GB2099594A - Indicating power developed by ultrasonic transducer - Google Patents

Abstract

In an ultrasonic cleaning system wherein an ultrasonic transducer of low mechanical Q is mounted on or in liquid-containing chamber which is to receive articles to be cleaned an indication of the power developed by the transducer is obtained by including a passive component (Lc), such as a resistance, inductance or capacitance, in series with the transducer and rectifying the voltage (VIc) appearing across the passive component (Lc) to obtain a d.c. signal (Vp) representative of the current in the radiation resistance (RA). As RA is substantially constant over a range of operating conditions the current measurement gives an indication of the power developed by the transducer. The d.c. signal (Vp) representative of the developed transducer power is used to adjust the vibration frequency of the transducer such as to maintain the power level at a predetermined level. <IMAGE>

Description

SPECIFICATION Improved ultrasonic generator The present invention relates to ultrasonic generators and is concerned in particular with such generators used in ultrasonic cleaning systems.

Ultrasonic cleaning systems employ a transducer which is mounted on or within a liquid-containing chamber into which articles to be cleaned are placed. The liquid is usually water dosed with suitable chemicals, such as soaps and detergents, for washing the articles in the chamber. To assist in dislodging dirt, oil, grease etc from the articles the transducer is caused to transmit ultrasonic vibrations to the articles via the liquid.

With presently known equipment of this type, monitoring of variables of the system, such as the liquid temperature and the concentration of the chemical additives is carefully monitored but virtually no monitoring of the ultrasonics is provided. The result is that, in the event of poor or zero functioning of the ultrasonic section of the equipment, such equipment can be observed to be losing its cleaning efficiency without the reason being apparent to the user.

The type of systems which are most vulnerable to underperforming ultrasonic sections are the newer applications where process cycle time is a key parameter and for which ultrasonic was proposed in the first place as a means of reducing the cycle time. Examples lie in the cleaning of plastics beer and soft drinks crates and aluminium and stainless steel beer kegs. In these examples, because of the dosing and other chemical monitoring devices installed, the ultrasonics comes under suspicion as the cause of poor performance in the overall system. Since the operator is presently unable to confirm or disprove his suspicions, ultrasonics is in danger of falling into disrepute and further implementation of the technology delayed.

Because ultrasonic drive currents and voltages are (1) of high frequency (25-50 kHz), (11) of complex shape, and (111) are modulated in both amplitude and frequency, conventional power meters capable of handling these waveshapes are both expensive and fragile. Perhaps because of the latter facts, ultrasonic equipment manufacturers have traditionally omitted power monitoring and have therefore ignored the power drops from the transducer which occur with rises in the surrounding liquid temperature, one argument being that since the cleaning solution itself becomes more effective with temperature not as much ultrasonic energy is needed.The drop in power developed is, however, typically of the order of 2030% so that considerable increases in cleaning efficiency by the chemicals must be obtained to achieve the desired result. In general this just does not happen.

Thus, a primary object of the present invention is to provide an inexpensive and rugged power meter capable of giving a usefui reading of the power developed by the ultrasonic transducer.

A secondary object is to provide a means of maintaining the power developed by the ultrasonic transducer at a predetermined constant level independent of changes of process temperature.

It has been appreciated by the present Applicants that in low Q ultrasonic transducers the foilowing conditions apply, namely: (a) the electrical 0 of the transducer dominates the mechanical Q; and (b) the acoustic radiation resistance (RA) of the ioaded transducer assembly remains substantially constant over a wide range of operating frequency and parts loading.

Because of (b) above, the current flowing through RA is proportional to the power developed.

Thus, in accordance with a first aspect of the present invention, an indication of the power developed by an ultrasonic transducer of low mechanical 0 is obtained by including a passive component (resistance, inductance or capacitance) in series with the transducer and rectifying the voltage appearing across the latter passive component to obtain a d.c. signal representative of the current in the radiation resistance (RA) and hence of the power developed by the transducer.

Preferably, the passive component is an inductance since this provides better operating characteristics, such as a reduced heating effect, better reliability and closer tolerance specification.

It has also been appreciated by the present Applicants that the power developed by an ultrasonic transducer of low mechanical Q bears a substantially linear relationship with the frequency of vibration over a relatively wide range of frequencies in the region of the normal operating frequency.

Thus, in accordance with a second aspect of the invention, the d.c. signal representative of the developed transducer power is used to adjust the frequency of vibration of the transducer so as to maintain the power level at a predetermined value.

Preferably this is achieved by comparing the actual power level with a reference power level in a comparator and using the error signal to vary the vibration frequency accordingly.

