125 results about "Frequency oscillation" patented technology

The invention discloses a current source type LCCL double-frequency resonance system and a parameter matching method thereof, which are suitable for synchronous double-frequency induction quenching of complex workpieces such as gears. The system is composed of two current type resonant inverters working at a low frequency and a high frequency respectively and a dual-frequency LCCL resonant cavity, and dual-frequency current superposition is realized through parallel output. The double-frequency LCCL resonant cavity has two parallel resonance points, a mode based on simplified topology is adopted to simplify parameter design, and meanwhile design errors are maintained at a low level. A power matching coefficient is introduced, the ratio of high-frequency output power to low-frequency output power is adjusted by adjusting the value of the power matching coefficient, and the problem of double-frequency power unbalance caused by frequency variation characteristics of load equivalent resistance and the like is solved. The superposed double-frequency current forms a low-frequency envelope embedded high-frequency oscillation composite waveform on the induction heating coil, cooperative control of tooth root deep quenching (low frequency) and tooth surface hard quenching (high frequency) is achieved, and the method has the advantages of being simple in parameter design, high in power decoupling performance, high in operation stability and the like.

The invention relates to the technical field of power system operation and control, and discloses an improved model-free adaptive flexible low-frequency power transmission oscillation suppression method, which performs zero-phase lag filtering on noisy signals through a nonlinear tracking differentiator, effectively solves the problem of control divergence caused by signal noise, and improves the suppression efficiency. A tight-format dynamic linearization and linear expansion state observer technology is adopted, accurate capture of dynamic characteristics of a strong nonlinear system and real-time observation and compensation of internal and external full-dimensional disturbance are achieved without dependence of a physical model, and particularly, an adaptive estimation algorithm of an output variable quantity penalty factor is introduced. The problem that model parameter identification jumps severely under the broadband oscillation working condition is solved, the robustness of a control system is enhanced, and the self-adaptive optimality of a control strategy is guaranteed and interference of long-term online operation on steady-state performance is avoided by combining parameter optimization of a cuckoo search algorithm and an event triggering mechanism of time-frequency double criteria.

The invention discloses a test pool for suppressing low-frequency oscillation. The test pool comprises a pool main body, a wave maker and a wave absorbing beach, the wave maker is arranged in the pool main body and is used for making waves in the pool main body; the pool main body comprises a shoal area; the wave absorbing beach is placed in the shoal area and right faces the wave maker. The wave absorbing beach is of a non-solid structure with pores; a solid back plate is arranged on one side, back to the wave maker, of the wave absorbing beach; the solid back plate covers the side surface of the wave absorbing beach; a lifting plate is arranged at the bottom of the pool body; the placing position of the wave absorbing beach can be changed in the direction close to or away from the wave maker, so that the distance between the wave maker and the wave absorbing beach is changed; the lifting plate can ascend and descend to change the depth of the pool body. In the scheme, the solid back plate on the back side of the wave absorbing beach can play a role in blocking waves, so that the length of the pool main body can be adjusted by moving the position of the wave absorbing beach, the natural oscillation period of the pool main body is further adjusted, and the interference of the natural oscillation period of the pool on a test is reduced.

The invention discloses a photovoltaic inverter control parameter optimization and broadband oscillation suppression method based on a seathead whale migration algorithm, and the method comprises the steps: S1, building each subsystem model, and building a sequence impedance model of a photovoltaic inverter; s2, selecting to-be-optimized control parameters in the sequence impedance model, calculating normalized sensitivity at each frequency point by using a multi-frequency-point central difference method, and performing full-frequency-band analysis; the global sensitivity of each control parameter is obtained through a global sensitivity calculation formula, and then the optimization range of each to-be-optimized control parameter is given through the global sensitivity; and S3, on the basis of the optimization range of the to-be-optimized control parameters calculated based on the global sensitivity, optimizing the control parameters by adopting a sedan whale migration algorithm. By adopting the method, the capability of avoiding local optimum and effective convergence is improved, the optimized and given parameter values can be effectively matched with the impedance of the photovoltaic inverter and the impedance of the power grid, and the stability of the photovoltaic grid-connected system is improved.

