Method for controlling ultrasonic transducer, electrosurgical generator and electrosurgical apparatus

Abstract

The invention relates to a method for controlling an ultrasonic transducer, an electrosurgical generator and an electrosurgical apparatus. The ultrasonic transducer (206) is driven by a total current signal (im) having a signal frequency (f), the driving of the total current signal (im) results in an alternating instrument voltage signal (um) of the transducer (206), the instrument voltage signal (um) having a transducer phase shift with respect to a main current component of the total current signal (im) as a function of a mechanical oscillation of the transducer, the ultrasonic transducer (206) having a fluctuating mechanical resonance frequency, the amplifier (204) is controlled to provide a total current signal (im) and to adjust the signal frequency (f) as a function of the transducer phase shift towards the resonant frequency, and in order to determine the transducer phase shift, signal coefficients of the main current component and of the instrument voltage signal (um) are calculated, the signal coefficients respectively representing a phase relationship of the main current component or of the instrument voltage signal with respect to the common reference signal, and calculating a transducer phase shift from the signal coefficients.

Description

Technical Field

[0001] The present invention relates to a method for controlling an ultrasonic transducer of an electrosurgical instrument. The present invention also relates to an electrosurgical generator for controlling an ultrasonic transducer of an electrosurgical instrument. The present invention also relates to an electrosurgical device comprising an electrosurgical generator and an ultrasonic transducer of an electrosurgical instrument connected to the electrosurgical generator. Background Art

[0002] The ultrasonic transducer is used to convert the alternating current signal into the mechanical oscillation of the electrosurgical instrument with ultrasonic frequency. To this end, the alternating current signal excites the mechanical oscillation substantially at the resonant frequency of the ultrasonic transducer.

[0003] Can be based on Figure 3 The equivalent circuit shown models the ultrasonic transducer. The model shows the mechanical resonant circuit as the electrical equivalent (CM+LM+RM) and the parallel capacitance (CE).

[0004] The ultrasonic transducer needs to be driven at its mechanical resonant frequency. Since the resonant frequency varies with the mechanical load, a control loop may be needed to track the changing resonant frequency. A PLL can be used to adjust the frequency based on the phase shift between the measured voltage across the transducer and the current of the electrical equivalent of the mechanical resonant circuit (which can be understood as the mechanical current). When the phase shift is positive, the resonant circuit is inductive and the frequency is too high. When the phase shift is negative, the resonant circuit is capacitive and the frequency is too low.

[0005] A possible control loop may contain an analog sensor, an ADC and a digital controller (FPGA) driving the amplifier at a calculated frequency. The controller can determine the phase shift between current and voltage by comparing their respective zero crossings in the time domain.

[0006] Since zero-crossing detection is very sensitive to noise, the current and voltage signals must be filtered very carefully using low-pass filters.

[0007] Filtering signals in the digital domain is challenging. On the one hand, the signal must be oversampled by a high factor to accurately determine the signal's zero crossings. On the other hand, a robust FIR low-pass filter requires many taps when the cutoff frequency is lower than the sampling frequency.

[0008] One possibility to solve this problem is by filtering the signal in the analog domain. This introduces the problem of phase shift of the filter-generated signal. Therefore, the two analog filter paths need to be matched very closely to ensure that no additional relative phase shift is introduced. This requires a complex filter structure with components with tight tolerances. Summary of the invention

[0009] It is therefore an object of the present invention to solve at least one of the above problems. In particular, a solution for improving the control of an ultrasonic transducer should be provided, in particular a solution for identifying the mechanical resonance frequency of an ultrasonic transducer in an improved manner and / or making it more reliable should be provided. At least one solution should be proposed which is an alternative solution to the known solutions.

[0010] According to the present invention, a method for controlling an ultrasonic transducer of an electrosurgical instrument is proposed. The ultrasonic transducer is driven by an alternating current signal having a signal frequency. The transducer can be driven primarily by a current source having a voltage limit.

[0011] This total current signal results in an alternating electrical instrument voltage signal of the transducer, depending on the mechanical oscillation of the transducer. In other words, an alternating total current signal, which can be a predominantly sinusoidal current signal, is applied to the transducer. Depending on the transducer, an instrument voltage signal is generated. The shape of the instrument voltage signal depends primarily on the mechanical oscillation of the transducer.

[0012] Depending on the mechanical oscillations of the transducer, the instrument voltage signal has a transducer phase shift relative to the main current component of the total current signal.Thus, depending on the mechanical oscillations of the transducer, there will be a resulting phase shift, ie a phase shift between the instrument voltage signal and the main current component.

[0013] The ultrasonic transducer has a fluctuating mechanical resonant frequency. This mechanical resonant frequency may be in the range of 45kHz-50kHz, in particular in the region of 46kHz-49kHz. The resonant frequency may vary within this range. Of course, it may also be outside this range. However, the mechanical resonant frequency fluctuates at least slightly, and this may depend on the use of the electrosurgical instrument. For example, the resonant frequency may differ depending on whether the electrosurgical instrument is in contact with tissue or not.

[0014] There is also an amplifier that is controlled to provide a total current signal and adjust the signal frequency towards the resonant frequency according to the transducer phase shift. Therefore, the transducer phase shift indicates whether and how the voltage signal and the current signal meet the mechanical resonant frequency. If there is no phase shift, the resonant frequency is met. Therefore, it is important to determine the transducer phase shift and adjust the signal frequency so that the magnitude of the phase shift is reduced. In particular, a positive phase shift indicates that the resonant circuit is inductive and the frequency is too high, and should therefore be reduced. On the other hand, when the phase shift is negative, the resonant circuit is capacitive, the frequency is too low and should be increased.

[0015] In order to determine the transducer phase shift, it is proposed to calculate signal coefficients of the current signal and the main voltage signal. Such signal coefficients will represent the phase relationship of the main current component or the device voltage signal, respectively, relative to the common reference signal. Thus, there are calculated signal coefficients for the main current component representing the phase relationship of the main current component relative to the common reference signal, and there are calculated signal coefficients for the device voltage signal representing the phase relationship of the device voltage signal relative to the common reference signal.

