Ultrasonic Vibration Device Having Piezo Sensor

Abstract

The present invention concerns an ultrasonic vibration device having a sonotrode and a converter, wherein the converter has a piezoelectric actuator for converting an electrical alternating voltage into a mechanical vibration, wherein there is provided a first piezoelectric sensor element which outputs a first measurement signal which varies in dependence on the vibration amplitude of the mechanical vibration. To provide an ultrasonic vibration device which allows measurement of at least one intensive state variable like for example the temperature or the ageing state of the piezoelectric actuator it is proposed according to the invention that there is provided a second piezoelectric sensor element which outputs a second measurement signal which varies in dependence on the vibration amplitude of the mechanical vibration, wherein the second piezoelectric sensor element differs in a physical property from the first piezoelectric sensor element.

Claims

1. An ultrasonic vibration device having a sonotrode and a converter, wherein the converter has a piezoelectric actuator for converting an electrical alternating voltage into a mechanical vibration, wherein there is provided a first piezoelectric sensor element which outputs a first measurement signal which varies in dependence on the vibration amplitude of the mechanical vibration, characterised in that there is provided a second piezoelectric sensor element which outputs a second measurement signal which varies in dependence on the vibration amplitude of the mechanical vibration, wherein the second piezoelectric sensor element differs in a physical property from the first piezoelectric sensor element.

2. An ultrasonic vibration device according to claim 1 characterised in that there is provided a comparison device which compares the first measurement signal to the second measurement signal.

3. An ultrasonic vibration device according to claim 1 characterised in that the first piezoelectric sensor element and the second piezoelectric sensor element have different pyroelectric properties.

4. An ultrasonic vibration device according to claim 1 characterised in that the first piezoelectric sensor element and the second piezoelectric sensor element have crystals with different crystallite sizes.

5. An ultrasonic vibration device according to claim 4 characterised in that one of the two piezoelectric sensor elements has a piezoelectric monocrystal and the other piezoelectric sensor element has a piezoelectric polycrystal.

6. An ultrasonic vibration device according to claim 1 characterised in that at least one piezoelectric sensor element is arranged in the converter.

7. An ultrasonic vibration device according to claim 6 characterised in that the piezoelectric actuator and the two piezoelectric sensor elements are arranged in mutually juxtaposed relationship.

8. An ultrasonic vibration device according to claim 1 characterised in that the piezoelectric actuator and the first piezoelectric sensor element comprise the same piezoelectric material.

9. An ultrasonic vibration device according to claim 1 characterised in that the piezoelectric actuator is of a greater extent in the longitudinal direction than the second piezoelectric sensor element.

10. A method of measuring a change in an intensive state variable of an element of an ultrasonic vibration system according to claim 1 characterised in that the difference of the first measurement signal multiplied by a first weighting factor and the second measurement signal multiplied by a second weighting factor is formed and the difference is used as a measure in respect of the variation in the intensive state variable.

11. An ultrasonic vibration device according to claim 6 characterised in that both piezoelectric sensor elements are arranged in the converter.

12. An ultrasonic vibration device according to claim 7 wherein the two piezoelectric sensor elements are arranged in mutually juxtaposed relationship in the longitudinal direction.

Description

[0030] Further advantages, features and possible uses of the present invention will be apparent from the description hereinafter of a preferred embodiment. In the drawing:

[0031] FIG. 1 shows a first embodiment of a converter of an ultrasonic vibration device according to the invention, and

[0032] FIG. 2 shows a second embodiment of a converter of an ultrasonic vibration device according to the invention.

[0033] FIGS. 1 and 2 show two different variants of a converter 10 and 10′ respectively. The converter can be connected to a sonotrode, optionally by way an amplitude transformer.

[0034] For the sake of simplicity only the converter is shown here. Coupling of the converter to the sonotrode or the amplitude transformer however is generally known and is not subject-matter of the present invention.

[0035] The converter 10 and 10′ respectively has main body elements 13 and 14, between which the piezoelectric actuator 2 and the piezoelectric sensor elements 3, 4 are disposed. Fixing is effected by means of the threaded rod 11 and the fixing nut 12. The lower main body element 13 has a holding flange 9 on which the converter can be held.

[0036] The piezoelectric actuator comprises two pairs of piezoelectric discs 2. Each pair is connected at its outside surfaces to a respective ground electrode 1. The mutually facing surfaces of each pair are connected to a +electrode 6, by way of which the actuator can be controlled. By means of the piezoelectric actuator, a mechanical vibration is generated in the converter and thus also in the connected elements, namely the sonotrode and possibly the amplitude transformer. For that purpose a generator (not shown) is connected to the piezoelectric actuator by way of the +electrodes. The generator delivers an electrical alternating voltage which is converted into a mechanical vibration by the piezoelectric actuator on the basis of the piezoelectric effect.

[0037] The piezoelectric actuator however is not an ideal converter, that is to say losses occur during operation, that lead to an increase in temperature of the piezoelectric actuator. The piezoelectric actuator varies its properties in dependence on temperature. In that respect only a part of the change is also reversible. As soon as the temperature exceeds a given value the piezoelectric actuator suffers irreversible damage. Therefore temperature measurement in the vibration structure is basically desirable in order to provide for efficient monitoring and possibly influence the temperature.

