Patent Publication Number: US-7211927-B2

Title: Multi-generator system for an ultrasonic processing tank

Description:
RELATED APPLICATIONS 
   The subject application is a continuation-in-part of commonly-owned U.S. patent application Ser. No. 09/370,302 filed Aug. 9 1999; now U.S. Pat. No. 7,004,016 which is a division of U.S. patent application Ser. No. 09/097,374 filed Jun. 15, 1998 (now U.S. Patent No. 6,016,821, granted Jan. 25, 2000); which is a continuation of U.S. patent application Ser. No.: 08/718,945 filed Sep. 24, 1996 (now U.S. Pat. No.: 5,834,871, granted Nov. 10, 1998) and U.S. Provisional Application No.: 60/049,717 filed Jun. 16, 1997). 

   BACKGROUND OF THE INVENTION 
   Ultrasonic systems for processing and cleaning parts are widely used by industry. Such systems typically include (a) a tank to hold the process chemistry such as cleaning solution, (b) an ultrasound generator, and (c) one or more transducers connected to the tank and the generator to deliver ultrasound energy to the process chemistry. These systems are generally adequate for low frequency operation, i.e., where the energy applied to the chemistry is around 20 khz. However, prior art ultrasound processing equipment has important technology limitations when operating at high frequencies and high power; and delicate parts such as disk drives for the computer industry require high frequency, high power ultrasound in order to effectively process components without damage. In one failure mode, for example, prior art transducers are known to fail when subjected to extended periods of operation, especially at high frequency and high power. In addition, prior art transducers are generally non-linear with respect to power output as a function of drive frequency. Therefore, prior art ultrasonic processing systems sometimes include costly electronics to compensate for such non-linearities. 
   There are other problems. For example, certain manufacturers require that a particular generator be matched to a particular tank since that combination is measured and known to provide particular process characteristics. However, this is cumbersome to an end user who cannot swap one generator for another in the event of a failure. More importantly, though, end users are not able to effectively monitor whether the system has degraded. Typically, for example, end users become aware of failure modes only after parts are damaged or destroyed within the process. There is a need, therefore, of monitoring systems which monitor processes in real-time. 
   It is, accordingly, one object of the invention to provide systems, apparatus and methods for delivering high frequency, high power ultrasound energy to process chemistries. Another object of the invention is to provide generators and systems which enable multi-frequency operation, selectively and without undue difficulty. Still another object of the invention is to provide improved transducer designs which increase system reliability and which improve power delivery. Yet another object of the invention is to provide systems, apparatus and methods for monitoring ultrasound processes in real-time or as a quality control (“QC”) step. 
   SUMMARY OF THE INVENTION 
   As used herein, “ultrasound” and “ultrasonic” generally refer to acoustic disturbances in a frequency range above about eighteen kilohertz and which extend upwards to over two megahertz. “Lower frequency” ultrasound, or “low frequency” ultrasound mean ultrasound between about 18 khz and 90 khz. “Megasonics” or “megasonic” refer to acoustic disturbances between 600 khz and 2 Mhz. As discussed above, the prior art has manufactured “low frequency” and “megasonic” ultrasound systems. Typical prior art low frequency systems, for example, operate at 25 khz, 40 khz, and as high as 90 khz. Typical prior art megasonic systems operate between 600 khz and 1 Mhz Certain aspects of the invention apply to low frequency ultrasound and to megasonics. However, certain aspects of the invention apply to ultrasound in the 100 khz to 350 khz region, a frequency range which is sometimes denoted herein as “microsonics.” 
   As used herein, “resonant transducer” means a transducer operated at a frequency or in a range of frequencies that correspond to a one-half wavelength (λ) of sound in the transducer stack. “Harmonic transducer” means a transducer operated at a frequency or in a range of frequencies that correspond to 1λ, 1.5λ, 2λ or 2.5λ of sound, and so on, in the transducer stack. “Bandwidth” means the range of frequencies in a resonant or harmonic region of a transducer over which the acoustic power output of a transducer remains between 50% and 100% of the maximum value. 
   As used herein, a “delicate part” refers to those parts which are undergoing a manufacture, process, or cleaning operation within liquid subjected to ultrasonic energy. By way of example, one delicate part is a semiconductor wafer which has extremely small features and which is easily damaged by cavitation implosion. A delicate part often defines components in the computer industry, including disk drives, semiconductor components, and the like. 
   As used herein, “khz” refers to kilohertz and a frequency magnitude of one thousand hertz. “MHz” refers to megahertz and a frequency magnitude of one million hertz. 
   As used herein, “sweep rate” or “sweep frequency” refer to the rate or frequency at which a generator and transducer&#39;s frequency is varied. That is, it is generally undesirable to operate an ultrasonic transducer at a fixed, single frequency because of the resonances created at that frequency. Therefore, an ultrasonic generator can sweep (i.e., linearly change) the operational frequency through some or all of the available frequencies within the transducer&#39;s bandwidth at a “sweep rate.” Accordingly, particular frequencies have only short duration during the sweep cycle (i.e., the time period for sweeping the ultrasound frequency through a range of frequencies within the bandwidth). “Sweep the sweep rate” or “double sweeping” or “dual sweep” refer to an operation of changing the sweep rate as a function of time. In accord with the invention, “sweeping the sweep rate” generally refers to the operation of sweeping (i.e., linearly changing) the sweep rate so as to reduce or eliminate resonances generated at the sweep frequency. 
   In one aspect, the invention provides ultrasound transducer apparatus. In the apparatus, at least one ceramic drive element is sandwiched between a front driver and a backplate. The drive element has electrical contacts or electrodes mounted on either face and is responsive to voltages applied to the contacts or electrodes so as to produce ultrasound energy. A connecting element—e.g., a bolt—connects the back plate to the front driver and compresses the drive element therebetween. In accord with the invention, the front driver and/or the backplate are shaped so that the apparatus produces substantially uniform power as a function of frequency over a range of frequencies. In another aspect, the shape of the driver and/or backplate are selected so as to provide a varying power function as a function of frequency. 
   In another aspect, a multi-frequency ultrasound generator is provided. In one aspect, the generator includes a constant power output circuit with means for switching the center frequency of the output signal selectively. The switching means operates such that little or no intermediate frequencies are output during transition between one center frequency and another. 
   Another multi-frequency generator of the invention includes two or more circuits which independently create ultrasound frequencies. By way of example, one circuit can generate 40 khz ultrasound energy; while another circuit-can generate 104 khz energy. A switching network connects the plurality of circuits such that the generator is shut down and relay switching takes place in a zero voltage condition. As above, therefore, the switching occurs such that little or no intermediate frequencies are output during transition between one center frequency and another. 
   In still another aspect, a two stage ultrasonic processing system is provided. The system includes (a) one or more transducers with a defined ultrasound bandwidth defined by an upper frequency and a lower frequency. The system further includes (b) a frequency generator for driving the transducers from the upper frequency to the lower frequency over a selected or variable time period and (c) a process tank connected with the transducers so as to generate ultrasound energy within the tank at frequencies defined by the generator. During a given cycle, the generator drives the transducers from the upper frequency to the lower frequency. Once the lower frequency is reached, a frequency control subsystem controls the generator so as to drive the transducers again from upper to lower frequency and without driving the transducers from lower to upper frequencies. In this manner, only decreasing frequencies—per cycle—are imparted to process chemistries. The system thus provides for removing contamination as the downward cycling frequencies cause the acoustic energy to migrate in an upwards motion inside the tank which in turn pushes contamination upwards and out of the tank. 
   In another aspect of the invention, the two stage ultrasonic processing system includes means for cycling from upper-to-lower frequencies every half cycle. That is, once the transducers are driven from upper to lower frequencies over a first half cycle, the generator recycles such that the next half cycle again drives the transducers from upper to lower frequencies. Alternatively, after driving the transducers from upper to lower frequencies for a first half cycle, the system inhibits the flow of energy into the tank over a second half cycle. 
   The two stage ultrasonic processing systems of the invention can be continuous or intermittent. That is, in one preferred aspect, the system cycles from upper to lower frequencies and then from lower to upper frequencies in a normal mode; and then only cycles from upper to lower frequencies in a contamination removing mode. 
   In still another aspect, the invention provides a process control probe which monitors certain process characteristics within an ultrasonic process tank. The probe includes an enclosure, e.g., made from polypropylene, that transmits ultrasound energy therethrough. The enclosure houses a liquid that is responsive to the ultrasonic energy in some manner such as to create free radicals and ions from which conductivity can be measured. This conductivity provides an indication as to the number of cavitation implosions per unit volume being imparted to the process chemistry within the tank. A conduit from the enclosure to a location external to the process chemistry is used to measure the characteristics of the liquid in response to the energy. In other aspects, a thermocouple is included within the enclosure and/or on an external surface of the enclosure (i.e., in contact with the process chemistry) so as to monitor temperature changes within the enclosure and/or within the process chemistry. Other characteristics within the tank and/or enclosure can be monitored over time so as to create time-varying functions that provide other useful information about the characteristics of the processes within the tank. 
   In one aspect, the invention provides an ultrasonic system for moving contaminants upwards within a processing tank, which holds process liquid. An ultrasonic generator produces ultrasonic drive signals through a range of frequencies as defined by an upper frequency and a lower frequency. A transducer connected to the tank and the generator responds to the drive signals to impart ultrasonic energy to the liquid. A controller subsystem controls the generator such that the drive signals monitonically change from the upper frequency to the lower frequency to drive contaminants upwards through the liquid. 
   In one aspect, the controller subsystem cyclically produces the drive signals such that the generator sweeps the drive signals from the upper frequency to the lower frequency over a first half cycle, and from the lower frequency to the higher frequency over a second one half cycle. The subsystem of this aspect inhibits the drive signals over the second half cycle to provide a quiet period to the liquid. 
   In other aspects, the first and second one-half cycles can have different time periods. Each successive one-half cycle can have a different time period such that a repetition rate of the first and second half cycles is non-constant. Or, the first one-half cycle can have a fixed period and the second one-half cycle can be non-constant. 
