Abstract:
An AC switch is created by switching devices to modify the output of an ultrasonic generator. The AC switch introduces a modification circuitry into and out of the output stage of the ultrasonic generator. The AC switch is placed in parallel with the modification circuitry when inserting the modification circuitry into a conduction line of the ultrasonic generator. It is placed in series when inserting the modification circuitry between two nodes of the ultrasonic generator. A control circuit is associated with the AC switch to turn on and off the ultrasonic generator, overcoming the inability of triacs to turn off power when conducting ultrasonic current. The introduction of the modification circuitry by the AC switch allows the modification of the frequency, amplitude, power, impedance and waveform of an ultrasonic generator.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The following U.S. Patents and pending U.S. Patent Applications are related to the present application, and hereby incorporated by reference: 
     U.S. application Ser. No. 08/718,945 filed Sep. 24, 1996, entitled “Apparatus and Methods for Cleaning and/or Processing Delicate Parts”, which claims priority of Provisional Application Ser. No. 60/023,150 filed Aug. 5, 1996, entitled “Apparatus and Methods for Processing and Cleaning Semiconductor Wafers and Other Delicate Parts”. U.S. application Ser. No. 08/718,945 issued in Nov. 10, 1998 as U.S. Pat. No. 5,834,871. 
     U.S. application Ser. No. 09/066,171 filed Apr. 24, 1998, entitled “Apparatus and Methods for Cleaning and/or Processing Delicate Parts”, which is a continuation of U.S. Pat. No. 5,834,871 and is issued in Dec. 14, 1999 as U.S. Pat. No. 6,002,195. 
     U.S. application Ser. No. 09/097,374, filed Jun. 15, 1998, entitled “Systems and Methods for Ultrasonically Processing Delicate Parts”, issued on Jan. 25, 2000 as U.S. Pat. No. 6,016,821, claiming priority to U.S. Provisional Patent Application Ser. No. 60/049,717 filed on Jun. 16, 1997, and entitled “Systems and Methods for Ultrasonically Processing Delicate Parts”. U.S. application Ser. No. 09/097,374 is also a continuation-in-part of U.S. application Ser. No. 08/718,945, filed on Sep. 24, 1996, entitled “Apparatus and Methods for Cleaning and/or Processing Delicate Parts”, which issued on Nov. 10, 1998 as U.S. Pat. No. 5,834,871. 
     U.S. application Ser. No. 09/066,158 filed Apr. 24, 1998, entitled “Apparatus and Methods for Cleaning and/or Processing Delicate Parts”, which is a continuation-in-part of U.S. Pat. No. 5,834,871 and is issued in Jan. 30, 2001 as U.S. Pat. No. 6,181,051 B1 U.S. application Ser. No. 09/066,158 also claims priority to U.S. Provisional application 60/023,150. 
     U.S. application Ser. No. 09/370,302, filed Aug. 9, 1999, entitled “Probe System for Ultrasonic Processing Tank”, still pending. 
     U.S. application Ser. No. 09/371,704, filed Aug. 9, 1999, entitled “Ultrasonic Generating Unit having a Plurality of Ultrasonic Transducers”, now issued Jan. 30, 2001 as U.S. Pat. No. 6,181,052 B1. 
     U.S. application Ser. No. 09/370,751, filed Aug. 9, 1999, entitled “Power System for Impressing AC voltage Across a Capacitive Element”, and is now issued Jan. 9, 2001 as U.S. Pat. No. 6,172,444 B1. 
     U.S. application Ser. No. 09/370,324, filed Aug. 9, 1999, entitled “Ultrasonic Transducer with Bias Bolt Compression Bolt”, issued on Sep. 11, 2001 as U.S. Pat. No. 6,288,476 B1. 
     U.S. application Ser. No. 09/370,301, filed Aug. 9, 1999, entitled “Ultrasonic Transducer with Epoxy Compression Elements”, now issued Jun. 5, 2001 as U.S. Pat. No. 6,242,847 B1. 
