Ultrasonic knife sweep frequency control method

Abstract

The present invention discloses an ultrasonic knife frequency sweep control algorithm, which controls the output of a DDS device, collects the phase difference of the output voltage and current, adjusts the output frequency of the DDS, and finally makes the ultrasonic knife work in a resonant state. The frequency of the output current is controlled by collecting the dynamic impedance of the ultrasonic knife head and the phase difference between the output voltage and the current. When the distance to the resonance point is far, the impedance of the circuit is large, and the frequency can be adjusted by the impedance at this time. When the distance to the resonance point is close, the change of impedance is small. At this time, the speed of adjusting the frequency by impedance will be relatively slow. At this time, the frequency is adjusted by the change of phase difference, and the efficiency and accuracy are significantly improved. Frequency tracking and frequency locking are achieved from the two angles of impedance size and phase difference change, so that the ultrasonic knife always works in a resonant state.

Description

Technical Field

[0001] The invention relates to an ultrasonic cutting knife, and in particular to an ultrasonic cutting knife sweep frequency control method used in the ultrasonic cutting knife. Background Art

[0002] The working principle of ultrasonic scalpel is to use the electrostrictive effect to convert the electrical energy of ultrasonic scalpel into mechanical energy, and drive the scalpel head to work through the amplification and coupling of the amplitude transformer, and radiate energy to the local tissue of the human body, so as to perform surgical treatment. The transducer is the key component that converts electrical energy into mechanical energy in the ultrasonic scalpel. The driving current of the transducer is provided by the power supply. When the frequency of the driving current is consistent with the natural frequency of the scalpel vibration system, the ultrasonic scalpel works in a resonant state, at which time the vibration effect is the best, the amplitude of the scalpel head is the largest, and the energy will be transferred to the scalpel head to the greatest extent; if it does not work in a resonant state, most of the energy will be consumed inside the transducer, causing the transducer to heat up, resulting in a decrease in the output power and efficiency of the vibration system, a decrease in the output amplitude, and a decrease in the quality of surgery.

[0003] In order to solve the above problems, it is necessary to adopt automatic frequency tracking technology. Although there are many frequency tracking methods such as self-excited oscillation, phase-locked loop, current feedback, vibration speed feedback, etc., no matter what method is used to achieve frequency tracking, the final performance is to make the cutter head work in a resonant state, and the so-called resonance means that the equivalent circuit is in a pure resistive state, the phase of the voltage and the current are equal, that is, the phase difference between the voltage and the current is 0, and the impedance of the circuit is the smallest, that is, the circuit is in a pure resistive characteristic. In theory, it can be judged whether it is in a resonant state from the size of the current, the size of the impedance, or the phase difference, but it is difficult to judge whether it is in a resonant state and realize frequency tracking in a single state; on the one hand, the speed of judging resonance is slow, and on the other hand, it will cause the system to oscillate at a certain amplitude, and it is ultimately difficult to achieve true resonance. Summary of the invention

[0004] In order to solve the problem that the existing ultrasonic cutting knives have the problem that the resonance frequency tracking and scanning control basically adopts a single state to judge whether it is resonant, which makes the resonance frequency tracking control difficult and it is difficult to effectively control the ultrasonic knife to work in the resonance state at all times, the present invention provides an ultrasonic knife scanning frequency control method which realizes frequency tracking from two angles of impedance size and phase difference change, can effectively control the ultrasonic knife to work in the resonance state at all times, and improves the efficiency, accuracy and reliability of ultrasonic knife surgery.

[0005] The specific technical solution adopted by the present invention to solve the above technical problems is: an ultrasonic knife frequency sweeping control method, characterized by: comprising the following control algorithm process;

[0006] S1. Set the starting frequency and the output frequency sweep current of the DDS device to 5-10 mA. During the frequency sweep, the system has not found the resonant frequency and is not easy to excite with high power.

[0007] S2. Obtain the output voltage, output current, dynamic impedance, and phase difference between voltage and current of the driving circuit. The output voltage and output current of the driving circuit are realized through a series of sampling circuits and processors. The dynamic impedance is the ratio of the output voltage to the output current. The phase difference between voltage and current needs to be processed by a specific circuit and FPGA device.

