CN102589627B - Absolute propagation time measuring method for ultrasonic flow meter - Google Patents

Abstract

The invention provides an absolute propagation time measuring method for an ultrasonic flow meter. The method comprises the following steps of: setting an energy converter 3 at the center of an opposite side between an energy converter 1 and an energy converter 2, which are arranged in the shape of V; if the distance between the energy converter 1 and the energy converter 2 is set to be L, setting the transverse distance between the energy converter 3 and the energy converter 1 and the transverse distance between the energy converter 3 and the energy converter 2 are both to be L/2; when the absolute propagation time in case of fair current is measured, leading the energy converter 1 to send out ultrasonic signals, and two receivers, i.e. the energy converter 2 and the energy converter 3, receive the signals; and obtaining the time difference of the signals received by the two receivers by a mode of getting the relative propagation time, wherein a metering result of the time difference is the time that the ultrasonic signals are propagated to the energy converter 2 from the energy converter 3, i.e. obtaining the fair current absolute propagation time in the case that the energy converter 2 and the energy converter 3 are distributed in the shape of Z, wherein the adverse current absolute propagation time measuring method is similar to the fair current absolute propagation time measuring method. The method is used without a timing module, i.e. absolute time measurement is converted into relative time measurement, so that the measurement precision is improved.

Description

A kind of absolute propagation time measuring method for ultrasonic flow meter

Technical field

The invention belongs to ultrasonic flow rate field of measuring technique, particularly a kind of three transducers of employing for ultrasonic flow meter and improved least error quadratic sum algorithm are measured the method for absolute propagation time.

Background technology

Ultrasonic technology is applied to flow measurement Main Basis: when ultrasound wave incides after fluid, the ultrasound wave of propagating in fluid will be loaded with the information of rate of flow of fluid.Generation, propagation and the detection of ultrasonic flow meter to signal has various method to set up, thereby form the ultrasonic flowmeter of different principle, it is broadly divided into propagation speed differential method (comprising: time difference method, phase difference method, frequency-difference method), Doppler method, correlation method, wave beam deflection method, etc.

Transit-time ultrasonic flow meter is to calculate flow velocity according to ultrasonic signal following current travel-time and adverse current travel-time and the difference of the two, and then tries to achieve flow.In the formula of calculating flow velocity, most important two parameters are absolute propagation time and relative travel-time.

Travel-time refers to relatively: the time difference in following current travel-time and adverse current travel-time.Calculate the relative travel-time and following current travel-time and adverse current travel-time can not be done to subtraction, the waveform that when waveform that should utilize following current time, transducer receives and adverse current, transducer receives, the time difference that adopts cross-correlation method, least error sum-of-squares scheduling algorithm to calculate these two groups of similar waveforms, fluctuating error scope is in 1ns.Wherein, least error sum-of-squares precision will be higher than cross-correlation method, but computing velocity is slightly inferior.

Absolute propagation time refers to: transmitter transmitting ultrasound wave arrives the time of receiver.When measurement, be all generally with two sensor synergism effects, transducer different arrangement mode outside pipeline, has Z-type, V-type, N-type, W type.No matter which kind of arrangement mode, all, receiver cannot obtain ultrasonic echo signal to the shortcoming of two sensors accurately, and therefore on calculating absolute propagation time, efficiency is not high.The common method of calculating absolute propagation time is counting method.Transmitter is launched hyperacoustic while, and receiver starts counting, when the sampled signal of receiver end is during higher than set threshold value, thinks and receives ultrasonic signal, and stop counting, the travel-time equals count value and is multiplied by the sampling period.The shortcoming of this method is that threshold value need rule of thumb set, and count value is integer, if the sampling period is 40ns, error is between positive and negative 40ns, and scope is larger, and out of true need to adopt new method to improve the precision of asking for of absolute propagation time.

Summary of the invention

In order to solve the lower problem of absolute propagation time precision of measuring, the present invention proposes a kind of method of utilizing 3 transducers to measure the time and tide travel-time, do not need timing module and threshold value is set, being equivalent to that absolute time measurement is converted to relative time measures, thereby reduce error, improved measuring accuracy.

