GB2192512A - Ultrasonic devices - Google Patents

Abstract

To ensure that an ultrasonic tool 16 is driven at an optimum frequency, the phases of the voltage applied to the transducer 14 and the current flowing through the transducer are compared, and an error signal is generated dependent upon the divergence of the phase difference from a desired difference, which may be zero phase difference. The error signal is used to vary the frequency at which the ultrasonic device is driven, in the direction to reduce the error signal, so that the ultrasonic device can be driven at zero error signal (i.e. the desired current/voltage phase relationship) and thus at maximum efficiency. The method may be used in tuning a horn or testing a transducer. <IMAGE>

Description

1 GB 2 192 512 A 1 SPECIFICATION To an extent this can be compensated for

bythe use of a compensation circuit in the power gener Ultrasonic devices ator, for example comprising a bridge circuit having the tra nsducer in o ne a rm, to vary the f req uency of This invention is concerned with improvements re- 70 the voltage of the power generator as the bridge cir lating to ultrasonic devices, particularly devices cuit detects capacitance/inductance across thetrans comprising a transducer and a vibrator unit attached ducer. Howeverthe ability of such a compensating to the transducer in such a mannerthat, on the app- mechanism to control the frequency applied to the lication of an oscillating voltage to drive thetransdu- transducer is limited.

cer, the vibrator unit (commonly known as a "horn") 75 Thus to a great extent in ultrasonic operationsthe vibrates at a high frequency. horn is driven at a frequency other than its resonant An efficientway of operating an ultrasonic device frequency, involving wastage of power, and un- is bythe use of a power generatorwhich drivesthe necessary heating a wear on the power generator.

horn at its resonant frequency. It is thus necessary According to this invention there is provided a forthe resonant frequency of a horn to be deter- 80 method of tuning the horn of an ultrasonic device in mined, and indeed in general horns are manufactu- which an oscillating voltage is applied to a transdu red having specific resonant frequencies. The manucerto which the horn is or may be connected, involv facture of a horn to a specific resonant frequency is a ing a comparison of the phase difference between delicate and time consuming operation. Convention- the voltage applied to the transducer and the current ally a horn is manufactured which has a resonantfre- 85 flowing through the transducer.

quency belowthe desired frequency, and the horn is Thus under normal circumstances thefrequency "tuned" by removal of metal so thatthe desired fre- of the oscillating voltage may be increased until, at quenGy is attained. resonance, the voltage and currentwill be in phase.

Conventionally this involves the use of a tunerto Since a comparison ofthe phases ofthe current which the horn may be attached, comprising a trans90 and voltage provides a simpler and more accurately ducer,to which the horn is connected, and a power determinable parameter, the resonant frequency of generatorto drive the transducer at a variable rate. the horn may be determined with a significantly The frequency at which the transducer is driven is greater accuracythan has heretofore been practic manually increased from a value belowthe resonant able.

frequency ofthe horn, and the voltage across the 95 Additionally however in more complicated circu transducer is measured. Below resonance the trans- itry which includes inductance and/or capacitance, a ducer is capacitive and above resonance the transdu- specific phase difference may be attained, cor cer is inductive, whilst at resonance, the transducer responding to a "tuning out" of the induction of cap is purely resistive. Thus the resonant point can be acitance of the circuit. A determination of the phase determined when the impedence across a transdu- 100 difference necessaryfor "tuning out" this induc cer is the minimum, and the frequency read off a tance or capacitance may readily be established by scale provided on the power generator. Dependent measuring the phase difference whilst powering a upon the exteritto which the resonant frequency is horn of known resonant frequency at its resonantfre belowthe desired frequency, the horn is machined to quency.

increase its resonantfrequency towards (but still 105 As will be appreciated, the invention described below) the desired frequency, and is re-tested. above may additionally be used forthetuning and Whetherthe point of minimum voltage is deter- testing of transducers themselves, and accordingly mined manually or automatically, this is a difficult this invention also provides a method of tuning and/ operation to accomplish wholly accurately, sincethe ortesting atransducer in which an oscillating vol- determination of the minimum value of a curve, as 110 tage is applied to the transducer, the method involv- with a maximum, involves a "hunting" operation, ing a comparison of the phase difference between and where, as is the case in the presentfield, the peak the voltage applied to the transducer and the current is sharp, the determined frequency is almost invark flowing through the transducer.

ablyto one side orthe other of the resonantfrequ- According to this invention there is also provided ency. Thus thefrequency to which the horn will be 115 an ultrasonic device comprising a power generator tuned will in fact be differentfrom the desired frequ- adapted to provide an oscillating voltage of variable ency. frequencyto a transducer means whereby the frequ Further, in circuits having capacitance or induc- ency may be changed, and meanswherebythe tance,the minimum voltage point may not in fact phases of the voltage across the transducer and the correspond exactlyto the point of minimum resist- 120 current through the transducer maybe compared.

