EP0442510A1 - Method and apparatus for ultrasonic liquid atomization - Google Patents

Method and apparatus for ultrasonic liquid atomization Download PDF

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Publication number
EP0442510A1
EP0442510A1 EP91102120A EP91102120A EP0442510A1 EP 0442510 A1 EP0442510 A1 EP 0442510A1 EP 91102120 A EP91102120 A EP 91102120A EP 91102120 A EP91102120 A EP 91102120A EP 0442510 A1 EP0442510 A1 EP 0442510A1
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EP
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Prior art keywords
frequency
signal
ultrasonic transducer
oscillator
control
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EP91102120A
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German (de)
French (fr)
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EP0442510B1 (en
Inventor
Martin Rüttel
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Siemens AG
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Siemens AG
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B06—GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
    • B06B—METHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
    • B06B1/00—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency
    • B06B1/02—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy
    • B06B1/0207—Driving circuits
    • B06B1/0223—Driving circuits for generating signals continuous in time
    • B06B1/0238—Driving circuits for generating signals continuous in time of a single frequency, e.g. a sine-wave
    • B06B1/0246—Driving circuits for generating signals continuous in time of a single frequency, e.g. a sine-wave with a feedback signal
    • B06B1/0253—Driving circuits for generating signals continuous in time of a single frequency, e.g. a sine-wave with a feedback signal taken directly from the generator circuit
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B06—GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
    • B06B—METHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
    • B06B2201/00—Indexing scheme associated with B06B1/0207 for details covered by B06B1/0207 but not provided for in any of its subgroups
    • B06B2201/70—Specific application
    • B06B2201/77—Atomizers

