Method and circuit for exciting an ultrasonic generator and the use thereof for atomizing a liquid

A method and a circuit for exciting an ultrasonic generator comprises a control loop which includes the ultrasonic generator itself and a voltage-controlled oscillator. The control loop keeps the active power consumption to a desired value, which is compared in a comparator with the instantaneous active power consumption. One output of a further rectangular oscillator is connected to the control input of the voltage-controlled oscillator. The rectangular oscillator is put into operation if in the control loop there are no control oscillations or only those which are smaller than a predetermined threshold. The output of the rectangular oscillator is connected across one diode to the control input of the voltage-controlled oscillator and across another diode to the controlled input of comparator. The additional signal is applied to the voltage-controlled oscillator, apart from the control signal of the control loop. The cycle of the additional signal is longer than the change time constant at the control input of the voltage-controlled oscillator and the additional signal swing is selected in such a way that the frequency of oscillator passes through a predetermined frequency range.

BACKGROUND OF THE INVENTION 
This invention relates to a method and a circuit for exciting an ultrasonic 
generator and the use thereof for atomizing a liquid. 
The possibility of atomizing liquids with the aid of piezoelectric 
ultrasonic generators is known. For example, the article by W. D. Drews 
"Flussigkeitszerstaubung durch Ultraschall" ("Liquid Atomization by 
Ultrasonic Energy") in "Elektronik" (1979), No. 10, pp. 83-90 briefly 
describes the principle of this method. An ultrasonic generator is 
utilized which is equipped with an atomizer disk or plate and a circuit 
for exciting this ultrasonic generator. 
However, the technical realization of the atomization of a liquid using an 
ultrasonic generator has been difficult due to a number of problems. 
An atomization is only possible close to the resonance of an ultrasonic 
generator (together with its atomizer disk), and the necessary exciting 
frequency must be very precisely maintained. The locking of the oscillator 
of the exciting circuit to an apparent resonance, which does not 
correspond to an effective atomization must be reliably prevented. 
The exciting circuit must be in a position to detect changes in the 
necessary exciting frequency as a function of different parameters. Such 
parameters are e.g. the manufacturing tolerances of the mechanical 
components of the ultrasonic generator (particularly its atomizer disk), 
the variations in the mechanical and electrical parameters of the 
piezoelectric ceramic used in its manufacture, the operating temperature 
of the ultrasonic generator (very important when used in burners), the 
aging of the ultrasonic generator, deposits formed thereon (such as e.g. 
soot and resins when used in burners) and the manufacturing adjustment and 
other tolerances in the exciting circuit. 
Reliable detection of a stoppage of atomization must be ensured. If 
stoppage is caused by droplets which have stuck to the atomizer disk, the 
centrifuging of these droplets from the disk must be ensured. 
A practical requirement with respect to industrial use is the 
interchangeability of the exciting circuit and the ultrasonic generator 
itself or optionally its atomizer disk and namely without any matching or 
adapting and without high tolerance requirements on the replacement parts 
of spares. 
To achieve the best possible efficiency the atomizing capacity of the 
ultrasonic generator or its atomizer disk must be automatically 
regulatable, without any action by an operator and without having to 
change e.g. the exciting voltage or the duty cycle of the drive frequency. 
Numerous methods and circuits have already been proposed for solving these 
problems. 
DE-3222425 proposes exciting the ultrasonic generator across a matching 
network, which inter alia serves to suppress the starting of oscillation 
of the ultrasonic generator to harmonics of its resonant frequency. The 
direct current component of the resonator current is used for regulating 
the exciting current and the alternating current component of the 
resonator current is used for regulating the exciting frequency, a band 
pass filter only permitting the passage of the frequency component at the 
desired resonant frequency of the ultrasonic generator. In the case of a 
resonance failure the exciting frequency is wobbled or swept, in order to 
pass through the resonance point and to obtain relocking. It is a 
disadvantage of the solution that the circuit is matched to the ultrasonic 
generator and particularly to its desired resonant frequency, so that the 
operation of the ultrasonic generator cannot follow the changes in certain 
of the aforementioned parameters and also the easy interchangeability of 
components is not ensured. A reliable operation is not ensured in the case 
of oscillation starting, particularly under load and with varying 
operating conditions, because the impedance and therefore the phase 
relationships between the current and the voltage of the ultrasonic 
generator vary considerably in the case of load changes and consequently 
it is not possible to track the optimum oscillating frequency, derived 
from the phase relationship between the current and the voltage in the 
ultrasonic generator. A true compensation of the capacitance of the 
ultrasonic generator by means of its inductance is not possible due to the 
capacitance changing during load changes. 
