AFM control device and control method for adaptive feedback loop adjustment

Abstract

The invention discloses an adaptive feedback loop adjustment AFM control device and control method. The device comprises a signal acquisition module, a digital signal processing module, a feedback control module, a signal generation module, a communication interface and an upper computer module. The method comprises the following steps: carrying out sweep frequency fitting processing on a probe cantilever to obtain probe characteristic parameters; generating a sine and cosine analog signal and driving the probe to vibrate at the resonant frequency to obtain a vibration feedback signal of the probe; performing signal preprocessing on the vibration feedback signal of the probe to obtain a preprocessed vibration feedback signal; pID control and self-adaptive feedback adjustment are performed in combination with the preprocessed vibration feedback signals and probe characteristic parameters, and resonance angular frequency variation is obtained; and determining the surface structure of the sample. According to the embodiment of the invention, the control coefficient can be optimized through a performance index minimization method, multiple parameter debugging tests are reduced, and the control precision of the system is improved. The method can be widely applied to the technical field of modern nano material research.

Description

Technical Field

[0001] The present application relates to the technical field of modern nanomaterial research, and in particular to an AFM control device and a control method for adaptive feedback loop regulation. Background Art

[0002] With the rapid development of modern nanotechnology, scanning probe microscopy (SPM) is an important tool for modern nanoscale research and a basic analytical equipment for modern nanotechnology research. Compared with scanning electron microscopy (STM) which can only be used for measuring conductive samples, atomic force microscopy (AFM) is compatible with conductive and insulating samples and has a wider range of applications. According to the detection principle of atomic force microscopy, in order to ensure the imaging quality, the probe needs to work in a resonant frequency state. Therefore, it is necessary to feedback-adjust the driving signal based on the relevant information (phase, amplitude, frequency, etc.) of the actual vibration signal of the scanning probe. Feedback adjustment is usually achieved through a PID control algorithm. The driving signal after feedback adjustment acts on the AFM probe again, and the relevant information of the sample surface can be inferred based on the frequency, phase and other physical quantities of the actual vibration signal of the probe.

[0003] The AFM sampling empirical method used in the related art is often used to find the appropriate feedback control coefficient, which is not only inefficient but also not universal. Alternatively, a traditional PID coefficient tuning model is used, such as a method and device for automatically setting PID parameters based on atomic force probe parameters, which uses a constant attenuation ratio method or a critical proportion method. These methods not only require additional experimental steps to determine some preliminary coefficients, especially the critical proportion method, which requires repeated experiments to find the proportional coefficient of the critical oscillation, but also the obtained coefficients are not necessarily reliable and often require further adjustment by the operator before use.

[0004] In summary, the technical problems existing in the relevant technologies need to be improved. Summary of the invention

[0005] The main purpose of the embodiments of the present application is to propose an AFM control device and control method with adaptive feedback loop regulation, which can optimize the control coefficient by minimizing the performance index method, reduce multiple debugging parameter tests, and improve the control accuracy of the system.

[0006] To achieve the above-mentioned purpose, an embodiment of the present application provides an AFM control device with adaptive feedback loop regulation on one hand, the device comprising a signal acquisition module, a digital signal processing module, a feedback control module, a signal generating module, a communication interface and a host computer module, the output end of the signal acquisition module is connected to the first input end of the digital signal processing module, the first output end of the digital signal processing module is connected to the first input end of the communication interface, the first output end of the communication interface is connected to the first input end of the host computer module, the output end of the host computer module is connected to the second input end of the communication interface, the second output end of the communication interface is connected to the second input end of the digital signal processing module, the second output end of the digital signal processing module is connected to the first input end of the signal generating module, the third output end of the digital signal processing module is connected to the input end of the feedback control module, the output end of the feedback control module is connected to the second input end of the signal generating module, wherein:

[0007] The signal generating module is used to generate sine and cosine analog signals and drive the probe to vibrate at a resonant frequency;

[0008] The signal acquisition module is used to acquire the vibration feedback signal of the probe and perform signal preprocessing to obtain a preprocessed vibration feedback signal;

[0009] The digital signal processing module is used to perform information extraction processing on the pre-processed vibration feedback signal to obtain information on the interaction force between the probe and the sample surface;

[0010] The feedback control module is used to track and adjust the sine and cosine analog signals according to the information of the interaction force between the probe and the sample surface to obtain an adjusted sine and cosine analog signal;

[0011] The communication interface is used for data transmission communication;

[0012] The host computer module is used to display the waveform of the adjusted sine-cosine analog signal.