Definitions

  • the invention relates to a method for triggering an ultrasonic transducer for atomizing a liquid, a trigger signal having an adjustable trigger frequency being fed to the ultrasonic transducer. It also relates to a device for controlling an ultrasonic transducer for atomizing a liquid with a controllable oscillator which emits a control signal with an adjustable control frequency and which is connected on the output side to the ultrasonic converter.
  • Piezoceramic ultrasonic transducers for atomizing liquids are used in various facilities, for example in inhalation devices or in humidifiers. In the latter, water is used to humidify the air. In all of these devices, it is of crucial importance that the excitation or control frequency for the ultrasound transducer is optimally adapted to it.
  • the optimum operating point is understood to be the operating state with respect to the feed current, feed voltage and drive frequency, in which the volume of liquid atomized per unit of time is the greatest for a given electrical power. This optimal operating point is normally at a resonance frequency of the ultrasound transducer. Due to the installation geometry or due to deviations of the ultrasonic transducer from an ideal predefined design, the point of greatest efficiency can be shifted slightly. This can only be insufficiently recognized and corrected by the previously known control principles for the ultrasonic transducer.
  • the first method involves the ultrasound transducer itself as a frequency-determining element in an oscillating circuit, for example in a power oscillator.
  • This principle is implemented, for example, in a commercially available ultrasonic liquid atomizer (ultrasonic atomizer EFE-HMV1R7M6E from Matsushita Electric, specification from Quick-Ohm GmbH, D-5600 Wuppertal).
  • a pulse code modulated transmitter with its own oscillator is used, which emits ultrasound waves with a frequency of 1.7 MHz onto a water surface via the ultrasound transducer.
  • the impact of the ultrasonic waves on the boundary layer between water and air causes the liquid to rise, which manifests itself as fine water dust or mist.
  • the ultrasonic transducer is attached to the lower part of a water tank.
  • One possibility for using the ultrasonic transducer as a frequency-determining element is, for example, the arrangement of the ultrasonic transducer in the feedback line of an oscillator. This is described for example in EP-AO, 240,360.
  • the amplitude and phase frequency response of the ultrasound transducer is then used to pull the drive frequency emitted by the oscillator to the resonance frequency of the ultrasound transducer.
  • This method has the disadvantage that the working frequency obtained in this way is also influenced by other circuit components and can therefore be noticeably adjacent to the optimum working frequency of the ultrasonic transducer.
  • a certain vibration quality of the ultrasonic transducer is required for reliable functioning, which places high demands on the manufacturing accuracy in the manufacture of the ultrasonic transducer.
  • a stable operating frequency is applied to the ultrasound transducer via a power amplifier with the aid of a separate oscillator, the frequency of which is set once.
  • the optimal working frequency can now be determined and set once on the device's own oscillator for the ultrasonic transducer.
  • the optimal working frequency is when the sound pressure has reached its maximum. If you run the device's own oscillator as a quartz-stabilized frequency synthesizer, you get a relatively stable control system with good efficiency.
  • the disadvantage is the high manufacturing effort, which is caused by the tuning process described. Due to the fixed frequency setting, frequency deviations due to aging of the ultrasonic transducer are not compensated for. This can cause the efficiency to deteriorate over the lifetime.
  • the present invention is based on the object of designing a method and a device of the type mentioned at the outset in such a way that it is possible to work at the optimum operating point, without being influenced by other circuit components and by signs of aging of the ultrasound transducer.
  • tracking of the control frequency of the ultrasonic transducer should be made possible during operation such that the point of greatest atomization efficiency is always maintained.
  • the stated object is achieved in that the control frequency is tracked as a function of the signal tapped at the ultrasound transducer.
  • the signal tapped at the ultrasound transducer is demodulated and then filtered, after which a current mean value signal is formed from the demodulated and filtered signal, which signal is used to set the drive frequency.
  • directional information that is to say information about whether the current working frequency is above or below the optimal working frequency (at which optimal atomization is obtained), is obtained at least when operation is started. This is important because the drive frequency must be reduced or increased accordingly.
  • the control frequency is experimentally tuned up or down from a predetermined frequency value, and that the test obtained in the course of time -Mean value signal is examined for the presence of a maximum. If a maximum is present, the "direction information" is obtained, and the drive frequency is then tracked, taking into account the "direction information", in accordance with the current mean value signal in the frequency range of the maximum.
  • the aforementioned tuning and searching for the maximum and the change in the actuation frequency are preferably carried out here with the aid of a microcomputer or microprocessor.
  • a frequency tracking branch is provided which connects the input of the ultrasonic transducer to the frequency control input of the oscillator.
  • the frequency tracking branch preferably comprises an amplitude demodulator and a downstream band filter.
  • the band filter should be followed by a microprocessor, the output of which is connected to the frequency control input of the oscillator.
  • the present method and the present device are based on the control of the ultrasound transducer with a control frequency that can be corrected during operation.
  • the essential thing here is that a signal is used for frequency tuning that is directly related to the atomizing power and contains all parasitic influences. This is the mentioned signal picked up at the ultrasound transducer, which reflects the reflection of the ultrasound waves on the liquid surface.
  • the ultrasound transducer a piezoelectric, preferably a piezoceramic ultrasound transducer, is used both for sending and for receiving.
  • FIG. 1 there is a liquid 4 to be atomized, in the present case water, in a vessel 2.
  • the liquid surface is designated 6.
  • a piezoelectric, preferably a piezoceramic, ultrasonic transducer 8 is arranged on the bottom of the vessel 2. During operation, it emits ultrasonic waves 10 in the direction of the water surface 6.
  • the radiation surface of the ultrasonic transducer 8 is curved. It is used for transmitting the ultrasonic waves 10, but at the same time also for receiving the ultrasonic waves reflected on the liquid surface 6.
  • the device for controlling the ultrasound transducer 8 comprises a controllable oscillator 12 which emits a control signal s with an adjustable control frequency f. It is preferably a sine wave oscillator.
  • the control frequency f is in the range from 0.5 to 5 MHz, preferably in the middle range from 2.5 MHz.
  • the control frequency f can be influenced by a control signal p at the frequency control input 14 of the oscillator 10.
  • the oscillator 12 is connected on the output side to the input of a power amplifier 16. Its output 18 is in turn connected to the ultrasound transducer 8.
  • a frequency tracking branch 20 is also provided, which connects the output 18 of the power amplifier 16 and thus the input of the ultrasound converter 8 to the frequency control input 14 of the oscillator 12.
  • this frequency tracking branch 20 comprises an amplitude demodulator 22 connected to the output 18, a downstream band filter 24 and a microprocessor 26 connected downstream thereof, the output of which is connected to the frequency control input 14 of the oscillator 12.
  • the frequency range of the bandpass filter 14 is in the range from 50 Hz to 10 kHz. It is intended to filter out the area below the useful frequency of approximately 2.5 MHz in which the maximum noise lies when sputtering occurs.
  • the demodulator 22 is a rectifier circuit, in particular a diode circuit.
  • the ultrasonic transducer 8 is supplied via the power stage 16 with the control signal s of the adjustable control frequency f from the controllable oscillator 12.
  • the ultrasonic transducer 8 then sends sound waves 10 through the liquid 4 to the surface 6 thereof.
  • the ultrasonic waves are reflected there, and some of these reflected ultrasonic waves are returned to the ultrasonic transducer 8, where they are converted into electrical signals.
  • These signals are superimposed on the control signal from the power amplifier 16 at the output 18 to the signal U.
  • the signal U tapped here arrives at the amplitude demodulator 22 and from there to the downstream band filter 24.
  • the envelope becomes of the output signal U, which is shown in FIGS.
  • a measuring voltage or a “current mean value signal” m is obtained.
  • This current mean signal m is used to control the oscillator 12. If there is "directional information" which is determined by the microprocessor 26, the signal p can be formed therefrom and from the signal m and applied to the frequency control input 14.
  • the control frequency f is experimentally changed with the aid of the microprocessor 26 from a predetermined frequency value fo upwards or downwards (test run).
  • a test mean signal m ' is then obtained as signal m in the course of time t. This is examined by the microprocessor 26 for the presence of a maximum.
  • the microprocessor 26 also determines whether the originally specified frequency value fo is above or below the frequency f * at which the maximum of the test mean signal m 'occurs. This is the "direction information" mentioned above.
  • the microprocessor 26 changes the control signal p such that the said maximum - which corresponds to the point of the greatest atomization efficiency - occurs and is then recorded.
  • the control frequency f is tracked in the frequency range of the maximum in accordance with the current mean value signal m.
  • the microprocessor 26 is thus able to determine that the maximum has been exceeded and is set up in such a way that the control signal p guides the control frequency f in the direction of the optimum frequency f *.
  • the liquid surface 6 remains calm.
  • the wave field 10 is then not disturbed and the signal U is not subject to any change over time.
  • the amplitude demodulator 22 supplies a pure DC voltage, and the measurement voltage m behind the bandpass filter 24 is almost zero.
  • the signal s - deviating from the preferred sinusoidal configuration - is shown as a triangular signal.
  • the liquid surface 6 becomes increasingly uneasy.
  • the wave field 10 is disturbed by this movement on the liquid surface 6.
  • the reflected signal component is thereby modulated with a low-frequency noise, which is in particular in the range from 50 Hz to 10 kHz.
  • This noise is illustrated by the envelopes h1 and h2 in FIG. 3. From this noisy signal U, the current average signal m is formed via the demodulator 22 and the bandpass filter 24, which is now no longer zero but has a value that can be measured. It could be called a "noise signal”.
  • this mean or measurement signal m becomes larger.
  • the bandwidth of this noise signal m also increases in addition to the amplitude.
  • maximum noise occurs.
  • the current mean value signal or noise signal m is used by the microprocessor 26 as a control signal p for frequency control of the oscillator 12.

Abstract

An oscillator (12) with frequency control input (14) transmits a drive signal (s) with adjustable drive frequency (f) to the ultrasonic transducer (8) provided in a liquid (4) for the purpose of atomisation. In order to maintain the optimum operating point (f<*>) in each case, a frequency adjustment branch (20) is provided according to the invention via which branch the drive frequency (f) is adjusted as a function of the signal (U) tapped off at the ultrasonic transducer (8). This frequency adjustment branch (20) preferably comprises an amplitude demodulator (22), a bandpass filter (24) and a microprocessor (26). The latter also serves to trigger a test pass at the start of an atomisation process in order to determine whether the current working frequency of the ultrasonic transducer (8) lies below or above the optimum working frequency (f<*>). <IMAGE>

Description

Die Erfindung bezieht sich auf ein Verfahren zur Ansteuerung eines Ultraschallwandlers zur Zerstäubung einer Flüssigkeit, wobei ein Ansteuersignal mit einstellbarer Ansteuerfrequenz dem Ultraschallwandler zugeleitet wird. Sie bezieht sich weiterhin auf eine Einrichtung zur Ansteuerung eines Ultraschallwandlers zur Zerstäubung einer Flüssigkeit mit einem steuerbaren Oszillator, der ein Ansteuersignal mit einstellbarer Ansteuerfrequenz abgibt und der ausgangsseitig an den Ultraschallwandler angeschlossen ist.The invention relates to a method for triggering an ultrasonic transducer for atomizing a liquid, a trigger signal having an adjustable trigger frequency being fed to the ultrasonic transducer. It also relates to a device for controlling an ultrasonic transducer for atomizing a liquid with a controllable oscillator which emits a control signal with an adjustable control frequency and which is connected on the output side to the ultrasonic converter.

