Title:
OSCILLATOR CIRCUIT FOR AN ULTRASONIC CLEANER, UTILIZING A SATURABLE CORE TRANSFORMER
United States Patent 3584244


Abstract:
A piezoelectric transducer for an ultrasonic cleaner is driven by a switching circuit having transistors connected in series. The transducer is connected in series with one transistor and the primary winding of a saturable core transformer providing feedback to the bases of the transistors.



Inventors:
VEST GARY W
Application Number:
04/831867
Publication Date:
06/08/1971
Filing Date:
06/10/1969
Assignee:
CLEVITE CORP.
Primary Class:
Other Classes:
331/116R, 331/158
International Classes:
B06B1/02; (IPC1-7): H01V7/00
Field of Search:
310/8.1 331
View Patent Images:



Primary Examiner:
Hirshfield, Wilton O.
Assistant Examiner:
Budd, Mark O.
Claims:
What I claim is

1. An oscillator comprising in combination a resonant transducer, electric power supply terminals, switch means having an element in circuit with said power supply terminals and said transducer, such switch means having control means, a feedback transformer having a saturable core, a primary winding means in circuit with the transducer, and secondary winding means coupled to the control means.

2. An oscillator as described in claim 1, in which the switch means comprises a pair of transistors in series with the power supply terminals, the transistors having bases constituting control means therefore and the secondary winding means comprise a pair of secondary windings each connected to one of said bases but with opposite polarity relationship.

3. An oscillator as described in claim 1 provided with means for shifting phase to regulate power.

4. An oscillator as described in claim 2 provided with means for regulating the power limit.

5. An oscillator as described in claim 3 wherein means are provided for shifting the frequency of operation to regulate impedance.

6. An oscillator as described in claim 5, wherein voltage is applied to the power supply terminals, and the relationship between voltage at the power supply terminals and the number of primary windings turns is chosen to give a rate of saturation of the saturable core transformer such that saturation time is of the order of magnitude of the duration of a cycle of the transducer resonant frequency.

7. An oscillator as described in claim 5 wherein at normal operation the circuit constants are adjusted to cause the time required to saturate the core of the transformer to correspond approximately to a half-cycle of the series resonant circuit of the transducer.

8. An oscillator as described in claim 7 wherein the time required to saturate the core of the transformer is slightly shorter than one-half cycle of the series resonant circuit so as to cause the circuit to operate on the high frequency side of the curve of transducer impedance plotted against frequency.

9. An oscillator as described in claim 1 wherein the impedance of the transducer is large in comparison with the impedance appearing across the primary winding of the saturable core transformer.

10. An oscillator as described in claim 9 wherein the ratio of impedance of the transducer to the impedance at the primary winding of the saturable core transformer is of the order of between 15 and 30 to 1.

Description:
INTRODUCTORY DESCRIPTION

It is an object of the invention to provide a drive circuit for an ultrasonic cleaner which is substantially self-regulating as to power consumption.

A further object of the invention is to provide a circuit for ultrasonic cleaning apparatus in which excessive variations in power consumption do not occur with variations in water depth and there is no danger of excessive power input to an ultrasonic transducer and to an object being cleaned in case of low water depth.

Other and further objects, features and advantages of the invention will become apparent as the description proceeds.

The circuit may be employed in apparatus of the general type disclosed in the copending application of John P. Arndt and Edmond G. Franklin, Ser. No. 660,262 filed Aug. 14, 1967 and assigned to the same assignee as the present invention.

In carrying out the invention in accordance with a preferred form thereof, a transducer drive circuit is employed comprising a pair of transistors in series energized by direct current which may be supplied by rectified, conventional alternating current lighting circuit. An ultrasonic transducer of the piezoelectric type is employed which is connected in series with one of the transistors, across the other, so that by switching the transistors alternately on and off, power of suitable frequency is supplied to the ultrasonic transducer. Switching is accomplished by means of a feedback transformer having a primary winding in series with the transducer and having a pair of secondary windings oppositely poled, each connected to the base of one of the transistors. The transformer is provided with a saturable core and the circuit constants are such as to drive the core into saturation which causes the circuit to operate in such a manner as to regulate the power.