     U.S. application Ser. No. 09/504,567, filed Feb. 15, 2000, entitled “Multiple Frequency Cleaning System”, issued on Nov. 6, 2001 as U.S. Pat. No. 6,313,565 B1. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an AC switch for connecting or disconnecting modification circuitry into and out of the power section of an ultrasonic generator. AC switching devices such as triacs, relays, silicon controlled rectifiers and/or transistors can be utilized. 
     2. Discussion of the Related Art 
     Ultrasonic generators are utilized in a variety of applications including but not limited to cleaning, plastic welding, cell disruption, sonochemistry, degassing, micro machining, and developing photosensitive polymers. This diversity in applications requires a versatile generator or a variety of ultrasonic generators. Frequency adjustment, amplitude control, power changes, waveform shaping, power control and output impedance selection are useful control parameters for ultrasonic generators that are designed for a variety of applications. It is therefore an object of the present invention to make ultrasonic generators more versatile by the switching of resistive, reactive, and active components. 
     Heretofore, different mechanisms and methods have been utilized to modify the parameters of an ultrasonic generator, like the use of linear amplifiers or drive circuits to accomplish frequency adjustment, amplitude control, power changes, waveform shaping and power control. The desired parameter(s) is formed in a low level analog or digital format and then amplified to the proper power level to drive the ultrasonic transducers. U.S. Pat. No. 5,076,854 is a typical example of this technique. The disadvantages of linear power amplifiers are that they are expensive, inefficient and physically large. 
     Another technique is to switch off the current to the output of the ultrasonic generator earlier for lower power and lower amplitude. This technique is known in the art to control power output and amplitude output from an ultrasonic generator. This has the disadvantage of switching losses in the semiconductor switching devices. These switching losses increase with increasing frequency, making this method even more disadvantageous as the ultrasonic frequency is increased. 
     Known devices and methods control of output amplitude by controlling the supply voltage to the ultrasonic generator or oscillator. U.S. Pat. No. 4,736,130 illustrates this method. The output voltage of the voltage regulator is changed as the output amplitude is also changed in the same fashion. A switching regulator can be used as the voltage regulator. Increased size and expense are disadvantages of this approach. Other known methods include the use of a linear regulator to regulate the voltage. The disadvantage of this method is inefficiency and the requirement for excess heat removal. 
     Other known devices have used AC switches in the output of ultrasonic systems to multiplex different transducers to an ultrasonic receiving and sending circuitry. An example is found in U.S. Pat. No. 6,051,895. This patent shows a series field effect transistor configuration used as the AC switch. Other AC switches, such as those formed from IGBTs, BJTs have been utilized. However, AC switching at the output of ultrasonic systems for the purpose of multiplexing a selected generator to a transducer array or multiplexing of a selected transducer to a generator or receiver suffers from the need for multiple generators or multiple transducers. The present invention can accomplish the same task with a single generator. 
     Various references demonstrate the versatility and switching abilities of triac switches. U.S. Pat. No. 5,892,314 discloses a triac switch that acts as a gate to an energy storage inductor to transfer the energy in a piezoelectric film to the energy storage inductor. In this patent the triac, however, is the active device in the generator circuit, not an AC switch used to modify the output power, amplitude, frequency or impedance of the generator circuit. U.S. Pat. No. 5,930,946 discloses a pest control device where a triac is used to generate an electromagnetic field in the AC wiring. U.S. Pat. No. 4,023,004 discloses a triac to control the power supply of a microwave oven. U.S. Pat. No. 5,592,073 discloses a circuit for controlling a triac switch. U.S. Pat. No. 5,734,289 describes another, yet different, circuit for controlling a triac switch. U.S. Pat. No. 4,027,226 shows a bipolar inverter that can use triacs as the switching mechanism. Finally, U.S. Pat. No. 4,845,391 discloses a circuit to simulate a triac switch. 
     In the above-identified patents the mechanisms and methods for changing the parameters of an ultrasonic generator suffer from numerous disadvantages, such as large size, inefficiency, switching losses. It is an object of the present invention to eliminate such shortcomings. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to the creation of an AC switch by electronic circuitry. The AC switch as presented in this invention will exchange a modifying circuitry (which contains resistive, reactive, and active components) into and out of the power section of an ultrasonic generator. Therefore, the output of the ultrasonic generator will be modified by the modification circuitry disclosed, by way of example, herein. The AC switch is operatively connected to the modification circuitry. It switches the modification circuitry into and out of the output stage of the generator. The control circuitry is associated with the AC switch and is adapted to turn off and turn on the AC switch. The AC switch will swap resistive, reactive and active components and networks of these components into and out of the power section of ultrasonic frequency generators. The present invention provides a simple and reliable manner to increase the number of parameters and diversify the capabilities of an ultrasonic generator. 
