Control method, electrosurgical generator, and electrosurgical instrument

Abstract

To identify more accurately the mechanical resonance frequency of an ultrasonic transducer.SOLUTION: There is provided a method for controlling an ultrasonic transducer of an electrosurgical instrument 202. Therein, an amplifier 204 is controlled to: provide an AC overall current signal im to the ultrasonic transducer; and adjust a signal frequency f towards a mechanical resonance frequency according to a phase shift of ultrasonic transducer. The control method comprises the steps of: calculating signal coefficients for the main current component and the AC equipment voltage signal um in order to determine the phase shift of ultrasonic transducer, the signal coefficients being representative for a phase relationship of the main current component or the AC equipment voltage signal um respectively with respect to a common reference signal; and calculating from the signal coefficients the phase shift of ultrasonic transducer.SELECTED DRAWING: Figure 2

Description

[Technical field]

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

[0002] Ultrasonic transducers are used to convert alternating electrical signals into mechanical vibrations of an electrosurgical instrument having ultrasonic frequencies. To accomplish this, the alternating electrical signal essentially stimulates mechanical vibrations at the resonant frequency of the ultrasonic transducer.

[0003] An ultrasonic transducer can be modeled according to the equivalent circuit shown in Figure 3. This model shows the mechanical resonant circuit as an electrical equivalent (CM+LM+RM) and a parallel capacitance (CE).

[0004] An ultrasonic transducer needs to be driven at its mechanical resonant frequency. Since the resonant frequency changes with the mechanical load, a control loop may be needed to track the changing resonant frequency. Using a PLL (Phase Locked Loop), the frequency can be adjusted according to the measured phase difference between the voltage across the ultrasonic transducer and the current in the electrical equivalent of the mechanical resonant circuit, which can be understood as the mechanical current. If the phase difference is positive, the resonant circuit is inductive and the frequency is too high. If the phase difference is negative, the resonant circuit is capacitive and the frequency is too low.

[0005] A control loop that may be applied may include an analog sensor, an ADC (Analog-to-Digital Converter), and a digital controller (FPGA: Field-Programmable Gate Array) that drives an amplifier at a calculated frequency. The controller may determine the phase difference between the current and the voltage by comparing their respective zero crossings in the time domain.

[0006] Because 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 difficult: on the one hand, the signal must be oversampled by a high factor to accurately determine the instances when the signal crosses zero, and on the other hand, if the cutoff frequency is lower than the sampling frequency, a robust FIR low-pass filter would require many taps.

[0008] One possibility to solve this problem is to filter the signal in the analog domain. This raises the issue that the filter will generate phase differences in the signal. Therefore, both analog filter paths need to be very closely matched to ensure that no additional relative phase differences are introduced. This requires complex filter structures with components with tightly limited tolerances. Summary of the Invention [Problem to be solved by the invention]

[0009] It is therefore an object of the present invention to address at least one of the above problems. In particular, a solution should be provided for improving the control of an ultrasonic transducer, in particular for identifying the mechanical resonance frequency of an ultrasonic transducer in an improved manner and / or more reliably. At the very least, a solution should be proposed that represents an alternative solution to known solutions. [Means for solving the problem]

[0010] According to the present invention, a method according to claim 1 is proposed, i.e. a method for controlling an ultrasonic transducer of an electrosurgical instrument. Such an ultrasonic transducer is driven by an alternating total current signal having a signal frequency. Such an ultrasonic transducer may be driven primarily by a current source having a voltage limit.

[0011] Such an alternating total current signal produces an alternating machine voltage signal of the ultrasonic transducer in response to the mechanical vibration of the ultrasonic transducer. In other words, an alternating total current signal, which may be a predominantly sinusoidal current signal, is applied to the ultrasonic transducer. The alternating machine voltage signal is produced in response to the mechanical vibration of the ultrasonic transducer. The shape of the alternating machine voltage signal depends primarily on the mechanical vibration of the ultrasonic transducer.

[0012] The AC instrument voltage signal has a phase difference of the ultrasonic transducer with respect to the main current component of the AC total current signal in response to the mechanical vibration of the ultrasonic transducer. Thus, in response to the mechanical vibration of the ultrasonic transducer, a phase difference occurs, i.e., a phase difference between the AC instrument voltage signal and the main current component.

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

[0014] There are also amplifiers that provide an AC total current signal and are controlled to adjust the signal frequency relative to the mechanical resonant frequency depending on the phase difference of the ultrasonic transducer. Thus, the phase difference of the ultrasonic transducer indicates whether and possibly how well the AC machine voltage signal and the AC total current signal match the mechanical resonant frequency. If there is no phase difference, the mechanical resonant frequency is matched. It is therefore important to determine the phase difference of the transducer and adjust the signal frequency so that the amplitude of the phase difference is reduced. In particular, a positive phase difference indicates that the resonant circuit is inductive and the frequency is too high and should be reduced. On the other hand, a negative phase difference indicates that the resonant circuit is capacitive and the frequency is too low and should be increased.

[0015] To determine the phase difference of the ultrasonic transducer, it is proposed to calculate signal coefficients of the main current component and the AC equipment voltage signal. Such signal coefficients shall represent the phase relationship of the main current component or the AC equipment voltage signal, respectively, to a common reference signal. Thus, there are calculated signal coefficients of the main current component that represent the phase relationship of the main current component to the common reference signal, and there are calculated signal coefficients of the AC equipment voltage signal that represent the phase relationship of the AC equipment voltage signal to the common reference signal.

[0016] Such a common reference signal may be pre-determined for comparison. However, such a common reference signal may simply be a theoretical or virtual reference signal. As will be further explained below, to calculate the signal coefficients of the main current component and the AC equipment voltage signal, calculations are used using the same algorithm for the same time interval and using the respective sampled values ​​of the main current component and the AC equipment voltage signal. In this way, by using the same time interval, the same boundary conditions for calculating the signal coefficients of both signals are obtained, resulting in the same common reference signal. Thus, when the same common reference signal is considered for both signals, or this common reference signal is not explicitly given, but this common reference signal is at least the basis for the calculation, a relationship regarding the phase difference of the ultrasonic transducer can be identified.

