Load-sensitive Generator For Driving Piezo-electric Transducers

Abstract

An oscillatory circuit, operating in the ultrasonic or high sonic frequencies, generates power for a piezo-electric transducer in proportion to mechanical resistances or loadings encountered by the transducer. High sensitivity and instantaneous power adjustment to a wide variety of loads is obtained, and instant "turn-on" and "turn-off" at the adjusted power settings are secured, by a circuit in which the input to the first stage of a two-stage oscillatory circuit comprises the sum of three feedback components plus a DC forward bias.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electrical generator structures or circuits, and more particularly to generators primarily for use as power supply systems for piezo-electric transducers.

2. Description of the Prior Art

One characteristic of the prior art generators, designed for driving piezo-electric transducers, is found in the lack of true sensitivity of said generators to widely varying mechanical resistances or loading encountered by the transducer during the normal operation thereof.

Transducers are conventionally pre-loaded mechanically, prior to start-up. It is necessary to correspondingly pre-adjust the power supply system to correspond to the pre-load. And, after start-up, the resistances encountered by the transducer vary, as a result of which the power may be too great or too little, reducing operating efficiency to a marked degree.

Another characteristic of prior art generators is in the means for turn-on and turn-off of the power supply. Conventionally, upon turn-on of the power supply, there is an undesirable time lag between closure of the on-off switch and the appearance of power at the requisite operating voltage. This is due to the fact that in the prior art systems, a large filter capacitor must be charged each time the power supply is turned on.

It has heretofore been suggested, in Der Pat. No. 3,129,367, that a power supply system be provided embodying means responding to changes of the resonant frequency of the transducer to change the power supply frequency toward the new resonant frequency of the transducer.

In that patent, as in the present invention, feedback components are sensitive to changes in the mechanical loading of the transducer to adjust the power supply.

However, the patented invention is not believed to relate the several feedback components in such fashion as to automatically regulate the load power with maximum efficiency. And, there is no suggestion in the patented invention for incorporating a coactive relationship between the self-regulating power supply circuitry and an improved means for turn-on and turn-off of power, calculated to provide power instantaneously, as compared to prior art arrangements in which time is lost during each operating cycle while a large filter capacitor is being charged.

SUMMARY OF THE INVENTION

Summarized briefly, the invention is an oscillatory electrical circuit operating in the ultrasonic or high sonic range, adapted to drive a piezo-electric converter or transducer which encounters mechanical loads of greatly varying impedance. The invention includes an input stage driving an output stage via a coupling transformer. The output stage drives a transducer through a step-up transformer, and the primary winding of a third transformer.

The input to the first stage comprises three AC components plus a DC forward bias. The summing of these at the input of the input stage, is productive of the self-regulating power characteristic, by reason of the fact that at least one is a current feedback component that is fully sensitive to, and varies proportionately to a wide range of mechanical loads or resistances encountered by the transducer during normal operation.

A second component is a conventional voltage feedback from the output of the second stage to the input of the first stage, while the third component feeds back from a point between the two amplifier stages (thus being unaffected by the mechanical load), contributing the important capability of starting under load.

All three components serve important individual functions, while coacting to provide in sum an automatic regulation of the power supply to the transducer fully responsive to changes in the mechanical loading of said transducer.

Summarized further, the invention includes a DC power supply combined with a relay-controlled turn-on circuit to leave the DC supply in a standby condition, ready to supply power instantaneously responsive to opening or closing of the relay contacts under the control of a user. This eliminates the highly undesirable characteristic in conventional power supply circuits wherein appreciable time is lost due to the need to charge a filter capacitor whenever the apparatus is turned on. The invention, rather, leaves the power supply up to voltage (charged) between duty cycles of the transducer, as compared to turning off the entire generator.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of a power supply system according to the present invention; and

FIG. 1a is a view showing a conventional D.C. operating voltage supply circuit by means of which power is supplied to the generator comprising the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The numeral 10 designates a typical high-power electro-acoustic converter or transducer, which when energized provides mechanical vibration energy, typically at a frequency of 20 kHz.

The illustrated transducer 10 is typical and does not per se constitute part of the present invention. A transducer of this type is frequently used, for example, in ultrasonic welding of plastics, an application in which heavy mechanical loading of the transducer occurs during each duty cycle thereof.

High frequency power is supplied by a generator 30, through a cable 32, to the transducer.

The generator 30 consists of two stages: a first or input stage 40 and a second or output stage 44 coupled by transformer 42 having primary winding 42a and secondary windings 42b, 42c. The output stage 44 drives the transducer 10 via a step-up transformer 46 having primary winding 46a and secondary or output windings 46b, 46c, and the primary winding 48a of a transformer 48.

A conventional DC power supply 60, shown in detail in FIG. 1a, supplies operating voltage to amplifier stages 40, 44 through conductors 65, 64 respectively.

The inductance of primary winding 48a added to the inductance of output winding 46b of transformer 46, resonates with the capacity of the transducer 10 and a shunt trimming capacitor C3 at the operating frequency.

