US3443130A - Apparatus for limiting the motional amplitude of an ultrasonic transducer - Google Patents

Description

A SHOH 9 APPARATUS FOR LIMITING THE MOTIONAL AMPLITUDE OF AN ULTRASONIC TRANSDUCER Filed Dec. 8'. 1964 Sheet of s are @OFF 20/ J2 '1 MW I INVENTOR.

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Blair @Burkies IIY'YORIVEYS Sheet 2 01'5 y 1969 A. SHOH APPARATUS FOR LIMITING THE MOTIONAL AMPLITU OF ANULI'RASONIC TRANSDUCER Filed Dec. 8, 1964 ay 6, 1969 SHOH 3,443,130

APPARATUS FOR LIMITING THE MOTIONAL AMPLITUDE 7 OF AN ULTRASONIC TRANSDUCER Filed Dec. 8, 1964 Sheet .9 of e FROM 0077 01 22$ CUIVV TER TRANSFORMER i 20 f F may 6, 1969 SHOH 3,443,130

A. APPARATUS FOR LIMITING THE MOTIONAL AMPLITUDE OF AN ULTRASONIC TRANSDUCER v Filed Dec. 8, 1964 Sheet 4 of s May 6, 1969 A. SHOH 3,443,130

APPARATUS FOR LIMITING THE MOTIONAL AMPLITUDE OF AN ULTRASONIC TRANSDUCER Filed Dec. 8, 1964 Sheet y 6, 1969 A. SHOH 3,443,130

APPARATUS FOR LIMITING THE MOTIONAL AMPLITUDE OF AN ULTRASONIC TRANSDUCER Filed Dec. 8, 1964 Sheet 6 of 6 con/warn? United States Patent 3,443,130 APPARATUS FOR LIMITING THE MOTIONAL AMPLITUDE OF AN ULTRASONIC TRANSDUCER Andrew Slroh, Stamford, COIIIL, assignor, by mesne assignments, to Branson Instruments, Incorporated, Stamford, Conn., a corporation of Delaware Continuation-impart of application Ser. No. 265,751, Mar. 18, 1963. This application Dec. 8, 1964, Ser. No. 416,816

Int. Cl. H01v 7/00 US. Cl. 310--8.1 10 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation-in-part of the application of Andrew Shoh, Ser. No. 265,751 filed Mar. 18, 1963, entitled, Ultrasonic Cleaning Apparatus, now Patent No. 3,293,456.

This invention relates to sonics, more particularly it relates to systems for controlling the power supplied to a sonic transducer system subjected to loads of greatly varying acoustic impedance. The invention comprises automatic self-regulating systems and apparatus for continuously delivering varying amounts of acoustic power that are the maximum that can be safely delivered to a transducer system operating into any acoustic impedance. Thus the invention provides systems that make the most efficient use of acoustical transducing means by insuring that they deliver the greatest acoustic power of which they are capable.

In recent years sonic energy has found wide use in science and industry for cleaning, soldering, welding, material treatment, homogenizing, dispersing, microbiological cell disruption and the like. For many years both magnetostrictive and electrostrictive (piezoelectric) transducers have been used in sonic cleaning systems. Magnetostrictive transducers utilize metal laminated core structures that change in length when subjected to a magnetic field provided by a coil. Electrostrictive or piezoelectric transducers utilize crystalline or ceramic elements that change their length under the influence of an applied electric field. Except for the above-mentioned use in cleaning tanks, up until a few years ago, the vast majority of sonic transducer systems for providing high sonic power levels were of the magnetostrictive type. However, the develop ment of piezoelectric ceramics, such as lead titanite lead zirconate, capable of withstanding the high temperatures developed in high powered transducer systems have lead to their use in high power general purpose systems. Such a general purpose transducer system or sonic converter is disclosed in the copending application of Stanley E. Jacke and Henry Biagini, Ser. No. 384,025 filed July 13, 1964 for Sonic Wave Generator, Patent No. 3,328,610.

In the above-identified application the sonic vibrations developed electrostrictively in a pair of ceramic wafers are concentrated and increased in amplitude by a concentrating horn or acoustic impedance transformers to provide at the end thereof extremely high power densities per unit area. This sonic converter is being used in many of the ultrasonic process applications listed above, for

example, for homogenizing, dispersing and disrupting biological cells, in ultrasonic welding, soldering and treatment of materials and the like.