     The AC switch introduces a modification circuit that is able to (1) maintain full power output from a multiple frequency ultrasonic generator as the center frequency of the generator is changed, (2) step sweep the output of an ultrasonic oscillator, and (3) vary the output power and amplitude of a non self-oscillating ultrasonic generator. A fixed frequency oscillator can be modified to accomplish certain of these functions and to sweep frequency. This is accomplished by the step sweeping and successive AC switching in of capacitors and/or inductors (i.e. modification circuitry). 
     This patent will suggest a number of applications in which the AC switch is created by triacs. A triac is a three terminal semiconductor, which controls current in either direction. The triac is suited to create a simple and less expensive AC switch than the use of transistors. Nevertheless, it will be obvious to those skilled in the art that other circuitry can be substituted for triacs. One example of such other circuitry, which simulates a triac, is one that includes back to back silicon-controlled rectifiers. Also, a series/parallel active device configuration or bi-directional lateral insulated gate bipolar transistor, can act as the AC switch. 
     The phrase “modification circuitry” as used herein is defined as resistive, reactive and active components and networks of these components. The circuitry will have two main leads and one or more control leads available for active components or networks containing active components. One of ordinary skill in the art will readily appreciate that it is possible to introduce a different value of a resistive or reactive component through the use of a transformer; therefore, in some cases a transformer winding or tap can be the part of the modification circuitry that is switched by the AC switch. 
     The modification circuitry is placed in parallel with an AC switch when it is required that the modification circuitry be inserted into a conduction line of the ultrasonic generator. The modification circuitry is placed in series with an AC switch when it is required that the modification circuitry be inserted between two nodes of the ultrasonic generator. When connected in series, the modification circuitry is inserted at any time in the cycle by turning on the AC switch. In the case of a parallel connection, the modification circuitry is removed from the generator when the AC switch is on. The reverse effect will happen when the AC switch is turned off. The addition of a control circuitry to the AC switch supplies turn on and off signals to the AC switch. Where the AC switch is a triac, the control circuitry will provide (1) a turn off signal to the ultrasonic generator for a period of time at least as long as the triac turn off time, (2) the turn off signal to the triac for a period of time at least as long as the triac turn off time, and (3) concurrent signals for a period of time at least as long as the triac turn off time. The use of this control circuitry is necessary due to the fact that the speed of triacs is too slow to allow them to go off when conducting an ultrasonic current. 
     Another embodiment of the invention includes modification circuitry capable of modifying the following parameters of the output of an ultrasonic generator: frequency; amplitude; power; impedance; and waveform. The parameter will change in accordance to the purpose of the application or generator. The modification includes at least one capacitor, one inductor, or one resistor. Finally, it can also include an active/passive network with a control circuitry adapted to control the active components in the network. 
     In another embodiment of the invention, a control circuitry capable of supplying a turn off signal to the AC switch for a duration D 1  is illustrated. If the AC switch is a triac, the control circuitry will also supply a turn off signal D 2  to the generator, where D 1  and D 2  are concurrent for a time equal to or greater than the triac turn off time. The same will apply if the AC switch is comprised of back to back silicon controlled rectifiers. In the case of the modification of the output frequency of an ultrasonic oscillator, the “controller” will represent the control circuit. This controller can be further modified to selectively activate or deactivate components so as to step sweep the output frequency of an oscillator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic diagram of a conduction line of an ultrasonic generator. 
     FIG. 2 shows a schematic diagram of an ultrasonic generator conduction line and the AC switch and modification circuitry, in a parallel connection. The control function of the AC switch is also shown. 
     FIG. 3 shows a schematic diagram of two nodes in the power section of an ultrasonic generator. 
