CN104374447A - Guided wave type radar level meter echo signal processing method based on first-order derivative with combination of reference curve - Google Patents
Guided wave type radar level meter echo signal processing method based on first-order derivative with combination of reference curve Download PDFInfo
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Abstract
The invention relates to a guided wave type radar level meter echo signal processing method based on a first-order derivative with the combination of a reference curve. The method includes the steps that firstly, the reference curve is constructed; secondly, echo signals are preprocessed; thirdly, level echo judgment is conducted; fourthly, propagation time is calculated; fifthly, a level value is calculated. Signals which comprise step parts which are not superposed with the level echo need to be stored in advance and serve as the reference curve for the subsequent level echo judgment; 9-point moving and average filtering preprocessing is conducted on the echo signals; first-order derivative calculation is conducted on the filtered echo signals, the echo is detected, multiple parameters are combined with the reference curve to judge the level echo, a level echo positioning point and a reference starting point are determined, and propagating time is calculated; three points are selected in a defined zone at will to be calibrated, and a level-propagating time model is determined; finally, the propagating time is substituted into a function relation between the level and the propagating time, and the level value is obtained.
Description
Technical field
The invention belongs to Level measurement technical field, being specifically related to a kind of take single-chip microcomputer as the method for the process guided wave formula radar levelmeter echoed signal based on first order derivative and in conjunction with reference curve of core.
Background technology
Guided wave formula radar levelmeter utilizes the echo ranging principle in Radar Technology to carry out level gauging, is widely used in industrial circle.The difficult point of guided wave formula radar levelmeter echo signal processing is the judgement of true thing position echo and the determination of echo-bearing point.Wherein, whether thing position echo determines measurement accurate, and echo-bearing point affects accuracy of measurement.
For the key issue of guided wave formula radar levelmeter signal transacting, famous producer of external a few family, such as, E+H company, Emerson Rosemount company and Siemens-Milltronics company decile do not propose different echo signal processing methods.Rosemount INC. (US) 12001 West 78th Street Prairie, Minnesota 55344 U.S.A. (" Method of determining a disturbance echo profile for a radar level gauge system ", U.S Patent:No.7924216 B2, Apr.12,2011) a kind of method based on reverberator is described, according to the reverberator waveform construction reference curve under slack tank condition, by comparing reverberator nearest on actual range and electrical distance determination liquid level, obtain reference curve range of application, the interference echo on liquid level can be eliminated, then according to maximal value determination thing position echo after subtracting each other.Rosemount INC. (US) 12001 West 78th Street Prairie, Minnesota 55344 U.S.A. (" System and method for filling level determination ", U.S Patent:No.7525476 B1, Apr.28,2009.) describe at guided wave bar known position installation reverberator, the change of velocity of propagation along guided wave bar is compensated, to improve accuracy of measurement by the known distance of a pair known reflectors and the difference of measuring distance.Rosemount INC.'s (" guided wave formula radar level gauge system with permittivity compensation by multi-mode propagation ", China, CN201120366374.7,2012-6-6.) describe a kind of special guided wave bar, be called incomplete outer dielectric transmission detector, the communication mode of two kinds or more friction speeds can be supported, then according to formula determination specific inductive capacity and velocity of propagation.
Chinese invention patent (" a kind of system and method processing guided wave formula radar levelmeter echoed signal ", the patent No.: ZL 201210150945.2, the applying date: 2012-9-12, authorize day: 2013-9-11) describe not jljl position echo determination methods for differing dielectric constant.The place that this patent is identical with above-mentioned patent is: according to slack tank signal configuration reference curve; The pre-service of Butterworth low pass ripple has been carried out to echoed signal section; The judgement of thing position echo can regard two steps as, first according to derivation detection echo, and arranges the interference of a width range eliminating part, then looks for remaining echo the maximal value determination thing position echo exceeding reference curve; Echo-bearing point is determined by the 2 rank Lagrange interpolation points of peak point and adjacent 2; Nominal data is linear interpolation or 3 matchings of 4 ~ 10.That, for little dielectric constant dielectric, reference curve needs the correction of filling and leading up coda wave with the difference of above-mentioned patent; Echoed signal section has been carried out slope and has been amplified to highlight variation tendency; Echoed signal section deducts reference curve and obtains clean echoed signal section, and process is afterwards all for this signal.
This patent Problems existing is: Butterworth low pass ripple is a kind of frequency domain filtering method, is applied to frequency domain character unconspicuous guided wave formula radar levelmeter echo signal processing, poor effect; Structure reference curve needs first to construct slack tank condition, when being applied to actual production, needs emptying material, and needs to upgrade, otherwise can not get rid of the new interference echo produced; Echoed signal and reference curve subtract each other the nonlinear moving that may cause echo-bearing point, affect measuring accuracy.
For this reason, application Chinese invention patent is (based on the guided wave formula radar levelmeter echo signal processing system and method for first order derivative, application number: 201310534738.1, the applying date: on November 1st, 2013) provide a kind of system and method processing the guided wave formula radar levelmeter echoed signal triggered without synchronizing pulse.Have employed based on first order derivative method detection of echoes, store the multiple parameter of echo, as end points, peak point, width, slope amplitude etc., and based on the organic assembling determination thing position echo to these parameters; Propose for the method for sampling of the guided wave formula radar levelmeter of triggerless sampling and the defining method with reference to starting point; Adopt at 3 o'clock, minute two sections of linear calibrations.This application patent of invention does not need slack tank condition, and practical operation is simple.
This application patent of invention Problems existing is: the restriction of echo parameter suppression ratio may cause getting rid of true echo or cannot causing sentencing mistake by exclusive PCR echo; Employing is carried out interpolation to true echo and is obtained peak-peak point as unique point, and carry out demarcating and measuring, its linearity is not best, and namely accuracy of measurement is not best; Carry out 60 median average filters to measurement result, response speed is slower.
