An AFM control device and control method with adaptive feedback loop regulation

Abstract

The application discloses an AFM control device and a control method of adaptive feedback loop adjustment. 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: performing sweep fitting processing on a probe cantilever to obtain probe characteristic parameters; generating a sine and cosine simulation signal and driving the probe to vibrate at a resonance 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; performing PID control and adaptive feedback adjustment on the preprocessed vibration feedback signal and the probe characteristic parameters to obtain a resonance angular frequency change; and determining the structure of a sample surface. The embodiment of the application can optimize the control coefficient by using the minimum performance index method, reduce the test of multiple debugging parameters, and improve the control precision of the system. The application can be widely applied to the field of modern nanomaterial research technology.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of modern nanomaterial research, and particularly relates to an AFM control device and a control method of self-adaptive feedback loop adjustment. BACKGROUND

[0002] With the rapid development of modern nanotechnology, a scanning probe microscope (SPM) is an important tool for modern nanoscale research and a basic analysis equipment for modern nanotechnology research. Compared with a scanning electron microscope (STM) which can only be applied to measurement of conductive samples, an atomic force microscope (AFM) can be compatible with conductive and insulating samples and has a wider application range. According to the detection principle of the atomic force microscope, in order to ensure the imaging quality, the probe needs to work in the resonant frequency state, and therefore a feedback adjustment needs to be made to the driving signal according to the related information (phase, amplitude, frequency, etc.) of the actual vibration signal of the scanning probe. The feedback adjustment is usually realized through a PID control algorithm. The driving signal after the feedback adjustment is re-applied to the probe of the AFM, and the related information of the sample surface can be calculated according to the frequency, phase and other physical quantities of the actual vibration signal of the probe.

[0003] The AFM of the related art usually finds suitable feedback control coefficients by using an empirical method, which is not only low in efficiency but also has no universality; or a traditional PID coefficient setting model is used, for example, an automatic setting PID method and device based on atomic force probe parameters. The parameter setting method used by the method and device is a constant damping ratio method or a critical proportion method. These methods not only need additional experimental steps to determine some preliminary coefficients, especially the critical proportion method, which needs to find the proportional coefficient of critical oscillation through repeated experiments, but also the obtained coefficients are not necessarily reliable and often need to be further adjusted by an operator before use.

[0004] In summary, the technical problems in the related art need to be improved. SUMMARY

[0005] The main purpose of the embodiments of the present application is to provide an AFM control device and a control method of self-adaptive feedback loop adjustment, which can optimize the control coefficients through the minimum performance index method, reduce the test of multiple debugging parameters, and improve the control precision of the system.

[0006] To achieve the above object, one aspect of the embodiment of the present application proposes an AFM control device with adaptive feedback loop adjustment, which 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 output end of the signal acquisition module is connected with the first input end of the digital signal processing module, the first output end of the digital signal processing module is connected with the first input end of the communication interface, the first output end of the communication interface is connected with the first input end of the upper computer module, the output end of the upper computer module is connected with the second input end of the communication interface, the second output end of the communication interface is connected with the second input end of the digital signal processing module, the second output end of the digital signal processing module is connected with the first input end of the signal generation module, the third output end of the digital signal processing module is connected with the input end of the feedback control module, and the output end of the feedback control module is connected with the second input end of the signal generation module, wherein:

[0007] The signal generation module is used for generating a sine simulation signal and driving the probe to vibrate at a resonance frequency.

[0008] The signal acquisition module is used for acquiring the vibration feedback signal of the probe and performing signal preprocessing to obtain a preprocessed vibration feedback signal.

[0009] The digital signal processing module is used for performing information extraction processing on the preprocessed vibration feedback signal to obtain information of the interaction force between the probe and the sample surface.

[0010] The feedback control module is used for performing tracking adjustment processing on the sine simulation signal according to the information of the interaction force between the probe and the sample surface to obtain an adjusted sine simulation signal.

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

[0012] The upper computer module is used for displaying the waveform of the adjusted sine simulation signal.

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

[0014] The first operational amplifier is used for performing amplification processing on the vibration feedback signal of the probe to obtain an amplified vibration feedback signal.