[0017] Furthermore, it is proposed to calculate the phase difference of the ultrasonic transducer from the signal coefficients.

[0018] That is, signal coefficients can be calculated that characterize a particular signal with respect to the phase relationship of both signals to a common reference signal, which phase relationship can also be expressed as a phase angle. These signal coefficients thus also identify the phase angles of both signals, and in particular, by subtracting one phase angle from the other, the phase difference of the ultrasonic transducer can be calculated. In this way, the phase difference of the ultrasonic transducer can be easily and reliably calculated without comparing the signal waveforms of the measurement signals. This 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 both signals for the entire time or at least half the time are taken into account. Thus, four coefficients are obtained, in particular two coefficients for each signal, calculated based on many sampled values. For example, each signal may be measured using 100 samples each for one time, or more. For this example, the two coefficients of one sampled signal are calculated based on 100 samples.

[0020] In this way, the problem of noise in the signal can at least be reduced.

[0021] According to one aspect, it is proposed to adjust the signal frequency to coincide with or approach the mechanical resonance frequency depending on the calculated phase difference of the ultrasonic transducer. Thus, the phase difference of the ultrasonic transducer is calculated, which phase difference indicates the deviation between the mechanical resonance frequency and the signal frequency. The signal frequency is then adjusted to coincide with the mechanical resonance frequency, or at least to coincide better with the mechanical resonance frequency. It has been found that it is not necessary to determine the mechanical resonance frequency, but only whether the signal frequency deviates from the mechanical resonance frequency.

[0022] In particular, it is proposed to determine whether the signal frequency is above or below the mechanical resonant frequency, and then increase or decrease the signal frequency accordingly to move closer to the mechanical resonant frequency.

[0023] The increase or decrease of the signal frequency to approach the mechanical resonant frequency can be performed in various ways. One possibility is to increase or decrease the signal frequency by a constant value. In practice, such increases or decreases are performed repeatedly so frequently that such a discrete method becomes an almost continuous method. Thus, due to the difference between the signal frequency and the mechanical resonant frequency, the signal frequency is changed, in particular changed slightly, to at least slightly approach the mechanical resonant frequency. This process is repeated continuously, including repeated calculations of the phase difference of the ultrasonic transducer. Thus, such an adjustment process is repeated very frequently, each time bringing the signal frequency a little 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. With the integral part of the control, the sought signal frequency maintains its value when the input is zero.

[0025] There are therefore different possibilities for designing such a feedback control loop: one is to use PI or PID control, another is to increase by a constant value. In both cases the feedback can be the calculated phase difference of the ultrasonic transducer or the calculated difference between the signal frequency and the calculated, at least roughly calculated, mechanical resonance frequency. In both cases the input value tends to be 0, where the signal frequency corresponds to the mechanical resonance frequency. It was found that the simplest is to simply feed back the calculated phase difference of the ultrasonic transducer and simply use a proportionality factor to transfer the phase difference of the ultrasonic transducer to the frequency difference between the signal frequency and the mechanical resonance frequency. Such a proportionality factor can be calculated or determined in a simulation or can be determined by measurement during a test run, when the mechanical resonance frequency is measured or estimated.

[0026] However, it is also possible and proposed as an embodiment to calculate the actual value of the mechanical resonant frequency according to the calculated ultrasonic transducer phase difference, and set the signal frequency according to the calculated mechanical resonant frequency, i.e., set the signal frequency to the mechanical resonant frequency. It has been found that the calculated ultrasonic transducer phase difference can be correlated with 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 very accurately when the AC total current signal is generated.

[0027] It turns out that the variation rate of the mechanical resonant frequency is not high, in particular less than 10% of the average mechanical resonant frequency, and in some cases less than 5% of such average mechanical resonant frequency. It can therefore be assumed that the calculated phase difference of the ultrasonic transducer is approximately proportional to the difference between the mechanical resonant frequency and the signal frequency. The mechanical resonant frequency can therefore be calculated in this way very easily and very accurately.

[0028] According to one aspect, it is proposed to calculate the signal coefficients on the basis of samples of the AC total current signal and of the AC machine voltage signal, so that these values ​​can be sampled during the control cycle and each time a new value can be calculated, a new transducer phase difference can be calculated and the signal frequency can be adapted accordingly.

[0029] In this manner, the AC machine voltage signal is measured and the AC total current signal may also be measured, however the AC total current signal may simply be known as part of the method, particularly since it is generated by the electrosurgical generator that generates such a current signal.

[0030] Based on such sampled values, a control cycle can be implemented that calculates the phase difference of the ultrasonic transducer for each sample time and further adjusts the signal frequency at each sample time.

[0031] According to one aspect, the signal coefficients characterize the first harmonic of the corresponding signals. 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 AC equipment voltage signal characterize the first harmonic of the AC equipment voltage signal. Such coefficients can be calculated quasi-continuously using a Fast Fourier Transform.

[0032] In this way, for each control cycle, and further for each sample time, updated signal coefficients of the main current components and the AC machine voltage signal can be calculated, based on which the ultrasonic transducer phase difference can be calculated.

[0033] In this way, a reliable calculation method is provided which avoids observing the zero crossings of a particular signal which is normally used to calculate a particular frequency and a particular phase angle.

[0034] According to one embodiment, for each signal, i.e., for the main current component and the AC machine voltage signal, the signal coefficients are calculated according to the following equations (1) and (2): 1 and b 1 It is proposed that:

[0035]

number

[0036]

number

[0037] Where: x[k] is the corresponding signal sample, N is the number of samples per signal, The samples are taken for a predetermined time interval and / or for a time interval of one period corresponding to the nominal value of the resonant frequency.

