Piezoelectric dental scaler

Abstract

The invention relates to a piezoelectric dental scaler (100) comprising a handpiece (110) for generating ultrasonic vibrations and for transmitting the ultrasonic vibrations to a tool (115) that can be connected to the handpiece, wherein the ultrasonic vibrations are generated by means of a piezoelectric element (112) which is supplied with alternating current by a control unit (10), and wherein the control unit (10) is designed to regulate the phase shift between the current and the voltage of the alternating current supplied to the piezoelectric element (112) to a predetermined target value and, independently of the regulation of the phase shift, to regulate the current of the alternating current supplied to the piezoelectric element (112) to a current target value corresponding to a predetermined operating intensity of the handpiece (110).

Description

[0001] KaVo Dental GmbH

[0002] P62433 / WO

[0003] 1

[0004] PIEZOELECTRIC TARTAR REMOVER

[0005] The present invention relates to a piezoelectric tartar remover and a method for controlling such a device. In particular, the invention relates to a method for controlling a piezoelectric tartar remover that enables the precise and efficient generation of vibrations of a tool driven by the piezoelectric element of the tartar remover.

[0006] A tartar remover, also known as a piezoscaler, is a dental instrument used to remove tartar and plaque from teeth through high-frequency mechanical vibrations. These vibrations are generated by piezoelectric elements that respond to electrical voltage and produce mechanical vibrations. Compared to a conventional hand instrument, a piezoscaler offers the advantage of minimally invasive, fast, and effective tooth cleaning, especially in hard-to-reach areas. Piezoelectric dental instruments are used for tartar removal, periodontal treatments, and root planing.

[0007] One such piezoscaler is described, for example, in EP 3 348332 Al. This known scaler uses a piezoelectric transducer connected to a driver circuit via a phase-locked loop. This phase-locked loop synchronizes the transducer's drive voltage with its resonant frequency to ensure maximum efficiency in vibration generation. Synchronization is achieved by phase-matching the current, but this only works reliably under optimal operating conditions. A similar solution is described in EP 3 381 402, which uses piezoelectric elements for dental scalers. Here, the so-called magnetostrictive effect is used as an alternative method for vibration generation.

[0008] To determine the resonant frequency of such a piezoscaler, which can change during operation, EP 2011 577 A1 proposes incorporating excitation pauses for continuous recalculation of the resonant frequency. EP 2338438, on the other hand, discloses applying a pulse to the handpiece to determine the resonant frequency, as proposed by KaVo Dental GmbH.

[0009] P62433 / WO

[0010] 2

[0011] to evaluate the impulse response. However, these methods require additional sensors or operational pauses, which can impair the efficiency of the respective tartar remover.

[0012] A key problem with the known state of the art is that monitoring vibrations often requires expensive and error-prone sensors. Furthermore, frequent operational interruptions are necessary to continuously monitor and adjust the resonant frequency, reducing the efficiency and continuous use of the scaler. The impulse response analysis described in EP 2338438 is also complex and likewise leads to operational interruptions. Precise and sensitive vibration control is therefore not possible with known solutions from the prior art.

[0013] Document US 2006 / 269900 Al concerns an ultrasonic tartar removal device that uses magnetostrictive materials or piezoceramics. Here, digital filter control loops adjust the current amplitude and frequency as setpoints using PWM. Phase shift determination is an additional element of the data processing.

[0014] The present invention therefore aims to provide a novel control concept for a piezo scaler that avoids the aforementioned disadvantages. In particular, it aims to enable precise and continuous control of the amplitude of the treatment tool without the need for expensive sensors or operational interruptions for determining the resonance frequency.

[0015] The problem is solved by a piezoelectric tartar remover having the features of claim 1. Advantageous further developments of the invention are described in the dependent claims.

[0016] The core idea of ​​the invention lies in a novel control of the vibrations generated by the piezoelectric element, which enables precise and stable operation.

[0017] In particular, a novel concept for controlling a piezo scaler is presented, which ensures sensitive control of the amplitude of the tool.

