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VARIABLE FREQUENCY ULTRASONIC
GENERATOR WITH CONSTANT POWER
FIELD OF THE INVENTION
This invention relates to ultrasonic cleaning, and more particularly to an improved generator for driving ultrasonic transducers used in cleaning.
BACKGROUND OF THE INVENTION
In ultrasonic cleaning, a transducer, usually piezoelectric but sometimes magnetostrictive, is secured to a cleaning tank to controllably impart ultrasonic vibrations to the tank. The tank is filled with a cleaning liquid and parts are immersed into the liquid to be cleaned by ultrasonic agitation and cavitation. Interaction between the ultrasonically agitated liquid and the contaminants carried by parts immersed in the liquid causes the contaminants to be dislodged.
Various circuits have been configured for driving the ultrasonic transducer and have provided a variety of features. Parameters which are available for adjustment or control are the ultrasonic frequency, the power level, amplitude or frequency modulation, and duty cycle control of power bursts, among others.
In ultrasonic cleaning, it is known that the output circuit, which usually includes a driver, the ultrasonic transducer, and the mechanical system which it drives have a resonant frequency. The mechanical system, of course, includes the liquid in the tank and the parts immersed in the liquid. Quite clearly, the mass and shape of the parts, the temperature of the liquid, and other factors, all influence the resonant frequency of the output circuit. In some cases, the driver circuit is tuned to the approximate resonant frequency of the load by selection of inductance and capacitance values, and in other cases, the driver can drive the load either on or off resonance.
The art has developed to allow users various controls over the frequency of the ultrasonic generator in an effort to match the resonant frequency of the generator to that of the mechanical system. Indeed, it has been proposed to allow the system to automatically tune to the resonant frequency of the system, but such automatic resonance tuning has not been entirely satisfactory, particularly when combined with other forms of ultrasonic generator control. In Ratcliff U.S. Pat. No. 4,554,477, for example, the output circuit which includes the driver and the load is tuned for automatic resonance tracking, but the power output is intentionally varied or modulated to produce peak power which is substantially higher than the average power output.
It has been proposed to allow the user to adjust a sweep frequency, i.e., a cyclical change in output frequency with respect to time. The aforementioned Ratcliff patent includes a sweep feature to allow sequential resonating of a number of ultrasonic transducers driven in series. Other controls which have been proposed include duty cycle control of the output frequency (bursts of ultrasonic pulses with duty cycle controlled on and off intervals), amplitude modulation of the ultrasonic pulses, and the like. Various forms of power controls have also been proposed. Krsna U.S. Pat. No. 4,864,547 exemplifies a typical approach of using a shunt resistor in one of the main power supply circuits as a general indicator of power delivered to the load. But the measure is indirect and inaccurate because it
relates primarily to input power and does not take account of output efficiency.
A further example of an ultrasonic generator including multiple controlled parameters can be found in Wil5 Ham Puskas, U.S. Pat. No. 4,736,130. That patent discusses adjustments for the center frequency of the ultrasonic drive, the on and off time of power bursts of the ultrasonic pulses, degas on and off time, as well as amplitude modulation of the ultrasonic power bursts. 10 It is understood that there is a relationship between the output frequency of the generator, its relationship to the resonant frequency of the system, and the power delivered to the output circuit. Various systems have attempted to monitor power by, in effect, measuring 15 input power to the generator. Thus, ultrasonic controls have been available which claim to be constant power, but which simply include a shunt resistor in the input power circuit which is, at best, a crude indicator of output power, since efficiency and the like are depen20 dent upon the degree to which the system is on or off resonance. It is known, for example, that as the frequency varies from a resonance point to an off resonance point the efficiency of the system decreases, and the power delivered to the load is also reduced.
Thus, it is not a simple matter to propose a multiply controlled ultrasonic generator which is capable of producing constant power, because variation of the frequency parameters, for example, has a direct impact 30 upon the power delivered to the output. Nor is it possible to simply measure input current to the system, and use that as a basis for suggesting constant output power, particularly in systems which allow the variation of center frequency of the ultrasonic drive or its sweep. 35 Thus, while it has been thought desirable to provide the user with features such as automatic resonance seeking, constant and adjustable power output, and the like, it has not been possible heretofore to provide those features in the same ultrasonic generator. Tradeoffs 40 were necessary due at least in part to the state of the art.
