EP0524275A1 - Electric circuit for a device for measuring the level in industrial tanks and the like - Google Patents

Electric circuit for a device for measuring the level in industrial tanks and the like

Info

Publication number
EP0524275A1
EP0524275A1 EP92902706A EP92902706A EP0524275A1 EP 0524275 A1 EP0524275 A1 EP 0524275A1 EP 92902706 A EP92902706 A EP 92902706A EP 92902706 A EP92902706 A EP 92902706A EP 0524275 A1 EP0524275 A1 EP 0524275A1
Authority
EP
European Patent Office
Prior art keywords
frequency
microprocessor
bandpass filter
level
low
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP92902706A
Other languages
German (de)
French (fr)
Inventor
Ronald Van Der Pol
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Krohne Messtechnik GmbH and Co KG
Original Assignee
Krohne Messtechnik GmbH and Co KG
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Krohne Messtechnik GmbH and Co KG filed Critical Krohne Messtechnik GmbH and Co KG
Publication of EP0524275A1 publication Critical patent/EP0524275A1/en
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
    • G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
    • G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
    • G01F23/28—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring the variations of parameters of electromagnetic or acoustic waves applied directly to the liquid or fluent solid material
    • G01F23/284—Electromagnetic waves
    • 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
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications

Definitions

  • the invention relates to an electrical circuit for a device for level measurement of industrial tanks and.
  • the like With an electronic transmitter and receiving part for microwaves and a waveguide provided with a window and an antenna, the signal reflected by the liquid level and the transmission signal being fed to a mixing stage, the low-frequency output signal of which is via an A / D converter the microprocessor calculating the fill level is abandoned.
  • Level measuring devices working with microwaves which emit signals in the microwave range, these being reflected on the surface of the medium in the container and being picked up by the receiver.
  • the distance between the fill level measuring device and the surface of the medium is determined from the signals with the aid of an electronic switching arrangement, which is also assigned to microprocessors and electronic computers for evaluation.
  • an electronic switching arrangement which is also assigned to microprocessors and electronic computers for evaluation.
  • a cylindrical waveguide window made of quartz glass or the like is arranged in the waveguide projecting through the tank roof of the tank and has a low dielectric loss factor which is favorable for the permeability of the microwaves.
  • a voltage-controlled ten oscillator generates a continuously changing transmission signal which uses a coupler to generate microwaves which are directed onto the liquid surface via the waveguide provided with a window and the antenna.
  • the electromagnetic waves reflected by the liquid surface are received by the antenna and converted into an electrical frequency signal by the head part, which is fed to a mixer.
  • the received signal experiences a frequency shift compared to the transmitted signal, which is directly proportional to the fill level.
  • the transmit and receive signals are converted by means of a mixing stage into a low-frequency signal, which is digitized and processed in the microprocessor.
  • the optical window arranged in the waveguide disadvantageously causes interference signals due to reflection of the microwaves, the amplitudes of which are substantially greater than those of the useful signal.
  • the output signal of the mixer stage contains a power density component which may be considerably higher than the component of the useful signal reflected by the liquid surface due to possible mode changes in the microwave signal and / or due to multiple reflections of the optical window.
  • the spectral lines of the power density spectrum which occur at different fill levels result in relatively high values at low frequencies, which are based on the reflection of the optical window, but lower values at higher frequencies due to the larger measuring section.
  • the high, low-frequency power components that arise in particular when the optical window is dirty are also present at a low fill level.
  • the fluctuations between the low-frequency and higher-frequency values of the power density can be between four and twenty times, in industrial tanks and the like. Like. Are even above it. 2
  • a circuit for a level measuring device without a waveguide window is known from DE-OS 31 34 243, in which a filter is arranged between a mixing stage and an A / D converter and is intended to filter out undesired frequencies. Since the distance information is the content of the mixed frequency, the filtering out of undesired frequencies would also filter out the corresponding useful signals and would not allow a distance determination in this frequency range at all.
  • the object of the invention is to design a circuit of the generic type in such a way that the interference signals are damped without impairing the detection of the distance information.
  • This object is achieved according to the invention in that a bandpass filter with a lower fundamental frequency damping the low-frequency interference frequencies is arranged between the mixer stage and the A / D converter.
  • the band-pass filter only the low-frequency range of the frequency band is specifically attenuated by the band-pass filter, in which the interference frequencies caused by the optical window and on the other hand the useful frequencies which arise at a relatively high level with a high power density are located.
  • the low-frequency interference frequencies are essentially eliminated and useful signals are transmitted in a power density suitable for the evaluation.
  • the useful signals which are above the fundamental frequency of the bandpass filter and which have only a lower power density due to the greater distance between the level measuring device and the surface of the medium are passed through in a damped manner.
  • a bandpass filter can be selected with a correspondingly fixed cut-off frequency, which captures the spectrum of the interference frequencies caused by the optical window.
  • a limit frequency as suitable, which corresponds to a distance frequency generated with a distance between the coupler of the level measuring device and the liquid level of approximately 0.1 to 3 m.
  • the lower cut-off frequency can advantageously also be controlled either by the microprocessor or a computer, or the bandpass filter can be adjustable to a plurality of selectable cut-off frequencies.
  • a frequency analysis can be carried out with the microprocessor and the limit frequency of the bandpass filter can be set depending on the proportion of the interference frequencies. Adjusting the cut-off frequency can also make sense depending on the level.