Summary of the invention
In view of the above problems, the invention provides a kind of based on first order derivative and in conjunction with the guided wave formula radar levelmeter echo signal processing method of reference curve, thus reach more reliable to the accurate judgement of thing position echo, reach reduce blind area, improve measuring accuracy, response speed requirement faster, and not high to hardware requirement, without the need to constructing slack tank condition.
A kind of comprise based on first order derivative and in conjunction with the guided wave formula radar levelmeter echo signal processing method of reference curve (1) constructs reference curve, (2) signal preprocessing, (3) thing position echo judges, (4) travel-time calculates, and (5) calculate thing place value; Concrete operations are: in advance storage bag contain do not superpose " step " part with thing position echo signal as with reference to curve, judge for follow-up thing position echo; 9 moving average filtering pre-service are carried out to echoed signal; Carry out first order derivative to filtered signal and calculate detection of echoes, multiparameter judges thing position echo in conjunction with reference curve, determines thing position echo-bearing point and calculates the travel-time with reference to starting point; In restriction is interval, select at 3 arbitrarily demarcate, determine thing position-travel-time model; Finally, is substituted into thing position and the functional relation between the travel-time travel-time, calculate thing place value.
Concrete technical solution is as follows:
For the guided wave formula radar levelmeter without synchronous trigger pulse, trigger ADC owing to there is no synchronizing signal and sample, therefore need to determine first sampling point, ensure to collect correct signal segment.A complete echoed signal comprises a square-wave pulse and is positioned at some reflection echos thereafter.Before this echoed signal and next square-wave pulse, have longer steady section, there is no obvious echo, and square-wave pulse has precipitous rising edge and negative edge.According to this feature of signal, judge sampled point in the interrupt service subroutine of ADC, when determining the negative edge collecting square-wave pulse, the data of 1000 length after preservation, as the echoed signal section of subsequent treatment.Comprise in echoed signal section material surface, sensor self the factor such as " step " interference, noise produce reflection echo, referred to as echo.The concrete treatment step of echoed signal is as follows:
(1) reference curve is constructed
2 interference for " step " intrinsic in level meter signal is formed: when 1) thing position is less than 25cm, thing position echo amplitude is less than noise level thereafter; 2) " step " feature own and thing position echo are similar to.Therefore, storage bag need contain the signal of " step " part do not superposed with thing position echo as with reference to curve in advance, judge for follow-up thing position echo; Tank height is definite value, when guided wave formula radar levelmeter be arranged on tank deck, guided wave bar vertically downward time, definition guided wave formula radar levelmeter flange undersurface is thing position to the distance on material surface.Making thing position for being greater than the arbitrary value of 70cm by increasing or reduce material, then random acquisition one group of echoed signal, intercepting from starting point to the signal segment comprising " step " that do not superpose with other echoes intrinsic signal usable reflection echoed signal partly; Such as, when Sampling starting point is square-wave pulse negative edge, to be starting point to sampled point be this part in 255 ~ 305 a bit.When Sampling starting point be square-wave pulse rising edge initial time, this part be extended to possibly from starting point to sampled point be in 450 ~ 500 a bit.
(2) 9 moving average filterings are carried out to echoed signal section, noise decrease, smoothly to shake.
(3) echo seek scope is set for comprising effective reflection echo part, such as, when being square-wave pulse negative edge for 1m stock, Sampling starting point, sampled point 230 ~ 500 point can be set to, eliminate the impact of the guided wave bar tie point reflection echo that section start is fixed, and reduce the length of unnecessary signal transacting.Calculate 1 order derivative signal of echoed signal section within the scope of this
, the computing formula after simplification is
In formula,
for echoed signal section,
for the point on signal.
(4) to 1 order derivative signal
judge from the off.If
,
contrary sign, and
be greater than 0, then determine that n is the starting point start [i+1] of terminal end [i] and next echo; If
,
contrary sign, and
be less than 0, and starting point determines mark set, then determine that n is peak point peak [i]; I represents echo sequence number.
(5) unique point of echo has been recorded, namely after the starting point of echo, peak point, terminal, calculate the width w [i] of this echo, the difference of first order derivative maxima and minima, referred to as rate of change amp [i], and compare with preset value, the echo being less than preset value is got rid of, and the echo of reservation is as alternative echo, can exclusive PCR echo, described interference echo is random narrow spike, slightly noise echo.The computing formula of above-mentioned each characteristic parameter is:
In formula,
datafor echoed signal section,
d_max
, d_min represents first order derivative maxima and minima respectively,
ifor echo sequence number.
(6) to 1 order derivative signal
in scope, all points judge according to step (4) ~ (5), obtain several alternative echoes.
(7) alternative echo is judged, the echo and reference curve that are positioned at " step " scope place are made comparisons, be greater than 100 ~ 110 numerical value if do not meet, then get rid of this echo; Otherwise, retain.
(8) degree of confidence is carried out to each alternative echo
confidencecalculate, formula is as follows:
In formula,
ifor alternative echo sequence number,
datafor echoed signal section,
peakfor alternative echo-peak point.
(9) determine that alternative echo corresponding to maximum confidence is thing position echo.
(10) minimum slope point is asked for thing position echo, and on first order derivative curve, carry out 2 rank Lagrange interpolation, determine thing position echo-bearing point; Select a bit near fixation reflex echo guided wave bar tie point reflection echo negative edge mid point, crossing with this echo negative edge with this place straight line, linear interpolation obtains with reference to starting point; Then calculate the travel-time, described travel-time=thing position echo-bearing point-reference starting point.
(11) limit filtration is carried out to the travel-time, get rid of the erroneous calculations result that random disturbance reason causes, and preserve.
(12) to same thing position, the travel-time of more than 40 times calculating preservation carries out median average filter, and end product is as the travel-time of this thing position correspondence.
The described travel-time is substituted in the thing position determined by demarcation and the functional relation between the travel-time, calculate thing place value.