As discussed in the above-identified copending application, one of the problems encountered in operating such a high power sonic converter is that when the device is energized and the tip of the concentrating horn is not coupled to any medium other than air the tip vibrates violently at very large amplitudes. Very little power is transferred to the air and substantially all of the power supplied to the converter must be dissipated therein. In contrast therewith, when the tip of the concentrating horn is coupled to a less compliant medium, such as a liquid, energy transfer occurs from the horn to such a medium, thus leaving a smaller amount of energy to be dissipated in the converter. Depending upon the efiiciency of the converter, the medium which receives the acoustic power and the degree of coupling achieved, the power to be dissipated in the converter may vary over a ratio of 10 to 1 from the condition of no power transfer to the load, to the other condition when good power transfer from the horn to the load is achieved.

In order to protect the transducers from the possibility of destruction by excessive power dissipation, the prior art employed principally two approaches. One method is to limit the power supplied to the converter to the amount of power which the converter safely can dissipate under the condition of no power transfer. As is readily apparent, this approach seriously limits the power which is available to the converter when good power transfer between the horn and the load is achieved. The other approach concerns the provision of manually adjustable power control means, for instance, selectable voltage levels in order to vary the power supplied to the converter. This latter method, as is readily apparent, is not satisfactory as the operator must judge the degree of power transfer and, in the event that too much power is applied to the converter, or if the converter is uncoupled from the load and inadvertently left to operate into air without power reduction, the converter may destroy itself.

As a consequence of these problems, converters of this type have been generally limited to low power levels. For example, the converter disclosed in the aboveidentified copending application has been limited to a maximum of approximately 100 watts. This being the greatest power the converter can internally dissipate when operating in air. Generally speaking the greater the impedance of the acoustic load presented to the converter the smaller the power supplied to the medium.

I have discovered methods, apparatus and systems for controlling the power supplied to a converter, specifically as disclosed in the above-identified copending application. In order to better understand my invention I have made the following observation utilizing an S- converter manufactured by Branson Sonic Power Division of Branson Instruments, Inc, in Danbury, Conn., in accordance to the above-identified copending application.

The Branson S-75 converter was connected to a power amplifier to form an oscillator in the manner described in the above-identified copending application, Ser. No. 265,751, now Patent No. 3,293,45 6. As explained therein, it thus operated at the frequency of maximum conversion efiiciency. The converter using a solid step horn acoustic transformer was operated first in air. Various voltages were supplied to the converter. The amplitude of the motion of the tip of the concentrating horn and the motional current supplied to the converter were measured. The motional current was derived in the manner described in the above-identified copending application. That is, a current equal to the clamped capacitance current of the converter was subtracted from the total :urrent. supplied to the converter. This produces a current equal to the motional current in the series branch equivalent circuit of the converter.

It was found that the motional current supplied to the converter and the amplitude of the motion of the tip of the concentrating horn were directly proportional throughout the useful operating range of the converter. It was further found that this proportionality was independent of the acoustic load presented to the transducer when the transducer was operated in air, Water or heavy oil.

It wasfurther found by driving the converter with a variable frequency oscillator that this proportionality was substantially independent of frequency at or near the natural resonance of the transducer.

It was further found that the internal dielectric losses of the converter were small and may be neglected. Furthermore it was found that the internal losses in the converter appear to the driving generator or amplifier as a constant impedance in series with a varying impedance directly proportional to the acoustic load presented to the converter.

It is therefore an object of the present invention to provide systems for keeping the heat dissipation in a sonic converter constant or below a predetermined maximum.

Another object of the invention is to provide systems for keeping the mechanical deformation in a sonic converter constant or below a predetermined maximum.

A further object of the invention is to provide systems for keeping the amplitude of the motion of the transducing elements of a sonic converter constant or below a predetermined maximum.

Still another object of the invention is to provide systems for keeping the power dissipated in a sonic converter constant or below a predetermined maximum.

A still further object of the invention is to provide systems for keeping an electrical quantity supplied to a sonic converter proportional to the quantities of the above objects constant or below a predetermined maximum.

Still another object of the invention is to provide systems for keeping motional current supplied to an electrostrictive sonic converter constant or below a predetermined maximum.

Another object of the invention is to provide systems for increasing the total power supplied to a sonic converter as the acoustic impedance into which the converter is working increases.

Still another object of the invention is to provide systems for keeping the total current supplied to a sonic converter employing electrostrictive elements substantially constant or below a predetermined maximum.

Yet another object of the invention is to provide methods, apparatus and systems for driving a high power sonic converter from an apparently high impedance source.

A further object of the invention is to control the power delivered to an ultrasonic transducer.

Another object of the invention is to compensate for the varying acoustic impedances into which an ultrasonic power transducer operates.