Piezokeramische Ultraschallwandler zur Zerstäubung von Flüssigkeiten werden in verschiedenen Einrichtungen eingesetzt, zum Beispiel in Inhalationsgeräten oder in Luftbefeuchtern. In letzteren wird Wasser zur Luftbefeuchtung herangezogen. Bei all diesen Einrichtungen ist es von entscheidender Bedeutung, daß die Anregungs- oder Ansteuerfrequenz für den Ultraschallwandler optimal an diesen angepaßt ist. Als optimaler Betriebspunkt wird dabei der Betriebszustand bezüglich Speisestrom, Speisespannung und Ansteuerfrequenz verstanden, in dem bei einer bestimmten zugeführten elektrischen Leistung das pro Zeiteinheit zerstäubte Flüssigkeitsvolumen am größten ist. Normalerweise liegt dieser optimale Betriebspunkt auf einer Resonanzfrequenz des Ultraschallwandlers. Bedingt durch die Einbaugeometrie oder durch Abweichungen des Ultraschallwandlers von einer idealen vorgegebenen Bauform kann jedoch der genannte Punkt des größten Wirkungsgrades leicht verschoben sein. Dies kann durch die bisher bekannten Ansteuerprinzipien für den Ultraschallwandler nur unzureichend erkannt und korrigiert werden.Piezoceramic ultrasonic transducers for atomizing liquids are used in various facilities, for example in inhalation devices or in humidifiers. In the latter, water is used to humidify the air. In all of these devices, it is of crucial importance that the excitation or control frequency for the ultrasound transducer is optimally adapted to it. The optimum operating point is understood to be the operating state with respect to the feed current, feed voltage and drive frequency, in which the volume of liquid atomized per unit of time is the greatest for a given electrical power. This optimal operating point is normally at a resonance frequency of the ultrasound transducer. Due to the installation geometry or due to deviations of the ultrasonic transducer from an ideal predefined design, the point of greatest efficiency can be shifted slightly. This can only be insufficiently recognized and corrected by the previously known control principles for the ultrasonic transducer.

Bisher sind zwei Verfahren zur Frequenzabstimmung gebräuchlich:
Das erste Verfahren bezieht den Ultraschallwandler selbst als frequenzbestimmendes Element in eine Schwingschaltung, zum Beispiel in einen Leistungsoszillator, ein. Dies Prinzip ist beispielsweise in einem käuflich erhältlichen Ultraschall-Flüssigkeits-Zerstäuber realisiert (Ultraschall-Zerstäuber EFE-HMV1R7M6E der Firma Matsushita Electric, Spezifikation der Firma Quick-Ohm GmbH, D-5600 Wuppertal). Hier wird ein puls-code-modulierter Sender mit eigenem Oszillator verwendet, der über den Ultraschallwandler Ultraschallwellen der Frequenz 1,7 MHz auf eine Wasseroberfläche strahlt. Das Auftreffen der Ultraschallwellen auf die Grenzschicht zwischen Wasser und Luft verursacht ein Aufsteigen der Flüssigkeit, was sich als feiner Wasserstaub oder Nebel bemerkbar macht. Der Ultraschallwandler wird hierbei am Unterteil eines Wassertanks befestigt. - Eine Möglichkeit zur Verwendung des Ultraschallwandlers als frequenzbestimmendes Element ist zum Beispiel auch die Anordnung des Ultraschallwandlers in der Rückkopplungsleitung eines Oszillators. Dies ist beispielsweise in der EP-A-O,240,360 beschrieben. Danach wird der Amplituden- und Phasenfrequenzgang des Ultraschallwandlers dazu benutzt, die vom Oszillator abgegebene Ansteuerfrequenz auf die Resonanzfrequenz des Ultraschallwandlers zu ziehen. Dieses Verfahren hat den Nachteil, daß die so erhaltene Arbeitsfrequenz auch von anderen Schaltungsbauteilen beeinflußt wird und somit merklich neben der optimalen Arbeitsfrequenz des Ultraschallwandlers liegen kann. Auch ist hier für eine sichere Funktion eine gewisse Schwinggüte des Ultraschallwandlers erforderlich, was an die Fertigungsgenauigkeit bei der Herstellung des Ultraschallwandlers hohe Anforderungen stellt.Two methods of frequency tuning are currently in use:
The first method involves the ultrasound transducer itself as a frequency-determining element in an oscillating circuit, for example in a power oscillator. This principle is implemented, for example, in a commercially available ultrasonic liquid atomizer (ultrasonic atomizer EFE-HMV1R7M6E from Matsushita Electric, specification from Quick-Ohm GmbH, D-5600 Wuppertal). Here a pulse code modulated transmitter with its own oscillator is used, which emits ultrasound waves with a frequency of 1.7 MHz onto a water surface via the ultrasound transducer. The impact of the ultrasonic waves on the boundary layer between water and air causes the liquid to rise, which manifests itself as fine water dust or mist. The ultrasonic transducer is attached to the lower part of a water tank. - One possibility for using the ultrasonic transducer as a frequency-determining element is, for example, the arrangement of the ultrasonic transducer in the feedback line of an oscillator. This is described for example in EP-AO, 240,360. The amplitude and phase frequency response of the ultrasound transducer is then used to pull the drive frequency emitted by the oscillator to the resonance frequency of the ultrasound transducer. This method has the disadvantage that the working frequency obtained in this way is also influenced by other circuit components and can therefore be noticeably adjacent to the optimum working frequency of the ultrasonic transducer. Here too, a certain vibration quality of the ultrasonic transducer is required for reliable functioning, which places high demands on the manufacturing accuracy in the manufacture of the ultrasonic transducer.

