Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S11/00—Systems for determining distance or velocity not using reflection or reradiation
  • G01S11/14—Systems for determining distance or velocity not using reflection or reradiation using ultrasonic, sonic, or infrasonic waves
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
  • G01F1/66—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by measuring frequency, phase shift or propagation time of electromagnetic or other waves, e.g. using ultrasonic flowmeters
  • G01F1/667—Arrangements of transducers for ultrasonic flowmeters; Circuits for operating ultrasonic flowmeters
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
  • G01F1/704—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow using marked regions or existing inhomogeneities within the fluid stream, e.g. statistically occurring variations in a fluid parameter
  • G01F1/708—Measuring the time taken to traverse a fixed distance
  • G01F1/712—Measuring the time taken to traverse a fixed distance using auto-correlation or cross-correlation detection means
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P5/00—Measuring speed of fluids, e.g. of air stream; Measuring speed of bodies relative to fluids, e.g. of ship, of aircraft
  • G01P5/18—Measuring speed of fluids, e.g. of air stream; Measuring speed of bodies relative to fluids, e.g. of ship, of aircraft by measuring the time taken to traverse a fixed distance
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P5/00—Measuring speed of fluids, e.g. of air stream; Measuring speed of bodies relative to fluids, e.g. of ship, of aircraft
  • G01P5/24—Measuring speed of fluids, e.g. of air stream; Measuring speed of bodies relative to fluids, e.g. of ship, of aircraft by measuring the direct influence of the streaming fluid on the properties of a detecting acoustical wave
  • G01P5/245—Measuring speed of fluids, e.g. of air stream; Measuring speed of bodies relative to fluids, e.g. of ship, of aircraft by measuring the direct influence of the streaming fluid on the properties of a detecting acoustical wave by measuring transit time of acoustical waves
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
  • G01F1/704—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow using marked regions or existing inhomogeneities within the fluid stream, e.g. statistically occurring variations in a fluid parameter
  • G01F1/708—Measuring the time taken to traverse a fixed distance
  • G01F1/7082—Measuring the time taken to traverse a fixed distance using acoustic detecting arrangements

