Method of tuning an ultrasonic device, ultrasonic device and machine for performing an ultrasonic tooling operation

Abstract

 To ensure that an ultrasonic tool is driven at an optimum frequency, the phases of the voltage applied to the tranducer and the current flowing through the tranducer are compared, and an error signal is generated dependant upon the divergence of the phase difference from a desired difference, which may be zero phase difference. The error signal is functional 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.

Description

[0001] This invention is concerned with improvements relating to ultrasonic devices, particularly devices comprising a transducer and a vibrator unit attached to the transducer in such a manner that, on the application of an oscillating voltage to drive the transducer, the vibrator unit (commonly known as a "horn") vibrates at a high frequency.

[0002] An efficient way of operating an ultrasonic device is by the use of a power generator which drives the horn at its resonant frequency. It is thus necessary for the resonant frequency of a horn to be determined, and indeed in general horns are manufactured having specific resonant frequencies. The manufacture of a horn to a specific resonant frequency is a delicate and time consuming operation. Conventionally a horn is manufactured which has a resonant frequency below the desired frequency, and the horn is "tuned" by removal of metal so that the desired frequency is attained.

[0003] Conventionally this involves the use of a tuner to which the horn may be attached, comprising a transducer, to which the horn is connected, and a power generator to drive the transducer at a variable rate. The frequency at which the transducer is driven is manually increased from a value below the resonant frequency of the horn, and the voltage across the transducer is measured. Below resonance the transducer is capacitive and above resonance the transducer is inductive, whilst at resonance, the transducer is purely resistive. Thus the resonant point can be determined when the impedence across a transducer is the minimum, and the frequency read off a scale provided on the power generator. Dependent upon the extent to which the resonant frequency is below the desired frequency, the horn is machined to increase its resonant frequency towards (but still below) the desired frequency, and is re-tested.

[0004] Whether the point of minimum voltage is determined manually or automatically, this is a difficult operation to accomplish wholly accurately, since the determination of the minimum value of a curve, as with a maximum, involves a "hunting" operation, and where, as is the case in the present field, the peak is sharp, the determined frequency is almost invariably to one side or the other of the resonant frequency. Thus the frequency to which the horn will be tuned will in fact be different from the desired frequency.

[0005] Further, in circuits having capacitance or inductance, the minimum voltage point may not in fact correspond exactly to the point of minimum resistance across the transducer.

[0006] In addition, whilst a horn may be tuned to a specific frequency to be driven by a power generator opeating at (or close to) that frequency, the resonant frequency of the horn will change to an extent dependant upon the load applied thereto, and power will be lost. Further, over longer periods of use, and/or with heavy loads, the resonant frequency of the horn may drift from its original resonant frequency, and the frequency may change with temperature.

[0007] To an extent this can be compensated for by the use of a compensation circuit in the power generator, for example comprising a bridge circuit having the transducer in one arm, to vary the frequency of the voltage of the power generator as the bridge circuit detects capacitance/inductance across the transducer. However the ability of such a compensating mechanism to control the frequency applied to the transducer is limited.

[0008] Thus to a great extent in ultrasonic operations the horn is driven at a frequency other than its resonant frequency, involving wastage of power, and unnecessary heating a wear on the power generator.

[0009] According to this invention there is provided a method of tuning the horn of an ultrasonic device in which an oscillating voltage is applied to a transducer to which the horn is or may be connected, involving a comparison of the phase difference between the voltage applied to the transducer and the current flowing through the transducer.

[0010] Thus under normal circumstances the frequency of the oscillating voltage may be increased until, at resonance, the voltage and current will be in phase.

[0011] Since a comparison of the phases of the current and voltage provides a simpler and more accurately determinable parameter, the resonant frequency of the horn may be determined with a significantly greater accuracy than has heretofore been practicable.

[0012] Additionally however in more complicated circuitry which includes inductance and/or capacitance, a specific phase difference may be attained, corresponding to a "tuning out" of the induction or capacitance of the circuit. A determination of the phase difference necessary for "tuning out" this inductance or capacitance may readily be established by measuring the phase difference whilst powering a horn of known resonant frequency at its resonant frequency.

[0013] As will be appreciated, the invention described above may additionally be used for the tuning and testing of transducers themselves, and accordingly this invention also provides a method of tuning and/or testing a transducer in which an oscillating voltage is applied to the transducer, the method involving a comparison of the phase difference between the voltage applied to the transducer and the current flowing through the transducer.

