DE2316437C2 - Device for measuring the speed or a speed component of the flow of a fluid - Google Patents

Description

The invention relates to an apparatus for measuring speed or a speed component a fluid of the type specified in the preamble of claim 1.

In British patent specification 12 85 175 a flow meter is described which has an output frequency which is proportional to the flow rate. This output frequency is the difference frequency fd of "upstream" and "downstream" - "sing-around" pulse chains. By "singaround" pulse trains are meant pulse trains that are generated by triggering the transducer again when a pulse is received by the receiver.

The frequency of the "upstream" pulse train
, 1 CV

The frequency of the »downstream sw pulse train
1 C + V

It is

V - velocity of the fluid,
C = speed of sound in the fluid,

L = distance between the transducers,
/ ι = "upstream" transit time - that is, the time it takes for impulses to be transmitted from the transmitter to the receiver,
t ₂ = »downstream SK running time,

From which follows
fa = / ι

2 V

The output time for such a flow meter is the time it takes to /, / to the required Measure accuracy. The long times required for this output or for this readout for low Flow velocities are required, can be a disadvantage in some applications.

The output line can be reduced by a technique using variable frequency oscillators as described in US Pat. No. 3,625,057 or US Pat. No. 3,420,102.

A disadvantage of the known devices is the use of separate paths through the fluid through for upstream and downstream measurements. Very small turbulence in the Flow or small fluctuations in fluid temperature or density between the two paths can invalidate the measurements made by comparing the two paths.

DE-OS 19 08 511 describes a system in which the same path through the fluid for upstream and downstream measurements is used. However, a direct analog time pulse measurement is used to compare the difference in the propagation times of the respective ultrasonic pulses im Fluid used.

However, there are a number of difficulties which the known devices have not sufficiently overcome will. So give; there are many reflections of ultrasound from different parts of the system, such as pipe walls, Turbulence, bubbles or other objects in the flowing medium. These can lead to many undesirable Signals or lead to the interruption of the desired signals. The accuracy is critical depending on the exact and consistent structure of the "sing-around" frequency. A retriggering from one unwanted signal can produce an error that, even if it is correct over a large number of

ίο re-triggered »sing-around« signals averaged becomes, is still large compared to the small measured value

The invention is based on the problem of solving the described difficulties in an elegant and precise manner to overcome with a device that with good stability against the many undesirable or other disruptive effects that so often affect the output of ultrasonic flow measuring systems can make imprecise or even incomprehensible.

This object is achieved by the characterizing features of claim 1; Appropriate training can be found in the subclaims.

The device according to the invention is particularly suitable for measuring the total flow in rivers or large ones Pipes advantageous, with the transducers spaced across the entire width of the river or pipe are on a sloping path. in order to have a speed component on the way in this way to introduce. With such large baselines, the response time of systems, such as those in the British Patent 12 85 175 are unacceptably slow.

However, the device according to the invention is also advantageous for measuring flow in small pipelines. Transducers can be attached to the outlet side of the pipeline, doing so in this way no disturbance to the flow is introduced. The transducers are arranged axially offset and so angled and coupled to the pipeline that taking into account breaks at the various "^ wipe the ultrasonic impulses a corresponding Take the path, inclined as steeply as possible to the radial direction, in the flowing fluid. A flow of about 1 cm / second in a pipe about 10 cm in diameter would be a Require response time in minutes if a »singaro'jnd« system and a coincidence count can be used. In the device according to the invention, the Response time can easily be reduced to a fraction of a second.

The invention will now be described with reference to the drawing which shows it by way of example
1 shows a block diagram of an embodiment,

2 shows a block diagram of a further embodiment,

3 is a block diagram in greater detail of part of the construction of FIG. 1,
F i g. 4 different waveforms to show the relative timing of various processes or operations during

Fig. 5 is a block diagram of a modification the construction according to FIG. 1 reproduces.

In Fig. I the essential components of a flow meter are shown.

The output of this flow meter is the differential frequency of two voltage-controlled frequency-variable oscillators VFOI and VFO2, whose frequency

sequence exactly

be an impulse so that

N x

and / Vx-

can be set by control circuits which compare t \ with / VfvFoi and also h with Mvfcm.

