Positioning device and method for operating such a positioning device

Abstract

The invention relates to a positioning device (1) comprising a positioning unit (2) with at least one piezoelectric actuator (4), a drive element (5) coupled to a positioned element (6) and a controller (3). The positioning device has a defect analysis device (16) for detecting defects in the positioning unit (2), the actuator (4) or actuators functioning as an ultrasonic generator (12) or receiver (13) in addition to functioning as a drive for the drive element (5), the defect analysis device (16) comprising a measurement signal generator (22) generating a sinusoidal voltage for exciting the generator (12) and a resonance analyzer (24) analyzing the electrical signal generated by the receiver (13). The invention also relates to a method for operating such a positioning device to detect defects in the positioning unit (2).

Description

Technical Field

[0001] The present invention relates to a positioning device according to claim 1 and a method of operating such a positioning device according to claim 14.

Background Art

[0002] Piezoelectrically driven positioning devices enable fine positioning in the range from a few tenths of a picometer to a few centimeters. In particular, piezoelectric stack actuators are used as adjustment components in these mechanical systems. A piezoelectric actuator is an electromechanical energy converter, and its operating principle is based on the change in the shape of a specific crystal under the influence of an electric field. The actuator is composed of a piezoelectric single crystal, a piezoelectric single crystal ceramic, or a piezoelectric polymer composite material.

[0003] Piezoelectrically driven positioning devices are used, among other things, in very complex and thus costly devices such as lithography apparatuses and optical star telescopes. Stack actuators mainly operate quasi-statically, i.e., well below the lowest resonance of the system. Nevertheless, like the solid joints (Festkoerpergelenke) commonly used in such positioning devices, they are subjected to a certain mechanical stress, and the displacement of the driven elements is realized by their elastic deformation. The contraction and expansion of the actuator, or the bending of the solid joint, can occur up to 20,000 times per second during operation. As the operating time increases, there is a possibility of microcracks, short circuits, delamination, or other defects occurring in the actuator, guide, joint, or other components of the micropositioning device, which can have an adverse effect on the operation of the device until complete failure. A sudden failure of the positioning device usually causes high costs.

[0004] Most of the aforementioned defects do not usually occur suddenly, but start from minor initial defects, develop and spread over time. All mechanical systems have specific resonance patterns, which depend on the shape, geometric dimensions, material properties, etc. of the individual system components. Defects cause changes in the resonance patterns of the system. The appearance of resonances that did not exist before, or the change or removal of resonances that previously existed within the system, indicates the presence of defects.

[0005] A method for non-destructively measuring the material properties of an object using ultrasonic waves is known from Patent Document 1. For this purpose, a transmitter is used to induce ultrasonic waves into the object to be inspected, and a receiver is used to measure the propagation time thereof. By analyzing the propagation of ultrasonic waves within the object, conclusions regarding its material properties can be obtained.

[0006] Non-Patent Document 1 (scientific paper) describes a method for non-destructively detecting cracks, delamination, or other defects in laminated piezoelectric ceramic capacitors. This method is based on the excitation of standing ultrasonic waves within the capacitor. The impedance of the capacitor is recorded using a laboratory impedance analyzer HP model 4192A. Changes in the impedance image of the laminated capacitor have been shown to be evidence of internal defects in the capacitor.

[0007] From Non-Patent Document 2 (scientific paper), a method for non-destructively detecting cracks, delamination, or other defects in laminated piezoelectric actuators is known. The change in the resonance pattern of the actuator due to internal defects is investigated. For this purpose, the impedance spectrum of the actuator is compared with direct measurements using an optical microscope and a scanning electron microscope. The impedance is recorded from below to above the resonance of the actuator using a laboratory impedance analyzer (Solartron 1260). It has been confirmed that by inspecting the impedance of the laminated piezoelectric actuator, defects within the actuator can be detected quickly and non-destructively.

[0008] Patent Document 2 describes a system for analyzing and suppressing unwanted vibrations in various machines such as turbines, motors, robots, etc. This system includes a number of piezoelectric vibration sensors, an actuator for generating vibrations, and a controller connected to the sensors and the actuator. The measured vibrations are transmitted to the controller by a feedback loop, dynamically changing the drive signal of the actuator. To suppress unwanted vibrations, the controller drives the actuator according to a vibration suppression algorithm.

[0009] An apparatus for nondestructively detecting structural damage is known from Patent Document 3. This apparatus includes a piezoelectric sensor and an actuator. The actuator generates sound waves within the structure to be inspected. The sensor receives the waves reflected from the structure. When the sensor signal is evaluated, the presence of structural damage is indicated.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0011]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

[0012] An object of the present invention is to provide a positioning device capable of predicting or detecting defects occurring within a positioning unit, and to provide a method for operating such a positioning device in order to predict or detect defects within the positioning unit. Thereby, in particular, in the worst case, the corresponding components of the positioning unit can be replaced in a timely manner before it completely fails.

[0013] The positioning device according to the present invention includes a positioning unit and a controller. Next, the positioning unit includes at least one piezoelectric, and preferably a stacked actuator, which serves to move a driving element. The movement of the driving element is caused by a target mechanical deformation of at least one actuator, and the movement of the driving element is provided for driving or positioning the element to be positioned.

[0014] The coupling between the driving element and the element to be positioned is provided to transmit the movement of the driving element to the element to be positioned. This coupling can be, for example, a rigid mechanical connection so as to directly convert the movement of the driving element to the element to be positioned. However, the coupling can also be realized as a frictional contact in some cases, especially as an intermittent frictional contact between the driving element and the element to be positioned. That is, during the driving step, there is friction or frictional contact between the driving element and the element to be positioned, and a mechanical coupling through the friction or frictional contact exists during this time interval. After each driving step, a stage where the driving element returns to its initial position follows in time, thereby preparing for the next step.

[0015] If the positioning unit consists of a single actuator, this actuator is configured or arranged to act or function as both a generator and a receiver of acoustic ultrasonic waves in addition to its driving function. If the positioning unit has several, that is, at least two actuators, at least one of the actuators is configured to act or function as at least a generator of acoustic ultrasonic waves, and at least one of the other actuators is configured to act or function as at least a receiver of acoustic ultrasonic waves.

[0016] In other words, the positioning device or its positioning unit according to the present invention has only one piezoelectric, and preferably laminated actuator, and in addition to the function of moving the drive element, it also functions as a generator and receiver of acoustic ultrasonic waves at the same time. Therefore, it combines the functions of a drive device, a generator, and a receiver by itself, or the positioning device or positioning unit has several, that is, at least two piezoelectric, and preferably laminated actuators, and only one of them, several, or all of them, in addition to the function of moving the drive element, also function as a generator and receiver of acoustic ultrasonic waves at the same time. Further, when the positioning device or positioning unit includes several actuators, one or more actuators function only as a generator of acoustic ultrasonic waves in addition to the driving function, and another actuator or several other actuators function only as a receiver of acoustic ultrasonic waves in addition to the driving function. Therefore, the functions of the drive device and the generator and the drive device and the receiver are divided between at least two different spatially separated actuators.

