EP0340470A1 - Method and circuit for driving an ultrasonic transducer, and their use in atomizing a liquid - Google Patents

Abstract

An ultrasonic transducer (1) is excited by the output signal of a voltage-controlled oscillator (4). The latter is controlled (6, 7) with a delta generator (5) in such a way that its frequency is periodically swept in a range enclosing the series resonance of the ultrasonic transducer. A measured variable (I, V) corresponding to the damping of the ultrasonic transducer is formed and compared with a maximum admissible damping. If the detected damping is smaller than the maximum admissible damping, the oscillator (4) is additionally controlled as a function of the measured variable. This is preferably a function of the excitation current (I) of the ultrasonic transducer. Preferably, an instantaneous value (24) and a delayed value (8) are formed therefrom and their difference compared (9) with a threshold value: if this difference is equal to the threshold value, the sweeping device (7, 5) is reversed. The power at the ultrasonic transducer can then be controlled by varying the operating voltage (U) of the output stage (2) connected upstream of the ultrasonic transducer. …<??>The method can be used for atomising a liquid by means of an ultrasonic transducer provided with an atomising plate. …<IMAGE>…

Description

The invention relates to a method for exciting an ultrasonic vibrator according to the preamble of claim 1, a circuit for exciting the ultrasonic vibrator according to this method, and the use of the method for atomizing a liquid.

The ability to atomize liquids using ultrasonic piezoelectric transducers is well known. For example, in the essay by W.-D. Drew's "Liquid atomization by ultrasound" in "Electronics" (1979), No. 10, pages 83-90 briefly describes the principle of this method, an ultrasonic oscillator provided with an atomizing plate and a circuit for exciting this ultrasonic oscillator.

The technical realization of the atomization of a liquid with the help of an ultrasonic vibrator is complicated by several problems:
Since atomization is only possible in the vicinity of the resonance of the unit consisting of the ultrasonic vibrator and its atomizing plate, the necessary excitation frequency must be observed very precisely. The locking of the oscillator of the excitation circuit to an apparent resonance that does not correspond to an effective atomization must be excluded with certainty.
The excitation circuit must be able to track the excitation frequency according to the changes in various parameters. Such parameters are, for example, the manufacturing tolerances of the mechanical components of the ultrasonic vibrator and in particular its atomizing plate, the variations in the mechanical and electrical parameters of the piezoceramic used in its manufacture, the operating temperature of the ultrasonic transducer (very important when used in burners), the aging of the ultrasonic transducer, the deposits that form on it (such as soot and resins during use) in burners), the properties of the medium to be atomized, and also the manufacturing, adjustment and other tolerances in the excitation circuit.
The cessation of atomization must be reliably recognized. If the exposure is caused by droplets remaining on the atomizing plate, it must also be ensured that they are thrown off the atomizing plate.
A practical requirement for industrial applicability is the free interchangeability of the excitation circuit and the ultrasonic oscillator itself or, if applicable, its atomizing plate, and without any adjustment work and without high tolerance requirements on the spare parts or on the components of the circuit (which is particularly important when individual components are replaced due to repairs is).
The active power of the ultrasonic vibrator or its atomizing plate must be adjustable in the largest possible dynamic range, and the control of active power and frequency must not influence each other. Furthermore, changes in the aforementioned parameters and the operating voltage must have little or no influence on the function of the control loops.

Various methods and circuits have already been proposed to solve these problems.

In DE-3222425 it was proposed to excite the ultrasonic oscillator via a matching network which, among other things, is intended to suppress the oscillation of the ultrasonic oscillator at harmonics of its resonance frequency. The direct current component of the resonator current is used to regulate the excitation current and the alternating current component of the resonator current is used to regulate the excitation frequency, a bandpass filter only permitting the frequency component at the desired resonance frequency of the ultrasonic oscillator. If the resonance fails, the excitation frequency is swept in order to pass through the resonance point and to achieve re-engagement. The disadvantage of this solution is that the circuit has to be matched to the ultrasonic vibrator and in particular to its resonance target frequency, so that the operation of the ultrasonic vibrator cannot be tracked to the changes in some of the parameters listed above and the easy interchangeability of components is not guaranteed . A reliable function is not guaranteed when starting up, especially under load and with changing operating conditions, since the impedance and thus the phase relationships between the current and voltage of the ultrasonic transducer change significantly with changes in load, and thus a tracking of the optimal oscillation frequency, derived from the phase relationships between current and Voltage in the ultrasonic transducer is not possible. It is not possible to achieve a real compensation of the capacitance of the ultrasonic vibrator by means of an inductor, because of the capacitance which changes during operation and in particular when there are changes in load.

