EP1875969A1 - Generator for exciting a piezoelectric transducer - Google Patents

Abstract

The generator has a digital processing unit e.g. microcontroller, comprising an oscillator to generate a clock frequency, where the generator is operated based on an iterative operation phase (200) comprising successive iterations. A transducer is excited by the generator, during each iteration, at a frequency variable in a frequency band around a set point frequency. Values of electrical quantities related to the excitation of the transducer are acquired for set of frequencies of the band. The values are analyzed to determine a resonance/anti-resonance frequency for following iteration.

Description

The present invention relates to a generator for exciting a piezoelectric transducer, for example a transducer for spraying a liquid.

Such a generator may be used, for example, to diffuse an odorant product, especially a perfume.

The excitation of the transducer must be done at a particular frequency if it is desired to obtain a satisfactory energy efficiency, particularly when the generator is powered by a power source such as a battery or a battery and that the power consumption must be reduced as much as possible.

This frequency is for example the resonant or anti-resonance frequency, and may vary depending on certain operating parameters such as, for example, the temperature or the rheological characteristics of the liquid to be sprayed.

It is thus known to seek to enslave the excitation frequency of the transducer by measuring an electrical quantity related to its operation, in order to keep the excitation frequency as close as possible to the optimal frequency.

Sputtering devices comprising a relatively complex analogue circuit for controlling the excitation frequency are known from the publications US 3,904,896 or JP 06-254455 .

The publications US 4,689,515 , EP 0 123 277 and US 2002/0129813 disclose spray devices having a digital processing unit responsive to variation of an electrical magnitude related to the operation of the transducer to maintain the excitation frequency at an optimum value.

The publication WO 00/51747 describes a sputtering device in which the excitation frequency of the transducer varies sawtooth in a predefined frequency band while the excitation voltage decreases exponentially.

The patent application French FR 2 802 836 describes another spraying device. During an initial phase, a search for an optimal excitation frequency is performed. This research is carried out initially with a scan in a relatively wide frequency band ranging from 1.7 MHz to 1.9 MHz with a step of 10 kHz. This frequency is determined by analyzing the intensity of the exciter current. Once this frequency value is determined, a frequency sweep with a step of 1 kHz is performed in order to determine more precisely, using the same criterion, the optimal excitation frequency. Then, in a subsequent operating phase, the excitation is carried out at the frequency thus determined, except when certain characteristics of the exciter current exceed predefined limit values. The measurement of the exciter current can be difficult to implement.

There is a need for further refinement of the sprayer generators in order to benefit from a spray device which has satisfactory sputtering and power consumption characteristics while being relatively inexpensive to manufacture.

There is also a need to benefit from a sprayer generator in which the transducer is excited in a manner extending its life.

The invention aims to meet all or part of these needs.

• exciter le transducteur à une fréquence variable dans une bande de fréquences autour d'une fréquence de consigne,
• acquérir des valeurs prises par au moins une grandeur électrique liée à l'excitation du transducteur pour une pluralité de fréquences de cette bande,
• analyser les valeurs ainsi acquises pour déterminer une nouvelle fréquence de consigne pour une itération suivante.

The object of the invention is thus, according to a first of its aspects, a generator for exciting a piezoelectric transducer, in particular for spraying a liquid, this generator comprising at least one digital processing unit and being arranged to operate at least according to an iterative operation phase comprising more than two successive iterations and for, during each iteration:

• excite the transducer at a variable frequency in a frequency band around a set frequency,
• acquiring values taken by at least one electrical quantity related to the excitation of the transducer for a plurality of frequencies of this band,
• analyze the values thus acquired to determine a new reference frequency for a next iteration.

Thanks to the invention, the power supply of the transducer can be performed in a relatively simple manner with a high probability that during each iteration, the transducer is excited at a frequency close to the frequency for which the spray efficiency is optimal. , namely the resonant frequency or anti-resonance depending on the case.

The Applicant has found that exciting the transducer during each iteration at a frequency close to the optimal frequency was sufficient to obtain a satisfactory spray result.

It is thus not necessary thanks to the invention that the transducer is permanently excited at the resonant or anti-resonance frequency.

The frequency variation in the frequency band during an iteration can be carried out according to a scan between extreme frequency values of the band, for example from a low value of the frequency to a higher value of the frequency and passing through the setpoint. Alternatively, the variation of the frequency can be performed during a random iteration in the frequency band.