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

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

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

[0018] The demodulator is configured to demodulate the preprocessed vibration feedback signal to obtain an amplitude of the vibration feedback signal.

[0019] The digital phase-locked loop is configured to perform frequency extraction and phase-locked loop calculation on the preprocessed vibration feedback signal to obtain a frequency of the vibration feedback signal and a phase value of the vibration feedback signal.

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

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

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

[0023] The PID controller is configured to adjust outputs of the voltage-controlled oscillator and the automatic gain controller according to errors between set values and actual measurement values of the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal, and the phase value of the vibration feedback signal.

[0024] The voltage-controlled oscillator is configured to control a frequency of the discrete sine and cosine digital signals generated by the direct frequency synthesis module.

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

[0026] To achieve the above object, another aspect of the embodiments of the present application proposes a control method of an AFM control device with adaptive feedback loop adjustment, which comprises the following steps:

[0027] The probe cantilever is subjected to sweep fitting processing to obtain probe characteristic parameters;

[0028] Sine and cosine simulation signals are generated and the probe is driven to vibrate at a resonance frequency to obtain a vibration feedback signal of the probe;

[0029] The vibration feedback signal of the probe is subjected to signal preprocessing to obtain a preprocessed vibration feedback signal;

[0030] PID control and adaptive feedback adjustment are performed on the preprocessed vibration feedback signal and the probe characteristic parameters to obtain a resonance angular frequency change amount;

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

[0032] In some embodiments, the PID control and adaptive feedback adjustment performed on the preprocessed vibration feedback signal and the probe characteristic parameters to obtain a resonance angular frequency change amount include:

[0033] The preprocessed vibration feedback signal is subjected to digital signal processing to obtain an amplitude of the vibration feedback signal, a frequency of the vibration feedback signal, and a phase value of the vibration feedback signal, and an actual measurement value is constructed;

[0034] PID control is performed according to the actual measurement value to generate optimized PID coefficients;

[0035] Based on the optimized PID coefficients, the output of the voltage-controlled oscillator and the automatic gain controller is adjusted according to the error between the set value and the actual measurement 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 to obtain an error adjustment result;

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

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

[0038] In some embodiments, the PID control performed 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 optimized PID coefficients includes:

[0039] Initial PID coefficients are determined according to the target time constant of the AFM control device;

[0040] The transfer function of the AFM control device and the transmission function of the PID controller are determined to construct a closed-loop transmission function model;

[0041] According to the closed-loop transfer function model, the initial PID coefficients are iteratively optimized by minimizing a performance index to generate optimized PID coefficients, and the amplitude of the vibration feedback signal, the frequency of the vibration feedback signal, and the phase value of the vibration feedback signal are controlled by the PID.

[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 a complex variable in Laplace transform, ω ext represents the external disturbance frequency, K d represents the derivative gain coefficient, ω f represents the cutoff frequency of the derivative filter, Q represents the quality factor of the resonator system, and A represents the external disturbance 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 represents the integral gain coefficient, K d represents the derivative gain coefficient, T f represents the time constant of the derivative filter, and s represents a complex frequency variable in Laplace transform.

[0048] The embodiment of the present application at least has the following beneficial effects: the present application provides an AFM control device and control method with adaptive feedback loop adjustment, which obtains probe characteristic parameters through sweep fitting processing on the probe cantilever, generates a sine and cosine simulation signal and drives the probe to vibrate at the resonance frequency, obtains the vibration feedback signal of the probe, further pre-processes the vibration feedback signal of the probe to obtain a pre-processed vibration feedback signal, combines the pre-processed vibration feedback signal with the probe characteristic parameters to perform PID control and adaptive feedback adjustment, and obtains the resonance angular frequency variation. The method of adaptive feedback loop adjustment can directly generate appropriate control coefficients according to the model of the controlled object, greatly saves the test time, and can make corresponding adjustment to the control coefficients according to the input target response speed, greatly improves the flexibility of the system, and optimizes the control coefficients by the minimum performance index method, so that the generated coefficients can be directly used, avoiding multiple test and adjustment of parameters. The structure of the sample surface is determined according to the resonance angular frequency variation, secondary development can be performed according to different test requirements, and the universality of the present application is greatly improved. BRIEF DESCRIPTION OF DRAWINGS