  • the interference frequencies caused by the optical window are expediently determined when the tank is empty, stored in the microprocessor and taken into account in the arithmetic signal processing.
  • the bandpass filter is advantageously assigned an amplifier stage controlled by the microprocessor, which can be arranged or integrated before or after the bandpass filter.
  • 1 shows a circuit arrangement for a level measuring device
  • Fig. 3 shows the low-frequency signal formed by a mixer
  • FIG. 4 shows the power density spectrum formed by an A / D converter.
  • a sawtooth generator 1 or the like controls a voltage-controlled oscillator 2, which generates an electrical output signal f a which changes continuously in frequency.
  • the time course of this frequency is represented, for example, in FIG. 2 by the curve f s .
  • the transmission signal f s is converted via a coupler 3 into electromagnetic waves (microwaves) which are directed by a waveguide 4 via an optical window 5 and an antenna 6 onto the surface of a liquid located in an industrial tank 7 or the like.
  • the microwaves reflected by the liquid surface are received again by the antenna 6 and converted by the coupler 3 into an electrical signal f r which is fed to a mixer 8. Due to the running time of the microwave from the coupler 3 to the liquid surface and back, there is a frequency shift £ dependent on the fill level with respect to the transmission signal (cf. FIG. 2). This frequency shift corresponds to equation (1).
  • the frequency shift is directly proportional to the level.
  • the transmission signal f s and the reception signal f r are fed to the mixer 8, the output signal of which is the low-frequency Contains the 4th distance signal £, which is shown in FIG. 3 as an ideal sinus curve.
  • the output signal contains further frequency components which are caused by reflections of the microels on the inner wall of the container, on the built-in containers and the like. Like arise.
  • Such interference frequencies can be determined and suppressed by means of a microprocessor processing the measurement signals in the context of so-called learning processes.
  • the low-frequency output signal ⁇ f of the mixer 8 is fed to an A / D converter 12 via a bandpass filter, consisting of a high-pass filter 9 and a low-pass filter 10, and digitized with a discrete number of adjustments.
  • a microprocessor 13 carries out a frequency analysis from the successive digital values and determines a discrete power density spectrum.
  • the power density spectrum would only consist of a few frequency lines, as indicated in FIG. 4, the two secondary lines being limited by the time the signal ⁇ f is present for the Frequency analysis are caused.
  • the output signal j f of the mixer 8 contains, due to possible changes in mode of the microwave signal and / or due to multiple reflections of the optical window, a power density component which can be considerably higher than the component of the useful signal reflected by the liquid surface.
  • the spectral lines of the power density spectrum which occur at different filling levels result in relatively high values at low frequencies, which are based on the reflection of the optical window, but lower values at higher frequencies due to the larger measuring path.
  • the high, low-frequency power components that arise in particular when the optical window is dirty are also present at a low fill level.
  • the fluctuations between the low-frequency and higher-frequency values of the power density can be between four and twenty times, for industrial tanks and the like. Like. Are even above it.
  • the interference frequencies that occur are first determined when the tank is empty, stored in the microprocessor 13 and taken into account in the arithmetic signal processing.
  • the bandpass filter has a lower cut-off frequency to dampen the low-frequency interference that occurs during measurement.
  • the cut-off frequency of the high-pass filter 9 and low-pass filter 10 are expediently designed in such a way that, at a maximum fill level, the relatively small output signal £ f is passed almost without attenuation, while the lowest cut-off frequency of the high-pass filter 9 occurs due to that which occurs when the tank is empty Frequency shift is determined.
  • the minimum sampling frequency of the A / D converter 12 is determined by the Shannon relationship.
  • a cut-off frequency can be chosen so that it corresponds to the low-frequency frequency shift ⁇ f at a fill level at which the distance between the liquid surface and the antenna is approximately 0.1 to 3.0 m.
  • the high-pass filter 9 is assigned a plurality of adjustable cut-off frequencies in order to adapt to different operating conditions and interference.
  • the high-pass filter 9 is preferably controlled by the microprocessor 13, that is to say a controllable high-pass filter is used. This allows a by changing operating conditions, for. B. suppression or the like. Increase interference as much as possible, the microprocessor 13 calculating such changes from the power density values and adapting the cut-off frequency of the high-pass filter 9 accordingly.
  • the associated control line is designated 14 in FIG. 1.
  • the A / D converter is to be protected against overdriving, which is caused by the maximum values of the low-frequency signals and which lead to poor resolution.
  • an amplifier stage 11 which is controlled by the microprocessor 13 via a line 15 and which prevents the A / D converter 12 from oversteering, is provided before or in or after the bandpass filter.
  • the fill level is essentially the same, it may be expedient to select a higher cut-off frequency of the bandpass filter. This can also be done through the adjustability or control of the cutoff frequency.
  • the power density spectrum does not consist of a discrete spectral line, but usually contains adjacent lines. Since a line corresponds to a discrete distance, the secondary lines can be used to calculate the fill level, for example by weighting the amplitudes of the secondary lines. A linear weighting would be suitable in the flat range of the frequency characteristic. If the frequency lines are in a non-constant area of the filter, the "roll-off" must be considered as a factor in the weighting. If the frequency output of the measuring section is known, the correction in the interpolation of the discrete spectral lines can be taken into account for very precise measurements.