[0008] S3. Determine the size of the dynamic impedance, and decide which parameter to use to adjust the frequency step according to the size of the dynamic impedance; when the impedance is greater than 5000 ohms, the frequency step is set to 200Hz; when the impedance is greater than or equal to 500 ohms, the frequency step is set by matrix query; when the impedance is less than 500 ohms, the frequency step is set by phase difference; the impedance node is set to 5000 ohms, and 500 ohms can be adjusted according to the actual situation, and even if the node value of the impedance is changed, it will not affect the implementation of the algorithm;

[0009] S4. Set the output frequency of the DDS device. At this time, the resonant frequency of the system has been found.

[0010] S5. Determine whether the frequency exceeds the range of 53.5kHz to 56.5kHz. If it exceeds, it is considered that there is a problem with the ultrasonic knife head;

[0011] S6. When the calculated dynamic impedance value is less than 500 ohms and the frequency step is 0, it indicates that the frequency sweep is finished;

[0012] S7. After the frequency sweep is completed, the system sets the output current according to the set gear. When the system is working, the fluctuation range of the output current frequency and the resonant frequency is small. At this time, the frequency of the output current is still dynamically adjusted according to the phase difference between the output voltage and the output current to keep the system in a resonant state.

[0013] By controlling the output of the DDS device, then collecting the phase difference of the output voltage and current, adjusting the output frequency of the DDS, and finally making the ultrasonic knife work in a resonant state. The frequency of the output current is controlled by collecting the dynamic impedance of the ultrasonic knife head and the phase difference between the output voltage and current. When the distance from the resonance point is far, the impedance of the circuit is large, and the frequency can be adjusted by impedance at this time. When the distance from the resonance point is close, the change of impedance is small. At this time, the speed of adjusting the frequency by impedance will be relatively slow. At this time, the change of phase difference is used to adjust the frequency, and the efficiency and accuracy are significantly improved. Both performance and life will be greatly improved. This scheme realizes frequency tracking and frequency locking from two angles: impedance size and phase difference change, so that the ultrasonic knife always works in a resonant state. Through this control algorithm, the resonant frequency of the ultrasonic knife system can be found quickly and accurately, and the frequency locking is stable, so that the ultrasonic knife always works in a resonant state, improving the efficiency of surgery and reducing transducer heating.

[0014] Preferably, in the above step S1, the starting frequency is set to 53.5kHz, and the output current of the DDS device is set to 7mA. Setting the output current amplitude to 7mA not only reduces the output power of the system, but also does not affect the calculation results of the system. It improves the effectiveness of the starting output frequency sweep and reduces the heating of the transducer.

[0015] Preferably, in the above step S3, if the resonance point is exceeded by 2kHz, it can be considered that the ultrasonic knife is faulty, so the initial scanning frequency is set to 53.5kHz, and when the impedance is greater than 5000 ohms, the frequency step is set to 200Hz.

[0016] Preferably, in the above step S3, when the impedance is less than 5000 ohms and greater than 500 ohms, the compensation is set by finding the set step size matrix according to the following formula:

[0017] dStepHz= Step_matrix[((Impedance-500) / 563 ) * 9 + abs(Phase) / 10]

[0018] Step_matrix is ​​an array of 72 elements, Impedance is the equivalent impedance of the ultrasonic knife, and Phase is the phase difference between the output voltage and the output current. Abs(Phase) is the absolute value of the phase difference; this adjustment is understood as coarse adjustment. Improve the effectiveness of the coarse adjustment control of the scanning frequency in this impedance area.

[0019] Preferably, in the above step S3, when the impedance is less than 500 ohms, the frequency setting step is determined by the rate of change of the phase difference to the frequency, and the adjustment can be understood as fine-tuning the frequency step;

[0020] When the impedance is less than 500 ohms, it means that the operating frequency is close to the resonant frequency. At this time, the frequency adjustment error is large and the speed is slow only by using impedance. A new method is needed to find the resonant frequency. Through a large number of experiments, the following frequency step fine-tuning formula is summarized. The variables used are still the impedance and phase difference of the system.