The invention is described further hereinafter, by way of example, with reference to the accompanying drawings, in which: Figs. 1 (a) to (c) are equivalent circuit diagrams used to explain the operation of a device in accordance with the present invention: Figs. 2(a) and (b) are further diagrams used to explain the operation; and Fig. 3 is a circuit diagram of part of a device in accordance with the invention which enables the operating frequency to be varied for maintaining the transducer power at a predetermined level.

As explained in detail below, the objective of the device described hereinafter is to enable the power delivered by the ultrasonic transducer to be monitored in a simple manner and then to arrange for the frequency of vibration of the transducer to be adjusted so as to maintain the transducer power at a predetermined level whereby, irrespective of, for example, temperature changes in the washing liquid, the transducer power can remain substantially unaltered.

Power measurement is achieved as follows.

A conventional ultrasonic generator and transducer system can be represented by the equivalent circuit shown in Fig. 1 (a) where L1 is the effective output inductance of the generator, Rp is the shunt capacitance of the piezo elements in the transducer, C1 represents the mechanical compliance of the transducer, L2 represents the mass of the transducer and RA represents the radiation resistance of the transducer. The construction of the transducer is such that it is of high electrical Q and low mechanical Q. This can be achieved in a known manner by, for example, using a pair of sandwich-type piezo transducers attached at laterally spaced apart positions on an aluminium bar.

Since the electrical quantities dominate the mechanical quantities in such a system, the circuit can be rerepresented as shown in Fig. 1 (b) wherein Cs is the series equivalent of Rp. The reactance X, is arranged to be greater than the reactance Xc, in order to avoid problems resulting from the increase in the internal temperature of the transducer during operation.

The circuit of Fig. 1 (b) can be redrawn as in Fig.

1 (c) wherein XLE=XLXCS The present system described hereinafter relies on the facts that (a) the electrical Q of the transducer dominates the mechanical Q and (b) the acoustic radiation resistance (RA) of the loaded transducer assembly remains substantially constant over a wide range of operating frequency and parts loading. As a result of RA being constant, the current flowing through it is proportional to the power developed by the transducer. Thus, (as shown in Fig. 2(a)) if a small value inductance Lc (e.g. 1 0,uH) is inserted in series with RA, the voltage developed across it can be used to derive a d.c. voltage Vp proportional to high-freqeuncy power.As shown in Fig. 2(b)), the simplest manner to achieve such a d.c. voltage is to include a diode D1 together with smoothing components R1 and C, in a series circuit across the inductance Lc. A d.c. meter M placed across C1, R, will then give a reading representative of the transducer power developed and can therefore be calibrated directly in watts.

The provision of such a visible indication of the power being developed by the transducer is immediately useful in showing to the system operator that the ultrasonic part of the system is (a) working or not, and (b) is working at a particular power level at which, if the other system parameters such as temperature and chemical dosage remain constant, can be expected to result in a certain cleaning performance.

In accordance with the second aspect of the invention, the power-representing voltage (Vp) is also used to as a control signal to adjust the operating frequency of the generator whereby to maintain the power at a predetermined operational level, the power developed by the transducer bearing a substantially linear inverse relationship with frequency of vibration over a wide frequency band in the region of the normal operation frequency. This is achieved as follows in the embodiment illustrated in Fig. 3.

As shown in Fig. 3, a conventional power oscillator for driving an ultrasonic transducer comprises a transformerT1 whose primary P, is connected in parallel with a capacitor C2 and in series with the collector-emitter path of a transistor Tr, across a D.C. supply provided via terminals A, B. The base of Tr, is connected to its emitter via a diode chain D2 D3. The operating frequency is controlled by a feedback path coupled to the base Tri and basically comprises a small secondary winding P3, a capacitor C2 and a pair of resistors R2, R3. A diode D4 connects the junction of the resistors R2, R3 to the transducer collector.

There can also be included optionally in the feedback loop a first inductance LA which is used to provide an automatic and repeated small sweep of the oscillation frequency irrespective of the power level being delivered. This is a known provision to attempt to maintain the transducer operating at peak power at least sometime during each frequency sweep. For this purpose, the inductance LA is the secondary of a transformer (not shown) to which a voltage of swept frequency is provided whereby LA is varied between saturation and an effective open-circuit condition during each frequency sweep.

The frequency adjustment provided in accordance with the present invention is achieved by way of an inductance LB which in this embodiment is provided by the primary of a transformerT2. The current in the secondary of transformers T2 is varied by way of a control element, indicated diagrammatically by the collector-emitter path of a transistor Tr2, whose base receives a control signal VEfrom a comparator E. The comparator derives the latter control signal VE by comparing the "actual" power sigal Vp from Fig. 2(b) with a predetermined reference level VA whereby the value of VE, and hence the operating frequency, is varied in order to maintain VA Vp=constant.