[0016] The above-mentioned common reference signal can be predetermined for comparison purposes. However, such a common reference signal can also be only a theoretical reference signal or a virtual reference signal. As will be further explained below, for the same time interval, the same algorithm is used and the sampled values ​​of the current signal and the instrument voltage signal are used to calculate the signal coefficients for the main current component and the instrument voltage signal, respectively. In this way, using the same time interval, there are the same boundary conditions for calculating the signal coefficients of the two signals, resulting in having the same common reference signal. Therefore, when the same common reference signal is taken into account or at least used as the basis for calculation for both signals, the relationship regarding the transducer phase shift can be identified without explicitly giving the common reference signal.

[0017] It is also proposed to calculate the transducer phase shift from the signal coefficients.

[0018] Thus, signal coefficients characterizing a specific signal can also be calculated for their phase relationship with respect to a common reference signal, and this phase relationship can also be expressed as a phase angle. Thus, these signal coefficients also specify the phase angle of the two signals, and in this way, the transducer phase shift can be calculated, in particular by subtracting one phase angle from the other. In this way, the transducer phase shift can be calculated in an easy and reliable manner without having to compare any signal waveforms of the measured signals. It can also be done without identifying zero crossings or other significant points of the two signals.

[0019] In particular, to calculate the signal coefficients, the sampled values ​​of the two signals are considered over a whole period or at least half a period. As a result, therefore, in particular, there are two coefficients per signal, and therefore four coefficients in total, which are calculated based on a number of sampled values. For example, each signal can be measured over a period, using 100 samples per period, or even more. With respect to this example, two coefficients of one sampled signal are calculated based on 100 samples.

[0020] In this way any noise issues with the signal may be at least reduced.

[0021] According to one aspect, it is proposed to adjust the signal frequency to match the mechanical resonant frequency or to be closer to the mechanical resonant frequency based on the calculated transducer phase shift. Therefore, the transducer phase shift is calculated and indicates the deviation between the mechanical resonant frequency and the signal frequency. Therefore, the signal frequency is adjusted to match the mechanical resonant frequency, or at least better match the mechanical resonant frequency. It is found that it is not necessary to identify the mechanical resonant frequency, but only to identify whether the signal frequency deviates from the mechanical resonant frequency.

[0022] It is particularly advisable to determine whether the signal frequency is above or below the resonant frequency and then to increase or decrease the signal frequency accordingly to get closer to the resonant frequency.

[0023] Raising or lowering the signal frequency to get closer to the resonant frequency can be done in different ways. One possibility is to raise or lower the signal frequency by a constant value. This discrete approach is actually almost a continuous approach, since there is a high repetition frequency for performing this raising or lowering. Thus, the difference between the signal frequency and the resonant frequency results in a change, in particular a small change in the signal frequency, so that it is at least slightly closer to the mechanical resonant frequency. This step is repeated continuously, including repeating the calculation of the transducer phase shift. Thus, this adjustment step is repeated very frequently and each time causes the signal frequency to become slightly closer to the mechanical resonant frequency, until it almost reaches the mechanical resonant frequency.

[0024] Another possibility is to use a feedback control loop with PID or PI control. Depending on the integral part of the control, the signal frequency is found to maintain its value when the input is zero.

[0025] There are therefore different possibilities for designing such a feedback control loop. One is to use a PI or PID control, another is to add a constant value. In both cases, the feedback can be a calculated transducer phase shift, or a calculated difference between the signal frequency and a calculated, at least roughly calculated, mechanical resonant frequency. In both cases, the input value tends to zero, where the signal frequency matches the mechanical resonant frequency. It has been found that it is easiest to feed back only the calculated transducer phase shift and to use only a proportionality factor in order to convert the transducer phase shift into a frequency difference between the signal frequency and the mechanical resonant frequency. When the mechanical resonant frequency is measured or estimated, such a proportionality factor can be calculated or determined in a simulation or by measurements during test operation.

[0026] However, as an aspect, it is also possible and advisable to calculate the actual value of the mechanical resonant frequency based on the calculated transducer phase shift and to set the signal frequency based on the calculated mechanical resonant frequency, i.e. to set the signal frequency to the mechanical resonant frequency. It is found that the calculated transducer phase shift can be related to the frequency difference between the signal frequency and the mechanical resonant frequency. Since the signal frequency is known, the mechanical resonant frequency can be calculated quite accurately due to the generation of the AC total current signal.

[0027] It is found that the variation of the mechanical resonant frequency is not high, in particular below 10% of the average mechanical resonant frequency, and possibly even below 5% of this average mechanical resonant frequency. It can therefore be assumed that the calculated transducer phase shift is almost proportional to the difference between the mechanical resonant frequency and the signal frequency. The mechanical resonant frequency can therefore be calculated quite easily and quite accurately in this way.

[0028] According to one aspect, it is proposed that the calculation of the signal coefficients is based on samples of the total current signal and samples of the instrument voltage signal. In this way, these values ​​can be sampled during a control cycle, and each time a new value can be calculated, a new transducer phase shift can be calculated, and the signal frequency can be adjusted accordingly.

[0029] Thus, the instrument voltage signal will be measured and the total current signal may also be measured. However, the total current signal may also only be known because it is generated as part of the method, in particular by an electrosurgical generator generating such a current signal.

[0030] Based on such sampling values, a control cycle can be implemented which calculates the transducer phase shift for each sampling time and thus also adjusts the signal frequency for each sampling time.

[0031] According to one aspect, the signal coefficients characterize the first harmonic of the corresponding signal. That is, the two signal coefficients of the main current component characterize the first harmonic of the main current component, and the two coefficients of the instrument voltage signal characterize the first harmonic of the instrument voltage signal. The coefficients can be calculated in a quasi-continuous manner using a fast Fourier transform.

[0032] In this way, for each control period and thus for each sampling time, updated signal coefficients of the main current component and of the instrument voltage signal may be calculated and the transducer phase shift calculated based on the updated signal coefficients.