'vfoi. fvFO2 are the time spans of the two controlled oscillators.

The output frequency is proportional to the flow velocity.

Ί h

2NV

For a similar water velocity and transducer spacing, this method results in an output proportional to a water velocity N times that of the method prescribed in British Patent 1285175, so that a readout is - times the time

can be done. A / can be any convenient number, such as 100 or 1000.

In the arrangement according to FIG. 1 the converters are not set up in such a way that they generate a "sing-around"; the transmitter is not retriggered by a received pulse. but by an impulse from a main oscillator 11, the oscillation period of which is greater than (i or tj

The speed resolution of this flow meter is, within limits, determined by the smallest Time difference between / | and / Vivroi or between fj and Mvfo2 can be resolved.

Electromechanical ultrasonic transducers I and 2 are arranged at a distance from one another so that they transmit sound wave pulses through the fluid in the direction in which the speed component is to be measured.

It appears expedient to change the mode of operation of the arrangement according to FIG. I for one direction of transmission to describe, with the opposite direction being similar

The main oscillator U supplies a chain of "start" pulses, the period of which is expediently about

10% or 20% longer than -p , where

L is the distance between transducers 1 and 2 and C is the speed of sound in the fluid

Each »start« pulse enables a trigger pulse for the converter I via a driver circuit TXI, as soon as the next pulse from the oscillator VFOl appears after this "start pulse".

A logic! Circuit compares the running time of the received pulse from the converter 2 through an amplifier RXl with the time at which the / V-th pulse from Oscillator VFOl appears depending on whether the received pulse is before or after the / V-th VFOl impulse appears becomes a control -! - circuit causes a small change in the control voltage to be made to the oscillator VFOl to increase or decrease its frequency, respectively. To a number of transmissions will be the af-te VFOl pulse in time coincidence with the received is where

fi is the time that the ultrasonic pulse is on his Way between the converters.

Similar transmissions are made in the opposite direction from converter 2 to converter 1, at a VFO2 frequency of

to obtain.

The individual parts for this process are the same as the corresponding, above-described items are provided with reference numerals, but using the Index number 2 instead of index number 1.

A differential circuit 12 provides an output the frequency of which is proportional to the velocity of the fluid between the transducers.

/ vfoi ~ / vfo2

2NV

A scanner 13 generates a pulse that each

jo start impulse after a specified time delay which is set by a delay circuit 14 follows. The sampling period is arranged so that includes the expected time of arrival of a received pulse. If no pulse received in this Ab tustzcit as a result of an obstacle in the fluid away, this is recognized by the logic as an error condition, and no change in the VFO control voltage is applied. A temporary disability or disorder that only

nigc transmissions impaired, may be admitted will. A permanent malfunction is recognized and reading out is blocked.

F i g. 3 shows some parts of FIG. 1 more in detail. Again, the process is only carried out in one direction of travel considered. The VFO frequency must be accurate

/ Vx-L
t \

where J _{1 is} the time it takes for an ultrasonic pulse to travel from the sending transducer to the receiving transducer

The transmitting transducer is triggered in time with a VFO pulse and a comparison is made between the line up to the / Vth VFO pulses and the time that elapses before a pulse is received at the receiving transducer. Any difference between these times creates an error voltage which controls the VFO frequency. After a sufficient number of transmissions, a time match is achieved between the Nth VFO and the received pulse, ie / vr> is exactly the same

/ vx-L

With reference to Fig.3 and Fig.4, the process is following:

A start pulse (referred to as "reset" in FIGS. 3 and 4) from the main oscillator 11 resets flip-flop circuits 15 and 16, resets the -f -N counter 17 and sets the start -Flip-flop 18 back.

At the end of the reset pulse, the - = - A / -Zäh-Lt. ' 17 to count on.

When a next VFO pulse (n · =]) is fed to the -f- / V-counter 17, the “start” flip-flop circuit 18 is set and its output (see TX-TRIGGER in Fig. 4 ) triggers the driver circuit TX of the transmitter, ie the transmitting converter is controlled in time with a VFO pulse.

Either the / Vth VFO pulse (output from the -7 · counter) can set the flip-flop circuit 16, or the received pulse RX can set the flip-flop circuit 15, whichever appears first. The first Fiiptiop graduation prevents the other.