[0038] This method of calculating the signal coefficients also uses samples of the corresponding signals, i.e. samples of the current signal and samples of the mains voltage signal. The variable k can also be understood as a discrete sample time, and the samples from k=0 to k=N-1 may reflect samples of one period of the signal frequency.

[0039] Thus, since an AC total current signal is generated and its frequency is known, it has also been found that the signal frequency is known. Therefore, the period of such an AC signal, i.e. a mainly sinusoidal signal, is also known. Thus, the above formula for calculating the signal coefficients is applied to a sampled signal of exactly one signal period, and this first harmonic Fourier trigonometric function coefficient a 1 and b 1 This gives very accurate calculation results.

[0040] Of course, it is also possible to use samples of two or more integer signal periods.

[0041] Even with such calculations using a complete signal period, fast calculations and therefore fast control are still possible, since the underlying signal frequency is expected to be in the range of 40 kHz to 50 kHz. The signal frequency could be a little higher or a little lower. However, with such high signal frequencies, the time required to sample the values ​​of one signal period is in the range of about 20 μs. Also, since it is only necessary to change the frequency of the current signal every signal period, it has been found that it is sufficient to take samples for one signal period.

[0042] According to one embodiment, the signal coefficients of each signal are the first harmonic Fourier trigonometric coefficients a 1 and b 1 If the absolute phase difference φ a is the formula φ a =atan2(a 1, b 1 ) and it is proposed that the ultrasonic transducer phase difference is calculated as the difference between the absolute phase differences of both signals, i.e., the ultrasonic transducer phase difference is calculated as the difference between the absolute phase differences between the main current components and the AC machine voltage signal.

[0043] This results in a very suitable calculation of the absolute phase difference, and therefore of the phase difference of the ultrasonic transducer, which provides good results and does not depend on the observation of zero crossings. Such a calculation is also resistant to any noise and other distortions of the corresponding signals, i.e. the current signals or the main voltage signals.

[0044] According to one aspect, it is proposed: The signal coefficients of each signal are the first harmonic Fourier trigonometric function coefficients a 1 and b 1 If The phase difference of the ultrasonic transducer is φ t is the quotient of both signals a 1 / b 1 In particular, φ is calculated by approximation as the difference between t is calculated using the following formula: φ t =a1,i / b 1,i -a 1,u / b 1,u Where: a 1,i and b 1,i is the first harmonic Fourier trigonometric coefficient a of the main current component 1 and b 1 and a 1,u and b 1,u is the first harmonic Fourier trigonometric function coefficient a of the AC device voltage signal 1 and b 1 It is.

[0045] In this way, the phase difference of the ultrasonic transducer can be easily calculated, and the use of trigonometric calculation atan2 (arc tangent) can be avoided. The phase difference of the ultrasonic transducer, i.e. the corresponding phase difference angle mentioned above, is expected to be very small, i.e. much less than 30°, and it was found that such an approximation is feasible. According to the proposed adjustment of the signal frequency to the mechanical resonance frequency, it was also found that such control allows the signal frequency to be well matched to the mechanical resonance frequency and to deal with the phase difference of the ultrasonic transducer to be small. In this way, the proposed approximation can be used.

[0046] It has also been found that the calculation of the ultrasonic transducer phase difference is only necessary to adjust the signal frequency to the mechanical resonant frequency. Therefore, the error of the calculated ultrasonic transducer phase difference does not affect, or does not affect significantly, the control of adjusting the signal frequency. In particular, it is expected that an error will occur only when the ultrasonic transducer phase difference is large. However, when the ultrasonic transducer phase difference is large, the error of adjusting the signal frequency is acceptable. If the signal frequency is close to matching the mechanical resonant frequency, a higher accuracy is required. However, at this stage, the error due to the approximation is also very small, and even if it is noticeable, the error of the control is quite small.

[0047] According to one aspect, calculating the phase difference of the ultrasonic transducer and / or adjusting the signal frequency to match or approach the mechanical resonant frequency is repeated periodically to track changes in the mechanical resonant frequency, and in particular it is proposed that this calculation or adjustment is repeated at a predefined repetition frequency, which is at least one repetition per 10 signal periods, preferably at least one repetition per 2 signal periods, in particular one repetition per signal period.

[0048] It has been found that this constant repetition allows the signal frequency to be controlled very close to the mechanical resonant frequency. By repeatedly calculating the phase difference and / or adjusting the signal frequency of the ultrasonic transducer, the ultrasonic transducer, and thus the corresponding electrosurgical instrument, can be operated effectively almost all the time.

[0049] 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, even a slight movement of the electrosurgical instrument during operation may change the above conditions and the mechanical resonant frequency may also change. However, since the signal period is in the range of about 20 μs, the calculation and / or adjustment is repeated at one iteration per signal period to rapidly track the changed conditions. In comparison, manual movement of the electrosurgical instrument during operation may be very slow when compared to the above high repetition frequency.

[0050] Thus, such changes in conditions can be tracked quickly enough to maintain good operation of the electrosurgical instrument. As explained above, such ultrasonic transducers used as electrosurgical instruments, and thus such ultrasonic electrosurgical instruments, only operate well if the AC overall current signal and the resulting AC instrument voltage signal closely match the mechanical resonant frequency. This rapid iteration of calculations and / or adjustments can ensure such good operation.

[0051] Of course, it is also possible to slow the repetition frequency down to 1 repetition per 10 signal periods or to a value between this and 1 repetition per 10 signal periods.

[0052] According to one embodiment, the signal coefficients of each signal, in particular the first harmonic Fourier trigonometric coefficients a 1 and b 1 It is proposed that the signal coefficients, which are: ##EQU1## are used to calculate the rms value of the current signal, in particular the main current component, and / or the rms value of the voltage signal and / or the active power value of both signals. The respective rms values ​​and / or active power values ​​of both signals are thus calculated on the basis of the first harmonic of the current signal and / or the voltage signal.