     FIG. 4 shows a schematic diagram of the AC switch and modification circuitry connected in series between two nodes in the power section of an ultrasonic generator. The control function of the AC switch is also shown. 
     FIG. 5 shows a schematic diagram of a triac circuit employing the invention as used in the output of a multiple frequency generator. 
     FIGS. 6A and 6B show a schematic diagram of a control circuit that produces on and off signals for the gates of the triacs in FIG.  5  and on and off signals for the frequency generation of the ultrasonic generator. 
     FIG. 7 shows a schematic diagram of an ultrasonic frequency oscillator with a triac network in the output to step sweep the frequency output of the oscillator. 
     FIG. 8 shows a schematic diagram of a control circuit that produces on and off signals for the gates of the triacs in FIG.  7  and on and off signals for the oscillator in FIG.  7 . 
     FIG. 9 shows a schematic diagram of an ultrasonic frequency oscillator with a triac network in the output using inductive, capacitive and resistive modification circuits. 
     FIGS. 10A to  10 C show schematic diagrams of AC switches formed from various active components. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings in detail, for the ease of the reader, like reference numerals designate identical or corresponding parts throughout the views depicted in the drawings. It should be noted that each embodiment of the present invention is not depicted by a drawing; nor are each of the notable applications of the present invention depicted by a drawing. FIG. 1 shows a schematic representation of a view of a conduction line  20  from a power section of an ultrasonic generator. FIG. 2 shows a box representation of a “parallel structure”. As used herein, a parallel structure refers to a modification circuitry  26  and an AC switch  25  with a control  23  where the two leads of the modification circuitry  26  are connected in parallel to the AC switch  25 . The “parallel structure” is connected into the conduction line  20  of the power section of an ultrasonic generator. As used herein, “power section of an ultrasonic generator”, “ultrasonic generator power section” or “output of an ultrasonic generator” is defined as that output circuitry of an ultrasonic generator where the ultrasonic frequency is present. Where AC switch  25  is comprised of a triac, lead number  1  of the modification circuitry  26  is connected to triac terminal MT 1 . Lead number  2  of the modification circuitry  26  is connected to triac terminal MT 2 . The triac gate is connected to the control  23 . In cases where the modification circuitry  26  contains active components, the additional control leads of these active components are also connected into the control  23 . In cases where the AC switch  25  is a configuration containing more than one active component, the leads of each of the active components are driven by control  23 , with proper isolation between the separate control lines where necessary. 
     FIG. 3 shows a schematic view of two nodes  27  and  28  in the power section of an ultrasonic generator. FIG. 4 illustrates a “series structure”. As used herein, a “series structure” refers to a modification circuitry  33  and an AC switch  34  in which the two leads of the modification circuitry  33  are connected in series with the leads of an AC switch  34 . This series structure is connected between two nodes in the power section of an ultrasonic generator as shown in FIG. 4. A control  29  is present to turn on and off the AC switch  34 . When the AC switch  34  is comprised of a triac, the leads are the MT 1  and MT 2  terminals of the triac. The third lead is the gate of the triac or AC switch  34  and is connected with the control system  29 . In cases where the modification circuitry  33  contains active components, the additional control leads of these active components are also connected into the control circuitry  29 . In cases where the AC switch  34  is a configuration containing more than one active component, the leads of each of the active components are driven by control  29 , with proper isolation between the separate control lines where necessary. 
     FIG. 5 illustrates the use of a triac circuit in a preferred embodiment of the invention as depicted in FIGS. 1 and 2. The triac circuit, of FIG. 5, is used to modify the output of a multiple frequency ultrasonic generator. In particular, the modification circuitry is comprised of five capacitor passive components  19 ,  36 ,  38 ,  40 , and  42  and associated triacs  35 ,  37 ,  39 ,  41 , and  43 . The triacs switch the modification circuitry into and out of the output stage of a multiple frequency ultrasonic generator. In a typical application, the output of an ultrasonic generator is connected between the +RF and −RF terminals, as shown in FIG.  5 . The ultrasonic transducer array is connected between the +RF and GND terminals. FIG. 5 also contains a more complex parallel structure defined by the modification circuitry formed by capacitors  19  and  36  and triac  37  in parallel with the AC switch, triac  35 . 