Definitions

• the invention concerns a method for measurement of the flow velocity, the volumetric flow, and/or of the mass flow of a gas that flows in a pipe, in which method sound detectors are fitted in the flow pipe at a certain distance from one another in the longitudinal direction of the measurement pipe, and in which method a long-wave sound that proceeds in the flow pipe downstream and upstream exclusively in the base mode as a plane-wave front is fed into the flow pipe by means of sound sources from outside the measurement distance defined by said detectors, and the flow quantities are determined from the travel times of said sound downstream and upstream over the measurement distance, from the cross-sectional area of the pipe, and from the density of the gas to be measured.
• the invention concerns a device for measurement of the gas flow velocity and/or of quantities derived from same, such as volumetric flow and/or mass flow, which device comprises a measurement pipe, in which the flow to be measured runs, and which device comprises loudspeakers as the transmitters of the wide-band, low-frequency sound signals that proceed exclusively as a plane-wave front in the base mode and power amplifiers for said loudspeakers, and microphones as the sound detectors and amplifiers for the microphone signals, and which device is provided with calculator devices and programs so as to calculate said flow quantity or quantities from the distance between the sound detectors in the longitudinal direction of the flow pipe, from the cross-sectional area of the measurement pipe, from the density of the flowing gas, and from the travel times of the sound signals downstream and upstream over the distance between the sound detectors.
• f c (c - v ma ⁇ )/(1.7*D) (1)
• c the travel speed of sound in the gas at rest
• v ma ⁇ the highest dimensioned flow velocity
• D is the diameter of the pipe.
• a sound that is transmitted in the form of frequency scanning and a filter connected to follow said sound in principle, also permit that sound is transmitted in both directions at the same time, but with a different momentary frequency, in which case the downstream signals and the upstream signals of the two sound detectors can be separated by means of a total of four scannable filters, as is described in the FI Pat. Appl. No. 916102.
• frequency scanning is, in principle, non-stationary, in which case the necessary filters are also time-dependent. This results in dispersion, i.e. the time delay of the filters is dependent on the frequency, which further readily results in an error in the determination of the travel time if the scanning is not chosen correctly, if the scanning is chosen too rapid, or if the beginning and the end of the scanning are not cut off from the measurement.
• These drawbacks can be avoided if, in stead of frequency scanning, wide-band transmissions are employed in which all frequencies sound constantly with an invariable amplitude. At the same time, this means that sound is transmitted in both directions simultaneously.
• the present DSP-technique permits even a versatile processing of signals of audio frequency in real time and provides a number of possibilities for determination of the travel-time information in a noisy environment.
• the method of the invention is mainly characterized in that said sound is transmitted into the measurement pipe as stationary, periodic sequences that proceed over the measurement distance simultaneously in both directions, downstream the sequence S d (t,T) and upstream the sequence S u (t,T), wherein T is the length of the period and t is the relative time measured from the beginning of each period, and that said sequences S d (t,T),S u (t,T) are orthogonal in relation to one another, i.e. they have no common frequency components different from zero.
• the device in accordance with the invention is mainly characterized in that the sound transmitted through said loudspeakers is composed of wide-band sequences of equal periods, which are transmitted during the period T simultaneously downstream and upstream, the downstream sequence S d (t,T) and the upstream sequence S u (t,T), which sequences S d (t,T),S u (t,T) are arranged as orthogonal in relation to one another, i.e. they include no common frequency components different from zero.
• Figure 1 is a general block-diagram illustration of an acoustic system of measurement in accordance with the invention
• Figure 2 illustrates sound sequences transmitted downstream and upstream
• Figure 3 the upper curve represents a correlation peak consisting of even frequencies and imagined as measured downstream
• the lower curve represents a correlation peak consisting of odd frequencies and imagined as measured upstream
• Figure 4 illustrates, in a way corresponding to Fig. 3, an application in which, in stead of a second sequence to be correlated, a Hubert transform of same has been chosen
• Figure 5 shows graphs of a phase difference obtained with even and odd frequencies in an application in which the modulation arises from reflection echoes alone
• Figure 6 shows the cumulative distributions corresponding to Fig. 5.
• Fig. 1 is a general block-diagram illustration of a system by whose means the invention can be carried into effect advantageously.
• the flow to be measured e.g. a natural-gas flow
• the sound sources 2a and 2b preferably loudspeakers
• 5a and 5b are signal amplifiers
• 6 is a real-time processor, preferably a digital signal processor provided with necessary analog inputs and outputs, by means of which processor 6 it is possible to carry out the procedures based on FFT-algorithms or on FIR-filter algorithms
• 7 is a system processor, through which the communication between the real-time processor 6, the display device 8 and a separate PC work station 9 is accomplished.
• the sounds that are transmitted simultaneously in both directions from the loudspeakers 2a and 2b do not interfere with each other and that the measurement is also as insensitive to the noise in the pipe 1 as possible.
• the principal idea of the invention is to transmit, from the loudspeakers 2a and 2b into the measurement pipe 1, downstream and upstream, such wide-band sound sequences S d (t,T),S u (t,T) of equal periods as do not include any common frequency component and as are, thus, orthogonal, i.e. their correlation function disappears identically:
• the signals of the microphones 3a and 3b herein, let D L (t,T) represent the signal of the microphone 3a at the left end of the measurement distance L, and let D R (t,T) represent the signal of the microphone 3b at the right end of the measurement distance L, with the exception of the background noise, can be reduced unequivocally to downstream and upstream components orthogonal in relation to one another:
• the principal idea of the present invention permits the solution of a number of problems that deteriorate the signal-to-noise ratio and leads to several different alternative embodiments.
• the proportion of the external pipe noise that occurs in these discrete frequency components is the smaller, the higher the number of the periods is over which the situation is examined.
• the periodic signal increases in proportion to N, whereas the incommensurable noise increases in proportion to the square root of N.
• phase angles of the discrete frequency components can be chosen freely, in principle. In order to avoid the effect of non-linearity of loudspeakers 2a and 2b or of equivalent sound sources and, for example, owing to safety regulations concerning natural-gas pipes, it is advisable to avoid momentary peaks of sound power. This aim is carried into effect readily if said phase angles are chosen at random.
• the frequency range to be used is limited at the upper end by the so-called cut-off frequency [formula (1)], at which, in addition to the piston mode, the first non-attenuating higher mode occurs, which has a different travel speed and which thereby disturbs the precise determination of the travel speed of the piston mode. At the lower end, it is often advisable to cut off the mains frequency of the AC electricity network and frequencies that are lower than its first upper harmonic.