[0021] dStepHz = 0.00207 * (Impedance * cos(DEG_TO_RAD(shPhaseGap))*PhaseGap;

[0022] This formula is derived by estimating the fitting curve of the effect of frequency on phase difference and impedance. This formula can further improve the data capacity and make certain changes; Impedance is the equivalent impedance of the ultrasonic knife; PhaseGap is the phase difference between the output voltage and the output current; DEG_TO_RAD (shPhaseGap) is the phase difference between the output voltage and the output current converted into radian values. Improve the effectiveness of the scanning frequency fine-tuning control for this impedance area.

[0023] Preferably, in the above step S3, when the impedance is less than 5000 ohms and greater than 500 ohms, the frequency coarse adjustment adopts the following query step matrix to determine the frequency step corresponding to the algorithm:

[0024] Step_matrix

[72] =

[0025] {

[0026] FIDUT_0P12, FIDUT_0P12, FIDUT_0P6, FIDUT_1P2, FIDUT_1P2, FIDUT_2P4,FIDUT_4P8, FIDUT_7P2, FIDUT_12,

[0027] FIDUT_0P12, FIDUT_0P24, FIDUT_0P6, FIDUT_1P2, FIDUT_2P4, FIDUT_4P8,FIDUT_7P2, FIDUT_10, FIDUT_12,

[0028] FIDUT_0P24, FIDUT_0P6, FIDUT_1P2, FIDUT_1P2, FIDUT_2P4, FIDUT_7P2,FIDUT_10, FIDUT_12, FIDUT_12,

[0029] FIDUT_0P24, FIDUT_0P6, FIDUT_1P2, FIDUT_2P4, FIDUT_4P8, FIDUT_10,FIDUT_10, FIDUT_12, FIDUT_12,

[0030] FIDUT_0P6, FIDUT_1P2, FIDUT_2P4, FIDUT_2P4, FIDUT_4P8, FIDUT_10,FIDUT_12, FIDUT_12, FIDUT_12,

[0031] FIDUT_0P6, FIDUT_1P2, FIDUT_2P4, FIDUT_4P8, FIDUT_7P2, FIDUT_10,FIDUT_12, FIDUT_12, FIDUT_12,

[0032] FIDUT_0P6, FIDUT_1P2, FIDUT_2P4, FIDUT_4P8, FIDUT_10, FIDUT_12,FIDUT_12, FIDUT_12, FIDUT_12,

[0033] FIDUT_1P2, FIDUT_2P4, FIDUT_4P8, FIDUT_10, FIDUT_12, FIDUT_12, FIDUT_12, FIDUT_12, FIDUT_12,

[0034] };

[0035] FIDUT_0P12 corresponds to 0.12Hz

[0036] FIDUT_0P6 corresponds to 0.6Hz

[0037] FIDUT_1P2 corresponds to 1.2Hz

[0038] FIDUT_2P4 corresponds to 2.4Hz

[0039] FIDUT_4P8 corresponds to 4.8Hz

[0040] FIDUT_7P2 corresponds to 7.2Hz

[0041] FIDUT_10 corresponds to 10Hz

[0042] FIDUT_12 corresponds to 12Hz.

[0043] Improve the effectiveness of frequency sweep step size query application control in matrix elements.

[0044] Preferably, the number of elements in the step length matrix is ​​not limited to the current 72, and other numbers of step length matrix data may be used to increase or decrease the amount of data according to actual conditions, thereby improving the flexibility and effectiveness of actual frequency sweep step length query application control.

[0045] As a preference, the formula for determining the step length matrix points and the data in the step length matrix can be adjusted appropriately according to different hardware circuits and are not limited to the current step length matrix data, thereby improving the flexibility and effectiveness of the actual frequency sweep step length query application control.

[0046] Preferably, 0.00207 in the frequency step length fine-tuning formula is a constant, which is appropriately adjusted according to different circuits or loads, so as to improve the flexibility and effectiveness of the actual frequency sweep step length fine-tuning control.

[0047] Preferably, in the above steps S1 to S6, the following control variables can be appropriately added, such as increasing the count of the number of frequency sweep failures. By analyzing the number of frequency sweep failures, a reference research basis for analyzing and adjusting the frequency sweep control algorithm is provided to improve the stability, reliability and effectiveness of the frequency sweep control.