[0013] In some embodiments, the signal acquisition module includes a first operational amplifier, a first low-pass filter and an analog-to-digital converter, the output end of the first operational amplifier is connected to the input end of the first low-pass filter, and the output end of the first low-pass filter is connected to the input end of the analog-to-digital converter, wherein:

[0014] The first operational amplifier is used to amplify the vibration feedback signal of the probe to obtain an amplified vibration feedback signal;

[0015] The first low-pass filter is used to filter the amplified vibration feedback signal to obtain a filtered vibration feedback signal;

[0016] The analog-to-digital converter is used to perform analog-to-digital conversion processing on the filtered vibration feedback signal to obtain a preprocessed vibration feedback signal.

[0017] In some embodiments, the digital signal processing module includes a demodulator, a digital phase-locked loop and a direct frequency synthesis module, wherein:

[0018] The demodulator is used to demodulate the preprocessed vibration feedback signal to obtain the amplitude of the vibration feedback signal;

[0019] The digital phase-locked loop is used to perform frequency extraction and locked phase calculation processing on the pre-processed vibration feedback signal, respectively, to obtain the frequency value of the vibration feedback signal and the phase value of the vibration feedback signal;

[0020] The direct frequency synthesis module is used to generate discrete sine and cosine digital signals.

[0021] In some embodiments, the signal generating module includes a digital-to-analog converter, a second operational amplifier and a second low-pass filter, the output of the digital-to-analog converter is connected to the input of the second operational amplifier, and the output of the second operational amplifier is connected to the input of the second low-pass filter.

[0022] In some embodiments, the feedback control module includes a PID controller, a voltage-controlled oscillator and an automatic gain controller, the first output end of the PID controller is connected to the input end of the voltage-controlled oscillator, and the second output end of the PID controller is connected to the input end of the automatic gain controller, wherein:

[0023] The PID controller is used to adjust the output of the voltage-controlled oscillator and the automatic gain controller according to the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal, and the error between the set value and the actual measured value of the phase value of the vibration feedback signal;

[0024] The voltage controlled oscillator is used to control the frequency of the discrete sine-cosine digital signal generated by the direct frequency synthesis module;

[0025] The automatic gain controller is used to control the amplitude of the discrete sine and cosine digital signal generated by the direct frequency synthesis module.

[0026] To achieve the above object, another aspect of the embodiment of the present application provides a control method for an AFM control device with adaptive feedback loop regulation, the method comprising the following steps:

[0027] Perform frequency sweep fitting processing on the probe cantilever to obtain the probe characteristic parameters;

[0028] Generate sine and cosine analog signals and drive the probe to vibrate at a resonant frequency to obtain a vibration feedback signal of the probe;

[0029] Performing signal preprocessing on the vibration feedback signal of the probe to obtain a preprocessed vibration feedback signal;

[0030] Combining the preprocessed vibration feedback signal with the probe characteristic parameters to perform PID control and adaptive feedback adjustment to obtain a change in the resonant angular frequency;

[0031] The structure of the sample surface is determined according to the change in the resonant angular frequency.

[0032] In some embodiments, the combining the pre-processed vibration feedback signal with the probe characteristic parameter to perform PID control and adaptive feedback adjustment to obtain the resonant angular frequency change includes:

[0033] Performing digital signal processing on the preprocessed vibration feedback signal to obtain the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal and the phase value of the vibration feedback signal, and constructing an actual measurement value;

[0034] Perform PID control according to the actual measured value to generate an optimized PID coefficient;

[0035] Based on the optimized PID coefficient, the outputs of the voltage-controlled oscillator and the automatic gain controller are adjusted according to the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal, and the error between the set value and the actual measured value of the phase value of the vibration feedback signal to obtain an error adjustment result;

[0036] The frequency and amplitude of the discrete sine and cosine digital signals are adjusted according to the error adjustment result to obtain a new resonant angular frequency of the vibration feedback signal;

[0037] The resonant angular frequency of the new vibration feedback signal is subtracted from the natural resonant angular frequency of the probe to obtain a change in the resonant angular frequency.