   In one aspect, the first half cycle corresponds to a first time period and the second one half cycle corresponds to a second time period, and the subsystem varies the first or second time periods between adjacent cycles. 
   Preferably, the subsystem includes means for shutting the generator off during the second one half cycle. 
   In another aspect, the subsystem includes an AM modulator for amplitude modulating the drive signals at an AM frequency. In one aspect, the AM modulator sweeps the AM frequency. In another aspect, the AM modulator sweeps the AM frequency from a high frequency to a low frequency and without sweeping the AM frequency from the low frequency to the high frequency. The subsystem can further inject a quiet or degas period before each monotonic AM frequency sweep. 
   In another aspect, there is provided an ultrasonic system for moving contaminants upwards within a processing tank including: a processing tank for holding process liquid, an ultrasonic generator for generating ultrasonic drive signals through a range of frequencies defined between an upper frequency and a lower frequency, at least one transducer connected to the tank and the generator, the transducer being responsive to the drive signals to impart ultrasonic energy to the liquid, and a controller subsystem for controlling the generator through one or more cycles, each cycle including monotonically sweeping the drive signals from the upper frequency to the lower frequency, during a sweep period, and recycling the generator from the lower frequency to the upper frequency, during a recovery period, the sweep period being at least nine times longer than the recovery period. 
   In one aspect, the controller subsystem varies a time period for each cycle wherein the time period is non-constant. 
   In still another aspect, an ultrasonic system is provided for moving contaminants upwards within a processing tank, including: a processing tank for holding process liquid; an ultrasonic generator for generating ultrasonic drive signals; at least one transducer connected to the tank and the generator, the transducer being responsive to the drive signals to impart ultrasonic energy to the liquid; and an amplitude modulation subsystem for amplitude modulating the drive signals through a range of AM frequencies characterized by an upper frequency and a lower frequency, the subsystem monotonically changing the AM frequency from the upper frequency to the lower frequency to drive contaminants upwards through the liquid. 
   In one aspect, the generator sweeps the drive signals from upper to lower frequencies to provide additional upwards motion of contaminants within the liquid. 
   In another aspect, the AM frequencies are between about 1.2 khz and a lower frequency of 1 Hz. The AM frequencies can also cover a different range, such as between about 800 Hz and a lower frequency of 200 Hz. 
   In another aspect, the invention provides a multi-generator system for producing ultrasound at selected different frequencies within a processing tank of the type including one or more transducers. A generator section has a first generator circuit for producing first ultrasonic drive signals over a first range of frequencies and a second generator circuit for producing second ultrasonic drive signals over a second range of frequencies. The generator section has an output unit connecting the drive signals to the transducers. Each generator circuit has a first relay initiated by a user-selected command wherein either the first or the second drive signals are connected to the output unit selectively. 
   In one aspect, a 24VDC supply provides power for relay coils. 
   In another aspect, each generator circuit has a second relay for energizing the circuit. Two time delay circuits can also be included for delay purposes: the first time delay circuit delaying generator circuit operation over a first delay period from when the second relay is energized, the second time delay circuit delaying discontinuance of the first relay over a second delay period after the generator circuit is commanded to stop. The first delay period is preferably longer than the second delay period such that no two generators circuits operate simultaneously and such that all generator circuits are inactive during switching of the first relay. 
   Each relay can include a 24 VDC coil. A selecting device, e.g., a PLC, computer, or selector switch, can be used to select the operating generator circuit At selection, 24 VDC connects to the two relays of this operating generator circuit Preferably, each relay coil operates at a common voltage level. 
   In one aspect, a variable voltage ultrasonic generator system is provided, including: an ultrasonic generator; a switching regulator for regulating a 300 VDC signal to +12V and +15V lines, the generator being connected to the +12V and +15V lines; and a power factor correction circuit connected to AC power. The power factor correction circuit provides 300 VDC output to the generator and to the regulator. The generator thus being automatically operable from world voltage sources between 86 VAC and 264 VAC. 
   In another aspect, a variable voltage ultrasonic generator system is provided, including: an ultrasonic generator; and a universal switching regulator (known to those skilled in the art), connected to AC power, for regulating a set of DC voltages to the generator. The generator thus being automatically operable from world voltage sources between 86 VAC and 264 VAC. 
   In another aspect, a double compression transducer is provided for producing ultrasound within an ultrasound tank. The transducer has a front plate and a backplate. At least one piezoceramic is sandwiched between the front plate and backplate. A bias bolt with an elongated bias bolt body between a bias bolt head and a threaded portion extends through the front plate and the piezoceramic and is connected with the backplate (either by screwing into the backplate or by a nut screwed onto the bias bolt adjacent the backplate). The bias bolt also forms a through-hole interior that axially extends between the head and the threaded portion. A second bolt with an elongated body between a second bolt head and a threaded tip is disposed within the bias bolt. The second bolt head is rigidly attached to the tank and a nut is screwed onto the threaded tip and adjacent to the backplate. The bias bolt thus provides a first level of compression of the piezoceramic. The second bolt provides a second level of compression of the front plate and the tank, particularly when epoxy is used to bond between the front plate and the tank. 
   In still another aspect, a variable voltage ultrasonic generator system is provided. The system includes an ultrasonic generator and a constant peak amplitude triac circuit connected to AC power. The triac circuit converts the AC power to a 121.6 voltage peak, or less, AC signal. A bridge rectifier and filter connects to the AC signal to rectify and filter the AC signal and to generate a DC voltage less than (86)(√{square root over ( )}2) volts. A switching regulator regulates the DC voltage to 12 VDC and 15 VDC; and the generator connects to the DC voltage, the 12 VDC and the 15 VDC. In this manner, the generator is thus automatically operable from world voltage sources between 86 VAC and 264 VAC. 
   The invention is next described further in connection with preferred embodiments, and it will become apparent that various additions, subtractions, and modifications can be made by those skilled in the art without departing from the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention may be obtained by reference to the drawings, in which: 
       FIG. 1  shows a cut-away side view schematic of an ultrasound processing system constructed according to the invention; 
       FIG. 2  shows a top view schematic of the system of  FIG. 1 ; 
       FIG. 3  shows a schematic illustration of a multi-transducer system constructed according to the invention and used to generate broadband ultrasound in a combined bandwidth;  FIG. 4  graphically illustrates the acoustic disturbances produced by the two transducers of  FIG. 3 ;  FIG. 5  graphically illustrates the broadband acoustic disturbances produced by harmonics of a multi-transducer system constructed according to the invention; 
       FIG. 6–16  show transducer and backplate embodiments for systems, methods and transducers of the invention; and  FIG. 17  shows representative standing waves within one transducer of the invention; 
       FIG. 18  illustrates preferential placement and mounting of multiple transducers relative to a process tank, in accord with the invention; 
       FIG. 19  illustrates a representative standing wave relative to the process tank as formed by the arrangement of  FIG. 18 ; 
       FIG. 20  illustrates another preferential pattern of placing transducers onto a mounting surface such as an ultrasound tank, in accord with the invention; 
       FIG. 21  illustrates, in a side view, the mounting of two transducers (such as the transducers of  FIG. 20 ) to a tank, in accord with the invention; 
       FIG. 22  shows an exploded side view of further features of one transducer such as shown in  FIG. 21 ; 
       FIG. 23  illustrates a two stage ultrasound delivery system constructed according to the invention; and  FIGS. 24 and 25  show alternative timing cycles through which the system of  FIG. 23  applies ultrasound from upper to lower frequencies; 
       FIGS. 26–30  show alternate sweep down cyclical patterns for applying a power-up sweep pattern in accord with the invention; 
       FIG. 31  schematically illustrates ultrasound generator circuitry for providing dual sweeping power-up sweep and variable degas periods, in accord with the invention; 
       FIGS. 32 and 33  show multi-frequency ultrasound systems constructed according to the invention; 
       FIG. 34  illustrates a process control system and ultrasound probe constructed according to the invention; 
       FIGS. 35 and 36  illustrate two process tanks operating with equal input powers but having different cavitation implosion activity; 
       FIG. 37  illustrates a process probe constructed according to the invention and for monitoring process characteristics within a process chemistry such as within an ultrasound tank; 
       FIG. 38  shows a schematic view of a system incorporating the probe of  FIG. 37  and further illustrating active feedback control of energy applied to an ultrasound tank, in accord with the invention; 
       FIGS. 39–41  illustrate alternative embodiments of ultrasonic generators with universal voltage input, in accord with the invention; 
       FIG. 42  graphically illustrates an AM burst pattern in accord with the invention; and  FIG. 43  illustrates one burst of primary frequency ultrasound within one of the non-zero AM periods; 
       FIG. 44  illustrates an AM sweep pattern, in accord with the invention; 
       FIGS. 45A–45C  schematically show one AM power up-sweep generator circuit constructed according to the invention; 
       FIG. 46  shows a prior art laminar tank; 
       FIG. 47  shows an improved laminar tank, constructed according to the invention; 
       FIG. 48  shows a quick dump rinse (QDR) tank constructed according to the invention; 
       FIG. 49  shows an improved high frequency transducer constructed according to the invention; 
       FIG. 50  illustrates, in a side exploded view, a double compression transducer constructed according to the invention; 
       FIG. 51  shows a prior art transducer with a bias bolt threaded into the upper part of the front driver; 
       FIG. 52  shows an improved transducer, constructed according to the invention; with a bias bolt threaded into a lower part of the front plate; 
       FIG. 53  illustrates one transducer of the invention utilizing a steel threaded insert to reduce stress on the front driver; 
       FIG. 54  shows a side view of a printed circuit board coupled with transducers as a single unit, in accord with the invention; and  FIG. 55  shows a top view of the unit of  FIG. 54 ; 
       FIG. 56  shows an acid-resistant transducer constructed according to the invention; 
       FIG. 57  schematically shows one power up-sweep generator circuit of the invention; 
       FIG. 58  illustrates a wiring schematic that couples a common voltage supply to one generator of a system that includes multiple generators, in accord with the invention;  FIG. 59  shows a wiring schematic to couple the generators to a single processing tank with transducers; and  FIG. 60  schematically shows a circuit coupled to the rotary switch of  FIG. 58 ; 
       FIG. 61  shows a multi-generator system constructed according to the invention. 
       FIG. 62  shows a waveform of a sweeping frequency signal according to the invention. 
   