Further, the present invention also utilizes a kind of improved least error sum-of-squares that cross-correlation method and least error sum-of-squares are combined, calculate the relative travel-time, improve the object of absolute propagation time precision thereby realized, and can in the situation that not affecting precision, reduce computing time.

In prior art, increase sound path in order to improve measuring accuracy, thereby produced transducer different arrangement mode outside pipeline, have Z-type, V-type, N-type, a W type.The present invention is based upon on the basis that V-type arranges.

The present invention adds a transducer 3 in the arrange positive offside center of lower transducer 1 and transducer 2 of V-type.If the distance between transducer 1 and transducer 2 is L, transducer 3 is L/2 apart from the lateral separation of transducer 1 and transducer 2.Transducer 1 and transducer 2 state transmitting and receiving that takes turns to operate, 3 of transducers are operated in accepting state, be that transducer 1 is launched ultrasonic signal, transducer 2 and transducer 3 receive signal simultaneously, and the signal that now transducer 2 and transducer 3 receive is the signal under following current state; Transducer 2 is launched ultrasonic signal, and transducer 1 and transducer 3 receive signal simultaneously, and the signal that now transducer 1 and transducer 3 receive is the signal under adverse current state.

For following current situation, transducer 1 is launched ultrasonic signal, and transducer 2 and 3 is receiver, and first the ultrasonic signal that transducer 1 is launched is received by transducer 3, is received again immediately after reflection by transducer 2, and therefore these two reception signal correction degree can be very high.Method when the relative travel-time is asked in utilization, as cross-correlation method etc. is asked for the difference between the signal that signal that transducer 3 receives and transducer 2 receive, its result of calculation is exactly ultrasonic signal propagates into transducer 2 time from transducer 3, has been equivalent to ask for the following current travel-time that transducer 2 and transducer 3 are arranged in situation according to Z-type.

In adverse current situation, in like manner can ask the time of ultrasonic signal from transducer 3 to transducer 1: transducer 2 is launched ultrasonic signal, now two receivers are that transducer 1 and transducer 3 receive signal; Equally, the mode that the relative travel-time is asked in employing is asked for the difference of the signal that two receivers receive, its result of calculation is exactly ultrasonic signal propagates into transducer 1 time from transducer 3, has been equivalent to obtain the adverse current travel-time that transducer 1 and transducer 3 are arranged in situation according to Z-type.

Due to the sound travel difference of asking for the signal in these two travel-times and passing through, therefore must guarantee that transducer 3 equates apart from the lateral separation of transducer 2 and transducer 1, guarantee that sound path equates.In addition, the method for asking for the relative travel-time is similar to classic method, only utilizes transducer 1 and transducer 2, but will be by the last time difference divided by 2, illustrates that result is transducer 3 the arrives transducer 2 relative travel-time to transducer 1 or transducer 3.

Beneficial effect

1, the present invention proposes a kind of method of utilizing 3 transducers to measure the time and tide travel-times, do not need timing module, and can eliminate circuit time and postpones the precision problem bringing, thereby realized the object of raising absolute propagation time precision.

2, the present invention proposes a kind of improved least error sum-of-squares, cross-correlation method and least error sum-of-squares are combined, can in the situation that not affecting precision, reduce the working time of least error sum-of-squares.

Accompanying drawing explanation

Fig. 1 is the transducer V-type schematic diagram of arranging;

Fig. 2 is the signal schematic representation that in Fig. 1, transducer 2 and transducer 3 receive;

Fig. 3 is that least error sum-of-squares extracts core signal schematic diagram;

Fig. 4 is calculation procedure process flow diagram;

Fig. 5 is for calculating the theoretical derivation schematic diagram of absolute propagation time.

Embodiment

Below in conjunction with the drawings and specific embodiments, the present invention is described in further details.

The invention provides a kind of 3 transducers method with improved least error quadratic sum algorithm measurement absolute propagation time utilized.