[0048] The beneficial effects of the present invention are: by controlling the output of the DDS device, then collecting the phase difference of the output voltage and current, adjusting the output frequency of the DDS, and finally making the ultrasonic knife work in a resonant state. The frequency of the output current is controlled by collecting the dynamic impedance of the ultrasonic knife head and the phase difference between the output voltage and the current. When the distance is far from the resonance point, the impedance of the circuit is large, and the frequency can be adjusted by the impedance at this time. When the distance is close to the resonance point, the change of the impedance is small. At this time, the speed of adjusting the frequency by impedance will be relatively slow. At this time, the change of the phase difference is used to adjust the frequency, and the efficiency and accuracy are significantly improved. When the distance is far from the resonance point, the impedance is large, and the frequency is adjusted according to the impedance size. When the distance is close to the resonance point, the frequency is adjusted by the phase difference. The natural frequency of the ultrasonic knife, also known as the resonant frequency, is usually around 55.5kHz. When the frequency of the driving current is consistent with the resonant frequency of the ultrasonic knife, the efficiency of the ultrasonic knife transducer is the highest, the heat generation of the transducer is the smallest, and the performance and life will be greatly improved. This scheme realizes frequency tracking and frequency locking from the two perspectives of impedance size and phase difference change, so that the ultrasonic knife always works in a resonant state. This control algorithm can quickly and accurately find the resonant frequency of the ultrasonic knife system and lock the frequency stably, so that the ultrasonic knife always works in a resonant state, improving the efficiency of the operation and reducing the heating of the transducer. DETAILED DESCRIPTION

[0049] In an embodiment, an ultrasonic knife frequency sweep control method includes the following control algorithm process;

[0050] S1. Set the starting frequency and the output frequency sweep current of the DDS device to 5-10mA. During the frequency sweep, the system has not found the resonant frequency and is not easy to excite with high power. Set the output current amplitude to 7mA, which reduces the output power of the system without affecting the calculation results of the system. The above DDS is the abbreviation of Direct Digital Synthesis, and its Chinese name is direct digital synthesis.

[0051] S2. Obtain the output voltage, output current, dynamic impedance, and phase difference between voltage and current of the driving circuit. The output voltage and output current of the driving circuit are realized through a series of sampling circuits and processors. The dynamic impedance is the ratio of the output voltage to the output current. The phase difference between voltage and current needs to be processed by a specific circuit and FPGA device. The above-mentioned FPGA is the abbreviation of Field Programmable Gate Array, and its Chinese name is field programmable logic array.

[0052] S3. Determine the size of the dynamic impedance, and decide which parameter to use to adjust the frequency step according to the size of the dynamic impedance; when the impedance is greater than 5000 ohms, the frequency step is set to 200Hz; when the impedance is greater than or equal to 500 ohms, the frequency step is set by matrix query, and when the impedance is less than 500 ohms, the frequency step is set by phase difference; the impedance node set is 5000 ohms, and 500 ohms can be adjusted according to the actual situation, and even if the node value of the impedance is changed, it will not affect the implementation of the algorithm;

[0053] S4. Set the output frequency of DDS. At this time, the resonant frequency of the system has been found.

[0054] S5. Determine whether the frequency exceeds the range of 53.5kHz to 56.5kHz. If it exceeds, it is considered that there is a problem with the ultrasonic knife head;

[0055] S6. When the calculated dynamic impedance value is less than 500 ohms and the frequency step is 0, it indicates that the frequency sweep is finished;

[0056] S7. After the frequency sweep is completed, the system sets the output current according to the set gear. When the system is working, the fluctuation range of the output current frequency and the resonant frequency is small, the output current fluctuation range is ±5%, and the resonant frequency fluctuation range is ±5Hz. At this time, the output current frequency is still dynamically adjusted according to the phase difference between the output voltage and the output current to keep the system in a resonant state.

[0057] In the above step S1, the starting frequency is set to 53.5kHz, and the output current of the DDS is set to 7mA.

[0058] In the above step S3, if the resonance point is exceeded by 2kHz, it can be considered that the ultrasonic knife is faulty, so the initial scanning frequency is set to 53.5kHz, and when the impedance is greater than 5000 ohms, the frequency step is set to 200Hz.