   DETAILED DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2  show schematic side and top views, respectively, of an ultrasound processing system  10  constructed according to the invention. An ultrasonic generator  12  electrically connects, via electrical paths  14   a ,  14   b , to an ultrasound transducer  16  to drive the transducer  16  at ultrasound frequencies above about 18 khz, and usually below 4 MHz. Though not required, the transducer  16  is shown in  FIG. 1  as an array of transducer elements  18 . Typically, such elements  18  are made from ceramic, piezoelectric, or magnetostrictive materials which expand and contract with applied voltages or current to create ultrasound. The transducer  16  is mounted to the bottom, to the sides, or within the ultrasound treatment tank  20  through conventional methods, such as known to those skilled in the art. A liquid (“process chemistry”)  22  fills the tank to a level sufficient to cover the delicate part  24  to be processed and/or cleaned. In operation, the generator  12  drives the transducer  16  to create acoustic energy  26  that couples into the liquid  22 . 
   Although the transducer  16  of  FIGS. 1 and 2  is shown mounted inside the tank  20 , those skilled in the art will appreciate that other mounting configurations are possible and envisioned. For example, an alternative configuration is to mount the transducer  16  to an outside surface of the tank  20 , typically at the bottom  20   a  of the tank  20 . The transducer elements  18  of the transducer  16  are of conventional design, and are preferably “clamped” so as to compress the piezoelectric transducer material. 
     FIG. 3  illustrates a two transducer system  30 . Transducer  32   a ,  32   b  are similar to one of the elements  18 ,  FIG. 1 . Transducer  32   a  includes two ceramic sandwiched elements  34 , a steel back plate  38   a , and a front drive plate  36   a  that is mounted to the tank  20 ′. Transducer  32   b  includes two ceramic sandwiched elements  34 , a steel back plate  38   b , and a front drive plate  36   b  that is mounted to the tank  20 ′. Bolts  39   a ,  39   b  pass through the plates  38   a ,  38   b  and screw into the drive plates  36   a ,  36   b , respectively, to compresses the ceramics  34 . The transducers  32  are illustratively shown mounted to a tank surface  20 ′. 
   The transducers  32   a ,  32   b  are driven by a common generator such as generator  12  of  FIG. 1 . Alternatively, multiple generators can be used. The ceramics  34  are oriented with positive “+” orientations together or minus “−” orientations together to obtain cooperative expansion and contraction within each transducer  32 . Lead-outs  42  illustrate the electrical connections which connect between the generator and the transducers  32  so as to apply a differential voltage there-across. The bolts  39   a ,  39   b  provide a conduction path between the bottoms  43  and tops  45  of the transducers  32  to connect the similar electrodes (here shown as “−” and “−”) of the elements  34 . 
   The length  40   a ,  40   b  of transducers  32   a ,  32   b , respectively, determine the transducer&#39;s fundamental resonant frequency. For purposes of illustration, transducer  32   a  has a fundamental frequency of 40 khz, and transducer  32   b  has a fundamental frequency of 44 khz. Transducers  32   a ,  32   b  each have a finite ultrasound bandwidth which can be adjusted, slightly, by those skilled in the art. Typically, however, the bandwidths are about 4 khz. By choosing the correct fundamental frequencies, therefore, an overlap between the bandwidths of the two transducers  32   a ,  32   b  can occur, thereby adding additional range within which to apply ultrasound  26   a ′,  26   b ′ to liquid  22 ′. 
   The acoustic energy  26 ′ applied to the liquid  22 ′ by the combination of transducers  32   a ,  32   b  is illustrated graphically in  FIG. 4 . In  FIG. 4 , the “x” axis represents frequency, and the “y” axis represents acoustical power. The outline  44  represents the bandwidth of transducer  32   a , and outline  46  represents the bandwidth of transducer  32   b . Together, they produce a combined bandwidth  43  which produces a relatively flat acoustical energy profile to the liquid  22 ′, such as illustrated by profile  48 . The flatness of the acoustic profile  48  within the bandwidth  43  is preferably within a factor of two of any other acoustic strength within the bandwidth  43 . That is, if the FWHM (full width, half maximum) defines the bandwidth  43 ; the non-uniformity in the profile  48  across the bandwidth  43  is typically better than this amount. In certain cases, the profile  48  between the two bandwidths  44  and  46  is substantially flat, such as illustrated in  FIG. 4 . 
   The generator connected to lead-outs  42  drives the transducers  32   a ,  32   b  at frequencies within the bandwidth  43  to obtain broadband acoustical disturbances within the liquid  22 ′. As described herein, the manner in which these frequencies are varied to obtain the overall disturbance is important. Most preferably, the generator sweeps the frequencies through the overall bandwidth, and at the same time sweeps the rate at which those frequencies are changed. That is, one preferred generator of the invention has a “sweep rate” that sweeps through the frequencies within the bandwidth  43 ; and that sweep rate is itself varied as a function of time (a phenomenon denoted herein as “sweep the sweep rate”). In alternative embodiments of the invention, the sweep rate is varied linearly, randomly, and as some other function of time to optimize the process conditions within the tank  20 ′. 
   With further reference to  FIGS. 1 and 2 , each of the elements  18  can have a representative bandwidth such as illustrated in  FIG. 4 . Accordingly, an even larger bandwidth  43  can be created with three or more transducers such as illustrated by transducers  32   a ,  32   b . In particular, any number of combined transducers can be used. Preferably, the bandwidths of all the combined transducers overlap-to provide an integrated bandwidth such as profile  48  of  FIG. 4 . As such, each transducer making up the combined bandwidth should have a unique resonant frequency. 
   Those skilled in the art understand that each of the transducers  18  and  32   a ,  32   b ,  FIGS. 1 and 3 , respectively, have harmonic frequencies which occur at higher mechanical resonances of a primary resonant frequency. It is one preferred embodiment of the invention that such transducers operate at one of these harmonics, i.e., typically the first, second, third or fourth harmonic, so as to function in the frequency range of 100 khz to 350 khz (see, e.g.,  FIG. 5 , which illustrates an applied ultrasonic bandwidth of 102 khz to 110 khz in a manner similar as in  FIG. 4 ). This frequency range provides a more favorable environment for acoustic processes within the liquid  22 ,  22 ′ as compared to low frequency disturbances less than 100 khz. For example, ultrasound frequencies around the 40 khz frequency can easily cause cavitation damage in the part  24 . Further, such frequencies tend to create standing waves and other hot spots of spatial cavitation within the liquid. 
     FIGS. 6–10  illustrate alternative backplate configurations according to the invention. Unlike the configuration of  FIG. 3 , the backplates of  FIGS. 6–10  are shaped to flatten or modify the power output from the entire transducer when driven over a range of frequencies such as shown in  FIG. 4 . Specifically,  FIG. 6  includes a backplate  58  that, for example, replaces the backplate  38  of  FIG. 3 . A portion of the bolt  39  is also shown. As illustrated, the backplate  58  has a cut-away section  60  that changes the overall acoustic resonance of the transducer over frequency. Similarly, the backplate  58   a  of  FIG. 7  has a curved section  60   a  that also changes the overall acoustic resonance of the transducer over frequency.  FIGS. 8 ,  9  and  10  similarly have other sloped or curved sections  60   b ,  60   c , and  60   d , within backplates  58   b ,  58   c  and  58   d , respectively, that also change the overall acoustic resonance of the transducer. 
   The exact configuration of the backplate depends upon the processing needs of the ultrasound being delivered to a tank. For example, it is typically desirable to have a flat or constant power over frequency, such as shown in  FIG. 4 . Accordingly, for example, the backplate and/or front driver can be cut or shaped so as to help maintain a constant power output such that the energy generated by the transducer at any given frequency is relatively flat over that bandwidth. Alternatively, the backplate can be cut or shaped so as to provide a varying power output, over frequency, such as to compensate for other non-linearities within a given ultrasound system. 
     FIG. 17  illustratively shows how standing waves are formed within one transducer  69  of the invention over various frequencies  61 ,  62 ,  63 . Because of the shaped surface  70  of the backplate  59 , there are no preferred resonant frequencies of the transducer  69  as standing waves can form relative to various transverse dimensions of the transducer  69 . By way of example, frequency  62  can represent 38 khz and frequency  63  can represent 42 khz. 
     FIG. 11  illustrates still another transducer  80  of the invention that provides for changing the power output as a function of frequency. The front driver  82  and the backplate  84  are connected together by a bolt  86  that, in combination with the driver  82  and backplate  84 , compress the ceramics  88   a ,  88   b . The configuration of  FIG. 11  saves cost since the front driver  82  has a form fit aperture-sink  90  (the bolt head  86   a  within the sink  90  are shown in a top view in  FIG. 12 ) that accommodates the bolt head  86   a . A nut  86   b  is then screwed onto the other end of the bolt  86  and adjacent to the backplate  84  such that a user can easily access and remove separate elements of the transducer  80 . 
   The front driver  82  and/or backplate  84  (the “backplate” also known as “back mass” herein) are preferably made from steel. The front driver  82  is however often made from aluminum. Other materials for the front driver  82  and/or the backplate  84  can be used to acquire desired performance characteristics and/or transducer integrity. 
     FIG. 13  shows another transducer  92  that includes a backplate  94  and a front driver  96 . A bolt  98  clamps two ceramic elements  97   a ,  97   b  together and between the backplate  94  and driver  96 ; and that bolt  98  has a bolt head  100  that is approximately the same size as the diameter “D” of the transducer  92 . The bolt head  100  assists the overall operation of the transducer  92  since there is no composite interface of the bolt  98  and the driver  96  connected to the tank. That is, the bond between the tank and the transducer  92  is made entirely with the bolt head  100 . By way of comparison, the bond between the tank and the transducer  80 ,  FIG. 11 , occurs between both the bolt  86  and the driver  82 . A sloped region  99  provides for varying the power output over frequency such as described herein. 
     FIG. 14  illustrates one end  102  of a transducer of the invention that is similar to  FIG. 13  except that there is no slope region  99 ; and therefore there is little or no modification of the power output from the transducer (at least from the transducer end  102 ). 
     FIGS. 15 and 16  show further transducer embodiments of the invention.  FIG. 15  shows a transducer  110  that includes a driver  112 , backplate  114 , bolt  116 , ceramic elements  118   a ,  118   b , and electrical lead-outs  120 . The backplate is shaped so as to modify the transducer power output as a function of frequency. The driver  112  is preferably made from aluminum. 
     FIG. 16  illustrates an alternative transducer  120  that includes a backplate  122 , driver  124 , bolt  126 , ceramic elements  128   a ,  128   b , and lead outs  130 . One or both of the backplate and driver  122 ,  124  are made from steel. However, the front driver  124  is preferably made from aluminum. The bolt head  126   a  is fixed within the driver  124 ; and a nut  126   b  is screwed onto the bolt  126  to reside within a cut-out  122   a  of the backplate  122 . The backplate  122  and front driver  129  are sealed at the displacement node by an O-ring  123  to protect the electrical sections (i.e., the piezoelectric ceramics and electrodes) of the transducer  120  under adverse environmental conditions. 
   The designs of  FIGS. 13–14  have advantages over prior art transducers in that the front plate in each design is substantially flush with the tank when mounted to the tank. That is, the front plates have a substantially continuous front face (e.g., the face  112   a  of  FIG. 15 ) that mounts firmly with the tank surface. Accordingly, such designs support the tank surface, without gap, to reduce the chance of creating cavitation implosions that might otherwise eat away the tank surface and create unwanted contaminants. 