[0033] In this way, a reliable calculation is provided, avoiding the observation of any zero crossings of the specific signal which would otherwise be used to calculate the specific frequency and the specific phase angle.

[0034] According to one aspect, it is proposed that for each signal, ie for the main current component and the device voltage signal, the signal coefficients are the first harmonic Fourier trigonometric coefficients a1 and b1 calculated according to the following formula

[0035]

[0036] in

[0037] x[k] is a sample of the corresponding signal, and

[0038] N is the number of samples per signal, and

[0039] The samples are acquired at predetermined time intervals and / or at time intervals corresponding to one period of the nominal value of the resonant frequency.

[0040] This way of calculating the signal coefficients also uses samples of the corresponding signals, namely samples of the current signal and samples of the main voltage signal. The variable k can also be understood as a discrete sampling time, and the samples from k=0 to k=N-1 can reflect samples of one cycle of the signal frequency.

[0041] It is therefore also found that the signal frequency is well known, since the total current signal is generated and therefore its frequency is known. Therefore, the alternating period, i.e., the mainly sinusoidal signal, is also well known. Therefore, the formula for calculating the signal coefficients is applied to the sampled signal of exactly one signal period, resulting in a fairly accurate result for calculating the first harmonic Fourier trigonometric coefficients a1 and b1.

[0042] Of course, samples of two or more integer signal periods may also be used.

[0043] Even for such calculations using a complete signal period, fast calculations and therefore fast control are still possible, since the fundamental signal frequency is expected to be in the range of 40 kHz to 50 kHz. It can also be a little higher or lower. However, since the signal frequency is so high, the time required to sample the value of one signal period is in the range of about 20 μs. It has also been found that sampling within one signal period is sufficient, since it is sufficient to change the frequency of the current signal for each signal period.

[0044] According to one aspect, it is proposed that for the signal coefficients of each signal as the first harmonic Fourier trigonometric coefficients a1 and b1, based on the formula To calculate the absolute phase shift of each signal And the transducer phase shift is calculated as the difference between the absolute phase shifts of the two signals. That is, the transducer phase shift is calculated as the difference between the absolute phase shifts of the main current component and the instrument voltage signal.

[0045] Therefore, the calculation of the absolute phase shift and hence the transducer phase shift is quite sufficient, which provides good results and does not rely on observing any zero crossings.This calculation is also tolerant to some noise and other distortions of the corresponding signal (ie the current signal or the mains voltage signal).

[0046] According to one aspect, it is recommended that:

[0047] - For each signal, the signal coefficients are the first harmonic Fourier trigonometric coefficients a1 and b1;

[0048] - The transducer phase shift is calculated by approximation, so that the transducer phase shift is calculated as the difference of the quotient a1 / b1 of the two signals, in particular using the following formula

[0049]

[0050] in

[0051] a 1,i and b 1,i are the first harmonic Fourier trigonometric coefficients a1 and b1 of the main current component; and

[0052] a 1,u and b 1,u are the first harmonic Fourier triangular coefficients a1 and b1 of the instrument voltage signal.

[0053] Thus, the transducer phase shift can be calculated in an easy way and the use of trigonometric calculation atan2 can be avoided. It was found that the expected transducer phase shift (i.e. the corresponding phase shift angle) is rather small, i.e. well below 30°, making this approximation feasible. It was also found that by adjusting the signal frequency towards the mechanical resonance frequency according to the suggestion, a good matching of the signal frequency to the mechanical resonance frequency can be ensured by this control, thus with a small transducer phase shift. Thus, the suggested approximation can be used.

[0054] It has also been found that only the transducer phase shift needs to be calculated to adjust the signal frequency towards the mechanical resonant frequency. Therefore, any inaccuracy in the calculated transducer phase shift will not affect or will not significantly affect the control of adjusting the signal frequency. In particular, inaccuracies are only expected for large transducer phase shifts. However, at large transducer phase shifts, phase shift inaccuracies in the adjustment of the signal frequency are acceptable. When the signal frequency is close to matching the mechanical resonant frequency, higher accuracy is required. However, at this stage, the inaccuracies due to the approximation are also relatively small, which results in relatively small control inaccuracies (if any are significant).

[0055] According to one aspect, it is suggested to periodically repeat the calculation of the transducer phase shift and / or adjust the signal frequency to match the mechanical resonant frequency or be closer to the mechanical resonant frequency to track changes in the mechanical resonant frequency, and in particular, repeat the calculation or adjustment, wherein the predetermined repetition frequency is at least once every 10 signal cycles, preferably at least once every 2 signal cycles, and in particular once every signal cycle.

[0056] It was found that by such permanent repetition, the signal frequency can be controlled to be very close to the mechanical resonance frequency.By repeating the calculation of the transducer phase shift and / or the adjustment of the signal frequency, the ultrasonic transducer and thus the corresponding electrosurgical instrument can operate effectively almost all the time.

[0057] The mechanical resonant frequency may depend on whether the electrosurgical instrument is in contact with tissue or other material or not in contact with any material. Thus, only small movements of the electrosurgical instrument during operation may change the conditions, and thus the mechanical resonant frequency may also change. However, repeating the calculation and / or adjustment with one repetition per signal period results in a fast tracking of the changing conditions, since the signal period is approximately in the range of 20 μs. In contrast, manual movement of the electrosurgical instrument during operation may be quite slow when compared to the high repetition frequency.

[0058] Thus, changes in this situation can be tracked quickly enough to maintain good operation of the electrosurgical instrument. As described above, such an ultrasonic transducer, and therefore such an ultrasonic electrosurgical instrument used as an electrosurgical instrument, only operates well if the total current signal and the resulting AC voltage signal nearly match the mechanical resonant frequency. By such rapid repetition of calculations and / or adjustments, such good operation can be ensured.

[0059] Of course, the repetition frequency may also be reduced to once every 10 signal cycles, or to a value between once every 10 signal cycles and once every 1 signal cycle.