Preferably, there is included a full-wave bridge of high-speed diodes (not shown) between Tr2 and the secondary of T2 in order to preserve the current balance in the latter.

A convenient practical realisation of the transformerT2 has a one-to-one turns ratio, wound on a toroidal iron powder former, with each winding having an inductance of 85yH. If the "secondary" winding of this transformer T2 is shortened out by connecting its ends together this effectively cancels the flux and hence the inductance B of the "primary" winding. By opening and short circuiting the secondary winding alternately, the operating frequency of the oscillator can be made to change by, for example, 1 kHz (from say 30 kHz to 31 kHz). This has been found to result in an output power variation of 25% which is adequate to accommodate the variations in power which the present system is intended to overcome.

Thus, the appropriate setting of the reference level VR the system can be arranged to maintain a predetermined power level by varying the vibration frequency accordingly.

Claims (Filed 17 May 1982) 1. An ultrasonic cleaning system which comprises an ultrasonic transducer of low mechanical Q mounted on or within a liquidcontaining chamber which, in use, receives articles to be cieaned, and wherein an indication of the power developed by the transducer is obtained by including a passive component in series with the transducer and rectifying the voltage appearing across the latter passive component to obtain a d.c. signal representative of the current in the radiation resistance (RA) and hence of the power developed by the transducer.

2. An ultrasonic cleaning system as claimed in claim 1, wherein the passive component is an inductor.

3. An ultrasonic cleaning system as claimed in claim 1 or 2, wherein the d.c. signal representative of the developed transducer power is used to adjust the frequency of vibration of the transducer such as to maintain the power level at a predetermined value.

4. An ultrasonic cleaning system as claimed in claim 3, wherein the actual measured power level is compared with a reference power level in a comparator to derive an error signal which is used to vary the vibration frequency.

5. An ultrasonic cleaning system as claimed in claim 4, wherein the transducer is driven by a power oscillator comprising a transformer whose primary winding lies in series with the emittercollector path of a transistor and whose secondary includes a feedback winding coupled to the transistor base and wherein the feedback circuit includes an inductor whose inductance is arranged to be varied in accordance with the magnitude of said error signal for varying the frequency of the oscillator.

6. An ultrasonic cleaning system as claimed in claim 5, wherein the inductor in the feedback circuit comprises one winding of a transformer, the current in a second winding of the latter transformer being arranged to vary in accordance with said error signal.

7. An ultrasonic cleaning system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. inductance B of the "primary" winding. By opening and short circuiting the secondary winding alternately, the operating frequency of the oscillator can be made to change by, for example, 1 kHz (from say 30 kHz to 31 kHz). This has been found to result in an output power variation of 25% which is adequate to accommodate the variations in power which the present system is intended to overcome. Thus, the appropriate setting of the reference level VR the system can be arranged to maintain a predetermined power level by varying the vibration frequency accordingly. Claims (Filed 17 May 1982)

1. An ultrasonic cleaning system which comprises an ultrasonic transducer of low mechanical Q mounted on or within a liquidcontaining chamber which, in use, receives articles to be cieaned, and wherein an indication of the power developed by the transducer is obtained by including a passive component in series with the transducer and rectifying the voltage appearing across the latter passive component to obtain a d.c. signal representative of the current in the radiation resistance (RA) and hence of the power developed by the transducer.

2. An ultrasonic cleaning system as claimed in claim 1, wherein the passive component is an inductor.

3. An ultrasonic cleaning system as claimed in claim 1 or 2, wherein the d.c. signal representative of the developed transducer power is used to adjust the frequency of vibration of the transducer such as to maintain the power level at a predetermined value.

4. An ultrasonic cleaning system as claimed in claim 3, wherein the actual measured power level is compared with a reference power level in a comparator to derive an error signal which is used to vary the vibration frequency.

5. An ultrasonic cleaning system as claimed in claim 4, wherein the transducer is driven by a power oscillator comprising a transformer whose primary winding lies in series with the emittercollector path of a transistor and whose secondary includes a feedback winding coupled to the transistor base and wherein the feedback circuit includes an inductor whose inductance is arranged to be varied in accordance with the magnitude of said error signal for varying the frequency of the oscillator.

6. An ultrasonic cleaning system as claimed in claim 5, wherein the inductor in the feedback circuit comprises one winding of a transformer, the current in a second winding of the latter transformer being arranged to vary in accordance with said error signal.

7. An ultrasonic cleaning system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.