A further object of the invention is to provide systems whereby a conventional ultrasonic power transducer is able to deliver more power into a load than heretofore possible.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combinations of elements and arrangements of parts which are all as exemplified in the following detailed disclosure. The scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed disclosure taken in connection with the accompanying drawings in which:

FIGURE 1 is a front view, partially cut away, of an ultrasonic power converter and electronic driver in accordance with the present invention.

FIGURE 2 is an equivalent circuit of the ultrasonic power converter shown in FIGURE 1.

FIGURE 3 is a block circuit diagram of another embodiment of the invention.

FIGURE 4 is a block circuit diagram of another embodiment of the invention.

FIGURE 5 is a block circuit diagram of another embodiment of the invention.

FIGURES 6 and 7 are detailed circuit diagrams of a preferred embodiment of the invention combining the em bodiments of FIGURES 3, 4 and 5.

FIGURE 8 is a block circuit diagram of another embodiment of the invention.

FIGURE 9 is a block circuit diagram of a preferred embodiment of the invention.

FIGURE 10 is a detailed circuit diagram of the embodiment of the invention of FIGURE 9.

The same reference characters refer to the same elements throughout the several views of the drawings.

GENERAL DESCRIPTION The ultimate objects of the present invention are to limit the dissipation in a sonic converter to a safe, maximum value for all loading conditions and to obtain a loading characteristic wherein the mechanical power output from the converter increases when the converter operates into a load of increased acoustical impedance.

The invention may be realized in its most perfect form by the use of feedback techniques to hold an electrical quantity proportional to acoustic deformation or motion in the converter constant.

In the case of converters employing electrostrictive elements such an electrical quantity is the motional current I This current may be derived from the electrical power supplied to the converter by compensating for the clamped capacitance of the converter in the manner described in Patent No. 3,293,456.

The ultimate objects of the invention are realized by providing means in the power generator connected to the converter such that the total current supplied to the converter is substantially constant as the converter operates into varying acoustic impedances. Thus, the power generator may be provided, in effect, with a high output impedance by connecting a large impedance in series between the generator and the converter; this impedance being much greater than the effective motional impedance of the converter when operating into a load of the largest acoustical impedance. This is shown in FIGURE 3.

The internal impedance of the generator may also be matched by means of a transformer with the total motional impedance of the converter operating into the highest acoustical impedance to which it is to be subjected as shown in FIGURE 4.

In another embodiment of the invention this object is achieved by operating the converter otf resonance so that the effective frequency dependent impedance of the converter becomes a large impedance in series with the load varying impedance of the converter as shown in FIG- URE 5.

In FIGURE 6 there is shown in detail a circuit for applying the methods of FIGURES 3, 4 and 5 to a conventional transistorized sonic generator amplifier shown in FIGURE 7. A nonlinear impedance is provided in the positive feedback oscillator circuit of FIGURE 6 that causes the converter to operate below its mechanical resonance and the operating resonance of the circuit when it is initially energized. This prevents oscillation at another undesired fairly strong mechanical resonance in the converter only slightly above the desired operating resonance. The entire oscillator feedback loop of the circuit of FIGURE 7 is characterized by a narrow band characteristic about the desired operating frequency so that operation at other nearby resonances is further inhibited.

In another embodiment of the invention a sonic converter may be connected in loop circuit with a linear amplifier utilizing negative current feedback as shown in FIGURE 8 to provide an effective high output impedance for the generator.

Another method of providing a generator with an effective high output impedance is to drive it from a constant current power supply as shown in FIGURES 9 and 10.

SPECIFIC DESCRIPTION Now referring to FIGURE 1, a high power density ultrasonic converter is generally indicated at 20. This is constructed in the manner detailed in the above-identified Patent No. 3,328,610. The converter 20 comprises a metal casing 22 supporting a perforated metal vent plate 24 integral with the transducer system generally indicated at 26. The transducer system is bolted together and comprises a pair of ceramic discs 28 separated by a metal plate 30. The ceramic discs 28 are backed by a massive metal back plate 32 and operate into a sonic energy concentrating step horn 34. Electrical energy is supplied to the transducer system 26 by means of a wire 36 connected to the metal plate 30 and the metal casing 22 connected to the vent plate 24. The converter also comprises a fan 38 for cooling the transducer system 26.

Power for the transducer system 26 and the fan 38 is supplied via an insulated cable 40 from a power generator generally indicated at 42. The power generator 42 is connected via line cable 44 to a supply of 115 volts, 60 cycle power.