[0053] This is advantageous because there is a clear relationship between these frequencies due to the first harmonic and the deflection of the ultrasonic transducer and / or the electrosurgical instrument, which is the ultrasonic electrosurgical instrument. This part of the signal corresponding to the mechanical deflection is therefore also a characteristic signal for considering this deflection. Therefore, by considering such rms values ​​and / or active power values, the best relationship is provided between the considered electrical signal and the mechanical behavior.

[0054] According to one aspect, it is proposed: the AC total current signal has a principal current component and a capacitive current component, the capacitive current component being dependent on the capacitance of the ultrasonic transducer; The main current component is calculated by subtracting the capacitive current component from the total alternating current signal, in particular the measured total alternating current signal of an ultrasonic transducer, and / or First harmonic Fourier trigonometric function coefficient a 1 and b 1 is used to calculate the capacitive current components, in particular capacitive current component i C is calculated in terms of capacitance C and frequency ω using the following formula: i C =C*ω*a 1 *cos(ω*t) or i C =-C*ω*b1 *sin(ω*t).

[0055] ω is the nominal mechanical resonant frequency ω 0 can be used, which relates to all the formulas discussed.

[0056] Thus, it has been found that the total current, in particular the measured current, has a main current component and a capacitive current component. Thus, in principle, there exists a main current component and a capacitive current component as an additive component. This capacitive current component depends on the capacitance of the ultrasonic transducer. This capacitive current component is essentially, simply speaking, not involved in driving the mechanical movement of the ultrasonic electrosurgical instrument.

[0057] However, such a capacitive current component causes a phase angle shift between the AC machine voltage signal and the AC total current signal, which does not indicate a shift between the signal frequency and the mechanical resonant frequency. Only the phase angle shift between the main current component and the AC machine voltage signal indicates a shift between the signal frequency and the mechanical resonant frequency. Thus, the AC total current signal is cleared from this capacitive current component.

[0058] Thus, the principal current component is calculated by subtracting the capacitive current component from the total AC current signal. The total AC current signal can be measured and the measured total AC current signal can be used as the total AC current signal. Of course, by generating an total AC current signal whose characteristics may be known, it may be possible to use these characteristics instead of measuring the total AC current signal, i.e. the total AC current signal.

[0059] With the known capacitance of the ultrasonic transducer, the capacitive current components can be calculated. One way to do this is to use the first harmonic Fourier trigonometric coefficient a 1 and b 1They can be calculated as explained above. In this way, essentially only the first harmonic of the current signal is taken into account, which is advantageous, since it is essentially only this first harmonic component that affects the phase difference caused by the capacitance. Furthermore, using the first harmonic Fourier trigonometric coefficients is reliable and resistant to noise and distortion on the signal. In particular, the given formula provides a simple way to calculate the capacitive current components.

[0060] According to a further aspect, it is proposed: To calculate the capacitance of an ultrasonic transducer, an AC total current signal is generated having a test frequency that differs from the nominal value of the resonant frequency by at least a minimum frequency difference to produce an AC equipment voltage signal; The signal coefficient is calculated based on the AC total current signal and the resulting AC equipment voltage signal having the test frequency; The capacitance of the ultrasonic transducer is calculated based on these signal coefficients, in particular The minimum frequency difference is at least 10 kHz, in particular at least 20 kHz.

[0061] Thus, it has been found that the main current component that drives the mechanical vibration of the ultrasonic transducer, and therefore the electrosurgical instrument, is less dominant when the signal frequency is significantly different from the mechanical resonant frequency. In this case, which is achieved by having a minimum frequency difference of at least 10 kHz, capacitance is essentially the only ultrasonic transducer characteristic that affects the AC total current signal driven through the ultrasonic transducer. Thus, the relationship between the AC total current signal and the resulting AC device voltage signal gives a clear indication of capacitance.

[0062] The selection of a minimum frequency of at least 10 kHz is based on the understanding that the mechanical resonance frequency is in the range of about 40 kHz to 50 kHz. Therefore, this minimum frequency of 10 kHz is a significant frequency difference and hardly induces mechanical vibrations of the ultrasonic transducer or electrosurgical instrument. The minimum frequency can also be at least 20 kHz. In this way, the frequency is further away from the mechanical resonance frequency. However, it should be noted that the test frequency should not deviate at least too much, since if the deviation is too large, the nonlinearity of the capacitance may no longer be negligible. In that case, errors may occur in the capacitance calculation.

[0063] According to one aspect, it is proposed: To calculate the signal coefficients, in particular for calculating the signal coefficients of AC equipment voltage signals, The AC machine voltage signal is modified to a modified voltage signal by adding a virtual voltage signal; The virtual voltage signal is calculated by multiplying the main current component by a predetermined virtual resistance, in particular As the main current components the previously calculated main current components are used, a previously calculated main current signal is calculated based on previously calculated signal coefficients for the voltage signal and the main current signal; and / or The modified voltage signal is calculated directly using previously calculated signal coefficients for the voltage signal and the main current signal, and / or The coefficients of the modified voltage signal are calculated directly using the signal coefficients previously calculated for the voltage signal and the main current signal, using a given virtual resistance.

[0064] The basic idea of ​​this aspect is that due to the noise often present in the measured voltage signal, the measured voltage may have a very small amplitude, and therefore the evaluation or calculation may give rise to problems, such as stability problems, if the noise level is too high with respect to the signal amplitude.

[0065] It has been found that such a situation can occur when the equipment is operating and is operating at or near its mechanical resonant frequency, in which case such a virtual voltage signal, free of noise, is added to the measured voltage signal, resulting in a signal with an amplitude significantly higher than the noise amplitude.

[0066] This additional virtual voltage signal does not accurately reflect the actual voltage at the ultrasonic transducer and may have a slightly different phase angle, resulting in a small change in the measured frequency. This may result in a small deviation of the frequency from the ideal mechanical resonant frequency, but such deviation is tolerable. It has also been found that the frequency fluctuations around the ideal value are small because the frequency setting is repeated frequently. This is tolerable because the ideal resonant frequency also varies depending on the load on the ultrasonic transducer, and the frequency must be adjusted very frequently.