     FIG. 18  shows one preferred arrangement (in a bottom view) for mounting multiple transducers  140  to the bottom  142   a  of a process tank  142 . Specifically, the lateral spacing between transducers  140 —each with a diameter X—is set to 2X to reduce the cavitation implosions around the transduces  140  (which might erode the generally expensive tank surface  142   a ). By way of example, if the transducer  140  has a two inch diameter (i.e., X=2″), then the spacing between adjacent transducers  140  is four inches. Other sizes can of course be used and scaled to user needs and requirements.  FIG. 9(d)  illustrates, in a cross sectional schematic view, a standing wave  144  that is preferentially created between adjacent transducers  140 ′ with diameters X and a center to center spacing of 2X. The standing wave  144  tends to reduce cavitation and erosion of the tank  142 ′ surface. 
   Surface cavitation is intense cavitation that occurs at the interface between the solution within the tank and the radiating surface upon which the ultrasonic transducers are mounted. There are several problems associated with surface cavitation damage. First, it is often intense enough to erode the material of the radiating surface. This can eventually create a hole in the radiation surface, destroying the tank The erosion is also undesirable because it introduces foreign materials into the cleaning solution. Surface cavitation further generates cavitation implosions with higher energy in each cavitation implosion than exists in the cavitation implosions in the process chemistry. If the cavitation implosions in the process chemistry are at the proper energy level, than there is the possibility that the higher energy cavitation implosions at the surface cavitation will cause pitting or craters in the parts under process. In addition, the energy that goes into creating the surface cavitation is wasted energy that is better used in creating bulk cavitation. 
     FIG. 20  illustrates a closed hex spacing pattern  149  of transducer elements  150  that causes the radiating membrane  151  (i.e., the surface of the tank to which the elements are bonded to) to vibrate in a sinusoidal pattern such that surface cavitation is prevented or reduced. In a side view,  FIG. 21  illustrates a G- 10  isolator  153  bonded between two of the transducers  150 ′ (and specifically the front driver  150   a ) and the radiating surface  151 ′, i.e., the wall of the tank  154  holding the process chemistry  156 . The G- 10   153  operates to further reduce unwanted surface cavitation, often times even when the closed hex spacing pattern of  FIG. 20  is not possible. Piezoelectric elements  155  are sandwiched between the front plate  150   a  and backplate  154 .  FIG. 22  shows an exploded side view of one of the G- 10  mounted transducer  150 ″ of  FIG. 21 . Layers of epoxy  160  preferably separate the G- 10  isolator  153  from the transducer  150 ″ and from the surface  152 ′. 
   Most ultrasonic processes, including cleaning, have two distinct stages. The first stage is usually preparation of the liquid and the second stage is the actual process. The system  200  of  FIGS. 23–25  reduces the time for liquid preparation and accomplishes the task to a degree where shorter process times are possible. 
   The invention of  FIG. 23  utilizes the sound fields as an upward driving force to quickly move contaminants to the surface  207   a  of the liquid  207 . This phenomenon is referred to herein as “power up-sweep” and generally cleans the liquid more quickly and thoroughly so that part processing can be done with less residual contamination. 
   More particularly,  FIG. 23  shows a system  200  constructed according to the invention. A generator  202  drives a plurality of transducers  204  connected to a process tank  206 , which holds a process chemistry  207 . The generator  202  drives the transducers  204  from an upper frequency (f upper ) to a lower frequency (f lower ), a shown in  FIG. 25 . Once f lower  is reached, a frequency control subsystem  208  controls the generator  202  so as to drive the transducers  204  again from f upper  to f lower  and without driving the transducers from f lower  to f upper . In this manner, only decreasing frequencies are imparted to the process chemistry  207 ; and acoustic energy  210  migrates upwards (along direction  217 ), pushing contamination  211  upwards and out of the tank  206 . 
   As shown in  FIG. 24 , the two stage ultrasonic processing system  200  can alternatively cycle the transducers  204  from f upper  to f lower  every other half cycle, with a degas, quiet or off half cycle  222  between each power burst. The control subsystem  208  of this embodiment thus includes means for inhibiting the flow of energy into the tank  206  over a second half cycle so that the quiet period  222  is realized. It is not necessary that the time periods of the first and second one-half cycles  222   a ,  222   b , respectively, be equal. 
     FIGS. 24 and 25  also show that the rate at which the frequencies are swept from f upper  to f lower  can vary, as shown by the shorter or longer periods and slope of the power bursts, defined by the frequency function  220 . 
   The generator  202  preferably produces frequencies throughout the bandwidth of the transducers  204 . The generator  202  is thus preferably a sweep frequency generator (described in U.S. Pat. Nos. 4,736,130 and 4,743,789) or a dual sweep generator (described in International Patent Application PCT/US97/12853) that will linearly or non-linearly change frequency from the lowest frequency in the bandwidth to the highest frequency in the bandwidth; and that will thereafter reverse direction and sweep down in frequency through the bandwidth. The invention of  FIG. 25  has an initial stage where the sweeping frequency only moves from the highest bandwidth frequency to the lowest bandwidth frequency. Once the lowest frequency is reached, the next half cycle is the highest frequency and the sweep starts again toward the lowest frequency. An alternative ( FIG. 24 ) is to shut the ultrasonics off when the lowest frequency is reached and reset the sweep to the highest frequency. After an ultrasonics quiet period  222 , another sweep cycle from high frequency to low frequency occurs. This. “off” period followed by one directional sweep is repeated until contamination removal is complete; and then the processing can start in a normal way. Alternatively, a power up-sweep mode can be utilized for improved contamination removal during processing. 
   The reason that contamination is forced to the surface  207   a  of the process chemistry  207  in the system of  FIG. 23  is because the nodal regions move upward as frequency is swept downward. Contamination trapped in nodal regions are forced upward toward the surface as nodes move upward. Generally, the system of  FIG. 23  incorporates a type of frequency modulation (FM) where frequency changes are monotonic from higher to lower frequencies. Transducers  204  mounted to the bottom of the process tank  206  generate an ever expanding acoustic wavelength in the upward direction  217  (i.e., toward the surface  207   a  of the process chemistry  207 ). This produces an acoustic force  210  which pushes contamination  211  to the surface  207   a  where the contamination  211  overflows the weirs  213  for removal from the tank  206 . 
   Those skilled in the art should appreciate that methods and systems exist for sweeping the applied ultrasound energy through a range of frequencies so as to reduce resonances which might adversely affect parts within the process chemistry. See, e.g., U.S. Pat. Nos. 4,736,130 and 4,743,789 by the inventor hereof and incorporated by reference. It is further known in ultrasonic generators to “sweep the sweep rate” so that the sweep frequency rate is changed (intermittently, randomly, with a ramp function, or by another function) to reduce other resonances which might occur at the sweep rate. By way of example, the inventor of this application describes such systems and methods in connection with  FIGS. 3 ,  4 ,  5 A,  5 B,  12 A,  12 B and  12 C of International Application No. PCT/US97/12853, which is herein incorporated by reference. 
   The variable slope of the frequency function  220  of  FIGS. 24 and 25  illustrates that the time period between successive power up sweeps, from f upper  to f lower , preferably changes so as to “sweep the sweep rate” of the power up sweep. Accordingly, the power up-sweep preferably has a non-constant sweep rate. There are several ways to produce a non-constant power up-sweep rate, including: 
   (a) As illustrated in  FIG. 28 , sweep down in frequency (i.e., from f upper  to f lower ) at a relatively slow rate, typically in the range of 1 Hz to 1.2 khz, and sweep up in frequency (i.e., from f lower  to f upper ) during the recovery time at a rate about ten times higher than the sweep down frequency rate. Vary the rate for each cycle. This cycle is repeated during processing. 
   (b) As illustrated in  FIG. 29 , sweep down in frequency at a relatively slow rate and shut the generator  202  off (such as through the control subsystem  208 ) at periods  225 ′ when the lowest frequency f lower  in the bandwidth (bandwidth=f upper −f lower ) is reached. During the off time  225 ′, a degassing period  222  can occur as in  FIG. 24  due to buoyancy of the gas bubbles; and the subsystem  208  resets the generator  202  to the highest frequency for another relatively slow rate of sweeping from f upper  to f lower , each time reducing contaminants. Vary the time of the degas period. Repeat this cycle during processing. 
   (c) As a function of time, change or “sweep” the power up-sweep rate at optimum values (1 Hz to 1.2 khz) of the rate, as shown in  FIG. 28 . The change in the upward sweep rate and the change in the downward sweep rate can be synchronized or they can be random with respect to one another. 
   (d) For the case where there is a degas period, as in  FIGS. 24 and 29  (i.e. the recovery period when the generator is off or unconnected while resetting from low frequency to high frequency), vary the length of the degas period  222  ( FIG. 24 ),  225 ′ ( FIG. 29 ) randomly or as a function of time such as through a linear sweep rate time function. This technique has an advantage for cases where there is one optimum power up-sweep rate (i.e., the rate of frequency change between f upper  and f lower ) and, accordingly, low frequency resonances are eliminated by changing the overall rate. In such a technique, the slope of the frequency function  220 ′ in  FIG. 29 , is constant, though the period of each degas period  225 ′ changes according to some predefined function. 
   (e) As shown in  FIG. 30 , sweep the rate with a combination of (c) and (d) techniques above. 
   Note that in each of  FIGS. 24–30 , the x axis represents time (t) and the y axis represents frequency f. 
     FIG. 31  shows a schematic  250  illustrating the most general form of generator circuitry providing both non-constant power up-sweep rate and non-constant degas period, as described above. 
   Extraction Tool Analysis 
   When evaluating one ultrasonic cleaner versus another as to its usefulness as an extraction tool, the slope between the first two points and the magnitude of the initial point are meaningful if the parts being extracted start out with identical contamination. If not, the results can be misleading. For example, consider two cleaners (e.g., tanks) that each remove 90% of the contamination on each trial. If cleaner A is tested with a part starting with 10,000 particles of contamination, point #1 will be 9,000 and point #2 will be 900. The slope is 8,100. Now if cleaner B is tested with a part starting with 1,000 particles, point #1 will be 900 and point #2 will be 90. Cleaner B thus has a slope of 810, which is ten times less than for cleaner A in removing the same percentage of contamination per run. 
   A preferred technique of the invention is to measure the slopes when the points are plotted on semi-log paper or to calculate log (count #1)−log (count #2) and compare figures between tanks. Since log (count #1)−log (count #2) equals log (count #1/count #2), a similar result is obtained if you compare the quotient of count #1 divided by count #2 for each cleaner. 
   The magnitude of the initial point does not provide significant information. However, the semi-log slope permits determining initial contamination count as long as the extraction time for each trial is short enough so the first three points are in a straight line. This line is extended back to the y-axis where x=0 to get the initial contamination count. 
   To evaluate two extraction tools, experimentation leads to a trail time that provides three points with each tool on a straight line when plotted on semi-log paper. For each tool, E for extraction is then calculated as log (count #1)−log (count #2). The tool with the largest E is the best 
   The procedure for evaluating part cleanliness may be different than for evaluating tools, such that the magnitude of point #1 is now significant. However, the technique can be similar: choose a trial time to give three points in a straight line on semi-log paper; extrapolate back to the y-axis to get the initial number of particles on the part; continue trials until the count levels off or becomes zero (minus infinity on a semi-log plot); if the count became zero, there is no erosion, therefore, add together all the particles removed and subtract this from the extrapolated initial number of particles, indicating the remaining contamination count on the part; if the count leveled off to an erosion level, calculate the remaining contamination on the part by the formula: 
           C   =       (     y   ⁢     -     ⁢     axis   intercept       )     -       ∑     i   =   1     n     ⁢     trialcount   i       +   nx           
where x=the erosion count per trial and n=the number of trials
 