SUMMARY OF THE INVENTION
In view of the foregoing, it is a general aim of the present invention to provide an ultrasonic generator 45 which provides a true constant power output over a useful operating power range, and at the same time provides for automatic resonance tracking.
Thus, it is an object of the present invention to provide an ultrasonic generator which includes a power 50 setting control which assures that the power delivered to the load is at the preset level. It is a corollary object to configure such a system along with automatic resonance tracking, so that even when the system changes the frequency of the ultrasonic generator to that de55 manded by the automatic resonance tracking system, the power output remains at the level set by the user.
A general objective is to provide a system which degasses the cleaning liquid in a quick and efficient manner. In accomplishing that objective, it is a further 60 object to provide user selectable frequency modulation of the ultrasonic frequency. In more detail, it is an object to intentionally sweep the ultrasonic frequency away from the automatically tuned resonance point, at a rate and for a deviation determined by the user, while 65 at the same time assuring that the output power is at the preset and desired level.
Thus, it is a resulting object to provide an ultrasonic generator which provides ease of use in that the user
need only set the power level which he understands is needed for a particular cleaning application, and the system automatically tunes to the frequency of the load which is in the tank at that time. Furthermore, in a situation where the user determines that frequency 5 modulation would be of value, such as for degassing at the start of a cleaning cycle, the user is capable of simply dialing in the frequency modulation parameters, without concern about altering the power level to be delivered to the load. 10
In accomplishing the various aims of the invention, it is a subsidiary object to provide a wattmeter circuit for use with an ultrasonic generator which measures the voltage, current and power factor in the output circuit to monitor the output power for comparison against a 15 desired settable output power level. In that respect, it is a further object to provide such a system in connection with a high efficiency generator which has no input isolation transformer, thereby requiring the provision of an isolated output circuit wattmeter. 20
It is a feature of the invention that the user need set only the output power level, and frequency modulation parameters if desired, and the system will automatically tune to the resonant frequency of the output circuit, thereby assuring high efficiency and reliable operation 25 of the generator. In that respect, it is a further feature that when frequency modulation is selected, the characteristics of the automatic resonance tracking circuit are configured to allow the intentional modulation of the resonance frequency to a predetermined amount off- 30 resonance. Thus, the systems cooperate to prevent the automatic resonance tracking elements from canceling or nullifying the user selected frequency modulation.
In that respect, it is a further feature that the system modulates the power to the output by means of phase 35 modulating the DC power supply for the output stage. Thus, the output power is modulated quite independently of the frequency, and even though the frequency of operation has an impact on the power delivered to the output, the mechanism for adjusting power output is 40 independent of the ultrasonic frequency. Thus, the user settable power level can be accomplished even though the system is automatically tuning the resonant frequency, without creating an unstable or unreliable control system. 45
Other objects and advantages will become apparent from the following detailed description when taken in conjunction with the drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an ultrasonic generator exemplifying the present invention; and
FIG. 2a-2d are more detailed circuit diagrams of the generator of FIG. 1.
DETAILED DESCRIPTION OF THE
While the invention will be described in connection with a preferred embodiment, there is no intent to limit it to that embodiment. On the contrary, the intent is to 60 cover all alternatives, modifications and equivalents included within the spirit and scope of the invention as defined by the appended claims.
Turning now to the drawings, FIG. 1 shows an ultrasonic generator 20 being powered from an AC line 21 65 and being adapted to drive a load 22 which includes the ultrasonic transducer and the mechanical system coupled thereto. As will become more apparent, the load is
tuned in the sense that it has a resonant frequency. The output frequency of the generator 20 is based on that established by a voltage controlled oscillator 26 which delivers an oscillating output signal to a predriver 27 and thence to an output stage or power amplifier 28. The power amplifier 28 and load 22 are sometimes collectively referred to herein as output circuit 29. In the output circuit 29, the power amplifier 28 is coupled to the load 22 for driving the load at the frequency established by the voltage controlled oscillator 26.