A better understanding of the invention will be afforded by the following detailed description considered in conjunction with the accompanying drawings in which:

DRAWINGS

FIG. 1 is a circuit diagram of a driving circuit for the transducer employed in the cleaning apparatus,

FIG. 2 is a view of a section of the apparatus cut by a vertical plane,

FIG. 3 is a graph of input impedance plotted against frequency of voltage applied to a piezoelectric resonator,

FIG. 4 is a plan view of the transducer and connections, and

FIG. 5 is an enlarged view of a portion of the graph of FIG. 3 near fundamental resonant frequency of the piezoelectric resonator, and

Like reference characters are utilized throughout the drawing to designate like parts.

MECHANICAL CONSTRUCTION

In the embodiment of the invention illustrated, the circuit is employed with a receptacle or tank 11 such as shown in FIG. 2 composed of a suitable material such as stainless steel, e.g., for holding a liquid for subjecting material to be cleaned to the effect of ultrasonic vibration. The tank is properly mounted to maintain the proper impedance characteristics.

For producing ultrasonic vibration of the tank 11 and the contained liquid 17 a suitable transducer 18 is provided. Although the invention is not limited to the use of a particular composition, it has been found that satisfactory results are accomplished by employing a polarized dielectric ceramic composed of lead titanate and lead zirconate with additives in proportions described in U.S. Pat. No. 2,906,710 issued to Kulcsar and Cmolik and manufactured in the manner described in said patent. For example, a disc may be employed comprising a solid solution of lead zirconate, lead titanate and additives. An alkaline earth element such as calcium and strontium is substituted for 1 to 30 atom percent of the lead. The mole ratio of lead and alkaline earth zirconate to lead and alkaline earth titanate in the solid solution is in the range from 63:65 to 45:44. A still lower loss material may be employed, if desired, such as lead titanate, zirconium titanate with additives and substituents as described in the copending application of Don Berlincourt and Lawrence R. Sliker, Ser. No. 651,875 filed July 7, 1967 and U.S. Pat. No. 3,068,177 issued to Sugden.

The transducer 18 may be in the form of a disc polarized transversely and driven at a frequency such that the drive frequency corresponds to the resonant vibration frequency of the disc in its radial mode. Accordingly, the tank 11 is formed with a suitable flat or plane surface to which the disc-shaped transducer 18 may be bonded.

It will be understood that piezoelectric transducers such as the disc 18 are customarily fabricated with silvered surfaces as shown in FIG. 4.

For making electrical connections to the surfaces of the piezoelectric transducer 18, conducting strips 22 may be applied to the upper silvered surface 23, the cemented surface. Good results are obtained when the tank has rounded corners, particularly around the bottom edge of the tank and the junction between the sidewalls and the bottom which carries the transducer disc.

Although I am not able to explain the exact theory involving this feature, I believe that when the corners are rounded, more ultrasonic energy is transmitted up the sides of the tank into the liquid, thereby improving the distribution of energy throughout the tank. Furthermore, I believe the geometry of this tank has an effect upon the electrical impedance characteristics as seen across the transducer terminals.

Better cleaning is ordinarily obtained with relatively low vibration frequencies, preferably relatively close to the audible limit of frequency. Such low frequency has heretofore been difficult to obtain by the operation of the transducer disc in the thickness mode. Such low frequencies would ordinarily require a very thick resonant oscillator disc or the attachment of a mass to lower the frequency. A thin disc with a large diameter when operated in the radial mode of vibration makes it possible to get along with very little piezoelectric material because the diameter of the disc controls the frequency.

Increasing the diameter of the piezoelectric disc may make it possible also to transfer a greater amount of power to the oscillator disc.

In order to enable the utilization of a larger diameter disc, the disc may, if desired, be made of a greater diameter than the bottom plane surface of the tank.

