DE102017005208A1 - Method for transit time measurement of an ultrasonic signal in a fluid - Google Patents

Abstract

The present invention relates to a method for ultrasonic transit time measurement in a fluid, in which
a fluid flowing through a measuring path is acted on by a first transmission signal generated by an ultrasonic transducer with a first frequency f ₁ and a second transmission signal generated by the ultrasonic transducer with a second frequency f ₂ ,
receive the first and second transmit signals from an ultrasound transducer and generate first and second receive signals each having a plurality of waves,
the transit time difference is determined from the received signals,
a phase difference (PD) between the first received signal having the first frequency f ₁ and the second received signal having the second frequency f _{2 is} measured, and
from the measured phase difference (PD), the position of a measuring point in the region of the waves (wave train) of the received signal is derived.

Description

The present invention relates to methods for measuring the transit time of an ultrasonic signal in a fluid or medium according to the preamble of claim 1.

Technological background

Ultrasonic flowmeters are based on the measurement of the propagation time of sound waves, which is measured either as an absolute transit time or as a difference from a reference transit time or as an absolute transit time and as a difference from a reference transit time. For absolute transit time measurement, so-called level and envelope methods are usually used. However, reliable measurements are associated with a considerable complexity of implementation. The implementation is mostly implemented using so-called ASICs (Application Specific Integrated Circuits).

In transit time difference method, the transit time difference of the sound waves is measured with and against the media movement. Due to the periodic repetition of the phase positions, however, only transit time differences can be measured which are smaller than the period of the signal frequency used. This can lead to problems at high medium flows. In addition, no absolute transit time measurements are possible. The transit time can be determined in this method by determining the transit time difference with respect to a reference time having a known transit time.

In addition, the transit times of ultrasonic signals in a fluid and derived therefrom runtime differences are also dependent on the temperature of the fluid. This results in the need either to detect the fluid temperature by means of a temperature sensor or to detect a variable correlated to the fluid temperature, such as the absolute signal delay for calculating the fluid temperature, and to include the fluid temperature in determining the flow rate.

Nearest prior art

From the DE 198 41 154 A1 a method according to the preamble of claim 1 is known. For measuring the sound propagation time in media with a strong or strongly fluctuating sound attenuation is proposed to radiate sound from a sound sensor two different frequencies and received again, wherein for each received signal, the phase shift with respect to a respectively associated reference signal measured and from the measured phase shifts Running time is calculated. The frequency of the one sound signal corresponds to the resonance frequency of the ultrasonic transducer, that of the other sound signal is about 10% higher.

In the US 4 527 432 is a method and apparatus for ultrasonic flow measurement of fuel for optimizing the fuel supply in a vehicle with an extended measuring range described in which in each case the phase difference between a transmission signal and a received signal of a first lower frequency and the phase difference between a transmission signal and a received signal second high frequency in each case both in and against the flow direction of the fluid can be measured. Subsequently, the phase difference of the lower frequency is subtracted from the phase difference of the higher frequency and from this the difference in the number of wave trains in each direction is determined. This difference is proportional to the flow rate.

From the DE 10 2011 016 963 A1 a method for ultrasonic flow measurement according to the transit time difference method is known. Here, too, a phase difference between a transmission signal and a reception signal of a first lower frequency and the phase difference between a transmission signal and a reception signal of a second higher frequency are respectively measured in and against the flow direction of the fluid. The measured phase differences are then checked under a coincidence condition and the flow rate calculated on the basis of the coincidence consideration. As a result, the measuring range can be increased to more than one period.

The DE 10 2011 004 830 B4 describes a method for measuring the propagation velocity of sound waves with a dynamic time window. In this case, at least two sound waves with at least two different frequencies are fed into a measuring path one after the other. Furthermore, the phase angle of the received sound waves relative to an associated reference signal of the same frequency is determined. The sound frequencies used must be chosen so that the occurring phase angle difference between two adjacent frequencies is always less than 2π.

Object of the present invention

The object of the present invention is to provide a novel method of the generic type which, with the possibility of a simple implementation, enables an exact determination of the transit time of an ultrasonic signal even at high flow rates.

Solution of the task

The object of the present invention is achieved by the subject matter of claim 1. Advantageous embodiments of the invention are claimed in the further claims.