The embodiment of the invention discloses a negative damping elimination method, device and equipment based on SVG and a storage medium, and the method comprises the steps: carrying out the impedance scanning of the SVG in a range of 1-3000Hz, obtaining the positive-sequence impedance and negative-sequence impedance of the SVG under different operation conditions, and determining the leading operation condition of the SVG impedance; determining the positive sequence equivalent impedance and the negative sequence equivalent impedance of the SVG grid-connected point according to the positive sequence dominant impedance and the negative sequence dominant impedance of the SVG under the dominant operation condition; determining the value ranges of the positive sequence damping correction value and the negative sequence damping correction value of the SVG; the maximum value of the intersection of the two value ranges is determined as SVG compensation damping, and the SVG compensation damping is merged into an SVG grid-connected point port; according to the scheme, only two unknown parameters including the positive sequence damping correction value and the negative sequence damping correction value are involved, the parameters can be calculated based on an analytic formula pure algebraic equation, the calculation mode is simple, the port impedance characteristic negative damping of the SVG in the full frequency band within the range of 1-3000 Hz can be eliminated, and the problem of high-frequency oscillation is thoroughly solved.

The invention discloses a VSG grid-connected system control method based on an improved three-parameter wave trap, and belongs to the technical field of power system control, and the method comprises the steps: firstly building a single-input single-output small signal model based on a VSG grid-connected system containing power coupling, and calculating an active open-loop transfer function; an improved active damping resonance suppression method is introduced, so that the transient stability of the system is improved; the expected system stability margin under the system oscillation frequency is obtained, and the parameter design of the three-parameter wave trap is completed based on the subsynchronous frequency oscillation characteristics; the three-parameter wave trap is remodeled, and active steady-state deviation caused by the virtual damping coefficient under power grid frequency disturbance is eliminated. According to the method, the stability of the system is improved, the system is ensured to have a relatively good stability margin, and active steady-state deviation caused by power grid frequency disturbance of the grid-connected system can be completely eliminated.

The invention relates to the technical field of nerve regulation and control, and discloses a nerve regulation and control method based on dynamic frequency band analysis, which is used for improving the accuracy of epileptic seizure judgment. The neural regulation and control method based on dynamic frequency band analysis comprises the following steps: acquiring a local field potential signal of a target brain region in a preset time period; performing dynamic frequency band analysis on the local field potential signal to obtain a pathological wave frequency band, a high-frequency oscillation band, a harmonic coupling band and a suppression wave band; calculating the energy ratio and the cross-band phase coupling strength of each feature sub-band, and generating a multi-dimensional feature vector; the epileptic seizure probability is calculated according to the multi-dimensional feature vector; and when the epileptic seizure probability is greater than a preset threshold value, selecting a corresponding stimulation parameter group according to the dominant characteristic sub-band type, and outputting an electrical stimulation pulse with a frequency band matching characteristic.

The invention discloses a VSG coordinated control method and device based on improved self-adaption and feed-forward compensation, and the method comprises the steps: building a model of a VSG optical storage grid-connected system, and analyzing a power angle characteristic and a frequency oscillation curve; introducing an arc tangent function to optimize a traditional adaptive control strategy, and smoothly adjusting rotational inertia and a damping coefficient; a feed-forward compensation control strategy is added, an active compensation expression and a rotor motion equation are established, a compensation coefficient and a filtering time constant are set in a coordinated mode, and configuration of a small-signal model change zero-pole pair of an active power reference value under disturbance is reconstructed; adjusting virtual rotational inertia and a damping coefficient based on the optimized adaptive control strategy, and correcting a power reference value in advance by predicting frequency fluctuation based on a feed-forward compensation control strategy; according to the method, power overshoot and frequency change deviation are effectively suppressed, smooth adjustment of rotational inertia and VSG dynamic performance optimization are realized, and the system response speed and stability are enhanced.

The invention relates to the technical field of wind power plant frequency support, and discloses a frequency coordination control method and system suitable for a non-array wind power plant in a complex terrain. Through multi-direction wake flow modeling and energy storage dynamic compensation technology, cluster energy storage optimization and complex terrain adaptability design are combined to realize full-period power regulation and control. According to the method, a multi-direction wake flow coupling model is established, and the frequency supporting precision is improved; an energy storage dynamic compensation mechanism is introduced, low-frequency oscillation is suppressed, and the frequency recovery time is shortened; an economic constraint-based cluster energy storage capacity optimization algorithm reduces the energy storage configuration cost; a terrain shielding effect compensation algorithm is developed, wake flow errors are reduced, and complex scene adaptability is enhanced; a full-period power smooth regulation and control strategy is constructed, the frequency recovery rate is larger than or equal to 0.3 Hz / s, and secondary drop is controlled within 0.05 Hz. The frequency supporting capacity and the system stability of the wind power plant under the complex terrain are remarkably improved, meanwhile, economical efficiency and reliability are considered, and the method is suitable for a power system with high-proportion renewable energy access.