[0038] In some embodiments, performing PID control according to the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal, and the phase value of the vibration feedback signal to generate an optimized PID coefficient includes:

[0039] Determine the initial PID coefficients based on the target time constant of the AFM control device;

[0040] Determine the transfer function of the AFM control device and the transfer function of the PID controller, and construct a closed-loop transfer function model;

[0041] According to the closed-loop transfer function model, in combination with the initial PID coefficient, the initial PID coefficient is iteratively optimized by minimizing the performance index method to generate the optimized PID coefficient, and PID control is performed on the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal and the phase value of the vibration feedback signal.

[0042] In some embodiments, the expression of the closed-loop transfer function model is specifically as follows:

[0043]

[0044] In the above formula, T(s) represents the closed-loop transfer function model, ω n represents the natural frequency of the resonator system, K p Represents the proportional gain coefficient, K i represents the integral gain coefficient, s represents the complex variable in Laplace transform, ω ext Represents the external interference frequency, K d represents the differential gain coefficient, ω f represents the differential filter cutoff frequency, Q represents the quality factor of the resonator system, and A represents the external interference amplitude.

[0045] In some embodiments, the expression of the output signal of the PID controller is specifically as follows:

[0046]

[0047] In the above formula, u(t) represents the feedback control signal, K p represents the proportional gain coefficient, Δθ represents the error between the set value and the actual measured value, K i Indicates the integral gain coefficient, K d Denotes the differential gain coefficient, T f represents the time constant of the differential filter, and s represents the complex frequency variable in the Laplace transform.

[0048] The embodiments of the present application include at least the following beneficial effects: The present application provides an AFM control device and control method with adaptive feedback loop adjustment. The scheme obtains the characteristic parameters of the probe by performing frequency sweep fitting processing on the probe cantilever, and then generates sine and cosine analog signals and drives the probe to vibrate at a resonant frequency to obtain a vibration feedback signal of the probe. The vibration feedback signal of the probe is further preprocessed to obtain a preprocessed vibration feedback signal. The preprocessed vibration feedback signal is combined with the characteristic parameters of the probe to perform PID control and adaptive feedback adjustment to obtain a change in the resonant angular frequency. The method of adaptively adjusting the feedback loop is adopted, and a suitable control coefficient can be directly generated according to the model of the controlled object, which greatly saves the test time, and the control coefficient can be adjusted accordingly according to the input target response speed, which greatly improves the flexibility of the system. The control coefficient is optimized by minimizing the performance index method, and the generated coefficient can be used directly, avoiding multiple debugging parameter tests. The structure of the sample surface is determined according to the change in the resonant angular frequency, and secondary development can be performed according to different test requirements, which greatly improves the versatility of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a structural schematic diagram of an AFM control device with adaptive feedback loop regulation provided in an embodiment of the present application;

[0050] Figure 2 It is a schematic flow chart of the steps of a control method of an AFM control device with adaptive feedback loop regulation provided in an embodiment of the present application;

[0051] Figure 3 It is a schematic diagram of a feedback loop automatic adjustment logic control framework provided by an embodiment of the present application;

[0052] Figure 4 It is a schematic diagram of the feedback loop automatic adjustment logic control flow provided in an embodiment of the present application.

[0053] Attached figure numbers: 110, signal acquisition module; 120, digital signal processing module; 130, feedback control module; 140, signal generation module; 150, communication interface; 160, host computer module; 101, first operational amplifier; 102, first low-pass filter; 103, analog-to-digital converter; 201, demodulator; 202, digital phase-locked loop; 203, direct frequency synthesis module; 301, PID controller; 302, automatic gain controller; 303, voltage-controlled oscillator; 401, second low-pass filter; 402, second operational amplifier; 403, digital-to-analog converter. DETAILED DESCRIPTION

[0054] In order to make the purpose, technical solutions and advantages of the present application clearer, the present application is further described in detail below in conjunction with the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the present application and are not intended to limit the present application. When the following description refers to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the embodiments of the present application. They are only examples of systems and methods consistent with some aspects of the embodiments of the present application as detailed in the attached claims.