Arrange as shown in Figure 1 transducer mounting means, add a transducer 3 in the arrange positive offside center of lower transducer 1 and transducer 2 of V-type.If the distance between transducer 1 and transducer 2 is L, transducer 3 is L/2 apart from the lateral separation of transducer 1 and transducer 2.Transducer 1 and transducer 2 state transmitting and receiving that takes turns to operate, 3 of transducers are operated in accepting state, be that transducer 1 is launched ultrasonic signal, transducer 2 and transducer 3 receive signal simultaneously, and the signal that now transducer 2 and transducer 3 receive is the signal under following current state; Transducer 2 is launched ultrasonic signal, and transducer 1 and transducer 3 receive signal simultaneously, and the signal that now transducer 1 and transducer 3 receive is the signal under adverse current state.

While Figure 2 shows that following current, ultrasonic signal sends from transmitter transducer 1, to transducer 3, arrives the schematic diagram of transducer 2 through reflection again.In theory, ultrasonic signal 3 should equate with the time of experiencing to transducer 2 from transducer 3 from transmitter to transducer.Transmitted waveform the unknown, transducer 3 is same transmitted wave transmission with the signal that transducer 2 receives, and therefore the degree of correlation is very high.Now can utilize the method for asking for the relative travel-time such as cross-correlation method, least error sum-of-squares etc. to ask in following current situation the time difference of ultrasonic signal between transducer 3 and transducer 2, be the following current absolute propagation time that transducer 3 and transducer 2 are arranged in situation according to Z-type.In adverse current situation in like manner.

Wherein, cross-correlation method is that two similar waveforms are carried out to cross-correlation calculation, obtain maximal correlation and put the time difference that corresponding horizontal ordinate is exactly two similar waveforms, but the method only can obtain time difference situation roughly, accurate not.

The specific implementation of current a kind of least error sum-of-squares is (referring to IEEE Transactions on Ultrasonics, Ferroelectrics, And Frequency Control, VOL.55, NO.9, September 2008):

For two similar waveforms that have a time difference, if one of them signal is reference signal S1, another signal is S2, extract core signal S1 and S2 from same a starting point, it is N that S1 extracts sampled point number, it is M that S2 extracts sampled point number, wherein N > M, and subsequent process is only relevant with core signal.This core signal refers to the signal that can include obvious crest, and the extraction window of core signal can not be too large, otherwise can extract more garbage signal, increases number of computations.Core signal extracts the concrete selection of window and can be determined or be adopted empirical value by test.

Then, the S1 of extraction is carried out to the Cubic Spline Fitting of following form, the formula after matching is as follows, and S2 is still discrete point;

$S_1\left(t\right)=\left\{\begin{array}{c}{f}_1\left(t\right)=a_1{(t/\&PartialD;-T_1)}^3+b_1{(t/\&PartialD;-T_1)}^2+c_1(t/\&PartialD;-T_1)+{\mathrm{<figure-callout\; id="1"\; label="d"\; filenames="HDA0000137931910000011.png,HDA0000137931910000032.png"\; state="\{\{state\}\}">d</figure-callout>}}_1.....................................0<t<1\&CenterDot;\&PartialD;\\ ...\\ f_i\left(t\right)=a_i{(t/\&PartialD;-T_i)}^3+b_i{(t/\&PartialD;-T_i)}^2+c_i(t/\&PartialD;-T_i)+d_i.....................................(i-1)\&CenterDot;\&PartialD;<t<i\&CenterDot;\&PartialD;\\ ...\\ f_{N-1}\left(t\right)=a_{N-1}{(t/\&PartialD;-T_{N-1})}^3+b_{N-1}{(t/\&PartialD;-T_{N-1})}^2+c_{N-1}(t/\&PartialD;-T_{N-1})+d_{N-1}....(N-2)\&CenterDot;\&PartialD;<t<(N-1)\&CenterDot;\&PartialD;\end{array}\right\}---\left(1\right)$

Wherein, f _i(t) be the fitting function of i segmentation (i sampled point is to the segmentation between i+1 sampled point, altogether), T _i=(i-1).Order

formula (1) is reduced to:

$\left\{\begin{array}{c}{f}_1\left(t^{\&prime;}\right)=a_1{\left(t^{\&prime;}\right)}^3+b_1{\left(t^{\&prime;}\right)}^2+c_1\left(t^{\&prime;}\right)+{\mathrm{<figure-callout\; id="1"\; label="d"\; filenames="HDA0000137931910000011.png,HDA0000137931910000032.png"\; state="\{\{state\}\}">d</figure-callout>}}_1.....................................0<t^{\&prime;}<1\\ ...\\ f_i\left(t^{\&prime;}\right)=a_i{\left(t^{\&prime;}\right)}^3+b_i{\left(t^{\&prime;}\right)}^2+c_i\left({t^{\&prime;}}_i\right)+d_i.....................................0<t^{\&prime;}<1\\ ...\\ f_{N-1}\left(t^{\&prime;}\right)=a_{N-1}{\left(t^{\&prime;}\right)}^3+b_{N-1}{\left(t^{\&prime;}\right)}^2+c_{N-1}\left(t^{\&prime;}\right)+d_{N-1}................................0<t^{\&prime;}<1\end{array}\right\}---\left(2\right)$

Can find out from formula (2), the Cubic Spline Fitting is here that the abscissa value of the starting point of each segmentation and end point is changed and move [0,1] fitting function of the each segmentation in situation, the time difference that subsequent calculations obtains is like this also in [0,1].

This t ' value is exactly two time differences between signal, for example, can adopt differentiate to ask the mode of extreme value to obtain t ' in order to make formula (3) for t ' value hour the time difference of two signals so;

$\mathrm{\&epsiv;}\left(t^{\&prime;}\right)=\underset{i=1}{\overset{M}{\mathrm{\&Sigma;}}}{(f\left(t^{\&prime;}\right)-S_2[i\left]\right)}^2---\left(3\right)$

Wherein, S ₂[i] is the amplitude of i element in S2 core signal, and for S2, the span of i is 1～M.

It is the time data after conversion and translation that above-mentioned steps obtains t ' value, and does not know which segmentation t ' belongs to, and therefore cannot adopt

be reduced to t.Therefore need S2 to the sampling period of direction translation near S1, if (fall behind S1 on the S2 time, by S2 to left, otherwise to the right), every translation once all adopts formula (3) to calculate ε t ' hour, carry out N-M time, finally from the ε of multiple minimums, obtain minimum one, its corresponding t ' is as net result.

Visible, above-mentioned least error sum-of-squares need to carry out repeatedly the calculating of t ' value, and calculated amount is huge.

The defect of bringing in order to solve independent use cross-correlation method and above-mentioned least error sum-of-squares, the present invention adopts improved least error sum-of-squares, it is that cross-correlation method and least error sum-of-squares are combined, and can in the situation that not affecting precision, reduce the working time of least error sum-of-squares.

The idiographic flow of improved least error quadratic sum algorithm comprises the steps, and in this flow process, establishing S1 is the signal that transducer 2 received signals receive, S2 is the signal that transducer 3 received signals first receive, the horizontal ordinate of S1 and S2 is sampled point sequence number, relatively good like this calculating.Referring to Fig. 4:

Step 1, utilize Fast Fourier Transform (FFT) FFT to calculate the discrete cross-correlation function of S1 and S2;

Step 2, ask the abscissa value k corresponding to maximum of points of discrete cross-correlation function, being S2 core signal need to count to the integer of right translation, the Position Approximate that S2 core signal overlaps with S1 core signal relevant portion;

Step 3, extract the core signal S1 and S2 from same a starting point, it is N that S1 extracts element number, and it is M that S2 extracts element number, and N is greater than M;

Step 4, the segmentation between each neighbouring sample point in S1 core signal is all moved to [0,1] and carries out Cubic Spline Fitting on transverse axis, S2 is still discrete point.Because the horizontal ordinate of S1, S2 in the present embodiment is sampled point sequence number rather than time, the f that therefore after Cubic Spline Fitting, the fitting function of i segmentation still illustrates for formula (2) _i(t '), T _i=(i-1), but t '=t-T _i, the value that t is horizontal ordinate is sampled point sequence number.

