METHOD FOR CONTROLLING AN ELECTROSURGICAL INSTRUMENT CAPABLE OF PRODUCING A CUTTING PLASMA AND CORRESPONDING ELECTROSURGICAL GENERATOR

Abstract

A method for controlling an electrosurgical instrument capable of producing a cutting plasma for cutting or vaporizing tissue when in operation at such tissue, wherein an electrosurgical generator generates a voltage signal being basically sinusoidal and the voltage signal is applied to the electrosurgical instrument, applying the voltage signal to the electrosurgical instrument results in an operating current signal capable of providing the cutting plasma, and the operating current signal including a linear current component being sinusoidal having a fundamental frequency and a nonlinear current component not having the fundamental frequency, wherein the operating current signal is controlled depending on the nonlinear current component in order to control the cutting plasma.

Claims

1.-13. (canceled)

14. A method for controlling an electrosurgical instrument capable of producing a cutting plasma for cutting or vaporizing tissue when in operation at such tissue, wherein an electrosurgical generator generates a voltage signal being basically sinusoidal and the voltage signal is applied to the electrosurgical instrument, applying the voltage signal to the electrosurgical instrument results in an operating current signal capable of providing the cutting plasma, and the operating current signal comprising a linear current component being sinusoidal having a fundamental frequency and a nonlinear current component not having the fundamental frequency, wherein the operating current signal is controlled depending on the nonlinear current component in order to control the cutting plasma.

15. The method according to claim 14, wherein for receiving the nonlinear current component, the linear current component is extracted from the operating current signal such that the nonlinear current component remains.

16. The method according to claim 14, wherein the linear current component is determined by calculating signal coefficients characterizing a first harmonic of the operating current signal and the linear current component is described using these signal coefficients.

17. The method according to claim 14, wherein the linear current component is determined by calculating first harmonic Fourier trigonometric coefficients a.sub.1 and b.sub.1 of the operating current signal according to the formulas: $a_1=\frac{2}{N}\underset{k=0}{\overset{N-1}{.Math.}}x\left[k\right]*\cos \left(2\frac{k}{N}\right)$ $\mathrm{and}$ $b_1=\frac{2}{N}\underset{k=0}{\overset{N-1}{.Math.}}x\left[k\right]*\sin \left(2\frac{k}{N}\right)$ wherein x[k] are samples of the operating current signal and N is the number of samples used, and wherein samples of the operating current signal are taken over a time interval of one period of the linear current signal or the voltage signal.

18. The method according to claim 14, wherein the operating current signal is controlled depending on an rms-value of the nonlinear current component and the rms-value of the nonlinear current component is calculated by calculating a square root of a difference between the square of a rms-value of the operating current signal and the square of an rms-value of the linear current component.

19. The method according to claim 14, wherein for controlling the operating current signal depending on a or the rms-value I.sub.nl of the nonlinear current component the rms-value I.sub.nl is calculated using the formula: $l_{\mathrm{nl}}=\sqrt{{l_{\mathrm{rms}}}^2-\frac{{a_1}^2+{b_1}^2}{2}}$ wherein I.sub.rms is the rms-value of the operating current signal and a.sub.1 and b.sub.1 being the first harmonic Fourier trigonometric coefficients of the operating current signal.

20. The method according to claim 14, wherein for controlling the operating current signal a or the rms-value of the nonlinear current component is controlled to a reference value, and/or an amplitude of the voltage signal is increased when the rms-value of the nonlinear current component is below the reference value or below a first reference value and the amplitude of the voltage signal is decreased when the rms-value of the nonlinear current component is above the reference value or above a second reference value.

21. The method according to claim 14, wherein in a first control stage for heating a saline, a or the rms-value of the nonlinear current component is controlled to a or the first reference value and in a second control stage for providing the cutting plasma the rms-value of the nonlinear current component is controlled to a second reference value, wherein the first reference value is larger than the second reference value.