Depending on whether the flip-flop circuit 16 or the flip-flop circuit 15 is operating, the VFO then requires a control voltage increase to reduce or increase its frequency, respectively.

When the flip-flop circuit 15 operates, this triggers a monostable circuit 19 which switches on an input current for an integrator 22 via a level shift circuit 21 which changes its output by a small positive increase. This change in the VFO control voltage increases the VFO frequency somewhat.

If, on the other hand, the flip-flop circuit 16 operates, then its monostable circuit 23 delivers over a level shift circuit 24 an input current of opposite polarity in order to this Way to change the integrator output by a small negative increment and lower the VFO frequency.

After a number of transmissions the VFÖ frequency becomes very accurate

, vx-L

40

adjusted, and successive transmissions produce errors of opposite polarity, so that the VFO frequency is alternately slightly lower and higher than

so

is regulated.

The integrator 22 is set to have a gives a sufficiently small increase when triggered by the level shift circuit 21 or the level shift sound 24 to obtain the best resolution to achieve that is compatible with the required response time and the other sources of error in the system However, such small increases can result in a long delay in achieving coincidence in the initial build-up if there is initially a widespread discrepancy between the arrival signs of the received RX pulse and the »A / th VFOw pulse is present. The integrator 22 is therefore at this Execution example! with a manually operated Control is provided for increasing the size of the increment so that an approximated coincidence quickly can be achieved.

The smallest time difference that is resolved between received RX and "Wth VFO" pulses is about 6 nanoseconds, and this gives a speed resolution that is better than that required for most use cases.

The speed resolution actually achieved is always better than indicated by considering the time resolution of the circuit. Even with stagnant fluids or very constant ones Flow velocities there are always changes in the transmission times of the ultrasound around the mean time. Although this time changes to a result in greater oscillation of the VFO frequency around the true mean value than the alternating positive or negative errors described above, see above the average VFO frequency is closer to this mean.

If there is an obstacle or a fault between the converters and no RX pulse is received, see below there is no correction of the VFO frequency. This obstacle or this disturbance is recognized by a fault circuit (not shown), which the operation of the Flip-flop circuit 26 blocks. The integrator has a long decay constant, so a small number of »Wrong transmissions generated a negligible error. A fault for more than a few transmissions is recognized by the "error" circuit, and the speed reading is prevented until the fault is eliminated.

F i g. Figure 2 shows another arrangement in which speed resolution is improved at the expense of response time. In this method, two voltage-controlled variable-frequency oscillators VFOl and VF02 are used, with their Frequencies are controlled so that they

and

but the control voltage is obtained by comparing between Mti and A / Mvfoi and between AZf ₂ and MNtVfO ₂ . M means how many times each channel is given the opportunity to perform a "singaround" after each main oscillator pulse. The period of this oscillator must be greater than Mt \ or Mh . In this way, the speed resolution is improved by a factor M over the system (as in FIG. 1), using multiplying VFO ¹ S without "sing-around". By selecting M accordingly, a satisfactory compromise between the speed resolution and the response time can easily be set for each selected measurement.

As with F i g. 1, it is appropriate to operate for the to describe a direction of transmission, the opposite direction being similar

The main oscillator 25 supplies a train of pulses, the period of which is greater than ~ q ~

Each start pulse from the main oscillator 25 gives a transmitter-driver circuit TX1 the opportunity to deliver a driver pulse to the converter 1 as soon as it is the next VFOl pulse after this start pulse appears.

The resulting ultrasonic pulse travels through the fluid through from converter 1 to converter 2. The received pulse at the amplifier RXl output immediately triggers the driver circuit TXl,

9 10

ν to add a further drive pulse for the converter 1. For each fault condition, a trigger pulse for the

■ generate j; d. h, the system performs a "sing-around". Driver circuit TX generated in order to achieve the "sing-around"

Ϊ The M-lc received pulse at the amplifier RXI trig- maintain, but there is no change in the

The driver circuit TX1 doesn’t get it again, so only VFO control pairing.