[0018] The piezoelectric tartar remover according to the invention comprises a handpiece for generating ultrasonic vibrations, wherein these vibrations are generated by a device inside the KaVo Dental GmbH

[0019] P62433 / WO

[0020] 3

[0021] The piezoelectric element located in the handpiece is generated. The piezoelectric element is controlled by a control unit that provides an alternating current. Crucially, the control of the piezoelectric element is achieved via two separate control loops.

[0022] Firstly, a phase-locked loop (PLL) is incorporated, whose function is to monitor and regulate the phase shift between the current and voltage of the piezoelectric element. This phase shift is crucial for maintaining the resonant frequency, which significantly contributes to the efficiency of vibration generation. If the system frequency changes due to operating conditions such as mechanical stress or heating, the phase-locked loop ensures that the drive frequency is adjusted to guarantee optimal phase alignment and thus stable vibrations.

[0023] In parallel to the phase-locked loop, a second control loop is used to regulate the current supplied to the piezoelectric element. Provided the phase shift is kept constant, this current is directly proportional to the amplitude of the scaler, thus enabling precise control of the tool's amplitude, independent of the control frequency. This represents a significant advantage over previously known control methods for piezoelectric scalers.

[0024] According to the present invention, a piezoelectric tartar remover with a handpiece for generating ultrasonic vibrations and for transmitting the ultrasonic vibrations to a tool connectable to the handpiece is proposed, wherein the ultrasonic vibrations are generated by means of a piezoelectric element arranged in the handpiece, which is supplied with alternating current by a control unit of the piezoelectric tartar remover, and wherein the control unit is configured to

[0025] a) to control the phase shift between the current and the voltage of the alternating current supplied to the piezoelectric element to a predetermined setpoint in a phase control loop and

[0026] b) independently of the phase shift control in a current control loop, the current of the alternating current supplied to the piezoelectric element is reduced to a current- KaVo Dental GmbH

[0027] P62433 / WO

[0028] 4

[0029] To regulate a setpoint that corresponds to a predetermined intensity for the operation of the handpiece.

[0030] The problems encountered in the prior art are thus avoided according to the invention by a combined control of frequency and current, in which the phase control loop ensures that the system's frequency is continuously adapted to the operating conditions without interrupting operation. This maintains the resonant frequency during operation and keeps the relationship between current and the tool's amplitude stable. Ultimately, this makes it possible to adjust the amplitude independently of the frequency via the current control loop, so that it can be precisely and sensitively adapted to the current working conditions at any time. These measures lead to optimized operation of the tartar remover, since, for example, the tool's amplitude can be reduced under increased load to prevent damage to the tooth enamel and / or the tool itself.

[0031] According to an advantageous further development of the invention, the control unit includes a unit for measuring the current value of the alternating current supplied to the piezoelectric element. The current values ​​determined in this way are supplied as input values ​​to both the phase-locked loop and the current-locked loop. Components for determining the actual values ​​required for control are thus advantageously used by both control loops, which reduces the overall hardware effort for implementing the control unit operating according to the invention. The control unit can also preferably include a unit for measuring the voltage of the supplied alternating current or be designed to model the voltage waveform of the alternating current based on the measured current, whereby in the case of the second alternative the required hardware components can be further reduced.

[0032] The actual value representing the current phase shift can be determined in various ways, a preferred embodiment being to calculate this value directly or to determine it via a Fourier transform of the voltage or current waveform. Furthermore, the value representing the actual phase shift can also be determined based on zero-crossing measurements of the voltage or current waveform, or the control unit can be designed to determine it based on the active power and apparent power supplied to the piezoelectric element.

[0033] P62433 / WO

[0034] 5

[0035] The alternating current determines a value that represents the actual value of the phase shift. In the latter case, the effort required to implement the two control loops is somewhat reduced, since no separate sensors or measuring circuits are needed for certain input values.

[0036] A target value for the phase shift can be set independently of a predetermined intensity for the operation of the handpiece. This target value is initially set and subsequently maintained continuously during operation using a phase-locked loop (PLL). The PLL can include a digital or analog phase-locked loop (PLL) circuit to control the phase shift between current and voltage as efficiently as possible. Furthermore, the PLL can regulate the phase shift by adjusting the frequency of a driver signal for a power amplifier stage that generates the alternating current supplied to the piezoelectric element. The power amplifier stage is preferably analog or a Class-D amplifier. The PLL adjusts the frequency of the driver signal for the power amplifier stage from a predetermined initial value.Preferably, this initial value can be an external starting value dependent on the handpiece and / or tool, or a pre-determined value obtained using a resonant frequency search unit. In the latter case, the initial value for the frequency of the driver signal is determined once at the start of operation, and no further interruptions are necessary during subsequent operation, as is the case in the prior art. The output stage for generating the alternating voltage for operating the piezoelectric element can include controllable switches connected to form at least a half-bridge.