[0017] When subsequent parts of the specification and claims refer to "actuator" or "the actuator" (i.e., the singular form), this should not be understood as being limited to a single actuator. Rather, the use of the singular form in relation to the term "actuator" (i.e., "actuator" or "the actuator") should be understood or interpreted to mean that the features regarding the actuator apply only to a single actuator, or in the case of multiple actuators, to all, only some of them, or only one of them. For example, when the configuration of the actuator is described, this description of the configuration applies exactly to this single actuator if there is a single actuator, or to one of the actuators, all of the actuators, only some of the actuators, or only one of the actuators if there are several actuators. The foregoing also equally applies to the use of the terms "generator" and "receiver" in this specification.

[0018] As used herein, the term "or" should be understood as an inclusive disjunction, i.e., a non-exclusive disjunction, unless otherwise specified. In this context, for example, the expression "the actuator has the function of a generator or a receiver" should be understood herein to mean that the actuator has the function of a generator or the function of a receiver, or the actuator has the functions of both a generator and a receiver.

[0019] The function of the mechanical adjustment member of the positioning unit is performed by an actuator or a plurality of actuators, whereby the number of actuators used is mainly determined by the application. In addition to the function of the adjustment member, when there is a single actuator, in addition to its driving function, it has both the function of a generator of acoustic ultrasonic waves and the function of a receiver. In this case, i.e., when there is a single actuator, it is provided with both an ultrasonic generator and a receiver. The function of the generator is to generate ultrasonic waves, and the function of the receiver is to receive ultrasonic waves.

[0020] The actuator is arranged, for example, between a plurality of solid joints and connected or coupled to the drive element via them. The movement or deformation of the actuator is transmitted to the drive element by the elastic deformation of the plurality of solid joints.

[0021] In the case of a stacked actuator, the actuator is composed of several layers, and each layer consists of two electrodes and a polarized piezoelectric material arranged between them. In this context, the stacked actuator is also mentioned.

[0022] The controller consists of a control regulator for an actuator or a positioning unit, a defect analysis device that excites an acoustic ultrasonic generator and stores and analyzes the signals of an acoustic ultrasonic receiver, and optionally a commutator. In the optional commutator, switching is performed between the actuator operation as a generator or as a receiver (i.e., generating the movement or deformation of the actuator) and the sensor operation of the actuator. Further, the controller may interface with a computer having a display screen on which a person can visually perform defect analysis.

[0023] The control regulator includes a power output stage for the actuator, a track and a signal generator, and a position controller for controlling the position and optionally the speed or acceleration of the positioning unit.

[0024] The defect analysis device has the functions of controlling and adjusting or controlling and regulating the actuator or the positioning unit, exciting the ultrasonic generator with a measurement signal, and processing the signals coming from the ultrasonic receiver. The defect analysis device consists of at least one measurement signal generator for generating an electrical sinusoidal voltage and a resonance analyzer for analyzing the signals generated by the actuator acting as a receiver.

[0025] The piezoelectric material of the actuator may be a single-crystal piezoelectric material, a polycrystalline piezoelectric ceramic, a piezoelectric polymer material, or other piezoelectric or electrostrictive materials. The positioning unit may comprise one or more actuators, preferably stacked actuators. If solid joints are used in the positioning unit, they may be bending joints or torsion joints (Biege- oder Torsionsgelenke).

[0026] The actuator is connected to the controller to excite the generator and process the signals received from the acoustic ultrasonic receiver. By using the actuator not only as an adjustment member but also as a generator or receiver of acoustic ultrasonic waves within the positioning unit, qualitatively new characteristics are imparted to the positioning unit. This avoids the installation of costly additional separate transmitters and receivers. Since there are no additional system components, the probability of the positioning unit or the positioning device failing is increased.

[0027] The resonance analyzer processes the signal from the current sensor, stores the signal from the current sensor and the signal from the measurement signal generator, and compares or analyzes the two recorded resonance signals or resonance spectra with each other. In order to compare or analyze efficiently, an appropriate neural network algorithm is used. When a defined deviation is detected in the analyzed resonance image (resonance image), a visual or other warning is issued. The data is also transmitted from the resonance analyzer to the computer screen via an optionally selected interface. The operator or operator can perform a visual analysis or inspection of the measurement if necessary.

[0028] An advantageous embodiment of the positioning device according to the invention provides that the measurement signal generator is designed to generate an electrical sinusoidal voltage with a periodic frequency sweep. Thereby, the positioning unit can be periodically excited within a specific range, and as a result, a resonance image is generated within a defined frequency range and such a resonance image can be analyzed.

[0029] Another advantageous embodiment of the positioning device according to the invention provides that the defect analysis device comprises a voltage or current amplifier with a linear or clocked broadband output that appropriately amplifies the signal generated by the measurement signal generator to excite the generator within the required frequency range. Furthermore, it may be advantageous for the defect analysis device to use the same power output stage that is used to drive the actuator from the control regulator as the output voltage or current amplifier of the measurement signal generator.

[0030] It may be advantageous for the defect analysis device to include a white noise generator, and it may be particularly advantageous if the measurement signal generator is suitable for generating this white noise. White noise includes frequencies of a wideband spectrum having a constant power density spectrum within a specified frequency range. This signal may be advantageously used as a broadband and efficient excitation of the positioning unit for acquiring resonance images.

[0031] Furthermore, the position controller or the trajectory of the controller and the signal generator may be implemented using an integrated circuit in the form of, for example, a digital signal processor (DSP) or a field programmable gate array (FPGA), and it may be advantageous if the measurement signal generator and the resonance analyzer are implemented as program modules within the same integrated circuit. In particular, for cost-sensitive applications, it is advantageous to incorporate the functions of the defect analysis device into the same integrated circuit that is already being used for the control or adjustment tasks of the positioning unit.

[0032] Furthermore, it may be advantageous for the resonance analyzer to include a data interface to a monitor for visually inspecting the resonance image. In this case, the data is transmitted from the resonance analyzer via the interface to the computer monitor. The operator can perform a visual analysis or inspection of the measurement data as needed.