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

[0050] Figure 2 is a step flowchart of a control method of an AFM control device with adaptive feedback loop adjustment provided by the embodiment of the present application;

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

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

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

[0054] In order to make the purposes, technical solutions and advantages of the present application clearer, the present application will be further described in details below with reference to the accompanying drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application. When the following description refers to the accompanying drawings, the same numbers in different drawings represent the same or similar elements unless otherwise indicated. The implementations described in the following exemplary examples do not represent all implementations consistent with 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 appended claims.

[0055] It can be understood that the terms "first", "second" and the like used in the present application can be used herein to describe various concepts, but unless specifically stated, 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 embodiments of the present application, the first information can also be referred to as the second information, and similarly, the second information can also be referred to as the first information. Depending on the context, the word "if" as used herein can be interpreted as "when" or "upon" or "in response to determining".

[0056] The terms "at least one", "multiple", "each", "any" and the like used in the present application include one, two or more than two, multiple includes two or more than two, 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 commonly understood by one skilled in the art to which the present application belongs. The terms used herein are only for the purpose of describing the embodiments of the present application and are not intended to limit the present application.

[0058] Referring to Figure 1 , Figure 1 The structure diagram of an AFM control device with adaptive feedback loop adjustment provided by the embodiments of the present application is shown in FIG. 1. 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. 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; and 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 and 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 comprises a digital-to-analog converter 403, a second operational amplifier 402 and a second low-pass filter 401, an output end of the digital-to-analog converter is connected with an input end of the second operational amplifier, and an output end of the second operational amplifier is connected with an input end of the second low-pass filter.

[0065] The feedback control module is used for tracking and adjusting the sine-cosine analog signal according to the information of the interaction force between the probe and the sample surface, so as to obtain an adjusted sine-cosine analog signal.

[0066] Specifically, the feedback control module comprises a PID controller 301, a voltage-controlled oscillator 303 and an automatic gain controller 302, a first output end of the PID controller is connected with an input end of the voltage-controlled oscillator, and a second output end of the PID controller is connected with an input end of the automatic gain controller, wherein the PID controller is used for adjusting the output of the voltage-controlled oscillator and the automatic gain controller according to the error between the set value and the actual measurement value of the amplitude, the frequency and the phase value of the vibration feedback signal; the voltage-controlled oscillator is used for controlling the frequency of the discrete sine-cosine digital signal generated by the direct frequency synthesis module; and the automatic gain controller is used for controlling 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 for displaying the waveform of the adjusted sine-cosine analog signal.

[0069] In summary, in the embodiment, the AFM control device constructed by the embodiment of the application comprises 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 signal acquisition module comprises an operational amplifier, a low-pass filter and an analog-to-digital converter, and is used for acquiring the feedback signal of the probe 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, so as to be a digital signal convenient for computer processing. The signal generation module comprises a digital-to-analog converter, an operational amplifier and a low-pass filter, and is used for generating a sine-cosine analog signal for driving the probe to vibrate. The digital signal processing module is used for processing the acquired probe feedback signal and extracting the information related to the interaction force between the sample surface and the probe. The feedback control module comprises a PID control algorithm, a voltage-controlled oscillator (VCO) and an automatic gain controller (AGC), and is used for adjusting the frequency of the driving signal to track the resonance frequency in real time. The host computer module can display the signal waveform and the feedback loop control effect through a third-party platform.

[0070] Please refer to Figure 2The embodiment of the application further provides a control method of the AFM control device with adaptive feedback loop adjustment, which can realize the AFM control device with adaptive feedback loop adjustment.

