GB2099146A - A phase difference flowmeter - Google Patents

Abstract

In conventional phase difference flowmeters acoustic waves are transmitted in directions upstream and downstream of the flow and the phase difference on reception is measured. This phase difference can be used to calculate the flow rate. Improved accuracy of measurement is provided by converting (5, 5A) the two received signals into square wave form. These square wave signals are then processed at 6 and 6A so as to produce further square wave signals having the same relative phase as the outputs of 5 and 5A but reduced in frequency by a common fixed value. Since the frequency is now considerably reduced the phase difference can easily be measured and this is done by diffentiators 7, 7A, flip flops 8, 8A and an AND gate 9, the latter producing an output pulse whose duration is proportional to the phase difference. This duration is measured by using the output of the gate 9 to enable a counter 10 which is fed with clock signals from the circuit 2. The output of counter 10 is a measure of the phase angle and from this, circuits 11, 12, 13 and 14 calculate the flow rate which is displayed at 15 and used to perform a control function at 16 and 17. <IMAGE>

Description

SPECIFICATION A phase difference flowmeter This invention relates to a phase difference flowmeter that is to say a flowmeter of the type comprising means for measuring the relative phase of first and second waves received after propagation in different directions through a medium flowing relative to the meter, thereby providing a signal related to the rate of flow.

Normally one of the said directions would be upstream and the other direction downstream.

Such flowmeters can be used for measuring the flow of fluids or solids and can be used in situations where the meter is stationary and the fluid or other medium is moving (e.g. when measuring the rate of flow of a fluid through a pipe) or in situations where the meter is moving and the fluid or other medium is relatively stationary (e.g. when measuring the air speed of an aircraft).

In existing phase difference flowmeters the technique used for measuring the phase difference is not sufficiently accurate for some purposes and it was with a view to meeting such requirements for accuracy that the present invention arose.

The invention provides a flowmeter of the type comprising means for measuring the relative phase of first and second waves receivedtafter propagation in different directions through a medium flowing relative to a meter, thereby providing a signal related to the rate of flow, characterised by means for producing, from the received first and second wave, respective first and second timing signals displaced by equal amounts below the frequencies of the received waves but having the same relative phase; and a timer for timing an intervale between a first and a second timing signal to produce a measure of the relative phase, this being related to the rate of flow.

The aforementioned waves are preferably acoustic or ultra-sonic.

One way in which the invention can be performed will now be described with reference to the accompanying drawing of an ultra-sonic flowmeter constructed in accordance with the invention.

Referring to the drawing, the output of 4 MHz oscillator 1, which output is in the form of a sine wave, is applied to a . 2 circuit 2 and the resulting 2 MHz sine wave is applied to ultra-sonic transducers 3 and 3A. The transmitter 3 transmits an ultra-sonic signal through a medium whose flow rate is to be measured, the direction of propagation of this ultra-sonic signal being downstream of the flow direction and at an angle which in the illustrated example is at 450 to the direction of flow. Similarly the transducer 3A transmits an ultra-sonic signal through the medium in a direction upstream of the direction of flow and at an angle of 450 relative to it. A transducer 4 receives the acoustic energy from the transducer 3 and a fourth transducer 4A receives the energy propagated from transducer 3A.It can readily be appreciated that, because of the flow of the medium, the time taken for the energy to flow from transducer 3 to transducer 4 will be less than the time taken for energy to travel from transducer 3A to transducer 4A, the time difference depending on the flow rate. Because of this difference in propagation time the output signals from transducers 4 and 4A will have a phase difference and the circuitry now to be described is for the purpose of accurately measuring this phase difference in order to derive a measure of the flow rate.

The outputs from the transducers 4 and 4A are passed to respective sine to square converters 5 and 5A which produce square waves at the same frequency and phase as the sine waves at their inputs.

The outputs of the converters 5 and 5A are passed to the D inputs of respective D-type flip flops 6 and 6A which receive their clock signals from the oscillator 1 via a s721 circuit 2A. Those familiar with the operation of D-type flip flop circuits will know that each clock input serves to set the Q output to the same binary level as the current value of the D input. The effect of this is that there appears at the output Q a waveform which is a square wave of a frequency equal to the frequency of the D input minus the frequency of the clock input. However, despite this frequency change the relative phases of the square wave outputs of the flip flops 6 and 6A is the same as the relative phase of their D inputs.

The outputs from the flip flops 6 and 6A are passed to respective differentiators 7 and 7A which act as leading edge detectors and produce timing signals coincident with the leading edges of their input waveforms.