[0014] According to this invention there is also provided 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 the current through the transducer may be compared.

[0015] Preferably the device comprises means by which a specific phase relationship may be determined, such means being manual (e.g. a super­imposition of the or part of the voltage and current traces on a screen) or automatic, e.g. by the use of microchip devices to compare the phases of specific parts of the traces, e.g. at maximum rates of increase or decrease.

[0016] The means may be such as to detect or assist in a manual detection of zero phase difference, or may be such as to detect or assist in a manual detection of a selected phase difference.

[0017] In the application of the invention above described to a tuning device, a more accurate determination of the resonant frequency of a horn may be determined by measurement of the frequency of generation at the point of the specific phase difference (e.g. zero in a simple circuit or equal to the "tuned out" inductance or capacitance in a complicated circuit).

[0018] However the invention may also be applied to devices utilising (rather than tuning) horns. Thus, an oscillating voltage of increasing or decreasing frequency may be applied to the transducer of an ultrasonic device, the voltage and current phase differences being continuously determined as hereinbefore described. Desirably the circuit includes a means to maintain the frequency at phase equality (or desired inequality) such as a circuit branch comprising a phase-lock loop, thus ensuring that the device is driven at maximum efficency.

[0019] Conveniently this is achieved by the conversion of both the voltage and current wave forms to square waves at their zero crossing points, and these signals are compared in frequency and phase relationship. At the resonant point both current and voltage signals will be in-phase, this point being detected by a zero shift in the DC level at the intergrated output of the phase detector, the voltage conveniently being buffered by a high impedence voltage follower FET circuit which in turn drives the voltage control sinusoidal oscillator.

[0020] In this manner unless the particular circumstances of use call for it, it is not necessary to utilise an accurately tuned horn, since the device will automatically determine the resonant frequency of the horn, and provide an oscillating voltage to the transducer at that frequency, and will follow any movement from that frequency as may occur during use.

[0021] According to this invention there is also provided a machine for performing an ultrasonic tooling operation (such as ultrasonic welding, or ultrasonic drilling) on a workpiece, the machine comprising a transducer, a tool driven at ultrasonic frequency by the transducer, power generating means for driving the transducer, means for clamping a workpiece in relation to the tool, and means for causing relative movement of approach and separation between the clamping means and the tool, characterised in that the frequency at which the transducer is driven by the power generator is controlled as hereinbefore described.

[0022] There will now be given a detailed description, to be read with reference to the accompanying drawings, of a power generator, and of a machine tool comprising the power generator, which are preferred embodiments of this invention, having been selected for the purposes of illustrating the invention by way of example.

[0023] In the accompanying drawings:

FIGURE 1 is an elevational view of the machine which is the preferred embodiment of the invention; and

FIGURE 2 is a block circuit diagram of a control circuit of the power generator of the machine.

[0024] The machine which is the preferred embodiment of this invention is a machine for performing an ultrasonic tooling operation on a workpiece, specifically an ultrasonic welding operation, and comprises a polygonal, specifically hexagonal, base 6, on a front panel 7 of which there are control operators provided, and on side panels adjacent to the front panel start buttons 8 are provided.

[0025] Extending upwardly from the base 6 are two rodless cylinders 10, 10, between which a bridge member 12 extends which carries a transducer 14, downwardly from which a vibrator unit or horn 16 is mounted, the lower face of the horn providing a tool to operate on a workpiece at ultrasonic frequency.

[0026] Located in the base 6 is a power generator, power being supplied therefrom to the transducer 14 by a cable 18.

[0027] Mounted on the base between the rodless cylinders 10 is a fixing plate 20, comprising conventional means by which a workpiece, to be operated on by the tool 16, may be secured by conventional clamping means.

[0028] In the use of the machine, power at a desired frequency is applied by the generator to the transducer 14, and the horn 16 is vibrated at the desired frequency. The bridge 12 is moved vertically between the cylinders 10 towards a workpiece mounted on the fixing plate 20, to perform the desired ultrasonic machining operation on the workpiece.

[0029] Desirably the two cylinders 20 are powered independently, so that one cylinder alone may be powered for the application of low loads between the tool and the workpiece, whilst both cylinders may be utilised when higher loads are required.

[0030] As will be seen the machine is desirably of modular construction, enabling (for example) cylinders of different power to be substituted for the cylinders 10, dependent upon the machining requirements.