[0060] According to one aspect, it is suggested that the signal coefficients, in particular the signal coefficients of each signal are the first harmonic Fourier trigonometric coefficients a1 and b1, which are used to calculate the rms value of the current signal, in particular the rms value of the main current component, and / or the rms value of the voltage signal, and / or the actual power values ​​of the two signals. Therefore, each rms value and / or the actual power value of the two signals is calculated based on the first harmonic of the current signal and / or the voltage signal.

[0061] This is advantageous because there is a clear relationship between these frequencies in terms of the first harmonic and the deflection of the ultrasonic transducer and / or the electrosurgical instrument as an ultrasonic electrosurgical instrument. Therefore, this part of the signal corresponding to the mechanical deflection is therefore also a characteristic signal for taking into account this deflection. Therefore, taking into account such rms values ​​and / or real power values ​​provides an optimal relationship between the electrical signal under consideration and the mechanical behavior.

[0062] According to one aspect, it is recommended

[0063] - the total current signal has a main current component and a capacitive current component, the capacitive current component being dependent on the capacitance of the transducer, and

[0064] - calculating the main current component by subtracting the capacitive current component from the total current signal, in particular from the measured alternating current signal of the transducer, and / or

[0065] - The first harmonic Fourier trigonometric coefficients a1 and b1 are used to calculate the capacitor current signal, where in particular

[0066] - Use the following formula to calculate the capacitive current component i for capacitance C and frequency ω C :

[0067] i c =C*ω*a1*cos(ω*t)or i c =-C*ω*b1*sin(ω*t).

[0068] The nominal frequency ω0 can be used for ω. This is relevant to any of the formulas discussed.

[0069] It is therefore found that the total current, in particular the measured current, has a main current component and a capacitive current component. Thus, there is essentially a main current component and a capacitive current component as an additional component. The capacitive current component depends on the capacitance of the transducer. The capacitive current component is essentially - in simple terms - not involved in the mechanical movement of the driving ultrasonic electrosurgical instrument.

[0070] However, this capacitive current component causes a deviation between the phase angle of the voltage signal and the total current signal, which does not indicate any deviation between the signal frequency and the mechanical resonant frequency. Only the deviation of the phase angle between the main current component and the instrument voltage signal indicates a deviation between the signal frequency and the mechanical resonant frequency. Therefore, the capacitive current component is removed from the total current signal.

[0071] Therefore, the main current component is calculated by subtracting the capacitive current component from the total current signal. The total current signal can be measured, and thus the measured alternating current signal can be used as the total current signal. Of course, by generating this total current signal, the characteristics of the total current signal may be known, and these characteristics can be used instead of measuring the alternating current signal, i.e. the total current signal.

[0072] With the known capacitance of the ultrasonic transducer, the capacitive current component can be calculated. One way of doing this is to use the first harmonic Fourier triangular coefficients a1 and b1. These coefficients can be calculated as described above. In this way, substantially only the first harmonic of the current signal is considered, which is advantageous because substantially only this first harmonic component affects the phase shift caused by the capacity. In addition, the use of the first harmonic Fourier triangular coefficients is reliable and tolerant of noise and distortion on the signal. In particular, the given formula provides a simple way to calculate the capacitive current component.

[0073] On the other hand, it is recommended

[0074] -To calculate the transducer capacitance,

[0075] - generating a total current signal having a test frequency differing from a nominal value of the resonant frequency by at least a minimum frequency difference, thereby obtaining an instrument voltage signal,

[0076] - calculating a signal coefficient based on the total current signal and the resulting device voltage signal having the test frequency, and

[0077] - Calculate the transducer capacitance based on these signal coefficients, where in particular

[0078] The minimum frequency difference is at least 10 kHz, in particular at least 20 kHz.

[0079] Thus, it is found that the main current component driving the mechanical oscillations of the transducer and thus the electrosurgical instrument is not very dominant when the signal frequency differs greatly from the mechanical resonant frequency. In this case, achieved by having a minimum frequency difference of at least 10 kHz, capacitance is essentially the only characteristic of the transducer that affects the total current signal driven by the transducer. Therefore, the relationship between the total current signal and the resulting instrument voltage signal gives a clear indication of capacitance.

[0080] The selection of a minimum frequency of at least 10 kHz is based on the understanding that the resonant frequency is in the range of about 40 kHz to 50 kHz. This minimum frequency of 10 kHz is therefore a significant frequency difference, resulting in almost no triggering of any mechanical oscillation of the transducer or electrosurgical instrument at all. The minimum frequency may also be at least 20 kHz. In this way, the frequency is even further away from the mechanical resonant frequency. However, it should be remembered that the test frequency should at least not deviate too much, because in the case of too high a deviation, any nonlinearity of the capacitance can no longer be ignored. In this case, inaccuracies in the calculated capacitance may result.

[0081] According to one aspect, it is recommended

[0082] - for calculating signal coefficients, in particular for calculating signal coefficients of device voltage signals,

[0083] - modifying the device voltage signal into a modified voltage signal by adding a virtual voltage signal, wherein

[0084] - calculating a virtual voltage signal by multiplying the main current component by a predetermined virtual resistance, and in particular,

[0085] - using the previously calculated main current component as the main current component, and

[0086] - calculating a previously calculated main current component based on signal coefficients previously calculated for the voltage signal and the main current component, and / or

[0087] - directly calculating the modified voltage signal using the signal coefficients previously calculated for the voltage signal and the main current component, and / or

[0088] - Using the signal coefficients previously calculated for the voltage signal and the main current component and using a predetermined virtual resistance to directly calculate the coefficients that modify the voltage signal.

[0089] The basic idea in this regard is that the measured voltage may have a very small amplitude, especially considering the noise that is typically present on the measured voltage signal. Therefore, the evaluation or calculation may lead to problems with respect to the signal amplitude, such as stability problems if the noise level is too high.

[0090] It was found that this may happen if the instrument is in operation and operates at or near the resonant frequency. In this case, this virtual voltage signal, which is not affected by the noise, is added to the measured voltage signal and a signal with a sufficiently high amplitude is obtained when compared to the noise amplitude.