This frequency is established mainly by transducer assembly 10, which has the highest Q factor of all the tuned elements in the loop.

The AC voltage developed across the primary inductance 48a at resonance is a sensitive function of the mechanical loading of the transducer and increases with the mechanical loading encountered by the transducer. A transformed fraction of this voltage is fed back to the first stage input, where in combination with other feedback components, it develops the desired self-regulating characteristic of the generator.

The input to the first stage consists of three AC feedback components plus a DC forward bias. Component E.sub.1 developed in primary winding 46a of the output transformer 46 is a conventional voltage feedback, necessary for oscillation but not sufficient for starting.

Component E.sub.2 developed in winding 42c of the first stage transformer is independent of and unaffected by loading forces. Being derived from an inner loop, it contributes the important capability of starting under load.

Component E.sub.3, derived from secondary windings 48b and 46c of transformers 48, 46 respectively, is a transformed fraction of the resonance voltage developed across the primary inductance of transformer 48. It is extremely sensitive to and varies widely with the mechanical loads encountered by the transducer, typically from about 5 percent of E.sub.1 when operating in air, to 50 percent of E.sub.1 when under heavy mechanical load. The combination of this component with feedback E.sub.1 develops automatic, load-sensitive power output characteristic of the present invention.

Another aspect of the invention relates to on-off control.

Conventionally, intermittently-operating generators are cycled on and off by means of a main control switch S.sub.1, located in the primary power circuit PC having conductors 50, 51 and controlling the flow of current from the illustrated 110 V-AC source, to the typical DC operating voltage supply 60 illustrated in detail in FIG. 1a. A substantial and undesirable lapse of time occurs at each turn-on, by reason of the necessity of charging a large filter capacitor 61 normally provided as part of the DC operating voltage supply. As a second and less critical characteristic of the conventional on-off control circuitry, a heavy current surge is drawn at each turn-on to charge the large filter capacitor and also, to start the oscillator circuit itself. This causes line voltage dips objectionable to others sharing the same power line, blown fuses and short life of the contacts of said switch.

However, the main undesirable characteristic of the conventional control circuitry, as noted above, is the excessive time needed to charge the filter capacitor 61.

In contrast, the disclosed generator is cycled on and off by closing a switch S.sub.2 to open light-duty, normally closed contacts of relay K.sub.1. Opening these contacts ungrounds the power supply of stages 40, 44 allowing turn-on bias to be applied. In turn, stage 40 when conducting supplies turn-on bias through a connector 49 to the output stage 44, resulting in completion of the oscillatory loop, and the generation of ultrasonic power output. Conversely, closing the relay contacts grounds the power supply to stages 40, 44, and thereby extinguishes oscillation and power output. At all times, however, filter capacitor 61 remains fully charged.

The generator is initially placed on standby, by closing switch S.sub.1 which charges the above-mentioned filter capacitor incorporated as part of the DC voltage supply. The charge, by reason of the inventive arrangement disclosed, produces an initial current surge of only about one-half the normal value. Subsequently, the oscillator is instantaneously supplied from this fully-charged filter capacitor, whenever relay K.sub.1 is operated, with an even smaller line current surge.

This eliminates the heavy line transient at every operating cycle with its attendant objectionable consequences and even more importantly, eliminates to all intents and purposes the highly undesirable time lapse now required to charge filter capacitor 61 at the beginning of each operating cycle.

The high-voltage secondary 46b of transformer 46 connects through primary winding 48a of transformer 48 to piezo-electric transducer 10. Capacitor C3 is a trimming value used with the other fixed components in the load circuit to establish resonance at the operating frequency, typically 20kHz.

The illustrated arrangement charges the filter capacitor 61 of the DC voltage supply with a moderate current surge when line switch S1 is closed, placing the generator on standby. The oscillator circuit is instantaneously cycled on, subsequently, by closing a switch S2. This may be a limit switch in the work stand, which closes when the desired pre-load has been applied to the work pieces (for example, when contact is made with plastic workpieces to be welded, not shown, by a horn tip of transducer 10). The capacitor 61 of the DC voltage supply supplies the starting current of the oscillator and greatly attenuates the line voltage transients which would be produced if the generator itself was cycled on and off. Switch S2 may be arranged to open at the conclusion of each duty cycle, awaiting application of the horn tip to the next following workpiece to be welded or otherwise acted upon.

There is a particular coaction between the feedback components and the turn-on means incorporating relay K1 and switch S2. A feedback signal, derived from an inductive component of the resonant circuit, is combined with conventional positive feedback in a sense and proportion yielding said power regulating characteristic. An additional feedback component, derived from the first stage, yields improved starting under load. And finally, turn-on of the generator is accomplished by completing the path between the DC voltage supply and the stages 40, 44 by closure of switch S2 during each duty cycle, in an arrangement in which filter capacitor 61 remains fully charged so that power will be instantaneously supplied, without the usual time lag noted in the prior art systems.