Abstract

Un circuit électrique est décrit pour un appareil de mesure de niveau de citernes industrielles et similaires ayant une partie électronique d'émission et de réception de micro-ondes et un guide d'ondes pourvu d'une fenêtre et d'une antenne. Le signal réfléchi par la surface du liquide et le signal d'émission sont transmis à un étage mélangeur (8) dont le signal de sortie basse fréquence est fourni par un convertisseur analogique/numérique (12) à un microprocesseur (13) qui calcule le niveau de remplissage. Afin de réduire l'influence de fréquences parasites générées par réflexion des micro-ondes dans la fenêtre optique ou similaire, un filtre passe-bande (9, 10) dont la fréquence limite inférieure amortit des fréquences parasites de basse fréquence est agencé entre l'étage mélangeur (8) et le convertisseur analogique/numérique (12). La fréquence limite inférieure peut être ajustée par le microprocesseur (13), ou le filtre passe-bande (9, 10) peut être réglé sur plusieurs fréquences limites sélectionnables. Afin d'éviter la saturation du convertisseur analogique/numérique, un étage amplificateur (11) commandé par le microprocesseur (13) est associé au filtre passe-bande (9, 10).An electrical circuit is described for a device for measuring the level of industrial tanks and the like having an electronic part for transmitting and receiving microwaves and a waveguide provided with a window and an antenna. The signal reflected by the surface of the liquid and the emission signal are transmitted to a mixing stage (8) whose low frequency output signal is supplied by an analog/digital converter (12) to a microprocessor (13) which calculates the filling level. In order to reduce the influence of spurious frequencies generated by reflection of microwaves in the optical window or the like, a bandpass filter (9, 10) whose lower limit frequency dampens low-frequency spurious frequencies is arranged between the mixer stage (8) and the analog/digital converter (12). The lower limit frequency can be adjusted by the microprocessor (13), or the bandpass filter (9, 10) can be set to several selectable limit frequencies. In order to avoid saturation of the analog/digital converter, an amplifier stage (11) controlled by the microprocessor (13) is associated with the band-pass filter (9, 10).

Description

Elektrische Schaltung für ein Gerät zur Füllstandmessunσ von Industrietanks u. dgl.Electrical circuit for a device for level measurement of industrial tanks and. like.
Die Erfindung betrifft eine elektrische Schaltung für ein Gerät zur Füllstandmessung von Industrietanks u. dgl. mit einem elektronischen Sender und Empfangsteil für Mikrowel¬ len und einem mit einem Fenster und einer Antenne versehenen Hohlleiter, wobei das vom Flüssigkeitsspiegel reflektierte Signal und das Sendesignal einer Mischstufe zugeführt wird, deren niederfrequentes Ausgangssignal über einen A/D-Wand¬ ler einem die Füllstandgröße berechnenden Mikroprozessor aufgegeben wird.The invention relates to an electrical circuit for a device for level measurement of industrial tanks and. The like. With an electronic transmitter and receiving part for microwaves and a waveguide provided with a window and an antenna, the signal reflected by the liquid level and the transmission signal being fed to a mixing stage, the low-frequency output signal of which is via an A / D converter the microprocessor calculating the fill level is abandoned.
Es sind mit Mikrowellen arbeitende Füllstandsmeßgeräte be¬ kannt, die Signale im Mikrowellenbereich aussenden, wobei diese an der Oberfläche des im Behälter befindlichen Medi¬ ums reflektiert und vom Empfänger aufgenommen werden. Aus den Signalen wird der Abstand zwischen dem Füllstandmeßge¬ rät und der Oberfläche des Mediums mit Hilfe einer elek¬ tronischen Schaltanordnung ermittelt, der auch Mikropro¬ zessoren und elektronische Rechner zur Auswertung zugeord¬ net sind. Zur Verwendung solcher Mikrowellen-Füllstandmeßge- rate für Behälter, wie Industrietanks, in denen bei hohen oder niedrigen Betriebstemperaturen Unter- oder Überdruck herrscht, insbesondere wenn diese explosible und/oder ag¬ gressive und/oder toxische Medien enthalten, ist es notwen¬ dig, den Behälterraum von dem elektronischen Sende- und Emp- fangsteil zu trennen. Hierfür ist in dem durch das Behäl¬ terdach des Tanks ragenden Hohlleiter ein zylinderförmiges Hohlleiterfenster aus Quarzglas o. dgl. angeordnet, das einen für die Durchlässigkeit der Mikrowellen günstigen nie¬ drigen dielektrischen Verlustfaktor besitzt. Bei Füllstand- meßgeräten dieser Art wird mittels eines spannungsgesteuer- ten Oszillators ein in seiner Frequenz sich kontinuierlich veränderndes Sendesignal erzeugt, das mittels eines Kopp¬ lers Mikrowellen erzeugt, die über den mit einem Fenster versehenen Hohlleiter und die Antenne auf die Flüssigkeits- Oberfläche gerichtet werden. Die von der Flüssigkeitsober¬ fläche reflektierten, elektromagnetischen Wellen werden von der Antenne empfangen und von dem Kopfteil in ein elek¬ trisches Frequenzsignal umgesetzt, das einem Mischer zu¬ geführt wird. Durch die vom Füllstand abhängige Laufzeit der Mikrowelle im Behälter erfährt das Empfangssignal gegenüber dem Sendesignal eine Frequenzverschiebung, die direkt der Füllstandhöhe proportional ist. Die Sende- und Empfangs¬ signale werden mittels einer Mischstufe in ein niederfre¬ quentes Signal überführt, das digitalisiert und im Mikropro- zessor verarbeitet wird. In nachteiliger Weise verursacht jedoch das im Hohlleiter angeordnete optische Fenster durch Reflexion der Mikrowellen Störsignale, deren Amplituden wesentlich größer als die des Nutzsignals sind. In der Praxis enthält das Ausgangssignal der Mischstufe durch