[0055] It is understood that the terms "first", "second", etc. used in this application can be used to describe various concepts in this article, but unless otherwise specified, these concepts are not limited by these terms. These terms are only used to distinguish one concept from another concept. For example, without departing from the scope of the embodiment of the present application, the first information may also be referred to as the second information, and similarly, the second information may also be referred to as the first information. Depending on the context, the words "if" and "if" as used herein can be interpreted as "at the time of" or "when" or "in response to determination".

[0056] The terms "at least one", "multiple", "each", "any", etc. used in this application, at least one includes one, two or more, multiple includes two or more, each refers to each of the corresponding multiple, and any refers to any one of the multiple.

[0057] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein are only for the purpose of describing the embodiments of this application and are not intended to limit this application.

[0058] Reference Figure 1 , Figure 1 A structural diagram of an AFM control device for adaptive feedback loop regulation provided by an embodiment of the present invention, referring to Figure 1The device includes a signal acquisition module 110, a digital signal processing module 120, a feedback control module 130, a signal generating module 140, a communication interface 150 and a host computer module 160, wherein the output end of the signal acquisition module is connected to the first input end of the digital signal processing module, the first output end of the digital signal processing module is connected to the first input end of the communication interface, the first output end of the communication interface is connected to the first input end of the host computer module, the output end of the host computer module is connected to the second input end of the communication interface, the second output end of the communication interface is connected to the second input end of the digital signal processing module, the second output end of the digital signal processing module is connected to the first input end of the signal generating module, the third output end of the digital signal processing module is connected to the input end of the feedback control module, and the output end of the feedback control module is connected to the second input end of the signal generating module, wherein:

[0059] The signal generation module is used to generate sine and cosine analog signals and drive the probe to vibrate at a resonant frequency;

[0060] Specifically, the signal acquisition module includes a first operational amplifier 101, a first low-pass filter 102 and an analog-to-digital converter 103, the output end of the first operational amplifier is connected to the input end of the first low-pass filter, and the output end of the first low-pass filter is connected to the input end of the analog-to-digital converter, wherein the first operational amplifier is used to amplify the vibration feedback signal of the probe to obtain an amplified vibration feedback signal; the first low-pass filter is used to filter the amplified vibration feedback signal to obtain a filtered vibration feedback signal; the analog-to-digital converter is used to perform analog-to-digital conversion on the filtered vibration feedback signal to obtain a preprocessed vibration feedback signal.

[0061] The signal acquisition module is used to obtain the vibration feedback signal of the probe and perform signal preprocessing to obtain the preprocessed vibration feedback signal;

[0062] Specifically, the digital signal processing module includes a demodulator 201, a digital phase-locked loop 202 and a direct frequency synthesis module 203, wherein the demodulator is used to demodulate the preprocessed vibration feedback signal to obtain the amplitude of the vibration feedback signal; the digital phase-locked loop is used to perform frequency extraction and locked phase calculation processing on the preprocessed vibration feedback signal to obtain the frequency of the vibration feedback signal and the phase value of the vibration feedback signal; the direct frequency synthesis module is used to generate discrete sine and cosine digital signals.

[0063] The digital signal processing module is used to extract information from the pre-processed vibration feedback signal to obtain information about the interaction force between the probe and the sample surface;

[0064] Specifically, the signal generation module includes a digital-to-analog converter 403, a second operational amplifier 402 and a second low-pass filter 401, the output of the digital-to-analog converter is connected to the input of the second operational amplifier, and the output of the second operational amplifier is connected to the input of the second low-pass filter.

[0065] The feedback control module is used to track and adjust the sine and cosine analog signals according to the information of the interaction force between the probe and the sample surface to obtain the adjusted sine and cosine analog signals;

[0066] Specifically, the feedback control module includes a PID controller 301, a voltage-controlled oscillator 303 and an automatic gain controller 302, wherein a first output end of the PID controller is connected to an input end of the voltage-controlled oscillator, and a second output end of the PID controller is connected to an input end of the automatic gain controller, wherein the PID controller is used to adjust the outputs of the voltage-controlled oscillator and the automatic gain controller according to the error between the set value and the actual measured value of the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal and the phase value of the vibration feedback signal; the voltage-controlled oscillator is used to control the frequency of the discrete sine-cosine digital signal generated by the direct frequency synthesis module; and the automatic gain controller is used to control the amplitude of the discrete sine-cosine digital signal generated by the direct frequency synthesis module.