Preferably, the frequency variation occurs around the setpoint value and the latter corresponds for example substantially to half of the frequency band defined by the low and high values of the frequency.

The electrical quantity whose values are acquired during an iteration may be a voltage or a current. The acquisition of a voltage, including the voltage across the transducer, can simplify the manufacture of the device relative to the reading of a current.

The values taken by the electrical quantity during each iteration can be acquired for all the frequencies of the frequency band. Thus, at each excitation frequency corresponds a measured value of the electrical quantity.

The digital processing unit may be arranged to generate a transducer excitation signal in pulse form. This excitation signal can drive a power stage connected to the transducer.

The generator may be arranged such that during an iteration, for each of the frequencies of the band, at least one burst of pulses at this frequency is emitted by the digital processing unit. A burst of pulses comprises for example between 50 and 250 pulses. Each burst of pulses can last between 10 ms and 100 ms, for example. Two bursts of pulses can be separated by a rest time of between 10 μs and 100 ms, for example. Two bursts of pulses can be separated by a rest period. The same iteration can comprise two bursts of pulses of the same frequency. The same iteration can also comprise bursts of pulses comprising the same number of pulses, the duration of a burst being for example determined by counting the pulses.

The generator may also be arranged to operate according to an initial operating phase before operating according to the iterative operation phase.

The analysis of the values of the measured electrical quantity can be carried out at least partially during a dead time during which the transducer is not excited, for example a dead time separating the iteration during which these values were acquired from the next iteration. This can make it possible to accept a lower calculation speed for the digital processing unit and thus to reduce the cost thereof.

The analysis of the values may comprise the determination at each iteration of an extremum of the electrical quantity, in particular a maximum which may correspond approximately to the resonance or anti-resonance frequency.

• exciter le transducteur à une fréquence variable dans une bande de fréquences par défaut plus large que la bande de fréquences de la phase itérative, autour d'une fréquence de consigne par défaut,
• acquérir les valeurs prises par au moins une grandeur électrique liée à l'excitation du transducteur pour une pluralité de fréquences de cette bande, et
• analyser les valeurs ainsi acquises pour déterminer la fréquence de consigne pour le début de la phase de fonctionnement itérative.

The digital processing unit can be arranged to compare the extremum thus determined with a reference value and, depending on the result of this comparison, to perform a new iteration of the iterative operation phase around the setpoint frequency determined at a given value. previous iteration, ie an initialization phase in which the generator is arranged for:

• exciting the transducer at a variable frequency in a default frequency band wider than the frequency band of the iterative phase, around a default reference frequency,
• acquiring the values taken by at least one electrical quantity related to the excitation of the transducer for a plurality of frequencies of this band, and
• analyzing the values thus acquired to determine the set frequency for the beginning of the iterative operation phase.

The digital processing unit may be arranged to perform a frequency sweep in the default frequency band. The scanning in the default frequency band can be carried out in a step larger than the scanning in the frequency band during the iterative operation phase.

During the initialization phase, the values of the electrical quantity related to the excitation of the transducer can be acquired for all the frequencies of the default frequency band.

The default frequency band can be at least +/- 3% wide on either side of the default setpoint frequency.

The frequency band of the iterative operating phase may have a width of at most +/- 20% on either side of the corresponding reference frequency, for example not more than +/- 10%.

The frequency band of the iterative operating phase may be centered around the corresponding reference frequency.

The digital processing unit may include a microcontroller, programmed to generate each excitation frequency by dividing a clock frequency of the microcontroller.

According to another embodiment of the invention, the piezoelectric transducer is arranged to generate vibrations to be transmitted to keratin materials.

• un transducteur piézoélectrique agencé pour pulvériser le liquide et un générateur tel que défini précédemment.

According to another of its aspects, the subject of the invention is also a device for spraying a liquid, comprising:

• a piezoelectric transducer arranged to spray the liquid and a generator as defined above.

The device may include an independent power supply.

• la figure 1 est une vue schématique avec coupe axiale partielle d'un exemple de dispositif de pulvérisation selon l'invention,
• la figure 2 est un schéma synoptique du générateur,
• la figure 3 illustre différentes étapes dans le fonctionnement du générateur,
• la figure 4 représente l'allure du signal d'excitation envoyé à l'étage de puissance du générateur,
• la figure 5 représente un détail de la figure 4, et
• la figure 6 est un schéma d'un exemple d'étage de puissance.