The invention provides a network construction converter control method, which comprises the following steps: decoupling an original transient damping coefficient and an original primary frequency modulation coefficient which are mutually coupled in an original VSG control system to obtain an active steady-state primary frequency modulation coefficient K [omega] and an active transient damping coefficient KD, and further respectively establishing active steady-state and transient independent regulation channels; active control steady-state deviation is realized through an active steady-state primary frequency modulation coefficient K [omega], response speed control is realized by combining a controller function Gdamp (s), active transient damping coefficient KD is adjusted to be combined with a second-order filter function GLPF (s) to control active low-frequency oscillation suppression, and independent control and adjustment of active steady state and transient state are realized. The adaptability of a network-forming converter control strategy to different power grid intensities, system inertias and load characteristics is further improved, and the problems that in an existing VSG control method, the control effect coupling of an original transient damping coefficient and an original primary frequency modulation coefficient is too high, and the requirement for independent adjustment of active power dynamic and steady-state characteristics cannot be met are solved.

The invention discloses a photovoltaic power station grid-connected point broadband oscillation modal parameter identification method and system based on an improved VMD-TKEO. The method comprises the steps of obtaining measurement data of broadband oscillation signals at a photovoltaic power station grid-connected point; taking the minimum envelope entropy as a target function, and determining the modal decomposition number of the variational modal decomposition (VMD) and the optimal parameter combination of the quadratic penalty factor through a whale optimization algorithm; performing variational mode decomposition on the obtained measurement data by using the optimal parameter combination to obtain an intrinsic mode component; performing noise reduction processing on the intrinsic mode component twice, performing time-frequency analysis on the intrinsic mode component IMF after noise reduction by using an energy operator TKEO, and extracting amplitude, frequency and damping ratio parameters of each mode for identification; according to the invention, the adaptive decomposition capability of the VMD on the complex broadband oscillation signal is obviously improved; noise interference and false mode components are effectively filtered out, and the signal-to-noise ratio of signals is remarkably improved; and high-precision identification of broadband oscillation modal parameters is realized.

This invention discloses a method for locating epileptogenic zones based on high-frequency oscillations and connectivity, comprising the following steps: 1. Acquiring SEEG data and selecting representative channels; 2. Filtering to acquire 54-200Hz signals and selecting baseline and target data; 3. Acquiring normalized high-frequency energy; 4. Calculating time and energy coefficients to obtain a high-frequency epileptogenicity index; 5. Filtering to acquire 12-45Hz signals; 6. Calculating nonlinear regression analysis; 7. Calculating total intensity; 8. Defining and calculating the connectivity high-frequency epileptogenicity index and evaluating performance. This invention can accurately locate epileptogenic zones in patients with different seizure patterns, and has potential application value in epilepsy treatment.

The application discloses a wide-frequency oscillation risk identification method and device. The method obtains the operation parameters of a power electronic device, the first amplitude-frequency response characteristic curve of the power electronic device in a preset frequency band range and the first transfer function of a power system, then determines the order of the transfer function of the power electronic device according to the first amplitude-frequency response characteristic curve, and then fits the second transfer function of the power electronic device according to the order of the transfer function and the operation parameters, so as to obtain the third transfer function for characterizing the performance of a grid-connected system by combining the first transfer function and the second transfer function, and finally identifies the wide-frequency oscillation risk of the grid-connected system according to the third transfer function. Through the implementation of the application, the transfer function identification efficiency can be improved, so as to improve the wide-frequency oscillation risk identification precision between the power electronic device and the power system.

The present application relates to the technical field of power system stability control, and particularly relates to a high-proportion wind power access system wide-frequency oscillation suppression method based on impedance shaping. The content comprises: constructing a frequency domain output impedance matrix, extracting a dominant wide-frequency oscillation coupling mode frequency band, introducing a mode suppression driven impedance shaping compensation mechanism, constructing a mode suppression driving quantity based on the mode direction in the dominant wide-frequency oscillation coupling mode, and performing local impedance shaping on the output impedance; based on the mode suppression driving quantity, quantifying the mode interference sensitivity between wind power converters, and introducing an impedance shaping gain correction mechanism based on the mode interference sensitivity, to correct the target output impedance after local shaping. The problems that the shaping process cannot effectively adjust dynamically according to the equipment capacity of the wind power converter and the frequency risk, resulting in no difference shaping and excessive compensation, and the problems that the interference coupling between the wind power converters cannot be effectively suppressed, resulting in excessive oscillation or suppression are solved.