In a somewhat different design, similar has been proposed in US-4275363, whereby the same disadvantages mentioned above also arise.

In DE-3314609 it was proposed to use the ultrasonic oscillator with clocked variable excitation power ("bursts") Operate using different tax times. The decaying vibration is used for frequency control. The disadvantage here is that for correct functioning, it is necessary either to adapt the circuit to the individual specimen of the ultrasonic oscillator used or to provide correspondingly low tolerances for the ultrasonic oscillator and the components of the circuit, and that the power is not constant over time.

In a somewhat different design, something similar was proposed in DE-3401735, the same disadvantages mentioned above also occurring.

In DE-3534853 it was proposed to operate the ultrasonic oscillator with clocked excitation power ("bursts") and to carry out a current measurement at very specific times for the automatic frequency adjustment. A disadvantageous and particularly costly is the need to temporarily store the values of the current measurement and the exact synchronization of the control processes.

In Swiss Patent Application No. 3155 / 87-0 dated August 17, 1987, it was proposed, among other things, to regulate the power with a constant excitation voltage on the ultrasonic oscillator by varying the operating frequency in the frequency range between the series resonance and the parallel resonance of the ultrasonic oscillator. The amplification of the control loop intended for this purpose is designed in such a way that slight control vibrations occur when the ultrasonic vibrator vibrates undamped. If, on the other hand, the control vibrations are missing, a wobble generator is switched on in order to shake off any drops attached to the ultrasonic vibrator and to search for the operating frequency again. The disadvantage here is that with ultrasonic transducers that do not reach the maximum adjustable power, the excitation circuit no longer locks onto the desired operating frequency when there is a high power requirement can. A further disadvantage is that the current / frequency characteristic near the parallel resonance, that is to say at low powers, has a low slope, which leads to the control oscillation being interrupted and, consequently, to the wobble generator being switched on for no reason. The dynamic range of the power control is severely limited by these two phenomena.

The object of the invention is to avoid the aforementioned disadvantages and to enable cost-effective and reliable control of an ultrasonic vibrator.

This object is achieved by the method specified in claim 1. A use of the method is specified in claim 8 and a circuit for executing this method in claim 9. Advantageous further developments of the method result from the dependent claims 2 to 7.

• Fig. 1 ein Blockschaltbild einer erfindungsgemässen Schaltung zur Anregung eines Ultraschallschwingers,
• Fig. 2 ein Beispiel einer Ausbildung der Endstufe der Schaltung gemäss Fig. 1, und
• Fig. 3 eine schematische Kennlinie des zeitlichen Mittel­werts des die Endstufe der Schaltung gemäss Fig. 2 durchfliessenden Stromes, gemessen als Spannungsab­fall an einem Strommesswiderstand, als Funktion der Anregungsfrequenz des Ultraschallschwingers.

An example of an embodiment of the invention is explained in more detail below with reference to the drawings. Show it:

• 1 is a block diagram of an inventive circuit for excitation of an ultrasonic vibrator,
• FIG. 2 shows an example of an embodiment of the output stage of the circuit according to FIG. 1, and
• 3 shows a schematic characteristic of the time average of the current flowing through the output stage of the circuit according to FIG. 2, measured as a voltage drop across a current measuring resistor, as a function of the excitation frequency of the ultrasonic oscillator.

1 shows a circuit for excitation of an ultrasonic vibrator 1, the atomizing plate of which is known per se (cf. for example in DE-3534853) is not shown. The An Excitation of the ultrasonic vibrator 1 takes place via an output stage 2, which is supplied with a current I from a source of a direct voltage U. The output stage 2 is controlled by a driver stage 3, which in turn receives a signal of frequency f from a voltage-controlled oscillator 4.

