ULTRASONIC OSCILLATOR

Abstract: In an ultrasonic oscillator which comprises an ultrasonic vibrator (6) for driving an ultrasonic vibrator horn of an ultrasonic atomizer, a balanced circuit (3, 4, 5) in which the ultrasonic vibrator (6) is connected so that a balanced condition is established with respect to the damped capacity of the ultrasonic vibrator, and amplifier means (1) for sending a signal to the balanced circuit and which includes a feedback circuit (8, 9) for positively feeding back the signal output from the balanced circuit, the feedback circuit is connected to means (18) for adjusting the feedback quantity so that, when the ultrasonic oscillator starts the oscillation, the signal feedback quantity output from the balanced circuit is adjusted to make the gain of the amplifier means much greater than 1, and when the oscillator is in the steadily oscillating condition, the signal feedback quantity output from the balanced circuit is adjusted to broaden the allowable frequency band width of the ultrasonic vibrator.

Description: BACKGROUND OF THE INVENTION This invention relates generally to an ultrasonic oscillator, and particularly to an ultrasonic oscillator suitable for use in various ultrasonic atomizers, for example, fuel injection devices for automobiles. Ultrasonic oscillators of this character are generally so constructed as to operate safely at a constant speed. They are roughly classified into two types; the load resistor type in which a resistor is connected in series with an ultrasonic vibrator that works as an electroacoustic transducer to drive an ultrasonic vibrator horn provided in an ultrasonic atomizer, and the feedback resistor type in which a resistor is connected in series with a feedback circuit of an amplifier circuit which constitutes the ultrasonic oscillator. FIG. 3 illustrates a typical circuit structure of an ultrasonic oscillator of the load resistor type. The oscillator comprises an amplifier circuit 1; a matching coil 2 and a load resistor R₂7 connected to the output of the amplifier circuit 1, a balanced bridge circuit having capacitors 3, 4, and 5 and a damped capacity R₁ of an ultrasonic vibrator 6 and connected to said matching coil 2 and said load resistor R₂7; a feedback circuit which comprises a capacitor 8 and a coil 9 connected in series, and which positively feeds an output signal of the balanced bridge circuit back to the input of the amplifier circuit 1; and an input resistor R₃10 connected to the input of the amplifier circuit 1. FIG. 4 illustrates a typical circuit of an ultrasonic oscillator of the feedback resistor type. Comparison of FIG. 4 with FIG. 3 reveals that the circuit of the ultrasonic oscillator of the feedback resistor type is generally the same as that of the load resistor type. The exception is that the feedback resistor type does not have the load resistor R₂ 7 employed in the load resistor type; instead, it has a feedback resistor R₄ 17 connected in series with the capacitor 8 and coil 9 in the feedback circuit. The ultrasonic oscillator of the load resistor type has disadvantages. Because the electric current (i.e., vibrator current) of the ultrasonic vibrator 6 flows directly into the load resistor R₂ 7 via the capacitor 5, the power loss at the load resistor R₂ 7 (i.e., the magnitude of electric power wastefully consumed by the resistor) reduces the efficiency of the oscillator and makes it increasingly difficult to drive the ultrasonic vibrator 6 at a constant speed as the load on the vibrator increases. Such disadvantages common to the ultrasonic oscillators of the load resistor type have been experimentally confirmed by the present inventors. FIG. 5 shows data measured by the present inventors. In the graph the ordinate represents the ratio of the vibration velocity v to the vibration velocity v₀ under the no-load condition of the vibrator 6. The abscissa represents the ratio of the equivalent series resistance R₁ of the ultrasonic vibrator 6 to the input resistance R₃ 10. Curves 11, 12, 13, 14, 15, and 16 represent the vibration velocity characteristics of the ultrasonic vibrator 6 using the ratios of the load resistance R₂ 7 to the input resistance R₃ 10 as parameters. The curves 11 through 16 represent the vibration velocity characteristics when R₂/R₃ = 0, 0.045, 0.09, 0.225, 0.45, and 0.9, respectively. FIG. 5 indicates that the ultrasonic vibrator 6 begins to work outside the constant-speed