ELECTRICAL DRIVE CIRCUIT

A suitable electrical circuit is utilized for applying voltage to the conductors 24 and 27 of the requisite frequency for maintaining ultrasonic vibration and means are provided for rendering the circuit substantially self-regulating with regard to power. Preferably a switching circuit is utilized employing a pair of series connected transistors 31 and 32.

A switching circuit for driving the piezoelectric transducer 18 is illustrated in FIG. 1. Utilizing a standard 115 volt alternating current as source, 100 to 150 volts of direct current may be made available in the circuit. The alternating current supply is represented by a cap 35. The transistors 31 and 32 are connected to the conductors 36 and 37 in series with a rectifier 38. Although the invention is not limited to the use of NPN transistors, in circuits shown by the way of illustration the transistors 31 and 32 are of the NPN type. The transistor 31 has a collector 39, an emitter 41 and a base 42. Similarly, the transistor 32 has a collector 43, an emitter 44 and a base 45.

For starting the switch circuit, positive current bias may be provided for the bases. This is accomplished in the circuit illustrated by providing resistors 51 and 52 each connected by a conductor 49 to the rectifier 38 and connected to the bases 42 and 45 respectively.

For driving the base 42 and 45 a transformer 54 is provided, having a primary winding 55 in series with the piezoelectric transducer 18 and a pair of secondary windings 56 and 57. As will be explained more fully hereinafter the use of a saturable core transformer permits causing the circuit to have such an operating point that the power is substantially regulated to within permissible limits to avoid damage either to the piezoelectric transducer or the objects being cleaned while providing sufficient power to accomplish the requisite cleaning action in spite of variations in the depth of the fluid in the tank or the properties of the material being cleaned. As indicated in the drawing the transformer may be a current stepdown transformer. The secondary winding 56 is connnected between the base 42 and the emitter circuit of the transistor 31, and the secondary winding 57 is connected between the base 45 and the emitter circuit of the transistor 32. As indicated by the conventional dot representation in the drawing, the polarities of the windings are such that the windings 56 and 57 are connected with opposite polarities to the transistor bases, and the upper ends of the windings 55 and 56 are of the same polarity.

The circuits of the secondary windings 56 and 57 include base resistors 58 and 59 respectively. For improving the wave form of the switching circuit, capacitors 61 and 62 are connected across the base resistors 58 and 59 respectively. The polarities of the secondary windings 56 and 57 are reversed on the two transistors 31 and 32; so that one is driven on while the other is driven off.

Although the apparatus is not limited to the use of water as a liquid in the cleaning tank 11, ordinarily water will be employed as the most economical liquid.

Since the piezoelectric disc 18 is closely coupled to the tank and the water load, the water depth in the tank effects the power transferred to the water and the resonant frequency of the transducer 18. The transducer impedance varies irregularly with water depth and there may be a discontinuity in the characteristics curve, and there may be one or more low impedance points.

In order to avoid excessive power at low impedance points the circuit is given such properties as to limit power by suitable phase shift or change in power factor instead of continuous operation at the zero phase shift, minimum impedance point. This is accomplished by use of the saturable core transformer with such a relationship between the input voltage and the primary turns as to cause the transformer to go into saturation during its operation. Moreover, a capacitor 86 is connected across the primary winding 55 and the capacitor 86 may also be shunted by a resistor 87. The combination of the saturable core winding 55 and the capacitor 86 also overcomes the problem of a possibility of the circuit jumping from one resonant frequency of operation to another.

The transducer 18, being of the piezoelectric type, constitutes a frequency-sensitive impedance with one or more resonances in series with the primary transformer winding 55, but in order to obtain the desired power regulation the impedance of the resonant device 18 is preferably large in comparison with the impedance at the primary winding 55. It may, for example, be of the order of 15 to 30 times as great. In the example illustrated by FIG. 1, the normal ratio is 20.

Regulation is provided by variation in operating frequency as a result of the rate of saturation of the core of the transformer 54. This is adjusted by the relationship between the primary voltage and primary turns which determines the time required for a core to saturate.