   The above analysis now provides the amount of contamination initially on the part (y-axis intercept), the contamination generated by erosion (nx), and the remaining contamination (C) on the part after all the extractions. 
   The energy in each cavitation implosion is the single most important characteristic of a high intensity ultrasonic field in a liquid used for cleaning or processing delicate parts. This energy value changes with chemistry characteristics, liquid temperature, and pressure and frequency of the ultrasound. Setting the center frequency of the ultrasonic generator to specific values over a wide range is the most practical way to choose the appropriate energy in each cavitation implosion for a given process. The invention of  FIG. 32  provides this function with a single generator. 
   Specifically,  FIG. 32  shows a system  300  including a generator  302  and transducers  304  that can be switched, for example, to either 72 khz or 104 khz operation. The transducers  304  operate to inject sonic energy  305  to the process chemistry  307  within the tank  306 . Because of the impedance characteristics at these frequencies, the generator  302  includes a constant power output circuit  306  that changes the center frequency output from the generator  302  while maintaining constant output power. The circuit  306  includes a switch section  308  that switches the output frequency from one frequency to the next with no intermediate frequencies generated at the output (i.e., to the transducers  304 ). 
   A similar system  310  is shown in  FIG. 33 , where switching between frequencies does not utilize the same power circuit. In  FIG. 33 , the generator  312  includes at least two drive circuits for producing selected frequencies f 1  and f 2  (these circuits are illustratively shown as circuit (f 1 ), item  314 , and circuit (f 2 ), item  316 ). Before the reactive components in either of the circuits  314 ,  316  can be switched to different values, the output circuit  318  shuts down the generator  312  so that stored energy is used up and the relay switching occurs in a zero voltage condition. 
   From the above, one skilled in the art should appreciate that the system  310  can be made for more than two frequencies, such as for 40 khz, 72 khz and 104 khz. Such a system is advantageous in that a single transducer (element or array) can be used for each of the multiple frequencies, where, for example, its fundamental frequency is 40 khz, and its first two harmonics are 72 khz and 104 khz. 
   An alternative system is described in connection with  FIG. 61 . 
     FIG. 34  illustrates a system  400  and process probe  402  constructed according to the invention. A generator  404  connects to transducers  406  to impart ultrasonic energy  403  to the process chemistry  407  within the tank  408 . The probe  402  includes an enclosure  410  that houses a liquid  412  that is responsive to ultrasound energy within the liquid  407 . The enclosure  410  is made from a material (e.g., polypropylene) that transmits the energy  403  therethrough In response to the energy  403 , changes in or energy created from liquid  412  are sensed by the analysis subsystem  414 . By way of example, the liquid  412  can emit spectral energy or free radicals, and these characteristics can be measured by the subsystem  414 . Alternatively, the conduit  416  can communicate electrical energy that indicates the conductivity within the enclosure. This conductivity provides an indication as to the number of cavitation implosions per unit volume within the process chemistry  407 . The conduit  416  thus provides a means for monitoring the liquid  412 . A thermocouple  420  is preferably included within the enclosure  410  and/or on the enclosure  410  (i.e., in contact with the process chemistry  407 ) so as to monitor temperature changes within the enclosure  410  and/or within the process chemistry  407 . Other characteristics within the tank  408  and/or enclosure  410  can be monitored by the subsystem  414  over time so as to create time-varying functions that provide other useful information about the characteristics of the processes within the tank  408 . For example, by monitoring the conductivity and temperature over time, the amount of energy in each cavitation explosion may be deduced within the analysis subsystem  414 , which preferably is microprocessor-controlled. 
   The prior art is familiar with certain meters which measure sound characteristics and cavitations within an ultrasonic tank. Each of the meters gives one number, usually in units of watts per gallon, and sometimes in undefined units such as cavities. However, the activity in a cavitating ultrasonic tank is very complex and no single number adequately describes this activity. For example, as shown in  FIGS. 35 and 36 , it is possible to have two ultrasonic tanks  420 ,  422 , both having the same input power (i.e. watts per gallon) but each having very different ultrasonic activity characteristics. The first tank  420  might have relatively few high energy cavitation implosions  420   a  while the second tank  422  has many low energy cavitation implosions  422   a  (specifically,  FIGS. 35 and 36  show cavitation implosions  420   a ,  422   a  during a fixed time period in the two tanks  420 ,  422  having equal input energies). At least two numbers are thus necessary to describe this situation: the energy in each cavitation implosion and the cavitation density. The energy in each cavitation implosion is defined as the total energy released in calories from a single cavitation event; and the cavitation density is defined as the number of cavitation events in one cubic centimeter of volume during a 8.33 millisecond time period. Note, in Europe and other countries with fifty Hz power lines, the cavitation events in one cubic centimeter are counted over a ten millisecond time period and multiplied by 0.833. This technique provides the most accurate measurement for the common ultrasonic systems that have their amplitude modulation pattern synchronized by two times the power line frequency. 
   In most ultrasonic systems, the cavitation density also varies as a function of time. Accordingly, this is a third characteristic that should be measured when measuring ultrasonic activity in a tank. 
     FIG. 37  thus illustrates one probe  650  of the invention which permits the calculation of these important parameters. Specifically, the probe  650  measures average conductivity conductivity as a function of time, and change in temperature. 
   A characteristic of ultrasonic cavitation in aqueous solutions is the production of free radicals, ions and super oxides. These by-products of the cavitation increase the conductivity of the aqueous solution. A measure of the conductivity is thus a function of the number of cavitation implosions present in the aqueous sample, and the time variation of this conductivity is a measure of how the cavitation density varies as a function of time. 
   Another characteristic of cavitation is that it heats the aqueous solution. This is because all the energy released during each cavitation implosion becomes heat energy. By measuring the change in temperature of the aqueous sample, therefore, and by knowing its mass and specific heat, one can calculate the total energy released from the cavitation by the following formula: energy (calories) equals specific heat (no units, i.e., a ratio) times mass (grams) times the change in temperature (° C.). When the amount of energy released is known, as well as the number of cavitation implosions that released this energy, a division of the quantities gives the energy in each cavitation implosion. 
   The probe  650  is similar in operation to the probe  402  of  FIG. 34  and includes a fixed sample volume of aqueous solution  652  (or other chemistry that changes conductivity in an ultrasonic field) contained in the probe tip  650   a . The probe tip  650   a  is designed to cause minimal disturbance to the ultrasonic field (e.g., the field  403  of  FIG. 34 ). Accordingly, the probe tip  650   a  is preferably made of a material that has nearly the same acoustic impedance as the liquid being measured and that has low thermoconductivity. Polypropylene works well since it and water have nearly the same acoustic impedance. 
   The probe  650  thus includes, within the probe tip  650   a , two electrodes  654 ,  656  to measure conductivity, and a temperature measuring probe (e.g., a thermocouple)  658  to monitor the temperature of the fixed mass of aqueous solution  652 . These transducers  654 ,  656  and  658  are connected to data wires for sampling of the transducer responses. A data collection instrument (e.g., an A/D sensor interface board and a computer) connects to the wires  670  out of the probe  650  to measure temperature rise as a function of time, ΔT=g(t), and to evaluate this quantity over a specific time period t′, in seconds, i.e., ΔT=g(t′). The data collection instrument also measures the initial conductivity, C 0 , without ultrasonics, and the conductivity as a function of time, C=h(t), within the ultrasonic field. Fixed constants associated with the probe should also be stored, including the specific heat (p) of the liquid  652 , the volume (V) of the liquid  652  (in cubic centimeters), the mass (m) of the liquid  652  (in grams), and the functional relationship n=f(C,C 0 ) between conductivity and the number of cavitation implosions occurring in the probe tip  650   a  in 8.33 milliseconds determined by counting the sonoluminescence emissions over a 8.33 millisecond period and plotting this versus the conductivity measurement. The instrument then calculates the ultrasonic parameters from this information according to the following formulas: 
   (a) cavitation density=D=n/V=f(C,C 0 )/V 
   (b) energy in each cavitation implosion=E=(0.00833)(p)(m)(g(t′))/V/f(C,C 0 )/t′ 
   (c) cavitation density as a function of time=f(h(t))/V 
   These three measured parameters are then fed back to the generator to continuously control the output of the generator to optimum conditions.  FIG. 38  shows a complete system  675  for monitoring and processing data from such a probe  650 ′ and for modifying applied ultrasound energy  676  applied to the process chemistry  678 . Specifically, the system  675  monitors the parameters discussed above and, in real time, controls the generator  680  to adjust its output drive signals to the transducers  682  at the tank  684 . The data collection instrument  685  connects to the wiring  670 ′ which couples directly to the transducers within the probe tip  650 ′. The instrument  685  generates three output signal lines corresponding to measured parameters: the “A” signal line corresponds to the energy in each cavitation implosion, the “B” signal line corresponds to the cavitation density output, and the “C” signal line corresponds to the cavitation density as a function of time. These signal lines A–C are input to separate comparators  686   a ,  686   b  and  686   c . The comparators  686   a–c  are coupled to signal lines D–F, respectively, so that the input signal lines A–C are compared to user selected optimum values for each of the parameters. Typically, the user employs empirical experimentation to arrive at the optimum values for a particular tank  684  and chemistry  678 . The results from the comparators  686  are input to the control system  690 , which controls the generator  680  (those skilled in the art should appreciate that the controller  690  and generator  680  can be, and preferably are, coupled as a single unit). 
   The energy in each cavitation implosion decreases as the frequency of the ultrasonics  676  increases and as the temperature of the solution  678  increases. The energy in each cavitation implosion is measured and compared to the optimum value (set by signal lines D–F) for the process, and if the measured value has a higher energy value than the optimum value, as determined by the comparators  686 , the center frequency of the generator  680  is increased (by the controller  690  receiving data at the “center frequency input control”) until the values are equal. If there is not enough range in the center frequency adjustment to reach the optimum value, then the temperature of the solution  678  is increased by the control system  690  until the optimum value is reached. An alternative is to utilize a switchable frequency generator, as described above, so as to change the drive frequency to one where the energy in each cavitation implosion is not greater than the optimum value, and without changing the solution temperature. 
   The cavitation density increases as the ultrasonic power into the tank  684  increases. Therefore, the cavitation density measurement fed back to the generator  680  is compared against the optimum value of cavitation density for the process; and if the measured value is lower than the optimum value, the generator output power is increased (by the controller  690  receiving data at the “power control”) until the two values are equal. If the measured value is greater than the optimum value, the generator output power is decreased until the values are equal. 
   Cavitation density as a function of time is controlled by the amplitude modulation (AM) pattern of the generator output  692 . Therefore the measured cavitation density as a function of time is measured and the generator&#39;s AM pattern is adjusted (via the controller  690  receiving data at the “AM Control”) until the measured function equals the optimum function. 
     FIG. 39–41  illustrate separate embodiments of universal voltage input ultrasonic generators, in accord with the invention. These embodiments are made to solve the present day problems associated with separate designs made for countries with differing power requirements (in volts A-C, or “VAC”), such as: 
   