22. The method according to claim 14, wherein the electrosurgical instrument comprises a single electrode for providing the cutting plasma between the electrode and a saline or a neighboring tissue or the electrosurgical instrument comprises two electrodes for providing the cutting plasma between these two electrodes.

23. An electrosurgical generator for controlling an electrosurgical instrument capable of producing a cutting plasma for cutting or vaporizing tissue when in operation at such tissue, wherein the electrosurgical generator is adapted for executing a method according to which the electrosurgical generator generates a voltage signal being basically sinusoidal and the voltage signal is applied to the electrosurgical instrument, wherein applying the voltage signal to the electrosurgical instrument results in an operating current signal capable of providing the cutting plasma, and the operating current signal comprising a linear current component being sinusoidal having a fundamental frequency and a nonlinear current component not having the fundamental frequency, wherein the operating current signal is controlled depending on the nonlinear current component in order to control the cutting plasma.

24. The electrosurgical generator according to claim 23, comprising a control device adapted to control the electrosurgical generator, wherein the electrosurgical generator is adapted to execute a method according to the method for controlling the electrosurgical instrument capable of producing the cutting plasma for cutting or vaporizing tissue when in operation at such tissue, wherein the electrosurgical generator generates the voltage signal being basically sinusoidal and the voltage signal is applied to the electrosurgical instrument, applying the voltage signal to the electrosurgical instrument results in the operating current signal capable of providing the cutting plasma, and the operating current signal comprising the linear current component being sinusoidal having the fundamental frequency and the nonlinear current component not having the fundamental frequency, wherein the operating current signal is controlled depending on the nonlinear current component in order to control the cutting plasma.

25. The electrosurgical generator according to claim 23, comprising a frequency converter for generating the voltage signal, being coupled to a or the control device, and/or an output port for connecting an electrosurgical instrument capable of producing a cutting plasma for cutting or vaporizing tissue when controlled by the generator and when in operation at such tissue and/or a current sensor for measuring an operational current and being coupled to the frequency converter and/or the control device.

26. An electrosurgical installation comprising an electrosurgical generator according to claim 23, and an electrosurgical instrument connected to the electrosurgical generator.

Description

[0081] The invention is now explained by way of example based on the accompanying figures.

[0082] FIG. 1 shows an electrosurgical generator.

[0083] FIG. 2 shows a simplified control structure.

[0084] FIG. 3 shows voltage and current signals.

[0085] FIG. 1 shows an electrosurgical generator 100 to which an electrosurgical instrument 102 is connected operating on a tissue 104, i.e. a working element, just schematically illustrated.

[0086] The electrosurgical generator 100 comprises a frequency converter 106 which is controlled by a control device 108. The frequency converter 106 when in operation provides at its output 110 a voltage signal which is applied to the attached electrosurgical instrument 102, resulting in an operational current.

[0087] There is a voltage sensor 112 for measuring the voltage signal and a current sensor 114 for measuring the operational current. These sensors 112, 114 are connected to the control device 108 for further consideration.

[0088] The control device 108 calculates from the current sample values provided by the current sensor 114 a nonlinear current component. Based on that, the control device 108 controls the frequency converter 106. Accordingly, at the frequency converter output, the voltage signal is generated depending on the calculated nonlinear current component.

[0089] The voltage sensor 112 might not be necessary as it is known what kind of voltage the voltage converter 106 generates. It is also possible that the voltage sensor 112 is connected to the frequency converter 106 directly. It is also possible that the control device 108 is part of the frequency converter 106.

[0090] FIG. 2 shows a schematic control structure 200. This control structure 200 illustrates that the control is based on a nonlinear current component I.sub.nl and a reference current I.sub.ref. These values are subtracted in the first summing element 202 resulting in a control error e. This control error e is passed to the control block 204 resulting in a set value for the voltage signal V.sub.set. This set value for the voltage signal V.sub.set is given to the frequency converter 206 which could be identical to the frequency converter 106 according to FIG. 1.