[0091] Since this added virtual signal does not exactly reflect the actual voltage at the transducer, it may have a slightly different phase angle and may result in a slight variation of the determined frequency value. This may result in a small frequency deviation from the ideal resonant frequency, but such deviations are acceptable. It has also been found that due to the frequent repetitions of setting the frequency, only small fluctuations about the ideal value of the frequency will occur. This has been found to be acceptable since, depending on the load of the transducer, the ideal resonant frequency will also fluctuate and the frequency will therefore have to be adjusted very frequently.

[0092] However, it has also been found that when the frequency is not too close to the resonant frequency, the measured voltage is also higher and high enough to be measured without problems, i.e. high enough to be distinguished from the noise level. Therefore, the virtual resistance can be selected so that the virtual voltage signal is only significant when the frequency is at or close to the resonant frequency. When the signal frequency deviates significantly from the resonant frequency, it is not significantly higher, i.e. lower than the measured voltage signal.

[0093] Therefore, when the signal frequency deviates significantly from the resonant frequency, the influence of the virtual resistance and / or the resulting virtual voltage can be ignored, and thus the deviation from the resonant frequency can be detected with high accuracy, thereby enabling the operating point to move toward the resonant frequency.

[0094] Preferably, the virtual voltage signal is selected so that its average rms value is between 3 and 20 times, in particular 5 to 10 times, the rms average value of the voltage signal at the resonant frequency. Based on the known average value of the voltage at the resonant frequency and the corresponding main current component, the virtual resistance can be calculated accordingly. The virtual resistance can be in the range of about 100-200 ohms. It can also be selected based on experience.

[0095] To calculate the main current component, the following formula can be used, which is explained further below:

[0096] i′(t)≈(a 1，i -CE·ω0·b 1，u )cos(ω0t)+(b 1，i +CE·ω0·a 1，u )sin(ω0t)

[0097] However, based on this, it is also possible to directly calculate the modified voltage, ie the modified voltage signal, using the signal coefficients previously calculated for the voltage signal and the main current component using the formula, which will be explained further below:

[0098] u′(t)≈(a 1，u +R virt (a 1，i -CE·ω0·b 1，u ))·cosω0t+(b 1，u +(b 1，i +CE·ω0·a 1，u ))sinω0t

[0099] Thus, even the modified voltage signal can be easily calculated using only these coefficients.

[0100] For the modified and compensated voltage signal a 1,u’ and b 1,u’ In terms of calculating the coefficients of directly using the signal coefficients previously calculated for the voltage signal and for the main current component and using a predetermined virtual resistance, the following relationship can be used, which is also explained in more detail below:

[0101]

[0102] Therefore, a 1,u’ and b 1,u’ It can be calculated as follows:

[0103] a 1，u’ =a 1,u +R virt (a 1，i –CEω0b 1,u )

[0104] b 1,u '=b 1,u +R virt (b 1,t +CEω0a 1,u )

[0105] According to one aspect, an electrosurgical generator for controlling an ultrasonic transducer of an electrosurgical instrument is also proposed, the electrosurgical instrument being adapted to perform a method according to any of the preceding aspects.

[0106] Thus, the electrosurgical generator may have any corresponding control unit, such as a digital controller, e.g., an FPGA, and an amplifier connected to and driven by the controller. The controller may provide a calculated frequency to be submitted to the amplifier, and the amplifier will then generate a corresponding current signal. The controller determines the phase shift between the current and the voltage, and will control the amplifier based on this determination.

[0107] Hence, any method according to the explained aspects may be at least partially implemented as a computer program on such a controller.

[0108] The electrosurgical generator may further comprise a current and voltage sensor for measuring the total current signal generated by the amplifier and for measuring the resulting instrument voltage signal.Such a sensor may in particular be connected to the controller and / or the amplifier.

[0109] An electrosurgical device comprising an electrosurgical generator and an electrosurgical instrument as described above is also proposed. The electrosurgical instrument is connected to the electrosurgical generator, and the electrosurgical instrument may have an ultrasonic transducer. In particular, the amplifier of the electrosurgical generator is connected to the ultrasonic transducer and provides a total current signal to the ultrasonic transducer. BRIEF DESCRIPTION OF THE DRAWINGS

[0110] The present invention will be explained in more detail by way of examples based on the accompanying drawings.

[0111] Figure 1 An electrosurgical generator with a connected electrosurgical instrument is shown.

[0112] Figure 2 A simplified control structure is shown.

[0113] Figure 3 The equivalent circuit of the ultrasonic transducer is shown.

[0114] Figure 4 Shown are two sin signals reconstructed by calculating the first-order Fourier coefficients. DETAILED DESCRIPTION

[0115] Figure 1 An electrosurgical generator 100 is shown to which an electrosurgical instrument 102 is connected, operating on tissue 104, shown schematically only.

[0116] The electrosurgical generator 100 comprises a frequency converter 106 controlled by a control device 108. When operating, the frequency converter provides at its output 110 a total current signal which is applied to the attached electrosurgical instrument 102, thereby generating an instrument voltage. Instrument voltage is a simplified term for, and therefore a synonym for, instrument voltage signal.

[0117] There is a voltage sensor 112 for measuring the instrument voltage signal and a current sensor 114 for measuring the total current signal. For further considerations, these sensors 112, 114 are connected to the control device 108.

[0118] The control device 108 calculates a signal coefficient from the current sample values ​​provided by the current sensor 114. Based on this, the control device 108 controls the frequency converter 106. Thus, at the frequency converter output, a total current signal is generated according to the calculated coefficient.

[0119] The voltage sensor 112 and / or the current sensor 114 may also be directly connected to the frequency converter 106. The control device 108 may also be a part of the frequency converter 106.