The converter 20 may be turned on by means of an on-ofi switch 46 or by means of a foot switch (not shown) which alternatively may be in series with the on-ofli switch 46. A jewel light 48 may be provided to indicate that the power generator 42 is energized. A power level control 50 may also be provided.

The experiments discussed in the introduction above indicate that at a given operating frequency the converter 20 may be represented by the equivalent circuit shown in FIGURE 2. This circuit comprises a clamped capacitance C representing the frequency independent capacitance of the converter. The clamp capacitance C is in parallel with a resistance R representing the dielectric or voltage dependent losses in the converter.

A third parallel arm of the equivalent circuit comprises a frequency dependent reactive impedance X which may be represented by the series connected capacitance C and inductance L. The resistance R represents the internal motional dependent losses in the converter. The resistance R represents the transformed acoustical impedance of the load into which the converter 20 is operated. The resistance R has been found to be directly proportional to the acoustic-a1 impedance of the load over the useful operating range of the converter 20. The resistance R that is the motionally dependent internal losses, has been found to be substantially constant over the useful operating range of the converter 20.

When a Branson Sonifier S-75 converter is operated at 20 kilocycles with a 1 /2 inch probe terminated by a step horn, R is found to be approximately 100 ohms. R when the converter operates into air is substantially zero. The total Q of the equivalent circuit is found to be approximately 800. When the converter is loaded, R may vary from 200 to several thousand ohms. R is so large that it has not been accurately measured but appears to be greater than 100,000 ohms. It can therefore be neglected. C is approximately .038 microfarad.

Thus, when operating at resonance, the current I through the clamped capacitance C is constant. The current I through the dielectric resistance R is substantially zero. The motional current I is substantially dependent on R since at a constant frequency, the reactive impedance of X is zero and R is constant.

Thus the total current I supplied to the converter may be vectorially added to the current passing through a capacitor having a value of .038 microfarad connected aCrOSS the source of the voltage V supplying the converter to derive the motional current 1 Methods for doing this are disclosed in detail in the above-identified Patent No. 3,293,456.

As disclosed in that application, the derived motional current I supplied to the converter may be utilized to provide a positive feedback signal for a linear power amplifier in the power generator 42 to cause the entire circuit to oscillate and thus the converter to operate at the natural mechanical resonance of the converter having the highest Q.

In the embodiments of the present invention, it is sought to maintain the motional current I through the converter substantially constant despite variations in the acoustical load to which the converter is coupled. When the motional current I is kept constant the amount of power delivered by the converter into a load will depend solely on the acoustic impedance of the load. That is, as the asoustic impedance of the load increases, R increases and the load dissipated power I R will increase. At the same time the internal motional losses in the converter I R will remain constant.

Constant current through the converter Now referring to FIGURES 3, 4 and 5, as previously explained, the objects of the invention can be somewhat imperfectly realized by keeping the total current I supplied to the converter 20 constant rather than the motional current, at least to the extent that the equivalent circuit of FIGURE 2 is an accurate representation of what is going on in the converter.

External impedance in series with generator and converter Thus, in FIGURE 3, the power amplifier 54 is connected tot he converter 20 through a reactance of impedance X very much greater than the sum of the internal motional losses R and the electrical transformation R of the external acoustical impedances into which the converter is to operate. That is:

In the system shown in FIGURE 3, the total current I will remain substantially constant even as R varies in accordance with the acoustical load into which the converter operates because the large impedance 160 will essentially determine the current I in the circuit. In this way the motional current I will also be held substantially constant.

Matching generator to highest acoustical load Another similar way of attaining the objects of the invention is shown in FIGURE 4. In this system the oscillator 162, which alternatively could be a power amplifier driven by a positive feedback as in the previously described self-tuning circuit, has an internal impedance R R is matched by means of transformer 164 to the maximum impedance of the converter 20 when operated into the maximum acoustic load to which it is subjected.

Thus, in the system of FIGURE 4 when the converter is unloaded, that is operating in air or a vacuum, because of the large disparity between the large internal impedance R of the oscillator 162 and the small internal impedance of the converter 20 only a small amount of power will be delivered thereto. However, as the acoustic load on the converter 20 increases, the converter will become increasingly matched to the internal impedance R of the oscillator 162 and the power delivered by the converter 20 will be at its maximum.