     
       
         
             
             
           
             
                 
             
           
          
             
               100 VAC 
               Japan, and intermittently during brown-outs in the U.S. 
             
             
               120 VAC 
               U.S. 
             
             
               200 VAC 
               Japan 
             
             
               208 VAC 
               U.S. 
             
             
               220 VAC 
               Most of Europe except Scandinavia and U.K. 
             
             
               240 VAC 
               U.S., U.K., Norway, Sweden and Denmark 
             
             
               “Z” VAC 
               Corresponding to unusual voltages found in France and other 
             
             
                 
               world locations 
             
             
                 
             
          
         
       
     
   
   These voltages are obviously problematic for industry suppliers of ultrasonic generators, who must supply the world markets. The invention of  FIGS. 39–41  eliminates the chance that a particular world consumer receives an incorrect generator by providing universal voltage generators that operate, for example, between 86 VAC and 264 VAC. 
   In  FIG. 39 , an ultrasonic generator  500  is shown connected to a 300 VDC source  501 . A power factor correction (PFC) circuit  502  connects to the front end of the generator  500  to produce a regulated 300 VDC. A switching regulator  504  regulates the 300 VDC to +12V and +15V. The generator  500  can be represented, for example, as the circuit of  FIG. 31 , except that the “high voltage supply” is replaced by the PFC circuit  502  and the +12V and +15V are replaced with control voltages from the regulator  504 . 
     FIG. 40  illustrates a generator  510  connected to a universal input switching regulator  512 . The regulator  512  generates a set  513  of DC voltages for the generator  510 . The generator  510  includes circuitry  514  that operates with the set  513 . The generator  510  can be represented, for example, as the circuit of  FIG. 31 , except that the “high voltage supply” and the +12V and +15V are replaced with output voltages from the regulator  512 . 
   Those skilled in the art should appreciate that methods and systems exist for utilizing the power line to acquire amplitude control for ultrasonic generators. By way of example, the inventor of this application describes such systems and methods in connection with  FIGS. 3 ,  4 ,  5 A,  5 B and  7  of International Application No. PCT/US97/12853. Specifically, an amplitude control subsystem is achieved by rectifying the AC power line and selecting a portion of the rectified line voltage that ends at the desired amplitude (such as between zero and 90° or between 180° and 270° of the signal). In this manner, amplitude modulation is selectable in a controlled manner as applied to the signal driving the transducers from the generator. For example, by selecting the maximum amplitude of 90° in the first quarter sinusoid, and 270° in the third quarter sinusoid, a maximum amplitude signal is provided. Similarly, a one-half amplitude signal is generated by choosing the 30° and 210° locations of the same sinusoids. By way of a further example, a one-third amplitude signal is generated by choosing 19.5° and 199.5°, respectively, of the same sinusoids. 
     FIG. 41  illustrates a generator  530  which operates at a DC voltage less than or equal to (86)(√{square root over ( )}2) volts. As in amplitude control, a triac  532  is used to select that portion of the power line voltage with an amplitude equal to the generator DC voltage requirements. The signal  534  is rectified and filtered by the bridge rectifier and filter  536  to obtain the constant DC voltage  538  in the range less than or equal to (86)(√{square root over ( )}2) volts. The generator  530  can be represented, for example, as the circuit of  FIG. 31 , except that the “high voltage supply” is replaced by the voltage from the bridge rectifier and filter  536  and the +12V and +15V are replaced with output voltages from the regulator  540 , as above. 
   In another embodiment, the selected AC voltage angle can be reduced to lower the DC voltage to reduce the amplitude of the ultrasonic drive signal. 
   The “power up sweep” features of the invention also apply to amplitude modulation, where an AM pattern of the AM frequency varies according to the power up-sweep techniques discussed above, and preferably at the same time with the techniques of “sweep the sweep rate”, as discussed herein. With power up-sweep AM, the AM pattern modulation creates an additional upward force on contamination while eliminating low frequency resonances. 
     FIG. 42  illustrates an AM (amplitude modulation) pattern  600  of the invention, where the frequency of the AM is constantly decreasing with increasing time t More particularly, ultrasonic bursts of energy (as shown in  FIG. 43 , with a frequency f) are contained within each of the non-zero portions  600   a  of the pattern  600 . As time increases, longer and longer bursts of energy are applied to the associated transducers. In the optimum case, the ultrasound frequency within each burst of  FIG. 43  varies with a power up sweep, from f upper  to f lower , as discussed above. 
     FIG. 44  shows a plot  610  of AM frequency verses time t. As shown, the AM frequency monotonicly changes from a high frequency, f high , to a low frequency, f low . When f low  is reached, a degas or quiet period  612  is typically introduced before the cycle  614  repeats. 
   Note that the sweep rate of the change of the AM frequency along the slope  616  can and preferably does change at a non constant sweep rate. The rate of AM frequency change can thus be non-constant. The degas period  612  can also be non constant. The degas period  612  can also be substantially “0”, so that no time is permitted for degas. 
   Generally, there are three ways to change the AM frequency. The burst length “L” ( FIG. 43 ) can be changed, the time between bursts can be changed (e.g., the periods  600   b ,  FIG. 42 , where the amplitude is zero); or both parameters can be changed simultaneously. 
     FIGS. 45A–45C  schematically illustrate electronics for one ultrasonic generator with AM power up-sweep capability, in accord with the invention. 
     FIG. 46  illustrates a prior art laminar tank  700 . Contamination within the tank  700  is a problem in critical cleaning operations because the contamination can redeposit on the part  701  under process. A common way to remove contamination from the cleaning solution  702  of the tank  700  is to build the tank  700  with overflow weirs  704  and to constantly add pure solution, or recirculate filtered solution, into the bottom of the tank at a solution inlet  706 . The solution injected through the inlet  706  travels through the tank volume and out over the overflow weirs  704 . Solution which overflows the weirs  704  exits through outlets  705  for disposal or filtering. 
   The problem with cleaning the solution  702  in this manner is that the cleaning time is excessive because there is mixing of pure or filtered solution with contaminated solution while solution passes through the volume of the tank  700 . The mixing causes a dilution of the contaminated solution by the pure or filtered solution. The result is that diluted solution overflows the weirs  704 ; and the contamination within the tank  700  is eliminated logarithmically rather than linearly. Logarithmic elimination theoretically takes an infinite amount of time to reach zero, whereas linear elimination has a theoretical finite time when the tank becomes contamination free. 
   The tank  720  of  FIG. 47 , constructed according to the invention, thus includes features which significantly reduce the afore-mentioned problems. Specifically, the tank  720  operates such that the solution  702 ′ in the tank  720  moves in a piston like fashion from the bottom  720   a  to the top  700   b  of the tank  700 , resulting in little or no mixing of contaminated solution with the new or filtered solution. Near linear removal of the contamination within the tank  700  results, providing for rapid clean up. 
   The tank  720  has a number of baffles that: reduce the velocity of the clean solution; equalize the pressure of the clean solution; and introduce the solution into the tank  720  with even distribution at the bottom  720   a  of the tank  720 . The first baffle  722  reduces the velocity of the solution injected through the inlet  706 ′. The second baffle  724  evenly distributes the solution at the bottom of the tank  720   a . Baffle  724  has a plate  726  with a large number of small holes  728  cut therethrough to give a minimum of 45% open area so that the pressure across any hole is minimized. 
   The combination of the baffles  722  and  724  operate to provide smooth movement of contaminated solution upwards and over the wirers  704 ′. The tank  720  thus augments, or provides an alternative to, the power up-sweep features discussed above. 
   The design of the tank  720  also benefits from alternative placement of the ultrasonic transducers  730  mounted with the tank. As illustrated, the transducers  730  are mounted to the sides  720   s  of the tank decreasing the disruption which might otherwise occur from bottom-mounted transducers interfering with the solution flow through the baffles  722 ,  724 . 
   A common feature in prior art tanks (ultrasonic and non-ultrasonic) is a quick dump rinse feature (QDR) where a large valve in the bottom of the tank opens to allow the solution in the tank to quickly drain out of the tank. This QDR feature reduces the contamination residing on the parts under process as compared to the contamination that would reside if the liquid were removed more slowly from the tank, or if the parts were pulled out of the tank. 
     FIG. 48  illustrates a QDR tank  800  modified in accord with the invention to speed up the rate of liquid removal from the tank. The large valve output  802  is connected to a vacuum reservoir  806  that is evacuated to a pressure below atmospheric pressure during the cleaning cycle. When the valve  802  is opened to dump the liquid  702 ″, the difference between atmospheric pressure and the pressure in the vacuum vessel  806  forces the liquid  702 ″ out of the tank  800 , thus shortening the drain time and further reducing the residual contamination. 
   The conventional stacked transducer consists of a front driver, active piezoelectric elements and a back mass. The length “L” of the transducer (from front plate to backplate) basically determines the transducer&#39;s primary and harmonic frequencies. As the fundamental frequency of the transducer becomes higher, the thickness of each of the transducer elements is reduced until they become impractical.  FIG. 49  shows a transducer  850  constructed according to the invention which reduces this impracticality. 
   In  FIG. 49 , the transducer  850  is shown connected to an ultrasound processing tank  852 , which holds process chemistry  854 . The transducer includes two piezoelectric elements  856  that are compressed between the backplate  858  and the tank  852 . Specifically, a bias bolt  860  connects through the transducer  850  and connects directly into a weld  861  at the tank  852 . Accordingly, there is no front plate; and thus the transducer length “L” can be divided between the piezoelectric elements  856  and the back mass  858 . This division makes it possible to make a stacked transducer  850  with a higher fundamental frequency (and higher harmonics too). 
   Most transducers discussed herein are longitudinal vibrators with elements sandwiched by a center bolt that holds the transducer assembly together and that provides a compressive bias to the active piezoelectric components (i.e., sandwiched between the a front plate and back mass or backplate). Since piezoelectric ceramic is strong under compression, but weak in tension, the constant compressive force provided by the spring constant of the bolt greatly improves the reliability of this transducer over other configurations. 
   The longitudinal vibrating transducer is normally connected to the tank or other surface that is to receive the sound energy by epoxy or brazing, or by a mechanical stud, or by a combination of these schemes. 
   The invention of  FIG. 50  illustrates a transducer  900  constructed according to the invention and shown in an exploded view. The transducer  900  has “double compression”, as discussed below, to increase its reliability over the prior art. Specifically, the bias bolt  904  has a through-hole  902  in its center. The center hole  902  receives a second bolt  906  that is welded to the surface of the tank  908  (illustrated by weld joint  927 ). When integrated, the second bolt  906  protrudes out past the tail mass  910  (i.e., the backplate) of the transducer  900  by way of a Belleville disc spring washer  912  and nut  914 , which screws onto bolt  906 . 
   As in other transducers herein, the transducer  900  includes piezoelectric ceramics  916 , associated electrodes  918 , and lead-outs  920  for the electrodes  918 . 
   The bias bolt  904  thus provides the first compressive force similar to other transducers herein. That is, the bolt  904  slides through the front driver  922  via the through-hole  924 , and continues on through the ceramics  916 . The back mass  910  has threads  910   a  which mate with the bolt  904 ; and thus the bolt  904  screws into the back mass  910 . By tightening the bolt  904  into the back mass  910 , the bolt  904  firmly seats into the counter-sink  922   a  of the front plate  922  and compression is applied to the ceramics  916 . 
   As an alternative, the threads in the back mass  910  can be thru-holed; and a nut against the back mass can replace the threads to support compression bias on the piezoceramic  916 . 
   The second compressive force derives from the operation of the second bolt  906 , which compresses the epoxy  926  after seating within the counter-sink  904   a  of the first bolt  904  and after tightening the nut  914  onto the bolt  906 . The front driver  922  is then bonded to the tank  908  via an epoxy layer  926 . The second compressive force keeps a compressive bias on the epoxy  926  bond between the front driver  922  and the tank surface  908 . 
   As an alternative, it is possible to eliminate the Belleville disc spring washer  912  and rely entirely on the spring tension in the second bolt  906 ; but the added feature of the Belleville disc spring washer  912  provides a larger displacement before tension goes to zero. 
   The second compressive bias of transducer  900  provides at least three improvements over the prior art. First, during the epoxy curing process, the bias keeps force on the epoxy bond  926  (even if the epoxy layer thickness changes during a liquid state) resulting in a superior bond. Second, during operation of the transducer  900 , the reliability of the bond  926  is enhanced because of the constant mechanical compressive force. That is, epoxy bonds are weakest in shear forces, and reasonably strong in tension but superior in compression. Third, during abnormal conditions (e.g., a mechanical jar to the bonding surface) that might dislodge a conventionally bonded transducer, the second compression force with its spring characteristics absorbs the mechanical shock and protects the epoxy bond. 