[0037] Within the framework of the independent current control according to the invention, it is preferably provided that the control unit is configured to adapt the duty cycle of a PWM signal for controlling the switches of the output stage. Furthermore, it is possible for the control unit to use a mathematical function and / or a so-called look-up table to adapt the target current to the current load or the operating mode of the handpiece in order to achieve a user-defined amplitude. The phase control loop and the current control loop are preferably implemented together in a microcontroller that executes both the control algorithms for frequency or phase control and for current control. This measure, in turn, can further reduce the material costs for implementing the control of the piezoelectric element according to the invention. KaVo Dental GmbH

[0038] P62433 / WO

[0039] 6

[0040] According to a further aspect of the present invention, a method for operating a piezoelectric tartar remover with a handpiece for generating ultrasonic vibrations and for transmitting the ultrasonic vibrations to a tool connectable to the handpiece is also proposed, wherein the ultrasonic vibrations are generated by means of a piezoelectric element arranged in the handpiece which is supplied with alternating current, and wherein in a phase control loop the phase shift between the current and the voltage of the alternating current supplied to the piezoelectric element is controlled to a predetermined setpoint and independently of this, in a current control loop the current of the alternating current supplied to the piezoelectric element is controlled to a current setpoint which corresponds to a predetermined intensity.

[0041] Preferably, a control signal from the current control loop and a control signal from the phase control loop are continuously supplied to a final stage that generates the alternating current supplied to the piezoelectric element.

[0042] Ultimately, the solution according to the invention makes it possible to achieve improved control for a piezoelectric tartar remover, which enables precise and stable control of the amplitude without the need for expensive sensors or interruptions in operation, thus enabling more efficient and gentler dental treatment overall.

[0043] The invention will now be explained in more detail with reference to the accompanying drawing. The drawing shows:

[0044] Figure 1 schematically shows the control of a piezoelectric tartar remover via a control unit;

[0045] Figure 2 shows the basic procedure for supplying a regulated alternating current to a piezoelectric tartar remover;

[0046] Figure 3 shows a schematic representation of the relationship between intensity and

[0047] Oscillation width of the piezoelectric and the corresponding target current;

[0048] Figure 4 shows the use of two independent control loops in generating a current supplied to the piezo scaler; and KaVo Dental GmbH

[0049] P62433 / WO

[0050] 7

[0051] Figure 5 shows the control concept according to the invention for controlling a piezo scaler.

[0052] Figure 1 shows an overview of a typical piezoelectric tartar remover 100, the main component of which is a handpiece 110. This handpiece 110 is designed to be detachably coupled to a tool specifically intended for treating tooth surfaces, the so-called scaler tip 115. The scaler tip 115 serves as the active element for the treatment and is set into vibration by the energy transmitted from the handpiece 115. These vibrations can be used to remove tartar and deposits from the tooth surface.

[0053] Inside the handpiece 110 is a piezoelectric element 112, which functions as a vibration generator. The piezoelectric element 112 is typically designed as a piezo stack, i.e., consisting of several layers of piezoelectric material. This element generates ultrasonic vibrations as soon as it is supplied with an electrical voltage. The generated vibrations are transmitted to the tool 115 via special coupling elements, which serve to optimally transmit and adapt the vibrations, enabling the tool 115 to be used for the precise and effective performance of dental treatments.

[0054] The handpiece 110, and in particular the piezoelectric element 112, is controlled by an external control unit 10, which is connected to the handpiece 110 via at least one cable and supplies the piezoelectric element 112 with a specially calibrated current responsible for generating the necessary vibrations. The control unit 10 can either be integrated into the treatment unit of a dental workstation or function as an independent, separate unit. In some cases, it is also conceivable that the control unit 10 provides additional media, such as air or water, to support the treatment, for example, by cooling and / or cleaning the treated area. However, the following discussion focuses exclusively on the control unit 10's function in operating the piezoelectric element 112.