operation range as the values of the load resistance R₂ 7 and the equivalent series resistance R₁ of the ultrasonic vibrator 6 increase, and that the ultrasonic vibrator 6 operates at a constant speed independently of the equivalent series resistance R₁ only when the load resistance R₂ 7 is zero. The data imply that, in addition to the aforementioned defects, the ultrasonic oscillator of the load resistor type has a disadvantage of a low upper limit to the load with which the oscillator can continue the oscillation without coming to a halt (or stall). The oscillatable frequency band width of the amplifier circuit 1 in the steady-state condition, governed by the capacitor 8 and coil 9 in the feedback circuit and by the resistors R₂ 7 and R₃ 10, is narrower than the resonance frequency band width of the ultrasonic vibrator 6 that varies with changes in the temperature and in the load. A substantial limitation is thus imposed on changing the oscillation frequency according to changes in the resonance frequency of the ultrasonic vibrator 6. A further possibility is that transistors and other semiconductor amplifier elements are damaged by an over-excited input to the amplifier circuit 1. As described above, the ultrasonic oscillator of the load resistor type has so many disadvantages that it is far from being of practical use. As compared with the load resistor type, the ultrasonic oscillator of the feedback resistor type may be said to be more trouble-free since it dispenses with the load resistor R₂ 7 that presents the above-mentioned disadvantages. Nevertheless, the present inventors have found that the feedback resistor type too has problems as described below. (1) When an element having a resistance of a substantially lower value than the input resistor R₃ 10 is used as the feedback resistor R₄ 17: Since the gain µβ of the circuit system is much greater than 1, the characteristics of the ultrasonic vibrator 6 at the start of oscillation, i.e., during the rise, and the upper limit of the load with which the oscillator continues the oscillation without stall is high. The circuit system further features improved constant-speed operation performance and improved efficiency common to the devices of the feedback resistor type. However, the frequency band width of the amplifier circuit 1 is so narrow that it cannot follow the vibrations in resonance frequency of the ultrasonic vibrator 6 with changes in the temperature and in the load, and semiconductor amplifier elements such as transistors can be broken down due to an over-excited input to the amplifier circuit 1. (2) When an element having a resistance substantially greater than that of the input resistor R₃ 10 is used as the feedback resistor R₄ 17: Contrary to the case (1) above, the amplifier circuit 1 has a broad oscillatable frequency band width, and there is no possibility of the semiconductor amplifier elements such as transistors being broken down by an over-excited input to the amplifier circuit 1. Moreover, the oscillator features the afore-mentioned advantages common to the feedback resistor type. However, since the feedback input voltage to the amplifier circuit 1 is divided by the feedback resistor R₄ 17 having a large resistance, the circuit system seldom produces a gain µβ » 1. The ultrasonic vibrator 6 starts to oscillate only when it is practically under no load, and the oscillator has a low upper limit of load with which it continues the oscillation without coming to a stall. As stated above, even the ultrasonic oscillator of the feedback resistor type shows variations in its characteristics with the value of the feedback resistance. It behaves contrariwise depending on whether the feedback resistance is small or large. It has been proposed in US-A 3,469,211 to connect in series with the vibrator means for increasing the loop gain during the start up period. This means may be a negative temperature coefficient resistor or a resistor with associated switching means which enable the removal of the resistor from the circuit. SUMMARY OF THE INVENTION The present invention has now been made to settle the afore-described problems of the prior art. Its object is to provide an ultrasonic oscillator which exhibits excellent characteristics both when the feedback resistance is substantially smaller, and when it is substantially larger, than the input resistance of the amplifier circuit. The object is achieved by an