Method And Apparatus For Controlling Frequency Of Piezoelectric Transducers

Abstract

The resonant frequency of a high Q piezoelectric transducer is selectively varied by adjusting the magnitude of the supply voltage for the said transducer. In one apparatus embodiment, the supply voltage to the transducer is automatically adjusted, so as to maintain the resonant frequency of the transducer nearly constant in the presence of external factors tending to shift the said frequency. The system is especially suited to transducers operating from a constant frequency supply system.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to piezoelectric transducers, and more specifically relates to methodology and apparatus for enabling more effective use of high Q sonic transducers.

In a series of recent patents assigned to the assignee of the present application, novel sonic transducers have been disclosed capable of delivering extremely high power, i.e., measurable in horsepower (or kilowatts) at acoustical frequency ranges. Reference may usefully be had in this connection, for example, to U.S. Pat. No. 3,368,085, to Robert C. McMaster, et al. The transducer therein disclosed is, in essence, a resonant horn structure excited internally relatively close to the vibrational node. The method of excitation is in contrast to the method of external excitation at the antinode common when horns are utilized in a sonic transducer system.

The capability of an electromechanical transducer -- such as the resonant structure disclosed in the aforementioned patent -- to perform work will be maximized when such transducer is driven at its resonant frequency, and accordingly it is normally desirable to drive such transducer at frequencies as close to its resonant frequency as is practicable. Unfortunately, however, when such a transducer is driven under load, various external factors tend to shift the resonant frequency. For example, any change in transducer temperature will affect the said transducer's resonant frequency. In the past it has been taken for granted that the resonant frequency of the transducer was not, as a practical matter, controllable. Accordingly it has in the past been proposed to adjust the frequency of the power supply potential -- e.g., by a feedback control signal -- so as to match the incoming power frequency to the changing resonant frequency of the transducer. It has been proposed, for example, that an electronic power supply be utilized for driving the transducer, as such power sources may be constructed as to within limits, possess variable frequency capabilities. Matching the frequency of the power supply to the shifting frequency of the transducer, is only, however, partially effective. In particular, the energy storage capabilities of a transducer are determined, to a great extent, by the constancy of the resonant frequency of the transducer. If both the transducer resonant frequency and supply and supply frequency are shifting constantly and together (the ideal case) the transducer is not able to store energy. The reason is that the energy storage capability can only be fully utilized in the duration of time that the frequencies are matched at any one frequency. The cost of electronic power supplies, moreover, is enormous as compared to that of a simple synchronous motor driven rotating power supply. But the latter type of power source is fixed in its frequency output, and up until the present time the problem of overcoming the frequency shifting phenomena noted above, whereby to efficiently couple the output from these low-cost, constant frequency power supplies to piezoelectric transducers, has not been overcome.

In accordance with the foregoing, it may be regarded as an object of the present invention to provide method and apparatus for selectively controlling the resonant frequency of a piezoelectric transducer.

It is a further object of the invention to provide method and apparatus whereby the external factors tending to shift the resonant frequency of a piezoelectric transducer under load may be compensated for, whereby said transducer maintains an essentially constant resonant frequency, and whereby inexpensive constant frequency power supplies may be efficiently used to drive said transducer.

It is another object of the invention to provide an arrangement for so coupling a high Q piezoelectric transducer to a constant frequency power supply, that external factors tending to shift the resonant frequency of said transducer, are automatically compensated for so as to maintain the resonant frequency of said transducer at an essentially constant value, in consequence of which inexpensive constant frequency power supplies may efficiently drive said transducer.