With a somewhat different construction much the same is proposed in U.S. 
Pat. No. 4,275,363, wherein the same, aforementioned disadvantages occur. 
DE-3314609 proposes operating the ultrasonic generator with timed bursts 
using different values thereof in each case. However, it is 
disadvantageous for the frequency matching and the control of the bursts 
to use the free dying out of an oscillation instead of the resonance 
behavior of the ultrasonic generator, because then it is not possible to 
obtain values varying in linear manner with the actual state. 
In a somewhat different construction much the same is proposed in 
DE-3401735 wherein, also, the same, aforementioned disadvantages occur. 
DE-3534853 proposes operating the ultrasonic generator with timed bursts 
and to carry out a current measurement during specific times for automatic 
frequency matching purposes. The necessary intermediate storage of the 
current measurement value and the precise synchronization of the 
measurement and control sequences are disadvantageous and, in particular, 
costly. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved method and 
circuit for exciting an ultrasonic generator, which would avoid the 
aforementioned disadvantages. 
Yet another object of the invention is to provide a method and circuit for 
exciting an ultrasonic generator, particularly used for the liquid 
automization, which would substantially reduce costs of conventional 
methods and circuits of the foregoing type. 
These and other objects of the invention are attained by a method for 
exciting an ultrasonic generator, comprising the steps of providing a 
voltage-controlled oscillator and exciting a frequency at its output, a 
control loop adjusting the active power at the ultrasonic generator by 
means of the exciting frequency between a series resonance and a parallel 
resonance of the ultrasonic generator, wherein, in addition to a 
regulating signal of the control loop, a periodic additional signal is 
applied to the voltage-controlled oscillator if no control oscillations or 
control oscillations appear in the control loop, which are below a 
predetermined threshold, the cycle of the additional signal being longer 
than a change time constant of the signal applied to the control input of 
the voltage-controlled oscillator and the additional signal swing is 
dimensioned in such a way that the frequency of the voltage-controlled 
oscillator passes through a predetermined frequency range, the middle 
value of which is roughly at the frequency of the series resonance and the 
width of which is approximately twice the frequency spacing between the 
series resonance and the parallel resonance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The ultrasonic generator exciting circuit shown in FIG. 1 comprises an 
ultrasonic generator 1 (whose per se known) the atomizer disk or plate of 
which is known per se and not shown. The ultrasonic generator 1 is excited 
across a transformer 2, which ensures a galvanic isolation of generator 1 
and optionally (as a function of its turns ratio) permits the excitation 
with different voltage values of the power source U. Two transistors 4, 5 
form a pushpull output stage of the circuit and alternately switch through 
the power source U to in each case half of the primary winding of 
transformer 2. 
The exciting circuit is closed across a precision resistor 18. A capacitor 
3 directly returns the current changes from transistors 4 and 5 to the 
power source U and consequently ensures that the voltage drop v occurring 
at the precision resistor 18 has a d.c. voltage component, which is 
proportional to the direct current consumption of the output stage. A 
driver 6 supplies the signals in proper phase necessary for transistors 4 
and 5. The voltage-controlled oscillator 7 generates the frequency f, with 
which the excitation of ultrasonic generator 1 takes place. 
As the losses in transistors 4 and 5, in transformer 2, in capacitor 3 and 
the losses in the secondary coil of transformer 2 caused by reactive 
currents (as a result of the generator capacitance) can be kept 
sufficiently small, the d.c. voltage drop at resistor 18 is a direct 
measure for the active power consumed by the ultrasonic generator 1. This 
is in turn a usuable measure of the liquid atomizing capacity. 
FIG. 2 shows the course of the d.c. voltage component, i.e. optionally the 
mean time value of the voltage V at precision resistor 18, i.e. also the 
course of the active power consumed by ultrasonic generator 1 as a 
function of the oscillating frequency f of generator 1. On the abscissa 
are plotted the oscillating frequencies f and on the ordinate, the voltage 
V measured at precision resistor 18. The characteristic curve shown in 
FIG. 2 corresponds to the good known impedance course (or reactance 
course) of a resonance system, such as that of a piezoelectric generator. 