Bei dem zweiten Verfahren (dies wurde bisher von der Anmelderin praktiziert) wird mit Hilfe eines separaten Oszillators, der in seiner Frequenz einmalig eingestellt wird, eine stabile Arbeitsfrequenz über einen Leistungsverstärker auf den Ultraschallwandler gegeben. Durch eine Messung des vom Ultraschallwandler erzeugten Schalldrucks kann nun die optimale Arbeitsfrequenz ermittelt und am geräteeigenen Oszillator für den Ultraschallwandler einmalig fest eingestellt werden. Die optimale Arbeitsfrequenz liegt dabei vor, wenn der Schalldruck maximal geworden ist. Führt man den geräteeigenen Oszillator als quarzstabilisierten Frequenzsynthesizer aus, erhält man ein relativ stabiles Ansteuersystem mit gutem Wirkungsgrad. Nachteilig ist jedoch der hohe Aufwand in der Fertigung, der durch den geschilderten Abstimmvorgang verursacht ist. Bedingt durch die feste Frequenzeinstellung werden hierbei auch Frequenzabweichungen durch Alterung des Ultraschallwandlers nicht kompensiert. Dies kann eine Verschlechterung des Wirkungsgrades über die Lebensdauer bewirken.In the second method (this has previously been practiced by the applicant), a stable operating frequency is applied to the ultrasound transducer via a power amplifier with the aid of a separate oscillator, the frequency of which is set once. By measuring the from the ultrasonic transducer generated sound pressure, the optimal working frequency can now be determined and set once on the device's own oscillator for the ultrasonic transducer. The optimal working frequency is when the sound pressure has reached its maximum. If you run the device's own oscillator as a quartz-stabilized frequency synthesizer, you get a relatively stable control system with good efficiency. The disadvantage, however, is the high manufacturing effort, which is caused by the tuning process described. Due to the fixed frequency setting, frequency deviations due to aging of the ultrasonic transducer are not compensated for. This can cause the efficiency to deteriorate over the lifetime.

Der vorliegenden Erfindung liegt die Aufgabe zugrunde, ein Verfahren und eine Vorrichtung der eingangs genannten Art so auszugestalten, daß ein Arbeiten am optimalen Betriebspunkt möglich ist, und zwar unbeeinflußt von anderen Schaltungsbauteilen und von Alterungserscheinungen des Ultraschallwandlers. Insbesondere soll eine Nachführung der Ansteuerfrequenz des Ultraschallwandlers während des Betriebs ermöglicht werden derart, daß der Punkt des größten Zerstäubungs-Wirkungsgrades stets eingehalten wird.The present invention is based on the object of designing a method and a device of the type mentioned at the outset in such a way that it is possible to work at the optimum operating point, without being influenced by other circuit components and by signs of aging of the ultrasound transducer. In particular, tracking of the control frequency of the ultrasonic transducer should be made possible during operation such that the point of greatest atomization efficiency is always maintained.

Die genannte Aufgabe wird bei dem Verfahren der eingangs genannten Art erfindungsgemäß dadurch gelöst, daß die Ansteuerfrequenz in Abhängigkeit von dem am Ultraschallwandler abgegriffenen Signal nachgeführt wird.In the method of the type mentioned at the outset, the stated object is achieved in that the control frequency is tracked as a function of the signal tapped at the ultrasound transducer.

Bevorzugt wird dabei so vorgegangen, daß das am Ultraschallwandler abgegriffene Signal demoduliert und anschließend gefiltert wird, wonach aus dem demodulierten und gefilterten Signal ein aktuelles Mittelwertsignal gebildet wird, das zur Einstellung der Ansteuerfrequenz verwendet wird.It is preferably carried out in such a way that the signal tapped at the ultrasound transducer is demodulated and then filtered, after which a current mean value signal is formed from the demodulated and filtered signal, which signal is used to set the drive frequency.

Von Bedeutung ist, daß eine "Richtungsinformation", das heißt eine Information darüber, ob die aktuelle Arbeitsfrequenz oberhalb oder unterhalb der optimalen Arbeitsfrequenz (bei der sich optimale Zerstäubung ergibt) liegt, zumindest bei Betriebsaufnahme erhalten wird. Dies ist wichtig, weil ja die Ansteuerfrequenz entsprechend reduziert bzw. vergrößert werden muß. Um die "Richtungsinformation" zu erhalten und zu berücksichtigen, ist nach einer besonders vorteilhaften Weiterbildung vorgesehen, daß zumindest zu Beginn eines Zerstäubungsvorgangs versuchsweise die Ansteuerfrequenz von einem vorgegebenen Frequenzwert aus nach oben oder unten durchgestimmt wird, und daß das hierbei im Verlaufe der Zeit erhaltene Test-Mittelwertsignal auf das Vorliegen eines Maximums untersucht wird. Bei Vorliegen eines Maximums wird die "Richtungsinformation" erhalten, und die Ansteuerfrequenz wird dann unter Berücksichtigung der "Richtungsinformation" nach Maßgabe des aktuellen Mittelwertsignals im Frequenzbereich des Maximums nachgeführt. Das genannte Durchstimmen und Aufsuchen des Maximums sowie die Veränderung der Ansteuerfrequenz wird hierbei vorzugsweise mit Hilfe eines Mikro-Computers oder Mikroprozessors durchgeführt.It is important that "directional information", that is to say information about whether the current working frequency is above or below the optimal working frequency (at which optimal atomization is obtained), is obtained at least when operation is started. This is important because the drive frequency must be reduced or increased accordingly. In order to obtain and take into account the "directional information", it is provided according to a particularly advantageous further development that at least at the beginning of an atomization process the control frequency is experimentally tuned up or down from a predetermined frequency value, and that the test obtained in the course of time -Mean value signal is examined for the presence of a maximum. If a maximum is present, the "direction information" is obtained, and the drive frequency is then tracked, taking into account the "direction information", in accordance with the current mean value signal in the frequency range of the maximum. The aforementioned tuning and searching for the maximum and the change in the actuation frequency are preferably carried out here with the aid of a microcomputer or microprocessor.