[0038] In addition ageing monitoring is desired. Ageing of the piezoelectric elements usually employed in power ultrasonic applications in the piezoelectric actuator is a matter of great interest to compensate for the reduction in the conversion of electrical to mechanical vibration, caused by ageing. With the current estimation methods for determining amplitude a lack of information relating to the ageing state prevents uniformly sufficient quality over the service life of the system.

[0039] The present invention is based on the notion that a combination of a plurality of different sensor materials in the converter differs explicitly in the physical property thereof: piezoelectric and pyroelectric to piezoelectric and non-pyroelectric for temperature measurement and monocrystalline and polycrystalline for service life monitoring. The difference in the physical properties also affords a difference in the dependency on an intensive state variable like for example temperature or the ageing state.

[0040] The piezoelectric sensors can also be used for amplitude measurement of the ultrasonic vibration, even if that is not absolutely necessary.

[0041] The examples shown in the Figures involve combined ageing, temperature and amplitude measurement with two piezoelectric sensor element types, of which the first piezoelectric sensor element 4 is piezoelectric and pyroelectric (and possibly also ferroelectric) and preferably comprises the same material as the piezoelectric actuator, and the second piezoelectric sensor element 3 is only piezoelectric but not pyroelectric.

[0042] In the illustrated example the first piezoelectric sensor element 4 comprises PZT and the second sensor element 3 comprises a monocrystal, like for example quartz, which ages substantially more slowly or not at all in comparison with a polycrystalline piezoelectric material like for example PZT.

[0043] Both piezoelectric sensor elements are best arranged close to a strain maximum in the vibration structure. That can occur for example in the converter near the piezoelectric actuator, as shown in the Figures. In FIG. 1 the first piezoelectric sensor element 4 is connected on a face to a ground electrode 1. The other face is connected to a measuring electrode 7, by way of which the first measurement signal can be taken off. In the same fashion the second piezoelectric sensor element 3 is connected on a face to a ground electrode 1 and at the other face to a measuring electrode 8. The second measurement signal can be taken off by way of the measuring electrode 8.

[0044] The two piezoelectric sensor elements 3, 4 are separated from each other by an insulating layer 5 to separate the two measurement electrodes.

[0045] The first piezoelectric sensor element 4 having piezoelectric and pyroelectric properties outputs a first measurement signal by virtue of the piezoelectric effect, which is substantially proportional to the vibration amplitude of the ultrasonic vibrator. In addition a further measurement signal component is output by virtue of the pyrotechnic effect, when the sensor element has an increase in temperature.

[0046] The second piezoelectric sensor element 3 which is admittedly piezoelectric but not pyroelectric or is only very weakly pyroelectric is placed near the first sensor element 4 and also outputs a measurement signal substantially proportional to the vibration amplitude of the ultrasonic vibration unit. As a departure from the first piezoelectric sensor element 4 however here a change in temperature does not have a significant influence on the second measurement signal so only a measurement signal proportional to the vibration amplitude of the ultrasonic vibration is output.

[0047] Basically it is also possible to omit the insulating element 5. If otherwise nothing is altered in the structure then the two measurement signals occur at the same electrode. If both sensor elements are arranged with the same polarisation then the two measurement sensor signals are subtracted so that the component which is proportional to the vibration amplitude is almost eliminated. Accordingly, only one measurement signal is obtained, which indicates a dependency on temperature. Possibly firstly scaling of the geometrical dimensions of both sensors is necessary so that the signal components caused by the mechanical vibration are also of equal size at both sensors for the same vibrations.

[0048] In the described example the second piezoelectric sensor element 3 comprises a monocrystal, for example of quartz, so that the ageing progresses substantially more slowly in this piezoelectric sensor element than in the first piezoelectric sensor element 4 in the piezoelectric actuator. Accordingly by the comparison of the high-frequency measurement signals between the two sensors or between the second piezoelectric sensor element 3 and the piezoelectric actuator it is possible to make an estimate about the ageing or the change in the piezoelectric constant, here in particular the d33 constant, and can possibly be compensated by means of the closed-loop control by way of the generator.

[0049] It is also possible as shown in FIG. 2 to dispense with the insulating element and nonetheless to detect two measurement signals separately from each other. In this embodiment both the first and also the second piezoelectric sensor elements 3, 4 each have two piezoelectric discs which are connected to a ground electrode 1 at their faces facing away from each other and are connected at their mutually facing inner faces to a measuring electrode 7, 8, by way of which the measurement signals can be taken off.

LIST OF REFERENCES

[0050] 1 ground electrode [0051] 2 piezoelectric actuator [0052] 3 second piezoelectric sensor element [0053] 4 first piezoelectric sensor element [0054] 5 insulating element [0055] 6 (+) electrode [0056] 7, 8 measuring electrode [0057] 9 holding flange [0058] 10, 10′ converter [0059] 11 threaded rod [0060] 12 fixing nut [0061] 13, 14 main body element