Operating converter 0]? resonance Now referring to FIGURE 5, another method of keeping the total current supplied to the converter constant is to operate it oh its natural resonant frequency. In the system illustrated in FIGURE 5 the operating frequency F supplied by an oscillator or determined by the total phase shift in a feedback circuit connected to the power amplifier 54 is different from the mechanical resonance frequency F of the converter 20. Since the converter is not operating at resonance, the reactive impedance X of the converter is not zero. It may be made as large as desired by operating as far as necessary off resonance. Thus, when the reactive impedance X is very much greater than the sum of the internal motional losses R and the electrical transformation R of the maximum external impedance into which the converter operates, the motional current I will be substantially determined by the reactive impedance X and will be independent of load.

A preferred embodiment A detailed circuit of a power generator embodying the invention as illustrated in FIGURES 3, 4 and 5 is shown in FIGURES 6 and 7. FIGURE 7 is a circuit diagram of a conventional two stage transistorized power amplifier 166.

Now referring to FIGURE 7, amplifier 166 is driven from a minus 25 volt DC. power supply connected to terminal 168. The first stage of the amplifier comprises four type 2N1905 transistors generally indicated at 170. The bases of one pair of transistors 170 are connected together and through parallel connected resistor 172 and capacitor 174 to terminal 176. The bases of another pair of transistors 170 are connected through resistor 178 and capacitor 180 to terminal 182. The emitters of all transistors are connected together and through parallel connected resistor 184 and capacitor 186 to terminal 188. The collectors of the upper pair of transistors 170 are connected together through the upper half of the center tapped primary winding of transformer 190 and through capacitor 192 to ground. The emitters of all transistors are grounded. Diodes 1-94 are connected between the collectors and emitters of each pair of transistors 174.

Resistors 172 and 178 are each 47 ohms, 1 watt resistors. Capacitors 174 and 180 are each .22 microfarad, 50 volt capacitors. Resistor 184 is a 68 ohm, /2 watt resistor. Capacitor 186 is a 10 microfarad, 150 volt capacitor. Capacitor 192 is a 50 microfarad, 150 volt capacitor. Diodes 194 are each type 1N2071. The first stage of amplifier 166 is connected to the power supply terminal 168 through resistors 196 and 198. Resistor 196 is a 390 ohm, 1 watt resistor. Resistor 198 is a 2 ohm, 10 watt resistor.

The secondary of transformer 190 is connected through a 2 microfarad, 200 volt capacitor 200 to the primary of'a second transformer 192 to form an interstage transformer from the first stage to the second stage of amplifier 166. Transformers 190 and 192 are wound on Allen Bradley type R034 cores, catalog number E-l102-142A. The primary of transformer 190 comprises 40 turns and is center tapped. The secondary comprises 30 turns. The primary of transformer 192 comprises 6 turns and its center tapped secondary comprises 6 turns. No. 24 wire is used in the primary of transformer 190. No. 22 wire is used in the secondary thereof. Both coils of transformer 202 are wound with No. wire.

The second stage of amplifier 166 comprises eight type 2N2076 transistors, generally indicated at 204. Transistors 204 are connected in parallel groups of four to provide a push-pull output stage. The bases of four of the transistors 204 are connected to the upper half of the secondary of transformer 202 and through the center tap thereof and resistor 206 to power supply terminal 168.

Resistor 206 is a 470 ohm, 1 watt resistor. The center tap of the secondary of transformer 202 is connected through the parallel combination of resistor 208 and capacitor 210 to ground. Resistor 208 is a 1 ohm, 10 watt resistor. Capacitor 210 is a 50 microfarad, 50 volt capacitor. The emitters of each of the transistors 204 are connected through eight parallel connected resistors, generally indicated at 212, to ground. Each of resistors 212 are .2 ohm, rated at 10 watts. The collectors of the four upper transistors 204 are connected together and through the upper half of the primary of an output transformer 214 and through the center tapthereof to terminal 168. Similarly, the collectors of the lower four transistors 204 are connected together and through the lower primary of transformer 204 to terminal 168. The secondary of transformer 214 is connected to terminals 216 and 218.

Transformer 214 is wound on an Allen Bradley type W04 core, catalog number U3000 D 137A. The primary comprises 12 center tapped turns of No. 12L Wire. The secondary comprises turns of No. 22 Wire and the primary and secondary are bifilarly wound.

As previously stated, amplifier 166 is conventional.