According to the invention, the position of a measuring point for the received signal of the first frequency and the position of a measuring point for the received signal of the second frequency relative to one another can be related to one another from the measured phase difference. Transmission signals of different frequencies propagate at the same speed in the medium to be measured. When these signals are received, a phase offset, which increases almost linearly from wave train to wave train of the received signal, can be detected directly between the received signals of different measuring frequency. From this phase difference, the exact position of a measuring point or measuring window in the received wave train can be derived exactly down to a few nanoseconds. In turn, the absolute signal propagation time of the measurement signals in the medium can be determined without additional measurement of the temperature of the medium. Temperature sensors for measuring the absolute transit time are therefore not necessary. Advantageously, the invention makes it possible to determine the transit time difference over several wave trains. From this, the flow can be clearly determined. As a result, the invention enables an effective measuring range extension, in particular towards high flow rates. The measuring window can be set or enlarged accordingly.

To generate different ultrasonic frequencies as measurement frequencies only a simple circuit technology is needed, which has no influence on the basic structure of the measuring system.

The ultrasound measurements with different frequencies are preferably carried out alternately.

Conveniently, the first ultrasonic frequency represents a main frequency, the second ultrasonic frequency is an additional frequency. The measurement with the second frequency can take place either only for one measuring direction or optionally also for both measuring directions.

Preferably, the first and second transmission signals are bursts of signals, each having a plurality of waves. In this case, the method according to the invention is particularly advantageous, since previous methods with the emission of short bursts always depended on carrying out each transit time measurement on exactly the same receiving wave.

Preferably, more than two measurement frequencies, such. B. three measurement frequencies are used.

Characterized in that a common reference clock is predetermined for the first and second transmission signals and the determination of the position of a measuring point or measuring window in the ultrasonic transit time measurement always takes place with reference to the common reference clock, the measuring times can always be compared with the reference clock or the reference clock for the Comparison of the measuring points or the measuring window are used.

Preferably, a window signal having a duration of preferably more than one period of the waves of the received signal is generated, the temporal position of the window signal is determined by a (empirical) receiving position based on previous measurements and within the window signal the time of a zero crossing of a particular wave of the first or second ultrasonic received signal evaluated.

Conveniently, at the position of the measuring point within the measuring window, a measuring pulse can be determined, which preferably extends from the zero crossing to the end of the measuring window.

According to the invention, it is further provided that the deviation of the first ultrasonic frequency f ₁ and second ultrasonic frequency f _{2 should} be set so low in each case to the resonance frequency of the ultrasonic transducer that no phase jump occurs along the signal periods of the signal packet. As a result signal periods with a high signal level can be selected as the operating point for the transit time measurement. This results in a high signal transmission efficiency very close to the main resonance frequency of the ultrasonic transducer. As a result, in addition to the determination of the absolute signal propagation times, transit time differences can also be determined very precisely.

If the method is operated with a center frequency to the first and second ultrasonic frequency, which is close to the resonance frequency, this ensures the additional advantage that this results in a larger frequency difference and thus an even better evaluable, larger phase difference.

It is advantageous if the first and second ultrasonic frequencies are approximately symmetrical to the main resonance frequency or center frequency of the ultrasonic transducer. The main resonance frequency is the frequency with the highest transmittable amplitude.

When using a center frequency, the latter can be used to determine the transit time difference, wherein the two additional frequencies above and below the center frequency are preferably used only for determining the phase difference of the two frequencies for measuring position determination.

From the absolute runtime, it is further preferable to derive a test value for a plausibility check and / or a fault check and / or the mixing ratio of a two- or multi-phase mixture and / or at least one fluid-specific physical quantity.

list of figures

• 1 einen vereinfachten prinzipiellen Schaltungsaufbau zur Durchführung des erfindungsgemäßen Verfahrens;
• 2 einen beispielhaften Zusammenhang zwischen Sendewellen, Empfangswellen, Referenztakt und Messpuls für eine Frequenz im Rahmen der Durchführung des erfindungsgemäßen Verfahrens;
• 3 stark vereinfachte Wellenverläufe (Sende- und Empfangswellen) von Ultraschallsignalen zweier unterschiedlicher Frequenzen und ein PD/Messpositions-Diagramm bei der Durchführung des erfindungsgemäßen Messverfahrens;

An expedient embodiment of the method according to the invention will be explained in more detail below. Show it:

• 1 a simplified basic circuit construction for carrying out the method according to the invention;
• 2 an exemplary relationship between transmission waves, receive waves, reference clock and measuring pulse for a frequency in the context of carrying out the method according to the invention;
• 3 greatly simplified waveforms (transmitting and receiving waves) of ultrasonic signals of two different frequencies and a PD / measurement position diagram in carrying out the measuring method according to the invention;

In the 1 illustrated device shows an example of a possible structure for implementing the measuring method according to the invention. A central control and processing unit 1 Any type (analog / digital logic, a microcontroller or the like) receives from a clock generator 2 the higher-level system clock (reference clock), from which all other processes in the measuring system are derived. First, the control and processing unit 1 on an adjustable clock pattern generator 3 the desired measuring frequency via a setting signal AS and supplies the clock pattern generator 3 with a reference clock RT synchronous clock signal.