f. ' M »sing-arounds« appear. In this embodiment s A temporary or transient fault that only has a

for example t / is chosen in such a way that not so many »single couple transmissions are impaired, can be tolerated

i |: arounds «exist as that of any truth. A longer fault is recognized and the AbIc-

; ':: apparently wedge of the coincidence between transmission and sung or output is prevented.

is'j receiving is introduced at one of the transducers. Since the converters do not receive pulses at the same time

Sk The logic 1 circuit compares the arrival time io gen, it is possible to use a single amplifier as a replacement

§ of the M-th received pulse with the time at which for the amplifiers RX1 AMP and RX2 AMP in FIG. 1

\\ ^l the MxN-th VFOl pulse after the start pulse to be used. A corresponding redesign of the

. ■; appears; d. That is, there is a comparison between Mt and the circuit shown in FIG. 5 illustrated which some additional

p] AWl VFOi instead of showing improvements.

Φ depending on whether the received impulse is before or after 15 after F i g. 5, a master oscillator 35 provides a chain

s | appears to the MAAten, a small change in pulses, the period of which is greater than L / C, is sent to each

fjj of the voltage is carried out by the control 1 circuit, the trigger TX1 and TX2, the pulses for triggering

t | to increase the VFOl frequency or lower TX2 relative to the pulses to trigger TXl time

£ add ». are offset; a meeting of receptionists

: After a number of these periods of the »sing and transmit pulses to be avoided at the transducers.

j! arounds «is the ΛίΛΖ-th VFOl pulse in time. A pulse from the main oscillator 35 controls the

coincidence with the M-th pulse from the amplifier RXI TXl trigger, which the driver circuit TXl when

'<so that, as in the previous procedure, the next VFOl pulse is triggered. The exit from

Trigger TX1 also sets up the 4 * ZV counter 36

/ yjQ, mfifxJ- [/, - * ■) 25 n-1 back.

; t \ \ CV) The resulting ultrasonic pulse travels through

the fluid through from the transducer 1 to

is converter 2. The received pulse is used to amplify

Transmissions are forwarded in the corer 37 in a similar manner.

opposite direction from transducer 2 to transducer 30. A precise time reference is received with this

1 carried out to detect the pulse by a zero crossing detector 38.

. ν builds. This is a feature which is an improved

/ vfoj -Nx- (I ₇ - ~ - ) resolution enabled.

/ j \ C + V / The output from zero crossing detector 38 becomes

35 via a switched scanner AND gate 39 to the logi-

to obtain. see! circuit and transmitted to control 1 circuit

The flow meter output, proportional to the number of parts corresponding to the components in operation in Water speed, is the difference frequency of the F i g. 1 are provided with the same reference numerals. So

both VFÖ's. the logic 1 circuit compares the time of arrival

ν ν 0 vi / ⁴⁰ ^ ^{β8 em} captured signal from converter 2 with the

/ * - / vpoi - / vfo2 ----- H-. Time at which the A / th pulse from the oscillator VFOl

* i h L, appears here. Depending on whether the pulse received

appears before or after the A / th VFOl pulse, will

It can be seen that the circuit of FIG. 1 and 3 causes the control 1 circuit to make a small change without further ado for this limited sing-around variation of the control voltage for the oscillator VFO1. The operation is similar, with the exception of increasing its frequency or taking it off after each main oscillator trigger pulse. After a number of transmissions a counter has to divide by Λ / Λ /, and the flip-flop switch, the A / th VFOl pulse will work in time with the MN-ttn VFO pulse and when minging with the received pulse. M-ttn received RX pulse. 50 The signal from the main oscillator 35 controls exceptionally For a given error in the VFO frequency, the connected scanner AND gate 39 is via a delay device 41 and a scanner 42, Das thus M times as large, which is an effective time-delayed sampling signal, both the response