[0067] However, it has also been found that if the frequency is not too close to the mechanical resonant frequency, the measured voltage is also high, high enough to measure without problems, i.e. high enough to distinguish it from the noise level. The virtual resistance can therefore be selected such that the virtual voltage signal is significant only if the frequency is the same as or close to the mechanical resonant frequency. If the signal frequency deviates significantly from the mechanical resonant frequency, the virtual voltage signal will not be significantly high, i.e. will be lower than the measured voltage signal.

[0068] Thus, if the signal frequency deviates significantly from the mechanical resonant frequency, the effects of the virtual resistance and / or the resulting virtual voltage can be ignored, the deviation from the mechanical resonant frequency can be detected with high accuracy, and the operating point can be moved towards the mechanical resonant frequency.

[0069] Preferably, the virtual voltage signal is selected such that its average rms value is 3 to 20 times, in particular 5 to 10 times, the rms average value of the voltage signal at the mechanical resonant frequency. Based on the known average values ​​of the voltage and the corresponding main current components at the mechanical resonant frequency, the virtual resistance can be calculated. The virtual resistance may be in the range of about 100-200 Ohms. It can also be selected empirically.

[0070] To calculate the principal current components, the following equation (3), which is further explained below, can be used.

[0071]

number

[0072] However, on that basis, it is also possible to directly calculate the corrected voltage, i.e. the corrected voltage signal, using the previously calculated signal coefficients for the voltage signal and the main current signal, using equation (4), which is further explained below.

[0073]

number

[0074] In this way, even the corrected voltage signal can be calculated very easily just by using these coefficients.

[0075] Corrected and compensated voltage signal a 1,u ' and b 1,u For the aspect where the coefficients of ' are calculated directly using the signal coefficients previously calculated for the voltage signal and the main current signal and using a given virtual resistance, the following relations can be used, which are explained in more detail below: u'(t)⇔a 1,u +R virt (a 1,i -CEω 0 b 1,u ),b 1,u +R virt (b 1,i +CEω0 a 1,u )

[0076] Therefore, a 1,u ' and b 1,u ' can be calculated as follows: a 1,u '=a 1,u +R virt (a 1,i -CEω 0 b 1,u ) b 1,u '=b 1,u +R virt (b 1,i +CEω 0 a 1,u )

[0077] According to one aspect, there is also proposed an electrosurgical generator for controlling an ultrasonic transducer of an electrosurgical instrument, adapted to perform a method according to any of the previous aspects.

[0078] 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 a controller. The controller may provide a calculated frequency that is sent to the amplifier, which then generates a corresponding current signal. The controller determines the phase difference between the current and the voltage and controls the amplifier based on that determination.

[0079] Thus, any of the methods according to the described aspects may be implemented, at least in part, as a computer program on such a controller.

[0080] The electrosurgical generator may also include current and voltage sensors for measuring the overall AC current signal generated by the amplifier and for measuring the resulting AC machine voltage signal. Such sensors may be connected to the controller and / or the amplifier, among others.

[0081] There is also proposed an electrosurgical installation comprising an electrosurgical generator and an electrosurgical instrument as described above. The electrosurgical instrument is connected to the electrosurgical generator, the electrosurgical instrument may comprise an ultrasonic transducer. In particular, an amplifier of the electrosurgical generator is connected to the ultrasonic transducer and supplies the ultrasonic transducer with an overall current signal.

[0082] The invention will now be explained in more detail, by way of example only, on the basis of the accompanying drawings, in which: [Brief description of the drawings]

[0083] [Figure 1] FIG. 1 shows an electrosurgical generator with an electrosurgical instrument connected thereto. [Diagram 2] FIG. 2 illustrates a simplified control structure. [Diagram 3] FIG. 2 is a diagram showing an equivalent circuit of an ultrasonic transducer. [Figure 4] 2 shows two sinusoidal signals reconstructed by calculating the first Fourier coefficients. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0084] FIG. 1 shows diagrammatically an electrosurgical generator 100 connected to an electrosurgical instrument 102 operating on tissue 104 .

[0085] The electrosurgical generator 100 includes a frequency converter 106 controlled by a controller 108. In operation, the frequency converter provides an AC total current signal at its output 110 that is applied to an attached electrosurgical instrument 102, resulting in an instrument voltage, which is a shortened term and synonym of the AC instrument voltage signal.

[0086] A voltage sensor 112 measures the voltage signal and a current sensor 114 measures the AC total current signal. These sensors 112, 114 are connected to the controller 108 for further consideration.

[0087] The controller 108 calculates a signal coefficient from the current sample values ​​provided by the current sensor 114. Based on this, the controller 108 controls the frequency converter 106. Thus, at the frequency converter output, an AC total current signal is generated according to the calculated coefficient.

[0088] It is also possible that the voltage sensor 112 and / or the current sensor 114 are directly connected to the frequency converter 106. It is also possible that the controller 108 is part of the frequency converter 106.

[0089] Fig. 2 shows a simplified control structure 200 for controlling an ultrasonic transducer of an electrosurgical instrument. To this end, Fig. 2 shows diagrammatically such an electrosurgical instrument 202 having an ultrasonic transducer. The electrosurgical instrument 202, and thus the ultrasonic transducer, is connected to an amplifier 204. The amplifier 204 may be the same as or similar to the frequency converter 106 of Fig. 1, or may be part of such a frequency converter.

[0090] The amplifier 204 delivers a total current i to the ultrasonic transducer 206 of the electrosurgical instrument 202. m The total current is a simplified term and synonym of the AC total current signal. This total current i m to drive the ultrasonic transducer 206, thereby generating an instrument voltage u m The total current i m , and therefore the device voltage u m has a signal frequency f. The ultrasonic transducer 206 works best when this signal frequency f matches the mechanical resonant frequency of the ultrasonic transducer 206. To ensure that, the total current i m and device voltage u m is measured using the voltage sensor 208 and the current sensor 210. Thus, the device voltage u m and the total current i m is measured. For simplicity, it is assumed that the total current and its measurement are the same, and the device voltage and its measurement are also the same.