et- waige Modeänderungen des Mikrowellensignals und/ oder durch Mehrfachreflexionen des optischen Fensters einen Leistungs¬ dichteanteil, der beträchtlich höher sein kann als der Anteil des von dem von der Flüssigkeitsoberfläche reflek¬ tierten Nutzsignals. Die bei unterschiedlichen Füllstandhö- hen auftretenden Spektrallinien des Leistungsdichtspetrums ergeben bei niedrigen Frequenzen relativ hohe Werte, die auf der Reflexion des optischen Fensters beruhen, jedoch bei höheren Frequenzen infolge der größeren Meßstrecke niedrigere Werte. Die insbesondere bei Verschmutzung des op- tischen Fensters entstehenden hohen niederfrequenten Lei¬ stungsanteile sind auch bei niedrigem Füllstand vorhan¬ den. Die Schwankungen zwischen den niederfrequenten und höherfrequenten Werten der Leistungsdichte können zwischen dem Vier- bis Zwanzigfachen, bei Industrietanks u. dgl. sogar noch darüber liegen. 2 Zwar ist aus der DE-OS 31 34 243 eine Schaltung für ein Füllstandmeßgerät ohne Hohlleiterfenster bekannt, bei der zwischen einer Mischstufe und einem A/D-Wandler einer Fil¬ ter angeordnet ist, der unerwünschte Frequenzen ausfiltern soll. Da die Abstandsinformation Inhalt der Mischfrequenz ist, würde das Ausfiltern unerwünschter Frequenzen auch die entsprechenden Nutzsignale ausfiltern und eine Abstandsbe¬ stimmung in diesem Frequenzbereich gar nicht zulassen.Level measuring devices working with microwaves are known which emit signals in the microwave range, these being reflected on the surface of the medium in the container and being picked up by the receiver. The distance between the fill level measuring device and the surface of the medium is determined from the signals with the aid of an electronic switching arrangement, which is also assigned to microprocessors and electronic computers for evaluation. To use such microwave level measuring devices for containers, such as industrial tanks, in which there is negative or positive pressure at high or low operating temperatures, in particular if they contain explosive and / or aggressive and / or toxic media, it is necessary to to separate the container space from the electronic transmitting and receiving part. For this purpose, a cylindrical waveguide window made of quartz glass or the like is arranged in the waveguide projecting through the tank roof of the tank and has a low dielectric loss factor which is favorable for the permeability of the microwaves. In the case of level measuring devices of this type, a voltage-controlled ten oscillator generates a continuously changing transmission signal which uses a coupler to generate microwaves which are directed onto the liquid surface via the waveguide provided with a window and the antenna. The electromagnetic waves reflected by the liquid surface are received by the antenna and converted into an electrical frequency signal by the head part, which is fed to a mixer. Due to the running time of the microwave in the container, which is dependent on the fill level, the received signal experiences a frequency shift compared to the transmitted signal, which is directly proportional to the fill level. The transmit and receive signals are converted by means of a mixing stage into a low-frequency signal, which is digitized and processed in the microprocessor. However, the optical window arranged in the waveguide disadvantageously causes interference signals due to reflection of the microwaves, the amplitudes of which are substantially greater than those of the useful signal. In practice, the output signal of the mixer stage contains a power density component which may be considerably higher than the component of the useful signal reflected by the liquid surface due to possible mode changes in the microwave signal and / or due to multiple reflections of the optical window. The spectral lines of the power density spectrum which occur at different fill levels result in relatively high values at low frequencies, which are based on the reflection of the optical window, but lower values at higher frequencies due to the larger measuring section. The high, low-frequency power components that arise in particular when the optical window is dirty are also present at a low fill level. The fluctuations between the low-frequency and higher-frequency values of the power density can be between four and twenty times, in industrial tanks and the like. Like. Are even above it. 2 A circuit for a level measuring device without a waveguide window is known from DE-OS 31 34 243, in which a filter is arranged between a mixing stage and an A / D converter and is intended to filter out undesired frequencies. Since the distance information is the content of the mixed frequency, the filtering out of undesired frequencies would also filter out the corresponding useful signals and would not allow a distance determination in this frequency range at all.
Demgegenüber liegt der Erfindung die Aufgabe zugrunde, eine Schaltung der gattungsgemäßen Art derart auszubilden, daß die Störsignale gedämpft werden, ohne die Erfassung der Ab¬ standsinformationen zu beeinträchtigen.In contrast, the object of the invention is to design a circuit of the generic type in such a way that the interference signals are damped without impairing the detection of the distance information.
Diese Aufgabe wird erfindungsgemäß dadurch gelöst, daß zwi¬ schen der Mischstufe und dem A/D-Wandler ein Bandpaßfilter mit einer die niederfrequenten Störfrequenzen dämpfenden unteren Grundfrequenz angeordnet ist.This object is achieved according to the invention in that a bandpass filter with a lower fundamental frequency damping the low-frequency interference frequencies is arranged between the mixer stage and the A / D converter.