[0067] The communication interface is used for data transmission communication;

[0068] The host computer module is used to display the waveform of the adjusted sine and cosine analog signal.

[0069] In summary, in this embodiment, the AFM control device constructed by the embodiment of the present invention includes a digital signal processing module, a signal acquisition module, a signal generation module, a feedback control module, a communication interface, and a host computer module. Among them, the signal acquisition module includes an operational amplifier, a low-pass filter, and an analog-to-digital converter. The module is used to collect the feedback signal when the probe moves on the sample surface. After the input signal is amplified and filtered, the analog signal is converted into a digital signal by the analog-to-digital converter. The digital signal generation module includes a digital-to-analog converter, an operational amplifier, and a low-pass filter. The module is used to generate a sine-cosine analog signal for driving the probe to vibrate; the digital signal processing module is used to process the collected probe feedback signal and extract information related to the interaction force between the sample surface and the probe; the feedback control module includes a PID control algorithm, a voltage-controlled oscillator (VCO), and an automatic gain controller (AGC). The module is used to adjust the frequency of the drive signal to track the resonant frequency in real time; the host computer module is built through a third-party platform, and can display the signal waveform and the feedback loop control effect.

[0070] See also Figure 2The embodiment of the present application also provides a control method of an AFM control device with adaptive feedback loop regulation, which can realize the above-mentioned AFM control device with adaptive feedback loop regulation. The control method includes the following steps:

[0071] S100, performing frequency sweep fitting processing on the probe cantilever to obtain probe characteristic parameters;

[0072] In some specific embodiments, the automatic adjustment coefficient function of the feedback control module requires characteristic parameters of the probe cantilever, which can be obtained through the system's frequency sweep function or by fitting the step response of the probe. The characteristic parameters include the probe's quality factor (Q value), amplitude gain Gain, damping coefficient c, linear stiffness coefficient k, and resonant system bandwidth Bandwidth or center frequency f0.

[0073] S200, generating a sine and cosine analog signal and driving the probe to vibrate at a resonant frequency to obtain a vibration feedback signal of the probe;

[0074] In some specific embodiments, the signal generating module generates a signal having a frequency of the natural resonance frequency f0 of the probe cantilever to drive the probe to vibrate. The specific expression is F drive (t) = F0cos(ω0t+φ), ω0 = 2πf0. Under the excitation of the driving signal, the motion of the probe cantilever can be expressed as z(t) = Acos(ωt+φ), where A is the amplitude and φ is the phase. Due to the influence of the probe-sample interaction force, the resonant frequency of the cantilever will change, and the change Δω of the resonant angular frequency ω can be expressed as: Δω = ω-ω0. The sample morphology can be inferred from the frequency offset. is the derivative of the probe-sample interaction force at the equilibrium position z0.

[0075] S300, performing signal preprocessing on the vibration feedback signal of the probe to obtain a preprocessed vibration feedback signal;

[0076] S400, combining the pre-processed vibration feedback signal with the probe characteristic parameters to perform PID control and adaptive feedback adjustment to obtain a change in the resonant angular frequency;

[0077] It should be noted that, in some embodiments, step S400 may include: S410, performing digital signal processing on the pre-processed vibration feedback signal to obtain the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal and the phase value of the vibration feedback signal, and constructing the actual measurement value; S420, performing PID control according to the actual measurement value to generate an optimized PID coefficient; S430, based on the optimized PID coefficient, adjusting the output of the voltage-controlled oscillator and the automatic gain controller according to the error between the set value of the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal and the phase value of the vibration feedback signal and the actual measurement value to obtain an error adjustment result; S440, adjusting the frequency of the discrete sine and cosine digital signal and the amplitude of the discrete sine and cosine digital signal according to the error adjustment result to obtain a new resonant angular frequency of the vibration feedback signal; S450, subtracting the resonant angular frequency of the new vibration feedback signal from the natural resonant angular frequency of the probe to obtain a change in the resonant angular frequency.

[0078] In step S410, it should be noted that for the input signal, for example, V in (t) = A sin(ωt+φ), and the demodulator reference signal V ref (t) = sin(ωt) and then multiply to get the product signal V product (t) = Asin(ωt+φ)sin(ωt), after expanding using the trigonometric formula, we get: The output after low-pass filtering is the DC component: This DC component represents the input signal amplitude A and phase φ.