In formula,
lfor thing position,
tfor the travel-time,
k,bfor demarcating relational expression coefficient,
t 1 for boundary time point.
Advantageous Effects of the present invention embodies in the following areas:
1. with HeFei University of Technology's patent (" a kind of system and method processing guided wave formula radar levelmeter echoed signal ": China, CN201210150945.2,2012-9-12.) compare, the present invention does not distinguish size specific inductive capacity, the structure of reference curve does not need slack tank condition, therefore easier, to have employed based on first order derivative and in conjunction with the method detection of echoes of reference curve, store the multiple parameter of echo, as end points, peak point, width etc., and form Rule of judgment determination thing position echo based on to the different modes combination of these parameters, arranging of reference curve can get rid of the intrinsic interference of sensor, the condition that multiple parameter is formed then is applicable to additionally, random interference, avoid the more new demand to reference curve, and pass through blind area (within 30cm), water under metal interference, oil (little specific inductive capacity), the checking of long range measurements echo, demonstrate validity, propose for the method for sampling of the guided wave formula radar levelmeter of triggerless sampling and the defining method with reference to starting point, adopt at 3 o'clock, minute two sections of linear calibrations, therefore practical operation is simpler, and maximum error of measuring is within ± 1cm,
2. apply for a patent (based on the guided wave formula radar levelmeter echo signal processing system and method for first order derivative with HeFei University of Technology, application number: 201310534738.1, the applying date: on November 1st, 2013) compare, the condition restriction that the present invention eliminates " suppression ratio ", add the contrast with reference curve, improve the reliability that thing position echo judges, when former patent occurs sentencing mistake, this patent can accurately judge; Starting sample point is decided to be the negative edge of transmit square waves pulse, decreases the unnecessary collection of 200; With the interpolation point of minimum slope point as thing position echo-bearing point, improve the linearity of thing position-travel-time model; 60 groups of median average filters are reduced to 40 groups of median average filters, improve response speed; Adopt 3 o'clock minutes two sections of linear calibrations, specify that the interval of 3 and the value of waypoint.
Accompanying drawing explanation
Fig. 1 is guided wave formula radar level gauge system theory diagram.
Fig. 2 is signal conditioning circuit figure.
Fig. 3 is the Single Chip Microcomputer (SCM) system hardware circuit diagram based on MSP430F541A.
Fig. 4 is the Single Chip Microcomputer (SCM) system software the general frame based on MSP430F5418A.
Fig. 5 is the Single Chip Microcomputer (SCM) system master monitor process flow diagram based on MSP430F5418A.
Fig. 6 is slack tank, 66cm, 99cm echoed signal.
Fig. 7 is 9 moving average filtering design sketchs.
Fig. 8 is coaxial echoed signal 9 moving average filtering effects.
Fig. 9 is 15cm water level and slack tank signal.
Figure 10 is 16cm and 100cm water level signal.
Figure 11 is thing position echo decision flow chart.
Figure 12 is 15cm water echoed signal.
Figure 13 is compared with the echoed signal under strong jamming.
Figure 14 is long range measurements echoed signal.
Figure 15 is travel-time sample sequence.
Figure 16 is travel-time sample probability density function.
Figure 17 is travel-time sample probability density function Gauss curve fitting.
Embodiment
Below in conjunction with accompanying drawing, the present invention is further illustrated.
See Fig. 1, guided wave formula radar level gauge system is by impulse sender, guided wave bar, equivalent time sampling circuit, signal conditioning circuit, form based on the Single Chip Microcomputer (SCM) system of MSP430F541A and liquid crystal display.Wherein, equivalent time sampling circuit is made up of pulse receiver, pulse daley sequence generator, frequency mixer.
Produce pulse waveform by impulse sender, this signal is divided into two-way, and shaping is amplified through preposition trigger circuit in a road, becomes the pulse signal that amplitude is comparatively large, forward position is comparatively trembled and delivers to transponder pulse source.Transponder pulse source sends periodic short pulse electromagnetic wave by emitting antenna along guided wave bar.Electromagnetic wave reflects to form a series of echoed signal on the interphase of different medium, is sent to equivalent time sampling circuit by receiving antenna after being received.Meanwhile, time base circuit sends another road pulse signal in frequency mixer, carries out mixing with the echo-pulse after postponing through tiny time.Signal after equivalent time sampling becomes medium and low frequency signal, and after modulate circuit, be sent to single-chip microcomputer process, result of calculation is by liquid crystal display.
Modulate circuit as shown in Figure 2.Power circuit adopts linear voltage stabilization chip TPS71530 U4 to provide the supply voltage of 3V for signal condition system, and the 3.3V voltage introduced by outside is stablized after power circuit, pure 3V voltage, is system power supply; Bleeder circuit is made up of transport and placing device U1A, resistance R6 and R7; The ratio of resistance R6 and R7 determines intrinsic standoff ratio, and transport and placing device U1A is adopted as series voltage negtive backword connection, has high input impedance and low output impedance; DC bias circuit is made up of transport and placing device U2A, U2B and resistance R8, R9 and R11, R12 and electric capacity C8; Electric capacity C8 plays every straight effect, and the ratio of resistance R8 and R9, R11 and R12 determines enlargement factor, after two-stage direct current biasing, and output voltage V0=Vref+Vin; Reference voltage generating circuit is made up of transport and placing device U1B, resistance R14 and R15, electric capacity C10; The ratio of resistance R14 and R15 determines the reference voltage level exported; Transport and placing device U1B rises and follows and buffer action.Because the amplitude of the echoed signal of equivalent time sampling circuit output is 0V to 4.7V, and MSP430F5418A single-chip microcomputer U3 only allows the electrode of input 0 to 3.3V, so, the echoed signal that equivalent time sampling circuit exports is nursed one's health; The echoed signal that equivalent time sampling circuit exports is connected to the left end of resistance R6 in signal conditioning circuit; Echoed signal enters bleeder circuit, and through dividing potential drop, its amplitude is reduced to 1/3 of open width value; Then, enter DC bias circuit, cut off the DC component in signal by electric capacity C8, and be raised to 1.25V by voltage offset electric circuit, and be the half of the reference voltage of ADC in MSP430F5418A singlechip chip.