• the division of the remaining frequencies between the sound sequences trans mitted downstream and those transmitted upstream can be carried out in different ways.
• the simplest way is to choose the even frequencies for one sequence and the odd frequencies for the other. This choice has the result that each sequence consists of two half-periods, of which the half-periods of the even frequencies are identical with each other, as is illustrated by the upper graph in Fig. 2, and the half-periods of the odd frequencies are also identical with each other when the signs of one of them are changed, as is illustrated by the lower graph in Fig. 2.
• the sounds that proceed downstream and upstream can be separated from each other simply by adding together an even number of half-periods, in one case as such, and in the other case by first changing the sigh of every second half-period.
• a second alternative is to allot the available frequencies irregularly but evenly among the downstream and the upstream sequences.
• the travel times of the measurement sound over the measurement distance L downstream and upstream can be determined in a number of different ways even from the same measurement data.
• the basic alternatives are to form a filtered correlation function of the signals of the microphones 3a and 3b as a function of the time difference or to examine the phase difference of successive frequency components of the signals concerned as a function of frequency.
• Rapid Fourier transforms of the digital signal processors with FFT-algorithms permit processing of a signal in a time and frequency space, if necessary, by means of fully the same apparatuses while interfering with the real-time program only.
• the determination of the travel time of the measurement sound from correlation functions takes place as follows. The situation is most ideal for determination of the middle point of a correlation peak when there is only one peak in the whole correlation function, i.e. the amplitudes of other possible peaks are so little that the distortion caused by them in the area of the main peak remains below the permitted level.
• the origin of interfering peaks may be in background noise that proceeds in the wrong direction, from which noise frequency components chosen for the main peak have been left over by the correlation filter, or connections and joints of the detector, which produce interfering echo peaks. If the interfering peaks cannot be attenuated to a sufficiently low level, attempts must be made to bring their location and range of effect outside the range of variation of the main peak.
• the location of the echoes can be affected by means of the locations of the connections of the detectors 3a and 3b, and the range of effect can be affected thereby that the frequency-dependence of the amplitudes of the frequency components is made even, while avoiding abrupt points of discontinuity.
• three different correlation functions can be formed:
• F d and F u denote filter operators, which pick up the downstream portion only or the upstream portion only from the detector signals
• Aver m means averaging over m periods
• n means averaging of correlation functions by integration over n periods.
• the first correlation function is the simplest one and substantially includes two peaks, one of them with a positive time-delay value and the other one with a negative time-delay value, corresponding to travel of sound downstream and upstream. This one is also most sensitive to disturbance.
• Each of the following, filtered correlation functions includes substantially one peak only, one representing the travel of sound downstream only, and the other one upstream only.
• FIG. 3 shows, in the upper half, a correlation peak consisting of even frequencies and imagined as measured downstream, and, in the lower half, a correlation peak consisting of odd frequencies and imagined as measured upstream.
• the small side peaks are produced by negative reflections from the loudspeaker branches.
• the signal of each microphone 3a and 3b can be correlated separately with a reference sequence R d (t,T) and R u (t,T) of invariable phase and of the desired distribution of amplitude, which sequences have been derived from corresponding transmission sequences S d (t,T) and S u (t,T) or from microphone sequences measured in a zero flow situation, the following correlation functions being produced:
• the sound travel time over the measurement distance L downstream is determined as the time difference between the middle points of the peaks of the correlation functions C Ld ( ⁇ ,T) and C Rd ( ⁇ ,T), and upstream as the time difference between the middle points of the peaks of the correlation functions C Lu ( ⁇ ,T) and C Ru ( ⁇ ,T).
• FTR-filters finite impulse response
• the downstream reference sequence being chosen as the coefficient vector for one of them, and the upstream reference sequence for the other one, respectively, or it is possible to carry out the operation in a frequency space as the following multiplications of Fourier-transformed vectors carried out component by component:
• Determination of the travel time as dependence on the phase angle takes place as follows. If the correlation function includes substantially one peak only, the phase angle of its Fourier transform is increased or decreased in a linear way as a function of the frequency, the angle coefficient being proportional to the shift of the peak away from the origin, for the shifting of time by means of the shift ⁇ is carried out in a time space by multiplication by the phase factor exp(2 ⁇ i* ⁇ 1* ⁇ ). An estimate for the travel time of the measurement sound over the measurement distance L is now obtained by forming the average of the phase-angle differences calculated over successive frequency steps of equal parity over the distance l 1 ...l 2 :
• the Fourier transforms of the signals of the microphones 3a and 3b have always been used as the frequency vectors.
• the vector derived from the signal of one of the detectors 3a,3b by the Fourier transform of the reference sequences R d (t,T) and R u (t,T), in which case the following four time shifts ⁇ Ld , ⁇ Rd , ⁇ Lu , and ⁇ Ru are obtained.
• the travel times downstream are obtained as the difference between the first two, and the travel time upstream as the difference between the latter two.
• the correlation function also includes other peaks, modulation occurs both in the phase-difference distribution and in the cumulative function.
• the risk of coining above or below these limits is reduced if the average value of the phase-angle difference remains near zero.
• it is advisable to compare the factual travel time for example, with the travel time in a zero flow situation in the reference pipe 1, in which case the travel-time difference produced by the flow velocity v is alone seen as a minor deviation from zero in the average of the distribution of the phase-angle difference.
• Fig. 5 shows the phase difference graphs both for even frequencies and for odd frequencies in a case in which the modulation arises from reflected echoes alone.
• Fig. 6 shows the corresponding cumulative distributions.
• phase-angle method In order that the phase-angle method were usable, it is required that the correlation function has a clearly distinguishable main peak and that, with every individual frequency, the phase angle of its Fourier transform comes reasonably close to the value that corresponds to the travel time of said peak.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Fluid Mechanics (AREA)
• Aviation & Aerospace Engineering (AREA)
• Acoustics & Sound (AREA)
• Multimedia (AREA)
• Electromagnetism (AREA)
• Radar, Positioning & Navigation (AREA)
• Remote Sensing (AREA)
• Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)
• Measuring Volume Flow (AREA)

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Cited By (12)

Citations (3)

• 1992
  • 1992-04-01 FI FI921448A patent/FI88208C/sv not_active IP Right Cessation
  • 1992-10-16 WO PCT/FI1992/000278 patent/WO1993020411A1/en active Application Filing
  • 1992-10-28 FI FI924880A patent/FI89835C/sv not_active IP Right Cessation

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