[0079] In some specific embodiments, Figure 3 As shown, in order to obtain the changed resonant angular frequency ω, the present invention adopts a digital phase-locked loop (PLL) tracking method. Since the phase-frequency relationship of the resonant system formed by the probe cantilever is fixed, the frequency can be calculated by locking the phase. The signal acquisition module receives the vibration signal of the probe, amplifies, filters and samples it, and then transmits it to the digital signal processing module. The digital signal processing module is mainly composed of a demodulator based on a combination of FPGA and ARM, a phase-locked loop, a direct frequency synthesis (DDS) and other parts.

[0080] For an input signal, for example, z(t)=Acos(ωt+θ), after multiplying it by the reference signal of the demodulator, the phase value of the input signal can be easily obtained.

[0081] The feedback control system makes corresponding adjustments according to the phase change Δθ to keep the probe cantilever vibrating at the resonance frequency. The feedback control signal can be expressed as:

[0082]

[0083] In the above formula, u(t) represents the feedback control signal, K p represents the proportional gain coefficient, Δθ represents the error between the set value and the actual measured value, K i Indicates the integral gain coefficient, K d Denotes the differential gain coefficient, T f represents the time constant of the differential filter, and s represents the complex frequency variable in the Laplace transform.

[0084] Information such as feedback control deviation and feedback control adjustment time can be transmitted back to the host computer module through the communication interface, so the feedback loop control effect can be clearly judged.

[0085] It should be noted that, in some embodiments, step S420 may include: S421, determining the initial PID coefficient according to the target time constant of the AFM control device; S422, determining the transfer function of the AFM control device and the transfer function of the PID controller, and constructing a closed-loop transfer function model; S423, according to the closed-loop transfer function model, in combination with the initial PID coefficient, iteratively optimizing the initial PID coefficient by minimizing the performance index method, generating optimized PID coefficients, and performing PID control on the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal, and the phase value of the vibration feedback signal.

[0086] During the operation of AFM, the signal adjustment speed is proportional to the signal noise, so the system adjustment speed in the present invention is adjustable, which provides a larger operating space compared to the traditional AFM control system. The system adjustment speed can be measured by a time constant, so before the feedback control module automatically adjusts the PID coefficient, it is also necessary to input the target time constant of the system, which parameter needs to be considered by compromising the required response time and signal-to-noise ratio.

[0087] Furthermore, if Figure 4 As shown, the PID control algorithm automatically calculates the appropriate PID coefficients based on the above input characteristic parameters and models. The AFM probe cantilever is a typical mechanical resonance system. The transfer function model of the mechanical resonance system can be expressed as follows:

[0088]

[0089] Where ζ represents the system damping ratio, ω n Represents the natural frequency of the resonator system. However, the damping ratio is a relatively abstract concept for practical applications. It is replaced by the characteristic parameter Q value of the resonator, and considering nonlinearity and external interference, the transfer function can be corrected to the following form:

[0090]

[0091] Furthermore, the control algorithm of the present invention gives priority to the PID control form by default, and can be adjusted to the PIDF control form according to demand. The transfer function model of the PID controller has the following form:

[0092]

[0093] Therefore, the closed-loop transfer function model composed of the PID controller and the probe cantilever system can be expressed as:

[0094]

[0095] In the above formula, T(s) represents the closed-loop transfer function model, ω n represents the natural frequency of the resonator system, K p Represents the proportional gain coefficient, K i represents the integral gain coefficient, s represents the complex variable in Laplace transform, ω ext Represents the external interference frequency, K d represents the differential gain coefficient, ω f represents the differential filter cutoff frequency, Q represents the quality factor of the resonator system, and A represents the external interference amplitude.

[0096] Furthermore, based on the above known model, the present invention optimizes the PID control coefficient by minimizing the performance index method, such as the integral absolute error (IAE), the integral square error (ISE), the integral time square error (ITSE) and the time multiplied absolute error index (ITAE), etc. These indices can quantify the control performance of the system, and the theoretically optimal controller parameters can be obtained by adjusting the PID coefficient and minimizing the error index.

[0097] Taking the time multiplied absolute error integral (ITAE) as an example, ITAE = ∫0 ∞ t·|e(t)|dt. The error e(t) is defined as the difference between the set value r(t) and the actual output y(t).