A highly desirable result is achieved in that power dissipated in the converter (transducer) remains at a low level, while still permitting power transferred to the load to vary over wide limits, on demand. With the self-regulating, load-sensitive characteristic of the generator herein described, the disintegration, power dissipation is low, thus preventing overheating and consequent destruction of the transducer and generator.

Further, the adaptability of the invention to start up instantaneously under mechanical load, increases the versatility and operating efficiency of the apparatus to a marked degree. For example, such a condition obtains when the transducer horn is immersed in a liquid, as for cleaning, cell disintegration, emulsifying and mixing. This condition is also encountered when the apparatus is used for welding plastics ultrasonically. Here, in order to avoid marring the work, it is desirable to pre-load the transducer horn against the plastic work pieces prior to startup. The invention, and particularly the improved self-regulating power characteristics thereof, greatly simplifies the task of establishing the pre-load, the power setting, and the operating interval.

It is worth considering, at this point, the particular relationship of components E1, E2, E3, in light of description already provided, to obtain a completely clear and full understanding of the relationship of these components one to another.

Component E1 is a transformed fraction of the amplifier output voltage applied to the resonant circuit that includes the transducer. This component is fed back with a positive sense, to sustain oscillation at the resonance frequency of the transducer. However, if the transducer is under a mechanical load, its reflected electrical impedance will usually reduce the output stage gain sufficiently to prevent the start of oscillation.

Component E2 is a transformed fraction of the output of the first stage of the amplifier, also with a positive sense. It is relatively insensitive to the transducer load, by virtue of the isolation afforded by the output stage.

The combination of components E1 and E2 is sufficient to reliably start and maintain oscillation under load, but does not have an inherent capability for load power regulation.

Component E3 is a transformed fraction of the resonance voltage developed across the inductor in series with the transducer. The inductor primary resonates with the capacity of the transducer and with a parallel trimming capacitance. Its value increases sharply with mechanical loading of the transducer. It is chiefly this component which imparts the self-regulating or load-sensing characteristic of the generator. Without this component, the power dissipated in the unloaded converter may approach or even exceed the power drawn under load.

Typical values of the component parts of the illustrated circuit are:

R1 15,000 ohms R2 330 ohms 61 500 mfd C1 0.1 mfd C2 0.02 mfd C3 0.005 mfd

The abstract of this application is not intended to constitute a comprehensive discussion of all the principles, possible modes or applications of the invention disclosed in this document and should not be used to interpret the scope of the claims which appear hereinafter.

Claims

1. An electrical generator for supplying power to a piezo-electric transducer, comprising an oscillatory circuit having an input and output connectable respectively to a power source and a transducer to be driven, said circuit including a feedback component taken from the output and sensitive to changes in the output voltage of said circuit, said component developing a proportionately adjusted voltage feeding back into the input end of said circuit for oscillation thereof, said generator including first and second amplifier stages in said circuit, said generator further including means for starting up said circuit under load, said means including a second feedback voltage component taken from the output of the

2. A generator as in claim 1 further including a third feedback component inductively derived from the output of said second stage and feeding back to the input of the first stage to adjust current feeding into the first

3. A generator as in claim 2, further including a first transformer connected with said second stage and a second transformer connected between the first transformer and the transducer, the primary inductance of said second transformer being in series between the first transformer and transducer to develop varying resonance voltages, which voltages are a sensitive function of varying mechanical loadings imposed upon the transducer and hence are productive of said one feedback component, said

4. A generator as in claim 3 wherein said second transformer includes a secondary winding in which said third feedback component is developed as a transformed fraction of the resonance voltage present in said primary

5. A generator as in claim 4 wherein the first and second stages are coupled by a third transformer having a secondary winding in which said

6. A generator as in claim 5 wherein the main power source is a DC bias

7. A generator as in claim 6 wherein there is a summing of the several

8. An electrical generator for supplying power to a piezo-electric transducer, comprising an oscillatory circuit having an input and output connectable respectively to a power source and a transducer to be driven, said circuit including a feedback component taken from the output and sensitive to changes in the output voltage of said circuit, said component developing a proportionately adjusted voltage feeding back into the input end of said circuit for oscillation thereof, said generator further including a DC bias current feed to the resonant circuit and an on-off control for the resonant circuit in the form of a relay having normally closed contacts opening responsive to energizing of the relay, said contacts when closed grounding the feed of DC bias current to the resonant circuit while leaving the biasing circuit in a stand-by condition, said contacts when opened breaking the grounding connection to initiate feed of the bias current to the resonant circuit without production of heavy line voltage current surge normally occurring in said DC bias current in the

9. A generator as in claim 8 wherein said DC bias includes a main on-off switch whereby closure of the main switch permits charging of the DC bias to put the same in stand-by condition with accompanying elimination of heavy line current surges during subsequent operation of the relay contacts to open positions, and a second on-off switch controlling the energization of the relay in the closed condition of the first switch so as to in turn control application of the DC bias to the resonant circuit.