The invention will be better understood on reading the following detailed description of an example of non-limiting implementation thereof, and on examining the appended drawing in which:

• FIG. 1 is a schematic view with partial axial section of an example of a spraying device according to the invention,
• FIG. 2 is a block diagram of the generator,
• FIG. 3 illustrates different steps in the operation of the generator,
• FIG. 4 represents the shape of the excitation signal sent to the power stage of the generator,
• FIG. 5 represents a detail of FIG. 4, and
• Fig. 6 is a diagram of an exemplary power stage.

The spraying device 1, shown in FIG. 1, comprises a generator 20 according to the invention and at least one reservoir containing a product P to be sprayed, supplying a piezoelectric transducer 4. The latter may be of any known type, being excited by the generator and arranged to vibrate a perforated membrane 6 to allow the formation of droplets 10 of the product P.

The generator 20 and the tank 2 may be integral with a housing 3.

The transducer 4 comprises, in the illustrated example, an annulus 11 of axis X made at least partially in a piezoelectric material, for example a ceramic, in particular zirconate (PZT) or metaniobate (PN) titanium or barium or zinc oxide. The ring 11 is for example the one marketed under the reference 27121 by the Danish company FERROPERM, the material being of the PZ27 type.

In the example considered, the piezoelectric material is polarized in the direction of its thickness, that is to say along the X axis, so that the application of a potential on its main faces, by means of Electrical conductors 18 and 19 connected to the generator 20 causes a vibration of the ring 11 in the radial direction which tends to reduce its internal diameter.

The perforated membrane 6 being in contact with the radially inner surface of the ring 11, the force exerted will cause a very slight bending of the perforated membrane 6, making it possible to create in its center the axial vibration necessary for the formation of the droplets 10.

The membrane 6 comprises, in its central region, openings 22 whose size and number are adapted to the particle size of the droplets and the flow rate that it is desired to generate.

The product feed of the perforated membrane 6 is effected for example by capillarity, by means of a wick 3 immersed in the reservoir 2, as illustrated in FIG. 1, or in the manner described in the patent US 5,518,179 or in the international application WO 00/53337 .

The generator 20 comprises, as can be seen in FIG. 2, a power stage 30 which delivers the excitation current to the transducer 4. This power stage, which can be produced according to the diagram given in FIG. 6, is attacked by a digital processing unit 40 which comprises for example at least one microcontroller.

The digital processing unit 40 comprises an oscillator 41 which generates a sufficiently high clock frequency to allow, after division by an integer n in a divider 42, to generate the control signal of the power stage, after implementation. is formed by a monostable 43. The clock frequency is for example greater than or equal to 30 MHz, being for example of the order of 60 MHz. If the frequency to be generated is for example of the order of 98.2 kHz, the integer n is for example between 580 and 640, for example 599 and 622, during the iterative phase. The difference between two successively generated frequencies is thus between 166 Hz and 155 Hz. The greater the clock frequency, the smaller the difference between two frequencies generated.

The digital processing unit 40 comprises first means 45 for measuring the values taken by at least one electrical quantity related to the excitation of the transducer as well as second means 46 for analyzing the values thus measured.

In the example considered, the first means 45 are arranged to measure the voltage across the transducer 4 and comprise an analog digital converter and a memory in order to allow the storage of the measured values when the excitation frequency varies and their subsequent analysis by the second means 46.

The first 45 and second 46 means are for example implemented by programming a microcontroller having a processor, a memory and an analog digital converter. The divider 42 can also be implemented by programming this microcontroller and the oscillator 41 is for example the clock circuit of the microcontroller. Monostable 43 may or may not be integrated with the microcontroller.

The generator 20 may be powered by an autonomous power supply 24, which may include one or more cells or batteries.

The operation of the generator 20 will now be described with reference to FIG. 3. This is arranged to operate initially according to an initialization phase 100 and then according to an iterative phase 200.

The initialization phase 100 begins with the start-up 110 of the generator. The latter can take place for example when the user presses a pushbutton or when an activation signal is received by the generator. This activation signal comes, for example, from the reception of an order transmitted by a remote control or a computer network or from the reading of a file containing an instruction intended to trigger the spraying in association with images and / or sounds. predefined, for example.