SUMMARY OF THE INVENTION

Now in accordance with the present invention, it has been discovered that the resonant frequency of piezoelectric transducers bears an inverse functional relationship to the potential utilized to excite the transducer. The changes in resonant frequency with applied voltage are unexpectedly large and provide in accordance with the invention a method for selectively controlling the transducer resonant frequency. The invention similarly enables apparatus configurations wherein selective controls of excitation voltage to the transducer is utilized to maintain an essentially constant resonant frequency at said transducer. In one apparatus embodiment of the invention, for example, automatic compensation for changes in resonant frequency of a transducer under load are effected by means of a reactor connected in series with the power supply and transducer. Externally caused reductions in resonant frequency of the said transducer, for example, result in an increase in current at that load, which in turn increases the reactor voltage drop, to thereby reduce the transducer voltage and raise its resonant frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is diagrammatically illustrated, by way of example, in the drawings appended hereto, in which:

FIG. 1 is a graph depicting minimum input impedance resonant frequency as a function of excitation voltage for several representative piezoelectric transducers;

FIG. 2 is a simplified electrical block diagram depicting an embodiment of the invention enabling maintenance of resonant frequency in a transducer under load; and

FIG. 3 is a simplified electrical block diagram depicting an embodiment of the invention whereby automatic compensation for changes in resonant frequency is enabled, even though the said changes result from external causes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 a series of curves, 1, 2, 3, and 4, are shown which indicate how the resonant frequency of four different transducers has been found to vary as a function of excitation voltage. The transducers in question, although comprised of differing materials, were all of the type disclosed in the aforementioned U.S. Pat. No. 3,368,085. The relationship indicated, however, is not unique to this particular type of transducer, but is displayed as well by other resonant structure piezoelectric transducers. Resonant frequency in FIG. 1 was determined by the minimum impedance (voltage dip method) using a broad-band, electronic, variable-frequency power supply. The applied voltage was measured at the transducer, on a standard Ballentine rms meter, at resonant frequency.

As can be seen in the figure, the transducer's resonant frequency varies in such a manner as to display an inverse functional relationship to the applied voltage. It will also be appreciated by those skilled in the art that the resonant frequency variations with respect to changes in applied voltage are of considerable magnitude. Thus, it will be noted that a change of from but 800 volts to 830 volts corresponds to a change in resonant frequency of approximately 5 Hz. Since the half-power points for a high Q transducer such as those from which the FIG. 1 data derives, are only approximately 10 Hz apart (under no load conditions) a variation of 5Hz is of considerable magnitude.

The ability to control the transducer resonant frequency under load, by external means, is particularly significant since, as has been pointed out, the energy storage capabilities of a transducer are to a great extent determined by the constancy of the resonant frequency thereof. Moreover, maintaining such frequency essentially constant enables effective use of a constant frequency power supply -- such as for example a synchronous motor driven supply -- to drive the piezoelectric transducer. Supplies of this type are extremely inexpensive in comparison to electronic power supplies, and are available for almost any practicable power level. Under typical load-operated conditions, however, increases or decreases in transducer resonant frequency tend to occur as a result of external factors. Such shift in resonant frequency may, for example, take the form of a frequency drop due to an increase in length of the transducer resulting from a temperature rise, or from coupling increments of a load mass into the transducer resonant structure, or frequency increases may be occasioned in consequence of coupling increments of load compliance into the transducer resonant structure.

In FIG. 2 a schematic electrical block diagram illustrates in simplified fashion how the present invention enables maintenance of resonant frequency in a piezoelectric transducer under load. As seen therein line power is provided at 5 to the power supply 6. In accordance with the invention the latter is a constant frequency rotating power supply -- e.g., of the type incorporating a synchronous motor. The output of power supply 6 then drives the piezoelectric sonic transducer 7. As has been indicated, transducer 7 under load will tend to shift its resonant frequency, as for example due to heating of the transducer structure. In order to compensate for such shifting, power transfer detector means 8 are connected to the transducer and in turn provide an error signal to potential level adjusting means 9. The said error signal is indicative of shift in power transfer between supply 6 and transducer 7, and this indicates departure of the resonant frequency of transducer 7 from its original value. Level adjusting means 9, in response to said error signal, then adjusts the voltage level output of power supply 6 so as to maintain transducer 7 at or close to its resonance frequency. Resonance detector means 8 may, as is known in the art, be based upon a wattmeter so connected as to instantaneously measure the power output from the supply 6 or the input to transducer 7. An arrangement of this type is shown, for example, in U.S. Pat. No. 3,434,074 to Ross C. Libby, which patent is assigned to The Ohio State University. As the object of the arrangement herein is to adjust the supply voltage to transducer 7 in accordance with the departure of the transducer from its resonant frequency, it will be evident that potential level adjusting means 9 and supply 6 could comprise varying arrangements enabling such result. For example, power supply 6 may be an adjustable autotransformer with means 9 serving to effectively rotate the adjustment arm thereof in accordance with the error signal. Similarly, supply 6 may include tapped transformers with means 9 serving to adjust points of tapping. In terms of the method set forth in the present invention, it will also be evident that simple manual adjustment of the voltage level to the loaded transducer can be used to adjust the resonant frequency, such adjustment being correlated with observation of measurements indicative of the proximity of the loaded transducer to its frequency of resonance.