[0091] Measured device voltage u m is the first harmonic Fourier trigonometric function coefficient a of the voltage signal 1,u and b 1,u Similarly, the measured total current i m is the first harmonic trigonometric coefficient a of the current signal 1,i and b 1,i The current is fed to a current Fourier transform block 214 which calculates

[0092] Therefore, the measured total current i m The first harmonic Fourier trigonometric coefficient a 1,i and b 1,i is the total current i m However, the mechanical resonance frequency f R For the calculation of the phase difference φ, which is related to determining the deviation of the signal frequency f from the total current i m can be cleared by a capacitive current component, which is the measured device voltage u m First harmonic Fourier trigonometric coefficient a 1,u and b 1,u and the capacitance C of the ultrasonic transducer 206 is known. If the capacitance C of the ultrasonic transducer 206 is not known, it can be measured as described above, but it may be known.

[0093] However, this first harmonic trigonometric coefficient a 1,u and b 1,u and the total current i m Based on the known capacitance C of the transducer 206, the total current i m Calculate the capacitive current component of the total current i m can be subtracted from

[0094] This calculation is performed in the correction block 216. Thus, the correction block 216 calculates the measured instrument voltage u m The first harmonic trigonometric coefficient a 1,u and b 1,uand calculates the first harmonic Fourier trigonometric coefficient a of the current signal. 1,i and b 1,I It also receives the capacitance C of the ultrasonic transducer 206 .

[0095] The total current i m One possibility to clear is to use these first harmonic trigonometric coefficients. The basic idea is to consider only the first harmonic of the total current and only the first harmonic of the equipment voltage, since only that is relevant for setting the signal frequency f and only that is relevant for considering the capacitive current component. Thus, the total current i m By using only the first harmonic, rather than using all of the first harmonic, it is possible to consider only the corresponding first harmonic coefficients of the total current and the equipment voltage. This is done in the correction block 216, where the first harmonic Fourier trigonometric function coefficient a 1,i and b 1,I a' 1,i and b' 1,i It was also found that the capacitive current component can be subtracted simply by modifying

[0096] Therefore, the total current i m is cleared from the capacitive current component by subtracting it, and the result is the remaining principal current component. Therefore, the first harmonic of this principal current component is expressed by the modified first harmonic Fourier trigonometric function coefficient a' 1,i and b' 1,i This is explained by:

[0097] Therefore, to calculate the phase difference between the main current components and the instrument voltage, the corresponding coefficient a' 1,i and b' 1,i And a 1,u and b 1,uis provided to a phase difference calculation block 218, which then calculates a phase difference φ, which is input to a frequency adjustment block 220. The frequency adjustment block 220 also receives the signal frequency f, which is the actual signal frequency, and adds a small offset Δf, or subtracts such a small offset Δf, or leaves the signal frequency f unchanged if the calculated phase difference φ is zero or less than a related threshold.

[0098] Thus, the result of the frequency adjustment block 220 is an adjusted frequency f a This adjustment frequency f a is provided to the amplifier 204, which then adjusts this tuning frequency f a Let f be the new signal frequency. Then, the amplifier 204 outputs a total current i m Thus, the total current i m is supplied to the electrosurgical instrument 202 and is induced by the instrument voltage u as described above. m The total current i m and device voltage u m With these new values ​​of , the next control cycle can begin.

[0099] Figure 3 shows the equivalent circuit of an ultrasonic transducer, which shows the so-called mechanical series resonant circuit provided by the mechanical capacitance CM, the mechanical inductance LM and the mechanical resistance RM. These three elements represent the electrical behavior of the ultrasonic transducer in so far as they depend on the mechanical behavior of the ultrasonic transducer.

[0100] In parallel there is a capacitance CE which describes the capacitive behavior of the ultrasonic transducer, independent of its mechanical behavior.

[0101] Thus, in operation, the equivalent circuit 300 is driven by an ultrasonic transducer to generate a capacitive current i C and the main current component i', mThe equivalent circuit 300 also represents the component of the device voltage u m Also shown.

[0102] FIG. 4 shows a graph of two sine signals with different phase angles, i.e., with phase differences relative to each other. Both sine signals contain white noise, and the first harmonic is calculated for both sine signals, and the first harmonic Fourier trigonometric function coefficients a 1 and b 1 The resulting sin signals A and B are shown in FIG. 4. Thus, the two signals A and B are reconstructed using their first harmonic Fourier trigonometric coefficients a 1 and b 1 and these Fourier coefficients can be converted to the actual phase difference between signals A and B using the atan2 function as explained above and further below.

[0103] Further embodiments and explanations are provided below that will help to better understand the invention.

[0104] It has been found that the phase difference between the relevant signals, i.e. between voltage and current, can be determined in the digital domain using a robust algorithm that does not require low-pass filtering to automatically extract the first harmonic of the signal. The idea is to calculate the first harmonic Fourier trigonometric coefficients a of the current and voltage according to the following equations (5) and (6): 1 and b 1 The aim is to calculate

[0105]

number

[0106]

number

[0107] where x[k] is the current / voltage sample and sin(2πk / N) is equal to the reference signal used by the amplifier and generated via a Numerically Controlled Oscillator (NCO). The cosine function samples can be efficiently generated with the same NCO.