[0098] Furthermore, since the above indicators are not intuitive, they are only used inside the control algorithm. The present invention feeds back the real-time control effect by calculating the open-loop and closed-loop responses as well as the amplitude-frequency response and phase-frequency response relationships.

[0099] S500, determining the structure of the sample surface according to the change in the resonance angular frequency.

[0100] In summary, compared with the prior art, the embodiments of the present invention have the following advantages:

[0101] 1) The adaptive adjustment feedback loop method is adopted, which can directly generate appropriate control coefficients according to the model of the controlled object, greatly saving test time, and can make corresponding adjustments to the control coefficients according to the input target response speed, greatly improving the flexibility of the system, and the control effect can be displayed in real time by the host computer.

[0102] 2) The system is composed of a digital signal processing module, a signal acquisition module, a signal generation module, a feedback control module, a communication interface, and a host computer module. The system has a high degree of integration and can be secondary developed according to different test requirements, which greatly improves the versatility of the embodiments of the present invention.

[0103] 3) The embodiment of the present invention adopts a feedback adjustment method based on a known model, and optimizes the control coefficient by minimizing the performance index method. The generated coefficient can be used directly, avoiding multiple debugging parameter tests and can better meet the conditions of different types of probes.

[0104] It can be understood that the contents of the above method embodiments are all applicable to the present system embodiments, the functions specifically implemented by the present system embodiments are the same as those of the above method embodiments, and the beneficial effects achieved are also the same as those achieved by the above method embodiments.

[0105] The preferred embodiments of the present invention are described above with reference to the accompanying drawings, but the scope of the rights of the present invention is not limited thereto. Any modification, equivalent substitution and improvement made by a person skilled in the art without departing from the scope and essence of the present invention should be within the scope of the rights of the present invention.

Claims

1. An AFM control device with adaptive feedback loop regulation, characterized in that: The device comprises a signal acquisition module, a digital signal processing module, a feedback control module, a signal generating module, a communication interface and a host computer module, wherein the output end of the signal acquisition module is connected to the first input end of the digital signal processing module, the first output end of the digital signal processing module is connected to the first input end of the communication interface, the first output end of the communication interface is connected to the first input end of the host computer module, the output end of the host computer module is connected to the second input end of the communication interface, the second output end of the communication interface is connected to the second input end of the digital signal processing module, the second output end of the digital signal processing module is connected to the first input end of the signal generating module, the third output end of the digital signal processing module is connected to the input end of the feedback control module, and the output end of the feedback control module is connected to the second input end of the signal generating module, wherein: The signal generating module is used to generate sine and cosine analog signals and drive the probe to vibrate at a resonant frequency; The signal acquisition module is used to acquire the vibration feedback signal of the probe and perform signal preprocessing to obtain a preprocessed vibration feedback signal; The digital signal processing module is used to perform information extraction processing on the pre-processed vibration feedback signal to obtain information on the interaction force between the probe and the sample surface; The feedback control module is used to track and adjust the sine and cosine analog signals according to the information of the interaction force between the probe and the sample surface to obtain an adjusted sine and cosine analog signal; The communication interface is used for data transmission communication; The host computer module is used to display the waveform of the adjusted sine-cosine analog signal.

2. The device according to claim 1, characterized in that The signal acquisition module includes a first operational amplifier, a first low-pass filter and an analog-to-digital converter, the output end of the first operational amplifier is connected to the input end of the first low-pass filter, and the output end of the first low-pass filter is connected to the input end of the analog-to-digital converter, wherein: The first operational amplifier is used to amplify the vibration feedback signal of the probe to obtain an amplified vibration feedback signal; The first low-pass filter is used to filter the amplified vibration feedback signal to obtain a filtered vibration feedback signal; The analog-to-digital converter is used to perform analog-to-digital conversion processing on the filtered vibration feedback signal to obtain a pre-processed vibration feedback signal.

3. The device according to claim 1, characterized in that The digital signal processing module includes a demodulator, a digital phase-locked loop and a direct frequency synthesis module, wherein: The demodulator is used to demodulate the preprocessed vibration feedback signal to obtain the amplitude of the vibration feedback signal; The digital phase-locked loop is used to perform frequency extraction and locked phase calculation processing on the pre-processed vibration feedback signal, respectively, to obtain the frequency value of the vibration feedback signal and the phase value of the vibration feedback signal; The direct frequency synthesis module is used to generate discrete sine and cosine digital signals.