Die genannte Aufgabe wird bei der Einrichtung zur Ansteuerung eines Ultraschallwandlers der eingangs genannten Art erfindungsgemäß dadurch gelöst, daß ein Frequenz-Nachführzweig vorgesehen ist, der den Eingang des Ultraschallwandlers mit dem Frequenzsteuereingang des Oszillators verbindet.The above object is achieved according to the invention in the device for controlling an ultrasonic transducer of the type mentioned at the outset in that a frequency tracking branch is provided which connects the input of the ultrasonic transducer to the frequency control input of the oscillator.

Bevorzugt umfaßt der Frequenz-Nachführzweig einen Amplituden-Demodulator und ein nachgeschaltetes Bandfilter. Dabei sollte dem Bandfilter ein Mikroprozessor nachgeschaltet sein, dessen Ausgang mit dem Frequenzsteuereingang des Oszillators verbunden ist.The frequency tracking branch preferably comprises an amplitude demodulator and a downstream band filter. The band filter should be followed by a microprocessor, the output of which is connected to the frequency control input of the oscillator.

Weitere vorteilhafte Ausgestaltungen der Erfindung sind in den Unteransprüchen gekennzeichnet.Further advantageous embodiments of the invention are characterized in the subclaims.

Das vorliegende Verfahren und die vorliegende Einrichtung basieren auf der Ansteuerung des Ultraschallwandlers mit einer Ansteuerfrequenz, die im Betrieb korrigiert werden kann. Das Wesentliche dabei ist, daß zur Frequenzabstimmung ein Signal benutzt wird, das direkt mit der Zerstäubungsleistung zusammenhängt und alle parasitären Einflüsse beinhaltet. Es ist dies das erwähnte, am Ultraschallwandler abgegriffene Signal, das die Reflektion der Ultraschallwellen an der Flüssigkeits-Oberfläche widerspiegelt. Der Ultraschallwandler, ein piezoelektrischer, vorzugsweise ein piezokeramischer Ultraschallwandler, wird ja sowohl zum Senden als auch zum Empfangen verwendet.The present method and the present device are based on the control of the ultrasound transducer with a control frequency that can be corrected during operation. The essential thing here is that a signal is used for frequency tuning that is directly related to the atomizing power and contains all parasitic influences. This is the mentioned signal picked up at the ultrasound transducer, which reflects the reflection of the ultrasound waves on the liquid surface. The ultrasound transducer, a piezoelectric, preferably a piezoceramic ultrasound transducer, is used both for sending and for receiving.

Ausführungsbeispiele der Erfindung werden im folgenden anhand von drei Figuren näher erläutert. Es zeigen:

FIG 1
eine Ultraschall-Flüssigkeits-Zerstäubungseinheit, die zur Luftbefeuchtung vorgesehen ist,
FIG 2
das am Ultraschallwandler abgegriffene Signal bei Fehlabstimmung, das heißt ohne Zerstäubung, und
FIG 3
das am Ultraschallwandler abgegriffene Signal bei optimaler Zerstäubung.
Exemplary embodiments of the invention are explained in more detail below with reference to three figures. Show it:
FIG. 1
an ultrasonic liquid atomization unit which is provided for air humidification,
FIG 2
the signal tapped at the ultrasonic transducer in the event of a mis-tuning, that is to say without atomization, and
FIG 3
the signal picked up at the ultrasonic transducer with optimal atomization.

Nach Figur 1 befindet sich in einem Gefäß 2 eine zu zerstäubende Flüssigkeit 4, vorliegend Wasser. Die Flüssigkeitsoberfläche ist mit 6 bezeichnet. Am Boden des Gefäßes 2 ist ein piezoelektrischer, vorzugsweise ein piezokeramischer Ultraschallwandler 8 angeordnet. Er sendet im Betrieb Ultraschallwellen 10 in Richtung auf die Wasseroberfläche 6 aus. Die Abstrahlfläche des Ultraschallwandlers 8 ist gekrümmt. Er wird zum Aussenden der Ultraschallwellen 10, gleichzeitig aber auch zum Empfangen der an der Flüssigkeitsoberfläche 6 reflektierten Ultraschallwellen eingesetzt.According to FIG. 1, there is a liquid 4 to be atomized, in the present case water, in a vessel 2. The liquid surface is designated 6. A piezoelectric, preferably a piezoceramic, ultrasonic transducer 8 is arranged on the bottom of the vessel 2. During operation, it emits ultrasonic waves 10 in the direction of the water surface 6. The radiation surface of the ultrasonic transducer 8 is curved. It is used for transmitting the ultrasonic waves 10, but at the same time also for receiving the ultrasonic waves reflected on the liquid surface 6.

Die Einrichtung zur Ansteuerung des Ultraschallwandlers 8 umfaßt einen steuerbaren Oszillator 12, der ein Ansteuersignal s mit einstellbarer Ansteuerfrequenz f abgibt. Es handelt sich bevorzugt um einen Sinusoszillator. Die Ansteuerfrequenz f liegt vorliegend im Bereich von 0,5 bis 5 MHz, vorzugsweise im mittleren Bereich von 2,5 MHz. Die Ansteuerfrequenz f kann durch ein Ansteuersignal p am Frequenzsteuereingang 14 des Oszillators 10 beeinflußt werden. Der Oszillator 12 ist ausgangsseitig an den Eingang eines Leistungsverstärkers 16 angeschlossen. Dessen Ausgang 18 wiederum ist an den Ultraschallwandler 8 angeschlossen.The device for controlling the ultrasound transducer 8 comprises a controllable oscillator 12 which emits a control signal s with an adjustable control frequency f. It is preferably a sine wave oscillator. The control frequency f is in the range from 0.5 to 5 MHz, preferably in the middle range from 2.5 MHz. The control frequency f can be influenced by a control signal p at the frequency control input 14 of the oscillator 10. The oscillator 12 is connected on the output side to the input of a power amplifier 16. Its output 18 is in turn connected to the ultrasound transducer 8.