[0055] The ultrasonic vibrations of the piezoelectric element 112 are generated by applying an alternating voltage to it. A known variant of a corresponding KaVo Dental GmbH

[0056] P62433 / WO

[0057] 8

[0058] The circuit arrangement often used to generate this alternating voltage is shown in Figure 2. This illustration particularly shows the electronic components that ultimately generate the alternating voltage to drive the piezoelectric element 112.

[0059] The alternating voltage is provided by a so-called output stage 40, which in the illustrated embodiment comprises two so-called half-bridges 41 and 42. Each of these half-bridges consists of two controllable switches (SI, S2 for the first half-bridge and S3, S4 for the second) connected in series between a supply voltage VCC and ground. The connection points of the switches within each half-bridge are configured as output lines and transmit the generated voltage to the handpiece 110.

[0060] Switches S1 to S4 of the two half-bridges 41 and 42 are each controlled by specific drivers 44 and 45, which are controlled by the central control unit 10. The control unit 10 ensures that switches S1 to S4, which in this example are implemented as controllable transistors, are alternately opened and closed. This alternating switching of the transistors generates a varying voltage on the respective phase lines, providing a two-phase power supply for the handpiece 110. This two-phase supply enables precise control and provides the necessary vibration excitation of the piezoelectric element 112.

[0061] To ensure that the voltage supplied to the piezoelectric element 112 remains within a predetermined, optimal range (typically around 200 V), a so-called matching stage 48 is provided in the illustrated embodiment. This matching stage 48 contains a transformer 49 that transforms the voltages generated by the half-bridges 41 and 42 and adapts them to a value suitable for the piezoelectric element 112. Alternatively, instead of this matching stage 48, it would also be possible to supply the half-bridges 41 and 42 with an input voltage VCC that is directly adapted to the level required by the piezoelectric element 112. In this case, the matching stage would be omitted, as the supply voltage would already have the desired value.

[0062] The use of such half-bridge arrangements for supplying piezoelectric elements in dental handpieces was already known. This technique enables KaVo Dental GmbH to

[0063] P62433 / WO

[0064] 9

[0065] The present invention relates specifically to an innovative method for regulating and adjusting the voltage profile of the voltage supplied to the piezoelectric element 112. This targeted modification of the voltage profile makes it possible to precisely control and optimize the operation of the tartar remover in order to ensure improved and controlled application.

[0066] During investigations, it was observed that at a fixed frequency and defined phase of the applied voltage, the oscillation velocity of the piezoelectric scaler, as well as the amplitude (as the integral of the oscillation velocity), are almost proportional to the applied current. In other words, it would be possible, in principle, to selectively influence the amplitude of the instrument 115 by appropriately adjusting the supply current.

[0067] However, this presents the problem that the operating frequency of a piezo scaler typically drifts during operation due to various physical influences. These influences can include, in particular, the mechanical stress or damping of the scaler during operation. Thermal heating or cooling of the piezoelectric element 112, mass changes of the vibrating system due to spray water, and / or the use of other tools are also factors that can lead to a change in the operating frequency.

[0068] These influences then result in the loss of the aforementioned advantageous, almost proportional relationship between current and amplitude during actual operation. However, by adjusting the frequency according to the invention based on the phase relationship between the piezo current and the piezo voltage, the desired frequency operating point can be readjusted, so that the advantageous proportional relationship between current and amplitude is maintained.

[0069] According to the present invention, it is therefore provided that a frequency or

[0070] Phase control achieves a decoupling between amplitude and frequency, which in turn makes it possible to subsequently adjust the amplitude via separate current control. Ultimately, this allows for targeted control of the amplitude of tool 115, which, according to KaVo Dental GmbH, then enables further training.

[0071] P62433 / WO

[0072] 10

[0073] The system's behavior, particularly its stiffness, can be precisely controlled. This means, for example, that it's possible to adjust the amount of power supplied under load. Furthermore, the system's load can be monitored during treatment, and the amplitude of the tool 115 can be selectively reduced or limited depending on the load to prevent damage to the tooth enamel or the tool itself.