In FIG. 3 a further embodiment of the invention is depicted in simplified electrical block diagram fashion. In considering this figure, understanding of the manner in which automatic compensation for resonance frequency shift occurs in will be facilitated by simultaneous reference to the graphs of FIG. 1. In FIG. 3, it is seen that a reactor 10 is placed in series with the power supply 6 (similar to that discussed in connection with FIG. 2) and sonic transducer 7. In accordance with the aspect of the invention herein discussed, the resonant frequency maintained nearly constant by the configuration depicted is chosen to be slightly higher than that of the (constant) supply frequency of supply 6. Referring to FIG. 1, abscissa 12 thus represents the said frequency of the power supply, and ordinate 13 represents the desired voltage to be applied to transducer 7. (Curve 1 is used to illustrate the present concept). It will now be appreciated that any reduction in resonant frequency of transducer 7 (e.g., due to temperature rise) will bring such frequency closer to the supply frequency and hence effect an increase in current at that load. This in turn will result in an increase in the voltage drop at reactor 10, to thereby reduce the transducer voltage and raise its resonant frequency. The net result tends to compensate automatically for externally caused changes in resonant frequency. For maximum compensation, the desired voltage on the transducer should equal the supply voltage (assumed constant) less the reactor voltage drop (vectorial subtraction) at a preselected load-current value. It should be noted that the automatic compensation arrangement described herein should be equally effective in compensating for increases or decreases in transducer resonant frequency caused by external factors.

While the present invention has been particularly described in terms of specific embodiments thereof, it will be apparent to those skilled in the art that numerous modifications are possible without departing from the spirit of the invention and the scope of the subjoined claims.

Claims

What is claimed is:

1. A system for maintaining a kilowatt power piezoelectric transducer at a nearly constant sonic resonant frequency in the presence of external factors tending to shift said frequency, comprising:

a. a high Q piezoelectric resonant structure transducer;

b. an alternating current power supply operable to provide an output voltage of a constant frequency and at a frequency slightly less than that of said resonant structure transducer;

c. means for applying said voltage to said transducer to drive said transducer; and

d. means to selectively adjust the level of said voltage applied to said transducer in accordance with shifts induced by said external factors,

e. said last named means including a reactor connected in series with said power supply and said transducer,

f. means selecting said voltage at the transducer to equal the supply voltage less the reactor voltage drop at a pre-selected load-current value, whereby to compensate for said shift and thereby maintain said transducer resonant frequency substantially constant.

2. A system in accordance with claim 1 wherein said power supply is a synchronous motor driven rotating type of supply.

3. A system in accordance with claim 1 wherein said adjusting means includes power transfer detector means at said transducer for detecting shifts in said resonance frequency and generating an error signal for controlling said potential level.

4. A method for maintaining a substantially constant difference in frequency between a constant frequency power supply and the resonant frequency of a high Q piezoelectric transducer, whereby to maintain the capacity of said transducer to perform work in the presence of external factors tending to shift said resonant frequency and lower said work capacity, said method comprising:

choosing the said resonant frequency at said transducer to be slightly higher than the frequency of said power supply; and

inserting a reactor in series with said power supply and transducer, whereby shifts in said resonant frequency effect changes in the potential drop across said reactor such as to vary the potential at said transducer in a direction tending to restore said resonant frequency to its original value.