Echoed signal after conditioning is connected to the pin 7 of MSP430F5418A U3 single-chip microcomputer, and pin 7 is P7.6/A14, as shown in Figure 3.The pin 7 of MSP430F5418A single-chip microcomputer is multiplexing pins, analog input A14-ADC is configured to, ADC in the sheet delivering to MSP430F5418A single-chip microcomputer U3 by echoed signal (indicating: ADC in the sheet that the following stated ADC is MSP430F5418A single-chip microcomputer) at this.The output Vref of the reference voltage generating circuit in Fig. 2 is connected to the Vref into DC voltage bias circuit, for DC voltage bias circuit provides reference voltage.
Based on MSP430F5418A Single Chip Microcomputer (SCM) system hardware circuit as shown in Figure 3, be made up of MSP430F5418A single-chip microcomputer U3, liquid crystal display (LCD) interface P2 and emulator interface P3.Select TI company MSP430F5418A single-chip microcomputer as core processor, because it has the features such as super low-power consumption, powerful processing power, abundant peripheral module, be applicable to the requirement of guided wave formula radar levelmeter signal transacting completely, and for guided wave formula radar levelmeter two-wire system realize provide possibility.
In Fig. 3, P2 is LCD interface.This LCD is dot matrix, parallel interface.The data transmission pin 3 ~ 10 of LCD is connected to the I/O mouth 42 ~ 35 of MSP430F5418A single-chip microcomputer U3 respectively,
(chip selection signal),
(reset),
(register selection),
(write enable signal),
(reading enable signal) is connected to pin 25-P2.0,26-P2.1,27-P2.2,28-P2.3, the 29-P2.4 of MSP430F5418A single-chip microcomputer U3 respectively, complete corresponding write operation by MSP430F5418A single-chip microcomputer U3, result is presented on LCD.
In Fig. 3, P3 is artificial mouth.Adopt personal computer and emulator, by this interface, complete the kernel operations to MSP430F5418A single-chip microcomputer U3, comprise program debug and programming etc.
Fig. 4 is the Single Chip Microcomputer (SCM) system software the general frame based on MSP430F5418A.System software adopts modular design method, and by master monitor module, interruption processing module, initialization module, echo signal processing module, module calculated by level meter and LCD display module forms.
Master monitor module in Fig. 4 is the total activation program of the Single Chip Microcomputer (SCM) system software based on MSP430F5418A, by calling each program module, realizes the function required by native system.
Fig. 5 is the Single Chip Microcomputer (SCM) system master monitor process flow diagram based on MSP430F5418A.It is a loop program.After system electrification, master monitor runs automatically, and whole program, by the mode by setting, carries out respective handling to each subroutine.Its basic process is: system electrification, carries out initialization to each functional module of MSP430F5418A single-chip microcomputer U3 and LCD display module variable; Open system interruption successively, ADC, then enter low-power consumption mode; In disconnected service routine, sampled data is judged in the adc.If collect the transmit square waves pulse falling edge of signal, start to preserve sampled data, and record sampling number; When collection 1000, exit low-power consumption mode, and close ADC; Echo signal processing is carried out to sampled signal, comprising: 9 moving average filterings carried out to echoed signal, filtered signal is carried out to the echo judgement of thing position, determine that echo-bearing point and reference starting point are to calculate the travel-time, to carry out filtering process to calculated value; Then, the travel-time is substituted into by demarcating the thing position determined and the functional relation between the travel-time calculates thing position; Subsequently, result by liquid crystal display out.
Lower mask body introduces echo signal processing, is divided into 5 major parts.
(1) reference curve is constructed
2 interference for " step " intrinsic in level meter signal is formed: when 1) thing position is less than 25cm, thing position echo amplitude is less than noise level thereafter; 2) " step " feature own and thing position echo are similar to.Therefore, storage bag need contain the signal of " step " part do not superposed with thing position echo as with reference to curve in advance, judge for follow-up thing position echo.Slack tank signal as shown in Figure 6,66cm, 99cm signal contrast, their " step " partially overlaps, visible, adjustable material thing position, any place after making it to be positioned at 70cm.At this moment gather echoed signal, intercept and store to sampled point 255 parts from starting point, as reference signal.This has effect same with storage slack tank signal, but, avoid structure slack tank condition.Comparatively speaking, do so more convenient and practical.In follow-up thing position echo judges, by comparing with reference curve, the interference of " step " can be got rid of.
(2) signal preprocessing
Adopt 9 moving average filterings.The echoed signal of guided wave formula radar levelmeter is typical time-domain signal, does not show obvious feature at frequency domain.Burr can be eliminated by moving average filtering, smoothly shake.Meanwhile, this filtering mode can cause signal amplitude to reduce, and echo rate of change diminishes, and therefore needs counting of choose reasonable moving average filtering.By comparing, select 9 moving average filterings, formula is:
(1)
In formula,
yfor filtered signal,
nfor the point on signal,
nfor signal is counted.
As can be seen from Fig. 7 and Fig. 8,9 moving average filterings substantially reduce signal jitter, and easily occur causing full width due to overflow when thing position is measured for coaxitron in Fig. 8, the crest of echoed signal is scabbled, the situation that echo-bearing point is not easily determined, after 9 moving average filterings, a more sharp-pointed crest can be obtained, be convenient to subsequent calculations.Therefore, 9 moving average filterings are selected in pre-service.
Table 1 compared for the filter effect of primary curve, Butterworth filtering, 5 moving averages, 9 moving averages, 21 moving averages.13 groups of echoed signals of random acquisition under same thing position are processed, same procedure is adopted to calculate the travel-time respectively, finally calculated to the difference (being designated as amplitude) of sample variance and max min this 13 group propagation time, observe the improvement effect of different pretreatments to signal stabilization degree, repeatability with this.