The voltage-controlled oscillator 4 is known per se (“VCO”) and is constructed using commercially available components. The permissible voltage swing at its control input is predetermined, the corresponding frequency swing at its signal output can be adjusted in a known manner by the value of frequency-determining components that can be connected to the oscillator 4 and are not shown in FIG. 1, which can be resistors and / or capacitors when using analog technology , when using digital technology can include a crystal oscillator and signal inputs to corresponding programming inputs.

The control input of the oscillator 4 receives a signal from a triangular generator 5, so that the frequency f is wobbled depending on the output voltage of the triangular generator 5. In accordance with the wobble direction, the triangle generator 5 increases or decreases its output voltage and the oscillator 4 its frequency f.

FIG. 2 shows an example of an embodiment of the output stage 2 of the circuit according to FIG. 1. The ultrasonic vibrator 1 is excited via a transformer 21, which ensures galvanic isolation of the ultrasonic vibrator 1 and, if necessary (depending on its winding conditions), the excitation with different voltage ranges Voltage U allowed. The output stage 2 comprises two transistors 22 and 23, which are driven by the driver stage 3 in push-pull and alternately switch the current I through to one half of the primary winding of the transformer 21. The driver stage 3 supplies the phases necessary for the transistors 22 and 23 correct signals depending on the signal of the frequency f fed to the driver stage 3. Such a driver stage is well known to the person skilled in the art and need not be described in more detail here. The excitation circuit for the ultrasonic oscillator 1 is closed via a current measuring resistor 24. A capacitor 25 leads the changes in the current I occurring at the frequency f directly from the transistors 22 and 23 back to the source of the voltage U, and thereby causes the voltage drop V occurring at the current measuring resistor 24 to be proportional to the temporal mean value of the current I, ie has no significant variation on frequency f.

3 shows a schematic, that is to say qualitatively represented, characteristic curve 31 of the temporal mean value of the current I as a function of the excitation frequency f of the ultrasonic oscillator. The excitation frequency f is plotted on the abscissa and the voltage drop V measured at the current measuring resistor 24 is plotted on the ordinate. The frequency range 32 of the control oscillation of a resonance detection control circuit of the circuit described below is also indicated in FIG. 3 when it is in the state locked onto the desired resonance frequency f _r .

Since the losses in the output stage 2 and in the transformer 21 can be kept sufficiently small compared to the useful power, the voltage drop V across the current measuring resistor 24 is also a direct measure of the electrical power supplied to the ultrasonic oscillator 1. This in turn is a useful measure of the atomizing power of the ultrasonic vibrator 1 when it is provided with an atomizing plate and is used to atomize a liquid.

The characteristic curve 31 shown in FIG. 3 corresponds to the well-known impedance curve (ie here also the reactance curve) of a resonance system like that of a piezo oscillator. The maximum 33 recognizable on the characteristic curve 31 corresponds to the series resonance resulting from the known equivalent circuit diagram of an oscillator, the recognizable minimum 34 corresponds to the parallel resonance resulting from the same equivalent circuit diagram. The ratio of the currents I at the maximum 33 and at the minimum 34 (thus also the ratio of the corresponding values of the voltage drop V) is essentially determined by the impedance behavior of the ultrasonic oscillator 1.

As shown in FIG. 1, the output voltage of the triangle generator 5 is fed to a window comparator 6. As will be seen from the following, this output voltage is always between the two window limits of the window comparator 6. If the output voltage of the triangle generator 5 now reaches the upper or lower window limit of the window comparator 6, this controls a set input or the set window or Reset input of a D flip-flop 7, which causes this flip-flop 7 to tilt (the conversion of the output signal of a window comparator into control signals for the inputs of a flip-flop is well known to the person skilled in the art and need not be described in more detail here). Thus, the window comparator 6 monitors the wobble direction of the oscillator 4, and the flip-flop 7 serves as a memory for the wobble direction.

An output of the flip-flop 7 supplies a signal to a control input of the triangle generator 5. This signal determines the wobble direction. Thus, when the output voltage of the triangle generator 5 reaches the upper or lower window limit of the window comparator 6, the tilting of the flip-flop 7 causes the wobble direction to be reversed, and the ultrasonic oscillator 1 is operated at a frequency which is between a lower frequency f and an upper frequency f _o (see FIG. 3) is wobbled uniformly as long as the clock input of the flip-flop 7 receives no pulses.

