparator being arranged to compare the phase of a loop signal of the phase-locked-loop circuit with the phase of the oscillating signal received at the other transducer after passing through the fluid flow, wherein the frequencies of both the loop signal and the received signal are divided by a predetermined factor before being compared by the phase comparator.

As a result of using this frequency dividing technique, the stability of the PLL as against input noise is improved, since spurious phase changes of the oscillating signal in the delay path between the transducers are averaged out.

Also; the frequency of the oscillating signal can be set to be well above the audible frequency range, thus eliminating the phenomenon known as "whistling".

In a preferred embodiment of the invention, to be described in greater detail hereinafter, a phase-setting circuit is provided for synchronising the first and second dividers so that their respective divided output signals are in-phase, so as to enable a successful acquisition by the PLL.

Another problem with the previously-proposed systems is that, when the transmitting and receiving transducers are disposed so as to allow the ultrasonic beam to pass through the fluid flow path within a pipe or conduit, the actual disposition of the transducers has caused a number of difficulties such as disturbance of the flow (which then affects the measurement result), reflection of ultrasonic energy from the conduit walls resulting in interference with the main ultrasonic beam, and/or, when the transducers are recessed in cavities away from the main fluid flow, resonance in the cavities interfering with the ultrasonic beam.

According to a further aspect of the invention there is provided a transducer arrangement in a conduit for measuring fluid flow within the conduit, wherein the conduit has first and second curved portions with first and second recessed sections in the first and second portions respectively, and therein first and second transducers are respectively disposed in the first and second recessed portions, the arrangement being such that there is a direct path between the transducers through the fluid flow path within the conduit.

It is found that a transducer arrangement of this type causes minimal disturbance on the fluid flow, avoids reflections off the walls of the conduit, and, due to the fact that the harmonic length of the recess in which each transducer is located varies around its periphery at the outlet to the main conduit (since the recess is located on a curved part of the conduit resonant effects do not occur.

The invention will now be further described by way of illustrative and non-limiting example, with reference to the accompanying drawings in which: Figure 1 is a block schematic diagram of a circuit according to a preferred embodiment of the invention; Figure 2 is a block diagram showing a modification to part of the circuit shown in Figure 1; and Figure 3 is a cross-sectional view of a conduit having an ultrasonic beam path arranged to optimise fluid flow measurement.

Referring to Figure 1, there is shown fluid flow measurement apparatus according to one embodiment of the invention. A phase-locked-loop (PLL) includes, in known manner, a phase comparator 10 for comparing the phases of a loop signal of the PLL and an input signal, a low-pass filter (LPF) 12 receiving the output of the phase comparator 10, and a voltage controlled oscillator (VCO) 14 receiving the filtered output of the LPF 12. In addition, the PLL includes a dividing circuit 16 in the loop path from the VCO 14 back to the phase comparator 10. The dividing circuit 16 is arranged to divide the frequency f1 of the oscillating signal produced by the VCO 14 by a factor n to provide a divided signal fn1 for phase comparison in the phase comparator 10.The dividing circuit 16 can in known manner be realised by a counter arranged to count up to n. The output of the VCO 14 is also connected to a transmitting transducer 18 from which the oscillating signal (at ultrasonic frequency) causes an ultrasonic beam to be directed through a fluid flow path to a receiving transducer 20.

Depending on the speed of fluid flow, the frequency f1 of the oscillating signal is changed to a received frequency f2 at the receiving transducer 20.

The signal from the receiving transducer 20 is applied to another dividing circuit 22, also arranged to divide by the factor n, the output signal from the dividing circuit 22 being at a frequency fn2. This signal is then fed to the phase comparator 10 for comparison with the divided signal at frequency fn1 from the dividing circuit 16 in the PLL.

The output of the VCO 14 may also be connected to an indicating means 24 which provides an indication of fluid flow speed dependent on the frequency f 1 of the oscillating signal from the VCO 14. The indicating means 24 can take any required form; for example, it can sum or integrate pulses of the oscillating signal whereby when a "unit" of fluid flow has been detected, a count in the indicating means is incremented by 1. As an alternative, the indicating means can be made responsive to the voltage supplied to the input of the VCO 14 whereby the signal indicative of fluid flow is a voltage rather than a frequency.