     The first structure  44  defined in FIG. 5 is formed by capacitor  19  and triac  35 . This first structure  44  is a parallel structure and is connected in the conduction line that typically connects −RF to GND. Thus, when triac  35  is off, the capacitor  19  is inserted between −RF and GND. When triac  35  is on, capacitor  19  is shorted out which effectively connects −RF to GND. The practical effect of this first structure  44  is to place capacitor  19  in series with the transducer array when triac  35  is off and to connect the transducer array directly to the ultrasonic generator when triac  35  is on. This arrangement is useful when generating the highest frequency in a multiple frequency ultrasonic generator. 
     Capacitor  36  and triac  37  demarcate the second structure  45  in FIG.  5 . This second structure  45  is a series structure and is connected between the nodes labeled −RF and GND. Thus, when triac  37  is on, capacitor  36  is inserted between −RF and GND. The reverse effect can be seen when triac  37  is off. When capacitor  36  is open circuited, capacitor  36  is effectively removed from the circuit. The practical effect of this second structure  45  is to place capacitor  36  in series with the transducer array when triac  37  is on. Assuming triac  35  is off, it will increase the capacitance, in series with the transducer array, to capacitors  19  and  36 . This is useful when generating the second frequency (counting down from the highest) in a multiple frequency ultrasonic generator. 
     The above two structures can form a more complex structure  46  which is an active/passive modification circuitry comprising capacitors  19 ,  36  and triac  37 . This modification circuitry is in parallel with triac  35  to form the third structure  46 , which is a parallel structure. The practical effect of this third structure  46  is to connect the ultrasonic generator output directly to the transducer array when triac  35  is on. When triac  35  is off, it will place a capacitance in series with the transducer array (either capacitor  19  or  19  plus  36  depending on the state of triac  37 ). This is useful when generating lower frequencies in a multiple frequency ultrasonic generator, because when triac  35  is on, it eliminates the higher frequency structures from the system. 
     The fourth structure  47  present, as shown in FIG. 5, is comprised of capacitor  38  and triac  39 , which form a series structure. When triac  39  is on, capacitor  38  is inserted between +RF and GND. In the case of triac  39  being off, capacitor  38  is open circuited, which effectively removes capacitor  38  from the circuit. The practical effect of this fourth structure  47  is to place capacitor  38  in parallel with the transducer array when triac  39  is on. The effect of this is to increase the capacitance in parallel with the transducer array. This is useful when generating the second frequency in a multiple frequency ultrasonic generator. It allows for the addition of the appropriate capacitance, making the power delivered at the second frequency equal to the power at the first frequency. 
     The fifth structure  48 , as shown in FIG. 5, comprises capacitor  40  and triac  41 . The fifth structure  48  has the same effect as the fourth structure, (i.e., it increases or decreases the amount of capacitance in parallel with the transducer array depending on the state of triac  41 ). This is useful when generating the third frequency in a multiple frequency ultrasonic generator. The power is kept equal to the first two frequencies by the increase or decrease of capacitance at the third frequency. 
     The sixth structure  49 , as shown in FIG. 5, is comprised of capacitor  42  and triac  43 . The sixth structure  49  is another series structure, which increases or decreases the capacitance in parallel with the transducer array depending of the state of triac  43 . This is useful when generating the fourth frequency in a multiple frequency ultrasonic generator. It adds sufficient capacitance to make the power at the fourth frequency equal to the first three frequencies. 
     The five gates of triacs  35  to  43  can be controlled individually, as are the gates as depicted in FIG.  7 . However, as shown in FIG. 5, the gates for triacs  35  and  41  are controlled by the same signal  50 . Similarly, the gates for triacs  37  and  39  are controlled by the same signal  51 . Finally, the gate for triac  43  is controlled independently by signal  52 . The reason for the mixture of dependent and independent control of the various gates is that, in the logic design of this particular circuit, the truth table for the gates of triacs  35  and  41  are identical. The same is true for the gates of triacs  37  and  39 . The signals from  50 ,  51  and  52  come from the control circuitry as depicted in FIG. 6A and 6B. 