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

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

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

[0074] In some specific embodiments, the signal generation module generates a signal with a frequency of the natural resonance frequency f0 of the probe cantilever to drive the probe to vibrate, and 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 represented 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 resonance frequency of the cantilever will change, and the change amount Δω of the resonance angular frequency ω can be represented as: Δω=ω-ω0. The sample topography can be inferred through 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, performing PID control and adaptive feedback adjustment on the preprocessed vibration feedback signal and the probe characteristic parameters to obtain a resonance angular frequency change amount;

[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 and amplitude of the discrete sine and cosine digital signals 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) after multiplication, we get the product signal V product (t) = Asin(ωt+φ)sin(ωt), which can be expanded using trigonometric formulas to obtain: 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, to obtain the changed resonant angular frequency ω, the present invention employs 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 probe's vibration signal, amplifies, filters, and samples it, and then transmits it to the digital signal processing module. The digital signal processing module primarily consists of a demodulator based on a combination of FPGA and ARM, a phase-locked loop, and direct frequency synthesis (DDS).

[0080] For an input signal, for example, z(t)=Acos(ωt+θ), after multiplying it with 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 based on the phase change Δθ to keep the probe cantilever vibrating at the resonant frequency. The feedback control signal can be expressed as:

[0082]

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

[0084] The feedback control deviation, the feedback control adjustment time and other information can be transmitted back to the host computer module through the communication interface, and the feedback loop control effect can be clearly judged.

[0085] It should be noted that in some embodiments, step S420 can include: S421, determining initial PID coefficients according to a target time constant of the AFM control device; S422, determining a transfer function of the AFM control device and a transfer function of the PID controller, and constructing a closed-loop transfer function model; S423, generating optimized PID coefficients by iteratively optimizing the initial PID coefficients through a minimum performance index method according to the closed-loop transfer function model and in combination with the initial 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 the AFM, the signal adjustment speed is proportional to the signal noise, so the system adjustment speed in the present application is adjustable, which provides a larger operable space relative to the conventional AFM control system. The system adjustment speed can be measured by a time constant, so the target time constant of the system needs to be input before the feedback control module automatically sets the PID coefficients. This parameter needs to be considered by compromising the required response time and the signal-to-noise ratio.

[0087] Further, as shown in Figure 4 , the PID control algorithm automatically calculates appropriate PID coefficients according to the above input characteristic parameters and models. The AFM probe cantilever is a typical mechanical resonant system, and the transfer function model of the mechanical resonant system can be represented by the following formula:

[0088]

[0089] wherein 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, and it is replaced by the characteristic parameter Q value of the resonator, and the transfer function is modified to the following form by considering the nonlinearity and external disturbance:

[0090]

[0091] Further, the control algorithm of the present application defaults to select the form of PID control, while it can be adjusted to the form of PIDF control according to the requirement. The transfer function model of the PID controller has the following form:

[0092]

[0093] Therefore, the closed-loop transfer function model formed by 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 a complex variable in Laplace transform, ω ext represents the external disturbance 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 disturbance amplitude.

[0096] Further, based on the known model described above, the present application optimizes the PID control coefficients by minimizing the performance index method. Common indexes are absolute error integral (IAE), square error integral (ISE), time squared error integral (ITSE), and time multiplied absolute error index (ITAE), etc. These indexes can quantify the control performance of the system. By adjusting the PID coefficients, the error index is minimized to obtain the theoretically optimal controller parameters.

[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] Further, since the above indexes are not intuitive, they are only used inside the control algorithm. The present application feeds back the real-time control effect by calculating the open-loop, closed-loop response, and the amplitude-frequency response, phase-frequency response relationship.

[0099] S500, determining the structure of the sample surface according to the resonant angular frequency variation.

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

[0101] 1) The adaptive feedback loop adjusting method is adopted, the appropriate control coefficient can be directly generated according to the model of the controlled object, the test time is greatly saved, the control coefficient can be adjusted according to the input target response speed, the flexibility of the system is greatly improved, and the control effect can be displayed in real time by the upper computer.

[0102] 2) The system has high integration degree, is composed of a digital signal processing module, a signal acquisition module, a signal generation module, a feedback control module, a communication interface and an upper computer module, secondary development can be carried out according to different test requirements, and the universality of the embodiment of the application is greatly improved.