[0059] In the above S3 step, when the impedance is less than 5000 ohms and greater than 500 ohms, the compensation is set by looking up the step size matrix according to the following formula:

[0060] dStepHz = Step_matrix[( (Impedance-500) / 563 ) * 9 + abs(Phase) / 10]

[0061] Where dStepHz is the calculated frequency step, Step_matrix is ​​a 72-element array, Impedance is the equivalent impedance of the ultrasonic knife, and Phase is the phase difference between the output voltage and the output current. Abs(Phase) is the absolute value of the phase difference; this adjustment is understood as coarse adjustment. The above formula and the corresponding matrix are obtained based on a large number of experimental studies.

[0062] In the above step S3, when the impedance is less than 500 ohms, the frequency setting step is determined by the rate of change of the phase difference to the frequency. This adjustment can be understood as fine-tuning the frequency step;

[0063] When the impedance is less than 500 ohms, it means that the operating frequency is close to the resonant frequency. At this time, the frequency adjustment error is large and the speed is slow only by using impedance. A new method is needed to find the resonant frequency. Through a large number of experiments, the following frequency step fine-tuning formula is summarized. The variables used are still the impedance and phase difference of the system.

[0064] dStepHz = 0.00207 * (Impedance * cos(DEG_TO_RAD(shPhaseGap))*PhaseGap;

[0065] This formula is derived by estimating the effect of frequency on phase difference and impedance through fitting curves. This formula can further improve the data capacity and make certain changes. Impedance is the equivalent impedance of the ultrasonic knife; PhaseGap is the phase difference between the output voltage and the output current; DEG_TO_RAD (shPhaseGap) is the phase difference between the output voltage and the output current converted into radians.

[0066] In the above step S3, when the impedance is less than 5000 ohms and greater than 500 ohms, the frequency coarse adjustment uses the following query step matrix to determine the frequency step corresponding to the algorithm:

[0067] Step_matrix

[72] =

[0068] {

[0069] FIDUT_0P12, FIDUT_0P12, FIDUT_0P6, FIDUT_1P2, FIDUT_1P2, FIDUT_2P4,FIDUT_4P8, FIDUT_7P2, FIDUT_12,

[0070] FIDUT_0P12, FIDUT_0P24, FIDUT_0P6, FIDUT_1P2, FIDUT_2P4, FIDUT_4P8,FIDUT_7P2, FIDUT_10, FIDUT_12,

[0071] FIDUT_0P24, FIDUT_0P6, FIDUT_1P2, FIDUT_1P2, FIDUT_2P4, FIDUT_7P2,FIDUT_10, FIDUT_12, FIDUT_12,

[0072] FIDUT_0P24, FIDUT_0P6, FIDUT_1P2, FIDUT_2P4, FIDUT_4P8, FIDUT_10,FIDUT_10, FIDUT_12, FIDUT_12,

[0073] FIDUT_0P6, FIDUT_1P2, FIDUT_2P4, FIDUT_2P4, FIDUT_4P8, FIDUT_10,FIDUT_12, FIDUT_12, FIDUT_12,

[0074] FIDUT_0P6, FIDUT_1P2, FIDUT_2P4, FIDUT_4P8, FIDUT_7P2, FIDUT_10,FIDUT_12, FIDUT_12, FIDUT_12,

[0075] FIDUT_0P6, FIDUT_1P2, FIDUT_2P4, FIDUT_4P8, FIDUT_10, FIDUT_12,FIDUT_12, FIDUT_12, FIDUT_12,

[0076] FIDUT_1P2, FIDUT_2P4, FIDUT_4P8, FIDUT_10, FIDUT_12, FIDUT_12, FIDUT_12, FIDUT_12, FIDUT_12,

[0077] };

[0078] FIDUT_0P12 corresponds to 0.12Hz

[0079] FIDUT_0P6 corresponds to 0.6Hz

[0080] FIDUT_1P2 corresponds to 1.2Hz

[0081] FIDUT_2P4 corresponds to 2.4Hz

[0082] FIDUT_4P8 corresponds to 4.8Hz

[0083] FIDUT_7P2 corresponds to 7.2Hz

[0084] FIDUT_10 corresponds to 10Hz

[0085] FIDUT_12 corresponds to 12Hz.

[0086] The number of elements in the step matrix is ​​not limited to the current 72, and other numbers of step matrix data can also be used to increase or decrease the amount of data according to actual conditions. The formula for determining the step matrix points and the data in the step matrix can be adjusted appropriately according to different hardware circuits and are not limited to the current step matrix data. The 0.00207 in the frequency step fine-tuning formula is a constant and should be adjusted appropriately according to different circuits or loads. In the above steps S1 to S6, the following control variables can be appropriately added in the middle, such as increasing the count of the number of failed frequency sweeps.