Can be found out by table 1 data, sample variance and the amplitude of 9 moving average filterings are minimum, and be significantly less than original signal, improvement result is obvious.
(3) thing position echo judges
Feature according to guided wave formula radar levelmeter signal: be 1) nonperiodic signal, do not have obvious characteristic at frequency domain, can not frequency domain technique be adopted; 2) signal does not have master pattern, interference echo and thing position echo shape similar, do not detect by relevant.Therefore, its signal transacting mainly carries out in time domain.
The difficult point of thing position echo identification comprises: 1) have many fixing interference, comprise the reflection echo of guided wave bar tie point, be positioned at " step " of sampled point 230 point ~ 255 point (when Sampling starting point is square-wave pulse negative edge), as shown in the arrow on Fig. 9 slack tank signal.The existence of " step " causes the thing position echo amplitude within 25cm or 30cm less, as 15cm signal in Fig. 9; 2) according to scene reflection, may there is random narrow high impulse interference in actual signal, amplitude is very large; 3) interference echo that causes such as chaff interference.Adopt the method for looking for maximum of points determination echo peak the earliest, therefore, have the blind area of 25cm or 30cm, and easily occur sentencing wrong situation, produce gross error.
Based on the guided wave formula radar levelmeter echo signal processing system and method for first order derivative, (application number: 201310534738.1, the applying date: on November 1st, 2013) propose a kind of thing position echo recognition methods of the more characteristic parameters based on first order derivative, utilize multiple parameters of echo, be bonded multiple Rule of judgment by different modes, enhance the reliability that echo judges.But found through experiments, when arranging suppression ratio and being 0.5, there will be erroneous judgement in some cases.Its reason is, in this case, the suppression ratio of thing position echo is also less than 0.5, as shown in Figure 10.Therefore, in " roughly selecting " stage by error exception.Such as, if suppression ratio reduced, 0.2, because the two difference is little, at some thing position place, because the situation that " step " can not be excluded may appear in the interference such as noise, erroneous judgement still can be caused.For this reason, the basis of the method is improved.
The judgement of thing position echo still adopts the thinking first got rid of, determine among a small circle more on a large scale, can regard two steps as: " roughly selecting " and " selected ".Detection of echoes when " roughly selecting ", and preserve starting point, peak and end point.By arranging the parameter exclusive PCR such as Echo width, determine alternative echo.The weighting sum such as peak point amplitude, width calculating alternative echo time " selected ", as degree of confidence, determines thing position echo.
In laboratory conditions statistical study is carried out to the echo character parameter of not jljl position, comprise peak point amplitude, width, rate of change etc.Echo width and peak point amplitude reflect backward energy; Rate of change is the difference of maximum slope and minimum slope on echo rising edge and negative edge, reflects echo change speed degree.From experimental data, in most cases thing position echo has maximum amplitude, and width is basicly stable, but echo superposes this situation and width can be caused to become large; Rate of change is comparatively large, but does not present regular change in time.Disturbance echo has different characteristics, is difficult to determine thing position echo by single condition.For this reason, the feature according to interference echo is classified, and forms different condition and gets rid of.
The Parameters variation interval of " roughly selecting " for getting rid of constant interval and the thing position echo with one or more parameter does not have overlapping interference echo, comprise random narrow spike, noise that width amplitude is very little, by arranging Echo width, the lower limit of rate of change gets rid of." selected " for getting rid of secondary or multiecho, echo that chaff interference produces and the larger noise of width, Parameters variation due to them is interval overlapping with having of thing position echo, therefore, do not get rid of by limited field, therefore adopt the method for comprehensive multiple calculation of characteristic parameters degree of confidence.
For " step " intrinsic in level meter signal interference, if this interference echo was not excluded in " roughly selecting " stage, then subsequent calculations degree of confidence can produce higher value, easily causes erroneous judgement.Therefore, before calculating degree of confidence, the alternative echo being positioned at " step " part is contrasted with the reference curve stored in advance, be greater than reference curve certain limit if do not meet, then think that this echo is itself " step ", got rid of.
Known to the characteristic parameter statistical study of " roughly selecting " residual echo afterwards, the echo that width is less, peak point amplitude is larger, rate of change is larger is that the possibility of thing position echo is larger.Therefore, set the coefficient of width during confidence calculations as negative, represent that width is larger, the possibility of thing position echo is less.The difference arranging these three parameter number magnitudes of Main Basis of weights, peak point amplitude is 10
3, rate of change is 10
2or 10, width is 10.Rule of thumb arrange the rate of change of confidence calculations, peak point amplitude, width weights, such as 0.6,0.1,0.3, the experimental formula obtaining confidence calculations is: degree of confidence=0.6 × rate of change+0.1 × peak point amplitude-0.3 × width.
Signal processing flow figure as shown in figure 11.