The actual start of an ultrasonic measurement takes place via the start signal SS to the clock pattern generator 3 , then a wave sequence over the transmission signal SES supplies. This transmission signal SES consists of waves of the selected measurement frequency, the measurement frequency is preferably selected so that they are possible in the range of ultrasonic transducers 8th . 9 resonant transferable frequencies, for example at the frequency of 1 MHz for 1MHz ultrasonic transducers. In this case, the transmission signal in the sense of a burst may contain only a few waves or, as a quasi-continuous excitation, any number of waves.

About the transmission amplifier 4 becomes the transmission signal SES power amplified and to the impedance of the ultrasonic transducer 8th . 9 customized. With the directional signal RS controls the control and processing unit 1 the multiplexer 5 so that the desired direction of measurement can be achieved and the necessary ultrasonic transducer 8th respectively. 9 with the transmission signal SES can be supplied.

In the example shown is about an ultrasonic transducer 8th a sound signal through the medium 12 to another ultrasonic transducer 9 be sent. The transmission signal from the multiplexer 5 is to the ultrasonic transducer 8th passed this into a corresponding sound signal (burst) 13 transforms.

During the transmission of the sound signal 13 or sound wave train in the medium 12 switches the control and processing unit after a set transmission time 1 the multiplexer 5 over the directional signal RS from the sending ultrasonic transducer 8th on the receiving ultrasonic transducer 9 around. After passing through the medium 12 The sound signal reaches over the measured transit time 13 or the sound wave train the ultrasonic transducer 9 deriving therefrom a corresponding electrical received signal IT generated and sent to the multiplexer 5 continues to exist.

From the multiplexer 5 the received signal arrives IT to an optional input amplifier 6 which amplifies the signal level as needed and the receiving path against the receiving ultrasonic transducer 9 terminates with the appropriate impedance.

With a zero crossing detector 7 , For example, a comparator, the incoming initially analog signal into a digital received signal OF converted for further processing. The control and computing unit generates for the desired time measurement 1 a window signal FS (Measuring window) of a duration of more than one wavelength, whose temporal position in the received signal OF is determined by a reception position based on previous measurements.

Within this measurement window is a zero crossing of the received signal OF in the functional unit measuring pulse generator 10 evaluated so that a measuring pulse MP results in time from the detected zero crossing to the given window end. This is the case of the control and processing unit 1 generated measuring window end synchronous to the reference clock RT from which also the transmission signal was generated. From the measurement window position, which represents a multiple of reference clocks, and the Length of the measuring pulse MP Summation can be used to determine the total signal transit time between the start of the signal transmission and the start of the received signal:

The signal transit time results from the number of reference clocks to the end of the measurement window multiplied by the time length of a reference clock period, minus the time pulse duration minus the number of receive waves up to the measurement window position, multiplied by the time length of a receive wave period.

The analog measuring pulse length is from a time-to-digital converter 11 for further digital processing by the control and processing unit 1 digitized.

The relationship described above between transmit waves, receive waves, reference clock RT and measurement window MF is in 2 exemplified outlined for a reference clock RT with 16-times transmission frequency, a burst transmission with 8 waves and a measuring window position for the evaluation of the 8th receiving shaft. How out 2 can be seen, has the window signal or measurement window MF a duration of more than one period, it is in the range of the zero crossing of the 8th wave.

In 3 are each on the left side two ultrasonic transmission signals of different frequency f ₁ respectively. f ₂ shown, which alternately in the form of a sound wave packet (burst) with a finite number of signal periods, the fluid or medium 12 run through.

In the right area of the 3 the respective resulting received signals are shown, which can be tapped at the ultrasonic transducer in each case in the form of an electrical signal. The respectively in the 3 shown received signal includes z. B. a total of 16 signal periods. In the steady state of the ultrasonic transducer in the region of the 8th wave, ie in the middle region of the respectively received ultrasonic packet, the signal level is highest.