[0074] The inventive principle of decoupled control of phase position and current is shown in more detail in Figure 4, which in particular shows the control processes of the inventive method carried out by the control unit 10.

[0075] The block 50, initially identifiable here as "Mathematical Function and / or Look-Up Table (LUT)", is responsible for outputting a setpoint for the power of the piezo scaler 110 and can also include, among other things, the previously mentioned extension for load-dependent and user-specific specification of the amplitude. This allows different power modes with varying stiffnesses and setpoint amplitudes to be specified, as shown schematically in Figure 3. The primary task of this block 50 is therefore to determine an adapted setpoint current based on any user input regarding a desired intensity or operating mode, as well as taking current and voltage into account. This setpoint then ultimately determines the amplitude or power at which the tool 115 of the handpiece 110 is to be operated.

[0076] Such calculations are preferably performed digitally using a suitably designed microcontroller; however, analog generation of a corresponding setpoint would also be conceivable. In both cases, the intensity can be amplified linearly to varying degrees depending on the mode, as indicated by Figure 3. Different offsets can also be specified for different operating modes. Furthermore, it would be conceivable to implement a different curve shape as an alternative to the linear relationship shown in Figure 3. For example, the slope could be made shallower with increasing power, so that the system reacts less aggressively, especially at peak power levels.

[0077] However, in order for the target values ​​for the amplitude provided by block 50 to be implemented appropriately, the KaVo Dental GmbH invention as described above must be carried out.

[0078] P62433 / WO

[0079] 11

[0080] Special regulation of the supply voltage to the piezo handpiece is required. As already mentioned, this is achieved by using two separate control loops to regulate the phase angle independently of the current.

[0081] For phase control, within the phase control loop designated with reference numeral 20, the phase difference between the current and voltage of the supply voltage supplied to the piezoelectric handpiece 110 is first determined using a suitable block 22. Like most of the other components described below, this measuring device for the phase position 22 can be implemented in software, thus minimizing the hardware effort required to perform the control according to the invention. As already mentioned, this block 22 requires input information regarding the current current and voltage profiles, whereby a value representing the current actual value of the phase shift is preferably determined based on a Fourier transform of the current current or voltage profile of the alternating current supplied to the piezoelectric element 112.However, a value representing the current actual value of the phase shift can also be determined based on the zero crossings of the current or voltage waveform of the alternating current supplied to the piezoelectric element 112. Furthermore, it would also be conceivable to determine the value representing the current actual value of the phase shift based on the active power and the apparent power of the alternating current supplied to the piezoelectric element 112.

[0082] The phase difference information obtained in this way is then fed to the actual controller 21, which adjusts the phase difference to a desired setpoint by appropriately adjusting the frequency of a driver signal used to control the power stage 40. For this purpose, the determined current phase difference is compared with another input value, which represents the setpoint of the phase difference and is preferably specified as a constant value "const" independent of the intensity for operating the handpiece 110. It would also be conceivable to make this setpoint modifiable, but this is not strictly necessary.

[0083] The actual control mechanism is implemented, for example, as an integral (I) controller, a pi (PI) controller, or a PID (Pulse Interlocked Direct Drive) or state controller. Controller 21 is part of the phase control loop 20 and is preferably implemented in software within a microcontroller, generally by KaVo Dental GmbH.

[0084] P62433 / WO

[0085] 12

[0086] Hardware implementation, for example using suitable operational amplifiers or FGPA, would also be conceivable. Crucially, a separate control loop 20 is implemented using a digital or analog phase-locked loop (PLL) circuit, which is solely responsible for controlling the frequency and thus for setting the phase difference of the supply current to the piezo scaler.

[0087] The frequency adjustment for the driver signal for the power amplifier 40 can be adapted starting from an initial value. This initial value can be fixed, for example, an externally specified starting value that depends on the handpiece 110 and / or the tool 115, or—as shown in the figures—a value determined in advance by means of a resonant frequency search unit 23. This unit 23, which in turn has information regarding the current voltage and current, determines a suitable starting value only once at the beginning of operation, unlike solutions known from the prior art. It is not required again during subsequent operation and, in particular, does not require any interruptions in operation.