[0033] Furthermore, it may be advantageous for the defect analysis apparatus to include a current sensor for detecting a signal generated by an actuator or a receiver. The current flowing through the actuator or the receiver contains information regarding the resonance image of the positioning unit or the positioning device. The current sensor converts the current generated by the receiver into a voltage U’i, amplifies it, and makes it available for use by the resonance analyzer. The conversion of the current can be performed using a resistor with subsequent amplification, or it can be performed by a transistor, a transformer, or an operational amplifier. For this purpose, an optocoupler can also be advantageously used. When an optocoupler or a transformer is used, the actuator is preferably DC-isolated from the defect analysis apparatus.

[0034] Furthermore, it may be advantageous to have at least one generator or receiver located between a plurality of solid joints. By arranging acoustic ultrasonic generators and receivers between a plurality of solid joints such as bending joints, torsion joints, or bending joint guides, better acoustic vibrations (akustische Flalterung) become possible. During the excitation or reception of ultrasonic waves, the stress on the side of the surrounding mechanical components is reduced for the generators and receivers. Therefore, their vibrations in the ultrasonic range are less affected. As a result, the mechanical quality of these mechanical oscillation circuits is improved, and the power required for the excitation of the generator is reduced. Consequently, the sensitivity of the receiver is also improved.

[0035] Here, it may be advantageous for the movement of the drive means to be guided by a plurality of solid joints between which the generator or receiver is arranged, or by an additional solid joint of the positioning unit.

[0036] Furthermore, it may be advantageous for the coupling of the drive element to the element to be positioned to be realized via a fixed connection or a frictional contact. A positioning unit in which the movement or deformation of the actuator is transmitted via the drive element to the element to be positioned by a frictional contact enables qualitative control of the frictional contact of the positioning unit. Thus, the deterioration of the frictional contact due to the soiling of the friction pair can be directly detected by acoustic analysis. Similarly, it is possible to detect the delamination of the friction rail of the element to be positioned where the drive element comes into or is about to come into frictional contact. When the drive element and the element to be positioned are fixedly connected, the movement of the drive element is directly transmitted to the element to be positioned, which is done very precisely and with high resolution, but due to the limited deformation of the actuator, a relatively short movement distance is possible.

[0037] Furthermore, in some cases it may be advantageous for the generator or receiver to be formed from at least part of the actuator. In the case of a stacked actuator, at least part of a plurality of layers of the actuator forms the generator or receiver. By applying stress partially or locally to the piezoelectric material or using the piezoelectric material as the generator or receiver, better acoustic matching of the ultrasonic waves to the mechanical environment can be achieved. In the case of a stacked actuator, for example, only one layer or approximately half of all the layers can be used as the generator or receiver. In this case, these layers are electrically connected accordingly, and the associated electrical wiring is drawn out from the positioning unit. The activation of the actuator and the generator and the transmission of the signals from the receiver are carried out within the controller.

[0038] In this regard, it may be found to be advantageous for the generator or receiver formed by at least part of the actuator to have no actuation function, i.e., no function of causing deformation or movement, and to be connected to the remaining part of the actuator by an acoustic connection having a low acoustic resistance. As a result, by separating the generator or receiver from the acoustic ultrasonic waves, a structure that can be used independently of the function of the actuator can be realized.

[0039] In order to detect a defect in a positioning unit, it may also be advantageous to use a generator of one actuator and a receiver of another actuator. Thus, in a positioning unit provided with a stacked actuator, an acoustic ultrasonic generator of one actuator and a receiver of another actuator can be used. The spatial separation, i.e., the distance, between the acoustic ultrasonic generator and the receiver thus provided enables the measurement of the propagation time of an acoustic measurement signal pulse. Measuring the flight time of an acoustic signal represents another method for detecting resonances or defects in a positioning unit.

[0040] The present invention also relates to a method of operating the above-described positioning device in order to predict or detect a defect occurring in the positioning device or its positioning unit. Each component of the positioning unit represents a mechanical oscillator having a resonance determined by its dimensions, material properties, and installation method. Cracks in the components of the positioning unit, delamination of the laminated structure of the stacked actuator, material fatigue of the solid joint, and other defects in the positioning unit lead to changes in the sound image. New resonances are caused, and existing resonances change or even disappear.

[0041] The method according to the present invention provides that, in addition to the actuator functioning as an adjustment member, i.e., generating the movement of a drive element that serves to position an element positioned by the deformation of the actuator, the actuator is used or operated so as to function as an ultrasonic generator or receiver for detecting a defect in the positioning unit.

[0042] For this purpose, an electrical measurement signal from a measurement signal generator of a defect analyzer is periodically applied to the actuator. The measurement signal may be an AC voltage or an AC current. The actuator or the generator excites acoustic ultrasonic waves throughout the positioning unit, and they are periodically recorded by the actuator or the receiver as mechanical resonances of the entire positioning unit and the actuator itself and can be processed by a resonance analyzer. The resonance analyzer processes the signal from an optional current sensor and stores it in the same way as the signal from the measurement signal generator.

[0043] In a further step, the resonance analyzer compares or analyzes two recorded signals or resonance spectra of the positioning unit with each other. During this comparison, resonances that previously existed and now have disappeared or changed are detected, and newly generated resonances are recorded. An appropriate and efficient neural network algorithm is used for the comparison or analysis. When a defined deviation is detected in the analyzed resonance image, a visual or other warning is issued.

[0044] The method according to the invention for predicting and detecting defects of the positioning unit and its components enables automatic monitoring of the state of the positioning unit and timely replacement or maintenance when a defect occurs. By using a piezoelectric actuator in the positioning unit not only as an adjustment member but also as an ultrasonic generator or receiver, the positioning unit is given qualitatively new characteristics. The positioning device according to the invention or the corresponding method according to the invention particularly omits the installation of additional separate transmitters and receivers that are costly.

[0045] In this method, a DC voltage or a low-frequency AC voltage is applied to the actuator by the power output stage, or a high-frequency AC voltage is applied to the actuator by the amplifier. In this context, low frequency means a voltage whose frequency is at least three times lower than the lowest resonance frequency of the positioning unit. In this context, high frequency means that the frequency of the voltage is approximately equal to or higher than the lowest resonance frequency of the positioning unit. When a voltage is applied to the piezoelectric material of the actuator, the actuator expands or contracts according to the sign of the voltage, thereby performing the positioning function. That is, the actuator transmits this expansion or contraction to the driving means connected thereto, and the driving means is provided for coupling with the element to be positioned, and through this coupling, the positioning movement of the element to be positioned can be achieved.

[0046] In addition to the function of the adjustment member, the actuator or a part thereof has the function of an ultrasonic generator or receiver. When a measurement signal from the amplifier is applied to the generator, ultrasonic waves are excited in its vicinity or within the positioning unit and are received by the receiver and converted into an electric current. The current flowing through the actuator reaches an optional current sensor, thereby being converted into a voltage Ui and further processed by a resonance analyzer.