   Those skilled in the art should appreciate that the double compression transducer  900  provides increased reliability when mounted with most any surface, and not simply an ultrasonic tank  908 . 
     FIG. 51  shows a cross-sectional view of a conventional stacked transducer  1000  with a bias bolt  1002  that screws into threads  1004  in the aluminum front driver  1006 . The threads  1004  are only within the top portion  1006   a  of the front driver  1006 . The transducer includes the normal piezo-ceramics  1007 , electrodes  1008 , and rear mass  1009 . 
     FIG. 52  shows an alternative transducer  1010  constructed according to the invention. In transducer  1010 , the threads  1012  within the front driver  1014  are at bottom portion  1014   a  so that bias pressure is not concentrated on the top threads (as in  FIG. 51 ) where the surface of the aluminum can be deformed in operation, decreasing bias pressure. The elements  1002 ′,  1007 ′,  1008 ′ and  1009 ′ have similar function as in  FIG. 51 ; except that they are sized and shaped appropriately to accommodate the thread repositioning at the bottom  1014   a  of the driver  1014 . 
     FIG. 53  illustrates a transducer  1020  that is similar to the transducer  1010 ,  FIG. 52 , except that a helical insert  1022  is used instead of the threads  1012 . The helical insert  1022  is preferably made from steel and will not plastically deform under normal transducer stresses. The helical insert  1022  thus prevents distortion of the aluminum driver  1014 ′ under the normal stresses of the transducer  1020 . Note that the a helical insert can similarly replace the threads  1004  of the prior art transducer  1000  to provide similar advantages in preventing distortion. 
     FIG. 54  illustrates a side view of one embodiment of the invention including a  5  printed circuit board (PCB)  1030  connected with ultrasonic transducers  1032  such as described herein (including, for example, piezoelectric ceramics  1034 ). The PCB  1030  contains circuitry and wiring so as to function as an ultrasonic generator and for the electrodes of the transducers  1032 . As such, the PCB  1030  can drive the transducers  1032  to produce ultrasound  1036  when powered. By way of example, the PCB  1030  can include the circuitry of  FIG. 31 . 
   The PCB  1030  and transducers  1032  are also substantially “integral” in construction so as to be a single unit. This provides structural integrity, and reduces the cost and size of the system. 
     FIG. 55  shows a top view of the PCB  1030  of  FIG. 54 . For purposes of illustration, the top surface  1030   a  of the PCB  1030  is shown with electrodes  1038  for the positive side of the piezoelectric ceramic  1034 . The electrodes  1038  are preferably connected by wiring  1048  (e.g., circuit board land patterns) to provide for common voltage input to the transducers  1032 . There is a similar electrode pattern on the bottom side (not shown) of the PCB  1030  that makes contact with the transducer&#39;s front driver  1032   b , which is in electrical contact with the bias bolt  1032   a  ( FIG. 54 ). The bolt  1032   a  connects through the transducer  1032  and into the back mass  1032   c , providing electrical feedthrough to the negative electrode of the piezoelectric ceramic  1034 . The PCB  1030  thus provides two electrodes for each transducer  1032  and all the interconnect wiring for the transducers  1032  such as by etching the PCB pattern. The ultrasonic generator is also provided with the PCB  1030  circuitry (illustrated by circuit board components  1040 ) with its output connected into the transducer electrodes as part of the PCB artwork. 
     FIG. 56  illustrates an acid resistant transducer  1050  with internal piezoelectric compression. By way of background, the above description has described certain transducers that utilize metal masses to lower the resonant frequency of the piezoelectric ceramics and a bolt to keep a compressive bias on the piezoelectric elements. In harsh environments, e.g., sulfuric acid process tanks, the metallic elements of the transducer are prone to acid attack and therefore are a reliability risk. The transducer  1050  of  FIG. 56  resolves this problem by eliminating the metal masses and the bolt. The compressive force on the piezoelectric ceramic  1058  is obtained by an epoxy  1052  that contracts upon curing. The metal “back mass” and the metal “front driver” such as described above are replaced by a non-metallic material  1060 . In  FIG. 56 , the front driver  1060   a  and back mass  1060   b  are thus both made from a non-metallic material such as quartz. 
   The internal piezoceramics  1058  connect to wiring to drive the elements  1058  in the normal way. To protect the wiring and ceramics, it can be made from Teflon which is soldered to the ceramic  1058  by known methods, such as illustrated by solder joint  1064 . 
     FIG. 57  illustrates a generator circuit  2000  used to implement power up-sweep such as described herein (e.g., such as described in connection with  FIG. 31 , except that  FIG. 31  uses IGBT&#39;s as the switching devices and  FIG. 57  uses MOSFET&#39;s). In  FIG. 57 , circuit  2000  includes a capacitive element  2012  with terminal  2012   a  connected to earth ground  2015   a  The other terminal  2012   b  connects to terminal  2040   b  of inductor  2040 . Terminal  2040   a  of inductor  2040  connects to terminal  2013   a  of the secondary  2013   c  of transformer  2013 . Terminal  2013   b  of secondary  2013   c  connects to earth ground  2015   b . The circuit  2000  includes two drive networks  2018  and  2020 , and a controller  2022 . 
   Drive network  2018  includes a blocking network  2028  and a multi-state power switch network  2030 , which is coupled to the controller  2022  by way of line  2022   a . The drive network  2020  includes a blocking network  2032  and a multi-state power switch network  2034 , which is coupled to the controller  2022  by way of line  2022   b.    
   In drive network  2018 , the blocking network  2028  and switch network  2030  provide a unidirectional current flow path characterized by a first impedance from the potential +V through the first primary winding  2013   d   1  of center-tapped primary winding  2013   d  of transformer  2013  when the switch network  2030  is in a first (conductive) state. The networks  2028  and  2030  provide an oppositely directed current flow path characterized by a second impedance from circuit ground  2023   a  through  2013   d   1  to the potential +V when the switch network  2030  is in a second (non-conductive) state. The first impedance of the flow path established when network  2030  is in its first state is lower than the second impedance of the flow path established when the network  2030  is in its second state. 
   In drive network  2020 , the blocking network  2032  and switch network  2034  provide a unidirectional current flow path characterized by a third impedance from the potential +V through the second primary winding  2013   d   2  of center-tapped primary winding  2013   d  of transformer  2013  when the switch network  2032  is in a first (conductive) state. The networks  2032  and  2034  provide an oppositely directed current flow path characterized by a fourth impedance from circuit ground  2023   b  through  2013   d   2  to the potential +V when the switch network  2034  is in a second (non-conductive) state. The third impedance of the flow path established when network  2034  is in its first state is lower than the fourth impedance of the flow path established when the network  2030  is in its second state. 
   The impedance (Z) of drive network  2018  when switch network  2030  is in its second state may be primarily determined by resistor  2028   b  (of value “R”), in which case Z has a value substantially equal to R for current flow in a direction toward +V, and a “near-infinity” value (i.e. relatively high) for current flow away from +V. In other embodiments, Z may be non-linear, normally lower at the beginning of operation in the second state and higher at times after the second state begins. For example, a metal oxide varistor (MOV) in parallel with a resistor (R) may be the primary determining factor for Z. In this case, at the beginning of operation in the second state when the voltage across Z is high, the low impedance of the on MOV primarily determines Z and later in the second state, as the voltage drops below the MOV&#39;s breakdown potential, Z is primarily determined by R. 
   A similar situation occurs for the impedance of drive network  2020  when switch network  2034  is in its second state. 
   Where the circuit  2000  is adapted to drive an ultrasonic transducer, the capacitive element  2012  may be an electrostrictive device suitable for use as an ultrasonic transducer. With such a configuration, for example, the controller  2022  may effectively control the circuit  2000  to drive such ultrasonic transducers at a selectively controlled frequency. In various forms of the invention, the controller  2022  may be adaptively controlled so as to track variations in the resonant frequency for the respective ultrasonic transducers, or to frequency modulate the frequency with a function such as a power up-sweep function, described above. 
   In operation, the controller  2022  cyclically switches the switch network  2030  between its first and second states at a frequency f (f=1/T), where f is less than or equal to f r  (f r =1/T r ), where f r  is the resonant frequency of the series LC network formed by  2012  and  2040 , approximately equal to 1/(2π(LC)^ 1/2 ). During each cycle, network  2030  is controlled to be in its first state for a period greater than or equal to T i /2, but less than or equal to T/2, at the beginning of each cycle. Network  2030  is controlled to be in its second state for the remainder of each cycle. 
   Similarly, the controller  2022  also cyclically switches the switch network  2032  between its first and second states at the frequency f (f=1/T). During each cycle, network  2032  is controlled to be in its first state for a period greater than or equal to T i /2, but less than or equal to T/2, at the beginning of each cycle. Network  2032  is controlled to be in its second state for the remainder of each cycle. In the presently described embodiment, the start time for each cycle of the switching of network  2030  is offset by T/2 from the start time for each cycle of the switching of network  2034 . In other forms, the start time for the cycle of the switching network  2030  may be offset by at least T r /2 and less than T r /2+D, where D equals T−T r . 
   An AC voltage waveform (V 0 ) at frequency f is impressed across the capacitive element  2012 . Generally, this voltage waveform V 0  passes from low to high and from high to low with a sinusoidal waveshape (at frequency f r ). After rising from its low peak level to its high peak level, the voltage waveform stays substantially at its high peak level (except for droop due to resistive losses) for a period ½ (T−T r ), or D/2, before passing from that high peak level to its low peak level. Similarly, upon returning to the low peak level, the voltage waveform V 0  remains at that level (except for droop due to resistive losses) for a period ½ (T−T r ), or D/2, before again passing to the high peak level. 
   Thus, the voltage impressed across capacitive element  2012  rises and falls at the resonant frequency f r  with the capacitive element  2012  being maintained in its fully charged state for a “dead” time which is adjustably dependent upon the switching frequency f of the controller  2022 . Accordingly, the drive frequency to the element  2012  may be adjustably controlled. 
   Where the element  2012  is an ultrasonic transducer, circuit  2000  is used to drive that transducer at a frequency adjusted to match the optimal drive frequency. In various embodiments, variations in that optimal drive frequency may be detected and the controller may be adjusted in closed loop fashion to adaptively track such variations. 
   Blocking network  2028  includes a diode  2028   a  in parallel with a resistor  2028   b , and the blocking network  2032  includes a diode  2032   a  and a resistor  2032   b . The single inductor (L)  2040  operates in resonance with the element  2012 . 
   Circuit  2000  is particularly useful with “fast” switching devices (such as bipolar, MOS and IGBT transistors) which do not require an extended turnoff time. In operation, the capacitive element  2012  and transformer  2013  function like the circuit of  FIG. 31 , except that circuit  2000  utilizes FETs instead of IGBTs (insulated gate bipolar transistors) for the terminal power switching devices. The power devices  2030 ,  2034  are also connected to circuit ground, eliminating the need for separate isolated power supplies, reducing the cost of the generator. 
   In another implementation of circuit  2000 ,  FIG. 57 , the inductor  2040  is not a separate component, but rather is incorporated into the transformer  2013  by way of leakage inductance. This leakage inductance performs the same function as inductor  2040  and the leakage inductance is controlled by the coupling of transformer  2013 , e.g., by setting a gap in the transformer&#39;s core as is known in the art. 
   With further reference to  FIG. 33 , one embodiment of the invention couples multiple generator frequencies to a common tank  306 ′ and transducers  304 ′.  FIG. 58  schematically shows additional switch circuitry  2098  compatible with this embodiment In  FIG. 58 , a common 24 VDC supply  2100  couples to a user-selectable switch  2102  (e.g., a rotary switch) to provide drive energy to remote connectors  2104   a–d  (each connector  2104  corresponding and connecting to a different generator frequency, e.g.,  2104   a  for 40 khz,  2104   b  for 72 khz,  2104   c  for 104 khz, and  2104   d  for 170 khz). Which ever generator thus connects to the 24 VDC supply between pins “1” and “2” on its corresponding remote connector  2104  will drive the common process tank, as shown in  FIG. 59 . The generators can have a remote on/off relay in the form of  FIG. 60 , which illustrates coupling between a Deltrol relay and the remove relay. The connector-to-tank wiring is further illustrated in  FIG. 59 . In  FIG. 59 , each generator within the system connects to each of the plurality of transducers  2106  within the tank; though only one generator actively drives the transducers  2106  depending upon the position of the switch  2102 . 
   In operation, power is applied to one generator (e.g., the 40 khz generator coupled to remote connector  2104   a ) via the 24VDC signal from the rotary switch  2102 . The following sequence then occurs with respect to  FIGS. 58–60 : 
   