The voltage drop V across the current measuring resistor 24 is delayed in an all-pass filter 8 by a frequency-dependent amount of time. The voltage drop V and the voltage drop delayed in the all-pass filter 8 are each fed to an input of a comparator 9, if appropriate after they have been freed from interference signals by filtering (not shown) and converted into suitable signals by an amplification (not shown).

If the signal corresponding to the delayed voltage drop is or is larger by a measure predetermined in the comparator than the signal corresponding to the instantaneous voltage drop, the comparator 9 supplies a signal to the clock input of the flip-flop 7, which causes the flip-flop 7 to tilt and thus the signal at the control input of the triangle generator 5 changes and reverses the respective wobble direction of the oscillator 4. The wobble direction is thus determined by the value of the instantaneous difference between the delayed voltage drop and the undelayed voltage drop, or by comparing this difference with a threshold.

The circuit described above works as follows.

If the ultrasonic oscillator 1 is operated in a state in which there is no resonance peak 33 or an insufficiently pronounced resonance peak, inter alia if the ratio of the currents I at the maximum 33 and the minimum 34 corresponds to an excessively damped impedance behavior of the ultrasonic oscillator 1, the wobble direction is reversed , when the triangle generator 5 delivers an output voltage which corresponds to the lower frequency limit f _u or the upper frequency limit f _o . This is the case, for example, if the ultrasonic vibrator 1 is used to atomize a liquid and is too strongly dampened by attached drops.

The wobble frequency can usefully be set in the area of the natural resonance of the attached drop in order to throw it off.

Together, the ultrasonic oscillator 1, the output stage 2, the current measuring resistor 24, the all-pass filter 8, the comparator 9, the flip-flop 7, the triangle generator 5, the oscillator 4 and the driver stage 3 form a control circuit which, due to its design, regulates the excitation frequency of the excitation frequency Ultrasonic vibrator 1 around its series resonance frequency f _r executes. By a suitable choice of the delay in the all-pass filter 8 and the switching thresholds in the comparator 9, it is achieved that only the course of the resonance curve, which is typical for proper atomization, is recognized as real resonance. This suitable choice results from the wobble frequency used, the frequency spacing between the series resonance 33 and the parallel resonance 34 of the ultrasonic oscillator, and from the control properties of the assemblies contained in the control loop. The wobble direction is reversed when the delayed voltage drop is greater than the undelayed voltage drop V by a predetermined amount. This has the result that the excitation frequency f of the ultrasonic vibrator 1 changes periodically in a region 32 around the resonance frequency, that is, this frequency control loop has a constant amplitude swings.

The instantaneous frequency and amplitude of the current I depend on the damping of the ultrasonic oscillator in the region of the resonance (cf. FIG. 3) in accordance with the selected sweep speed. A low attenuation corresponds to a high instantaneous amplitude and high instantaneous frequency, with a correspondingly large difference between the undelayed and the delayed voltage drop. If one now selects the corner frequency of the all-pass filter 8 and the hysteresis of the comparator 9 in such a way that it is only attenuated if there is sufficient attenuation, that is to say in the case of the on application for ultrasonic atomization when the transducer is not "flooded" switches and reverses the wobble direction so that a snap-in to the resonance frequency f _{r is achieved} . Otherwise, feedback is automatically received that a drop is hanging on the ultrasonic oscillator, since the excitation frequency f then reaches the lower frequency limit f _u or the upper frequency limit f _o , as a result of which the window comparator 6 responds, as has been described above.

The circuit thus causes the excitation frequency f of the ultrasonic oscillator to sweep through a predetermined frequency range, namely until the resonance detection snaps into place, ie the wobble direction is reversed before the frequency limit f _u or f _o is reached. Then the excitation frequency f of the ultrasonic oscillator locks into its resonance frequency f _r , provided that this resonance is of a sufficiently high quality (for example, the ultrasonic oscillator is not dampened by attached drops). The criterion for the engagement is the rate of change of the current I flowing through the output stage 2, which is in a first approximation proportional to the active power of the ultrasonic vibrator 1.