The maximum shown in FIG. 2 corresponds to the series resonance obtained 
from the known equivalent circuit diagram of a generator and the minimum 
corresponds to the parallel resonance occurring with the same equivalent 
circuit diagram. The ratio between maximum and minimum is essentially 
established by the impedance behavior of the ultrasonic generator 1. 
Between the maximum and the minimum is located the falling edge or side of 
the characteristic curve, on which e.g. at a frequency f.sub.1, a large 
atomizing capacity is obtained, whereas at a frequency f.sub.2 a small 
atomizing capacity is obtained. All the exciting frequencies, which lead 
to resonances of the ultrasonic generator 1 during the practical operation 
thereof, are located between a lower cut-off frequency f.sub.A and an 
upper cut-off frequency f.sub.B, whose mean value f.sub.M =(f.sub.A 
+f.sub.B)/2 is in the vicinity of the maximum active power. 
Oscillator 7 in FIG. 1 is a voltage-controlled oscillator constructed with 
commercially available components. The permitted voltage swing at its 
control input is predetermined and the corresponding frequency swing on 
its frequency output is adjustable in known manner through the value of 
resistors and/or capacitors connectable to oscillator 7 and not shown in 
FIG. 1. 
The voltage V tapped at the precision resistor 18 is compared with a 
voltage in comparator 21 adjustable on a potentiometer 19. The output 
signal of comparator 21 is smoothed by the RC network formed by a resistor 
9 and a capacitor 8 and is supplied to the oscillator 7 as a control 
voltage. Thus, with potentiometer 19 it is possible to set and maintain a 
clearly defined operating point on one side of the characteristic curve of 
FIG. 2. Oscillator 7, driver 6, transistors 4, 5, capacitor 3, transformer 
2, resistor 18, comparator 21, resistor 9 and capacitor 8 together form 
the regulator and, together with the latter, a regulating section provided 
through the ultrasonic generator 1 forms a control loop. 
Oscillator 7 is now set in such a way that with the control voltage swing 
which can be produced by comparator 21 at its controlled input, (i.e. also 
at capacitor 8), it is only possible to produce frequencies between 
f.sub.A and f.sub.B, i.e. only in a narrow range around the series 
resonance and the parallel resonance. It is even better if the 
frequencies, which can be produced, are in a range, which is within the 
range between the series resonance and the parallel resonance and is 
significantly smaller, such as e.g. the range between f.sub.1 and f.sub.2. 
The locking of the generator circuit to additional resonances, which can 
result from a matching between the transformer 2 and the ultrasonic 
generator 1 and which do not lead to an effective atomization is 
consequently prevented. Thus, a special matching between the transformer 2 
and the ultrasonic generator 1 is neither necessary, nor desired and 
consequently there is also no need for a filter in a resonance detection 
circuit. 
The large gain at comparator 21 gives in conjunction with the control 
voltage swing produceable by it a two-positioned control. Thus, the 
ultrasonic generator 1 is only operated at a frequency corresponding to a 
predetermined desired active power consumption. Moreover, due to the 
two-position control characteristic, the operation of the ultrasonic 
generator 1 is only possible at one of the two frequencies corresponding 
to the desired active power consumption (e.g. on the higher frequency side 
of the characteristic curve shown in FIG. 2 and at frequency f.sub.1). 
The above-defined control loop is designed in such a way that clearly 
defined control oscillations occur. This is essentially achieved in that 
the control voltage swing produced by comparator 21 is only incompletely 
smoothed by the RC network formed by resistor 9 and capacitor 8. The 
corresponding control oscillations, which are shown by a sweep of the 
exciting frequency and the oscillating frequency f of the ultrasonic 
generator 1 and consequently an a.c. voltage component superimposed on the 
d.c. voltage component in the voltage drop V occurring at the precision 
resistor 18, are given by the cooperation of the aforementioned RC network 
formed by resistor 9 and capacitor 8 with the precision resistor 18 and 
capacitor 3, as well as the gain at comparator 21 and the active power 
characteristic curve of the ultrasonic generator 1. 