Then, by the core signal of S2 k the point that move to right, be equivalent to k the point that move to left by S1, obtain f _i' (t ')=f _i+k(t ').If S2 is transducer 2 received signals in practice, S1 is transducer 3 received signals, and S1 is in advance in S2, the core signal of S2 should be moved to left k point, f of this step _i' (t ')=f _i-k(t ').Move to left and represent to move to the little direction of sampled point sequence number.

Adopt following revised least error quadratic sum formula (4) to calculate the time difference t of two signal S1 and S2 _k, time difference t _kbe t ' value hour for making the ε of formula (4), t _kbelong to interval [0,1];

$\mathrm{\&epsiv;}\left(t^{\&prime;}\right)=\underset{i=1}{\overset{M}{\mathrm{\&Sigma;}}}{({f_i}^{\&prime;}\left(t^{\&prime;}\right)-S_2[i\left]\right)}^2=\underset{i=1}{\overset{M}{\mathrm{\&Sigma;}}}{(f_{i+k}\left(t^{\&prime;}\right)-S_2[i\left]\right)}^2---\left(4\right)$

Wherein, S ₂[i] is i element in S2 core signal.For the situation shown in Fig. 3, f _i+kdata in (t ') expression employing S1 dotted line frame and the data in S2 dotted line frame ask poor.The time difference t that adopts formula (2) and (4) to obtain _kexpress that between S1 ' and S2 ', to need mutual translation how much can make the error minimum between the two be that registration is the highest.Correspondingly, k the point if S2 moves to left, the f in formula (4) _i+k(t ') adopts f _i-k(t ') replaces.

Step 5, according to t _kcan try to achieve the time difference t_sse of the time difference two receiver received signals:

t_sse＝(k+t _k)·T(5)

The error of thick translation distance k obtaining due to cross-correlation method, within a sampling period, therefore can be thought, the t finally obtaining _kshould be in one-period [0, T], therefore adopt (5) can obtain the final time difference and ask for result.

So far, this flow process finishes.

Can know by foregoing description, the present invention adopts 3 sensors absolute time measurement can be changed into theoretical validation that relative time measures to reduce measuring error as shown in Figure 5.The error of bringing due to circuit delay in reality is often very large, can not ignore.The problem that the method for utilizing 3 sensors to ask for absolute propagation time does not exist circuit time to postpone.As shown in Figure 5,1,2,3 are respectively the numbering of 3 transducers, e _a, e _brepresent the time delay of radiating circuit when transducer 1 and transducer 2 are transmitter, e ₁, e ₂, e ₃represent the time delay of receiving circuit when transducer 1,2,3 is receiver.Because the present invention based on circuit on, radiating circuit is obstructed, but receiving circuit only has one, therefore e ₁=e ₂=e ₃.T _up, t _dnbe respectively in co-current flow and counter-current flow situation, ultrasonic signal sends the time that reflexes to receiver through tube wall from transmitter, the T.T. of propagating in fluid.T _up/ 2, t _dn/ 2 are respectively following current time and the adverse current time that ultrasonic signal is propagated to transducer 1 from transducer 3 to transducer 2 with from transducer 3 fluid.T ₃, t ₃' be respectively actual ultrasonic signal and 3 surpass ripple to transducer 1 and the travel-time from transducer 3 to transducer 2 from transducer, now to consider circuit delay.For example, in following current situation, transducer 1 is launched ultrasonic signal, through the delay e of radiating circuit _aafter, ultrasonic signal enters fluid, through t _up/ 2 time arrives the position of transducer 3, and the ultrasonic signal that system acquisition receives to transducer 3 need to be through the delay e of receiving circuit ₃.Through tube wall reflection, ultrasonic signal passes through t again _up/ 2 time arrives the position of transducer 2, and the ultrasonic signal that system acquisition receives to transducer 2 need to be through the delay e of receiving circuit ₂.In adverse current situation, in like manner.Computing formula is as follows:

${\mathrm{<figure-callout\; id="3"\; label="t"\; filenames="HDA0000137931910000011.png,HDA0000137931910000032.png"\; state="\{\{state\}\}">t</figure-callout>}}_3=\frac{t_{\mathrm{up}}}{2}+e_1-{\mathrm{<figure-callout\; id="3"\; label="e"\; filenames="HDA0000137931910000011.png,HDA0000137931910000032.png"\; state="\{\{state\}\}">e</figure-callout>}}_3$

${\mathrm{<figure-callout\; id="3"\; label="t"\; filenames="HDA0000137931910000011.png,HDA0000137931910000032.png"\; state="\{\{state\}\}">t</figure-callout>}}_3^{\&prime;}=\frac{{\mathrm{<figure-callout\; id="2"\; label="t\; dn"\; filenames="HDA0000137931910000011.png,HDA0000137931910000021.png"\; state="\{\{state\}\}">t</figure-callout>}}_{\mathrm{dn}}}{2}+e_2-{\mathrm{<figure-callout\; id="3"\; label="e"\; filenames="HDA0000137931910000011.png,HDA0000137931910000032.png"\; state="\{\{state\}\}">e</figure-callout>}}_3$

From above formula, calculate t ₃, t ₃' postponing to have nothing to do with radiating circuit, the signal of 3 transducers connects same receiving circuit, therefore e simultaneously ₁=e ₂=e ₃.Therefore,

${\mathrm{<figure-callout\; id="3"\; label="t"\; filenames="HDA0000137931910000011.png,HDA0000137931910000032.png"\; state="\{\{state\}\}">t</figure-callout>}}_3=\frac{t_{\mathrm{up}}}{2}+e_1-{\mathrm{<figure-callout\; id="3"\; label="e"\; filenames="HDA0000137931910000011.png,HDA0000137931910000032.png"\; state="\{\{state\}\}">e</figure-callout>}}_3$

${\mathrm{<figure-callout\; id="3"\; label="t"\; filenames="HDA0000137931910000011.png,HDA0000137931910000032.png"\; state="\{\{state\}\}">t</figure-callout>}}_3^{\&prime;}=\frac{{\mathrm{<figure-callout\; id="2"\; label="t\; dn"\; filenames="HDA0000137931910000011.png,HDA0000137931910000021.png"\; state="\{\{state\}\}">t</figure-callout>}}_{\mathrm{dn}}}{2}+e_2-{\mathrm{<figure-callout\; id="3"\; label="e"\; filenames="HDA0000137931910000011.png,HDA0000137931910000032.png"\; state="\{\{state\}\}">e</figure-callout>}}_3$

From above formula, t ₃, t ₃' calculating and circuit delay irrelevant.

In conjunction with actual acquired data, measurand parameter is:

Internal diameter of the pipeline: 18mm

Outer diameter tube: 23mm

Pipeline material: stainless-steel tube

Ultrasonic transducer: piezoelectric type

Transonic path and pipeline axial angle: 45 degree

Fluid media (medium): pure water

Threshold level method is calculated absolute propagation time by counting, and the height of level is determined by experience.As level is set to 2v or 2.2v, the difference of 0.2v can cause too much counting.Suppose that the value that experience arranges is very accurate, because sampled signal is discrete point, counting method also can because of amplitude break through the point of set level and timing point inconsistent and cause error because sampling rate is 25MHz, this error is between positive and negative 40ns.And by three sensor measurement absolute propagation times of the present invention, can be converted into and measure the relative travel-time, error is in nanosecond.Take following current signal absolute propagation time under a certain group of fixed flow rate as example,

The stability of visible 3 sensor measurements of the present invention and precision are better than 2 traditional sensor measurement absolute times and obtain method.

Every group of signal comprises a following current signal and an adverse current signal, and each signal comprises 2048 sampled points.Utilize respectively traditional least error quadratic sum (SSE) method and the improved least error sum-of-squares of the present invention to calculate the mistiming between every group of signal.Calculate consuming time as shown in the table:

Tradition SSE	1.0994s	1.0338s	1.0632s	1.0432s	1.0435s
						The present invention	0.4897s	0.5032s	0.4753s	0.4831s	0.4772s

Visible, calculate N-M repetitive operation owing to not doing, the consuming time of improved least error quadratic sum algorithm reduces to some extent than before improving.