In addition, the apparatus shown in Figure 1 includes a phase-setting circuit 26 for initially putting the dividing circuits 16 and 22 into phase with each other so that the divided signals fn1 and fn2 are in phase to enable comparison by the phase comparator 10 and successful acquisition by the PLL. The phase-setting circuit 26 is also connected to the low-pass filter 12 so that, initially, the dividing circuits 16 and 22 can be held in a reset state and the output of the low-pass filter 12 can provide a control voltage to the VCO 14 which is approximately at a middle value, until the transmitted and received signals are approximately in phase.At that point, the reset state of the dividing circuits 16 and 22 is inhibited and the output of the low-pass filter 12 is no longer held at the previous value but is allowed to respond to the output of the phase comparator 10.

In more detail, the operation of the circuit shown in Figure 1 is as follows.

The VCO 14 generates an ultrasonic oscillating signal at the frequency f1 and which is arranged to produce n full waves in the delay path between the transducers 18 and 20. As mentioned previously, it is desirable to maintain a plurality of full waves in the flow path, in order to achieve higher resolution and sensitivity than would be possible if only one wave were to be used. In practice, it has been found that an effective number of waves between the transducers is 7. However, operating the PLL at a frequency arranged to provide seven full standing waves in the path between the transducers leads to a slowing in the response of the PLL.If this were not the case, there would be a risk that a rapid change in the fluid flow speed might cause the path between the transducers to have substantially a different number of waves (for example six or eight if the intended number is seven) whereby the PLL would lock onto this incorrect number of waves, resulting in inaccurate measurement.

In order to overcome this, the PLL is designed to operate in a manner as if there were only one standing wave in the delay path between the transducers 18 and 20. This is achieved by the use of the two dividing circuits 16 and 22 which divide the frequencies f1, f2 of the respective oscillating signals by the factor n (n being 7 in the example given). In other words, each dividing circuit outputs the nth subharmonic of the frequency f1 or f2. It is these two divided frequencies fn1 and f n2 which are locked in phase by the operation of the PLL comprising the phase comparator 10, the low-pass filter 12 and the VCO 14. Accordingly, the PLL operates at the lower divided frequency and in the same way as if there were only one standing wave in the delay path between the transducers 18 and 20.

In order to aid fast acquisition, the phase-setting circuit 26 is operative upon start-up or any interruption in operation such as loss of tracking, by providing reset signals to the dividing circuits 16 and 20, and holding the VCO control voltage at an intermediate value between its extreme values. Only when the wave form has progressed through the delay path between the transducers 18 and 20, and the input and output signals are approximately in phase, are the reset signals removed from the dividing circuits 16 and 22 and the VCO 14 allowed to respond to the control voltage within the PLL.

The circuit as described has the advantage that the stability of the PLL is improved with respect to the input noise, since spurious phase changes of the oscillating signal in the delay path between the transducers will be averaged out. Also, since the frequency at which the PLL operates is a subharmonic of the actual oscillating signal transmitted between the transducers, the transmission frequency can be well above the audible frequency range thus eliminating "whistling".

In order to average out errors arising from the physical conditions of the flow measurement such as the geometrical arrangement of the transducers relative to the fluid flow, it is possible to alternate the functions of the transducers 18 and 20 between transmission and reception.

Figure 2 shows an arrangement of this type, in which a switching circuit is connected between, on the one hand, the output of the VCO 14 and the input of the dividing circuit 22, and on the other hand, the transducers 18 and 20.

Connection to the transducers 18 and 20 is alternated by the switching circuit 28 such that for part of the time the transducer 18 transmits while the transducer 20 receives, and for the rest of the time, the transducer 20 transmits and the transducer 18 receives. Further details of the switching circuit 28 can be derived from US Patent No 4,320,666.

Figure 3 shows a particularly effective arrangement of transducers associated with a pipe or conduit, for measurement of the fluid flow through the conduit. The conduit is preferably arranged in a U-shape, with curved portions 30 and 32 having respective recesses 34 and 36 arranged in the outer walls of the curved portions 30 and 32. The recesses 34 and 36 open out to house the transducers 18 and 20, respectively. The recesses 34 and 36 are arranged relative to the conduit such that the beam path of the oscillating signal between the transducers passes through, aligned with and parallel to, the fluid flow. Thus, no geometric correction needs to be applied to the flow speed indication, as is necessary with arrangements where the ultrasonic signal path is along a diagonal with respect to the fluid flow path. Also, it is found that the arrangement of Figure 3 produces minimal interference of the fluid flow thereby ensuring accurate measurement results. Furthermore, since there is no constant harmonic length in the recesses, since the depth of each recess varies around its periphery at the outlets to the curved portion of the conduit, resonant effects (which may have interfered with the oscillating signal) do not occur.