[0103] 3) The embodiment of the application adopts the feedback adjusting method based on the known model, the control coefficient is optimized by the minimum performance index method, the generated coefficient can be directly used, the test of multiple debugging parameters is avoided, and different types of probes can be better satisfied.

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

[0105] The preferred embodiments of the embodiments of the application are described above with reference to the drawings, and the scope of the right of the embodiments of the application is not limited thereto. Any modification, equivalent replacement and improvement made by those skilled in the art without departing from the scope and essence of the embodiments of the application shall be within the scope of the right of the embodiments of the application.

Claims

1. 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 on the probe cantilever to obtain the probe characteristic parameters; Generate sine and cosine analog signals and drive 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 adaptive feedback adjustment are performed in combination with the preprocessed vibration feedback signal and the probe characteristic parameters to obtain a change in the resonant angular frequency; wherein the process of performing PID control includes: determining an initial PID coefficient according to a target time constant of the AFM control device; determining a transfer function of the AFM control device and a transfer function of the PID controller, and constructing a closed-loop transfer function model; according to the closed-loop transfer function model, in combination with the initial PID coefficient, iteratively optimizing the initial PID coefficient by minimizing a performance index method to generate an optimized PID coefficient, and performing PID control on the amplitude of the preprocessed vibration feedback signal, the frequency of the preprocessed vibration feedback signal, and the phase value of the preprocessed vibration feedback signal; determining the structure of the sample surface according to the change in the resonant angular frequency; The expression of the closed-loop transfer function model is specifically as follows: In the above formula, represents the closed-loop transfer function model, represents the natural frequency of the resonator system, represents the proportional gain coefficient, represents the integral gain coefficient, represents the complex variable in the Laplace transform, Indicates the external interference frequency, represents the differential gain coefficient, represents the cutoff frequency of the differential filter, represents the quality factor of the resonator system, Indicates the external interference amplitude.

2. The method according to claim 1, 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 a change in the resonant angular frequency includes: Performing digital signal processing on the preprocessed vibration feedback signal to obtain the amplitude, frequency and phase 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 coefficients, the outputs of a voltage-controlled oscillator and an automatic gain controller are adjusted according to the errors between the set values ​​and actual measured values ​​of the amplitude, frequency, and phase of the vibration feedback signal to obtain an error adjustment result; Adjusting the frequency and amplitude of the discrete sine and cosine digital signals according to the error adjustment result to obtain a new resonant angular frequency of the vibration feedback signal; The resonance angular frequency of the new vibration feedback signal is subtracted from the natural resonance angular frequency of the probe to obtain a change in the resonance angular frequency.

3. The method according to claim 1, characterized in that The expression of the output signal of the PID controller is specifically as follows: In the above formula, represents the feedback control signal, represents the proportional gain coefficient, Indicates the error between the set value and the actual measured value. represents the integral gain coefficient, represents the differential gain coefficient, represents the time constant of the differential filter, and s represents the complex frequency variable in the Laplace transform.

4. An AFM control device with adaptive feedback loop regulation, characterized in that: The AFM control device is applied to the control method according to any one of claims 1 to 3, and the device includes 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, 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 obtain 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 about 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 and communication; The host computer module is used to display the waveform of the adjusted sine and cosine analog signals.

5. The device according to claim 4, characterized in that The signal acquisition module includes a first operational amplifier, a first low-pass filter and an analog-to-digital converter, wherein the output of the first operational amplifier is connected to the input of the first low-pass filter, and the output of the first low-pass filter is connected to the input 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 preprocessed vibration feedback signal.

6. The device according to claim 4, 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 pre-processed 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 to obtain the frequency value and phase value of the vibration feedback signal; The direct frequency synthesis module is used to generate discrete sine and cosine digital signals.

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

8. The device according to claim 6, characterized in that The feedback control module includes a PID controller, a voltage-controlled oscillator, and an automatic gain controller, wherein the first output terminal of the PID controller is connected to the input terminal of the voltage-controlled oscillator, and the second output terminal of the PID controller is connected to the input terminal 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 sinusoidal and cosine digital signals 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.

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