The measuring method is used in the example of 3 with two ultrasonic signals of different frequency f ₁ . f ₂ carried out. It can be seen that propagate signals of different frequencies at the same speed in the medium or fluid to be measured and when receiving these signals of different frequency from wave to wave of the received signal almost linearly increasing phase shift PD can be detected directly between the signals of different measuring frequency ie measured. From the measured phase difference t ( f ₁ ) - t ( f ₂ ), the exact position of a measuring point in the received wave train can be derived to within a few nanoseconds and placed in relation to the measuring point of the other frequency.

Due to the measurement point assignment on the basis of the phase frequency measurement at at least two different frequencies, the runtime can be unambiguously determined even over several signal periods (wave trains), which effectively leads to a measurement range extension to high fluid flow rates.

In addition, a specific determination of the frequencies f ₁ respectively. f ₂ the two ultrasonic signals are made. Advantageously, the frequency is f ₁ of an ultrasonic signal only slightly above the main resonance frequency of the ultrasonic transducer, whereas the frequency f ₂ of the second ultrasonic signal is only slightly below the main resonance frequency of the ultrasonic transducer. The difference between the respective frequency and the main resonant frequency of the ultrasound transducer should expediently be so small that no phase jump occurs along the signal periods of the received signal packet.

The frequencies used f ₁ respectively. f ₂ should preferably deviate in a range of less than 10% from the main resonance frequency of the ultrasonic transducer. Particularly preferably, the frequencies are each in the range of a deviation of 3 to 8% to the main resonance frequency. As a result, in the steady state of the ultrasonic transducer, the signal periods located there are available for a measuring window with particularly high level strengths for the measuring method.

The determination of the two ultrasonic frequencies f ₁ respectively. f ₂ to the main resonant frequency of the ultrasonic transducer should also be symmetrical beyond expediently. This makes it possible, with regard to the signal generation and evaluation, to provide a hardware structure common to both ultrasonic signals. This in turn ensures that errors or distortions cancel due to the opposition.

Does the ultrasonic transducer has a main resonant frequency of z. B. 1010 MHz, so z. B. the first ultrasonic frequency f ₁ at about 980 MHz and the second ultrasonic frequency f ₂ at about 1040 MHz.

How out 3 can be seen, for carrying out the measuring method at a first frequency f ₁ eg the zero crossing into the positive of a certain signal period (eg the in 3 exemplified third signal period) as a reference point REF in the measurement window MF used become. Alternatively, another signal period, for example the eighth signal period, at which the signal level is highest, could also be used. When measuring at a second frequency f ₂ that differ from the first frequency f ₁ differs, results for the corresponding zero crossing a time offset (phase difference PD ) against the reference point REF from which the position of the shaft (eg the third wave) in the receive packet (sum of all received waves) can be determined. Here is the phase difference PD between the two frequencies f ₁ respectively. f ₂ with respect to the selected reference point RP approximately directly proportional to the number of waves received to the reference point REF ,

In a measurement of the type described above in the forward and backward direction and a corresponding averaging, the absolute sound propagation time can be determined therefrom without the influence of a medium velocity. For a known length of the measuring section results for a known medium from the absolute sound propagation time directly the medium temperature.

For calibration of the measuring method, the characteristic order of magnitude of the phase difference to be evaluated can be determined in a simple manner using a known type of medium, with a known medium temperature and known medium properties PD be determined by reference measurements for the desired reception positions in the wave packet.

From the electronically determined difference of the signal propagation times with / against the fluid flow, ie the transit time difference, the flow rate of the fluid in the control and computing unit 1 be determined.

Advantageously, for a denoising of the measured variable phase offset multiple measurements with the frequencies f ₁ respectively. f ₂ preferably be carried out quickly in a row.

Likewise, a common measurement window can be used for a forward and backward measurement. Alternatively, two separate measurement windows can be used for the forward and backward measurement. The latter is at widely spaced phase values of z. B. more than one μs appropriate. Alternatively, two measurement windows for the transit time difference and the phase difference measurement can be provided.

Likewise, from the absolute duration of the first or second ultrasonic signal z. B. a test value for a plausibility check and / or a fault check and / or the mixing ratio of a two- or multi-phase mixture and / or at least one fluid-specific physical quantity are derived.