[0047] It may be advantageous to detect the newly generated resonance, or the disappearance or change of a previously existing resonance, by forming the magnitude of the electrical impedance |Z| of the positioning unit as a function of frequency. For this purpose, the frequency of the measurement signal voltage U MG is changed according to a frequency sweep from an initial value f A to a final value f E , the current value I A flowing through the receiver and the phase angle value φ between the current and the voltage U A are measured as functions of frequency and recorded together with the voltage. To detect resonance, from a series of measurement values, the change in the impedance value |Z| = U A / I AIt is formed. Then, the resonance analyzer determines the presence of new mechanical resonances or the absence of previously existing mechanical resonances from the impedance change Z (|Z| = f(f)) and the change φ(f).

[0048] All resonances of the positioning unit are included in the impedance curve. When a defect occurs in the system components of the positioning unit, new resonances appear in the positioning unit or the previously existing resonance pattern changes. These changes can be easily recognized from the impedance curve. They can be advantageously identified by the resonance analyzer using a pattern recognition neural network algorithm.

[0049] During the frequency sweep, it may also be advantageous to measure the current value flowing through the receiver and the phase angle value between the current and the voltage in a frequency-dependent manner and record them together with the voltage. Thereby, from a series of measurement values for resonance detection, the magnitude of the impedance |Z| and the phase angle f are represented in the Nyquist diagram. Since the Nyquist diagram can simultaneously represent the magnitude of the impedance together with the phase angle, the frequency image (frequency image) of the positioning unit can be represented particularly advantageously.

[0050] Furthermore, in the frequency sweep, the initial frequency value of the measurement signal is equal to or slightly less than the lowest measurable resonance frequency value of the actuator, and the final resonance frequency value of the measurement signal is equal to or slightly greater than the highest measurable resonance frequency value of the actuator. Both the lowest resonance frequency value and the highest resonance frequency value may belong to different types of ultrasonic waves such as, for example, the longitudinal direction, bending, radial, shear, or other vibration modes of the actuator.

[0051] Preferably, the stacked piezoelectric actuator is an essential component of the positioning unit of the positioning device. The actuator has specific eigenmodes and associated eigenresonances. The eigenresonances of the actuator can be determined before it is attached to the positioning unit. After attachment, the eigenresonances can also be determined as they are included in the resonance image of the positioning unit when excited. By exciting the positioning unit in the frequency range of the resonance frequency of the actuator, defects in this actuator can be specifically detected.

[0052] Furthermore, in the frequency sweep, it may be advantageous if the initial frequency value of the measurement signal is equal to the lowest resonance frequency value of the actuator determined by the length of the actuator, and the final frequency value of the measurement signal is equal to twice the resonance frequency value determined by half of the length of the actuator.

[0053] The actuator usually undergoes particularly strong longitudinal expansion during operation. This type of stress causes cracks and delamination between the individual layers in the case of a stacked actuator. Excitation and resonance analysis of the positioning unit in the frequency range of the longitudinal vibration mode and the bending vibration mode of the actuator can be used to specifically detect this type of defect.

[0054] Furthermore, in the frequency sweep, it may be advantageous to vary the frequency of the electrical measurement voltage logarithmically or according to another convenient function from the initial value to the final value. This enables a rapid defect test of the positioning unit to be performed or the detection of the resonance of the positioning unit to be adjusted using a specific sound image.

[0055] Furthermore, it may be advantageous if the measurement signal is white noise and the current flowing through the actuator is measured and recorded. Then, for the purpose of detecting resonance, a series of measured values are Fourier-transformed, discrete Fourier-transformed (DFT), or fast Fourier-transformed (FFT). Newly occurring resonances, disappearing resonances, or changing resonances are detected. The Fourier transform is efficiently implemented within a DSP or FPGA, enabling rapid analysis. As a result of the Fourier transform, resonances can be easily recognized as amplitudes, so changes in the resonance image of the positioning unit can also be efficiently detected.

[0056] Furthermore, the frequency value of the measurement signal is equal to the measurable resonance frequency value of the actuator, and the resonance may belong to various types of ultrasonic waves such as, for example, the longitudinal direction, bending, radial, shear, or other vibration modes of the actuator. After exciting the generator for a short time at this resonance frequency, the attenuation of the positioning unit is recorded by the receiver. Furthermore, it may be advantageous if the recorded attenuation curve is compared with a previously recorded attenuation curve to detect resonance changes.

[0057] The current of the positioning unit driven by the piezoelectric actuator decays approximately according to the function I = I0EXP(-λt)sin(ωt) when the actuator is pushed or excited by resonance. Here, I is the current, I0 is the initial current, λ is the attenuation constant, ω is the angular frequency, and t is the time. When one of the resonances changes due to a defect, this change can be detected by the attenuation behavior or a change in the amplitude attenuation function A i = I0EXP(-λt), the attenuation time, or the change in the attenuation oscillation frequency. The attenuation time and attenuation function of the vibration can be quickly and easily recorded by a microprocessor-based measuring device. Defects occurring in the actuator can be detected by comparing the attenuation curves.

[0058] It may be advantageous if the frequency value of the measurement signal is equal to the measurable resonance frequency value of the positioning unit, and after briefly exciting the generator, the attenuation behavior of the positioning unit is recorded and compared with the previously recorded attenuation behavior to detect resonance changes. By observing changes in the natural resonance of the positioning unit, defects in components or structural parts can be detected.

[0059] While the generator is being excited to detect resonance changes when the frequency value of the measurement signal is equal to the measurable resonance frequency value of the positioning unit, the internal resistance RF of the positioning unit i =U A / I Ar is determined and may also advantageously be compared with the previously recorded internal resistance.

[0060] When a defect occurs in the positioning unit, part of the previously existing resonance changes. The resonance curve of the mechanical oscillator is characterized by its loss resistance R v (here referred to as internal resistance R i ). Each resonance of the positioning unit represents an oscillator with an internal resistance R i . R i changes due to the occurrence of a defect. The internal resistance R i can be determined by recording the voltage of the actuator U Ar and the current flowing through the actuator r at the specified resonance frequency f r . The recording of the voltage U Ar and the current I Ar can be easily realized and quickly analyzed by a microcontroller-based measuring device.

[0061] It may be even more advantageous if, while or after exciting the generator to detect resonance changes when the frequency value of the measurement signal is equal to the measurable resonance frequency value of the positioning unit, a reflected pulse is received by the receiver and its parameters are compared with the parameters of the previously received reflected pulse.