[0108] The relative phase difference of the signals is then atan2(a 1 ,b 1 ) can be calculated as

[0109] In total, four coefficients need to be calculated, two of which are related to the measured current i meas (t), and the other is the measured voltage u meas (t). i meas (t)⇔a 1,i ,b 1,i u meas (t)⇔a 1,u ,b 1,u

[0110] Therefore, the phase difference between the voltage and the current is atan2(a 1,u ,b 1,u )-atan2(a 1,i ,b 1,i ) The two atan2 functions can be calculated separately, or the atan2 subtraction rule can be used to reduce the calculation to the single atan2 function: atan2(a 1,u ,b 1,u )-atan2(a 1,i ,b 1,i )=atan2(a 1,u b 1,i -a 1,i b 1,u ,b 1,u b 1,i +a 1,u a 1,i )

[0111] The atan2 function can be efficiently calculated in digital logic using the CORDIC algorithm, but because a phase-locked loop (PLL) always forces the phase difference close to zero, the atan2 function can be reduced to a division by linearizing the function at φ=0, as in equation (7) below.

[0112]

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[0113] Typically, ultrasonic inverters implement voltage, current and power limits. As a side catch, the calculated Fourier coefficients can be directly converted to RMS current / voltage and active power according to the following equations (8) and (9):

[0114]

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[0115]

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[0116] Further improvements are proposed as follows:

[0117] The following improvements are essentially independent of the phase difference detection algorithm, but can be implemented very efficiently when using the Fourier coefficient based algorithm mentioned above.

[0118] Compensation for the capacitance of the electric transducer can be performed as follows.

[0119] The current through the device consists of a main current component, which can be represented as the "mechanical" part, and a capacitive component, which can be represented as the "capacitive" part or capacitive current flowing through a capacitance. This capacitive current must be compensated for two reasons. First, because the phase difference between the voltage and the "mechanical" current must be calculated. Second, because ultrasonic amplifiers usually operate in a constant "mechanical" current mode.

[0120] The current can be compensated by subtracting the current through the capacitance, which is equal to the derivative of the measured voltage multiplied by the capacitance of the device. This is done by letting i'(t) be the main current component, i.e. the current without the capacitive part, and i meas (t) is the total current, and u meas (t) is the equipment voltage, and i C It is expressed by the following equation (10) where (t) is the capacitive current component.

[0121]

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[0122] Calculating the derivative in the time domain, i.e. differentiating in the time domain, is very sensitive to noise. Instead, we can differentiate in the frequency domain using the Fourier coefficients.

[0123] i meas (t) / u meas The first harmonic Fourier coefficient of (t) is a 1,i ,b 1,i and a 1,u ,b 1,u , the current and voltage can be described by an equation describing a Fourier series. i meas (t)=a 1,i cos(ω 0 t)+b 1,i sin(ω 0 t)+residual u meas (t)=a 1,u cos(ω 0 t)+b 1,usin(ω 0 t)+residual

[0124] Since the residual represents further harmonics and only the first harmonic is considered, the residual can be neglected. Therefore, the above equations can be written as follows:

[0125]

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[0126]

number

[0127] On this basis, the coefficients are considered below, implying that the above formula is the basis for consideration: Thus, this relationship can be symbolized and described as follows, with the double arrows indicating that the coefficients on the right hand side are used in the corresponding formula, as above: i meas (t)⇔a 1,i ,b 1,i u meas (t)⇔a 1,u ,b 1,u

[0128] Based on this, the Fourier coefficients of the voltage derivative can be calculated using the properties of the Fourier series (see above) as shown in the following equations (13) to (15).

[0129]

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[0130]

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[0131]

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[0132] Differentiation in the time domain becomes multiplication in the frequency domain. The last equation is merely for clarity, since for further considerations it may be sufficient to use altered coefficients.

[0133] Therefore, the compensation of the above-mentioned capacitive current component requires a coefficient CEω, which is equal to the magnitude of the capacitive admittance at the mechanical resonant frequency. 0 (CE*ω 0 ) is needed. If the capacitance is unknown, it can be measured by driving the device off resonance and calculating:

[0134]

number

[0135] That is, the measured admittance at the measurement frequency of the instrument, measured by the ratio of the actual mechanical resonant frequency to the measurement frequency.

[0136] A further proposal is to use virtual machine loads.

[0137] When the mechanical load of the equipment is low, the amplitude of the voltage can be very small. This can make it difficult to determine the phase difference between the voltage and the current. To make the algorithm more robust, a virtual resistance can be added to the equipment by adding a virtual voltage proportional to the (compensated) current to the measured voltage. Thus, the virtual resistance R virt The corrected voltage u'(t) is obtained by taking into account u'(t)=u meas (t)+R virt i'(t)

[0138] This can also be achieved quite easily in the frequency domain. Using the above definitions, i meas (t)⇔a 1,i ,b 1,i umeas (t)⇔a 1,u ,b 1,u i'(t)⇔a 1,i -CEω 0 b 1,u ,b 1,i +CEω 0 a 1,u Corrected (compensated) voltage Fourier coefficients: u'(t)⇔a 1,u +R virt (a 1,i -CEω 0 b 1,u ),b 1,u +R virt (b 1,i +CEω 0 a 1,u ) That is, the following equation (17) results.

[0139]

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[0140] "Approximately equal" indicates that further harmonics are ignored. [Explanation of symbols]

[0141] 100 Electrosurgical Generator 102 Electrosurgical Instruments 104 Organization 106 Frequency converter 108 Control device 110 Output 112 Voltage Sensor 114 Current Sensor 200 Control Structures 202 Electrosurgical Instruments 204 Amplifier 206 Ultrasonic Transducer 208 Voltage Sensor 210 Current Sensor 212 Voltage Fourier Transform Block 214 Current Fourier Transform Block 216 Correction Block 218 Phase difference calculation block 220 Frequency Adjustment Block 300 Equivalent Circuit

Claims

1. 1. A control method for controlling an ultrasonic transducer of an electrosurgical instrument, comprising: the ultrasonic transducer is driven by an alternating total current signal having a signal frequency; driving the ultrasonic transducer with the alternating total current signal produces an alternating machine voltage signal of the ultrasonic transducer in response to mechanical vibration of the ultrasonic transducer; the alternating current machine voltage signal has a phase difference of the ultrasonic transducer relative to a main current component of the alternating current total current signal in response to mechanical vibration of the ultrasonic transducer; the ultrasonic transducer has a varying mechanical resonant frequency; The control method includes: An amplifier is controlled to supply the AC total current signal to the ultrasonic transducer and adjust the signal frequency relative to the mechanical resonant frequency in response to a phase difference of the ultrasonic transducer; To determine the phase difference of the ultrasonic transducer, calculating signal coefficients of the main current components and the AC machine voltage signal; the signal coefficients represent a phase relationship of each of the main current components or the alternating current machine voltage signals to a common reference signal; A control method for calculating a phase difference of the ultrasonic transducer from the signal coefficients.