The application provides a kind of energy storage converter control system stability analysis method, the method will model predictive control as the virtual synchronous generator control upper control system of energy storage converter, will model predictive control output value as the active power, reactive power reference value of virtual synchronous generator.This application realizes real-time correction virtual synchronous generator power, improves the stability of control system, avoids frequency oscillation.

The application discloses a kind of power system low-frequency oscillation characteristic signal extraction methods, comprising: the electrical variation data of acquisition each generator outlet is preprocessed;Through complementary ensemble empirical mode decomposition, variation data is decomposed, and noise component is filtered out;SPA smoothing algorithm is used to process the electrical variation signal after denoising;Prony method is used to decompose the signal after smoothing, and the amplitude and phase of each frequency component are extracted;The application has more excellent noise suppression capability and signal fidelity, can effectively solve the problem of modal aliasing, can adaptively identify and filter out various noise components, and shows good adaptability to noise interference under different operating conditions of power system;SPA smoothing algorithm reduces the error introduced by complementary ensemble empirical mode decomposition while effectively retaining the key features of low-frequency oscillation;Combined with Prony method, the characteristic parameters of the oscillation signal can be accurately extracted in a strong noise environment, providing a reliable data basis for low-frequency oscillation source identification.

The application discloses a test pool for inhibiting low-frequency oscillation, which comprises a pool main body, a wave maker and a wave absorbing beach. The wave maker is arranged in the pool main body and is used for generating waves in the pool main body. The pool main body comprises a shoal area. The wave absorbing beach is arranged in the shoal area and faces the wave maker. The wave absorbing beach is a non-solid structure with pores. A solid back plate is arranged on the side of the wave absorbing beach which is opposite to the wave maker. The solid back plate covers the side of the wave absorbing beach. A lifting plate is arranged at the bottom of the pool main body. The wave absorbing beach can change the placement position in the direction of approaching or moving away from the wave maker, so as to change the distance from the wave maker to the wave absorbing beach. The lifting plate can be lifted to change the depth of the pool main body. In the scheme, the solid back plate on the back side of the wave absorbing beach can block the waves, so that the length of the pool main body can be adjusted by moving the position of the wave absorbing beach, and then the inherent oscillation period of the pool main body is adjusted, and the interference of the inherent oscillation period of the pool on the test is reduced.

This invention discloses a method for achieving phase-frequency synchronization based on a coupled neuron oscillator system. First, a cross-frequency nonlinear coupling model is constructed by establishing a two-connection structure based on exogenous input and local interaction in the Poincaré model. By introducing nonlinear coupling, phase-frequency synchronization of the coupled neuron oscillators is achieved. Spectral analysis is then used to determine the effect of the coupling strength between low-frequency and high-frequency neurons on phase-frequency synchronization. Theoretical analysis is then used to determine the influence of the low-frequency oscillation phase on the high-frequency oscillation frequency and its mechanism.

The invention discloses a method and a device for positioning a low-frequency oscillation source of a wind-thermal bundling system. The method comprises the following steps: acquiring multi-source signals of the wind-thermal bundling system; carrying out adaptive wavelet threshold anti-noise preprocessing; carrying out adaptive noise intensity set empirical mode decomposition; screening a wind-fire coupling key mode; calculating a wind-fire adaptive dissipated energy flow; and oscillation source determination based on phase coupling: constructing a multi-condition fusion determination logic in combination with a phase coupling relationship between a dissipated energy flow trend and a wind-fire key mode and a phase coupling relationship between the dissipated energy flow trend and an electric energy transmission signal so as to realize oscillation source type identification. According to the invention, accurate identification of oscillation sources (a wind power side, a thermal power side and a tie line) in a frequency band of 0.1-2Hz is realized, and both positioning accuracy and engineering real-time performance are considered.