Concrete steps are as follows:
A () arranges echo seek scope for comprising effective reflection echo part,
Tank height is definite value, when guided wave formula radar levelmeter be arranged on tank deck, guided wave bar vertically downward time, definition guided wave formula radar levelmeter flange undersurface is thing position to the distance on material surface; Making thing position for being greater than the arbitrary value of 70cm by increasing or reduce material, then random acquisition one group of echoed signal, intercepting from starting point to the usable reflection echoed signal comprising " step " that do not superpose with other echoes part intrinsic signal;
Such as, for guided wave pole length be 1m, Sampling starting point be square-wave pulse negative edge time, sampled point 230 ~ 500 point can be set to, eliminate the impact of the guided wave bar tie point reflection echo that section start is fixed, and reduce the length of unnecessary signal transacting.Calculate 1 order derivative signal of echoed signal section within the scope of this
.Because the time interval of neighbouring sample point is fixed and very short, replace first order derivative to reach accuracy by first order difference, its computing formula can be approximately:
(2)
Because consecutive point are spaced apart 1, so formula (2) can be reduced to:
(3)
In formula,
for echoed signal,
for the point on signal;
B () is to first derivative signal
judge from the off, if
,
contrary sign, and
be greater than 0, then determine that n is the starting point start [i+1] of terminal end [i] and next echo; If
,
contrary sign, and
be less than 0, and starting point determines mark set, then determine that n is peak point peak [i]; I represents echo sequence number;
C () has recorded the unique point of echo, namely after the starting point of echo, peak point, terminal, calculate the width w [i] of this echo, the difference of first order derivative maxima and minima, referred to as rate of change amp [i], and compare with preset value, the echo being less than preset value is got rid of, and the echo of reservation is as alternative echo.Can exclusive PCR echo, described interference echo is random narrow spike, slightly noise echo etc.The computing formula of above-mentioned each characteristic parameter is:
(4)
(5)
In formula,
datafor echoed signal section,
d_max
, d_min represents first order derivative maxima and minima respectively,
ifor echo sequence number;
D () is to first derivative signal
in scope, all points judge according to step (b) ~ (c), obtain several alternative echoes;
E () judges alternative echo, will be positioned at " step " scope place, and namely sampled point is that the echo of 230 ~ 255 and reference curve are made comparisons.If do not meet and be greater than 100 ~ 110, then get rid of; Otherwise, retain;
F (), to alternative echo " selected ", namely calculate degree of confidence to each alternative echo, formula is:
(6)
G () determines that echo corresponding to maximum confidence is thing position echo.
As shown in Figure 12 ~ 14, one group of △ and a zero corresponding alternative echo in figure, the echo after namely " roughly selecting ", △ represents starting point, and zero represents end point, the thing position echo range determined after vertical dashed lines labeled " selected ".Figure 12 is 15cm water level echo signal, and thing position echo amplitude is less than noise thereafter.Thing position echo accurately cannot be judged by peak-peak method; The suppression ratio of this echo is less than 0.5, is more or less the same with the suppression ratio of itself " step ".Get rid of " step " interference according to setting " suppression ratio " parameter, will inevitably be there is the two and be excluded the problem that maybe can not be excluded simultaneously.With this patent institute extracting method, just can get rid of the larger noise of amplitude in conjunction with " the roughly selecting " after reference curve, only remain thing position echo.Figure 13 is 65cm water level echo signal, produces comparatively strong jamming with the retort stand directly contacted at the bottom of tank.Two alternative echoes are obtained after " roughly selecting ".Because the width of interference echo is larger, confidence value is less, is determined thing position echo.Figure 14 is the echoed signal of 8m long guided wave bar 650cm thing position, reduce because signal energy increases with thing position, therefore thing position echo amplitude is less than the noise level before being positioned at it, as seen from the figure, have to an echo after " roughly selecting ", efficiently solve this situation.
(4) travel-time calculates
Travel-time calculates the determination containing thing position echo-bearing point and beginning point of reference, thing position echo-bearing point and the terminal in travel-time.
Existing method is using peak point as thing position echo-bearing point, there is the problem that fluctuation is large, precision is low, also can left and right skew 1 ~ 2 point under same liquid level.Carry out 2 rank Lagrange interpolation to it to be equivalent to compensate for the low problem of sample frequency, can precision be improved.
To the unique point of echo, as maximum slope point, minimum slope point and test using these 2 points of the regional center as border, observe the linear case of not jljl position echo-bearing point.Wherein, central point computing formula is:
(8)
In formula,
midfor regional center point horizontal ordinate,
yfor echo curve,
ifor horizontal ordinate sampled point sequence number.
27 liquid levels have been randomly drawed to 20 ~ 100cm liquid level, gather one group of signal respectively, after 9 moving average filtering pre-service are carried out to signal, with peak point, peak point 2 rank Lagrange interpolation point, maximum slope point, minimum slope point, central point as thing position echo-bearing point, all adopt least square fitting, using the thing position that each echo-bearing point is determined-linearly model root-mean-square error as the measurement index of the linearity, as shown in table 3.
From table 3 data, the root-mean-square error of carrying out least square fitting as thing position echo-bearing point with minimum slope point is minimum, the highest for the accuracy calculating thing position, is secondly 2 rank Lagrange interpolation points of peak point.Identical with the object that peak point carries out 2 rank Lagrange interpolation, the first order derivative curve of signal carries out 2 rank Lagrange interpolation to minimum slope point, can further improve precision.
The starting point in travel-time is the transmit square waves pulse moment in theory, but in actual signal, exomonental starting point shakes severity usually, is flooded by noise, not easily accurately determines, the travel-time error calculated like this is larger.
If the starting point corresponding moment is
t 0 , in the echo-bearing point corresponding moment of two known substance positions, namely time of reception is respectively
t 1 ,
t 2 , thing position is respectively
h 1 ,
h 2 , then the expression formula of linear fit
middle parameter is:
(9)
From formula (9), level meter is calculated and is had nothing to do with the starting point moment.Therefore, the selection of starting point is minimum for criterion to calculate fluctuation.
In gathered signal, first reflection echo is fixation reflex echo, is produced by guided wave bar tie point, width is little, amplitude is large, rate of change is large feature that it has.Select the straight line of different amplitude crossing with this negative edge, calculate intersection point as with reference to starting point by the method for linear interpolation, computing time is poor, known respectively, amplitude more close to pulse falling edge mid point, mistiming calculated value more stable.Therefore, select a bit near guided wave bar tie point reflection echo negative edge mid point, crossing with this echo negative edge with this place straight line, linear interpolation obtains with reference to starting point.
Determine with reference to after starting point and thing position echo-bearing point, can the travel-time be calculated further: travel-time=thing position echo-bearing point-reference starting point.