[0120] Figure 2 A simplified control structure 200 for controlling an ultrasonic transducer of an electrosurgical instrument is shown. To this end, Figure 2 Such an electrosurgical instrument 202 with an ultrasonic transducer is schematically shown. The electrosurgical instrument 202 and its ultrasonic transducer are connected to an amplifier 204. The amplifier 204 may be connected to Figure 1 The frequency converter 106 may be the same as or similar to the frequency converter 106 of FIG. 1 , or may be a part of such a frequency converter.

[0121] Amplifier 204 provides a total AC current i m , which is thus transmitted to the ultrasonic transducer 206 of the electrosurgical instrument 202. The total current is a simplified term for the total current signal and is therefore a synonym for it. m Generating device voltage u m The total current i m and the corresponding device voltage u m The signal frequency is f. When the signal frequency f matches the mechanical resonant frequency of the transducer 206, the transducer 206 works best. To ensure this, the voltage sensor 208 and the current sensor 210 are used to measure the total current i m and device voltage u m Therefore, the measuring device voltage u m and the total current i m For simplicity, it is assumed that the total current and its measured value are the same, and the device voltage and its measured value are also the same.

[0122] Measured device voltage u m is given to the voltage Fourier transform block 212 to calculate the first harmonic Fourier trigonometric coefficient a of the voltage signal 1,u and b 1,u Similarly, the total current i m is given to the current Fourier transform block 214 to calculate the first harmonic trigonometric coefficient a of the current signal 1,i and b1,i .

[0123] Therefore, the total current i m The first Fourier trigonometric coefficient a 1,i and b 1,i Indicates the total current i m However, in order to calculate and identify the signal frequency f and the mechanical resonance frequency f R The deviation-related phase shift The capacitive current component can be removed from the total current. The measured device voltage u m The first harmonic Fourier trigonometric coefficient a 1,u and b 1,u And the capacitive current component is calculated using the known capacitance C of the ultrasonic transducer 206. If the ultrasonic transducer capacitance C is unknown, it can be measured as described above, but it can also be known.

[0124] However, based on the first harmonic trigonometric coefficient a 1,u and b 1,u With the known capacitance C of the transducer 206, the total current i can be calculated m The capacitive current component and the total current i m Subtract the capacitor current component from the

[0125] This calculation is done in modification block 216. Thus, modification block 216 receives the measured device voltage u m The first harmonic trigonometric coefficient a 1,u and b 1,u , and receive the first harmonic Fourier trigonometric coefficient a of the current signal 1,i and b 1,i And the capacitance C of the ultrasonic transducer 206.

[0126] The total current i is removed from its capacitive current component m One possibility is to use these first harmonic trigonometric coefficients. The basic idea is to consider only the first harmonic of the total current and also only the first harmonic of the device voltage, since only it is relevant to the setting signal frequency f and only it is relevant to consider the capacitive current component. Therefore, instead of using the entire total current i m , but only the first harmonic is used, so only the corresponding coefficients of the first harmonic of the total current and the first harmonic of the device voltage can be considered. This is done in modification block 216, and it is also found that the Fourier trigonometric coefficient a of the first harmonic can be used to 1,i and b 1,i Modified to a' 1,i and b' 1,i to subtract the capacitive current component.

[0127] Therefore, by subtracting the capacitive current component from the total current im The capacitive current component is removed from the main current component, and the result is the remaining main current component. Therefore, the first harmonic of the main current component is determined by the modified first harmonic Fourier triangular coefficient a' 1,i and b' 1,i describe.

[0128] Therefore, to calculate the phase shift between the main current component and the device voltage, the corresponding coefficient a' 1,i and b' 1,i and a 1,u and b 1,u is given to the phase shift calculation block 218. The phase shift calculation block 218 thus calculates the phase shift And the calculated phase shift is input to the frequency adjustment block 220. The frequency adjustment block 220 also receives the signal frequency f as the actual signal frequency, and if the calculated phase shift If it is zero or below the relevant threshold, a small offset Δf is added or subtracted or the signal frequency f is kept unchanged.

[0129] Therefore, the result of the frequency adjustment block 220 is an adjusted frequency f a The adjusted frequency f a is given to the amplifier 204, and the amplifier 204 converts the adjusted frequency f a As a new signal frequency f. Therefore, the amplifier 204 generates and outputs a total current i having the signal frequency f m Therefore, the total current i m is provided to the electrosurgical instrument 202, thereby obtaining the instrument voltage u as described above m . Using the total current i m and device voltage u m With these new values, the next control cycle can begin.

[0130] Figure 3 An equivalent circuit of an ultrasonic transducer is shown, which shows a mechanical series resonant circuit given by the so-called mechanical capacitance CM, mechanical inductance LM and mechanical resistance RM. These three elements represent the electrical behavior of the ultrasonic transducer, which in turn depends on the mechanical behavior of the ultrasonic transducer.

[0131] In parallel, there is a capacitance CE representing the capacitive behavior of the ultrasonic transducer which is independent of the mechanical behavior of the ultrasonic transducer.

[0132] Thus, when in operation, the equivalent circuit 300 explains the total current i delivered to the ultrasonic transducer. m components, which are divided into capacitive current components i C and the main current component i'. Therefore, the equivalent circuit 300 also shows the device voltage um .

[0133] Figure 4 A diagram of two sin signals having different phase angles and thus phase shifted relative to each other is shown. The two sin signals comprise white noise and for both sin signals the first harmonics are calculated and thus reconstructed using the first harmonic Fourier trigonometric coefficients a1 and b1. The resulting sin signals A and B are Figure 4 Thus, the two signals A and B are each given by their first harmonic Fourier triangular coefficients a1 and b1, and these Fourier coefficients can be converted to the actual phase shift between signals A and B using the atan2 function as described above and explained further below.

[0134] Other aspects and explanations that help to better understand the present invention are given below.

[0135] It was found that the phase shift between related signals (i.e., voltage and current) can be determined in the digital domain using a robust algorithm that does not require low-pass filtering, since it automatically extracts the first harmonic of the signal. The idea is to calculate the first harmonic Fourier trigonometric coefficients a1 and b1 of the current and voltage according to the following formulas:

[0136]

[0137] where x[k] is the current / voltage sample, and is equal to the reference signal used by the amplifier and generated via a numerically controlled oscillator (NCO). The same NCO can be used to effectively create samples of the cosine function.