In practicing one aspect of the invention, the system includes a resonance follower 30 having an input 31 coupled to the output circuit 29. In the illustrated embodiment, the input 31 of the resonance follower is coupled to a wattmeter circuit 32 which provides signals related to the voltage or current in the load. The change in the signal on input 31 with respect to a change in the driving frequency is a measure of whether the load 22 is at or is approaching resonance. Thus, if the signal on line 31 corresponds to load voltage, as that signal peaks while adjusting frequency, it will be appreciated that the system is approaching resonance. When the voltage across the load is at a maximum, the system is being driven at about resonance. In an alternative system, if the signal on the input 32c relates to load current, as the signal reaches a null (with a corresponding increase or decrease in frequency), it will be appreciated that the system is also approaching resonance, and the resonance point is at the null in the signal. The resonance follower 30 thus responds to the magnitude or change in magnitude of the signal on input 31 and produces a signal on the output 34 which is coupled to the control input of the voltage controlled oscillator 26 for tuning the voltage controlled oscillator to the frequency at which the output signal resonates the load. Preferably, the resonance follower includes means for sweeping the output frequency of the voltage controlled oscillator from one end of its range toward the other; encountering the resonant frequency then terminates the sweep.
The circuit thus far described is capable of automatically tuning the voltage controlled oscillator to the resonant frequency of the load. In some cases, however, it is desirable to operate at frequencies which sweep about the resonant frequency. In accordance with the invention, frequency modulation of the ultrasonic output provides an enhanced degassing capability. In the past, degassing has been accomplished by means such as pulsing the ultrasonic output on and off for controllable periods. This has been found to be less satisfactory than the degassing system according to the invention. In accordance with this aspect of the invention, frequency modulation of the resonantly tuned output is provided which controllably sweeps the ultrasonic frequency about the resonant frequency. Tests have indicated that a modulating frequency in a range between about 100 Hz. and 300 Hz. is most effective and can accomplish degassing efficiently to remove excess bubbles from the liquid before commencing of the cleaning cycle.
Thus, having tuned the voltage controlled oscillator 26 to a frequency which corresponds to the resonant frequency of the load, means are provided to modulate that tuned frequency in a controlled fashion. To that end, a signal generator 38 is provided having an output 39 coupled to a control input of the voltage controlled oscillator 26. Preferably, the output 39 of generator 38 is a sine wave used for frequency modulating the ultrasonic frequency about the automatically tuned reso
nance frequency. For example, if the voltage controlled that the maximum power will be delivered to the load
oscillator tunes the system to, for example, 40 kHz., the when it is driven at about its resonant frequency. Thus,
operator has the ability to frequency modulate the 40 the wattmeter circuit 32 which is connected directly in
kHz. ultrasonic frequency at a rate and to a depth which the output circuit will sense this actual output variation
are both controllable. Thus, the operator may select a 5 in power as the resonance follower 30 tunes the voltage
modulating frequency of 300 Hz., and the voltage con- controlled oscillator 26 to achieve resonance. While the
trolled oscillator will be caused to produce an output resonance follower is performing its function to achieve
which is substantially a 40 kHz. carrier modulated at resonance, the error amplifier 42 is also performing its
300 Hz., thus sweeping between set limits (established function to attempt to maintain the output power level
by a bandwidth control) at the 300 Hz. rate selected by 10 at about the level selected by the user. The time con
the operator. stants of the respective circuits are adjusted to primarily
The time constants of the elements are coordinated allow resonance to be found quickly, while a longer
such that the modulation frequency can be applied to time constant is associated with the power output cir
the voltage controlled oscillator by the modulator 38, cuit so that, once resonance is found, the proper power
while the resonance follower 30, operating with a 15 output level is maintained.