Now referring to FIGURE 6, the power control elements of the invention, generally indicated at 220 and the converter 20 are shown connected in circuit with the amplifier 166 of FIGURE 7 to attain the objects of the invention. Current transformer 222 is the input transformer to the first stage of amplifier 166 in FIGURE 7. Transformer 222 comprises a 22 turn, center tapped, primary and a 14 turn, center tapped, secondary Wound on one U and one bar shaped Allen Bradley type W04 cores, catalog numbers U2375 D 129C and B2375 D 307A, and is wound with No. 20 wire. The cores are spaced apart by an air gap of approximately .022 inch. The secondary of transformer 222 is connected, as shown, to the input terminals 176, 182 and 188 of the first or driver stage of amplifier 166 of FIGURE 7.

The output terminals 216 and 218 of the output transformer 214 of the amplifier 166 of FIGURE 7 are connected in circuit with the converter from terminal 216 through a 1.32 millihenry inductor 224 through the converter 20 to ground, from ground through the lower half of the primary of transformer 222 to terminal 218 of the output transformer 214 shown in FIGURE 7. Thus the total current I supplied to the converter 20 passes through the lower half of the primary of transformer 222. The clamped capacitance compensating current I passes through capacitor C of .0038 microfarad, 2500 volt rating through the upper half of the primary of transformer 222. The current in the secondary of transformer 222 is therefore directly proportional to the motional current T The indicator 224 compensates for the reactive nature of the load in the manner well known in the art. This inductor is wound on an Allen Bradley W04 core, catalog number U2375 D 129C and comprises 149 turns of No. 22L wire.

The ouptut transformer 214 of FIGURE 7, constructed in the manner described, is not matched to the converter 20 when it is operating in air. In the design illustrated in FIGURES 6 and 7, it is approximately matched to the converter when the converter is operating as a plastic Welder. Thus, the overall circuit operates in part in the manner shown in FIGURE 4.

Furthermore, the converter 20 is caused to operate off its natural mechanical resonance. In the case illustrated, this is a few hundred cycles below its natural resonance of approximately 20 kilocycles.

This off resonance operation is in part accomplished by choosing inductor 224 to be too large to precisely compensate for the reactive load of capacitor C and converter 20 and by means of capacitors 226 and 228 and resistor 230 connected across the lower primary of transformer 222.

Capacitor 226 is 3 microfarads rated at 200 volts and capacitor 228 is 1 microfarad also rated at 200 volts. Variable resistor 230 is 50 ohm maximum and is rated at 4 watts. Capacitor 226 and 228 tune with the lower half primary of transformer 222 to form a narrow band pass filter so that the entire loop circuit comprising the converter 20, the feedback network 220 and the amplifier 166 of FIGURE 7 can only operate around a narrow band of frequencies near the natural resonance of the converter 20.

Additionally, this circuit of capacitors 226 and 228 and the lower half of the primary of transformer 222 is tuned to a slightly higher frequency than the natural frequency of the converter 20. This additional phase shift is partially responsible for the converter operating at a slightly lower frequency than its natural frequency. The frequency of operation may be adjusted by adjusting variable resistor 230. The off resonance operation results in the converter having a large internal reactive impedance K as shown in FIGURE 5.

Because the converter 20 has a natural resonance at approximately 26 kilocycles which is quite strong, an additional precaution is taken to insure that when the converter is first turned on its begins to oscillate at the desired 20 kilocycle natural frequency. This is accomplished by adjusting variable resistor 230 so that the frequency of operation is several hundred cycles lower than that desired for steady state operation.

An inductor 232 connected in series with bac'k-to-back diodes 233 is also connected across the lower half primary of transformer 222. When power is initially applied to the converter 20, the voltage across the lower half primary of transformer 222 will be lower than the forward voltage of diodes 233 and inductor 232 will not be in the circuit. Only after the oscillations build up in the circuit at the frequency well below the desried natural resonance of the converter 20 will the forward voltage of diodes 233 be exceeded. Then, inductor 232 will be in the circuit and will partially compensate for the capacitance of capacitors 226 and 228. The frequency of operation of the converter then increases to the desired operating frequency. Inductor 232 comprises 32 turns of No. 26 wire on one leg of a C core, Allen Bradley type W04, catalog number U2375 D 1290. Diodes 232 are each type 1N207l.

The overall circuit of FIGURES 6 and 7 controls converter 20 such that, when operated in air, approximately 8.0 watts are dissipated therein and as the acoustic impedance of the load increases, up to 250 watts may be dissipated in the transducer load combination. No more than approximately 60 watts of this total is dissipated internally. Thus, the converter 20 can deliver into a load up to as much as approximately 190 watts, whereas the converter operated without the cotnrol circuit illustrated in FIGURE 6, using the same amplifier, can deliver perhaps as little as ten percent of this amount into a load.