[0091] The result of the frequency converter 206 is the voltage signal V.sub.sig. This voltage signal is passed to the electrosurgical instrument 208 which could be identical to the electrosurgical instrument 102 according to FIG. 1. The electrosurgical instrument 208 is also operating on a schematically illustrated tissue 210, i.e. on a working element.

[0092] Applying the voltage signal V.sub.sig to the electrosurgical instrument 208 results in an operational current i.sub.OP. This operational current i.sub.OP is measured by means of a current sensor 214. This operational current lop is thus the sampled current sampled by the current sensor 214. This sampled current is given to the Fourier transform block 216 and to the rms-block 218. The Fourier transform block 216 calculates first harmonic Fourier trigonometric coefficients and in this way identifies the linear current component lin. The rms-block 218 calculates the root mean square value I.sub.rms and thus the effective value of the sampled current lop. In the second summing element 220, a difference of the rms-value of the operational current I.sub.rms and the linear current component I.sub.lin is calculated resulting in the nonlinear current component Int. Accordingly, the nonlinear current component I.sub.nl is also an rms-value. However, this calculation of the nonlinear current component I.sub.nl is just a simplified illustration in FIG. 2 not showing all calculation steps and preferably the current component I.sub.nl is calculated by taking the root of the difference of the square of the rms-value of the operating current I.sub.rms and the square of the linear current component I.sub.lin. I.e. the second summing element 220 is a simplified illustration of I.sub.nl=sqrt(I.sub.rms.sup.2I.sub.lin.sup.2).

[0093] However, in this way, the nonlinear current component I.sub.nl is calculated and provided to the first summing element 202 as explained above.

[0094] The control block 204 can be designed such that depending on the error signal e, i.e. the result of the comparison of the reference value I.sub.ref and the nonlinear current component I.sub.nl, the voltage set value V.sub.set is either increased or decreased. The behavior might be similar to a PI-control. However, preferably, this can also be realized in a digital manner such that in each control cycle the voltage set value can be increased by a certain increment, if the error signal e is positive, or it can be decreased by a particular increment, which can be the same as for the explained increase, when the error signal is negative. Accordingly, this can be different to a I-behavior, as according to one aspect the amplitude of the error signal might not be taken into account, but only whether it is negative or positive. However, considering the particular value of the error signal might also be an option.

[0095] It is to be noted that the possibility of increasing the voltage set value by a constant increment per control cycle will result in almost a continuous behavior as the cycle frequency can be at 200 kHz or above.

[0096] FIG. 3 shows recorded voltage and current data during TURis operation. The voltage is approximately sinusoidal while the current consists of a linear part and nonlinear part corresponding to plasma breakthroughs. The nonlinear current component can also be understood as plasma current and cannot be measured directly. The idea is therefore to separate the linear current component from the nonlinear current component, i.e. from the plasma current and control on the plasma current only.

[0097] One way to separate the currents is to calculate the first harmonic of the measured current using the Fourier transform. This includes any capacitive and ohmic currents. Subtracting this measured current from the totally measured current yields an estimate of the nonlinear current component, i.e. of the plasma current which can then be used for control. Since only the rms-value of the plasma current is relevant, it can be calculated from the rms-value of the total current and the calculated Fourier coefficients a.sub.1 and b.sub.1 according to:

[00003]$l_{\mathrm{nl}}=\sqrt{{l_{\mathrm{rms}}}^2-{l_{\mathrm{lin}}}^2}=\sqrt{{l_{\mathrm{rms}}}^2-\frac{{a_1}^2+{b_1}^2}{2}}$

[0098] FIG. 3: The solid lines show measured voltage V.sub.sig and current lop data during TURis operation of an electrosurgical generator. The current is a combination of a capacitive current, which is 90 phase shifted with respect to the voltage, and current peaks which are in phase with the voltage. During ignition, there is a very high current in phase with the voltage. The dashed line shows the first harmonic component lin of the sampled current i.sub.op.