(5) to calculated value filtering
In actual job site, due to the existence of various interference, even if thing position is constant, travel-time calculated value still constantly changes.First, eliminate gross error by limit filtration, namely restriction travel-time calculated value fluctuating range gets rid of the erroneous calculations result that the factor such as random disturbance, mistake ripple causes.For thing position echo-peak point, generally about also meeting is located in same thing position, offseting 1 ~ 2 point, can amplitude limit be therefore 5.
Liquid level is constant, continuous acquisition 5000 group propagation time result of calculation, as shown in figure 15.Can be calculated the waving interval in travel-time for [233.4419,234.6059], maximum offset is 1.1640, and through converting corresponding 1.5609cm, namely the measured value maximum offset of same thing position is 1.5609cm.According to the computing method of probability density, travel-time sample fluctuation interval is divided into 30 sections, calculates the probability density on interval after sample is sorted, as shown in figure 16.
As shown in Figure 16, the probability density function of travel-time sample does not meet Gaussian distribution.Therefore, can not noise decrease effectively with linear filtering.
Because medium filtering has special advantage in eliminating abnormal data, calculating probability density function is again carried out after medium filtering to travel-time sample sequence, and Gauss curve fitting is carried out as shown in figure 17 to it, can find out that the sample probability density after medium filtering meets Gaussian distribution substantially, then be averaged and can reduce stochastic error.Median average filter combines the advantage of medium filtering and average filter, more easy in realization, carries out median average filter to go singular value, to reduce stochastic error to repeatedly calculated value.
Moving average filtering pre-service is carried out to signal, limit filtration and median average filter are carried out to calculated value, Stochastic choice 30 groups of liquid level recorder liquid levels and corresponding travel-time calculated value, the data obtained is as shown in table 3 by least square fitting determination root-mean-square error.By contrast table 3 and table 4 known, contribute to after carrying out median average filter reduce error, improve accuracy in computation.
Based on the guided wave formula radar levelmeter echo signal processing system and method (application number: 201310534738.1 of first order derivative, the applying date: on November 1st, 2013) propose median average filter calculated value being carried out to 60 groups, can stochastic error be effectively reduced.But find in actual applications, the median average filter of 60 groups will cause calculated value refreshing too slow.Such as, detect through reality, often performing a major cycle needs to spend 90ms, and namely often collection 1 signal calculates the time of 1 thing place value.So liquid crystal refreshes 1 value needs 90ms*60=5.4s, obviously cannot follow the tracks of the change of liquid level fast.For this reason, median average filter window is decreased to 40 groups, as calculated, liquid crystal refresh time is 90ms*40=3.6s.
Through above echo signal processing, obtain the travel-time corresponding with a certain thing position.Then, is substituted into the thing position determined by demarcation and the functional relation between the travel-time this travel-time, just can calculate effluent place value.
Calibration process completes before level meter dispatches from the factory.Effluent position and the functional relation between the travel-time is determined by demarcating.In theory, thing position and be linear relationship between the travel-time.In fact then there is nonlinear relationship.For this reason, piece-wise linearization process is carried out.For the guided wave bar that 1m is long, better to the linearity of bar end from guided wave bar centre position; For long guided wave bar, the partial linear degree after 1m is fine.Therefore, the method for piece-wise linearization is adopted.Consider the convenience demand of practical operation, preferably carry out 3 demarcation.For the disposal route using minimum slope point as thing position echo-bearing point, these 3 be selected within 30cm respectively, [30cm, 35cm] ∪ [55cm, 60cm], and near end 100cm.With the 1st with the coefficient of the 2nd matching as the 1st section; 2nd with the coefficient of the 3rd matching as the 2nd section.The separation of two sections is 30cm, and the travel-time of this thing position correspondence can be obtained by any point calibration point rough calculation, without the need to additionally increasing calibration point.Determine expression formula
, wherein,
lfor thing position,
tfor the travel-time,
k,bfor demarcating relational expression coefficient,
t 1 for boundary time point.After write Single Chip Microcomputer (SCM) program, when actual measurement, the travel-time obtained by echo signal processing substitutes into functional relation, just can calculate effluent place value.
Claims (3)
1. based on first order derivative and in conjunction with the guided wave formula radar levelmeter echo signal processing method of reference curve, it is characterized in that: comprise (1) structure reference curve, (2) signal preprocessing, (3) thing position echo judges, (4) travel-time calculates, and (5) calculate thing place value; Concrete operations are: in advance storage bag contain do not superpose " step " part with thing position echo signal as with reference to curve, judge for follow-up thing position echo; 9 moving average filtering pre-service are carried out to echoed signal; Carry out first order derivative to filtered signal and calculate detection of echoes, multiparameter judges thing position echo in conjunction with reference curve, determines thing position echo-bearing point and calculates the travel-time with reference to starting point; In restriction is interval, select at 3 arbitrarily demarcate, determine thing position-travel-time model; Finally, is substituted into thing position and the functional relation between the travel-time travel-time, calculate thing place value.