An initial relationship is established between the current flow in the primary winding and the time required to saturate. Then the circuit maintains that relationship. The time corresponds approximately to a half-cycle of the series resonant circuit, that is the circuit of the piezoelectric transducer 18. Preferably the time is made just a little shorter so as to cause the circuit to operate on the high frequency side of the curve of transducer impedance plotted against frequency. This eliminates the necessity for zero phase shift and makes it possible to operate on the slope of the resonant curve instead of at the minimum impedance point so as to avoid excessive power and burn-out of the equipment.

MODE OF OPERATING

The principle of operation of the circuit will be better understood from a consideration of the graphs shown in FIGS. 3 and 5. FIG. 3 is a graph of idealized characteristics of piezoelectric resonators which are applicable to the operation of an ultrasonic cleaner transducer. In FIG. 3 the relative input impedance of a piezoelectric device is plotted in a vertical direction against relative frequency of applied voltage plotted in a horizontal direction. In any piezoelectric material, it will be found that the input impedance varies as a function of frequency and as explained by FIG. 3 will appear capacitive for very low frequency. As frequency increases, the magnitude of impedance decreases toward a minimum at the fundamental series resonant frequency fr (as appears in FIG. 3), where it becomes purely resistive in nature. A further increase in frequency results in an increase in impedance but now inductive, which continues to increase until it reaches a maximum at the "antiresonant" frequency where it again is pure resistance. It will thus be found that with continual increases in frequency, electrical characteristics will undergo similar changes in impedance phase angle and magnitude in passing through various overtones and other characteristic resonances associated with the given piezoelectric device.

In the case of an ultrasonic generator, one is concerned with the behavior of a piezoelectric transducer at the fundamental and perhaps the lower order overtone frequencies.

FIG. 3 indicates one distinct minimum impedance point to exist at the fundamental resonant frequency which, as just stated, is also a frequency of pure resistance. While FIG. 3 may be an approximate representation of a simple piezoelectric resonator, it will be found that certain types of ultrasonic transducers are likely to have not just one minimum impedance or zero phase shift frequency, but rather a number of such frequencies, both in the general vicinity of the expected resonance and scattered across the spectrum. It is also to be expected that as the loading on a transducer is varied (that is by changing water depth, number of objects in a tank, or type of object being cleaned), some of these resonant frequencies will disappear and new ones will be formed. Likewise, the impedance levels at the different frequencies will vary considerably with changes in loading.

On all ultrasonic cleaning systems, some means must be employed to control the frequency at which the generator drives the transducer. If the generator is made to operate at a fixed frequency, poor performance will be encountered, since it is generally not possible to select one frequency which will always provide optimum operating conditions.

If a means is employed to enable the circuit to sense a zero phase shift or minimum impedance point (resonance), and thus force operation at that frequency, some improvement will result but again the situation is not optimum since there may be a problem in controlling at which resonance mode the circuit operates, as well as which particular resonant frequency within the mode. If this problem is somehow overcome, an additional problem to be encountered is that operation exactly at resonance is in itself not an optimum situation since output power will vary considerably with transducer impedance changes under varying loading conditions. In order to illustrate this effect, the circled portion of the curve of FIG. 3 has been enlarged and shown in FIG. 5.

As a hypothetical example, three variations of the curve in the vicinity of fr have been sketched in FIG. 5 to indicate possible values Zr might assume under different loading conditions. If an ultrasonic generator were used to drive the transducer exactly at fr with a fixed operating voltage of say 70 Vrms across the transducer, it would be found that under conditions defined by curve A, approximately 40 watts of power would be delivered to the transducer. Under the conditions indicated by curve C, however, 240 watts would be delivered. Such extreme variations in resonant impedance are realistic and constitute a serious problem in design of an ultrasonic cleaner. In some cases, damagingly high power levels may be applied to the object being cleaned, while in other situations the power might not even be adequate. In addition, power ratings for components in the ultrasonic generator must be selected on the basis of the "worst case conditions," which is likely to increase the cost and size of the generator beyond that required for more normally encountered conditions.