comparison between i and M _VFO1 results, which the i-FA = · ^{outsid hen} f ^{order. the} expected time of arrival of a ^{6 6} M 55 trapped impulses nerum, as well as the cherc corresponds to the one that is possible without a limited "sing switch to operate the received signal to the around". One thus achieves an increase in the logic 1 circuit, from M in the flow velocity resolution . The operation for a transmission from the transducer 2. Each trigger output to the driver circuit TX occurs in parallel with that for the transducer 1, and also generates a sampling pulse which is delayed 60 with the exception that the received signal is off the wall that a received pulse is within this sampler 1 arrives at the sampling time via the connected sampler AND gate 39 The trigger is blocked, unless logic 2-circuit is passed on during the sampling pulse in order to corresponding to Mi, to prevent re-triggering by unwanted signals and denoted by VF02 O / P, corresponding to Mi, if no pulse is received during the sampling pulse ic , a differential logic circuit 43 is received because of its obstacle or leads to the flow rate is indicated by a difference in a disturbance in the fluid path between the output (Mi-Mj). In this embodiment wall · im, it is indicated as an error The flow direction is determined by logical

Outputs are displayed that show either Nf \> Nf ₂ or Nf \ <Nf ₂ .

As in the arrangement according to FIG. 1, the integrator has a sufficient time constant, and it does not mind if some pulses are obstructed. The received pulses are monitored by the amplifier 37 and, indicated as RX1 0 / P and RX2 0 / P, transmitted to a logic error circuit 44. This circuit indicates an error and blocks reading out if less than a preset minimum percentage of pulses is received. In the typical case it has been found satisfactory to set the error circuit so that it comes into operation when less than 10% of the transmitted pulses are received by one of the transducers. In practice it usually turns out that either all pulses are received or that there is some interference if none are received. One vorübsr ^cfi "h £ r dfi, volatile fault! Beis as ⁿ ielsweise a voi beifahrendes boat where the river currents to überwac'Sen is, some pulses may unierbrechen, but will not interfere with the reading in general.

It goes without saying that the arrangement according to FIG. 2 can be modified to include a single amplifier in one of the F i g. 5 similar ways to use.

The invention is. not limited to details of the exemplary embodiments described above. For example it is preferred to have two variable frequency oscillators and associated control circuits too so that the measurements can proceed in parallel in the opposite directions, where the only condition is that there is a sufficient difference in timing between individual pulses in order to to avoid a coincidence of sending and receiving at any transducer with the following recipient block. The main advantages of this type of measurement are as follows:

(1) The ultrasonic measuring impulses plant themselves almost entirely Mg at the same time across the fluid path, so that in this way any possible effects on the transit speed measurement due to temperature, density or other changes in the fluid are minimized, and

(2) it's a straightforward and simple thing, a frequency difference between two oscillators operating at the same time, and the output is a continuous display of the flow rate although flow rate changes is not followed immediately, they are followed very quickly, especially with the Embodiment according to FIG. 1.

55

If desired, however, it would be possible to use a single voltage controlled oscillator with associated control circuitry and successive determinations of, initially, the frequency necessary for N pulses to fit into the length of time it takes for sound waves to pass from the v / andler 1 to get to transducer 2, and, below, the frequency that is required so that N pulses fit into the period of time that sound waves need to get from transducer 2 to transducer 1. Such a system would. Require memories to hold the first frequency while the second frequency is determined, after which the difference frequency can be found by subtraction.

In addition, it is undoubtedly extremely convenient and simple to set up the number Λ / νοη pulses of the variable frequency oscillator so that it is the same within each time period, so that the difference frequency has a simple proportionality to the flow rate. However, a different number of pulses could be used, for example N \ pulses for the upstream measurement and Nj pulses for the downstream measurement. ? »- measurement, use. It would then simply be necessary to adjust the differential output by a constant factor when calculating the flow velocity.

For this purpose 4 sheets of drawings

Claims (9)