2. as claimed in claim 1, to it is characterized in that in conjunction with the guided wave formula radar levelmeter echo signal processing method of reference curve based on first order derivative:
For the guided wave formula radar levelmeter without synchronous trigger pulse, trigger ADC owing to there is no synchronizing signal and sample, therefore need to determine first sampling point, ensure to collect correct signal segment; A complete echoed signal comprises a square-wave pulse and is positioned at some reflection echos thereafter; Before this echoed signal and next square-wave pulse, have longer steady section, there is no obvious echo, and square-wave pulse has precipitous rising edge and negative edge; According to this feature of signal, judge sampled point in the interrupt service subroutine of ADC, when determining the negative edge collecting square-wave pulse, the data of 1000 length after preservation, as the echoed signal section of subsequent treatment; Comprise in echoed signal section material surface, sensor self the factor such as " step " interference, noise produce reflection echo, referred to as echo; The concrete treatment step of echoed signal is as follows:
(1) reference curve is constructed
2 interference for " step " intrinsic in level meter signal is formed: when 1) thing position is less than 25cm, thing position echo amplitude is less than noise level thereafter; 2) " step " feature own and thing position echo are similar to; Therefore, storage bag need contain the signal of " step " part do not superposed with thing position echo as with reference to curve in advance, judge for follow-up thing position echo; Tank height is definite value, when guided wave formula radar levelmeter be arranged on tank deck, guided wave bar vertically downward time, definition guided wave formula radar levelmeter flange undersurface is thing position to the distance on material surface; Making thing position for being greater than the arbitrary value of 70cm by increasing or reduce material, then random acquisition one group of echoed signal, namely intercepting from starting point to the signal segment comprising " step " that do not superpose with other echoes intrinsic signal reflection echo signal partly; When Sampling starting point is square-wave pulse negative edge, to be starting point to sampled point be this part in 255 ~ 305 a bit; When Sampling starting point be square-wave pulse rising edge initial time, this part be extended to possibly from starting point to sampled point be in 450 ~ 500 a bit;
(2) 9 moving average filterings are carried out to echoed signal section, noise decrease, smoothly to shake;
(3) echo seek scope is set for comprising effective reflection echo part, for guided wave pole length be 1m, Sampling starting point be square-wave pulse negative edge time, can arrange sampled point is 230 ~ 500 points, eliminate the impact of the guided wave bar tie point reflection echo that section start is fixed, and reduce the length of unnecessary signal transacting; Calculate 1 order derivative signal of echoed signal section within the scope of this
, the computing formula after simplification is
In formula,
for echoed signal section,
for the point on signal;
(4) to 1 order derivative signal
judge from the off, if
,
contrary sign, and
be greater than 0, then determine that n is the starting point start [i+1] of terminal end [i] and next echo; If
,
contrary sign, and
be less than 0, and starting point determines mark set, then determine that n is peak point peak [i]; I represents echo sequence number;
(5) unique point of echo has been recorded, namely after the starting point of echo, peak point, terminal, calculate the width w [i] of this echo, the difference of first order derivative maxima and minima, referred to as rate of change amp [i], and compare with preset value, the echo being less than preset value is got rid of, and the echo of reservation is as alternative echo, can exclusive PCR echo, described interference echo is random narrow spike, slightly noise echo; The computing formula of above-mentioned each characteristic parameter is:
In formula,
datafor echoed signal section,
d_max
, d_min represents first order derivative maxima and minima respectively,
ifor echo sequence number;
(6) to 1 order derivative signal
in scope, all points judge according to step (4) ~ (5), obtain several alternative echoes;
(7) alternative echo is judged, the echo and reference curve that are positioned at " step " scope place are made comparisons, be greater than 100 ~ 110 numerical value if do not meet, then get rid of this echo; Otherwise, retain;
(8) degree of confidence is carried out to each alternative echo
confidencecalculate, formula is as follows:
In formula,
ifor alternative echo sequence number,
datafor echoed signal section,
peakfor alternative echo-peak point;
(9) determine that alternative echo corresponding to maximum confidence is thing position echo;
(10) minimum slope point is asked for thing position echo, and on first order derivative curve, carry out 2 rank Lagrange interpolation, determine thing position echo-bearing point; Select a bit near fixation reflex echo guided wave bar tie point reflection echo negative edge mid point, crossing with this echo negative edge with this place straight line, linear interpolation obtains with reference to starting point; Then calculate the travel-time, described travel-time=thing position echo-bearing point-reference starting point;
(11) limit filtration is carried out to the travel-time, get rid of the erroneous calculations result that random disturbance reason causes, and preserve;
(12) to same thing position, the travel-time of more than 40 times calculating preservation carries out median average filter, and end product is as the travel-time of this thing position correspondence;
The described travel-time is substituted in the thing position determined by demarcation and the functional relation between the travel-time, calculate thing place value;
In formula,
lfor thing position,
tfor the travel-time,
k,bfor demarcating relational expression coefficient,
t 1 for boundary time point.
3. adopt the disposal system of guided wave formula radar levelmeter echo signal processing method described in claim 1 or 2, it is characterized in that: comprise impulse sender, guided wave bar, equivalent time sampling circuit, signal conditioning circuit, based on the Single Chip Microcomputer (SCM) system of MSP430F541A and liquid crystal display; Produce pulse waveform by impulse sender, this signal is divided into two-way, and shaping is amplified through preposition trigger circuit in a road, becomes the pulse signal that amplitude is comparatively large, forward position is comparatively trembled and delivers to transponder pulse source; Transponder pulse source sends periodic short pulse electromagnetic wave by emitting antenna along guided wave bar; Electromagnetic wave reflects to form a series of echoed signal on the interphase of different medium, is sent to equivalent time sampling circuit by receiving antenna after being received; Meanwhile, time base circuit sends another road pulse signal in frequency mixer, carries out mixing with the echo-pulse after postponing through tiny time; Signal after equivalent time sampling becomes medium and low frequency signal, and after modulate circuit, be sent to single-chip microcomputer process, result of calculation is by liquid crystal display; During for level gauging, carry out according to the step of first demarcating rear measurement; Fixed at the advance rower that dispatches from the factory, timing signal, for the long guided wave bar of 1m, 3 thing sites are selected within 30cm respectively, [30cm, 35cm] ∪ [55cm, 60cm], and near end 100cm; Obtain and the travel-time corresponding to these 3 thing sites according to signal transacting step according to claim 1, as nominal data, be divided into two sections and carry out linear fit, with the 1st with the coefficient of the 2nd matching as the 1st section; 2nd with the coefficient of the 3rd matching as the 2nd section; The separation of two sections is 30cm, and the travel-time of this thing position correspondence can be obtained by any point calibration point rough calculation, without the need to additionally increasing calibration point; The functional relation write Single Chip Microcomputer (SCM) system that matching obtains.
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