Gemäß Figur 1 ist weiterhin ein Frequenz-Nachführzweig 20 vorgesehen, der den Ausgang 18 des Leistungsverstärkers 16 und damit den Eingang des Ultraschallwandlers 8 mit dem Frequenzsteuereingang 14 des Oszillators 12 verbindet. Dieser Frequenz-Nachführzweig 20 umfaßt vorliegend einen mit dem Ausgang 18 verbundenen Amplituden-Demodulator 22, ein nachgeschaltetes Bandfilter 24 und einen diesem nachgeschalteten Mikroprozessor 26, dessen Ausgang mit dem Frequenzsteuereingang 14 des Oszillators 12 verbunden ist. Der Frequenzbereich des Bandfilters 14 liegt dabei im Bereich von 50 Hz bis 10 kHz. Es ist dazu vorgesehen, denjenigen Bereich unterhalb der Nutzfrequenz von etwa 2,5 MHz herauszufiltern, in dem das maximale Rauschen liegt, wenn Zerstäubung eintritt. Bei dem Demodulator 22 handelt es sich um eine Gleichrichterschaltung, insbesondere um eine Dioden-Schaltung.According to FIG. 1, a frequency tracking branch 20 is also provided, which connects the output 18 of the power amplifier 16 and thus the input of the ultrasound converter 8 to the frequency control input 14 of the oscillator 12. In the present case, this frequency tracking branch 20 comprises an amplitude demodulator 22 connected to the output 18, a downstream band filter 24 and a microprocessor 26 connected downstream thereof, the output of which is connected to the frequency control input 14 of the oscillator 12. The frequency range of the bandpass filter 14 is in the range from 50 Hz to 10 kHz. It is intended to filter out the area below the useful frequency of approximately 2.5 MHz in which the maximum noise lies when sputtering occurs. The demodulator 22 is a rectifier circuit, in particular a diode circuit.

Im Betrieb wird der Ultraschallwandler 8 über die Leistungsstufe 16 mit dem Ansteuersignal s der einstellbaren Ansteuerfrequenz f aus dem steuerbaren Oszillator 12 versorgt. Der Ultraschallwandler 8 sendet dann Schallwellen 10 durch die Flüssigkeit 4 an deren Oberfläche 6. Dort werden die Ultraschallwellen reflektiert, und ein Teil dieser reflektierten Ultraschallwellen gelangt wieder zurück auf den Ultraschallwandler 8, wo sie in elektrische Signale umgesetzt werden. Diese Signale werden dem Steuersignal vom Leistungsverstärker 16 am Ausgang 18 zum Signal U überlagert. Das hier abgegriffene Signal U gelangt auf den Amplituden-Demodulator 22 und von dort auf das nachgeschaltete Bandfilter 24. Hier wird aus der Hüllkurve des Ausgangssignals U, das in den Figuren 2 und 3 bei Fehlabstimmung bzw. optimaler Abstimmung in Abhängigkeit der Zeit t dargestellt ist, eine Meßspannung oder ein "aktuelles Mittelwertsignal" m gewonnen. Dieses aktuelle Mittelwertsignal m wird zur Steuerung des Oszillators 12 verwendet. Liegt eine "Richtungsinformation" vor, welche vom Mikroprozessor 26 ermittelt wird, so kann daraus und aus dem Signal m das Signal p gebildet und dem Frequenzsteuereingang 14 aufgeschaltet werden.In operation, the ultrasonic transducer 8 is supplied via the power stage 16 with the control signal s of the adjustable control frequency f from the controllable oscillator 12. The ultrasonic transducer 8 then sends sound waves 10 through the liquid 4 to the surface 6 thereof. The ultrasonic waves are reflected there, and some of these reflected ultrasonic waves are returned to the ultrasonic transducer 8, where they are converted into electrical signals. These signals are superimposed on the control signal from the power amplifier 16 at the output 18 to the signal U. The signal U tapped here arrives at the amplitude demodulator 22 and from there to the downstream band filter 24. Here the envelope becomes of the output signal U, which is shown in FIGS. 2 and 3 in the case of a mis-tuning or optimal tuning as a function of the time t, a measuring voltage or a “current mean value signal” m is obtained. This current mean signal m is used to control the oscillator 12. If there is "directional information" which is determined by the microprocessor 26, the signal p can be formed therefrom and from the signal m and applied to the frequency control input 14.

Zu Beginn eines Zerstäubungsvorgangs wird versuchsweise die Ansteuerfrequenz f mit Hilfe des Mikroprozessors 26 von einem vorgegebenen Frequenzwert fo aus nach oben oder unten zeitlich verändert (Testlauf). Als Signal m erhält man dann im Verlaufe der Zeit t ein Test-Mittelwertsignal m'. Dieses wird vom Mikroprozessor 26 auf das Vorliegen eines Maximums untersucht. Der Mikroprozessor 26 ermittelt dabei auch, ob der ursprünglich vorgegebene Frequenzwert fo oberhalb oder unterhalb derjenigen Frequenz f* liegt, bei der das Maximum des Test-Mittelwertsignals m' auftritt. Dies ist die oben erwähnte "Richtungsinformation". Abhängig von dieser Information und vom Signal m verändert der Mikroprozessor 26 das Ansteuersignal p so, daß das genannte Maximum - diesem entspricht der Punkt des größten Zerstäubungs-Wirkungsgrades -eintritt und anschließend festgehalten wird. Mit anderen Worten: Bei Vorliegen des Maximums in der gewählten Richtung (nach oben oder unten) wird die Ansteuerfrequenz f nach Maßgabe des aktuellen Mittelwertsignals m im Frequenzbereich des Maximums nachgeführt. Der Mikroprozessor 26 ist also imstande festzustellen, daß das Maximum überschritten wurde, und er ist so eingerichtet, daß das Ansteuersignal p die Ansteuerfrequenz f in Richtung auf die optimale Frequenz f* führt.At the beginning of a sputtering process, the control frequency f is experimentally changed with the aid of the microprocessor 26 from a predetermined frequency value fo upwards or downwards (test run). A test mean signal m 'is then obtained as signal m in the course of time t. This is examined by the microprocessor 26 for the presence of a maximum. The microprocessor 26 also determines whether the originally specified frequency value fo is above or below the frequency f * at which the maximum of the test mean signal m 'occurs. This is the "direction information" mentioned above. Depending on this information and on the signal m, the microprocessor 26 changes the control signal p such that the said maximum - which corresponds to the point of the greatest atomization efficiency - occurs and is then recorded. In other words: when the maximum is present in the selected direction (up or down), the control frequency f is tracked in the frequency range of the maximum in accordance with the current mean value signal m. The microprocessor 26 is thus able to determine that the maximum has been exceeded and is set up in such a way that the control signal p guides the control frequency f in the direction of the optimum frequency f *.