Negative current feedback amplifier Now referring to FIGURE 8, another embodiment of the invention for keeping the total current supplied to the converter substantially constant comprises utilizing negative feedback with a power amplifier 54 so that it forms in effect a constant current amplifier 234. The input to the constant current amplifier 234 may be I as derived from a clamped compensating capacitor C, converter 20 and summing network 56, in the manner previously described, or may be supplied by an oscillator at the natural resonance frequency of the converter 20.

Power amplifier 54 is supplied with negative feedback voltage derived across resistor R in a well-known manner. This is combined with the input I by the resistor network R and R as will be well understood by those skilled in the art. This negative feedback results in amplifier 234 having an effective large internal impedance so that I the current supplied to the converter and capacitor C is substantially constant. Thus, I is also held constant.

Current controlled power supply The object of obtaining a power amplifier having constant current source characteristic may also be obtained in the manner shown in FIGURE 9. There the power amplifier 54 is supplied from a constant current power supply 236 connected to the power line. Constant current power supply 236 produces a constant power supply current I to the power amplifier 54 and, as is well known to those skilled in the art, this will cause the output current I and current I supplied to the con- 10 verter 20 to be substantially constant. The power amplifier may be driven from an oscillator or a feedback circuit as has been previously described.

A detailed circuit operating in the manner shown in FIGURE 9 is shown in FIGURE 10. Now referring to FIGURE 10, line plug 236 is connected to a source of 115 volt, 6*0 cycle AC. Power is supplied to the power supply transformer 238 through a fuse 240 and a switch 242. The secondary of transformer 238 is center tapped. The secondary produces 50 volts A.C. across rectifiers 244, both type 1N2071. They are connected to a filter network comprised of a 5 ohm resistor 246 and a microfarad capacitor 248 in the well known manner.

The constant current characteristic to keep the output current I of the power supply constant is obtained by connecting a transistor 250 type 2N350A in the power supply circuit, in the manner shown. By means of resistors 252 connected to the base of transistor 250, variable resistor 254, and fixed resistor 256, the bias between the emitter and the base of transistor 250 is fixed in accordance with the forward voltage drop across diodes 258. Resistor 252 is 470 ohms, variable resistor 254 has a maximum resistance of 5 ohms and resistor 256 is 1 ohm. Diodes 258 are each type 1-N2071.

The emitter base bias of transistor 250 may be adjustably fixed by adjusting resistor 254 at approximately 1 volt and the power supply current I may be fixed at 0.6 amps in accordance with the characteristic of transistor 250. The amplifier 260, which is driven from the power supply, is a 10 watt self-tuning amplifier for use with a converter 251 used in microbiological cell disruption, and the like, where only low power total power levels are desired. The amplifier comprises a single pushpull stage comprising two transistors generally indicated at 262, these are type 2N 1905. The emitters of transistors 262 are connected through 1 ohm resistors 264 to the collector of transistor 250'. The collectors of transistor 262 are connected across the primary of an output transformer 265. The bases of transistors 262 are connected across a center tapped secondary of an input current transformer 266. The center tap of the current transformer 266 is connected through a 10 microfarad capacitor 268 and a 68 ohm resistor 270 to the collector of transistor 250. The center tap of current transformer 266 is also connected to the other side of the power supply through a 1 kilohm resistor 272. The center tap of output transformer 265 is also connected to this common terminal of the power supply.

The secondary of output transformer 265 is connected in circuit with the converter 251 and one-half of the primary of current transformer 266. It is also connected in circuit through a compensating capacitor 274 and the other side of the primary of current transformer 266. The small converter 251 has a clamped capacitance of .002 microfarad and therefore the compensating capacitor 274 has the same capacitance. The primary of transformer 266 comprises 8 turns, center tapped. The secondary comprises 16 turns, center tapped. The primary of output transformer 265 comprises 30 turns, center tapped and the secondary also comprises 30 turns.

It will be seen that the input signal applied to the transistors 262 from the secondary of transformer 266 is proportional to the motional current I in the converter 251 and the circuit therefore operates at the frequency of maximum conversion efiiciency of the converter. The output current I of transformer 265 will be substantially constant as will the current I supplied through the converter. This is due to the constant current operation of the amplifying transistors 262.

SUMMARY OF THE INVENTION It will thus be seen that I have provided apparatus and systems for keeping the dissipation in a high power density sonic converter, due to motion therein, substantially constant by keeping an electrical quantity derived from the power supplied to the converter constant. In the case of sonic converters employing electrostrictive elements, this electrical quantity may conveniently be the motional current in the converter.