Claims (2)

1. for an absolute propagation time measuring method for ultrasonic flow meter, it is characterized in that, comprising:

Step 1: the positive offside center of the first transducer (1) and the second transducer (2) arranges the 3rd transducer (3) under V-type is arranged, and the 3rd transducer (3) is only operated in accepting state; If the distance between the first transducer (1) and the second transducer (2) is L, the 3rd transducer (3) is L/2 apart from the lateral separation of the first transducer (1) and the second transducer (2);

Step 2: while measuring in following current situation absolute propagation time, the first transducer (1) transmitting ultrasonic signal, now i.e. the second transducer (2) and the 3rd transducer (3) the reception signal of two receivers; The mode that the relative travel-time is asked in employing is asked for the time difference of the signal that two receivers receive, its result of calculation is exactly ultrasonic signal propagates into the second transducer (2) time from the 3rd transducer (3), has been equivalent to obtain the following current absolute propagation time that the second transducer (2) and the 3rd transducer (3) are arranged in situation according to Z-type;

While measuring in adverse current situation absolute propagation time, the second transducer (2) transmitting ultrasonic signal, now i.e. the first transducer (1) and the 3rd transducer (3) the reception signal of two receivers; Equally, the mode that the relative travel-time is asked in employing is asked for the time difference of the signal that two receivers receive, its result of calculation is exactly ultrasonic signal propagates into the first transducer (1) time from the 3rd transducer (3), has been equivalent to obtain the adverse current absolute propagation time that the first transducer (1) and the 3rd transducer (3) are arranged in situation according to Z-type.

2. the method for claim 1, is characterized in that, the step that the mode that the relative travel-time is asked in described employing is asked for the time difference of the signal that two receivers receive is:

The 1st step: utilize Fast Fourier Transform (FFT) FFT to calculate the discrete cross-correlation function of S1 and S2; Wherein, S1 is in described two receiver received signals, and S2 is another in described two receiver received signals; The horizontal ordinate of S1 and S2 is sampled point sequence number;

The 2nd step: abscissa value k corresponding to maximum of points that asks discrete cross-correlation function;

The 3rd step: extract the core signal S1 and S2 from same time starting point, it is N that S1 extracts element number, and it is M that S2 extracts element number, and N is greater than M;

The 4th step: the segmentation between each neighbouring sample point in the core signal of S1 is all moved to [0,1] and carries out Cubic Spline Fitting on transverse axis, and S2 is still discrete point; The fitting function f of i segmentation after Cubic Spline Fitting _i(t') adopt following formula to represent, t'=t-T _i, T _i=(i-1), the value that t is horizontal ordinate;

$\left\{\begin{array}{c}{f}_1\left(t^{\&prime;}\right)=a_1{\left(t^{\&prime;}\right)}^3+b_1{\left(t^{\&prime;}\right)}^2+c_1\left(t^{\&prime;}\right)+d_1......0<t^{\&prime;}<1\\ ...\\ f_i\left(t^{\&prime;}\right)=a_i{\left(t^{\&prime;}\right)}^3+b_i{\left(t^{\&prime;}\right)}^2+c_i\left(t^{\&prime;}\right)+d_i......0<t^{\&prime;}<1\\ ...\\ f_{N-1}\left(t^{\&prime;}\right)=a_{N-1}{\left(t^{\&prime;}\right)}^3+b_{N-1}{\left(t^{\&prime;}\right)}^2+c_{N-1}\left(t^{\&prime;}\right)+d_{N-1}......0<t^{\&prime;}<1\end{array}\right\}$

Adopt least error quadratic sum formula calculate the time difference t of two signal S1 and S2 _k, time difference t _kbe t' value hour for making above-mentioned least error quadratic sum formula, adopt differentiate to ask the mode of extreme value to obtain t', t _kbelong to interval [0,1]; S ₂[i] is i element of S2 core signal, ± selection principle be: when S2 is the signal first receiving, choose plus sige, otherwise choose minus sign;

The 5th step: according to t _kcan try to achieve the time difference t_sse:t_sse=(k+t of two receiver received signals _k) T.

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