longer time constant, is prevented from overriding the However, it will also be appreciated that the output modulation. Thus, even though the modulating genera- circuit can experience a change in resonant frequency tor 38 is driving the voltage controlled oscillator to during operation. For example, when parts to be frequencies off the resonant frequency of the load, and cleaned are added to or removed from the cleaning the resonance follower 30 can sense that the system is 20 tank, the mass of the mechanical system changes, and moving off-resonance, the time constant of adjustment that will impact the resonant frequency. Similarly, a of the resonance follower is such that the modulation change in the temperature of the bath in the ultrasonic will be accomplished without having the resonance cleaning tank will have an impact on the ultrasonic follower 30 correct for the intentionally introduced frequency. During these normal operating conditions, modulation. 25 the resonance follower 30 will continue to tune the In practicing another important aspect of the inven- voltage controlled oscillator 26 to maintain the output tion, the system is adapted to allow user setting of a frequency at the resonant frequency of the load. At the desired power level, to monitor the actual output power same time, the wattmeter 32 will continue to monitor delivered to the transducer, and to maintain that output the actual power in the output circuit, and the error power at the preset level. Thus, a manual power control 30 amplifier 42 will continue to modulate the input voltage 40 is provided which produces a desired power level to maintain the output power at the preselected level, input signal 41 for an error amplifier 42. The second As an example, assume the system is operating in a input 43, to be compared against the desired power quiescent condition at about the resonance frequency level signal 41, is derived from the wattmeter circuit 32. and delivering the magnitude of power demanded by The wattmeter circuit 32 in turn is connected in the 35 the user. Conditions then change such as by adding output circuit 29 to measure voltage, current and phase liquid or parts to the ultrasonic cleaning tank. Both the angle in the load 22, thus to provide an indication of resonance follower 30 and the wattmeter 32 detect the actual power delivered to the load. A signal having a change in the mechanical system which is reflected in magnitude related to the actual delivered power is out- the output circuit. The resonance follower attempts to put by the wattmeter circuit 32 onto the line 43 and 40 tune the voltage controlled oscillator 26 to find a new coupled to the error amplifier 42. The error amplifier 42 resonance frequency if it exists. At the same time, the in turn produces an output signal on line 44 which is wattmeter 32 signals the error amplifier 42 to adjust the coupled to a power regulator circuit 45. The output power level to cause the power delivered to the load to signal on line 44 has a magnitude which adjusts the match that selected by the input 40. As the voltage power regulator circuit 45 to modulate the output of a 45 controlled oscillator 26 is tuned, that also has an impact power supply 46 thereby to modulate the level of the on the power measured by the wattmeter 32, and that DC supply provided to the power amplifier 28. Thus, if can cause a further change in the output of error amplithe actual power in the load is higher than that de- fier 42. In the end, however, the resonance follower 30 manded by the user control 40, the power regulating will have monitored the voltage (or current) in the load circuit 45 causes the power supply modulator 46 to 50 to establish the operating frequency of the voltage conreduce the level of the power supply to the amplifier 28, trolled oscillator 26 at the resonant frequency of the and thereby reduce the output power delivered to the load, while the error amplifier 42 will have adjusted the load. Similarly, if the power measured by the wattmeter level of DC power delivered to the amplifier 28 so that 32 is less than that demanded by the power control 40, the actual ultrasonic power in the load circuit, in addithat condition is sensed by the error amplifier 42 which 55 tion to being at the resonant frequency, is at the level increases the output of the power regulator circuit 45, demanded by the manual power control 40. thereby causing the power supply modulator 46 to raise A subsidiary feature of the invention is a duty cycle the voltage of the supply for the output amplifier 28, control 50 which is adapted to provide a user adjustable and thereby increase the power in the load to that de- control of the duty cycle of certain characteristics of manded by the power setting. 60 the ultrasonic output. In one mode, the duty cycle conIt is significant to recall that the power delivered to trol operates through a first output 51 to control the the load is also highly dependent on the frequency of power output of the system. Thus, the output 51 is the ultrasonic generator. Thus, as the frequency of the connected to the manual power control 40 to alternate generator varies, assuming a constant input DC voltage the power demand signal between a first level selected to the output stage, the actual power delivered to the 65 by the power control 40, and a second level of substanload will vary both as a function of frequency and the tially zero. Thus, when operating in the duty cycle fact that the change in frequency is driving the system control mode over power, the system delivers power in either closer to or farther from resonance. It is known the output circuit as actually measured by the wattme