This invention discloses a method and system for frequency oscillation transfer analysis under multi-machine islanding. The method includes: establishing a transfer function; substituting the operating parameters of each operating condition into the transfer function; if stable, determining whether it is stable; if stable, it indicates that the system on the second VSG side is stable; if unstable, jumping to S3.1 to determine the cause of frequency oscillations on the second VSG side and the first VSG side; if unstable, it indicates that the frequency oscillations on the second VSG side are caused by insufficient stability margin of the second VSG side itself. The advantage of this invention is that it more accurately explains the cause of low-frequency oscillations in the system, thereby guiding the design of virtual synchronous machine controller parameters.

The invention discloses a measurement regulation and control method and system of an atomization tester, and aims to solve the problem that a Peltier temperature control system is easy to generate overshoot and low-frequency oscillation at a critical point. The method comprises the following steps: S1, setting a target temperature and a precision tolerance interval; s2, rapidly cooling to a dynamic switching temperature; s3, switching to a precise approaching mode, and driving a Peltier by adopting a frequency self-adaptive micro-current pulse sequence based on the residual heat to realize overshoot-free temperature positioning; and S4, when the temperature meets the steady-state conditions of the mean value, the standard deviation and the trend slope, switching to a steady-state holding mode, performing spectral analysis on the temperature sequence to distinguish high-frequency disturbance and low-frequency drift, and only applying intelligent micro-compensation subjected to nonlinear filtering to the low-frequency drift. According to the invention, through multi-stage strategy matching, pulse control based on energy prediction and intelligent disturbance identification, rapid, accurate and stable control of the temperature of the condensation plate is realized, and the reproducibility of a test result is significantly improved.

The invention discloses a method for extracting characteristic parameters of a low-frequency oscillation mode of a power system, and in the power system, a time sequence signal generally refers to records of parameters such as voltage, frequency and the like in the system along with time change. In order to effectively analyze the signals, singular spectrum analysis is firstly carried out, the singular spectrum analysis comprises the steps of converting the time sequence signals into a track matrix, then carrying out singular value decomposition on the track matrix, and reconstructing a time empirical orthogonal function obtained after singular value matrix decomposition, and at the moment, a denoised reconstructed signal is obtained; and performing matrix pencil analysis on the reconstructed signal, and extracting characteristic parameters of the low-frequency oscillation mode. In this way, the characteristic parameters of the low-frequency oscillation mode can be accurately extracted under the condition that the identification integrity is guaranteed.

The application provides a kind of high-frequency oscillation detection method based on band adaptive anti-noise power system, comprising: installing three-phase high-frequency voltage transformer at resonance sensitive point, selecting data acquisition window width, collecting three-phase voltage data and constructing three-phase voltage tensor mathematical model;Voltage tensor in three-phase voltage tensor mathematical model is decomposed to Tucker to realize voltage denoising;The voltage tensor after denoising is converted into three-phase voltage, and peak detection is carried out using dynamic measure;According to the detected high-frequency oscillation signal and its frequency range, wavelet packet decomposition is carried out on three-phase voltage, so that high-frequency oscillation signal and low-frequency fundamental wave and related harmonic components are separated to the wavelet coefficients of each frequency band;The wavelet coefficients containing oscillation signal are subjected to oscillation signal restoration;The restored oscillation signal is subjected to exponential decomposition using Prony method, and high-frequency oscillation characteristics are obtained.The application can improve the detection accuracy of high-frequency oscillation.

The invention discloses a thermal power generating unit low-frequency oscillation fault diagnosis method and system based on multi-source data analysis, and mainly relates to the technical field of thermal power generating unit fault diagnosis. Comprising the steps that a multi-source signal acquisition network is constructed, multi-source unit operation sensing of a preset monitoring window is executed on a target thermal power unit, and a speed regulator oil pressure pulsation sequence, a shaft system torsional vibration angular displacement sequence, a boiler heat storage coefficient sequence and a power grid power angle sequence are obtained; cascade time-frequency analysis is carried out; performing joint frequency matching and mode identification to obtain a plurality of abnormal low-frequency oscillation modes; and performing fault scene diagnosis on the plurality of abnormal low-frequency oscillation modes to obtain a target diagnosis result. The low-frequency oscillation fault diagnosis method has the beneficial effects that the technical problem of high error rate of low-frequency oscillation fault diagnosis caused by lack of reliable analysis of signals in the prior art is solved, and the technical effect of improving the fault diagnosis accuracy is achieved.