[0062] Defects may be detected using the propagation time, amplitude, or shape of the reflected pulse. The ultrasonic pulse can be transmitted by a generator of one actuator and received by a receiver of another actuator. By exciting a short pulse and detecting the resonance change of the positioning unit by evaluating the parameters of the reflected pulse, defects in the positioning unit can be quickly identified. In this case, the measurement signal may contain several pulses. The pulse can also be amplitude or phase modulated.

[0063] It may also be advantageous for the detection of resonance and the analysis of defects to be performed in the normal operating mode of the positioning unit. In this case, the resonance image of the positioning unit is recorded once during initial startup and then further recorded periodically or repeatedly during normal operation. The recorded measurements are compared and analyzed with the first recording. This allows the state of the positioning unit to be observed without interrupting its operation.

[0064] Furthermore, it may be advantageous for the analysis of the recorded resonance images to be performed visually by an operator. In this way, when non - characteristic resonance images occur, human intervention can be enabled or the replacement of the defective positioning unit or its defective components can be initiated. For this purpose, the device according to the invention may include a computer with a screen or monitor.

[0065] Further details, advantages, and features of the present invention will become apparent from the following description and drawings, but all details not described in the text are explicitly referred to.

Brief Description of the Drawings

[0066]

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Embodiments for Carrying Out the Invention

[0067] FIG. 1 schematically shows a positioning device 1 according to the present invention for detecting defects of a positioning unit 2 driven by an actuator 4. The positioning device 1 includes a controller 3 in addition to the positioning unit 2. The positioning unit 2 includes, in addition to a moving or driving function, a single piezoelectric stacked actuator 4 designed to have the functions of a generator 12 and a receiver 13 for acoustic ultrasonic waves, and a driving element 5 that is moved or driven by the actuator 4 and is coupled to an element 6 positioned by a fixed connection. Further, the positioning unit 2 includes a position sensor not shown in FIG. 1.

[0068] The actuator 4 is supported on holding elements 21 at both ends thereof, and the holding elements 21 are connected to a frame surrounding the actuator via a solid joint 9. As a result, the driving element 5 integrated with the frame is coupled to the actuator 4 via the solid joint 9, and the movement or deformation of the actuator 4 can be transmitted to the driving element 5.

[0069] The piezoelectric actuator 4 is composed of several layers 11, and each layer consists of two electrodes and a polarized piezoelectric material disposed therebetween. The possible polarization directions of the individual layers are indicated by the arrow P in FIG. 1.

[0070] The controller 3 has the function of controlling or adjusting the actuator 4 or the positioning unit 2, exciting the generator 12 with a measurement signal, and processing the signal from the receiver 13, and includes a control and regulation controller 14, a defect analysis device 16 that excites the generator 12 and records and analyzes the signal from the receiver 13, and optionally a commutator 31. In the commutator 31, switching is performed between the operating operation of the actuator 4 and the detection operation in which the actuator or a part thereof serves as the generator 12 and the receiver 13, respectively. Further, the controller 3 may interface with a computer 29 having a display screen on which an operator can visually perform defect analysis.

[0071] The control and regulation controller 14 includes a power output stage 15 for the actuator 4, a trajectory and signal generator 19, and a controller 18 for the position of the positioning unit 2 and optionally the speed and acceleration.

[0072] The defect analysis device 16 includes a current-voltage amplifier 17 for the generator 12, a measurement signal generator 22, a current sensor 23 for the signal generated by the receiver 13, and a resonance analyzer 24.

[0073] Figure 2 shows a schematic view of a preferred embodiment of a piezoelectric actuator 4 provided with a generator 12 and a receiver 13. The plurality of layers 11 of the actuator 4 are formed by conductive metallized surfaces and polarized piezoelectric materials located between those surfaces. In one possible variation of the electrical polarization of the plurality of layers 11, the polarization vectors of adjacent layers are directed in opposite directions to each other. The vector of the electrical polarization is indicated by an arrow P in Figure 2 and other corresponding figures. The plurality of layers 11 are electrically in parallel and mechanically connected in series.

[0074] Figure 3a) shows the voltage U of a measurement signal generator having a variable frequency MG and Figure 3b) shows a FEM model of the laminated actuator 4 with delamination between the layers of the layer structure. Figure 3c) shows an exemplary curve of the electrical impedance of the intact actuator as a function of the frequency f. Here, resonances of three vibration modes of the actuator 4, namely the first, third, and fifth longitudinal modes, are seen. Figure 3d) shows an exemplary curve of the electrical impedance of the actuator 4 with delamination as a function of the frequency f. Here, additional resonances generated by the delamination are seen.

[0075] Figure 4a) shows a FEM model of the positioning unit 2 provided with the actuator 4 according to Figure 1 having delamination between the layers of the layer structure. Figure 4b) shows an exemplary curve of the magnitude of the electrical impedance of the positioning unit 2 with an intact actuator 4 and a cracked actuator 4 as a function of the frequency f. Due to the delamination of the actuator 4, the curves of the magnitude of the impedance are significantly different. The resonances that existed when the actuator was intact have disappeared, and new resonances due to the delamination have been added.

[0076] Figure 5a) shows the amplitude spectrum of the positioning unit excited by white noise, and Figure 5b) corresponds to the currents flowing through the actuator 4 and the receiver 13 respectively, which are Fourier-transformed and recorded as a function of the frequency.

[0077] FIG. 6 shows exemplary current decay curves of the positioning unit 2 with an undamaged actuator and an actuator with delamination. The amplitude decay function A i2 The decay curve of the positioning unit with a damaged actuator having the amplitude decay function A changes in the resonance frequency and the decay constant λ2, and is faster than that of the A with an undamaged actuator. Due to delamination, the vibration period T2 of the amplitude decay function A i1 is decreasing. i2

[0078] FIG. 7 shows the current resonance curve of the positioning unit 2 according to FIG. 1 excited at one of its plurality of resonances by a sinusoidal measurement signal of amplitude U Ar . The current I Ar flowing through the receiver 13 is measured. R i =U A / I Ar is obtained by the defect analyzer and compared with the previously measured value. When a specified deviation occurs, a warning is output.

[0079] FIG. 8 shows different shapes of the ultrasonic pulse emitted by the generator 12 and the ultrasonic pulse reflected by the positioning unit 2 and detectable by the receiver 13. According to FIG. 8a), the ultrasonic pulse can have an exponential increase and an exponential decay. According to FIG. 8b), the ultrasonic pulse can have only an exponential decay. Or according to FIG. 8c), the ultrasonic pulse can also have different increase and decay functions. The ultrasonic pulses are characterized by their amplitude A P , frequency f P , duration τ, increase and decay functions f An , f Ap , and execution time t P .