Erfindungsgemäß wird durch das Bandpaßfilter gezielt nur derjenige niederfrequente Bereich des Frequenzbandes ge¬ dämpft, in dem einerseits die vom optischen Fenster verur¬ sachten Störfrequenzen und anderseits die Nutzfrequenzen liegen, die bei relativ hohem Füllstand mit einer hohen Lei- stungsdichte entstehen. Hierdurch werden die niederfrequen¬ ten Störfrequenzen im wesentlichen eliminiert und Nutzsi¬ gnale in einer für die Auswertung geeigneten Leistungsdich¬ te durchgelassen. Hingegen werden die über der Grundfre¬ quenz des Bandpaßfilters liegenden Nutzsignale, die wegen des größeren Abstand zwischen dem Füllstandmeßgerät und der Oberfläche des Mediums nur eine geringere Leistungsdichte haben, umgedämpft durchgelassen.According to the invention, only the low-frequency range of the frequency band is specifically attenuated by the band-pass filter, in which the interference frequencies caused by the optical window and on the other hand the useful frequencies which arise at a relatively high level with a high power density are located. As a result, the low-frequency interference frequencies are essentially eliminated and useful signals are transmitted in a power density suitable for the evaluation. On the other hand, the useful signals which are above the fundamental frequency of the bandpass filter and which have only a lower power density due to the greater distance between the level measuring device and the surface of the medium are passed through in a damped manner.
Erfindungsgemäß kann ein Bandpaßfilter mit einer entspre- chend festen Grenzfrequenz ausgewählt werden, welche das Spektrum der durch das optische Fenster verursachten Stör¬ frequenzen erfaßt. Für die Praxis erweist sich eine Grenz- frequenz als geeignet, die eine mit einem Abstand zwischen dem Koppler des Füllstandmeßgeräts und dem Flüssigkeits- spiegel von etwa 0,1 bis 3m erzeugten Abstandsfrequenz ent¬ spricht. Die untere Grenzfrequenz kann in vorteilhafter Wei- se auch entweder vom Mikroprozessor oder einem Rechner ge¬ steuert werden, oder das Bandpaßfilter kann auf mehrere wählbare Grenzfrequenzen einstellbar sein. Mit dem Mikro¬ prozessor läßt sich eine Frequenzanalyse durchführen und in Abhängigkeit von dem Anteil der Störfrequenzen die Grenzfre- quenz des Bandpaßfilters einstellen. Eine Anpassung der Grenzfrequenz kann auch in Abhängigkeit von dem Füllstand sinnvoll sein. Da bei einem hohen Füllstand die Frequenzver¬ schiebung sehr gering ist, sollte in diesem Meßbereich so¬ weit wie möglich ohne Dämpfung gearbeitet werden. Die durch das optische Fenster verursachten Störfrequenzen werden zweckmäßig bei leerem Tank ermittelt, im Mikroprozessor ge¬ speichert und bei der rechnerischen Signalverarbeitung be¬ rücksichtigt.According to the invention, a bandpass filter can be selected with a correspondingly fixed cut-off frequency, which captures the spectrum of the interference frequencies caused by the optical window. In practice, there is a limit frequency as suitable, which corresponds to a distance frequency generated with a distance between the coupler of the level measuring device and the liquid level of approximately 0.1 to 3 m. The lower cut-off frequency can advantageously also be controlled either by the microprocessor or a computer, or the bandpass filter can be adjustable to a plurality of selectable cut-off frequencies. A frequency analysis can be carried out with the microprocessor and the limit frequency of the bandpass filter can be set depending on the proportion of the interference frequencies. Adjusting the cut-off frequency can also make sense depending on the level. Since the frequency shift is very small at a high fill level, it should be worked as far as possible without damping in this measuring range. The interference frequencies caused by the optical window are expediently determined when the tank is empty, stored in the microprocessor and taken into account in the arithmetic signal processing.
Um eine Übersteuerung des A/D-Wandlers zu verhindern, ist zweckmäßig dem Bandpaßfilter eine vom Mikroprozessor ge¬ steuerte Verstärkerstufe zugeordnet, wobei diese vor oder nach dem Bandpaßfilter angeordnet oder integriert sein kann.In order to prevent overloading of the A / D converter, the bandpass filter is advantageously assigned an amplifier stage controlled by the microprocessor, which can be arranged or integrated before or after the bandpass filter.