     The FIGS. 6A and 6B illustrate a control circuit for the circuits in FIG.  5 . In FIG. 6A, the inputs  54  and  55  accept a binary code to determine the state of the triacs in FIG.  5 . The logic in FIG. 6B decodes the binary code to generate the gate drive signals for the triacs in FIG.  5 . The drive signal can be a positive voltage to the gate that will turn on the triac allowing the triac to conduct. The turn off signal is more complicated. To keep a triac conducting or in the on state, a current above a minimum current or the threshold current is sufficient. Therefore, to turn off a triac, the current flow has to be zero or less than the threshold current. The gates of the triac also need an off signal, usually zero volts. The “triac turn off time” as used herein is defined as the time required to accomplish the turn off of the triac with the gate at zero and with no current flow in the triac. The generator control line  63  in FIG. 6A goes low when the generator must be turned off to allow a triac to turn off (that is, when the generator is turned off, the output current decays to zero which lowers the current through the triac to below its threshold current, thus allowing the triac to turn off). The controller functions as follows. When the signal to inputs  54  or  55  is changed, one or more of the monostable multivibrators  56 ,  57 ,  58  or  59  triggers a high level output for approximately 37 milliseconds. These outputs proceed into NOR gate  60  and lower the voltage to the generator control line  63  for 37 milliseconds. The time the generator control line  63  is lowered depends on the time required for the energy stored in reactive components to decay, as well as on the application energy feedback. For example, in the case of a cleaning tank, the sound energy in the tank feeds back into the transducer, which will generate an AC ultrasonic voltage on the output stage of the generator. This feedback will typically take about 20 milliseconds to decay below the threshold of the triac. It is for this reason than the monostable multivibrators  56 ,  57 ,  58 , or  59  will output a signal for approximately 37 milliseconds, allowing for the above-mentioned conditions to be met. This 37 millisecond signal has the effect of turning the generator off and therefore stops the ultrasonic current from flowing through the “on” triacs. The signal change representing the new binary code is delayed about 50 microseconds. This delay is accomplished by either a resistor and capacitor combination  61  or by resistor and capacitor combination  62  or by both. The purpose of this delay is to make sure that the generator has accomplished its turn off sequence before the binary code is decoded into the new set of triac gate signals. It is acceptable to have the zero gate signal to the triac applied at any time with respect to the generator off signal. The only mandatory condition for the generator off signal is that the triac current be below the threshold (referred to herein as D 2 ) and that it and the triac zero gate signal (referred to herein as D 1 ) be concurrent for a time equal to or greater than the triac turn off time. The logic in FIG. 6B decodes the signals in a way that is well known to those familiars with NAND and invert logic. The gate signals are output onto  50 ,  51  and  52 , as shown in FIG.  5 . The high level outputs provide the on signal for the respective triacs, which will be turned on, and a low level output on the gates of the other triacs. 
     The binary code for the logic in FIGS. 6A and 6B is (P 1 , P 2 )=(0,0) for the highest frequency, (P 1 , P 2 )=(1,0) for the second frequency, (P 1 , P 2 )=(0,1) for the third frequency, and (P 1 , P 2 )=(1,1) for the fourth frequency. 
     FIG. 7 depicts another preferred embodiment of this invention. The output frequency of an ultrasonic oscillator  10  is changed by the addition of three series structures ( 78 ,  79 , and  80 ) to the output of the oscillator. The first series structure  78  consists of capacitor  83   a  and triac  83   b . The second series structure  79  consists of capacitor  84   a  and triac  84   b.  Finally, the third series structure  80  consists of capacitor  85   a  and triac  85   b . A controller  12  turns the oscillator  10  on and off by way of isolated lines  72  and  73 . The turn off and turn on signals are applied according to the circuit being a short circuit or an open circuit. The short circuit turns the oscillator off and the open circuit turns the oscillator on. The controller  12  also turns the triacs,  83   b ,  84   b  and  85   b , on and off by way of lines  74 ,  75  and  76 . Lines  74 ,  75 ,  76  are functionally similar to  50 ,  51  and  52  from FIG. 6B of this application. The controller  12  can contain circuitry similar to FIGS. 6A and 6B, so as to provide the turn off and on signal to the triacs, as shown in FIG.  7 . An alternative to control function  12  of FIG. 7 is depicted in FIG.  8 . 