The systems that I have provided for obtaining relatively constant motional current utilize means for creating a large impedance effectively in series between the sonic power generator and the motional resistance of the sonic converter.

It will be obvious to those skilled in the art that the systems, circuits, methods and apparatus disclosed herein may be modified in many ways for use in practical sonic converter systems specifically designed for particular working conditions. It will also be obvious to those skilled in the art that the methods, apparatus and systems disclosed herein may be modified for use with sonic converters employing magnetostrictive elements.

It will further be noted that the detailed systems disclosed herein all employ positive feedback derived from the motional current in sonic converters employing electrostrictive elements for controlling the frequency of operation of the converter in the manner disclosed in my above-identified Patent No. 3,293,456.

It will further be seen that in the present application I teach the use of an amplifier system connected in such positive feedback circuits having a narrow band pass characteristic centered about the desired operating frequency of the converter with which it is connected. Furthermore, I provide novel means for causing the converter, when initially turned on, to operate otf its normal operating frequency so that as oscillations build up in the converter it approaches its operating frequency from either a lower or a higher initial frequency of operation in order to avoid operation of the converter at an undesired nearby mechanical resonance.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efiiciently attained and, since certain changes may be made in the above methods, systems, apparatus and circuits without departing from the scope of the invention it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention, which, as a matter of language, might be said to fall therebetween.

Having described my invention what I claim as new and desire to secure by Letters Patent is:

1. Apparatus of the class described comprising, in combination:

(A) an electroacoustic sonic converter, having at least one electrostrictive element;

(B) a sonic power generator connected in circuit with said electroacoustic converter; and

(C) an impedance in series circuit with the motional dependent resistive impedance of said converter and said generator,

(1) said series impedance being substantially greater than the internal impedance of said converter plus the impedance of the acoustic load on said converter, whereby the total current supplied to said converter when presented with varying acoustic loads is substantially constant,

(2) the current through said series impedance being essentially equal to the current through said concerter resistive impedance.

2. The apparatus defined in claim 1 which further includes (D) feedback circuit means supplying a signal corresponding to the motional current in said converter to said generator such as to control the operating frequency of said converter.

12 3. The apparatus defined in claim 1 wherein said series impedance comprises an internal impedance of said sonic power generator.

4. The apparatus defined in claim 1 wherein said series impedance comprises a reactive impedance in said converter created by operation of said converter at a frequency different from its natural mechanical resonant frequency.

5. The apparatus as defined in claim 1 wherein a transformer is connected in circuit between said sonic power generator and said sonic converter such that the power generator is reflected as a high impedance source.

6. The apparatus of the class described comprising, in combination:

(A) a power amplifier having an input and an output transformer;

(B) an electroacoustic sonic converter;

(C) an inductor;

(1) connected in series circuit with the secondary of said output transformer and said converter and a portion of the primary winding of said input transformer,

(2) the secondary of said output transformer also connected in circuit with said inductor, a capacitor, and another portion of the primary of said input transformer, whereby the current in the secondary of said input transformer is proportional to the motional current in said converter;

and

(D) a reactive impedance connected in circuit with the primary of said input transformer and tuned therewith to cause said converter to operate at a frequency other than its resonant frequency.

7. The apparatus as claimed in claim 6 wherein said inductor has a value to detune its circuit whereby the con verter operates at a frequency diflering from its natural mechanical resonance.

8. The apparatus as claimed in claim 7 wherein said output transformer causes the internal impedance of said power amplifier to be mismatched from said converter when unloaded and to be matched with said converter when said converter is loaded with the largestacoustic impedance to which it is subjected.

9. Apparatus as claimed in claim 6 which further includes (E) an additional reactive impedance connected in circuit with nonlinear responsive means and with the primary of said input transformer whereby said additional impedance means has no effect on the primary of said input transformer until the current therethrough reaches a critical value as determined by said nonlinear responsive means to thus connect said additional impedance means in circuit wtih the primary of said input transformer to cause the frequency of operation of said converter to change.

10. Apparatus as claimed in claim 9 wherein said capacitor is tuned with the primary of said input transformer to produce a narrow bandpass frequency characteristic in said power amplifier.

References Cited UNITED STATES PATENTS 3,129,366 4/1964 Fry 318-116 X 2,917,691 12/1959 Prisco 318-118 3,177,416 4/1965 Pijls 318-118 3,151,284 8/1964 Kleesattel 318-118 3,223,907 12/1965 Blok 318-118 FOREIGN PATENTS 580,273 7/1959 Canada.

I. D. MILLER, Primary Examiner.

US. Cl. X.R.

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