As the ultrasonic generator 1 is an integral component of the control loop, 
said control oscillations can only occur if the generator 1 has the 
characteristic curve shown in FIG. 2. This is only the case when it is 
correctly atomizing. If it is excessively damped by droplets which have 
stuck, then it cannot have a marked resonance behavior in accordance with 
the characteristic curve of FIG. 2 and then the control oscillations 
either do not occur, or occur in a very weak and irregular manner. 
Thus, the appearance of clearly defined control oscillations of the control 
loop can be looked upon as a reliable criterion for a correct atomization. 
In order to be able to detect these control oscillations, the a.c. voltage 
component in the voltage drop V occurring at the precision resistor 18 is 
decoupled through a capacitor 17 and amplified by an amplifier 16. A 
rectifier 15 supplies a d.c. voltage as a measure of the amplitude of the 
amplified control oscillations. A comparator 13 decides by comparing this 
d.c. voltage with a desired voltage adjustable by a potentiometer 14 
whether the control oscillations are sufficiently large. If the control 
oscillations are not present or are too weak (which e.g. occurs on 
switching on the generator), then an oscillator 12, which in the present 
example is a rectangular oscillator, is started, so that alternately a 
higher and a lower voltage appears at its output. However, if the control 
oscillations are sufficiently large, then the oscillator 12 is switched 
off or remains switched off and is decoupled from the control loop by 
diodes 10 and 11. 
When the higher voltage appears at the output of oscillator 12 by means of 
diode 10 and a resistor 30 the control voltage at the control input of 
oscillator 7 (i.e. also at capacitor 8) is raised, so that after a time 
constant given by resistor 9, resistor 30 and capacitor 8, oscillator 7 
produces the upper cut-off frequency f.sub.B. Simultaneously the desired 
current requirement at the input of comparator 21 is raised across the 
diode 11 and a resistor 31. This forces an operating point of ultrasonic 
generator 1 in the upper region of the characteristic curve of FIG. 2. 
During the following appearance of the lower voltage at the output of 
oscillator 12 the latter is decoupled across diodes 10 and 11 from the 
control loop. Capacitor 8 discharges across the resistor 9, because the 
desired voltage at comparator 21 is higher at this time than the actual 
voltage and therefore the comparator output carries the lower output 
voltage (the desired voltage is at the inverting input). Thus, the 
frequency produced by oscillator 7 drops from f.sub.B towards f.sub.A. The 
cycle of oscillator 12 compared with the time constant of the discharge of 
capacitor 8 is chosen sufficiently large to ensure that there is a passage 
through the full frequency range between f.sub.B and f.sub.A. 
For as long as the cause of the detuning of ultrasonic generator 1 is not 
removed, (for as long as generator 1 is not locked on the exciting 
frequency or the stuck droplet has still not been shook off), a frequency 
sweep takes place between f.sub.B and f.sub.A. If the droplet has been 
shook off and ultrasonic generator 1 achieves a resonance behavior 
according to the characteristic curve of FIG. 2 or optionally re-achieves 
it, then the control oscillations appear, oscillator 12 is switched off 
(its output is set to the lower voltage) and is decoupled from the control 
loop through diodes 10 and 11. 
A power regulation at the ultrasonic generator 1 takes place in that the 
oscillating frequency f of generator 1 defined by the exciting frequency 
is displaced between the series resonance and the parallel resonance. The 
smallest atomizing capacity is obtained on exciting in parallel resonance 
(large reactive power, low active power) and the maximum atomizing 
capacity is obtained at series resonance (small reactive power and large 
active power). Thus, neither the exciting voltage, nor the duty cycle have 
to be changed for regulating the power. 
The invention has been described hereinbefore in connection with an 
ultrasonic generator, particularly a piezoelectric ultrasonic generator, 
whose use is e.g. in the field of liquid atomization. However, the 
invention can also be used on other resonance systems, whose resonance 
takes place in a narrow frequency band and consequently changes strongly 
as a function of a physical quantity, said quantity having to be 
maintained as precisely as possible. Thus, the invention is generally 
suitable for maintaining constant a physical quantity by means of a 
control loop, which comprises a reasonatable body, whose resonance 
behavior in a narrow frequency band is greatly influenced by the physical 
quantity and is used for detecting changes thereof.