Alternate timing signals from the differentiator 7 serve to set the output of a flip flop 8 to logic states 1 and 0 respectively so that the output of the flip flop 8 is at level 1 for time intervals equal to the repetition period of the square wave output of the D-type flip flop 6. Similarly the output of a flip flop 8A is at logic state 1 for times equal to the repetition period of the square wave output of the D-type flip flop 6A.

The outputs of the flip flops 8 and 8A are passed to an AND circuit 9 which produces output pulses whose durations represent the phase difference between the signals received by transducers 4 and 4A. This time period is measured using a counter 10 which is enabled by the output from gate 9 to count pulses received from the oscillator 1 via the . 2 circuit 2. In this particular example a count of 721 in the counter 10 represents a 3600 phase change and a count of zero indicates a zero phase change. The relationship between the count and phase angle is linear. Means is provided for resetting the counter 10 but this in not illustrated.

The output from the counter, representing the phase angle, is a digital signal consisting of ten bits presented in parallel. In order to calculate from this phase angle the velocity of the medium a simple mathematical process is carried out by circuits 11, 12, 13 andl4. The circuit 11 multiplies the output from the counter by a constant and the circuit 12 adds another constant to the output of the counter. The output of circuit 11 is divided by the output of circuit 12 at 13 and the result of this division is multiplied by another constant at 14 to produce a number which gives the velocity of flow in suitable units. This is displayed on a display 1 5. The output from the circuit 14 is also passed to a pump control circuit 1 5 which defines upper and lower thresholds.

When the flow rate as indicated by the output 14 falls below the lower threshold the pump control circuit increases the speed of operation of a pump 1 6 thereby increasing the flow rate. Conversely when the output of circuit 14 rises above the upper threshold the speed of operation of the pump 16 is reduced thereby reducing the flow rate. It will of course be appreciated that in some circumstances an automatic control system such as shown at 1 5 and 1 6 would not be required and that in other circumstances a visual display such as shown at 15 would not be required.

Claims (5)

CLAIMS 1. A flowmeter of the type comprising means for measuring the relative phase of first and second waves received after propagation in different directions through a medium flowing relative to the meter thereby providing a signal related to the rate of flow, characterised by means for producing, from the received first and second waves, respective first and second timing signals displaced by equal amounts below the frequencies of the received waves but having the same relative phase; and a timer for timing an interval between a first and second timing signal to produce a measure of the relative phase, this being related to the rate of flow. 2. A flowmeter according to claim 1 in which the said waves are acoustic or ultra-sonic. 3. A flowmeter according to claim 1 or 2 in which the means for producing the timing signals comprises: transducers for producing from the received first and second wave, first and second electrical signals in the form of sine waves; sine to square wave converting circuits for converting the said first and second electrical signals to square wave form; and means for reducing by equal amounts the frequencies of the square wave signals; and wherein the timer comprises a counter arranged to count clock signals during an interval between an edge in the first square wave signal and an edge in the second square wave signal. 4. A flowmeter substantially as described with reference to the accompanying drawing and substantially as illustrated in the accompanying drawing. New claims or amendments to claims filed on 25/9/81. Supersed claims 1-5. New or amended claims: CLAIMS

1. Apparatus for sensing flow rates by measuring the phase of a wave after propagation through a flow path relative to a reference signal comprising means for producing a square wave, from the first-mentioned wave, a logic circuit for receiving the square wave and for producing from it a reduced frequency signal whose frequency is a fixed amount less than that of the square wave and whose phase is related to that of the square wave and means for comparing the phase of the reduced frequency signal with the reference signal thereby obtaining an output related to the flow rate.

2. Apparatus according to claim 1 in which the signal propagated through the flow path is an acoustic ultrasonic wave.

3. Apparatus according to claim 1 or claim 2 including reference signal generating means comprising: means for propagating a second wave through the flow path in a different direction to that of the first wave; means for producing a second square wave from the said second wave; and a second logic circuit for receiving the second square wave and for producing from it a second reduced frequency signal whose frequency is less than that of the second square wave by the said fixed amount and whose phase is related to that of the second square wave; this second square wave constituting the reference signal.

4. Apparatus according to any preceding claim in which the or each logic circuit is a bistable and in which means is included to supply clock signals to the or each bistable, the bistable being constructed and arranged so that on receipt of a clock signal its output is set to a value dependent on the value of its square wave input.

5. A flowmeter substantially as described with reference to the accompanying drawing and as illustrated in the accompanying drawing.