ultrasonic oscillator according to the invention as defined in claim 1. In brief, the invention resides in an ultrasonic oscillator which comprises: an ultrasonic vibrator for driving an ultrasonic vibrator horn of an ultrasonic atomizer; a bridge circuit in which the ultrasonic vibrator and capacitor elements are connected so that a balanced condition is established with respect to the damped capacity of the ultrasonic vibrator; and amplifier means for sending a signal to the bridge circuit and which includes a feedback circuit for positively feeding back the signal output from the bridge circuit; said feedback circuit having means connected thereto for adjusting the feedback quantity so that, when the ultrasonic oscillator starts the oscillation, the signal feedback quantity output from the bridge circuit is adjusted to make the gain of the amplifier means much greater than 1, and when the oscillator is in the steadily oscillating condition, the signal feedback quantity output from the bridge circuit is adjusted to broaden the allowable frequency band width of the ultrasonic vibrator. The dependent claims show particular embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of an ultrasonic oscillator of the feedback resistor type embodying the present invention; FIG. 2 is a graph showing dynamic characteristics of an ultrasonic vibrator when the oscillator shown in FIG. 1 is incorporated in an ultrasonic atomizer; FIG. 3 is a circuit diagram of a conventional ultrasonic oscillator of the load resistor type; FIG. 4 is a circuit diagram of a conventional ultrasonic oscillator of the feedback resistor type; and FIG. 5 is a graph showing the vibration velocity characteristics of the load resistance and equivalent series resistance of the ultrasonic vibrator of the oscillator shown in FIG. 3. DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will now be described in conjunction with the drawings. FIG. 1 is a diagram illustrating the circuit of an ultrasonic oscillator of the feedback resistor type embodying the present invention, similar to that of the feedback resistor type shown in FIG. 4. The ultrasonic oscillator of the feedback resistor type embodying the invention comprises an amplifier circuit 1; a balanced bridge circuit which consists of inductance elements, i.e., capacitor elements or capacitors 3, 4, and 5 connected to a matching coil 2 connected to the output of the amplifier circuit 1, and a damped capacity of an ultrasonic vibrator 6; a feedback circuit which consists of a capacitor 8, a coil 9, and feedback quantity adjusting means, i.e., a positive temperature coefficient nonlinear resistance element 18, all connected in series to feed positively the output signal from the balanced bridge circuit back to the input of the amplifier circuit 1; and input resistor R₃ 10 connected to the input of the amplifier circuit 1. The afore-mentioned amplifier means consists of the amplifier circuit 1 and the feedback circuit. The output from the amplifier circuit 1 is set to a constant voltage, and the inductance of the matching coil 2 is so set as to attain conjugate matching with the load side (i.e., with the balanced bridge circuit consisting of capacitors 3, 4, 5 and the damped capacity of the ultrasonic vibrator 6) as viewed from the output terminal of the amplifier circuit 1 at a resonance frequency of the ultrasonic vibrator 6. The capacitors 3, 4, and 5 forming the balanced bridge circuit are such that they establish a balanced bridge condition C₃.C₅ = C₄.C d between their capacitances C₃, C₄, C₅ and the damped capacity C d . The nonlinear resistance element 18 is connected in series with the capacitor 8 and coil 9 in the feedback circuit. It divides part of the output signal from the amplifier circuit 1 between itself and the input resistor R₃ 10. The output signal from the amplifier circuit 1 is input to the balanced bridge circuit and thence positively fed back from the bridge circuit to the input of the amplifier circuit 1 via the capacitor 8 and coil 9. At the start of oscillation, i.e., during the rise of the circuit system, the nonlinear resistance element 18 has a resistance much smaller than that of the input resistor R₃ 10. Consequently, the circuit system satisfies a requirement µβ » 1 (meaning that the gain of the circuit system is much greater than unity) necessary for the rise of oscillation. The circuit system thus exhibits good rising