[0138] The relative phase shift of the signals can then be calculated as atan2(a1, b1).

[0139] In total, four coefficients need to be calculated, two for the measured current i meas (t), two voltages u for measurement meas (t), as follows:

[0140]

[0141] Therefore, the phase shift between voltage and current is atan2(a 1，u , b 1，u )-atan(a 1，i , b 1，i ). The two atan2 functions can be calculated separately, or the atan2 difference equation can be used to simplify the calculation to a single atan2 function:

[0142] atan2(a 1，u , b 1，u )-atan2(a1，i , b 1，i )=stan2(a 1，u b 1，i -a 1，i b 1，u , b 1，u b 1，i +a 1,u a 1，i )

[0143] Although the atan2 function can be computed efficiently with digital logic using the CORDIC algorithm, it can be The atan2 function can be simplified to division by linearizing the function, because the phase-locked loop (PLL) ensures that the phase shift is always close to 0:

[0144]

[0145] Typically, ultrasonic inverters implement voltage, current and power limiting. As a side-catch, the calculated Fourier coefficients can be directly converted to RMS current / voltage and real power according to the following formula:

[0146]

[0147] P=a1,a 1，u +b 1，i b 1，u

[0148] Other suggestions for improvement are as follows:

[0149] The following improvements are essentially independent of the phase shift detection algorithm, but can be implemented very efficiently when the above-described Fourier coefficient-based algorithm is used.

[0150] Compensation of the electrical transducer capacitance can be performed as follows.

[0151] The current through the instrument consists of a main current component, and can be expressed as a "mechanical" part, and a capacitive component, and can be expressed as a "capacitive" part or a capacitive current flowing through a capacitor. This capacitive current should be compensated for two reasons. First, the phase shift between the voltage and the "mechanical" current must be calculated. Second, ultrasound amplifiers are usually operated in a constant "mechanical" current mode.

[0152] The current can be compensated by subtracting the current through the capacitor, which is equal to the derivative of the measured voltage multiplied by the capacitance of the device. This is expressed in the following formula, where i'(t) is the main current component, that is, the current without the capacitive part, and i meas (t) is the total current, u meas (t) is the device voltage, and i c (t) is the capacitive current component.

[0153]

[0154] Computing the time domain derivative, and therefore the time domain differential, is very sensitive to noise. In contrast, the Fourier coefficients can be used to differentiate in the frequency domain.

[0155] If i meas (t) / u meas The first harmonic Fourier coefficient of (t) is given by a 1,i 、b 1，i and a 1,u 、b 1，u Given, the current and voltage can be described by the formula describing the Fourier series:

[0156] i meas (t) = a 1，i ·cos(ω0t)+b 1,i ·sin(ω0t)+residual

[0157] u meas (t) = a 1,u ·cos(ω0t)+b 1，u ·sin(ω0t)+residual

[0158] The residual represents the additional harmonics, and since only the first harmonic is considered, the residual can be ignored. Therefore, the formula can be written as:

[0159] i meas (t)≈a 1，i ·cos(ω0t)+b 1,t ·sin(ω0t)

[0160] u meas (t)≈a 1，u ·cos(ω0t)+b 1，u ·sin(ω0t)

[0161] On this basis, the coefficients are considered as follows, which means that the above formula is the basis for consideration. Therefore, the relationship can be symbolically explained as follows, where the double arrow indicates that the coefficients on the right side should be used in the corresponding formula given above:

[0162]

[0163] Based on this, the Fourier coefficients of the voltage derivative can be calculated using the properties of the Fourier series (above):

[0164]

[0165] Right now:

[0166] i′≈(t)≈(a 1,i -CE·ω0·b 1，u )cos(ω0t)+(b 1,i +CE·ω0·a 1，u ))sin(ω0t)

[0167] Differentiation in the time domain has become multiplication in the frequency domain. The last formula is only for illustration, as it may be sufficient for further considerations using only the changed coefficients.

[0168] Therefore, compensation of the capacitive current component mentioned above requires a factor CEω0 (CE*ω0), which is equal to the magnitude of the capacitor admittance at the resonant frequency. When the capacitance is unknown, it can be determined by driving the machine out of resonance and calculating

[0169]

[0170] That is, the measured admittance of the device at the measurement frequency is scaled by the ratio between the actual resonant frequency and the measurement frequency.

[0171] Another suggestion is to use virtual machine loads.

[0172] When the mechanical load on the device is low, the amplitude of the voltage can become very low. This can make it difficult to determine the phase shift between the voltage and the current. To make the algorithm more robust, a virtual resistance can be added to the device by adding a virtual voltage proportional to the (compensating) current to the measured voltage. Thus, considering the virtual resistance R virt , and obtain the modified voltage u'(t):

[0173] u′(t)=u meas (t)+R virt ·i′(t)

[0174] Again, this can be easily implemented in the frequency domain. Using the above definitions:

[0175]

[0176] The Fourier coefficients of the modified (compensated) voltage are obtained:

[0177]

[0178] Right now:

[0179] u′(t)≈(a 1，u +R virt (a 1,i -CE·ω0·b 1，u ))·cosω0t+(b 1，u +(b1,i +CE·ω0·a 1,u )sinω0t

[0180] The approximate equality indicates that further harmonics are neglected.

Claims

1. A method for controlling an ultrasonic transducer (206) of an electrosurgical instrument (102, 202), wherein - Through the total current signal (i) with signal frequency (f) m ) to drive the ultrasonic transducer (206), - transmitting the total current signal (i m ), and obtain the instrument voltage signal (u m ), - According to the mechanical oscillation of the ultrasonic transducer, the instrument voltage signal (u m ) has a relative current signal (i m ) of the transducer phase shift of the main current component, - the ultrasonic transducer (206) has a fluctuating mechanical resonance frequency, - controlling the amplifier (204) to provide the total current signal (i m ) and adjusting the signal frequency (f) toward the mechanical resonant frequency according to the transducer phase shift, and To determine the transducer phase shift: - Calculate the main current component and the device voltage signal (u m ), the signal coefficient representing the phase relationship of the main current component or the device voltage signal relative to a common reference signal, and - Calculating the transducer phase shift from the signal coefficients.