ance across thetransducer. Preferablythe device comprises means bywhich a In addition, whilst a horn may be tuned to a specific phase relationship may be determined, such specific frequencyto be driven by a power generator means being manual (e. g. a super - imposition of the operating at (or close to) thatfrequency, the resonant or part of the voltage and currerittraces on a screen) frequency of the horn will change to an extent depen- 125 or automatic, e. g. by the use of microchip devices to dant upon the load applied thereto, and powerwill compare the phases of specific pprts of the traces, be lost. Further, over longer periods of use, and/or e.g. at maximum rates of increase or decrease.

with heavy loads, the resonant frequency of the horn The means may be such as to detect or assist in a may driftfrom its original resonant frequency, and manual detection of zero phase difference, or may be the frequency may change with temperature. 130 such asto detector assist in a manual detection of a 2 GB 2 192 512 A 2 selected phase difference. allyan ultrasonic welding operation,and comprises In the application of the invention above described a polygonal, specifically hexagonal, base 6, on a to a tuning device, a more accurate determination of front panel 7 of which these are control operators the resonant frequency of a horn may be determined provided, and on side panels adjacentto thefront by measurement of the frequency of generation at 70 panel start buttons 8 are provided.

the point of the specific phase difference (e.g. zero in Extending upwardlyfrom the base 6 are two rod a simple circuit or equal to the "tuned out" induc- less cylinders 10, 10, between which a bridge tance or capacitance in a complicated circuit). member 12 extends which carries a transducer 14, Howeverthe invention may also be applied to devdownwardlyfrom which a vibrator unit or horn 16 is ices utilising (ratherthan tuning) horns. Thus, an os- 75 mounted, the lowerface of the horn providing a tool cillating voltage of increasing or decreasing frequto operate on a workpiece at ultrasonicfrequency.

ency may be applied to thetransducer of an Located in the base 6 is a power generator, power ultrasonic device, the voltage and current phase dif- being supplied therefrom to thetransducer 14 by a ferences being continuously determined as here- cable 18.

inbefore described. Desirably the circuit includes a 80 Mounted on the base between the rodless cylin meansto maintain the frequency at phase equality ders 10 is a fixing plate 20, comprising conventional (or desired inequality) such as a circuit branch corn- means bywhich a workpiece, to be operated on by prising a phase-lOck loop, thus ensuring thatthe de- the tool 16, may be secured by conventional clam vice is driven at maximum efficiency. ping means.

Conveniently this isachieved bythe conversion of 85 In the use of the machine, powerata desiredfrequ boththevoitage and current waveforms to square ency is applied bythe generator to the transducer 14, waves attheirzero crossing points, andthesesignals andthe horn 16 isvibrated atthe desired frequency.

are compared infrequencyand phase relationship. The bridge 12 is movedvertically between the cylin Atthe resonant point both current and voltage ders 10towards aworkpiece mounted onthefixing signalswill be in-phase,this point being detected by 90 plate20,to perform the desired ultrasonic machining a zero shift in the DC level atthe integrated outputof operation on the workpiece.

the phase detector, the voltage conveniently being Desirablythetwo cylinders20 are powered inde buffered bya high impedance voltage follower FET pendently, sothatone cylinderalone maybe pow circuitwhich inturn drives the voltage control sinus- ered for the application of lowloads between thetool oidal oscillator. 95 and the workpiece, whilst both cylinders maybe In this manner unless the particular circumstances utilised when higher loads are required.

of use call for it, it is not necessaryto utilise an ac- As will be seen the machine is desirably of mod curately tuned horn, since the device will automatic- ular construction, enabling (for example) cylinders a] ly determine the resonant frequency of the horn, of different powerto be substituted forthe cylinders and provide an oscillating voltage to the transducer 100 10, dependent upon the machining requirements.

atthatfrequency, and will follow any movement Figure 2 illustrates the circuitry of the power gener from that frequency as may occur during use. ator, illustrating the power line of the circuitry, the According to this invention there is also provided a whole system being grounded to earth, in a con machine for per-forming an ultrasonic tooling oper- ventional manner.

ation (such as ultrasonic welding, or ultrasonic dril- 105 Poweris input into the circuitat point30to a ling) on aworkpiece,the machine comprising a MOSFET power amplifier: the advantagesof utilis transducer, atool driven atultrasonicfrequencyby ing a MOSFET power amplifier is that, when several thetransducer, powergenerating meansfordriving M OS FET transistors are used,theyshare powerand thetransducer, means for clamping aworkpiece in limit tendency for cascade overloadto occur.

relation to the tool, and means for causing relative 110 From the power amplifier 32 power is applied movement of approach and separation between the through a current monitor 34to an output 36to the clamping means and the tool, characterised in that transducer, a loop extending backwardlyto the the frequency atwhich the transducer is driven by amplifier 32 through a short circuit and overload pro the power generator is controlled as hereinbefore tection device 33. Line 37 from the current monitor34 described. 115 is applied to an indicator 38, which is switchableto There will now be given a detailed description,to indicate eitherthe current orthe voltage atthe output be read with reference to the accompanying stage.