Die Erfindung ist in der Zeichnung beispielsweise darge¬ stellt; es zeigtThe invention is shown in the drawing, for example; it shows
Fig. 1 eine Schaltanordnung für ein Füllstandmeßgerät,1 shows a circuit arrangement for a level measuring device,
Fig. 2 die zwischen Sende- und Empfangssignal auftretende Frequenzverschiebung in einem Diagramm,2 shows the frequency shift occurring between the transmission and reception signal in a diagram,
Fig. 3 das von einer Mischstufe gebildete niederfrequente Signal undFig. 3 shows the low-frequency signal formed by a mixer and
Fig. 4 das von einem A/D-Wandler gebildete Leistungsdich¬ te-Spektrum. Wie Fig. 1 zeigt, wird mittels eines Sägezahngenerators 1 o. dgl. ein spannungsgesteuerter Oszillator 2 angesteuert, der ein sich kontinuierlich in seiner Frequenz änderndes elektrisches Ausgangssignal fa erzeugt. Der Zeitverlauf die- ser Frequenz ist beispielsweise in Fig. 2 durch die Kurve fs dargestellt. Das Sendesignal fs wird über einen Koppler 3 in elektromagnetische Wellen (Mikrowellen) gewandelt, die durch einen Hohlleiter 4 über ein optisches Fenster 5 und eine Antenne 6 auf die Oberfläche eines in einem Industrie- tank 7 o. dgl. befindlichen Flüssigkeit gelenkt werden. Die von der Flüssigkeitsoberfläche reflektierten Mikrowellen werden von der Antenne 6 wieder empfangen und von dem Kopp¬ ler 3 in ein elektrisches Signal fr umgesetzt, das einem Mischer 8 zugeführt wird. Durch die Laufzeit der Mikrowelle vom Koppler 3 zur Flüssigkeitsoberfläche und zurück findet eine vom Füllstand abhängige FrequenzVerschiebung £ gegen¬ über dem Sendesignal statt (vgl. Fig. 2). Diese Frequenz- Verschiebung entspricht der Gleichung (1) .4 shows the power density spectrum formed by an A / D converter. As shown in FIG. 1, a sawtooth generator 1 or the like controls a voltage-controlled oscillator 2, which generates an electrical output signal f a which changes continuously in frequency. The time course of this frequency is represented, for example, in FIG. 2 by the curve f s . The transmission signal f s is converted via a coupler 3 into electromagnetic waves (microwaves) which are directed by a waveguide 4 via an optical window 5 and an antenna 6 onto the surface of a liquid located in an industrial tank 7 or the like. The microwaves reflected by the liquid surface are received again by the antenna 6 and converted by the coupler 3 into an electrical signal f r which is fed to a mixer 8. Due to the running time of the microwave from the coupler 3 to the liquid surface and back, there is a frequency shift £ dependent on the fill level with respect to the transmission signal (cf. FIG. 2). This frequency shift corresponds to equation (1).
wobei für den Ausdruck τ^ in erster Näherung die Laufzeit der elektromagnetischen Wellen giltwhere the term τ ^ applies in a first approximation to the transit time of the electromagnetic waves
(2) τd =(2) τ d =
mit h = Abstand Koppler - Flüssigkeitsoberfläche c = Lichtgeschwindigkeitwith h = distance coupler - liquid surface c = speed of light
Aus den Gleichungen (1) und (2) ergibt sichFrom equations (1) and (2) follows
•_) x Λ - — 2h. dfs (3) Δf - c~ cfE-• _) x Λ - - 2h. dfs (3) Δf - c ~ cfE-
Danach ist die Frequenzverschiebung der Füllstandhöhe direkt proportional.After that, the frequency shift is directly proportional to the level.
Das Sendesignal fs und das Empfangssignal fr werden dem Mi¬ scher 8 zugeführt, dessen Ausgangssignal das niederfrequen- 4 te Abstandssignal £ enthält, das in Fig. 3 als ideale Si¬ nuskurve dargestellt ist. In der Praxis enthält das Aus¬ gangssignal weitere Frequenzanteile, die durch Reflexionen der Mikro elle an der Behälterinnenwand, an Behältereinbau- ten u. dgl. entstehen. Solche Störfrequenzen lassen sich mittels eines die Meßsignale verarbeitenden Mikroprozessors im Rahmen sogenannter Lernvorgänge ermitteln und unterdrük- ken. Hierfür wird das niederfrequente Ausgangssignal ^f des Mischers 8 über ein Bandpaßfilter, bestehend aus einem Hoch- paßfilter 9 und einem Tiefpaßfilter 10, einem A/D-Wandler 12 zugeführt und mit einer diskreten Anzahl von Auf astun¬ gen digitalisiert. Aus den aufeinanderfolgenden digitalen Werten führt ein Mikroprozessor 13 eine Frequenzanalyse durch und ermittelt ein diskretes Leistungsdichtespektrum.The transmission signal f s and the reception signal f r are fed to the mixer 8, the output signal of which is the low-frequency Contains the 4th distance signal £, which is shown in FIG. 3 as an ideal sinus curve. In practice, the output signal contains further frequency components which are caused by reflections of the microels on the inner wall of the container, on the built-in containers and the like. Like arise. Such interference frequencies can be determined and suppressed by means of a microprocessor processing the measurement signals in the context of so-called learning processes. For this purpose, the low-frequency output signal ^ f of the mixer 8 is fed to an A / D converter 12 via a bandpass filter, consisting of a high-pass filter 9 and a low-pass filter 10, and digitized with a discrete number of adjustments. A microprocessor 13 carries out a frequency analysis from the successive digital values and determines a discrete power density spectrum.
Hätte das Ausgangssignal &f idealerweise einen reinen si¬ nusförmigen Verlauf, würde das Leistungsdichtespektrum nur aus einigen Frequenzlinien bestehen, wie es in Fig. 4 ange¬ deutet ist, wobei die beiden Nebenlinien durch die begrenz- te Zeit des Anliegens des Signals ^f für die Frequenzanaly¬ se verursacht werden.If the output signal & f ideally had a purely sinusoidal profile, the power density spectrum would only consist of a few frequency lines, as indicated in FIG. 4, the two secondary lines being limited by the time the signal ^ f is present for the Frequency analysis are caused.