Es wurde bereits erwähnt, daß ein Teil der an der Oberfläche 6 reflektierten Ultraschallwellen wieder auf den Ultraschallwandler 8 zurückgelangt. In der Flüssigkeit 4 bilden sich stehende Wellen aus. Da der Ultraschallwandler 8 nicht nur elektrische Energie in Ultraschall, sondern auch umgekehrt Ultraschall in elektrische Energie umwandeln kann, wirkt sich der reflektierte Ultraschall unmittelbar auf das Ausgangssignal am Ausgang 18 aus. Je nach Amplitude und Phasenlage der Reflektionen ergibt sich am Innenwiderstand des Leistungsverstärkers 16 ein Spannnungsabfall U, der sich aus der Addition des Ausgangssignals des Verstärkers 16 mit dem reflektierten Signal ergibt.It has already been mentioned that some of the ultrasound waves reflected on the surface 6 return to the ultrasound transducer 8. Standing waves form in the liquid 4. Since the ultrasound transducer 8 not only converts electrical energy into ultrasound, but also vice versa into ultrasound can convert electrical energy, the reflected ultrasound has a direct effect on the output signal at the output 18. Depending on the amplitude and phase position of the reflections, a voltage drop U results at the internal resistance of the power amplifier 16, which voltage drop U results from the addition of the output signal of the amplifier 16 with the reflected signal.

Solange die Ansteuerfrequenz f weit vom optimalen Arbeitspunkt f* des Ultraschallwandlers 8 entfernt liegt, bleibt die Flüssigkeitsoberfläche 6 ruhig. Das Wellenfeld 10 wird dann nicht gestört, und das Signal U unterliegt keiner zeitlichen Änderung. Dies ist in Figur 2 gezeigt. In diesem Fall liefert der Amplituden-Demodulator 22 eine reine Gleichspannung, und die Meßspannung m hinter dem Bandpaßfilter 24 ist nahezu Null. In Figur 2 ist das Signal s - abweichend von der bevorzugten sinusförmigen Ausbildung - als Dreieckssignal gezeigt.As long as the control frequency f is far from the optimal operating point f * of the ultrasonic transducer 8, the liquid surface 6 remains calm. The wave field 10 is then not disturbed and the signal U is not subject to any change over time. This is shown in Figure 2. In this case, the amplitude demodulator 22 supplies a pure DC voltage, and the measurement voltage m behind the bandpass filter 24 is almost zero. In Figure 2, the signal s - deviating from the preferred sinusoidal configuration - is shown as a triangular signal.

Wird nun vom Mikroprozessor 26 die Ansteuerfrequenz f des Ansteuersignals s in Richtung auf die optimale Arbeitsfrequenz f* des Ultraschallwandlers 8 verschoben, wird die Flüssigkeitsoberfläche 6 zunehmend unruhiger. Durch diese Bewegung an der Flüssigkeitsoberfläche 6 wird das Wellenfeld 10 gestört. Der reflektierte Signalanteil wird dadurch mit einem niederfrequenten Rauschen moduliert, das insbesondere im Bereich von 50 Hz bis 10 KHz liegt. Dieses Rauschen ist durch die Hüllkurven hl und h2 in Figur 3 verdeutlicht. Aus diesem so verrauschten Signal U wird über den Demodulator 22 und das Bandpaßfilter 24 das aktuelle Mittelwertsignal m gebildet, das nun nicht mehr Null ist, sondern einen durchaus meßbaren Wert aufweist. Es könnte als "Rauschsignal" bezeichnet werden.If the control frequency f of the control signal s is now shifted by the microprocessor 26 in the direction of the optimum working frequency f * of the ultrasound transducer 8, the liquid surface 6 becomes increasingly uneasy. The wave field 10 is disturbed by this movement on the liquid surface 6. The reflected signal component is thereby modulated with a low-frequency noise, which is in particular in the range from 50 Hz to 10 kHz. This noise is illustrated by the envelopes h1 and h2 in FIG. 3. From this noisy signal U, the current average signal m is formed via the demodulator 22 and the bandpass filter 24, which is now no longer zero but has a value that can be measured. It could be called a "noise signal".

Bei weiterer Annäherung an den optimalen Arbeitspunkt f* (Punkt des größten Wirkungsgrades) wird dieses Mittelwert- oder Meßsignal m größer. Bei Einsetzen der Zerstäubung nimmt neben der Amplitude dieses Rauschsignals m auch dessen Bandbreite zu. Im optimalen Arbeitspunkt, der charakterisiert wird durch die Arbeitsfrequenz F*, tritt maximales Rauschen auf. Bei passender Dimensionierung des Bandpaßfilters 24 kann ein sehr genaues Abstimmverhalten erzielt werden. Das aktuelle Mittelwertsignal oder Rauschsignal m wird dabei vom Mikroprozessor 26 als Steuersignal p zur Frequenzsteuerung des Oszillators 12 verwendet.When the optimal operating point f * (point of greatest efficiency) is approached further, this mean or measurement signal m becomes larger. When the atomization begins, the bandwidth of this noise signal m also increases in addition to the amplitude. In the optimal working point that is characterized due to the operating frequency F *, maximum noise occurs. With a suitable dimensioning of the bandpass filter 24, a very precise tuning behavior can be achieved. The current mean value signal or noise signal m is used by the microprocessor 26 as a control signal p for frequency control of the oscillator 12.