drawings, of a power generator, and of a machine Between the power amplifier 32 and the current tool comprising the powergenerator, which are pre- monitor 34 a voltage monitor40 is connected, the ferred embodiments of this invention, having been 120 voltage monitorfeeding a signal through a root selected forthe purposes of illustrating the invention mean square to DC conversion generator 42, which bywayofexample. is applied to the indicator 38, and also to a voltage In the accompanying drawings: signal squaring and zero crossing detector44. A sim Figure 1 is an elevational view of the machine which ilar signal is applied from the current monitor 34to a Is the preferred embodiment of the invention; and 125 current signal squaring and zero crossing detector Figure2 is a block circuit diagram of a control cir- 46.

cuit of the power generator of the machine. Outputs from the detectors 44 and 46 are applied The machine which is the preferred embodiment to a phase comparator and error pulse generator 48, of this invention is a machine for performing an ul- which compares the signals produced bythe det- trasonic tooling operation on a workpiece, specific- 130ectors 44 and 46, and produces an error signal prop- 3 GB 2 192 512 A 3 ortional to the phase difference, or to the departure the current through the transducer maybe compa of the phase difference from a desired, preset phase red.

Claims (4)

1. difference. The error signal is applied to an error 5. A device according

   to Claim 4 comprising pulse intergrator 50, the output of which is applied to means by which a specific phase relationship maybe an automatic/manual and reset circuit 52. When the 70 determined.

   circuit 52 is on manual, an output is applied to a ten- 6. A device according to Claim 5 wherein the turn calibrated frequency controller 54 by which a means bywhich a specific phase relationship may be manually/controlled signal is applied to a voltage- determined is manual orautomatic.

   oscillator58. 7. A device according to one of Claims4and 5 When the circuit 52 is on automatic, a preset signal 75 wherein the means is such asto detector assist in the is applied to a high impedance buffer and voltagefol- manual detection of a zero phase difference, or may lower 56, which in turn drives the voltage control os- be such as to detect or assist in a manual detection of cillator58. a selected phase difference.

   A phase-lock loop circuit line 51 is applied back- 8. Adevice according to anyone of Claims 3to 6 wardlyfrom the circuit 52 to the integrator 50, to en- 80 comprising a circuit which includes a means to - main able the circuit to continually" hunt" fora no-error tain the frequency at phase equality or at a desired signal situation. phase in equality.

   Adisplay60 is powered bythe oscillator 58, and 9. A device according to Claim 7 wherein said the outputfirom the oscillator 58 is applied to a vol- means provides for the conversion of both the vol- tage controlled linear attenuator 62. A comparator 85 tage and current waveforms to square waves at their 64, which comprises a device 66 at which a set output zero crossing points, and these signals are compared voltage is applied, receives a signal from the conver- in frequency and phase relationship.

   tor42, and applies a comparative signal to the atten- 10. A machine for performing an ultrasonictool uator 60, ensuring that the voltage delivered by the ing operation on a workpiece, the machine compris- attenuator 60 to the amplifier 32 is asset. 90 ing a transducer, a tool driven driven at by the trans In this manner a desired voltage is applied to the ducer, power generating means for driving the output 36, at a frequency which ensures that the vol- transducer, means for clamping a workpiece in rela tage and current signals are in-phase, or at a desired tion to the tool, and means for causing relative move phase difference, and thus that the vibrator unit is ment of approach and separation between the clam- operating at its resonant frequency.

   95 ping means and the tool, characterised in that the Whilstthis is important in relation to initial tuning frequency at which the transducer is driven by the of ultrasonic tools, it is especially important in mach- power generator is controlled in accordance with ines, since the application of a load to a machine tool Claim 1 or Claim 3.

   vibrating at high frequency will in fact change the re sonant frequency of the tool.

   Thus by the use of the present invention a machine Printed for Her Majesty's Stationery Office by Croydon Printing Company (U K) Ltd, 11187, D8991685.

   tool maybe utilised in a manner which ensure max- Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, imum efficiency. from which copies maybe obtained.

   CLAIMS 1. A method of tuning the horn of an ultrasonic device in which an oscillating voltage is applied to a transducerto which the horn is or may be connected, involving a comparison of the phase difference between the voltage applied to the transducer and the currentf lowing through the transducer.
2. 2. A method of tuning and/or testing a transducer in which an oscillating voltage is applied to the trans- ducer,the method involving a comparison of the phase difference between thevoltage applied tothe transducerand the currentflowing through the transducer.
3. 3. Amethod according to one of Claims land 2 wherein the phases of the voltage and current are compared, and an error signal is generated which is dependent upon the divergence between the phase difference and a desired phase difference (which may be zero), and varying the voltage frequencyto reducethe errorsignal.
4. 4. An ultrasonic device comprising a power generator adapted to provide an oscillating voltage of variable frequency to a transducer, means whereby the frequency may be changed, and means whereby the phases of the voltage across the transducer and