The next step is a reading step 120 of default values of the set frequency f _i and the expected maximum voltage V _i across the transducer 4.

In the next step 130, the digital processing unit 40 is arranged to perform a frequency sweep in a default frequency band around the default reference frequency f _i .

This scanning is performed with a relatively high scanning pitch, for example about 300 Hz, from a low frequency value of about 0.9 f _i to a high frequency value of about 1.1 f _i , in the example considered.

During this scanning, for each discrete value f _j of the frequency, the digital processing unit 40 stores the corresponding voltage across the transducer 4.

The frequency sweep can be carried out by generating for each frequency f _j a burst 51 of pulses of frequency f _j , as illustrated in FIG. 4, these bursts being separated by rest times 52.

The duration of a burst 51 is for example between 10 microns and 100 ms. Two bursts of pulses are separated for example by a rest time of between 10 μs and 100 ms. In the example described, the duration of a burst 51 is about 1.5 ms and that of a rest time 52 is of the order of 3.5 ms. The frequency f _j corresponds to a particular value n _j of the division ratio n.

The digital processing unit 40 is arranged to change the division ratio n of the clock frequency in the divider 42 during the idle time, so as to generate the next frequency. The division ratio is for example incremented by one or two units, during the initial phase.

Each burst 51 may contain, for example, the same number of pulses 53.

Once the frequency sweep has been carried out, the generator proceeds to a step 140 during which the reference frequency f _m for the beginning of the iterative phase 200 is determined, by analyzing the voltage across the transducer 4 for each frequency f _j excitation of the initial phase 100, in order to select the frequency which corresponds to the maximum amplitude of voltage across the piezoelectric transducer 4.

, où V_i est la tension maximale attendue par défaut.In a subsequent step 150, which marks the end of the initial phase 100, the generator 20 determines whether the amplitude of the voltage across the transducer 4 is greater than a predefined threshold which is in the example considered equal to $\frac{{\mathrm{<figure-callout\; id="2"\; label="V\; i"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">V\; </figure-callout>}}_{\mathrm{i}}}{\mathrm{2}}$

, where V _i is the maximum voltage expected by default.

If so, the generator starts the iterative operation phase 200 and in the negative the generator returns to step 110 to restart the initialization phase 100.

During the iterative operation phase 200, the transducer is excited at a frequency f _j variable in a frequency band centered on the previously determined set value f _m , this frequency band extending between less than 2% of the frequency of setpoint f _m and more than 2% of the latter.

The scanning is thinner than during the initialization phase, with for example a scanning step of the order of 150 Hz, the division ratio n being for example decremented by one unit at each iteration.

At each iteration of the iterative operating phase 200, a new reference frequency f _m is determined, by analysis at a step 220 of the values of the voltage across the transducer, so as to take as a new reference frequency the frequency for which the voltage is maximum.

. Dans l'affirmative, le générateur recommence une itération à l'étape 210 avec comme nouvelle valeur de fréquence de consigne celle qui vient d'être déterminée. Dans la négative, le générateur recommence la phase d'initialisation 100.In step 230, as in step 150, the generator 20 determines whether the maximum amplitude of the voltage detected during the frequency sweep is greater than $\frac{{\mathrm{<figure-callout\; id="2"\; label="V\; i"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">V\; </figure-callout>}}_{\mathrm{i}}}{\mathrm{2}}$

. If so, the generator starts an iteration again at step 210 with the newly determined frequency value as the newly determined frequency. If not, the generator resumes the initialization phase 100.

The operation during the iterative phase 200 may comprise spray cycles comprising one or more iterations and a dead time during which the transducer is not excited, of duration for example between 1 and 5 ms between two successive cycles. This dead time can take place in step 230 for example.

The dead time can improve the performance of the spray by allowing in particular product P to go back into the wick 3 between two spraying cycles.

Of course, the invention is not limited to the example just described.

In particular, the scanning in the frequency band during the iterative operation phase 200 can be carried out in a wider or narrower range.

The frequency band in which the scanning takes place, both during the initial phase 100 and during the iterative phase 200, may not be exactly centered on the reference frequency.

The scanning in a frequency band can be done by increasing the frequency from the low frequency to the high frequency or vice versa, or randomly or otherwise.

The default values f _i and V _i can be stored in the generator 20 during the manufacture of the spraying device, in particular during operation tests.