2. The method according to claim 1, wherein: - Based on the calculated transducer phase shift - adjusting the signal frequency (f) to match the mechanical resonant frequency or to be closer to the mechanical resonant frequency, in particular - determining whether the signal frequency (f) is above or below the mechanical resonance frequency, and - correspondingly raising or lowering the signal frequency (f) to be closer to the mechanical resonant frequency, or - calculating the actual value of the mechanical resonant frequency from the calculated transducer phase shift, and - Setting the signal frequency (f) according to the calculated mechanical resonance frequency.

3. The method according to claim 1 or 2, wherein: - the calculation of the signal coefficient is based on samples of the total current signal and samples of the instrument voltage signal, in particular samples of the measured current signal and samples of the instrument voltage signal determined from a measured alternating voltage of the ultrasonic transducer (206).

4. A method according to any one of the preceding claims, wherein: - The signal coefficient characterizes the first harmonic of the corresponding signal.

5. A method according to any one of the preceding claims, wherein: - for each signal, in particular for the main current component and the device voltage signal (u m ), the signal coefficients are the first harmonic Fourier trigonometric coefficients a1 and b1 calculated according to the following formula: in x[k] is a sample of the corresponding signal, and N is the number of samples per signal, and The samples are acquired at predetermined time intervals and / or at time intervals corresponding to one period of a nominal value of the resonant frequency.

6. A method according to any one of the preceding claims, wherein: - For each signal, the signal coefficients are the first harmonic Fourier trigonometric coefficients a1 and b1 - Calculate the absolute phase shift for each signal based on the following formula - and said transducer phase shift is calculated as the difference of said absolute phase shifts of the two signals.

7. A method according to any one of the preceding claims, wherein: - For each signal, the signal coefficients are the first harmonic Fourier trigonometric coefficients a1 and b1 - Calculate the transducer phase shift by approximation so that the transducer phase shift is calculated as the difference of the quotient a1 / b1 of the two signals, in particular using the following formula in a 1,i and b 1,i are the first harmonic Fourier trigonometric coefficients a1 and b1 of the main current component, and a 1,u and b 1,u is the device voltage signal (u m )'s first harmonic Fourier trigonometric coefficients a1 and b1.

8. A method according to any one of the preceding claims, wherein: - regularly repeating the calculation of the transducer phase shift and / or adjusting the signal frequency (f) to match the mechanical resonant frequency or to be closer to the mechanical resonant frequency to track changes in the mechanical resonant frequency, and in particular, The calculation or adjustment is repeated at a predetermined repetition frequency, which is at least once every 10 signal cycles, preferably at least once every 2 signal cycles, and in particular once every signal cycle.

9. A method according to any one of the preceding claims, wherein: The signal coefficients, in particular the first harmonic Fourier trigonometric coefficients a1 and b1 of each signal, are used to calculate - the rms value of the current signal, and / or - the rms value of the voltage signal, and / or - actual power values ​​of the current signal and the voltage signal, wherein in particular, Each rms value and / or real power value of the current signal and the voltage signal is calculated based on a first harmonic of the current signal and / or the voltage signal.

10. A method according to any one of the preceding claims, wherein: - the total current signal (i m ) has a main current component and a capacitive current component, the capacitive current component being dependent on the capacitance (C) of the ultrasonic transducer (206), and - By taking the total current signal (i m ), in particular, subtracting the capacitive current component from a measured alternating current signal of the ultrasonic transducer (206), and / or - using the first harmonic Fourier trigonometric coefficients a1 and b1 to calculate the capacitive current signal, wherein in particular, - The capacitive current component i is calculated for capacitance (C) and frequency ω using the following formula: C : i C =C ＊ w ＊ a1 ＊ cos(w ＊ t)or ic=-C ＊ wb1 ＊ sin(w ＊ t)。 11. A method according to any one of the preceding claims, wherein: - in order to calculate the transducer capacitance (C) or to calculate said transducer capacitance (C), - generating the total current signal (i m ), the test frequency of the total current signal differs from the nominal value of the resonant frequency or the nominal value by at least a minimum frequency difference, thereby generating an instrument voltage signal (u m ), - Based on the total current signal (i m ) and the resulting device voltage signal (u m ) to calculate the signal coefficients, and - Calculate the transducer capacitance (C) based on these signal coefficients, where in particular The minimum frequency difference is at least 10 kHz, in particular at least 20 kHz.

12. A method according to any one of the preceding claims, wherein: - to calculate the signal coefficients, in particular to calculate the device voltage signal (u m ) signal coefficient, - Add a virtual voltage signal to the device voltage signal (u m ) is modified to modify the voltage signal, where - calculating the virtual voltage signal by multiplying the main current component by a predetermined virtual resistance, and in particular, - using the previously calculated main current component as said main current component, and - calculating a previously calculated main current component based on said signal coefficients previously calculated for said voltage signal and said main current component, and / or - directly calculating the modified voltage signal using the signal coefficients previously calculated for the voltage signal and the main current component, and / or - using the signal coefficients previously calculated for the voltage signal and the main current component and directly calculating the coefficients of the modified voltage signal using the predetermined virtual resistance.

13. An electrosurgical generator (100) for controlling an ultrasonic transducer (206) of an electrosurgical instrument (102, 202), the electrosurgical generator being adapted to perform the method for controlling an ultrasonic transducer (206) of an electrosurgical instrument (102, 202) according to any one of the preceding claims.

14. An electrosurgical device, comprising: - an electrosurgical generator (100) according to claim 13, and - An electrosurgical instrument (102, 202) having an ultrasonic transducer (206) connected to said electrosurgical generator (100).

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