[0080] Figure 9 shows the principle structure of a circuit implementation capable of simultaneously executing the normal operation and the resonance analysis mode of the positioning device or positioning unit. According to Figure 9a), the power output stage 15 is connected to the actuator 4 via an inductance L. The current or voltage amplifier 17 excites the actuator 4 or the generator 12 via a capacitance C. On the other hand, the capacitance C insulates the power output stage 15 from the amplifier 17 in a DC manner. On the other hand, the inductance L insulates the amplifier 17 from the power output stage 15 in an AC manner. According to Figure 9b), the power output stage 15 is connected to the actuator 4 via the secondary winding of the transformer T. The current or voltage amplifier 17 excites the generator 12 via the primary winding of the transformer T. The transformer T insulates the power output stage 15 from the amplifier 17 in a DC manner.

[0081] Figure 10 shows the positioning device 1 provided with the positioning unit 2 according to Figure 1. Here, the same power output stage 15 used to drive the stacked actuator by the control regulator is used as the output voltage or current amplifier of the measurement signal generator 22 by the defect analyzer. In this case, the power output stage 15 has a bandwidth sufficient to satisfy a plurality of requirements for generating ultrasonic waves by the actuator or the generator 12. Thereby, the cost and space for installing a separate amplifier are saved.

[0082] Figure 10 further shows an embodiment of the positioning device 1 in which the measurement signal generator 22 and the resonance analyzer 24 are housed in the same integrated circuit 30 as the control regulator 14. Figure 11 shows the principle structure of a possible circuit for the first processing of the electrical signal coming from the receiver 13 in the form of current or voltage. According to Figure 11a), the current I coming from the receiver 13 A can be represented by a voltage U using a resistor. i Using the circuit of Figure 11b), the current I coming from the receiver 13 A is converted into a voltage U' by a transistor. i In Figure 11c), the current I coming from the receiver 13 A is converted into a voltage U' using an optocoupler. iis mapped to. In the circuit arrangement shown in Fig. 11d), the current I A or voltage U A coming from the receiver 13 is converted to the voltage U’ i using a transformer. The circuit arrangement shown in Fig. 11e) uses an operational amplifier to convert the current I A or voltage U A coming from the receiver 13 to the voltage U V .

[0083] Figs. 1, 7, and 10 show schematic views of the positioning unit 2, where a single laminated piezoelectric actuator 4, which also forms the generator 12 and the receiver 13, is arranged between a plurality of solid joints 9.

[0084] Fig. 12 shows a schematic view of the positioning unit 2 with several, i.e., a total of 4 layers of laminated piezoelectric actuators 4, each actuator being arranged within or between a plurality of solid joints 9. The plurality of ends of each actuator 4 are in contact with a plurality of holding elements 21. The transmission of the movement from the drive element 5 of the actuator 4 to the element 6 to be positioned is effected via frictional contact, and the drive element 5 will either be in frictional contact with or in frictional contact with the friction rail 26 of the element 6 to be positioned. The element 6 to be positioned is here linearly mounted and guided via a guide device 20. A position sensor 28 is used to detect the position of the element 6 to be positioned.

[0085] Figure 13 shows the actuator 4 with a stacked design, where only some of the layers 11 are used to realize the generator 12 and the receiver 13. To realize the generator 12, it is conceivable to use a plurality of layers different from the number of layers used to realize the receiver 13. At the upper position of the commutator 31, a plurality of layers of the actuator are connected to the power output stage 15. The actuator is in the operating or driving mode. By switching to the lower position, a plurality of layers of the actuator for the purpose of the function of the generator or the receiver are connected to the power output stage, forming the generator 12 or the receiver 13 respectively. The current of the actuator is converted by the current sensor 23 into a voltage U i and further processed by the resonance analyzer 24.

[0086] Figure 14 shows an exemplary realization of the generator 12 or the receiver 13 as part of being connected to the actuator, i.e., it does not deform when a voltage is applied but acts as an actuator itself. The plurality of layers of the ultrasonic generator 12 and the receiver 13 have polarities opposite to each other. The generator 12 and the receiver 13 are connected to the remaining part of the actuator by an acoustic connection having a low acoustic resistance so that the ultrasonic waves are not substantially reflected or attenuated by the boundary layer. Such a connection can be realized, for example, by sintering the actuator to the generator or the receiver. Similarly, the components in the furnace can be connected by easily fusible glass or a similar hard material.

[0087] Referring to FIG. 1, the operating mode of the positioning device 1 according to the present invention or the method according to the present invention will be described. In a first step, a reference measurement (Etalonmessung) is performed. For this purpose, during the initial startup of the intact positioning unit 2, its resonance image is recorded by the defect analyzer 16. In this process, the piezoelectric actuator 4, in its function as an acoustic ultrasonic generator 12, is actuated by the measurement signal generator 22 using an electrical measurement signal. The measurement signal represents a voltage of frequency f. The measurement signal voltage is amplified by the current-voltage amplifier 17. The measurement signal voltage is amplified by the current-voltage amplifier 17 and sent to the generator 12 via the commutator 31. Thereby, the generator 12 is excited to generate ultrasonic waves radiated to the positioning unit. By the propagation of the ultrasonic waves, resonance vibrations are excited in a plurality of components of the positioning unit and in the actuator itself. These resonance vibrations then generate acoustic ultrasonic waves, which reach the receiver 13 together with the reflected ultrasonic waves and are detected by the receiver 13 in the form of a current change.

[0088] The current I from the receiver 13 A reaches the current sensor 23 of the defect analyzer, thereby converting it into a voltage U i and sending it to the defect analyzer 16. In the defect analyzer, the current I A or its image, the voltage U i , the voltage U from the measurement signal generator 22 MS , and the current I A and the phase angle value φ between the current I MS and the voltage U are recorded and stored, and from these, the resonance image (resonance image) of the positioning unit is created.

[0089] Thereafter, the positioning unit begins to operate to perform a plurality of intended positioning tasks. Here, the trajectory and the signal generator 19 control the actuator 4 using a control signal amplified by the power output stage 15 or conducted via the commutator 31. The actuator positions and moves the drive element 5 and the positioned element coupled thereto. The positioning can be controlled by the controller 18 using the position sensor 28.

[0090] After a certain operating time, the state of the positioning unit or a defect diagnosis is carried out. For this purpose, measurements are taken according to the first step of the method according to the invention. An electrical measurement signal is applied to generator 12 by measurement signal generator 22. As a result, generator 12 is excited to generate ultrasonic waves that are radiated to the positioning unit. As a result of the propagation of the ultrasonic waves, resonant vibrations are excited in a plurality of components of the positioning unit and in the actuator itself. These resonant vibrations then generate acoustic ultrasonic waves that reach receiver 13 together with the reflected ultrasonic waves and are detected by the receiver in the form of a current change.