The invention is not limited to a spraying device and the generator may be used with other devices comprising piezoelectric transducers, for example devices arranged to transmit ultrasonic vibrations to keratin materials such as ultrasonic transducers. used in devices to promote the penetration of active ingredients into the epidermis.

In this case, the clock frequency could be higher, for example greater than 100 MHz, the excitation frequency of the transducer being for example of the order of 1.5 MHz.

The expression "comprising one" must be understood to include "comprising at least one" and "between" as being included boundaries.

Claims (25)

Generator (20) for exciting a piezoelectric transducer (4), in particular for spraying a liquid, this generator comprising at least one digital processing unit (40) and being arranged to operate at least according to an iterative operating phase (200) comprising more than two successive iterations and, during each iteration: to excite the transducer (4) at a variable frequency in a frequency band around a reference frequency (f _m ), during the excitation of the transducer, acquiring values taken by at least one electrical quantity related to the excitation of the transducer (4) for a plurality of frequencies of this band, analyzing the values thus acquired to determine a new setpoint frequency (f _m ) for a next iteration. Generator according to claim 1, the new reference frequency corresponding approximately to the resonant frequency of the transducer (4). Generator according to claim 1, the new reference frequency corresponding approximately to the anti-resonance frequency of the transducer. Generator according to any one of the preceding claims, the frequency variation in the frequency band during an iteration taking place according to a frequency sweep between extreme frequency values of the band. Generator according to any one of the preceding claims, the measured electrical quantity being the voltage across the transducer (4). Generator according to any one of the preceding claims, the values taken by the electrical quantity during each iteration being acquired for all the frequencies of the band. Generator according to any one of the preceding claims, the analysis of the values taken by the electrical quantity being performed at least partially during a dead time during which the transducer is not excited. Generator according to any one of the preceding claims, the digital processing unit (40) being arranged to generate a pulse excitation signal (51). Generator according to any one of the preceding claims, being arranged such that during an iteration, for each of the frequencies of the band, at least one burst (51) of pulses at this frequency is emitted by the digital processing unit (40). Generator according to the preceding claim, each burst of pulses (51) lasting between 10 μs and 100 ms. Generator according to one of claims 9 and 10, two bursts of pulses (51) being separated by a rest time (52) of between 10 μs and 100 ms. Generator according to any one of claims 9 to 12, the bursts (51) having the same number of pulses (53). Generator according to any one of the preceding claims, the digital processing unit (40) being arranged to determine at each iteration an extremum of the measured electrical quantity. Generator according to the preceding claim, the digital processing unit (40) being arranged to compare the extremum thus determined to a reference value. Generator according to the preceding claim, wherein according to the result of the comparison, the digital processing unit (42) performs either a new iteration of the iterative operation phase (200) or returns to an initialization phase (100). ). Generator according to any one of the preceding claims, being arranged to operate according to an initial operating phase (100) before operating according to the iterative operating phase (200). Generator according to the preceding claim, wherein the generator is arranged, during the initial operating phase (100), for: - Exciting the transducer at a variable frequency in a default frequency band, wider than during the iterative operation phase (200), around a default reference frequency (f _i ), acquiring the values taken by at least one electrical quantity related to the excitation of the transducer (4) for a plurality of frequencies of this band, analyzing the values thus acquired to determine the set frequency (f _m ) for the beginning of the iterative operation phase (200). Generator according to the preceding claim, the digital processing unit (40) being arranged to perform a frequency sweep in the default frequency band. Generator according to claims 4 and 17, the scanning in the default frequency band being effected in a frequency step wider than scanning in the frequency band during the iterative operation phase (200). Generator according to any one of claims 17 to 19, the default frequency band having a width of at least 3% on either side of the default reference frequency. Generator according to any one of the preceding claims, the frequency band during the iterative operation phase (200) having a width of at most 20% on each side of the corresponding reference frequency. Generator according to any one of the preceding claims, the frequency band during the iterative operation phase (200) being centered around the corresponding reference frequency. Generator according to any one of the preceding claims, the digital processing unit (40) comprising a microcontroller programmed to generate each excitation frequency by dividing a clock frequency of the microcontroller. Device (1) for spraying a liquid comprising a transducer (4) arranged to spray the liquid and a generator (20) as defined in any one of the preceding claims. Device according to the preceding claim, comprising an autonomous power supply (24).

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