[0087] The "sweep frequency" described in this article means that under a given initial drive current frequency, the drive frequency is gradually adjusted according to the detected impedance and phase difference, so that the frequency of the drive current is finally consistent with the natural frequency of the ultrasonic knife, and the circuit is in a resonant state. When the ultrasonic knife is working, the resonant frequency of the system will change according to the change of the load, and "frequency locking" means that after finding the resonant point, the system needs to dynamically adjust the frequency of the output drive circuit according to the load to keep the system in a resonant state.

[0088] The above content and structure describe the basic principle, main features and advantages of the product of the present invention, which should be understood by those skilled in the art. The above examples and descriptions are only for explaining the principle of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which are within the scope of the present invention. The scope of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for controlling an ultrasonic knife frequency sweep, characterized in that: Including the following control algorithm process; S1. Set the starting frequency and the output frequency sweep current of the DDS device to 5-10 mA. During the frequency sweep, the system has not found the resonant frequency and is not easy to excite with high power. S2. Obtain the output voltage, output current, dynamic impedance, and phase difference between voltage and current of the driving circuit. The output voltage and output current of the driving circuit are realized by a sampling circuit and a processor. The dynamic impedance is the ratio of the output voltage to the output current. The phase difference between voltage and current needs to be processed by a circuit and an FPGA device. S3. Determine the size of the dynamic impedance, and decide which parameter to use to adjust the frequency step according to the size of the dynamic impedance; when the impedance is greater than 5000 ohms, the frequency step is set to 200Hz; when the impedance is less than 5000 ohms and greater than 500 ohms, the frequency step is set by matrix query; when the impedance is less than 500 ohms, the frequency step is set by phase difference; S4. Set the output frequency of the DDS device. At this time, the resonant frequency of the system has been found. S5. Determine whether the frequency exceeds the range of 53.5kHz to 56.5kHz. If it exceeds, it is considered that there is a problem with the ultrasonic knife head; S6. When the calculated dynamic impedance value is less than 500 ohms and the frequency step is 0, it indicates that the frequency sweep is finished; S7. After the frequency sweep is completed, the system sets the output current according to the set gear. When the system is working, the fluctuation range of the output current frequency and the resonant frequency is small. At this time, the output current frequency is still dynamically adjusted according to the phase difference between the output voltage and the output current to keep the system in the resonant state; In the above step S3, when the impedance is less than 500 ohms, the frequency setting step is determined by the rate of change of the phase difference to the frequency, and this adjustment is understood as the frequency step fine-tuning; The step size fine-tuning formula is: dStepHz = 0.00207 * (Impedance * cos(DEG_TO_RAD(shPhaseGap))*PhaseGap); Wherein, dStepHz is the frequency step; Impedance is the equivalent impedance of the ultrasonic knife; PhaseGap is the phase difference between the output voltage and the output current; DEG_TO_RAD (shPhaseGap) is the phase difference between the output voltage and the output current converted into radians; In the above S3 step, when the impedance is less than 5000 ohms and greater than 500 ohms, the step size is set by looking up the step size matrix according to the following formula: dStepHz = Step_matrix[((Impedance-500) / 563 ) * 9 + abs(Phase) / 10] Among them, dStepHz is the frequency step, Step_matrix is ​​an array of 72 elements, Impedance is the equivalent impedance of the ultrasonic knife, Phase is the phase difference between the output voltage and the output current, and Abs(Phase) is the absolute value of the phase difference; this adjustment is understood as coarse adjustment.

2. The ultrasonic knife frequency sweep control method according to claim 1, characterized in that: In the above step S1, the starting frequency is set to 53.5kHz, and the output sweep current of the DDS is set to 7mA.

3. The ultrasonic knife frequency sweep control method according to claim 1, characterized in that: In the above step S3, the initial scanning frequency is set to 53.5 kHz, and when the impedance is greater than 5000 ohms, the frequency step is set to 200 Hz.

4. The ultrasonic knife frequency sweep control method according to claim 1, characterized in that: In the above steps S1 to S6, a control variable for counting the number of times the frequency sweep fails is added in the middle.