[0088] The activity of this phase control loop 20 ultimately leads to the achievement of the desired linear relationship between the strength of the current supplied to the piezo handpiece 110 and the resulting amplitude of the treatment instrument 115.

[0089] This, in turn, opens up the possibility of specifically influencing the amplitude of the instrument 115 by regulating the current supplied to the piezoelectric element 112 independently of the phase control using a second current control loop 30. For this purpose, the current value can be determined via the output stage 40 described above (as can be seen from the schematic representation in Figure 4, the voltage plays no role in this part of the control; it is only supplied to the phase control loop 20 and, if necessary, to block 50, which is responsible for the setpoint specification) and supplied to the actual controller 31. As already mentioned, the controller 31 receives the setpoint information output by block 50, and the current is then regulated by appropriately adjusting the duty cycle for the control signals of the driver of the output stage 40.While the phase difference is regulated by adjusting the frequency of the drive signal for the output stage, the current is set by adjusting the duty cycle. Ultimately, this allows for targeted influence on KaVo Dental GmbH.

[0090] P62433 / WO

[0091] 13

[0092] The oscillation range, and thus the power of the tartar remover, can be reduced, enabling significantly more sensitive and precise work.

[0093] As already mentioned, it is preferably intended to implement a large proportion of the components relevant to the inventive method in software. Figure 5 shows in this context that the hardware effort for realizing the concept according to the invention is actually hardly increased compared to previously known control concepts. Thus, all components shown within the frame representing the control 10 can be implemented in software. In terms of hardware, primarily the output stage 40 (and, if applicable, the adaptation stage 48) must be implemented, which is required for the actual physical generation of the alternating voltage supplied to the piezo handpiece 110.Furthermore, in the illustrated embodiment, two analog-to-digital converters 61, 62 are required, which transmit the analog information regarding current and voltage received via the output stage 40 into digital information, which is then processed by the control unit. This processing within the framework of the two decoupled control loops 20, 30 ultimately leads to the aforementioned control information regarding frequency and duty cycle, which is output by the control unit 10 using a digitally controllable oscillator 63 to control the output stage 40.

[0094] It should be noted that in the illustrated case, both a unit for measuring the current and a unit for measuring the voltage of the alternating current supplied to the piezoelectric element 112 are provided. Alternatively, however, it would also be conceivable to perform only a current measurement and to model the voltage curve from this. In this case, the separate measurement of the voltage can then be omitted, thus further reducing the material costs for implementing the concept according to the invention.

[0095] Thus, according to the present invention, the behavior of a piezoelectric tartar remover can be significantly optimized with relatively little effort, since the power and therefore the amplitude of the tool can now be specifically influenced. Furthermore, changes occurring during operation, which previously could only be taken into account with considerable effort, can now be easily compensated for. Novel possibilities for defining the behavior of a piezoelectric tartar remover are also now available, which additionally prevent undesirable damage. KaVo Dental GmbH

[0096] P62433 / WO

[0097] 14

[0098] This can prevent damage to both the tool or handpiece and, in particular, to the teeth of the patient being treated.

Claims

KaVo Dental GmbH P62433 / WO 15 Claims 1. Piezoelectric tartar remover (100) with a handpiece (110) for generating ultrasonic vibrations and for transmitting the ultrasonic vibrations to a tool (115) that can be connected to the handpiece, wherein the ultrasonic vibrations are generated by means of a piezoelectric element (112) arranged in the handpiece (110), which is supplied with alternating current by a control unit (10) of the ultrasonic treatment device (100), and wherein the control unit (10) is designed to a) in a phase control loop (20) to control the phase shift between the current and the voltage of the alternating current supplied to the piezoelectric element (112) to a predetermined setpoint and b) independently of the control of the phase shift in a current control loop (30) to control the current of the alternating current supplied to the piezoelectric element (112) to a current setpoint that corresponds to a predetermined intensity for the operation of the handpiece (110).

2. Piezoelectric tartar remover (100) according to claim 1, characterized by that the control unit (10) has a unit for measuring the current value of the alternating current supplied to the piezoelectric element (112), wherein current values ​​determined by the unit are supplied as input values ​​to both the phase control loop (20) and the current control loop (30).

3. Piezoelectric tartar remover (100) according to claim 2, characterized by that the control unit (10) has an additional unit for measuring the voltage of the alternating current supplied to the piezoelectric element (112).