In der Praxis enthält das Ausgangssignal j f der Mischstufe 8 durch etwaige Modeänderungen des Mikrowellensignals und/ oder durch Mehrfachreflexionen des optischen Fensters einen Leistungsdichteanteil, der beträchtlich höher sein kann als der Anteil des von dem von der Flüssigkeitsoberfläche re¬ flektierten Nutzsignals. Die bei unterschiedlichen Füll¬ standhöhen auftretenden Spektrallinien des Leistungsdicht- spetrums ergeben bei niedrigen Frequenzen relativ hohe Wer¬ te, die auf der Reflexion des optischen Fensters beruhen, jedoch bei höheren Frequenzen infolge der größeren Meßstrek- ke niedrigere Werte. Die insbesondere bei Verschmutzung des optischen Fensters entstehenden hohen niederfrequenten Lei- stungsanteile sind auch bei niedrigem Füllstand vorhanden. Die Schwankungen zwischen den niederfrequenten und höher- frequenten Werten der Leistungsdichte können zwischen dem Vier- bis Zwanzigfachen, bei Industrietanks u. dgl. sogar noch darüber liegen. Die auftretenden Störfrequenzen werden zunächst bei leerem Tank ermittelt, im Mikroprozessor 13 gespeichert und bei der rechnerischen Signalverarbeitung berücksichtigt. Zur Dämpfung der beim Messen auftretenden niederfrequenten Stör- anteile besitzt der Bandpaßfilter eine untere Grenzfrequenz. Die Grenzfrequenz des Hochpaßfilters 9 und Tiefpaßfilters 10 sind zweckmäßig derart ausgelegt, daß bei einer maxima¬ len Füllhöhe das darin relativ kleine Ausgangssignal £f na¬ hezu ohne Dämpfung durchgelassen wird, während die niedrig- ste Grenzfrequenz des Hochpaßfilters 9 durch die bei leerem Tank auftretende Frequenzverschiebung bestimmt wird. Die minimale Abtastfrequenz des A/D-Wandlers 12 wird durch die Shannonsche Beziehung bestimmt. In der Praxis kann eine Grenzfrequenz so gewählt werden, daß sie der niederfrequen- ten Frequenzverschiebung ^f bei einer Füllstandhöhe ent¬ spricht, bei der der Abstand zwischen der Flüssigkeitsober¬ fläche und der Antenne etwa 0,1 bis 3,0 m beträgt.In practice, the output signal j f of the mixer 8 contains, due to possible changes in mode of the microwave signal and / or due to multiple reflections of the optical window, a power density component which can be considerably higher than the component of the useful signal reflected by the liquid surface. The spectral lines of the power density spectrum which occur at different filling levels result in relatively high values at low frequencies, which are based on the reflection of the optical window, but lower values at higher frequencies due to the larger measuring path. The high, low-frequency power components that arise in particular when the optical window is dirty are also present at a low fill level. The fluctuations between the low-frequency and higher-frequency values of the power density can be between four and twenty times, for industrial tanks and the like. Like. Are even above it. The interference frequencies that occur are first determined when the tank is empty, stored in the microprocessor 13 and taken into account in the arithmetic signal processing. The bandpass filter has a lower cut-off frequency to dampen the low-frequency interference that occurs during measurement. The cut-off frequency of the high-pass filter 9 and low-pass filter 10 are expediently designed in such a way that, at a maximum fill level, the relatively small output signal £ f is passed almost without attenuation, while the lowest cut-off frequency of the high-pass filter 9 occurs due to that which occurs when the tank is empty Frequency shift is determined. The minimum sampling frequency of the A / D converter 12 is determined by the Shannon relationship. In practice, a cut-off frequency can be chosen so that it corresponds to the low-frequency frequency shift ^ f at a fill level at which the distance between the liquid surface and the antenna is approximately 0.1 to 3.0 m.
Zur Anpassung an verschiedene Betriebsbedingungen und Stör- einflüsse sind dem Hochpaßfilter 9 mehrere einstellbare Grenzfrequenzen zugeordnet. Doch vorzugsweise wird der Hoch¬ paßfilter 9 von dem Mikroprozessor 13 gesteuert, also ein steuerbarer Hochpaßfilter verwendet. Hierdurch läßt sich ein durch veränderte Betriebsbedingungen, z. B. Verschnürt- zung o. dgl. erhöhter Störanteil bestmöglichst unterdrüc¬ ken, wobei der Mikroprozessor 13 aus den Leistungsdichte¬ werten derartige Veränderungen berechnet und die Grenzfre¬ quenz des Hochpaßfilters 9 entsprechend anpaßt. Die zuge¬ hörige Steuerleitung ist in Fig. 1 mit 14 bezeichnet.The high-pass filter 9 is assigned a plurality of adjustable cut-off frequencies in order to adapt to different operating conditions and interference. However, the high-pass filter 9 is preferably controlled by the microprocessor 13, that is to say a controllable high-pass filter is used. This allows a by changing operating conditions, for. B. suppression or the like. Increase interference as much as possible, the microprocessor 13 calculating such changes from the power density values and adapting the cut-off frequency of the high-pass filter 9 accordingly. The associated control line is designated 14 in FIG. 1.
Der A/D-Wandler ist gegen eine Übersteuerung zu schützen, die durch die Höchstwerte der niederfrequenten Signale ver¬ ursacht werden und die zu einer schlechten Auflösung führen. Anstelle eines aufwendigen hochauflösenden A/D-Wandlers ist vor oder im oder nach dem Bandpaßfilter eine vom Mikropro¬ zessor 13 über eine Leitung 15 gesteuerte, die Übersteue¬ rung des A/D-Wandlers 12 verhindernde Verstärkerstufe 11 vorgesehen. d Bei einem im wesentlichen gleichen Füllstand kann es zweck¬ mäßig sein, eine höhere Grenzfrequenz des Bandpaßfilters zu wählen. Auch dies kann durch die Einstellbarkeit oder Steu¬ erung der Grenzfrequenz geschehen.The A / D converter is to be protected against overdriving, which is caused by the maximum values of the low-frequency signals and which lead to poor resolution. Instead of a complex high-resolution A / D converter, an amplifier stage 11, which is controlled by the microprocessor 13 via a line 15 and which prevents the A / D converter 12 from oversteering, is provided before or in or after the bandpass filter. d When the fill level is essentially the same, it may be expedient to select a higher cut-off frequency of the bandpass filter. This can also be done through the adjustability or control of the cutoff frequency.