Claims (12)

Verfahren zur Ansteuerung eines Ultraschallwandlers (8) zur Zerstäubung einer Flüssigkeit (4), wobei ein Ansteuersignal (s) mit einstellbarer Ansteuerfrequenz (f) dem Ultraschallwandler (8) zugeleitet wird, dadurch gekennzeichnet, daß die Ansteuerfrequenz (f) in Abhängigkeit von dem am Ultraschallwandler (8) abgegriffenen Signal (U) nachgeführt wird.Method for controlling an ultrasonic transducer (8) for atomizing a liquid (4), wherein a trigger signal (s) with adjustable trigger frequency (f) is fed to the ultrasonic transducer (8), characterized in that the trigger frequency (f) in dependence on the Ultrasonic transducer (8) tapped signal (U) is tracked. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß das am Ultraschallwandler (8) abgegriffene Signal (U) demoduliert und anschließend gefiltert wird.Method according to Claim 1, characterized in that the signal (U) picked up at the ultrasound transducer (8) is demodulated and then filtered. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß aus dem demodulierten und gefilterten Signal (m) ein aktuelles Mittelwertsignal (p) gebildet wird, das zur Einstellung der Ansteuerfrequenz (f) verwendet wird.Method according to Claim 2, characterized in that a current mean value signal (p) is formed from the demodulated and filtered signal (m) and is used to set the control frequency (f). Verfahren nach Anspruch 3, dadurch gekennzeichnet, daß zumindest zu Beginn eines Zerstäubungsvorgangs versuchsweise die Ansteuerfrequenz (f) von einem vorgegebenen Frequenzwert aus nach oben oder unten durchgestimmt wird, daß das hierbei im Verlaufe der Zeit (t) erhaltene Test-Mittelwertsignal auf das Vorliegen eines Maximums untersucht wird, um eine Richtungs-Information zu erhalten, daß bei Vorliegen eines Maximums aus dem aktuellen demodulierten und gefilterten Signal (m) und aus der Richtungs-Information das aktuelle Mittelwertsignal (p) qebildet wird, und daß die Ansteuerfrequenz (f) dann nach Maßgabe des aktuellen Mittelwertsignals (p) im Frequenzbereich des Maximums nachgeführt wird.A method according to claim 3, characterized in that at least at the beginning of an atomization process, the control frequency (f) is tuned up or down from a predetermined frequency value in such a way that the test mean value signal obtained in the course of time (t) indicates the presence of a Maximum is examined in order to obtain directional information, that when there is a maximum from the current demodulated and filtered signal (m) and from the directional information, the current mean signal (p) is formed, and that the drive frequency (f) then is tracked in the frequency range of the maximum in accordance with the current mean value signal (p). Verfahren nach einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, daß das dem Ultraschallwandler (8) zugeleitete Ansteuersignal (s) sinusförmig ist.Method according to one of claims 1 to 4, characterized in that the control signal (s) fed to the ultrasound transducer (8) is sinusoidal. Einrichtung zur Ansteuerung eines Ultraschallwandlers (8) zur Zerstäubung einer Flüssigkeit (4) mit einem steuerbaren Oszillator (12), der ein Ansteuersignal (s) mit einstellbarer Ansteuerfrequenz (f) abgibt und der ausgangsseitig an den Ultraschallwandler (8) angeschlossen ist, dadurch gekennzeichnet, daß ein Frequenz-Nachführzweig (20) vorgesehen ist, der den Eingang (18) des Ultraschallwandlers (8) mit dem Frequenzsteuereingang (14) des Oszillators (12) verbindet.Device for controlling an ultrasonic transducer (8) for atomizing a liquid (4) with a controllable oscillator (12) which emits a control signal (s) with an adjustable control frequency (f) and which is connected on the output side to the ultrasound converter (8), characterized in that in that a frequency-Nachführzweig (20) is provided to the frequency control input (14) connects the input (18) of the ultrasonic transducer (8) of the oscillator (12). Einrichtung nach Anspruch 6, dadurch gekennzeichnet, daß der Frequenz-Nachführzweig (20) einen Amplituden-Demodulator (22) und ein nachgeschaltetes Bandfilter (24) umfaßt.Device according to claim 6, characterized in that the frequency tracking branch (20) comprises an amplitude demodulator (22) and a downstream band filter (24). Einrichtung nach Anspruch 7, dadurch gekennzeichnet, daß dem Bandfilter (24) ein Mikroprozessor (26) nachgeschaltet ist, dessen Ausgang mit dem Frequenzsteuereingang (14) des Oszillators (12) verbunden ist.Device according to claim 7, characterized in that the band filter (24) is followed by a microprocessor (26), the output of which is connected to the frequency control input (14) of the oscillator (12). Einrichtung nach einem der Ansprüche 6 bis 87 dadurch gekennzeichnet, daß zwischen dem Oszillator (12) und dem Ultraschallwandler (8) ein Leistungsverstärker (16) angeordnet ist, an dessen Ausgang (18) der Frequenz-Nachführzweig (20) angeschlossen ist.Device according to one of claims 6 to 87, characterized in that a power amplifier (16) is arranged between the oscillator (12) and the ultrasonic transducer (8), to whose output (18) the frequency tracking branch (20) is connected. Einrichtung nach einem der Ansprüche 6 bis 97 dadurch gekennzeichnet, daß der Ultraschallwandler (8) ein piezoelektrischer, vorzugsweise ein piezokeramischer Ultraschallwandler ist.Device according to one of claims 6 to 97, characterized in that the ultrasonic transducer (8) is a piezoelectric, preferably a piezoceramic, ultrasonic transducer. Einrichtung nach einem der Ansprüche 6 bis 107 dadurch gekennzeichnet, daß der Oszillator (12) ein Sinusoszillator ist.Device according to one of claims 6 to 107, characterized in that the oscillator (12) is a sinusoidal oscillator. Einrichtung nach einem der Ansprüche 7 bis 11, dadurch gekennzeichnet, daß die vom Oszillator (12) abgegebene Frequenz im Bereich von 075 bis 5 MHz und daß der Frequenzbereich des Bandfilters (24) im Bereich von 50 Hz bis 10 KHz liegt.Device according to one of Claims 7 to 11, characterized in that the frequency emitted by the oscillator (12) is in the range from 075 to 5 MHz and in that the frequency range of the bandpass filter (24) is in the range from 50 Hz to 10 KHz.
EP91102120A 1990-02-14 1991-02-14 Method and apparatus for ultrasonic liquid atomization Expired - Lifetime EP0442510B1 (en)

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DE4004541A DE4004541A1 (en) 1990-02-14 1990-02-14 METHOD AND DEVICE FOR ULTRASONIC LIQUID SPRAYING

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ATE117599T1 (en) 1995-02-15
DE4004541A1 (en) 1991-08-22
EP0442510B1 (en) 1995-01-25
DE59104350D1 (en) 1995-03-09

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