Patent claims: 1. Device for measuring the speed or a speed component of the flow s a fluid consisting of a first and a second unit, which are arranged at a distance from each other and each emit and receive sound waves that travel over a path through the Flow means pass through in the direction in which the speed or speed component is to be measured, from a timing device, from a trigger device for controlling the transmission of pulses in both directions via the path between the first and two th unit as well as from a device that provides an output that corresponds to the difference in the transit times of the Impulses in the respectively opposite directions on the way are related thereby characterized in that the trigger device (TRIG; TRIGGER 1, TRIGGER 2; TX1 -TRIGGER, TX2-TRIGGER) operates under the control of the timing device (11; 25; 35) to allow sound waves from the first unit (transducer 1) to the second Unit (converter 2) separately, but via the same path through the fluid as from the second unit (converter 2) to the first unit (converter 1) are transmitted that a control device (controller 1; controller 2) controls the frequency of a frequency-variable oscillator device (VFOl; VFO2) to a predetermined plurality ΛΛ of oscillations or .impulse- ", in a first period of time that corresponds to that a sound wave pulse on its way ve; the first unit (transducer 1) to the second unit (transducer 2) needs and a predetermined plurality Ni of vibrations or pulses to generate in a second period of time which corresponds to that which a sound wave pulse on its way from the second unit (converter 2) to the first unit (converter 1) and that the device for supplying an output (12; Nf \ - AWj; 43) the difference in acT frequency between the oscillations or pulses generated in the first period of time , and the vibrations or pulses generated in the second period of time. 2. Apparatus according to claim 1, characterized in that the control device (control 1; control 2) has a logic circuit (15, 16) for comparing the times of appearance of the N \ -th oscillation odsr of the Λ / ι-th pulse with the arrival time of the sound wave pulse at the second unit (transducer 2) and for comparing the time of appearance of the Λ / 2-th oscillation or the / vVth pulse following a given pulse transmission and the arrival time of the sound wave pulse at the first unit (transducer 1), where the oscillation or pulse repetition frequency is adjusted in such a way that it strives to bring these times of appearance of the N \ «th or / Vrth oscillation bo or of the Λ / ι-th or / Vj-th pulse into agreement with the arrival time of the corresponding sound wave pulse, wherein the difference determining device (12; Nf \ - Nfr, 43) the difference in frequency between the vibrations or pulses generated in the first (, 5th time period and the cr / cugicn oscillations or impulses in the second period of time. 3. Apparatus according to claim 2, characterized by a locking device (LOGIC 1; LOGIC 2) for Locks the setting of the oscillation or pulse repetition frequency in the event that none Sound wave pulse is received within a predetermined period of time which includes the expected time of arrival 4. Apparatus according to any one of claims 1 to 3, characterized in that N \ - Λ / 2— Ν 5. Apparatus according to one of claims I to 4, characterized in that the control device (Control 1; Control 2) the frequency of the variable frequency Oziüatoreinrichtung (VFOl; VFO2) by a fixed step change in the frequency each time one is sent and received Sound wave pulse regulates 6. Apparatus according to claim 5, characterized in that means for temporary Adjustment of the size of the step change in the frequency is provided 7. Apparatus according to one of claims ί to 5, characterized in that the operation is controlled is that each subsequent sound wave pulse across the path through the fluid is in the opposite direction to the previous sound wave pulse Direction runs 8.G erät according to claim 1, characterized in that the trigger device has a first (TRIGGER1) and a second (TRIGGER2) trigger, that first trigger control devices (RX 1, -fMJ are provided which cause the first trigger (TRIGGER 1) of the trigger devices, the first unit (converter 1 to trigger) upon receipt of a pulse by the second unit (transducer 2) again, and the 1) prevent the first trigger (tRIGGER to a further re-triggering when a predetermined plurality M \ of pulses from the first unit (transformer 1) to second unit (converter 2) has been transmitted that second trigger control devices (RX2, -r- M) are provided, which cause a second trigger (TRIGGER 2), the second unit (converter 2) when a pulse is received by the first unit ( Converter I) to trigger again, and which prevent the second trigger (TRIGGER 2) from being triggered again if a predetermined number Mi of pulses from the second unit (Wa transmitter 2) has been transmitted to the first unit (wall! er!) that the time control device (25) is set with respect to the pluralities M \ and Mi in such a way that any probability of coincidence between transmission and reception at each unit is avoided, and that said variable frequency Oszillatoreinrichtiing (VFO 1, VF02) is controlled so that it generates a plurality M \ N \ vibrations or pulses in the time of M \ "sing-around" transmission ": n of acoustic wave pulses from the first unit to the second unit is required, and to generate a large number of M2N2 oscillations or pulses in the time required for M ₂ "sing-around" transmission of sound wave pulses from the second unit to the first unit. 9. Apparatus according to any one of claims 1 to 8, characterized in that a time reference value at received sound wave pulses is built up by a Nullcliirchgang detector (38).