Wie in Fig. 4 angedeutet ist, besteht das Leistungsdichte¬ spektrum nicht aus einer diskreten Spektrallinie, sondern enthält in der Regel benachbarte Linien. Da eine Linie ei¬ nem diskreten Abstand entspricht, können die Nebenlinien zu Berechnung der Füllstandshöhe beispielsweise durch eine Wichtung der Amplituden der Nebenlinien herangezogen wer¬ den. Eine lineare Wichtung würde im flachen Bereich der Frequenzcharakteristik geeignet sein. Falls die Frequenzli¬ nien sich in einem nicht konstanten Bereich des Filters be- finden, ist bei der Wichtung der "Roll-off" als Faktor zu berücksichtigen. Bei bekanntem Frequenzausgang der Me߬ strecke kann für sehr genaue Messungen die Korrektur bei der Interpolation der diskreten Spektrallinien berücksich¬ tigt werden. As indicated in FIG. 4, the power density spectrum does not consist of a discrete spectral line, but usually contains adjacent lines. Since a line corresponds to a discrete distance, the secondary lines can be used to calculate the fill level, for example by weighting the amplitudes of the secondary lines. A linear weighting would be suitable in the flat range of the frequency characteristic. If the frequency lines are in a non-constant area of the filter, the "roll-off" must be considered as a factor in the weighting. If the frequency output of the measuring section is known, the correction in the interpolation of the discrete spectral lines can be taken into account for very precise measurements.

Claims

Patentansprüche Claims
1. Elektrische Schaltung für ein Gerät zur Füllstandmessung von Industrietanks u. dgl. mit einem elektronischen Sen¬ de- und Empfangsteil für Mikrowellen und einem mit einem Fenster und einer Antenne versehenen Hohlleiter, wobei das vom Flüssigkeitsspiegel reflektierte Signal und das Sendesignal einer Mischstufe zugeführt wird, deren nie¬ derfrequentes Ausgangssignal über einen A/D-Wandler ei¬ nem die Füllstandsgröße berechnenden Mikroprozessor auf¬ gegeben wird, dadurch gekennzeichnet, daß zwischen der Mischstufe (8) und dem A/D-Wandler (12) ein Bandpaßfil¬ ter (9,10) mit einer die niederfrequenten Störfrequenzen dämpfenden unteren Grenzfrequenz angeordnet ist.1. Electrical circuit for a device for level measurement of industrial tanks and. Like. With an electronic transmitter and receiver for microwaves and a waveguide provided with a window and an antenna, the signal reflected by the liquid level and the transmission signal is fed to a mixer, the low-frequency output signal via an A / D converter A microprocessor which calculates the fill level is given, characterized in that a bandpass filter (9, 10) with a lower cut-off frequency damping the low-frequency interference frequencies is arranged between the mixer stage (8) and the A / D converter (12) is.
2. Elektrische Schaltung nach Anspruch 1, dadurch gekenn- zeichnet, daß die untere Grenzfrequenz des Bandpaßfil¬ ters (9,10) einer mit einem Abstand zwischen dem Koppler des Füllstandmeßgeräts und dem Flüssigkeitsspiegel von etwa 0,1 bis 3 m erzeugten Abstandsfrequenz entspricht.2. Electrical circuit according to claim 1, characterized in that the lower cut-off frequency of the bandpass filter (9, 10) corresponds to a distance frequency generated with a distance between the coupler of the fill level measuring device and the liquid level of about 0.1 to 3 m.
3. Elektrische Schaltung nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß die untere Grenzfrequenz des Band¬ paßfilters (9,10) vom Mikroprozessor (13) gesteuert wird.3. Electrical circuit according to claim 1 or 2, characterized in that the lower cut-off frequency of the Band¬ pass filter (9,10) is controlled by the microprocessor (13).
4. Elektrische Schaltung nach Anspruch 1 oder 2, dadurch ge- kennzeichnet, daß der Bandpaßfilter (9,10) auf mehrere4. Electrical circuit according to claim 1 or 2, characterized in that the bandpass filter (9,10) to several
Grenzfrequenzen einstellbar ist.Limit frequencies is adjustable.
5. Elektrische Schaltung nach einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, daß vor oder im oder nach dem Bandpaßfilter (9,10) eine vom Mikroprozessor (13) ge¬ steuerte, die Übersteuerung des A/D-Wandlers (12) ver¬ hindernde Verstärkerstufe (11) vorgesehen ist. 5. Electrical circuit according to one of claims 1 to 4, characterized in that before or in or after the bandpass filter (9, 10) one of the microprocessor (13) controlled ge, the overload of the A / D converter (12) ver ¬ preventing amplifier stage (11) is provided.
EP92902706A 1991-02-12 1992-01-21 Electric circuit for a device for measuring the level in industrial tanks and the like Withdrawn EP0524275A1 (en)

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