The application discloses a wide-frequency oscillation monitoring method and system based on waveform data. First, the coherent spectrum analysis is carried out on the waveform data of each node of the system collected by the wide-area measurement system. The oscillation is judged by detecting the coherent spectrum peak value. If it is judged that the oscillation occurs, the oscillation sample is input into the CNN model. The number of data is compressed through the convolution layer and the pooling layer. The oscillation characteristics are extracted. The overfitting is reduced. The full connection layer is used to summarize and output the feature data set D. Finally, the oscillation characteristics of the wide-frequency oscillation signal output by the convolution neural network are used as the input of the XGBoost. The oscillation frequency and the attenuation factor are identified. The application greatly reduces the monitoring cost, solves the online monitoring problem of the oscillation signal in the wide-frequency range, discriminates the oscillation and the noise by using the coherent spectrum of the multi-node waveform data, effectively reduces the misjudgment caused by the strong noise, constructs the CNN in an unsupervised manner, omits the setting of the label quantity, and improves the generalization ability and the calculation efficiency of the model.

The method for modeling impedance of DAB converter based on multi-LC resonant unit and predicting high-frequency oscillation frequency comprises the following steps: step 1, measuring impedance characteristics of a high-frequency transformer and an external inductor according to a determined DAB converter topology; step 2, calculating parasitic parameters of the high-frequency transformer and the external inductor according to the measurement results of the impedance characteristics; and step 3, judging the number of high-frequency oscillation frequencies for different configurations and parameter sizes of the external inductor and the leakage inductance of the high-frequency transformer, and selecting a high-frequency oscillation frequency prediction formula to obtain a result. The frequency variation effects of the external inductor and the leakage inductance of the high-frequency transformer are comprehensively considered, the parasitic parameters are effectively extracted through accurate modeling, and the oscillation prediction formula is derived based on the transfer function, so that the deep physical mechanism of the high-frequency oscillation is revealed, and systematic and guiding theoretical basis and engineering practice support are provided for the high-frequency oscillation suppression scheme under different structural topologies and parameter combinations.

This invention discloses a method and system for suppressing broadband oscillations in power systems based on improved virtual impedance. The method includes the following steps: acquiring grid-side and DC-side operating signals and converter output signals from the power system, and obtaining digital signals after preprocessing; performing spectral analysis on the digital signals to extract oscillation frequency characteristics, determining the current oscillation type in the power system, and outputting corresponding control signals; switching the operating mode of the filter in the dual-mode bandpass filter; outputting the corresponding oscillation component; matching and calling a pre-tuned virtual impedance parameter model, and generating virtual impedance amplitude and phase adjustment commands; converting the virtual impedance into a converter drive signal to regulate the converter output impedance characteristics and suppress broadband oscillations. This invention, through oscillation type identification and coordinated switching of two types of dual-mode filters, can regulate the converter output impedance without additional hardware, effectively suppressing broadband oscillations in the power system.

The invention provides a fire hazard wind field and spread dynamic prediction method and system based on radar echo evolution characteristics. The method comprises the following steps: step 1, carrying out ground clutter suppression, radial high-frequency oscillation filtering, speed deblurring and time-space consistency check on reflectivity Z, radial speed V, spectral width W and dual polarization quantity of a plurality of weather radars; 2, under a three-dimensional variational 3DVAR framework, taking a numerical mode background field as prior, and performing inversion to obtain a boundary layer three-dimensional wind field; 3, establishing an anisotropic cellular automaton (CA) and live wire particle swarm coupling model under the constraints of a digital elevation model (DEM), combustible material types, water content and meteorological elements; 4, performing real-time correction and short-term and temporary extrapolation on the fire point and live wire states; and step 5, result output and early warning. By applying the technical scheme, the minute-level short-term and temporary prediction capability can be improved, and the safety of key assets such as power transmission channels is ensured.

A method for controlling a microfluidic pump having a pump body defining a cavity, and an actuator arranged to generate pressure oscillations of a fluid contained within the cavity in order to cause fluid flow through an inlet and an outlet of the cavity, the method comprising: oscillating the actuator at a first frequency and determining an indication of a peak velocity of the a peak velocity of the actuator at said first frequency; oscillating the actuator at a second frequency and determining an indication of a peak velocity of the actuator at said second frequency; and adjusting the frequency of oscillation of the actuator to said first or second frequency for which the lowest peak velocity of the actuator was determined.