Alternatively, the method can also be operated with a center frequency near the resonant frequency f ₀ for determining the transit time difference, with two additional frequencies f ₁ and f ₂ above and below the center frequency can only be used to determine the phase difference of the two frequencies for position determination. This offers the advantage that due to the larger frequency difference between f ₁ and f ₂ an even better evaluable, larger phase difference arises.

LIST OF REFERENCE NUMBERS

1

Control and computing unit

2

clock generator

3

Clock pattern generator

4

transmission amplifier

5

multiplexer

6

input amplifier

7

Zero crossing detector

8th

ultrasound transducer

9

ultrasound transducer

10

Measuring pulse generator

11

Time-to-digital converter

12

medium

13

Sound signal (burst) Measuring direction 1

14

Sound signal (burst) Measuring direction 2

IT

receive signal

OF

digital received signal

FS

window signal

MF

measurement window

MP

measuring pulse

PD

phase difference

RS

direction signal

RT

reference clock

AS

adjustment

SS

start signal

SES

send signal

REF

reference point

f ₁

frequency

f ₂

frequency

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 19841154 A1 [0005]
• US 4527432 [0006]
• DE 102011016963 A1 [0007]
• DE 102011004830 B4 [0008]

Claims (14)

Method for ultrasonic transit time measurement in a fluid, in which a fluid flowing through a measuring path is acted upon by a first transmission signal generated by an ultrasonic transducer with a first frequency f ₁ and a second transmission signal generated by the ultrasonic transducer with a second frequency f ₂ , the first and second transmission signals received by an ultrasonic transducer and first and second received signals, each with a plurality of waves are generated from the received signals, the transit time difference is determined, a phase difference (PD) between the first received signal at the first frequency f ₁ and second received signal at the second frequency f _{2 is} measured characterized in that the position of a measuring point in the region of the waves (wave train) of the received signal is derived from the measured phase difference (PD). Method according to Claim 1 , characterized in that a reference clock common to the first and second transmission signals (RT) is predetermined and the determination of the position of the measuring point or measuring window in the ultrasonic transit time measurement is performed with reference to the common reference clock (RT). Method according to Claim 1 or 2 , characterized in that the phase differences to be evaluated are determined by preliminary measurements under known environmental conditions. Method according to Claim 1 or 2 , characterized in that at the position of the measuring point and / or measuring window in the region of the waves (wave train) of the received signal, a measuring pulse (MP) is generated. I Method according to at least one of the preceding claims, characterized in that the first ultrasonic frequency f ₁ above and the second ultrasonic frequency f ₂ below the resonance frequency f ₀ or a center frequency f _{m of} the ultrasonic transducer. Method according to at least one of the preceding claims, characterized in that the deviation of the first ultrasonic frequency f ₁ or second ultrasonic frequency f _{2 in} each case to the resonance frequency f ₀ or a center frequency f _{m of} the ultrasonic transducer is ≤ 10%, preferably in the range of 3-8%. lies. Method according to at least one of the preceding claims, characterized in that the first and second ultrasonic frequency f ₁ or f ₂ to the resonant frequency f ₀ or center frequency f _{m of} the ultrasonic transducer, preferably at least substantially, are symmetrical. Method according to at least one of the preceding claims, characterized in that an ultrasound frequency (eg the ultrasound frequency f ₁ ) is used as the fundamental frequency and the further ultrasound frequency (eg the ultrasound frequency f ₂ further) is used as additional frequency and the measurement of the additional frequency either only for one measuring direction or for both measuring directions. Method according to at least one of the preceding claims, characterized in that the position of two zero crossings of the first and second ultrasonic signal is used as a reference point for determining the absolute transit time of the first and / or second ultrasonic signal. Method according to at least one of the preceding claims, characterized in that the first ultrasound signal and the second ultrasound signal are alternately generated and received by the ultrasound transducer. Method according to at least one of the preceding claims, characterized in that the measuring range of the ultrasonic flow meter is greater than a wave train of the first and / or second ultrasonic signal. Method according to at least one of the preceding claims, characterized in that the method is operated without measuring the fluid temperature. Method according to at least one of the preceding claims, characterized in that the fluid temperature and / or the speed of sound in the fluid is derived from the absolute transit time of the first or second ultrasonic signal. Method according to at least one of the preceding claims, characterized in that from the absolute duration of the first or second ultrasonic signal, preferably in conjunction with a temperature measurement, either a test value for a plausibility check and / or a fault check and / or the mixing ratio of a two- or Multi-phase mixture and / or at least one fluid-specific physical quantity is derived.

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