[0091] The current I from receiver 13 A reaches current sensor 23 of the defect analysis device, thereby converting it into voltage U i and sending it to defect analysis device 24. In the defect analysis device, the current I A or its image, the voltage U i , the voltage U coming from measurement signal generator 22 MS , and the phase angle value φ between the current I A and the voltage U MS are recorded, stored, and a resonance image of the positioning unit is created from them.

[0092] In the next process step, resonance analyzer 24 compares the currently created resonance image of the positioning unit with the resonance image of an undamaged positioning unit. The presence of new mechanical resonances, changes or absences of previously existing mechanical resonances are determined. For this purpose, an appropriate algorithm, for example an algorithm of a neural network, is implemented in the resonance analyzer. If a defined deviation from the reference measurement is detected in the current measurement, this indicates an impending failure of the positioning unit and a warning is issued by the defect analysis device.

[0093] To create a resonance image of the positioning unit, various advantageous methods can be used. For example, the positioning unit 2 can be supplied with an electrical measurement signal representing a voltage of variable frequency f from a measurement signal generator 22 (see FIGS. 3 and 4). The frequency f varies here from an initial value to a final value. In the resonance analyzer 24, the current I A flowing through the receiver 13 or its image, the voltage U i , the voltage U MS coming from the measurement signal generator 22, and the current I A and the phase angle value φ between the voltage U MS are recorded as a function of the frequency f. For the purpose of detecting the resonance of the positioning unit, from the saved series of measurement values, a function of impedance with respect to frequency |Z| = UA / IA is formed. From the transition of the frequency dependence of the impedance, the resonance analyzer creates a resonance image of the positioning unit and generates a prediction or diagnosis of defects.

[0094] Furthermore, the positioning unit 2 can be supplied with an electrical measurement signal representing a voltage of a specific frequency f from the measurement signal generator 22. Resonance mapping is performed based on the parameters of individual resonances (see FIGS. 6 and 7).

[0095] In another advantageous method, the measurement signal generator 22 applies an electrical measurement signal with a short duration and at least a specific frequency f to the positioning unit 2. The resonance image of the positioning unit is created based on the parameters of the reflected ultrasonic waves (see FIG. 8). In this case, the resonance analyzer records and analyzes the duration, amplitude, propagation time, or shape of one or more reflected pulses.

[0096] When creating and visually analyzing the resonance image of the positioning unit manually, the data is output from the defect analysis device to a computer 22 having a display screen and analyzed by an operator.

Explanation of Signs

[0097] 1 Positioning device 2 Positioning unit 3 Controller 4 Actuator 5 Driving Element 6 Element to be Positioned 9 Solid Joint 11 (Piezoelectric Layer of Actuator 4) 12 Acoustic Ultrasonic Generator 13 Acoustic Ultrasonic Receiver 14 Control Regulator 15 Power Output Stage 16 Defect Analyzer 17 Current or Voltage Amplifier 18 Position, Velocity, or Acceleration Controller 19 Trajectory and Signal Generator 20 Guide Device 21 (Holding Element of Actuator 4) 22 Measurement Signal Generator 23 Current Sensor 24 Resonance Analyzer 25 Friction Rail 26 Position Sensor 29 Computer 30 Integrated Circuit (e.g., FPGA, DSP) 31 Electronic Commutator 32 Layer of Ultrasonic Generator and Receiver 33 Connection Layer between Actuator and Ultrasonic Generator and Receiver

Claims

1. A positioning device (1) comprising a positioning unit (2) provided with a piezoelectric actuator (4), a drive element (5) movable by the actuator (4) and provided for coupling to an element (6) to be positioned, and a controller (3), The positioning device (1) comprises a defect analysis device (16) for detecting defects of the positioning unit (2). When the positioning unit (2) comprises a single actuator (4), the actuator (4) comprises an acoustic ultrasonic generator (12) and a receiver (13). When the positioning unit (2) comprises a plurality of actuators (4), at least one of the plurality of actuators (4) comprises at least one ultrasonic generator (12), and at least one other of the plurality of actuators (4) comprises at least one ultrasonic receiver (13). The defect analysis device (16) comprises a measurement signal generator (22) for generating a measurement signal in the form of a voltage for exciting the generator (12) or generators (12), The defect analysis device (16) comprises a resonance analyzer (24) for analyzing the electrical signal generated by the receiver (13) or receivers (13), In order to predict or detect defects occurring within the positioning unit, the measurement signal generator (22) is configured to determine resonances that previously existed but have now disappeared or changed by comparing a resonance image created before normal operation with a resonance image created during operation, and to record newly created resonances, The measurement signal generator (22) is configured to perform a comparison between the resonance image created before the normal operation and the resonance image created during the operation according to one of the following (A), (B), (C), (D), (E), (A) The measurement signal generator (22) is configured to generate an electrical sinusoidal voltage whose frequency periodically changes from an initial value to a final value in order to determine resonances that previously existed but have now disappeared or changed, (B) The measurement signal generator (22) is, The frequency value of the measurement signal is equal to the measurable resonance frequency value of the actuator (4), the resonance frequency belongs to various types of ultrasonic waves, after the generator (12) is excited for a short time at the resonance frequency, the attenuation of the positioning unit (2) is recorded using the receiver (13), and then, in order to detect a change in resonance, the recorded attenuation curve is compared with a previously recorded attenuation curve. (C) The measurement signal generator (22) The frequency value of the measurement signal is equal to the measurable resonance frequency value of the positioning unit (2), after the generator (12) is excited for a short time, in order to detect a change in resonance, the attenuation behavior of the positioning unit (2) is recorded and compared with a previously recorded attenuation behavior. (D) The measurement signal generator (22) The frequency value of the measurement signal is equal to at least one measurable resonance frequency value of the positioning unit (2), during the excitation of the generator (12), in order to detect at least one resonance change, the internal resistance Ri = UA / IAr of the positioning unit (2) is determined and compared with the value of the internal resistance of the previously recorded positioning unit (2). (E) The measurement signal generator (22) The frequency value of the measurement signal is substantially equal to the measurable resonance frequency value of the positioning unit (2), during or after a short-time excitation of the generator (12), a reflection pulse is detected by the receiver (13), a plurality of parameters of the reflection pulse are recorded in order to detect a resonance change, and compared with a previously recorded plurality of parameters, positioning device (1).

2. The defect analysis device (16) comprises a broadband linear or clock-output voltage or current amplifier (17), the positioning device (1) according to claim 1.

3. The controller (3) comprises an output stage (15) for driving the positioning unit (2), and the same output stage (15) also serves to supply electricity to the measurement signal generator (20). The positioning device (1) according to claim 1, characterized in that.

4. The defect analysis device (16) comprises a white noise generator. The positioning device (1) according to claim 1, characterized in that.