[0031] Figure 2 illustrates the circuity of the power generator, illustrating the power line of the circuitry, the whole system being grounded to earth, in a conventional manner.

[0032] Power is input into the circuit at point 30 to a MOSFET power amplifier: the advantages of utilising a MOSFET power amplifier is that, when several MOSFET transistors are used, they share power and limit tendency for cascade overload to occur.

[0033] From the power amplifier 32 power is applied through a current monitor 34 to an output 36 to the transducer, a loop extending backwardly to the amplifier 32 through a short circuit and overload protection device 33. Line 37 from the current monitor 34 is applied to an indicator 38, which is switchable to indicate either the current or the voltage at the output stage.

[0034] Between the power amplifier 32 and the current monitor 34 a voltage monitor 40 is connected, the voltage monitor feeding a signal through a root mean square to DC conversion generator 42, which is applied to the indicator 38, and also to a voltage signal squaring and zero crossing detector 44. A similar signal is applied from the current monitor 34 to a current signal squaring and zero crossing detector 46.

[0035] Outputs from the detectors 44 and 46 are applied to a phase comparator and error pulse generator 48, which compares the signals produced by the detectors 44 and 46, and produces an error signal proportional to the phase difference, or to the departure of the phase difference from a desired, preset phase difference. The error signal is applied to an error pulse intergrator 50, the output of which is applied to an automatic/manual and reset circuit 52. When the circuit 52 is on manual, an output is applied to a ten-turn calibrated frequency controller 54 by which a manually/controlled signal is applied to a voltage-oscillator 58.

[0036] When the circuit 52 is on automatic, a preset signal is applied to a high impedence buffer and voltage follower 56, which in turn drives the voltage control oscillator 58.

[0037] A phase-lock loop circuit line 51 is applied backwardly from the circuit 52 to the integrator 50, to enable the circuit to continually "hunt" for a no-­error signal situation.

[0038] A display 60 is powered by the oscillator 58, and the output from the oscillator 58 is applied to a voltage controlled linear attenuator 62. A comparator 64, which comprises a device 66 at which a set output voltage is applied, receives a signal from the convertor 42, and applies a comparative signal to the attenuator 60, ensuring that the voltage delivered by the attenuator 60 to the amplifier 32 is as set.

[0039] In this manner a desired voltage is applied to the output 36, at a frequency which ensures that the voltage and current signals are in-phase, or at a desired phase difference, and thus that the vibrator unit is operating at its resonant frequency.

[0040] Whilst this is important in relation to initial tuning of ultrasonic tools, it is especially important in machines, since the application of a load to a machine tool vibrating at high frequency will in fact change the resonant frequency of the tool.

[0041] Thus by the use of the present invention a machine tool may be utilised in a manner which ensure maximum efficiency.

Claims

1. A method of tuning the horn of an ultrasonic device in which an oscillating voltage is applied to a transducer to which the horn is or may be connected, involving a comparison of the phase difference between the voltage applied to the transducer and the current flowing through the transducer.

2. A method of tuning and/or testing a transducer in which an oscillating voltage is applied to the transducer, the method involving a comparison of the phase difference between the voltage applied to the transducer and the current flowing through the transducer.

3. A method according to one of Claims 1 and 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 frequency to reduce the error signal.

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 the current through the transducer may be compared.

5. A device according to Claim 4 comprising means by which a specific phase relationship may be determined.

6. A device according to Claim 5 wherein the means by which a specific phase relationship may be determined is manual or automatic.

7. A device according to one of Claims 4 and 5 wherein the means is such as to detect or assist in the manual detection of a zero phase difference, or may be such as to detect or assist in a manual detection of a selected phase difference.

8. A device according to any one of Claims 3 to 6 comprising a circuit which includes a means to maintain the frequency at phase equality or at a desired phase in equality.

9. A device according to Claim 7 wherein said means provides for the conversion of both the voltage and current wave forms to square waves at their zero crossing points, and these signals are compared in frequency and phase relationship.

10. A machine for performing an ultrasonic tooling operation on a workpiece, the machine comprising a tranducer, a tool driven at by the transducer, power generating means for driving the transducer, means for clamping a workpiece in relation to the tool, and means for causing relative movement of approach and separation between the clamping means and the tool, characterised in that the frequency at which the transducer is driven by the power generator is controlled in accordance with Claim 1 or Claim 3.

Drawing