As shown in Figure 3, it is preferable for a straight section 38 of the conduit to be provided between the curved portions 30 and 32, in order to ensure a sufficiently long delay path between the transducers 18 and 20.

Claims

1. Apparatus for measurement of fluid flow, the apparatus comprising: first and second transducers arranged along the path of the fluid flow such that an ultrasonic beam between the transducers can pass through the fluid flow; and a phase-locked-loop circuit including a phase comparator and a voltage controlled oscillator providing an oscillating signal to one of the transducers for transmission through the fluid flow, the phase-locked-loop circuit being responsive to the oscillating signal received at the other of the transducers after passing through the fluid flow;; wherein the phase-locked-loop circuit includes a first dividing means in the loop path from the voltage controlled oscillator to the phase comparator for dividing the frequency of the oscillating signal from the voltage controlled oscillator by a predeterminded factor and supplying the resulting divided signal to the phase comparator, and a second dividing means is arranged to divide the frequency of the oscillating signal received at the other of the transducers by the predetermined factor and to supply the resulting divided signal to the phase comparator, whereby the operating characteristics of the phase-locked-loop circuit are those associated with a frequency equal to the frequency of the voltage controlled oscillator divided by the predetermined factor.

2. Apparatus according to claim 1, including a phase-setting circuit for synchronising the first and second dividing means so that their respective divided signals are in phase to enable a successful acquisition by the phase-locked-loop circuit.

3. Apparatus according to claim 2, wherein the phase-setting circuit is arranged to hold the dividing means in a reset state and to hold the voltage controlled oscillator at a value intermediate its extreme values until the transmitted oscillating signal has been received at the other of the transducers, and the transmitted and received signals are approximately in phase.

4. Apparatus according to claim 1, claim 2 or claim 3, comprising direction switching means for alternately switching connections to the first and second transducers so that in a first state the first transducer transmits and the second transducer receives the oscillating signal, and in a second state the second transducer transmits and the first transducer receives the oscillating signal.

5. Apparatus according to any onevof the preceding claims, wherein the first and second transducers are located in a conduit through which the fluid is to flow, the conduit having first and second curved portions with first and second recessed sections in the first and second curved portions respectively, and wherein the first and second transducers are respectively disposed in the first and second recessed sections, the arrangement being such that there is a direct path between the transducers through the fluid flow path within the conduit.

6. Apparatus according to claim 5, wherein the path between the transducers is in parallel alignment with the fluid flow path.

7. Apparatus according to claim 6, wherein the conduit includes a straight portion between the curved portions, the path between the transducers extending through and parallel to the straight portion.

B. A method of measuring fluid flow in which a phase-locked-loop circuit having a phase comparator provides an oscillating signal transmitted as an ultrasonic beam from one transducer to another transducer through the fluid flow, the phase comparator being arranged to compare the phase of a loop signal of the phase-locked-loop circuit with the phase of the oscillating signal received at the other transducer after passing through the fluid flow, wherein the frequencies of both the loop signal and the received signal are divided by a predetermined factor before being compared by the phase comparator.

9. A transducer arrangement in a conduit for measuring fluid flow within the conduit, wherein the conduit has first and second curved portions with first and second recessed sections in the first and second portions respectively, and wherein first and second transducers are respectively disposed in the first and second recessed sections, the arrangement being such that there is a direct path between the transducers through the fluid flow path within the conduit.

10. Apparatus for measurement of fluid flow, the apparatus being substantially as hereinbefore described with reference to Figure 1, Figures 1 and 2, Figures 1 and 3 or Figures 1 to 3 of the accompanying drawings.

11. A method of measuring fluid flow, the method being substantially as hereinbefore described with reference to Figure 1, Figures 1 and 2, Figures 1 and 3 or Figures 1 to 3 of the accompanying drawings.

12. A transducer arrangement in a conduit for measuring fluid flow within the conduit, the transducer arrangement being substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.