However, an oscillator ultrasonic cleaner drive circuit utilizing a saturating core in accordance with the invention can synchronize to the resonant frequency of an ultrasonic transducer in such a manner as to overcome the previously discussed limitations of prior art ultrasonic generators.

The generator in accordance with the invention as indicated by the circuit of FIG. 1, in effect is biased to operate in the vicinity of a series resonant or minimum impedance frequency of the transducer, rather than for instance at anti resonance or elsewhere, because of the fact that load current feedback is used to drive the circuit.

In addition to the current feedback, other means are employed to provide the generator with unique characteristics not existent in prior art ultrasonic generators. One of the most significant is the ability of the generator to shift its operating frequency automatically closer to or further from the resonant frequency of the transducer and thereby compensate for load impedance variations. The generator is also capable of discriminating between various resonant frequencies in near vicinity of each other and will operate at the one providing optimum power delivery. In addition, it is capable of discrimination between the "fundamental" resonance and "overtone" resonances and thereby can be made to operate at a preferred resonance mode (which in most cases will be the fundamental for the following reasons).

1. Cavitation, the cleaning mechanism in ultrasonic generators, can be produced at lower power levels.

2. Cleaning effectiveness is generally better because of larger size cavitation "implosions"

3. Transducer efficiency is higher.

4. Transistorized drive circuits are generally more efficient and less expensive at lower frequencies.

A fundamental factor which makes the generator possible is that operating frequency is controlled by the impedance level of the ultrasonic transducer in combination with the saturation time of a transformer core and does not require that zero phase shift exist around the feedback path.

For the sake of illustration, one may consider how a circuit similar to that of FIG. 1 would operate if the feedback transformer were not permitted to saturate and if current feedback were the only frequency determining mechanism employed. If one assumes that no phase shift is introduced within the feedback loop itself, the circuit will drive the transducer exactly at resonance. If, however, there were some phase shift present, operation would be fixed above or below resonance by an amount necessary to offset this phase shift. Hence, it can be shown that while the correct use of feedback would establish operation at a minimum impedance frequency, a necessary requirement for operation (and also a frequency determining mechanism) in this illustration is that zero phase shift must be made to exist around the feedback loop.

In the actual situation, the feedback transformer 54 of FIG. 1 is driven into saturation and saturating time required establishes a frequency at which the circuit operates. In this situation, zero phase shift need not exist around the feedback loop and the transducer impedance magnitude rather than the phase angle becomes the frequency determining factor.

The effect which saturation of the feedback transformer has in controlling frequency is to cause the transistors to switch state immediately after the core saturates. The reason for this is that saturation reduces coupling between the primary and secondary windings of the transformer, resulting in negligible base drive to the transistors. The initially conducting transistor thus turns off, also stopping transducer current. A reverse polarity secondary voltage is then generated from the collapsing magnetic field within the feedback transformer, causing the previously "off" transistor to begin conducting, and at the same time further forcing the other transistor to an "off" condition.

It is believed that to use this type of system most advantageously, the core of the feedback transformer should saturate in a time period of slightly less than what would be the normal half-cycle time period of the transducer itself. This in essence means that the preferred operating frequency of such a generator would be slightly higher than the actual resonant frequency of the transducer. This does not imply that a saturating core ultrasonic generator must operate only on a frequency higher than the resonant frequency may be advantageous for reasons which will become more obvious hereinafter.

In order to obtain the proper saturation time as well as other necessary operative conditions, a number of parameters in the circuit may be varied among which would be magnetic characteristics of the transformer core, the number of primary turns, transformer turns ratio, the secondary impedance, the primary impedance, the series impedance of the transducer, and the circuit supply voltage level. The basic consideration in determining the saturating time of the transformer core is the ratio of the applied voltage to the number of "primary turns" (The higher the voltage the sooner the core saturates.) Once designed, the primary turns remain fixed, but the magnitude of primary voltage becomes the function of the voltage divider action between the impedance presented at the transformer primary and the transducer impedance in series with it. Since the primary impedance is fixed in value and is much the smaller of the two, the primary voltage is essentially a function of the transducer impedance (or transducer current), which, as already stated, is subject to considerable fluctuation with loading conditions, operating frequency, etc.