     When the capacitance of the transducer  77  is defined to be a capacitance value  77 , then with all the triacs in their off state, oscillator  10  produces a frequency approximately equal to f 1  where        f1   =     1     2      π          (     L1        (     81   +   77     )       )                                  
     When triac  83   b  is turned on by the controller  12 , thereby putting a high level on line  74  during operation of the oscillator (while maintaining the high level on line  74  or while maintaining the current flow through triac  83   b  or maintaining both of these conditions, i.e., maintaining the on state of triac  83   b ), the oscillator changes frequency from the above value to approximately f 2 , where        f2   =     1     2      π          (     L1        (       83      a     +   81   +   77     )       )                                  
     Therefore, the oscillator frequency made a step change from frequency f 1  to a lower frequency f 2 . 
     In a similar fashion, when triac  84   b  is then turned on by the controller  12 , thereby putting a high level on line  75  during operation of the oscillator (while maintaining the on state of triacs  83   b  and  84   b ), the oscillator changes frequency from the above value to approximately f 3 , where        f3   =     1     2      π          (     L1        (       83      a     +     84      a     +   81   +   77     )       )                                  
     Therefore, the oscillator frequency made a step change from frequency f 2  to a lower frequency f 3 . 
     In a similar fashion, when triac  85   b  is then turned on by the controller  12 , thereby putting a high level on line  76  during operation of the oscillator, the oscillator changes frequency from the above value to approximately f 4 , where        f4   =     1     2      π          (     L1        (       83      a     +     84      a     +     85      a     +   81   +   77     )       )                                  
     Therefore, the oscillator frequency made a step change from frequency f 3  to a lower frequency f 4 . 
     The above examples show a method to step sweep the output frequency of an oscillator from a high frequency to a lower frequency by successively turning on additional series structures comprising a capacitor modification circuitry and a triac. According to the invention, it is then necessary for the controller  12  to output a short circuit between lines  72  and  73  to turn the oscillator  10  off before the triacs  83   b ,  84   b  and  85   b  can be turned off. In a preferred embodiment, the controller  12  turns off all the triacs during this generator off time. The generator off time is timed to be at least as long as the triac turn off time plus the decay time of the sound field. Then the cycle of turning on the triacs one at a time to step sweep from the highest frequency f 1  to the lowest frequency f 4  can occur again. The controller then starts another oscillator off time where all the triacs are turned off and the cycle repeats. This step swinging operation can be accomplished with the control circuit, as shown in FIG.  8 . 
     It is clear to those skilled in the art that the circuit in FIG. 7 can produce other frequency cycles. With three series structures ( 78 ,  79 ,  80 ) having unequal values for capacitors  83   a ,  84 a and  85   a , a total of eight different frequencies are possible. The four listed above and        f5   =     1     2      π          (     L1        (       84      a     +   81   +   77     )       )                   f6   =     1     2      π          (     L1        (       83      a     +     85      a     +   81   +   77     )       )                   f7   =     1     2      π          (     L1        (       84      a     +     85      a     +   81   +   77     )       )                   f8   =     1     2      π          (     L1        (       85      a     +   81   +   77     )       )                                  
     Any permutation of these eight frequencies (8! or 40,320 permutations) can be organized into a cycle by the controller  12  and supplied to the transducer. It should be noted that for any frequency change that does not require a triac to be turned off, the frequency change can be accomplished without the controller  12  turning off the oscillator. However, if any frequency change occurs where one or more triacs have to be turned off, then the controller  12  concurrently turns off the oscillator for a time at least as long as the turn off time of the triacs plus the decay time of the sound field. 