5. The controller (3) comprises a position controller (18) and a track and signal generator (19). The position controller (18) or the track and signal generator (19) is realized by an integrated circuit, and the measurement signal generator (22) and the resonance analyzer (24) are realized as program modules within the same integrated circuit. The positioning device (1) according to claim 1, characterized in that.

6. The resonance analyzer (24) comprises a data interface to a display screen for visually controlling the resonance image. The positioning device (1) according to claim 1, characterized in that.

7. The defect analysis device (16) comprises a current sensor (23) for detecting the electrical signal generated by the receiver (13). In order to detect the current, a resistor, a transistor, a transducer, an optocoupler, or an operational amplifier is used. The positioning device (1) according to claim 1, characterized in that.

8. The actuator (4) is configured as a stacked piezoelectric actuator. The positioning device (1) according to claim 1, characterized in that.

9. The actuator (4) is arranged between a plurality of solid joints (9), and the transmission of the deflection of the actuator (4) to the drive element (5) is realized by the elastic deformation of the plurality of solid joints (9) without friction. The positioning device (1) according to claim 1, characterized in that.

10. The generator (12) or the receiver (13) forms part of the actuator (4) and is characterized by having no operating function, the positioning device (1) according to claim 1.

11. A part of the actuator (4) forming the generator (12) or the receiver (13) is connected to the remaining part of the same actuator (4) by an acoustic connection having a low acoustic resistance, the positioning device (1) according to claim 10.

12. The generator (12) is formed in one actuator (4), and the receiver (13) is formed in another separate actuator, the positioning device (1) according to claim 1.

13. A method of operating a positioning device (1) comprising a positioning unit (2) provided with a piezoelectric actuator (4), a drive element (5) provided to be movable by the actuator (4) and coupled to an element (6) to be positioned, and a controller (3), The positioning device (1) comprises a defect analysis device (16) for detecting a defect in the positioning unit (2). When the positioning unit (2) comprises a single actuator (4), the actuator (4) comprises an acoustic ultrasonic generator (12) and a receiver (13). When the positioning unit (2) comprises a plurality of actuators (4), at least one of the plurality of actuators (4) comprises at least one ultrasonic generator (12), and at least one other of the plurality of actuators (4) comprises at least one ultrasonic receiver (13). The defect analysis device (16) comprises a measurement signal generator (22) for generating a measurement signal in the form of a voltage for exciting the generator (12) or generators (12), The defect analysis device (16) comprises a resonance analyzer (24) for analyzing an electrical signal generated by the receiver (13) or receivers (13), An electrical measurement signal of the measurement signal generator (22) is periodically supplied to the generator (12) in the form of an alternating voltage, the mechanical resonance of the positioning unit (2) is periodically detected by the receiver (13), and in order to predict or detect a defect in the positioning unit (2), the resonance analyzer (24) compares a resonance image created before normal operation with a resonance image created during operation, so that the appearance of a new resonance or the disappearance or change of a previously existing resonance is detected and analyzed. The comparison between the resonance image created before the normal operation and the resonance image created during the operation is carried out according to one of the following (A), (B), (C), (D), and (E). (A) The measurement signal generator (22) generates an electrical sinusoidal voltage whose frequency periodically changes from an initial value to a final value in order to determine the resonance that previously existed but now has disappeared or changed. (B) The measurement signal generator (22) generates a measurement signal, the frequency value of the measurement signal is equal to a measurable resonance frequency value of the actuator (4), the resonance frequency belongs to various types of ultrasonic waves, after exciting the generator (12) for a short time at the resonance frequency, the attenuation of the positioning unit (2) is recorded using the receiver (13), and then, in order to detect a change in resonance, the recorded attenuation curve is compared with a previously recorded attenuation curve. (C) The measurement signal generator (22) generates a measurement signal, the frequency value of the measurement signal is equal to a measurable resonance frequency value of the positioning unit (2), after exciting the generator (12) for a short time, the attenuation behavior of the positioning unit (2) is recorded in order to detect a change in resonance and is compared with a previously recorded attenuation behavior. (D) The measurement signal generator (22) generates a measurement signal, the frequency value of the measurement signal is equal to at least one measurable resonance frequency value of the positioning unit (2), during the excitation of the generator (12), the internal resistance Ri = UA / IAr of the positioning unit (2) is determined in order to detect at least one resonance change and is compared with the value of the internal resistance of the positioning unit (2) recorded previously. (E) The measurement signal generator (22) generates a measurement signal, the frequency value of the measurement signal being substantially equal to the measurable resonance frequency value of the positioning unit (2). During or after a short excitation of the generator (12), a reflection pulse is detected by the receiver (13), and a plurality of parameters of the reflection pulse are recorded to detect a resonance change and compared with a plurality of previously recorded parameters. Method. **Claim 14**: In alternative (A), the frequency of the measurement signal is changed from an initial value to a final value, whereby the current value flowing through the receiver (13) and the phase angle value between the current and the voltage are measured in a form dependent on the frequency and recorded together with the voltage. From a series of measured values for detecting resonance, a function of the impedance amount |Z| dependent on the frequency is formed, and from the impedance amount |Z|, the presence of a new mechanical resonance or a change or absence of a previously detected mechanical resonance is determined. The method according to claim 13, characterized in that. **Claim 15**: In alternative (A), together with the phase angle, the function of the impedance amount |Z| dependent on the frequency is represented in a Nyquist diagram, and from this, the presence of the new mechanical resonance or a change or absence of the previously detected mechanical resonance is determined. The method according to claim 14, characterized in that. **Claim 16**: In alternative (A), the initial frequency value of the measurement signal is equal to the lowest resonance frequency value detectable by the actuator (4), and the final frequency value of the measurement signal is equal to the highest resonance frequency value measurable by the actuator (4). Both the lowest resonance frequency value and the highest resonance frequency value belong to different types of ultrasonic waves. The method according to claim 14, characterized in that. **Claim 17**: In alternative (A), the initial frequency value of the measurement signal is equal to the lowest resonance frequency value of the actuator (4) determined by the length of the actuator (4), and the final frequency value of the measurement signal is equal to twice the resonance frequency value determined by half the length of the actuator. The method according to claim 14, characterized in that. **Claim 18** The method according to claim 14, characterized in that the frequency of the measurement signal changes logarithmically from the initial value to the final value.

19. The method according to claim 13, characterized in that the measurement signal is white noise, the current flowing through the receiver (13) is measured and recorded, and the measurement results are Fourier-transformed, discrete Fourier-transformed, or fast Fourier-transformed in order to detect newly emerging or vanished resonances.

20. The method according to claim 13, characterized in that the analysis of the recorded resonance image is performed visually by an operator.

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