The desired power of the ultrasonic vibrator can be set by setting the operating voltage U of the output stage 2, for example using an adjustable voltage source (not shown). If the active power of the ultrasonic vibrator is also to be controlled, e.g. the current I is multiplied by the operating voltage U and the result of this multiplication is compared with the desired power.

The method described here has the advantage, among other things, that the functions of the resonance detection and the power control are separate, ie the resonance detection can work over a dynamic range of more than 1:10. The process also works continuously, so it is not external time processes bound and can therefore easily follow changes such as changes in operational damping, power, resonance frequency, etc. Attached drops are shaken off faster than with the use of previously known methods, since wobbling is not rigid across the entire frequency range, but the switching between a frequency of the highest and the highest at a certain level of damping, for example, even with a partially shaken off drop Active power and one of the two frequency limits wobbles.

In analogy to the circuit that was described in connection with FIG. 1, the analysis of the characteristic curve 31 can also be carried out by directly deriving the current I or the voltage drop V according to the frequency f. This is again equivalent to deriving the current I or the voltage drop V over time, since the frequency f determined by the triangle generator 5 varies linearly with time. Such a derivation is only the limiting case of the previously described difference formation in the event of a time delay tending towards zero at the all-pass filter 8.

In the above, the invention has been described in connection with an ultrasonic oscillator, in particular a piezoelectric ultrasonic oscillator, the use of which, e.g. is atomizing a liquid. However, the invention is also advantageously applicable to other resonance systems, the resonance of which lies in a defined frequency range and can change depending on various parameters and in particular on the usual variation in the properties of ultrasonic oscillators and circuit components produced in series.

Claims (9)

1. A method for excitation of an ultrasonic oscillator by the output signal of a voltage-controlled oscillator, the control voltage of the oscillator being regulated in such a way that its frequency is periodically wobbled in a predetermined range that encloses the frequency of the series resonance of the ultrasonic oscillator, characterized in that Corresponding measurement variable of the ultrasonic oscillator is formed and compared with a predetermined threshold value which corresponds to a predetermined maximum permissible damping, and if the comparison shows that the damping of the ultrasonic oscillator is smaller than the maximum permissible damping, the control voltage is additionally regulated as a function of the measured variable. 2. The method according to claim 1, characterized in that the measurement variable is the value of a function of the excitation current of the ultrasonic oscillator. 3. The method according to claim 2, characterized in that a current value and a delayed value of the measured variable and a difference between these values are formed, this difference is compared with the threshold value, and if this difference becomes equal to the threshold value, the direction of change of the control voltage is reversed becomes. 4. The method according to claim 3, characterized in that the delay of the measured variable is carried out by an all-pass filter, the characteristic curve for the delay is selected as a function of the frequency so that the difference can only be equal to the threshold value if the damping of the ultrasonic transducer is smaller than the maximum permissible damping. 5. The method according to claim 2, characterized in that the measurement variable is the derivative of the excitation current according to the frequency. 6. The method according to claim 1, characterized in that the limits of the range in which the frequency of the oscillator is swept are selected such that, taking into account the tolerances for the characteristic values of the ultrasonic oscillator, its useful resonance frequencies within this range, but its interference resonances outside of this Range come to rest. 7. The method according to claim 1, characterized in that the power at the ultrasonic oscillator is regulated by varying the operating voltage of an output stage connected between the oscillator and the ultrasonic oscillator. 8. Use of the method according to claim 1 for atomizing a liquid by an ultrasonic vibrator provided with an atomizing plate. 9. Circuit for carrying out the method according to claim 1, with a control circuit (5, 6, 7) for a voltage-controlled oscillator (4) for exciting an ultrasonic oscillator (1) via a driver stage (3) and an output stage (2), and with a current measuring resistor (24), characterized by an all-pass filter (8), to which the voltage (V) at the current measuring resistor (24) is supplied and which supplies a correspondingly delayed voltage, a comparator (9), the voltage (V) on Current measuring resistor (24) compares with the delayed voltage in the all-pass filter (8) and supplies a signal which is fed to the clock input of a flip-flop (7), while a signal occurring at an output of the flip-flop (7) is fed to a triangle generator (5) , which determines the frequency (f) of the voltage controlled oscillator (4).

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