However, if the circuit parameters previously mentioned have been properly chosen, when the generator is used to drive a transducer with "irregular" characteristics an equilibrium situation will be established whereby the generator operating frequency automatically adjusts itself to maintain a near constant transducer current, (or impedance) for varying conditions of operation such as power supply voltage changes, different loading situations, and shifts in the transducer resonant frequency. Since for line operation the power supply voltage remains relatively stable, this regulating effect to maintain a constant current will automatically result in power regulation as well.

The dynamic operation of the regulating effect can be illustrated as follows. By referring to FIG. 5, suppose that circuit parameter values are selected on the basis of curve B so as to allow the core to saturate in a time period of slightly less than one-half cycle of the transducer resonant frequency when the transducer impedance is, for example, 125 ohms (a typical value). With these conditions established, the generator will automatically operate at a frequency in the vicinity of point number 2 on curve B and will not drift higher or lower owing to the slope of this characteristic curve. This is because any tendency for frequency to increase would cause the transducer impedance to increase, reducing the feedback transformer primary voltage and thus automatically cancel the tendency (that is, lower the frequency) by lengthening the core saturation time. The same reasoning applies to tendencies for the frequencies to drift lower.

Next may be considered the effect of a change in resonant impedance of the transducer. If Zr were to decrease or if the first transducer were replaced by one having a lower Zr (that is curve C), the lower impedance would cause the core to saturate sooner, thus forcing operation further off resonance, but still at approximately the same 125 ohm impedance level. Likewise, any increase in Zr would result in the generator operating closer to resonance in order to maintain the same impedance (curve A). Thus the circuit parameter value should be selected on the basis of an expected range of variations in transducer impedance to obtain the most satisfactory regulating action. The locus operating points is within the rectangle 91 instead of within the rectangle 92 as in a "Zero phase shift" oscillator.

The generator discriminates between two or more resonances in the vicinity of each other in the following manner: Because of the relationship between saturating time and impedance, the circuit will first show preference to operate at the resonant frequency with the lowest value of Zr. This in most cases is the only deciding factor required. When two resonant frequencies exist, not extremely close in value to each other, but each having the same value of Zr, the generator will generally operate at that frequency which is closest to the originally established saturating time of the transformer core. If two resonant frequencies do exist, it is not uncommon in devices not employing my invention for the generator operating frequency to jump back and fourth between the two in a manner such as to obtain inadequate power delivery. However, where more than one resonance exists, my circuit will tend to operate closest to one in which it can most easily deliver the required amount of power--a situation which is highly desirable in an ultrasonic cleaner.

The preference the circuit shows in operating in the vicinity of some initially selected frequency is also desirable in suppressing tendencies to run at overtone frequencies. In some circumstances, however, this effect might not be great enough to completely prevent operation at the overtone. For instance, if the series resonant impedance of the fundamental frequency is slightly greater than at the first overtone, the circuit preference for lower impedance could take precedence and result in operation at the overtone were the shunt capacitor 86 not employed.

Since the capacitor 86 is connected across the feedback transformer primary, its capacitive reactance lowers the impedance to overtone frequencies. This results in a lower primary voltage or, more directly, an increase in core saturation time. Thus, any tendencies to run an overtone are adequately suppressed, forcing the circuit to run at the fundamental.

In the example given, it is desirable to operate at the fundamental frequency. Nevertheless, if operation at an overtone is desired, this may be accomplished by suitable selection of circuit elements and constants, and the principles of the invention may be employed to enhance operation at the preferred overtone.

FREQUENCY MODULATION EMBODIMENT

If frequency modulation of the transducer oscillation is desired, this may be accomplished by selecting a relatively small capacitance for the element 66, for example one microfarad.

In accordance with the provisions of the patent statutes the principle of operation of the invention has been described together with the apparatus now believed to represent the best embodiment thereof, but it is to be understood that the apparatus shown and described is only illustrative and that the invention may be carried out by other arrangements.