4. The device according to claim 1, characterized in that The signal generating module comprises a digital-to-analog converter, a second operational amplifier and a second low-pass filter. The output of the digital-to-analog converter is connected to the input of the second operational amplifier, and the output of the second operational amplifier is connected to the input of the second low-pass filter.

5. The device according to claim 1, characterized in that The feedback control module includes a PID controller, a voltage-controlled oscillator and an automatic gain controller, wherein the first output end of the PID controller is connected to the input end of the voltage-controlled oscillator, and the second output end of the PID controller is connected to the input end of the automatic gain controller, wherein: The PID controller is used to adjust the output of the voltage-controlled oscillator and the automatic gain controller according to the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal, and the error between the set value and the actual measured value of the phase value of the vibration feedback signal; The voltage controlled oscillator is used to control the frequency of the discrete sine-cosine digital signal generated by the direct frequency synthesis module; The automatic gain controller is used to control the amplitude of the discrete sine and cosine digital signals generated by the direct frequency synthesis module.

6. A control method for an AFM control device with adaptive feedback loop regulation, characterized in that: The method comprises the following steps: Perform frequency sweep fitting processing on the probe cantilever to obtain the probe characteristic parameters; Generate sine and cosine analog signals and drive the probe to vibrate at a resonant frequency to obtain a vibration feedback signal of the probe; Performing signal preprocessing on the vibration feedback signal of the probe to obtain a preprocessed vibration feedback signal; Combining the preprocessed vibration feedback signal with the probe characteristic parameters to perform PID control and adaptive feedback adjustment to obtain a change in the resonant angular frequency; The structure of the sample surface is determined according to the change in the resonant angular frequency.

7. The method according to claim 6, characterized in that The combining of the pre-processed vibration feedback signal and the probe characteristic parameter to perform PID control and adaptive feedback adjustment to obtain the resonant angular frequency change includes: Performing digital signal processing on the preprocessed vibration feedback signal to obtain the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal and the phase value of the vibration feedback signal, and constructing an actual measurement value; Perform PID control according to the actual measured value to generate an optimized PID coefficient; Based on the optimized PID coefficient, the outputs of the voltage-controlled oscillator and the automatic gain controller are adjusted according to the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal, and the error between the set value and the actual measured value of the phase value of the vibration feedback signal to obtain an error adjustment result; The frequency and amplitude of the discrete sine and cosine digital signals are adjusted according to the error adjustment result to obtain a new resonant angular frequency of the vibration feedback signal; The resonant angular frequency of the new vibration feedback signal is subtracted from the natural resonant angular frequency of the probe to obtain a change in the resonant angular frequency.

8. The method according to claim 7, characterized in that The step of performing PID control according to the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal and the phase value of the vibration feedback signal to generate an optimized PID coefficient includes: Determine the initial PID coefficients based on the target time constant of the AFM control device; Determine the transfer function of the AFM control device and the transfer function of the PID controller, and construct a closed-loop transfer function model; According to the closed-loop transfer function model, in combination with the initial PID coefficient, the initial PID coefficient is iteratively optimized by minimizing the performance index method to generate the optimized PID coefficient, and PID control is performed on the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal and the phase value of the vibration feedback signal.

9. The method according to claim 8, characterized in that The expression of the closed-loop transfer function model is specifically as follows: In the above formula, T(s) represents the closed-loop transfer function model, ω n represents the natural frequency of the resonator system, K p Represents the proportional gain coefficient, K i represents the integral gain coefficient, s represents the complex variable in Laplace transform, ω ext Indicates the external interference frequency, K d represents the differential gain coefficient, ω f represents the differential filter cutoff frequency, Q represents the quality factor of the resonator system, and A represents the external interference amplitude.

10. The method according to claim 8, characterized in that The expression of the output signal of the PID controller is specifically as follows: In the above formula, u(t) represents the feedback control signal, K p represents the proportional gain coefficient, Δθ represents the error between the set value and the actual measured value, K i Indicates the integral gain coefficient, K d Denotes the differential gain coefficient, T f represents the time constant of the differential filter, and s represents the complex frequency variable in the Laplace transform.

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