4. Piezoelectric tartar remover (100) according to claim 2, characterized by KaVo Dental GmbH P62433 / WO 16 that the control unit (10) is designed to model the voltage profile of the alternating current supplied to the piezoelectric element (112) on the basis of the measured current.

5. Piezoelectric tartar remover (100) according to one of the preceding claims, characterized in that that the control unit (10) is designed to determine a value representing the actual value of the phase shift on the basis of a Fourier transformation of the current waveform or the voltage waveform of the alternating current supplied to the piezoelectric element (112).

6. Piezoelectric tartar remover (100) according to one of claims 1 to 4, characterized in that, that the control unit (10) is designed to determine a value representing the actual value of the phase shift on the basis of zero crossings of the current waveform or the voltage waveform of the alternating current supplied to the piezoelectric element (112).

7. Piezoelectric tartar remover (100) according to one of claims 1 to 4, characterized in that, that the control unit (10) is designed to determine a value representing the actual value of the phase shift on the basis of the active power and the apparent power of the alternating current supplied to the piezoelectric element (112).

8. Piezoelectric tartar remover (100) according to one of the preceding claims, characterized in that that a target value for the phase shift is independent of a given intensity for the operation of the handpiece (110).

9. Piezoelectric tartar remover (100) according to one of the preceding claims, characterized in that that the phase-locked loop (20) includes a digital or analog phase-locked loop (PLL) circuit to control the phase shift between current and voltage. KaVo Dental GmbH P62433 / WO 17 10. Piezoelectric tartar remover (100) according to one of the preceding claims, characterized in that that the phase control loop (20) is configured to adjust the phase shift by adjusting the frequency of a driver signal for a final stage (40) which generates the alternating current supplied to the piezoelectric element (112), wherein the final stage (40) is preferably designed as an analog or as a Class-D amplifier.

11. Piezoelectric tartar remover (100) according to claim 10, characterized by that the phase control loop (20) adjusts the frequency of the driver signal for the output stage (40) starting from a predetermined initial value, wherein the initial value is in particular an externally specified starting value which depends on the handpiece (110), or a value determined in advance by means of a resonance frequency search unit (23).

12. Piezoelectric tartar remover (100) according to claim 10 or 11, characterized by that the final stage (40) comprises controllable switches (SI, S2, S3, S4) which are interconnected to form at least one HB back (42).

13. Piezoelectric tartar remover (100) according to claim 12, characterized by that the control unit (10) is designed to adapt the duty cycle of a PWM signal for controlling the switches (SI, S2, S3, S4) as part of current control.

14. Piezoelectric tartar remover (100) according to one of the preceding claims, characterized in that that the control unit (10) includes a mathematical function and / or a look-up table (50) for adjusting the set current to the current load or operating mode of the handpiece (110) in order to achieve a user-defined amplitude.

15. Medical ultrasound treatment device (100) according to one of the preceding claims, characterized by KaVo Dental GmbH P62433 / WO 18 that the phase control loop (20) and the current control loop (30) are implemented on a microcontroller which executes the control algorithms for both frequency and phase control as well as current control.

16. Method for operating a piezoelectric tartar remover (100) with a handpiece (110) for generating ultrasonic vibrations and for transmitting the ultrasonic vibrations to a tool (115) connectable to the handpiece (110), wherein the ultrasonic vibrations are generated by means of a piezoelectric element (112) arranged in the handpiece (110), which is supplied with alternating current, and wherein in a phase control loop (20) the phase shift between the current and the voltage of the alternating current supplied to the piezoelectric element (112) is controlled to a predetermined setpoint, and independently of the control of the phase shift in a current control loop (30) the current of the alternating current supplied to the piezoelectric element (112) is controlled to a current setpoint corresponding to a predetermined intensity.

17. Method for operating a piezoelectric tartar remover (100) with a handpiece (110) for generating ultrasonic vibrations and for transmitting the ultrasonic vibrations to a tool (115) connectable to the handpiece (110) according to claim 16, wherein a control signal from the current control circuit (30) and a control signal from the phase control circuit (20) are continuously supplied to a final stage (40) which generates the alternating current supplied to the piezoelectric element (112).

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