     FIG. 8 shows a schematic diagram of a control circuit representing the controller  12  of FIG.  7 . Since in the discussion of FIG. 7 above the main functional characteristics of FIG. 8 were mentioned, only a brief description of the main elements will be discussed herein below. The controller  12  (or  101  from FIG. 9) produces on/off signals for the gates of the triacs and on/off signals for the oscillator. The signal to turn on/off the oscillator  10  is sent by way of lines  116  and  117  (these lines are equivalent to lines  72  and  73  in FIG.  7 ). This on/off signal is generated by element  115  when the output is a short circuit, thereby turning off oscillator  10 . The component  118  decodes the signal to be output onto  119 ,  120  and  121  (these lines are equivalent to lines  74 ,  75  and  76  of FIG. 7) which is the signal sent into the triacs ( 83   b ,  84   b , and  85   b ). The element  122  is in charge of sending the signals to be interpreted by  118  and  115 . 
     FIG. 9 shows that an inductive modification circuit, a resistive modification circuit and a parallel structure can also modify an oscillator  10 . The operation of FIG. 9 is similar to that described for FIG.  7 . The control  101  for FIG. 9 can be similar to the control shown in FIG.  8 . 
     With reference to FIG. 9, the series structure  107 , comprising inductor  110   a  and triac  110   b , will increase the frequency of the oscillator when triac  110   b  is turned on. The series structure  108  comprising resistor  111   a  and triac  111   b  will decrease the output amplitude and power when triac  111   b  is turned on. The parallel structure  109  comprising capacitor  112   a  and triac  112   b  will increase the frequency when triac  112   b  is turned on. 
     Another application of the present invention is to change the output power and amplitude of an ultrasonic generator. With some ultrasonic generators that are not of the self-oscillating type (FIG. 7 is an example of a self-oscillating type, U.S. Pat. No. 4,743,789 is an example of a non self-oscillating type) their output power and amplitude are dependent on the total amount of capacitance connected to their outputs. Connecting series structures, comprising a capacitor and a triac, as shown, for example, in FIG. 7, to the output of these non self-oscillating generators allows the power and amplitude to be changed by controlling the state of the triacs. With n series structures,  2  raised to the power n power levels and amplitude levels can be programmed into the controller. 
     FIGS. 5 through 9 illustrate triacs utilized as the AC switch. However, as one skilled in the are will readily appreciate, any AC switch can be used (not just triacs). There are many ways to build AC switches, such as from transistors, including bipolar junction transistors (BJTs), metal oxide semiconductor field effect transistors (MOSFETs), and insulated gate bipolar transistors (IGBTs). Additionally, suitable AC switches can be constructed from thyristors, such as gate tum-off thyristors (GTOs), silicon controlled rectifiers (SCRs), MOS controlled thyristors (MCTs), and asymmetrical silicon controlled rectifiers (ASCRs). Other AC switches or devices with forced turn off and turn on capability, such as a bidirectional lateral insulated gate bipolar transistor or a relay, can be used. Such a transistor is described in U.S. Pat. No. 5,977,569. Triacs are preferred because they are inexpensive and have only one gate lead. As is well know in the art, most of the other AC switches, including transistors and thyristors, require more than one control lead to be driven. Often these multiple drives have to be isolated from one another. Gate turn off thyristors (GTOs) can make suitable AC switch, particularly if the cost of two control leads can be justified, because GTOs can be forced off by their gate leads. 
     FIG. 10A shows an AC switch in a series transistor configuration where BJTs (one N channel BJT and one P channel BJT) are used. FIG. 10B shows an AC switch made in a parallel thyristor configuration where SCRs are used. This FIG. 10B circuit is commonly known as back to back SCRs. Those skilled in the art can readily appreciate the use any active components (i.e., active components that can function as a switch) either in a parallel configuration or in a series configuration to form an AC switch. Typically, diodes are needed in the series or parallel configuration to pass current or to protect the active device. FIG. 10C shows a transistor parallel configuration using IGBTs where the AC switch comprises four diodes. As used herein, the phrase “series/parallel active device configuration” means active components either in series or in parallel. The active components can be a transistor configuration or a thyristor configuration or a combination of active devices and zero or more diodes. The active devices in series or parallel configuration will form an AC switch where one active device conducts current during one half of an AC cycle and the other active device conducts current during the other half of the AC cycle. 
     Although the invention is described by reference to specific preferred embodiments, it is clear that variations, modifications and adaptations to this invention can be made without departing from the spirit of the invention as claimed.