2. According to the calculated phase difference of the ultrasonic transducer, Adjusting the signal frequency to match or approach the mechanical resonant frequency; determining whether the signal frequency is above or below the mechanical resonant frequency; In response to this, the signal frequency is increased or decreased to approach the mechanical resonant frequency; Or, According to the calculated phase difference of the ultrasonic transducer, Calculating an actual value of the mechanical resonant frequency; The method of claim 1 , further comprising setting the signal frequency in response to the calculated mechanical resonant frequency.

3. 2. The method of claim 1, wherein the calculation of the signal coefficient is based on samples of the measured AC total current signal and samples of a mains voltage signal determined from the measured AC machine voltage signal of the ultrasonic transducer.

4. The method of claim 1 , wherein the signal coefficients characterize a first harmonic of a corresponding signal.

5. For each of the main current component and the AC machine voltage signal, the signal coefficients are calculated according to the following equations (1) and (2): １ and b １ wherein the samples are taken for a predetermined time interval and / or a time interval of one period corresponding to a nominal value of the mechanical resonant frequency; x[k] is the corresponding signal sample, N is the number of samples per signal, [0010] [0025] The control method according to claim 3.

6. The signal coefficients of the main current component and the AC device voltage signal are the first harmonic Fourier trigonometric function coefficients a １ and b １ If The absolute phase difference φ between the main current component and the AC machine voltage signal ａ But, φ ａ =a tan2(a １， b １ ) is calculated based on The method of claim 1 , wherein the phase difference of the ultrasonic transducer is calculated as the absolute phase difference of the main current component and the AC machine voltage signal.

7. The signal coefficients of the main current component and the AC device voltage signal are the first harmonic Fourier trigonometric function coefficients a １ and b １ If The phase difference of the ultrasonic transducer is φ ｔ is the quotient a of the main current component and the AC device voltage signal １ / b １ and the phase difference φ is calculated by approximation as the difference between ｔ is φ ｔ = a １，ｉ / b １，ｉ -a １，ｕ / b １，ｕ It is calculated using a １，ｉ and b １，ｉ is the first harmonic Fourier trigonometric function coefficient a of the main current component １ and b １ and a １，ｕ and b １，ｕ is the first harmonic Fourier trigonometric function coefficient a of the AC device voltage signal １ and b １ The control method according to claim 1 .

8. Calculating the phase difference of the ultrasonic transducer and / or adjusting the signal frequency to match or approach the mechanical resonant frequency is repeated periodically to track changes in the mechanical resonant frequency; 2. The method of claim 1, wherein the calculating or adjusting is repeated at a predetermined repetition frequency of at least one repetition per 10 signal periods, at least one repetition per two signal periods, or one repetition per signal period.

9. The signal coefficients of the main current component and the AC device voltage signal are the first harmonic Fourier trigonometric function coefficients a １ and b １ If The signal coefficients of the main current components and the AC machine voltage signal are the rms value of the principal current component, the rms value of the AC device voltage signal; used to calculate at least one of the main current components and the real power value of the AC machine voltage signal; 2. A method according to claim 1, wherein the respective rms values ​​of the main current component and the alternating current appliance voltage signal and / or the active power value are calculated based on a first harmonic of the main current component and / or the alternating current appliance voltage signal.

10. the alternating total current signal has a principal current component and a capacitive current component, the capacitive current component being dependent on the capacitance of the ultrasonic transducer; The principal current component is calculated by subtracting the capacitive current component from the measured total alternating current signal of the ultrasonic transducer; and / or First harmonic Fourier trigonometric function coefficient a １ and b １ is used to calculate the capacitive current component, The capacitive current component i Ｃ But, Ｃ = C * ω * a １ *cos(ω*t) or i Ｃ = -C * ω * b １ 2. The method of claim 1, wherein the capacitance C and the mechanical resonant frequency ω are calculated using:

11. To calculate the capacitance of the ultrasonic transducer, the AC total current signal is generated having a test frequency that differs from the nominal mechanical resonant frequency by at least a minimum frequency difference to produce the AC machine voltage signal; the signal coefficients are calculated based on the AC total current signal and the resulting AC equipment voltage signal having the test frequency; A capacitance (C) of the ultrasonic transducer is calculated based on the signal coefficients; The method of claim 1 , wherein the minimum frequency difference is at least 10 kHz, or at least 20 kHz.

12. [0023] Calculating a signal coefficient of the AC machine voltage signal the AC machine voltage signal is modified by adding a virtual voltage signal to a modified voltage signal; the virtual voltage signal is calculated by multiplying the principal current component by a predetermined virtual resistance; As said main current component the previously calculated main current component is used, a previously calculated main current component is calculated based on the alternating current machine voltage signal and the previously calculated signal coefficients for the main current component; the modified voltage signal is calculated directly using the signal coefficients previously calculated for the alternating current machine voltage signal and the main current component; and / or 2. The method of claim 1, wherein coefficients of the modified voltage signal are calculated directly using the signal coefficients previously calculated for the alternating current machine voltage signal and the main current component, using a predetermined virtual resistance.

13. An electrosurgical generator for controlling an ultrasonic transducer of an electrosurgical instrument, adapted to carry out a control method according to any one of claims 1 to 12.

14. An electrosurgical generator according to claim 13; and an electrosurgical instrument having an ultrasonic transducer connected to the electrosurgical generator.

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