     
       
         
             
             
             
           
             
                 
                 
             
             
                 
               Time 
               Event 
             
             
                 
                 
             
           
          
             
                 
                 7 milliseconds 
               Remote relay #1 energizes starting the 1/2 
             
             
                 
                 
               sec. timer #1 
             
             
                 
                10 milliseconds 
               Deltrol relay #1 connects the tank to the 
             
             
                 
                 
               40 khz generator 
             
             
                 
               0.5 seconds 
               1/2 sec. timer #1 starts the 40 khz generator, 
             
             
                 
                 
               the tank runs at 40 khz 
             
             
                 
                 
             
          
         
       
     
   
   If the rotary switch  2102  is turned to the next position, e.g., to the 72 khz generator position, the following sequence occurs (assuming, worst case, that the rotary switch is moved very fast so there is zero time between the 40 khz position and the 72 khz position): 
   
     
       
         
             
             
             
           
             
                 
                 
             
             
                 
               Time 
               Event 
             
             
                 
                 
             
           
          
             
                 
                 0 milliseconds 
               24VDC is removed from remote relay #1 
             
             
                 
                 0 milliseconds 
               24VDC is removed from Deltrol relay #1 
             
             
                 
                 5 milliseconds 
               40 khz generator turns off 
             
             
                 
                 7 milliseconds 
               72 khz remote relay #2 energizes starting the 
             
             
                 
                 
               1/2 sec. timer #2 
             
             
                 
                 10 milliseconds 
               Deltrol relay #2 connects tank to 72 khz 
             
             
                 
                 
               generator 
             
             
                 
                250 milliseconds 
               Deltrol relay #1 disconnects 40 khz 
             
             
                 
                 
               generator from the tank 
             
             
                 
                0.5 seconds 
               1/2 sec. timer #2 starts the 72 khz generator, 
             
             
                 
                 
               the tank runs at 72 khz 
             
             
                 
                 
             
          
         
       
     
   
   To avoid this “worst case” scenario, extra margin is provided by providing an off position between each rotary switch generator position. That is, the rotary switch can be labeled as follows:
 
OFF-40 khz-OFF-72 khz-OFF-104 khz-OFF-170 khz
 
   Generators connected within this system preferably have a four socket reverse sex square flange AMP CPC receptacle with arrangement 11-4 (AMP part number 206430-1) installed on the rear of the generator. The mating four pin plug (AMP part number 206429-1) has the following pin connections: 
   
     
       
         
             
             
             
           
             
                 
                 
             
           
          
             
                 
               Pin#1 
               +24 VDC referenced to Pin #2 connects the 
             
             
                 
                 
               generator or power module to the 
             
             
                 
                 
               transducers and turns the generator on 
             
             
                 
               Pin#2 
               return for 24 VDC signal, can be grounded 
             
             
                 
               Pin#3 
               anode of LED to indicate RF current flow 
             
             
                 
               Pin#4 
               cathode of LED to indicate RF current flow 
             
             
                 
                 
             
          
         
       
     
   
   The cable from the AMP plug is for example a Manhattan/Cot PIN M39025 control cable with four #24 AWG wires, with the following color codes: Pin#1 red; Pin#2 green; Pin#3 blue; and Pin#4 white. 
   Generators within this system can have a nine socket reverse sex square flange AMP CPC receptacle with arrangement 17-9 (AMP part number 211769-1) installed on the rear of the generator according to the following connections.
         Socket #1: +RF output   Socket #2: not used   Socket #3: +RF output   Socket #4: −DC test point   Socket #5: −RF output, ground   Socket #6: cable shield, ground   Socket #7: +DC output interlock   Socket #8: +DC input interlock   Socket #9: waveform test point       

   The mating nine pin plug (AMP part number 211768-1) can have the following pin outs and color code when supplied with a three wire RF cable.
     Pin#1: +RF output red   Pin#3: +RF output red   Pin#5: −RF output green/yellow   

   All pin#5s can for example be wired together and connected to the −RF transducer lead. All pin #1&#39;s are then connected together and connected to the +RF transducer lead coming from one-half of the transducers. All pin #3&#39;s are then connected together to the +RF transducer lead coming from the other one-half of the transducers. The only exception to this is when the generators do not all drive the same number of transducers. 
     FIG. 61  schematically shows a multi-generator system  3000  used to drive common transducers  3002 . One advantage of the system  3000  is that multiple generators  3004  can alternatively drive the transducer  3002 ; and it is assured that no two generators operate simultaneously. Each generator  3004  preferably represents a different drive frequency. Generator  3004   a  represents, for example, the generator set forth by circuitry of  FIG. 31  (except that preferably, a ½ second delay is installed into circuit  250  by adjusting capacitor  3006  to one microfarad instead of 1/10 microfarad, which provides only 50 ms delay). The relays  3008   a ,  3008   b  for example can be implemented similar to the relay schematic of  FIG. 60 . 
   The rotary switch  3010  (e.g., similar to the switch  2102 ,  FIG. 58 ) permits user selection between any of the generators  3004 . Generator  3004   b  can thus be switched in to drive the transducer  3002  with a different frequency. Those skilled in the art should appreciate that additional generators  3004   c ,  3004   d , . . . can be installed into the system  3000  as desired, with additional frequencies. Those skilled in the art should appreciate that the rotary switch  3010  can be replaced by a PLC or computer control to provide similar generator selection. 
   The invention thus attains the objects set forth above, among those apparent in the preceding description. Since certain changes may be made in the above description 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 be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are to cover all generic and specific features of the invention described herein, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween. 
   In an ultrasonic or microsonic cleaning or processing liquid, it is known that a particular frequency or a set of closely spaced frequencies will resonate a certain size population of bubbles or voids within the liquid. A conventional sweeping frequency ultrasonic or microsonic cleaning or processing signal produces a particular frequency or a set of closely spaced frequencies followed by the next particular frequency or set of closely spaced frequencies adjacent to the first particular frequency or set of closely spaced frequencies. 
   Unfortunately, cavitation efficiency suffers with this type of conventional sweeping frequency ultrasonic or microsonic cleaning or processing signal because the first particular frequency or set of closely spaced frequencies depletes members of that certain size population of bubbles or voids within the liquid leaving a smaller population for the second adjacent particular frequency or set of closely spaced frequencies to resonate. 
   There is shown in  FIG. 62 , a sweeping frequency drive signal  3100  that overcomes the above-described cavitation efficiency limitation of the prior art. When a certain size population of bubbles or voids within the liquid begins to be depleted causing a loss in cavitation efficiency, drive signal  3100  jumps, changes or rapidly sweeps to a non adjacent frequency within the bandwidth of the transducer array, such that the process continues with improved cavitation allowed by the new bubble population associated with this new non adjacent particular frequency or set of closely spaced frequencies. 
   In a preferred embodiment, drive signal  3100  can be maintained in the upper half of a bandwidth. The bandwidth is typically 10% of the center frequency (unless the system employs a special design / procedure, e.g., overlapping transducers frequency ranges). Therefore, for a center frequency at the high end of the microsonic frequency range (350 kHz), the bandwidth is typically 35 kHz. For 40 kHz ultrasonic transducers, the bandwidth is typically about 4 kHz. After a defined period of time (i.e., before cavitation efficiency suffers) at point  3102 , the frequency is changed to a new frequency that is typically one half bandwidth lower than the current frequency. This change in frequency may occur by sweeping the frequency to the new lower frequency (not shown; wherein the sweep time is typically less than 25% of the defined period of time), or stepping the frequency to the new lower frequency, as shown in  FIG. 62 . The length of this “defined period of time” is dependent on the frequency, power density, sweep rate, type of chemistry and chemistry conditions such as temperature. “Defined periods of time” vary inversely with respect to frequency and span the range from ten microseconds to two milliseconds. At point  3104 , this sweeping frequency continues from this new lower frequency. After the defined period of time (described above) at point  3106 , the frequency jumps to a new higher frequency (point  3108 ) that is typically one half bandwidth higher than the current frequency. 
   While a one half bandwidth frequency jump is typical, other amounts are possible. For example, the frequency may be jumped by a much larger percentage of the bandwidth, e.g., to a frequency proximate the lower limit of the bandwidth, such as point  3109 . 
   Further, while the system is described above as sweeping the frequency between points  3104  and  3106 , other configurations are possible. For example, the frequency maybe maintained constant (not shown) during the defined period of time. Alternatively, the frequency may be changed (between points  3104  and  3106 ) via one or more frequency steps (shown in phantom); or the set of closely spaced frequencies between points  3104  and  3106  may be random frequencies (not shown). 
   This frequency sweeping and frequency jumping continues until striking the lowest frequency in the bandwidth (at point  3110 ). At this point, the frequency jumps to the highest frequency in the bandwidth (to point  3112 ), and the sweeping and jumping process is repeated until the lowest frequency in the bandwidth is reached again (not shown). This high cavitation efficiency process is repeated and continued for the time needed in that particular bandwidth. 
   If a multiple generator system is driving a transducer array with a set of defined bandwidths (e.g., multiple harmonic bandwidths), then after the time needed in a particular bandwidth has elapsed, the drive signal from a different generator may produce a similar high cavitation efficiency signal in a different bandwidth.