characteristics. While the circuit system is in a steadily oscillating condition, the nonlinear resistance element 18 provides a resistance much greater than that of the input resistor R₂ 10 to satisfy the requirement µβ = 1. The circuit system, therefore, is capable of changing the oscillation frequency to follow the changes in resonance frequency of the ultrasonic vibrator 6. The gain µβ = 1 is obtained and the circuit system stabilized at a point where the oscillation frequency of the circuit system is matched with the varied resonance frequency of the ultrasonic vibrator 6. A preferred example of the nonlinear resistance element 18 is a tungsten resistance used for lamps, for example, or an N-type valence-controlled semiconductor called Posistor (trademark of a product by Murata Mfg. Co.). When the tungsten resistance is employed, it is most desirable that the resistance be used with less than 70% of the rated current and that such a lamp be adopted that the resistance R (Ω) and the feedback current I (mA) satisfy a relation R = 7.36 e 0.015 I . FIG. 2 shows dynamic characteristics of the ultrasonic vibrator in an ultrasonic atomizer which incorporates the ultrasonic oscillator of the feedback resistor type built as above. In FIG. 2, a circle 19 represents a fundamental wave oscillation mode, and a circle 20 represents a spurious oscillation mode. Numerical values 38.132, 38.135, 38.141, and 45.709 represent frequencies (KHz) at the points indicated by solid dots. In the spurious mode 20, oscillation is suppressed by filtration through the capacitor 8 and coil 9 in the feedback circuit. Table 1 clearly shows the differences between the effects achieved by the ultrasonic oscillator of the feedback resistor type embodying the present invention and the effects of the existing oscillators. According to the present invention as described above, the feedback circuit is connected with feedback quantity adjusting means to adjust the feedback quantity of the signal output from the balanced circuit so that the gain of the amplifier means will become much greater than unity when the oscillator starts to oscillate, and to adjust the feedback quantity of the signal output from the balanced circuit so that the allowable frequency band of the ultrasonic vibrator is broadened when the oscillator is in the steadily oscillating condition. An ultrasonic oscillator is thus provided which exhibits excellent characteristics both when it starts oscillation and when it is in the steadily oscillating condition.

Claims: An ultrasonic oscillator comprising an ultrasonic vibrator (6) for driving an ultrasonic vibrator horn of an ultrasonic atomizer; a bridge circuit (3, 4, 5) in which the ultrasonic vibrator and capacitor elements are connected so that a balanced condition is established with respect to the damped capacity of the ultrasonic vibrator; and amplifier means (1) for sending a signal to the bridge circuit and which includes a feedback circuit (8, 9) for positively feeding back the signal output from the bridge circuit, the feedback circuit having means (18) connected therein for adjusting the feedback quantity so that, when the ultrasonic oscillator starts the oscillation, the signal feedback quantity output from the bridge circuit is adjusted to make the gain of the amplifier means much greater than 1, and when the oscillator is in the steadily oscillating condition, the signal feedback quantity output from the bridge circuit is adjusted to broaden the allowable frequency band width of the ultrasonic vibrator. An ultrasonic oscillator as claimed in claim 1, characterized in that an inductance element (2) is connected between the output of said amplifier means and said bridge circuit to obtain conjugated matching with the load side as viewed from the output of said amplifier means at a resonance frequency of said ultrasonic vibrator. An ultrasonic oscillator as claimed in claim 1 or claim 2, characterized in that said feedback quantity adjusting means (18) consists of a nonlinear resistance element. An ultrasonic oscillator as claimed in claim 3, characterized in that said nonlinear resistance element consists of a tungsten resistance or an N-type valance-controlled semiconductor.

Inventors: ENDOH, MASAMI, TAKEZI, HIROYUKI, TANAKA, YASUHISA, YONEDA, MAKOTO
Applicants: TONEN CORPORATION, ULTRASONIC ENGINEERING CO.,
Application year: 1987
Main classification: B06B1
IPC codes: B06B1, H04R3, B05B17